1. Definition and examples of “truncation and onlap” unconformity
Stratigraphic unconformity is a window for understanding tectonic deformation, sea level change as well as climate fluctuation[1]. It’s also the foundation for identifying tectonic movement (events) stages[2], establishing chronology of regional tectonic movements, and analyzing stratified geologic structure of sedimentary basin[3]. Fluids like oil and gas usually migrate along unconformity, so unconformity is often an important place for accumulation of oil, gas, metallic or non-metallic minerals[4,5,6]. Doubtlessly, unconformity research takes an important position in earth science.
Researchers have carried out long-term investigation on forming time, spatial and geometric variables of unconformity[7,8,9,10,11], a lot of researches on the structure of unconformity and its ore-controlling mechanism have also been done[12,13,14,15]. They have identified three types of unconformities with different contact relationships (nonconformity, angular unconformity and parallel unconformity), and different distribution ranges (interval, regional, and local, etc.). Besides, they also examined weathered crust above, lithologic combination above and below and structural types of unconformities[12,14]. Among the various types of unconformity surfaces, there is a type called “truncation and onlap” unconformity[16], in which, the stratum below the unconformity surface is truncated, and the stratum above the unconformity overlaps the unconformity surface. Usually onlap is more common than downlap (Fig. 1).
Fig. 1.
Fig. 1.
Oil & gas reservoirs and typical stratigraphic truncated unconformities in marine cratonic basins of China. Z1d—Sinian Doushantuo Fm.; Z2dn—Sinian Dengying Fm.; —C—Cambrian; —C1q—Lower Cambrian Qiongzhusi Fm.; —C1с—Lower Cambrian Canglangpu Fm.; —C1l—Lower Cambrian Longwangmiao Fm.; O1-2—Middle to Lower Ordovician; —C2-3-O—Middle to Upper Cambrian-Ordovician; O3—Upper Ordovician; S—Silurian; P—Permian; P1—Lower Permian; T—Triassic; J—Jurassic; K1s—Lower Cretaceous Shushanhe Fm.; K1b—Lower Cretaceous Baxigai Fm.; K1bs—Lower Cretaceous Bashijiqike Fm.; E1-2km—Kumugeliemu Group Paleocene-Eocene; N+Q—Neogene+ Quaternary.
Fig. 1a shows the geological profile of the Yingmaili- Xiqiu area in the northern Tarim Basin. The Shushanhe Formation (Abbr. Fm.) of the Lower Cretaceous overlaps the lower strata of different geologic age. In the north of Yangtake, the Shushanhe Fm. overlaps the lower strata from north to south gradually. In the area of Well Quele1, the Triassic and Jurassic overlaying the lower stratum from north to south can also be seen. To date, under this unconformity surface, many stratigraphic truncation reservoirs of Cambrian, Ordovician, Silurian, and Permian have been discovered, and above the unconformity, sandstone onlap reservoir of Shushanhe Formation has been discovered. Fig. 1b shows the stratigraphic truncation structure below the Carboniferous Benxi Fm. in the Jingxi area of the Ordos Basin. The fifth and fourth members of the Majiagou Fm. of Lower Ordovician were eroded from east to west, and an incised valley is visible at Well Shan 39. Influenced by Yanshan tectonic movement, this unconformity now dips westward, and hydrocarbon from the Carboniferous source rocks migrates laterally or vertically, resulting in natural gas accumulations in multiple layers, mainly in the form of stratigraphic truncated oil and gas reservoirs. Fig. 1c shows the geological profile of the newly discovered giant gas field in the central Sichuan Basin, the Anyue gas field. In the Gaoshiti-Moxi area and Weiyuan-Ziyang area of this field, the Dengying Fm. of Sinian was eroded, at two slopes of the Deyang-Anyue rift trough, the Maidiping Fm. and the Qiongzhusi Fm. of Cambrian gradually overlapped the erosion surface until it’s all covered, and a typical “truncation and onlap” structure is shown in the Cambrian and Sinian. What’s more, in the Weiyuan, Gaoshiti, and Moxi areas (which are called “Deyang-Anyue rift trough”, now known as the “Chuanzhong paleo-uplift”), the Longwangmiao Fm. of the Lower Cambrian and Silurian were eroded layer by layer under the Permian unconformity surface. At the two slopes of the Chuanzhong paleo-uplift, the phenomenon of Ordovician and Silurian onlap can be identified on seismic profiles. Long-term inheritance characteristic of the Chuanzhong paleo-uplift can also be seen on the seismic profile. Currently, in the Moxi area, an intraplatform beach gas field has been discovered in the Longwangmiao Fm. which is also characterized by obvious stratigraphic truncation and unconformity (Fig. 1c).
Besides the above examples, the “truncation and onlap” unconformity is also a common phenomenon in outcrops and seismic profiles, and it also appears in various tectonic and sedimentary environments such as sedimentary basins or the flank of the orogenic belts. However, the genetic mechanisms and oil and gas accumulation models of this kind of unconformity still need further investigation. Several unconformities often superimpose or merge at structural highs, forming a superimposed unconformity as shown in the three examples in Fig. 1[17,18]. But little is known on the types and geologic significance of the superimposed unconformity. In the process of long-term observation of field geological outcrops and interpretation of seismic profiles of several sedimentary basins of China[15-17, 19-23], the author has systematically analyzed and summarized this kind of unconformity. Types of this kind of unconformity under different tectonic environments have been summarized according to the superimposing phenomenon and genetic mechanism, the impact of tectonic activities on the development of superimposed unconformity has also been probed based on the analysis of the transition of sedimentary center.
2. Types of “truncation and onlap” unconformities and superimposing styles of overlying and underlying strata
“Truncation and onlap” unconformities may develop under different stress mechanisms, including extension, compression and strike-slip. Due to the joint effect of ancient landform undulation, sea level fluctuation, and climate, the overlying strata may overlap the higher position of the paleo-geomorphology or prograde to the remote domain, forming down-lap, which finally results in “truncation and onlap" unconformity. Clearly, under different tectonic settings, this kind of unconformity has different structural styles and characteristics.
2.1. “Truncation and onlap” unconformity as well as superimposing styles of overlying and underlying strata in extensional environment
In extensional environment, half-graben or graben are the most common structures, also called fault-sags, which are controlled by the geometry of the boundary normal faults and the episodic tectonic events. With the migration and development of boundary faults in different periods, there are mainly three basic superimposing styles of fault-sag structures.
2.1.1. Parallel-superimposing style with breaking outward
In this kind of superimposing style, fault-sag zones of different periods shift, which is manifested in the form of breaking outward. This phenomenon is common in the Cretaceous fault-sags in the Songliao Basin and Songnan area, as well as the underlying fault-sags on the passive continental margins on both sides of the Atlantic.
As shown in Fig. 2a-1, the half graben formed in the phase I rifting was filled with layers 1-3. Thereafter, structural reverse occurred, and layers 4-6 were eroded and missing. Entering the phase II rifting, the fault activity shifted to the right boundary fault, layers 7-10 filled the right fault-sag, and layers 7-9 gradually overlapped the underlying unconformity surface from right to left, while the layer 10 covered the entire half graben. In the Fig. 2b-1, the chronostratigraphic framework profile shows that the unconformity at the bottom of the layers 7-10 and the unconformity at the top of the layers 1-3 have comparable distribution range, and there are approximately 3-9 layers of strata missing. The upper and lower strata with same dipping directions formed the “truncation and onlap” contact relationship.
Fig. 2.
Fig. 2.
Three basic superimposing styles of “truncation and onlap” unconformities in extension environment.
2.1.2. Superimposing style with rotation
Due to the change of strike of the boundary fault in different fault-sag periods, the axial directions of the early and late fault-sags rotate, resulting in a superimposed unconformity, which is common in basins like Beihai and Songliao. In Fig. 2a-2, the half graben formed in the phase I rifting was filled with layers 1-4. Thereafter structural reverse occurred, and layers 5 and 6 were eroded and missing. Entering the phase II rifting, the fault activity shifted to the right boundary fault, but the new/right boundary fault was not parallel to the old/left boundary fault. And the new fault-sag was filled with layers 7-10, the layers 7-9 gradually overlapped the underlying unconformity surface from right to left. The layer 10 covered the entire half graben. This kind of superimposing relationship has some similarities with the parallel-superimposing with breaking outward in Fig. 2a-1. But the main subsidence center of the former has rotated and migrated laterally, besides, the main sediment dispersion mode is also different from the latter one, that is, the source-sink system has changed.
Chronostratigraphic framework profile in the Fig. 2b-2 shows that the unconformity at the bottom of the layers 7-10 has similar distribution range with the unconformity at the top of the layers 1-4, and there are approximately 2 to 8 layers of strata missing between the two sets of strata. The overlying and underlying strata with similar tilt direction constitute the “truncation and onlap” contact relationship.
2.1.3. Parallel-superimposing style breaking inward
The migrating process of different periods of fault-sags appears in breaking inward pattern. This kind of superimposing has been observed in Bohai Bay, Songliao and Beihai Basins etc. The fault-sag transition mode between the Langgu Sag and the Baxian Sag in the northern part of the Jizhong Depression presents the breaking inward characteristic[24,25,26]. In Fig. 2a-3 the half graben formed in the phase I rifting was filled with layers 1-3. Thereafter structural reverse happened, resulting in missing of layers 4-6. Entering the phase II rifting, fault activity shifted to the left fault in the same strike and the new fault-sag was filled with layers 7-10, and layers 7-9 gradually overlapped the underlying unconformity surface from right to left, then, layer 10 covered the entire half graben. In the northern part of the Bozhong depression, activities of faults like the Daxing fault, Niudong-Hexiwu fault all ended in this kind of structure style[24,25,26]. The early faulted strata between the two boundary faults can be the material source area of the later fault-sag.
Chronostratigraphic framework profile in the Fig. 2b-3 shows that the unconformity surface under the layers 7-10 is narrowly distributed, while the unconformity surface at the top of the layers 1-3 has a wide distribution range. Approximately 3 to 9 layers of strata missed between the two unconformities, the overlying and underlying strata with same dipping direction form the “truncation and onlap” contact relationship.
2.2. “Truncation and onlap” unconformity and superimposing styles of overlying and underlying strata in compressional environment
In compressional environment, thrust faults, thrust fault- related folds and foreland basins etc. are the most common structural deformation. The unconformity surfaces formed under this environment vary a lot (Fig. 3), with growth unconformity surfaces developing in some cases.
Fig. 3.
Fig. 3.
Four basic styles of “truncation and onlap” unconformities in compressional environment.
2.2.1. Overlap superimposing over fold
This type is common in the Yubei-Tangnan area of the Tarim Basin, the Zhongguai bulge and the Beisantai bulge of the Junggar Basin, the Appalachian Basin and the foreland of Atlas Mountains of North Africa. Controlled by compressive stress, anticline is the most common structural deformation, its axial surfaces are usually inclined toward each other. As shown in Fig. 3a-1, the fold formed after the deposition of the layers 1-3, and layers at the top of them were eroded, leading to layers beneath the layer 1 denuded. Sedimentary hiatus took place on the unconformity surface, and layers 4 and 5 are missing. After entering another tectonic cycle, layers 6-9 deposited. Because of the anticline core was at higher position, the strata 6-9 then overlapped the structural high gradually, and the upper section of layer 9 eventually covered the whole anticline core. During the deposition period of layers 6-9, there might be no strong tectonic activity, forming a simple overlap structure and the phenomenon of “truncation and onlap” in the limbs of the anticline; the anticline might increase in amplitude under continuous compression. In this case, layers 6-9 might rotate and result in the decrease of the dip angle of layers 6 to 9 gradually. This phenomenon is also known as “growth strata” or syn-depositional structure, in which the dip angle of the strata will change with the strength of the tectonic compression. The angular unconformity surface between the two sets of strata (layers 1-3 and layers 6-9) shown in Fig. 3a-1 was folded, indicating that tectonic activity happened during the deposition stage of layers 6-9.
Chronostratigraphic framework profile in the Fig. 3b-1 shows that the strata 4 and 5 are missing regionally. In the limbs of the fold, unconformity with the same dipping direction developed, hiatus only occurred at the core of the fold. The missing strata form a symmetrical “bell” shape.
2.2.2. Syn-depositional fold superimposing style
This kind of unconformity is mainly found in the foreland thrust belt, especially the position where detachment fold develops, for example the Yaken anticline belt in the Kuche depression of the Tarim Basin, the Anjihai anticline belt in the southern margin of the Junggar Basin, the Xinchang, Jiulongshan and the Tongnanba anticline belts in the Sichuan Basin. As shown in Fig. 3a-2, at the first deposition stage, layers 1-3 deposited first, then a sedimentary hiatus followed, with layers 4-5 missing regionally. Thereafter, the secondary deposition stage came, when structural compression took place, forming a syn-depositional anticline. Since the high position of the paleo-geomorphology have changed with time, the layers 6-8 presents a feature of limb rotation and the dip angle of layers 6-8 gradually decrease. At the end of the deposition stage of layer 8, tectonic compression ceased.
The chronostratigraphic framework profile in Fig. 3b-2 shows that the missing strata form a typical “bell” shape. The formation at the anticline core below the unconformity might be eroded. In this type of superimposing, the overlap phenomenon in the higher position of fold is more distinct, while the truncation at the lower position only occurred at the core of the fold.
2.2.3. Anticline-syncline superimposing style
This kind of unconformity mainly appears in the background of multi-stage compression. In the early compression stage, the anticline fold and corresponding erosion unconformity surface formed. While in the later stage fold superimposing pattern changed, which developed into a typical superimposed unconformity.
As shown in Fig. 3a-3, the layers 1-3 deposited firstly, and fold activity mainly happened at the end of the depositional stage, producing an unconformity surface. During the deposition period of layers 4-5, fold activity took place again, and layers 4 and 5 overlapped toward the unconformity surface (could appear in the form of down-lap). After the deposition of layer 5, tectonic activity ceased and layers 6-8 filled the accommodation space formed by the tectonic activity, which resulted in the thickening of strata in the syncline and thinning in the anticline position.
Chronostratigraphic framework profile in the Fig. 3b-3 shows that the missing strata between the bottom of layers 4-5 and the top of layers 2-3 constitute a “lenticular” morphology. This kind of unconformity is characterized by low dip angle of the upper overlap layers and the lower truncated layers.
2.2.4. Superimposing style in overlap-filling incised valley on top of fold
This kind of unconformity is common in basins like the Williston baisn, Persian Gulf, Sichuan basin, Ordos basin, and Tarim Basin [27,28]. Fig. 1a and 1c has this feature, the lower strata were truncated more obviously while the upper strata overlapping the unconformity surface were horizontal or low in dip angle.
In Fig. 3a-4 after the deposition of layers 1-4, tectonic compression occurred, forming an anticline. Thereafter a deposition discontinuity happened during the depositional period corresponding to the layers 5-7. Affected by paleo-geomorphological, hydrodynamic, climatic factors, the top of the anticline was eroded more seriously, forming a typical incised valley, which was filled with layers 8-11 later. The sediment first filled the valley, to the depositional period of layer 11, the sediment covered the entire fold.
Chronostratigraphic framework profile in the Fig. 3b-4 shows that unconformity at the bottom of the layers 8-11 is nearly parallel with the unconformity at the top of the layers 1-4, and the absence of strata at the core of the anticline constitutes a “funnel” shape.
2.3. “Truncation and onlap” unconformity in strike slipping environment
Observation of strike-slip sedimentary basins reveals that this kind of basin, like Ridge, San Joaquin, Los Angeles, Vienna, Pannonia and Qaidam, has unique geological structure. Due to the steep occurrence of boundary faults and quick change of depositional facies, the lateral migration of depositing center is obvious. Besides, distribution range of the “truncation and onlap” unconformity is narrow and the evolution time is short, but the superimposition of unconformity can also occur at the edge of strike-slip basin.
3. Basic styles of superimposed unconformity
The “truncation and onlap” unconformities formed in different geological periods often superimpose with each other in three dimensional space (Fig. 4). The geological superimposing process of them can be divided into three basic types.
Fig. 4.
Fig. 4.
Basic types of superimposed unconformity.
3.1. Continuous uplifting type
Inherited uplifts are more common in Craton Basins, such as the Tazhong uplift, Chuanzhong uplift, and the central platform of the Western Texas Basin in USA. At some stages of the basin’s development, sediments gradually filled the accommodation space existed previously, just like layers 1-4 of sequence I (SQI) shown in Fig. 4a-1. At the end of the deposition period of SQI, tectonic compression or differential uplifting activities happened. In Fig. 4a-1, uplifting at left resulted in unconformity, and layers 1-4 were denuded from left to right, with erosion area reaching the center of the original sedimentary zone possibly, and only layer 1 was preserved at the most left of the profile.
During the deposition of sequence II, uplifting at the end of the left was still going on, but at the same time layers 5-7 deposited. Because of the change of structure and geomorphology, layers 4 and 5 in the central part of the sedimentary zone deposited continuously, while at the higher position in the left, layers 5-7 overlapped the lower strata layer by layer, forming a typical “truncation and onlap” unconformity. The layers 5 and 6 overlapped the layer 4 sequentially, the layers 6 and 7 overlapped the layer 3, while the exposed zone of layers 1 and 2 was erosion area, layer 3 was the source area during the depositional period of layers 5 and 6. At the end of the deposition period of layer 7, the erosion area was expanded again, and layers 6 and 7 were denuded.
During the deposition stage of sequence III (SQIII), the layers 8-10 deposited, sequentially onlapping the lower strata from right to left, the layer 8 overlapping the layers 7 and 4, the layer 9 overlapping the layer 3, the layer 10 overlapping the layer 2 or layer 1 sometimes. At the end of the deposition period of SQIII, regional tectonic movement occurred, forming a low-angle unconformity or hiatus in the whole region, and strata under the layer 10 was eroded.
During the depositing of sequence IV (SQIV), the depositional surface was relatively flat and the layer 11 or younger strata covered the whole area.
As shown in Fig. 4a-1, the unconformities between SQ I and SQ II, SQ II and SQ III, SQ III and SQ IV superimposed toward the left. The chronostratigraphic framework profile in Fig. 4b-1 shows the temporal and spatial contact relationships of strata. The unconformity at the bottom of the layers 5-10 is diachronous, under which the top of SQ I was eroded all the time, and the most left part had experienced the longest erosion process. The development of this kind of unconformity geometry is the result of continuous uplifting of the left part of the section in Fig. 4a-1.
3.2. Migration of uplift
In the marginal uplift belts of sedimentary basin, such as the paleo-uplifts of Tabei and Hetian, and the central paleo-uplifts in Ordos and Sichuan Basins, this kind of phenomenon is common.
SQ II includes layers 5-7, there is a gap between SQ I and SQ II, the layers 3 and 4 are missing. The regional tectonic activity after the deposition of layer 7 resulted in the regional stratigraphic unconformity, erosion area expanded as a result, and layers 5-7 were eroded from left to right at the higher position of the uplift.
SQ III comprises layers 10-13, which overlays the unconformity at the top of the SQ II and I from left to right. The layers 8 and 9 between SQ II and III are missing. The SQ III and SQ I, SQ II form “truncation and onlap” unconformity with same dipping direction.
The regional tectonic activity prior to the deposition of the SQ IV caused unconformity at the bottom of the SQ IV and missing of layers 14-16. As shown in the Fig. 4a-2, increase of uplifting speed in the left part resulted in higher topography. SQ IV comprises layers 17-21, which overlapped the unconformity from right to left at the bottom of the SQ IV.
The chronostratigraphic framework profile in Fig. 4b-2 shows that the four sets of sequence space from each other, with strata missing between any adjacent two. The unconformity at the bottom of SQ III and SQ IV form superimposed unconformity with the underlying layers, respectively. A “truncation and onlap” unconformity structure with opposite dipping direction formed between SQ IV and SQ I.
It can also be seen in Fig. 4a-2 that the “truncation and onlap” unconformity which is dipping in the same direction and the one which is opposite-dipping constitute superimposed unconformity in space due to structural high migration or differential uplifting. This phenomenon has been found in the Kongque River Slope in the northeastern part of Tarim Basin, where the unconformity at the bottom of the Jurassic is superimposed unconformity. The Ordovician-Triassic dips to the interior of the basin, at the same time, a number of unconformity surfaces exist at the bottom of the Silurian, Carboniferous and the Triassic, which constitute superimposed unconformity with same dipping direction, while the Jurassic dips to the margin of the basin, overlapping the basin from margin to center, constituting opposite-dipping superimposed unconformity.
3.3. Superimposed unconformity under propagation of deposition center
When deposition center of basin migrates toward the same direction, the unconformities would migrate toward the outside of basin and superimpose one by one. This is common in foreland basin, extensional basin or strike-slip basin.
Taking the foreland basin shown in Fig. 4a-3 as an example, the thrust structure moved at the margin of the basin, resulting in flexural subsidence of foreland depression and uplifting of forebulge.
During the development of SQ I, layers 1-4 deposited, overlying the strata underlying layer by layer from left to right. In the deposition stage of SQ II, layers 6-9 deposited in the same manner, but the distribution range was wider. During the deposition of SQ III, the layers 12-15 overlapped the substrata from left to right, expanded over the forebulge, and resulting in deposition of layers 14 and 15 in the outer depression. The layer 5 is missing between the SQ I and SQ II, and the layers 10 and 11 are missing between the SQ II and SQ III. At the highest position of the forebulge, only the latest deposited layer 15 remains. The unconformity at the bottom of the foreland basin expanded to the right, forming a superimposed diachronous unconformity.
4. “Truncation and onlap” unconformity formed under the transition of basin subsidence center
The transition or migration of subsidence center in sedimentary basin is a common tectonic phenomenon. In strike- slip basin, due to the movement of strike-slip fault, the expansion of basin and migration of subsidence center are more common. In superimposed basins, the migration of subsidence center in different period is also common due to changes of boundary conditions and subsiding mechanisms.
4.1. Unconformity formed under the transition of paleo-geomorphy or subsidence center
The migration or transition of basin subsidence center is a general phenomenon in superimposed basin. In Fig. 5a-1, layers 1-4 deposited in basin stage I. The layers 1 and 2 are relatively uniform in thickness. When the layers 3 and 4 was depositing, the subsidence center was mainly at the left part of Fig. 5a-1, so the layers 3 and 4 are thicker at left and thinner at right.
Fig. 5.
“Truncation and onlap” unconformity formed under the transition of basin subsidence center.
Before the deposition of layer 9, the layers 5-8 were missing, which may be a sedimentary discontinuity. Maybe it’s some tectonic activity that resulted in the uplift of the right part of Fig. 5a-1. In the basin stage II, the subsidence center was mainly at the right part, and layers 9-12 deposited, covering the underlying strata from right to left layer by layer, when the layer 12 deposited, the unconformity surface was overlapped entirely.
The chronostratigraphic framework profile in the Fig. 5b-1 shows that there is a large discontinuity between the unconformity surface between the bottom of the layers 9-12 and the unconformity surface at the top of the layers 1-4, with nearly 5 to 8 layers of strata missing. The opposite-dipping overlying and underlying strata constitute the “truncation and onlap” unconformity.
4.2. Unconformity formed under the migration of fault-sag
In the evolution of fault basin, because of the differential activities of boundary faults, regional unconformities often formed in different faulted structural layers. In the Fig. 5a-2, fault activity mainly occurred at the right side in fault-sag phase I, which controlled the formation of the right half graben filled with layers 1-5. Thereafter, tectonic reverse occurred, which resulted in the missing of layers 6-7.
Entering the fault-sag phase II, the activity of the right boundary fault ceased, but the left boundary fault started to move, the left half graben was filled with layers 8-12, and deposition area had been gradually enlarged at the same time. When the layer 12 was deposited, the accommodation space of half graben was entirely filled, and the uplifted wall was also covered by layer 12. The layers 8-12 and the layers 1-5 were reversely dipping, forming a contact relationship similar to the “seesaw”.
The chronostratigraphic framework profile in Fig. 5b-2 shows that the unconformity surface at the bottom of the layers 8-12 is widely distributed, forming “funnel” shape, while the unconformity surface at the top of the layers 1-5 has limited distribution. There are roughly 7 layers of strata missing, and the overlying and underlying strata constitute an opposite-dipping “truncation and onlap” unconformity.
This kind of unconformity is more common in the Alashan block, the Yabulai basin, etc. (Fig. 6). As shown in Fig. 6, above the basement of pre-Jurassic, the Jijigou Fm. of Lower Jurassic, the Qingtujing and Xinhe Fm. of Middle Jurassic, and the Shazaohe Fm. of Upper Jurassic deposited in the Saertai area of the southern Yabulai basin. After that, the Heiciwan bulge in the northern part of the basin uplifted and then eroded, forming an unconformity surface. During Cretaceous, the north of the basin subsided, resulting in the deposition of river-lake sediments of the Miaogou Fm. of Lower Cretaceous and the Jingangquan Fm. of Upper Cretaceous[29]. Unconformity at the base of the Cretaceous finally tilted.
Fig. 6.
Characteristic of unconformity formed under the migration of fault-sag in the Saertai area, Yabulai basin. Anj—Pre-Jurassic; J2q—Middle Jurassic Qingtujing Fm.; J2x1— 1st Member of Middle Jurassic Xinhe Fm.; J2x2—2nd Member of Middle Jurassic Xinhe Fm.; J3s—Upper Jurassic Shazaohe Fm.; K— Cretaceous; E—Eocene; Q—Quaternary.
4.3. Unconformity formed under the slipping of detachment fault
This kind of unconformity occurs generally in fault-sag basins and passive continental margin basins. The boundary fault of basin is a stepped structure composed by fault ramp and fault flat. When the hanging wall slides along the fault plane, the fault-ramp anticline and fault-ramp syncline may occur, and the syncline area will be a new sedimentary zone, which in turn may result in the transition of deposition center.
In the Fig. 5a-3, the normal boundary fault at the right side was active in the fault-sag phase I, which controlled the formation of the right half graben, the half graben was filled with layers 1-7 later. Subsequently, the structural reversion resulted in the missing of layers 8-10.
Entering the fault-sag phase II, activity of the boundary fault on the right side ceased, and the boundary fault on the left side began to move, but this fault was a large-scale ramp-flat fault, which had greater control on the fault-sag development than the right boundary fault, and then the new accommodation space was filled with layers 11-15. Controlled by the geometry of the boundary normal fault, a fault-ramp anticline formed on the ramp, and a fault-ramp syncline formed at the right of it. The overlying strata, layers 11-15 deposited in the left half graben, in which the layers 11-14 overlapped the unconformity surface formed at the end of the fault-sag phase I. In the area of fault-ramp syncline, the layers 12-14 overlapped the higher position of the fault-ramp anticline layer by layer, under which the layers 8-11 were missing. However, in the area of fault-ramp anticline, the top unconformity surface formed in the fault-sag phase I was folded, and layers 11-14 overlapped the higher position of the fault-ramp anticline layer by layer from two directions. In the depositional period of the layer 15, the fault-sag was covered entirely, and the whole area converted to depression process.
The chronostratigraphic framework profile in Fig. 5b-3 shows that the unconformity surface at the bottom of the layers 11-15 constitutes an upper convex “bell” shape, but the unconformity surface at the top of the layers 1-7 is narrowly distributed, and there are roughly 5 to 9 layers of strata missing between them. The overlying and underlying strata at the left side of the fault-ramp anticline constitute the opposite-dipping “truncation and onlap” unconformity; while on the right side of the fault-ramp anticline, this kind of unconformity is dipping in the same direction.
5. The temporal and spatial transition of the “truncation and onlap” unconformity
Stratigraphic unconformities of different stages, properties and styles may superimpose each other spatially. Whether it is extension converting to compression, or compression converting to extension[30], the transition surfaces between them are mostly superimposed unconformity surfaces.
The profile of Mahu sag in Junggar Basin, Saertai sag in Yabulai Basin and Kaiping sag in Pearl River Basin all show the temporal and spatial changes of superimposed unconformities, but because of differences in structural environment, they differ widely from each other.
The proven and controlled stratigraphic/lithologic reservoirs in the Mahu sag of the Junggar Basin have the petroleum resource amount of more than 12×108 t. There are many unconformities in the Permian and Triassic. Stratigraphic truncation phenomenon is common in the western slope of the Mahu sag, several stratigraphic truncated unconformities superimposed each other[31]. By contrast, in the eastern slope of the Mahu sag, the “truncation and onlap” unconformity is common (Fig. 7). In the Mahu sag, The Fengcheng Fm. of Lower Permian overlays the western part of the Luliang Island Arc Uplift Belt[31], 4 sets of onlap seismic reflection groups are visible in the Fengcheng Fm., and the distribution range gradually expands. There are 4-5 sets of onlap seismic reflection groups in the Xiazijie Fm. of the Middle Permian. The Xiazijie Fm. and the Fengcheng Fm. which are dipping in the same direction constitute “truncation and onlap” unconformity. To the higher position of the western part of the Luliang uplift, unconformities at the bottom of the Fengcheng Fm., and the Xiazijie Fm. superimposed with unconformity at the Lower Member of the Wuerhe Fm., and finally three of them merge into the unconformity at the base of the Triassic. The Lower Wuerhe Fm. is thick, which comprises four sets of onlap strata overlapping the western part of the Luliang Uplift, which form low-angle “truncation and onlap” unconformity with same dipping direction. These factors show that in this period, tectonic activities were strong, and the supply of sedimentary sources correspondingly changed greatly, resulting in the multi-stage development of fans. For these fans, the lateral superimposition and transition changed quickly. In the eastern slope of the Mahu sag, the Upper Wuerhe Fm. is missing. And the Lower Triassic Baikouquan Fm. also overlaps to higher position from lower part of Mahu sag, which shows the process of the deposition range expansion too. The sedimentary thickness of the Lower Member of Middle Triassic Karamay Fm. generally is stable, but the Upper Member of this formation thinned toward the western part of the Luliang Uplift. The formation of the above-mentioned unconformity structure was also obviously affected by the multi-phase and long-term activity of the western part of the Luliang Uplift. The formation and evolution of the western part of the Luliang Uplift is closely related to the activity of the underlying fault (Fig. 7). In seismic profiles, the dip angle from Fengcheng Fm. to Karamay Fm. reduce obviously. Because of the long-term and multi-phases uplifting of the western part of the Luliang Uplift, strata at the slopes of the uplift are characterized by multi-period overlapping, off-lapping and truncation, thus forming a series of “truncation and onlap” unconformity structures. The sediment denuded from the western part of the Luliang Uplift stacked nearby, forming a number of sand-conglomerate fans. The hydrocarbon generated by the Fengcheng Fm. and Jiamuhe Fm. in the Mahu sag migrated along these unconformities towards the western section of the Luliang Uplift, and finally accumulated in the sand-conglomerate reservoirs of the Xiazijie Fm., the Lower Wuerhe Fm., the Baikouquan Fm., and the Karamay Fm. Exploration so far has shown that the eastern slope of the Mahu sag is a large-scale stratigraphic unconformity oil and gas enrichment belt with oil and gas accumulated in multiple layers, such as the Permian and Triassic, which has great exploration potential.
Fig. 7.
Stratigraphic superimposed unconformity characteristics of eastern slope of Mahu depression, Junggar basin. P1f— Lower Permian Fengcheng Fm.; P2x—Middle Permian Xiazijie Fm.; P2w—Middle Permian Lower Wuerhe Fm.; P2w1—1st Member of Middle Permian Lower Wuerhe Fm.; P2w2—2nd Member of Middle Permian Lower Wuerhe Fm.; P2w3—3rd Member of Middle Permian Lower Wuerhe Fm.; P2w4—4th Member of Middle Permian Lower Wuerhe Fm.; T1b—Lower Triassic Baikouquan Fm.; T1b1—1st Member of Lower Triassic Baikouquan Fm.; T1b2—2nd Member of Lower Triassic Baikouquan Fm.; T1b3—3rd Member of Lower Triassic Baikouquan Fm.; T2k—Middle Triassic Karamay Fm.
In the above example, the deposition of the Fengcheng Fm. happened in tectonic extensional environment, and the Jiamuhe Fm. or the Carboniferous could be seen overlapping the western part of the Luliang Uplift under the unconformity surface at the bottom of the Fengcheng Fm. At the end of the depositional period of Fengcheng Fm., this area changed to tectonic compression, and the Xiazijie Fm. and the Lower Wuerhe Fm. deposited in foreland basin. Unconformities at the bottom of the Lower Wuerhe Fm. and Xiazijie Fm. superimposed with the unconformity at the bottom of the Fengcheng Fm., the nature of superimposition is obvious[32]. This kind of structure also has some significance for oil and gas accumulation.
Stratigraphic unconformity would change with tectonic development. For example, in Fig. 1a, the unconformity at the bottom of the Lower Cretaceous Shushanhe Fm. increasingly inclined to the north; in Fig. 1b, the unconformity at the bottom of the Upper Carboniferous in the Ordos Basin gradually tilted to the west since the Late Jurassic; in Fig. 1c, the bottom of Permian and the bottom of Cambrian sharply tilted toward the Longmen Mountains since the Yanshan Movement; in Fig. 6, the unconformity at the bottom of the Cretaceous has tilted with a large angle, which was caused by quick migration of the subsidence center; in Fig. 7, unconformities at the bottom and inner of the Permian gradually steeped, which was related to the subsidence of the Mahu sag and the uplift and rotation of the Luxi Bulge.
The Kaiping sag in the Pearl River Mouth Basin (Fig. 8) is a basin developed on detachment faults. The convex “anticline” at the left end of the Fig. 8 is located below the bottom of the Upper Oligocene Zhuhai Fm. The bottom reflection of Oligocene (T80), the bottom reflection of the Upper Member of Upper Wenchang Fm. (T81) and the bottom reflection of the Middle Member of the Upper Wenchang Fm. (T82) of this anticline are all truncated, the seismic stratigraphic surface of T80, T81 and T82 are all “truncation and onlap” unconformity surfaces. Three of these unconformities superimpose with the bottom unconformity of the Zhuhai Fm. (T70) at the top of the bulge. The restoration of this structure shows that it’s formed under the large-scale slipping of detachment fault and under the moving of the overlying fault block as well as the gradual rotating of stratigraphy, that is, these unconformities are not formed in present position, they are the result of tectonic development. The KP11-1-1 at the central of Fig. 8 is fault-ramp anticline, where unconformities occur at the base of the Zhuhai Fm. (T70), the base of the Upper Member of Enping Fm. (T71), the base of the Middle Member of Enping Fm. (T72), besides, unconformities at the base of the Upper Wenchang Fm. (T83) and the base of the Middle Member of the Upper Wenchang Fm. (T82) rotated at large angle in the limbs of anticline. In the Kaiping main sag at the right part of Fig. 8, a number of unconformities developed from the base of the Cenozoic (Tg) to the base of the Zhuhai Fm. all of which are characterized by “truncation and onlap” unconformity, and they not only moved with the underlying detached faults but also rotated with the geometric change of the large-scale ramp-flat fault at the bottom, the occurrence of these unconformities gradually become more gentle upward. The fault-sag shown in Fig. 8 experienced long-distance movement, which results in the extreme development of stratigraphic unconformity, the stratigraphic hiatus time of the unconformities usually is 0.5-1.0 Ma, and the hiatus time of the unconformity at the base of the Upper Member of Enping Fm. is up to 3.0 Ma.
Fig. 8.
Sketch of north-south geological profile crossing Well KP11-1-1 in Kaiping Sag, Pearl River Mouth Basin. Tg—base of Cenozoic; T85—base of the Middle Member of the Lower Wenchang Fm.; T84—base of the Upper Member of the Wenchang Fm.; T83—base of the Upper Wenchang Fm.; T82—base of the Middle Member of the Upper Wenchang Fm.; T81—base of the Upper Member of the Upper Wenchang Fm.; T80—base of the Oligocene; T72—base of the Middle Member of the Lower Oligocene Enping Fm.; T71—base of the Upper Member of the Enping Fm.; T70—base of the Upper Oligocene Zhuhai Fm.; T60—base of the Lower Miocene Zhujiang Fm.; T50—base of the Upper Member of the Zhujiang Fm.; T40—base of the Middle Miocene Hanjiang Fm.; T35—base of the Upper Member of the Hanjiang Fm.; T32—base of the Upper Miocene Yuehai Fm.
6. Geological significance of “truncation and onlap” unconformity
The formations overlying and underlying the “truncation and onlap” unconformity are favorable places for oil and gas accumulation[4,14,16], which has been proven by a series of discoveries, such as the Panhandle-Hugoton oil and gas field in North America; the Orinoco heavy oil belt in Venezuela in South America; the Cambrian gas field in North Africa etc.; and in China, Hudson Oilfield in the Tarim Basin[14]; Silurian oil and gas reservoirs on the northern slope of the Tazhong area; Shinan 21, Shinan 31 oilfields on the Luliang Uplift in the Junggar Basin; the stratigraphic unconformity large oil field recently found in the Upper Wuerhe Fm. in the western slope of the Mahu sag and the Zhongguai bulge[15, 33-35]; the Ordovician and the Lower Permian gas reservoirs in the eastern region of the southern section of the central paleo-uplift in the Ordos Basin; the oil reservoirs preserved in the Ordovician and 1st Member Shahejie Fm. in the Lixian slope of the Raoyang depression Bohai Bay Basin; and the oil reservoirs in the western slope of the Songliao Basin. Based on the “truncation and onlap” unconformity, the lower stratigraphic truncated oil and gas reservoirs and the upper stratigraphic overlying oil and gas reservoirs may form. Because of the differential development of the weathered crust and differential sealing capacity of the cap rock, oil and gas reservoirs above and below the unconformity vary significantly in scale[14].
The three examples shown in Fig. 1 are typical. In Fig. 1a, in the Tarim Basin, the oil and gas preserved under the unconformity at the base of the Cretaceous Shushanhe Fm. came from the southern Cambrian-Ordovician source rocks which belongs to Marine intra-craton depression sequence, hydrocarbon generated from these source rocks migrated from south to north. However, above the unconformity, the oil and gas originated from the Triassic-Jurassic terrestrial source rock in the Baicheng Sag, migrated from north to south. Oil and gas in these two systems experienced opposite-directional long-distance migration. This unconformity plays an important role in the north to south long-distance migration of oil and gas, evolving into Tabei complex oil and gas accumulation area.
Fig. 1b shows that in the Ordos Basin the gas generated from the coal-measure source rock of the Benxi Fm. Upper Carboniferous-Shanxi Fm. Permian. Because of the westward rotation of the bottom unconformity surface of the Upper Carboniferous Benxi Fm. the lateral hydrocarbon supply window came about in the weathered crust between the Benxi Fm. and the Ordovician, the probability of hydrocarbon accumulation in the Ordovician stratigraphic truncation traps has greatly increased, which has been proven by recent discoveries in the Jingxi area again, and it is predicted that the eastern slope of the central paleo-uplift of the Ordos is giant natural gas enrichment zone of this type.
There are many discussions about the example of the Sichuan Basin in Fig. 1c[36,37,38,39], the hydrocarbon generated in the Deyang-Anyue rift trough[36], and accumulated in the Sinian platform high-energy beach facies and the Lower Cambrian intraplatform beach facies[37,38,39], these hydrocarbon systems are also typical examples of lateral migration and accumulation. Due to the development of Weiyuan anticline in the Cenozoic, the natural gas migrated and adjusted westward. Because of severe hydrocarbon dissipation in the Weiyuan anticline, the Weiyuan gas field decreased in scale. In contrast, the tectonic background is relatively stable in the Anyue area, so large gas field was preserved. According to this model, the Sinian, Cambrian, Permian and Triassic in the northern Moxi area and the Ziyang area have great exploration potential, where multiple stratigraphic “truncation and onlap” unconformity gas reservoirs could constitute a compound oil and gas field group.
The role of stratigraphic unconformity in hydrocarbon accumulation has been discussed extensively[4,14,40-41]. Sandstone and glutenite above the unconformity surface can form long- distance migration channels due to “channel effect” [42]. This is common in Cratonic Basin and gentle slope of half graben as well as foreland basin slope. At present, a corresponding model has been established for oil and gas long-distance migrating mechanism along the unconformity[42,43,44].
A comprehensive analysis shows that the petroleum geological significance of unconformity is: (1) The karst system often develops under the unconformity surface, which is helpful in forming weathered crust reservoirs, and multi-stage karst systems can superimpos on each other to form a karst- fractures system with a vertical thickness of 200-300 m, such as the Tahe Oilfield and Jingbian Gas Field (Fig. 1b). (2) A series of traps usually occurs along a stratigraphic unconformity, such as stratigraphy truncation traps and stratigraphy overlap traps, the unconformity can also combine with anticlines and faults to form structural-stratigraphic, structural-lithologic, and other traps[14]. (3) Because of the unconformity structure and the bottom conglomerate above the unconformity, the stratigraphic unconformity often act as a channel for long-distance hydrocarbon migration[19]. (4) The unconformity increases the probability of fluid exchange between different tectonic layers, so oil and gas can accumulate in multiple tectonic periods. The example in Fig. 1 presents this characteristic, for example, the Jingbian gas field and Jingxi gas field, and Xinlongtai Archaeozoic gneiss buried-hill oil field in Liaohe fault depression even has oil sourcing from Tertiary. (5) The regional stratigraphic unconformity is superimposition interface of sedimentary basin, at this interface, a set of hydrocarbon accumulations may emerge. Such unconformities are often the key positions for oil and gas accumulation in the superimposed basins (Fig. 1).
The formations above and below the “truncation and onlap” unconformity are also favorable sites for metal mineral deposits[5,6], such as lead-zinc, gold, uranium, etc. There is a large Cretaceous sandstone uranium mine in Dongsheng area, Ordos Basin, which is related to the dominant migration pathway along the unconformity surface.
The “truncation and onlap” unconformity surface is clear reflection of tectonic events. Whether it is in a sedimentary basin or an orogenic belt, the tracing and analyzing of them is the key to understanding the orogenic process and basin developing mechanism, and especially an important window to understanding the intra-continental deformation[32, 45-46]. In the Mahu sag of Junggar Basin[32], Permian and Triassic strata at the west slope can be seen truncated[47], while stratigraphic overlapping can be seen on the eastern slope (Fig. 7), which reflects the west slope of the Mahu sag experienced strong activity of syn-depositional faulting, while the east slope near the paleo-uplift experienced tilting and uplifting.
7. Conclusions
“Truncation and onlap” unconformity is a common geological phenomenon. In extension tectonic setting, there are three basic unconformity types, i.e., parallel superimposing with breaking outward, parallel superimposing with breaking inward and superimposing with rotation. They are commonly seen in basins like Bohai Bay Basin, Songliao Basin in eastern China; and overseas like North Sea Basin, and passive continent margin basins on both sides of the Atlantic ocean. In compressional tectonic environment, there are four types of unconformities, and so four superimposing styles of overlying and underlying strata, overlap fold superimposing, syn-depositional fold superimposing, anticline-syncline superimposing, as well as superimposing in overlap-filling incised valley, which are common in Tethys belt, Appalachian, and foreland basin of Rocky Mountain Belt.
“Truncation and onlap” unconformity surfaces often superimpose with each other in space, forming superimposed unconformity. Superimposed unconformities include types associated with continuous uplifting, transition and uplifting, as well as propagation of deposition center.
The “truncation and onlap” unconformity may change with time, such as tilting, rotating, folding and migrating, etc. In sedimentary basin, this kind of unconformity phenomenon caused by the migration of the sedimentary center is also very common, which may include unconformity formed under the transition of paleo-geomorphy or subsidence center, unconformity formed under the migration of fault-sag, as well as unconformity formed under the slipping of detachment fault.
The “truncation and onlap” unconformity surfaces are main pathways for hydrocarbon migration. Multiple types of traps may form above and below these unconformity surfaces, which are important sites for accumulation of mineral resources. The accumulation of oil and gas along these unconformity surfaces is a major feature of superimposed basins.
Reference
Principles of stratigraphy
Unconformity analysis and its significance in the study of continental basin tectonics
Three topics are discussed in this paper as follows:(1)unconformity analysis and its significance in the study of basin formation and evolution;(2)unconformity analysis and its relation with intensity of tectonic movement and basin modification;(3)types of stratigraphic traps related with unconformities. From above mentioned discussions, it is concluded that:(1)normal cycle and onlap above unconformity were occurred at extensional and neutral tectonic settings, whereas reverse cycle, offlap and truncation unconformity were developed at compressional tectonic environments;(2)composite, progressive and syntectonic unconformities are related to accelerated and lessened uplifts in the margin of intracontinental foreland basins;(3) based on the structural deformations below truncation unconformities and eroded thickness of strata, the degree of basin modification resulted from tectonic movement, and the types of stratigraphic traps related to unconformities can be recognized.
Significance of unconformity to oil and gas migration and accumulation
The significance of unconformity and its later transfer movement in gold metallogenesis: An example from gold deposits in the contiguous area of Yunan-Guizhou-Guangxi provences
5 angular and parallel unconformities have occurred in strata in the contiguous area of Yunnan－ Guizhou－ Guangxi provinces since Palaeozoic era.Angular unconformity belongs to an intraplate folded－ orogenic deformation.Parallel unconformity is the record of intraplate epeirogenic deformation.Both are an unstable contact and always transfer into brittle detachment fault(when located at mid－ superstructure level),ductile shear fault(while located at mid－ infrastructure level) during later tectonic movement.The formation of gold deposits in the area are affected highly by the later transformation.These affections find expression in improving interaction of fluid－ rock,promoting the re－ migration and re－ concentration of gold－ bearing fluid and providing the depositional location for gold metallogensis.
Mineralization mechanism and prospection on unconformable contacts
DOI:10.1007/BF02911033
URL
[Cited within: 2]
Unconformity contains two types: disconformity and clino-unconformity. Mineralization occurred near the unconformable contact is an ubiquitous phenomenon. The unconformable contact was usually considered as a good transporting channel and a depositing room for ore-forming fluid. However, it has not yet been seriously considered that the structural movement of later stages occured in the unconformable contact might play a big role in the mineralization(or the transportation of ore-forming fluids). On the basis of lots of ore deposits that occurred on unconformable contact, we have summed up the main characteristics of them as follows: all ore deposits occurred on unconformable contact or near basement and cover strata, usually distributed as mineral concentration regions with large reserves and high grade; the mineralization process of these mineral deposits is of two stages: the sedimentary stage and the hydrothermal superposing stage, in which the alteration of wall rocks occurred mainly under low temperature; the ore-forming elements of this kind of deposit are nearly the same as those of other types of deposits in the same area; but in two different types of unconformable contact the shape of ore body and the place of mineral deposits are different. Finally, we discuss why there are no unconformable type of uranium deposit in China, and why the deposits occurred in the unconformable contact in China are mainly characterized by epithermal types.
Seismic stratigraphy and global changes of sea level, part 3: Relative changes of sea level from coastal onlap//PAYTON C W. Seismic stratigraphy applications to hydrocarbon exploration
Seismic stratigraphic interpretation and petroleum exploration
DOI:10.1111/j.1365-2753.2010.01469.x
URL
[Cited within: 1]
Abstract OBJECTIVES: Inappropriate use of antibiotics may increase antibiotic resistance and health care service load. Few studies have been conducted to investigate physician non-adherence to antibiotic prescription guidelines for treating urinary tract infections (UTIs) in child patients. This study aimed to examine the rates of and factors associated with non-adherence to antibiotic prescription guidelines for treating uncomplicated UTIs in child patients. METHODS: This is a cross-sectional study in which a random sample of 8921 children who received antibiotics prescription for UTIs (ICD-9-CM: 590.1, 595.0, 595.9 or 599.0) at age 17 years or less, between 2000 and 2007, were analysed. Data analysed were retrieved from Taiwan's National Health Insurance database. Non-adherence was determined by antibiotic prescription not recommended by the Infectious Disease Society of Taiwan. Multivariate logistic regression model was employed to assess the potential predictors for non-adherence, including various characteristics of patients, physicians and medical institutions. RESULTS: The overall non-adherence rate was estimated at 20.05%. Older patients, older physicians and physicians from community clinics were associated with higher rates of non-adherence. Compared with pediatricians who had the lowest rate (13.15%) of non-adherence, certain specialties were found to have significantly elevated adjusted odds ratio (AOR) of non-adherence, with the highest one noted for gynecologists (35.11%, AOR = 2.29, 95% confidence interval: 1.89-2.77). We also observed that the most frequently prescribed antibiotics not recommended on guidelines varied with physician specialty. CONCLUSIONS: Special attention should be concentrated on older physicians, gynecologists and physicians who practiced at community clinics to reduce non-adherence of antibiotic prescription for treating UTIs in child patients. 2010 Blackwell Publishing Ltd.
The missing time calculation of the hiatus surface: A case study of the upper Permian reefs section at the platform margin, Ziyun, Guizhou
DOI:10.1111/j.1755-6724.1997.tb00365.x
URL
[Cited within: 1]
Taken as an example the Upper Permian reef section on the carbonate platform margin at Ziyun, Guizhou, the paper discusses the missing time of a hiatus surface, an important problem in chemical sequence stratigraphy, with the concept of cosmic chemistry. Then the paper proposes a series of new concepts for chemical sequence stratigraphy, including the condensation surface, relative compaction factor and time missing factor. Finally a quantitative curve of Late Permian relative sea090009level change in the Ziyun area is presented with time coordinates.
Review of the studies on the hiatus surfaces in the records of stratigraphy.
Review on the dating of the weathering profiles
Weathering plays a significant role in the evolution of geochemistry of the Earth's surface and region environment.Precise dating of the weathering profiles may provide important information on these evolutions. However, direct dating of weathering events and profiles have been considered an international difficult problem. The dating methods of weathering crust has made great advances in last decades, such as dating by the 未 18 O values of clay minerals, the K/Ar and 40 Ar/ 39 Ar study of cryptomelanes, the chemical weathering rates, and so on. Some new important methods of dating weathering crust were introduced and reviewed in this paper.
Characteristics and oil- controlling significance of unconformity structure layer on top of carbonate rock
Above and under the unconformity at the top of carbonate rocks in North Tarim (Tabei) uplift show that three structural layers were developed widely They are sedentary deposit layer, seepage flow layer, and under flow layer. Related studies of distribution and oil-controlling factors indicate: sedentary deposit layer occurs widely in both above and under the unconformity. It is the best path to regional oil and gas migration; the seepage layer mainly distributes on high part of palaeoarch, it is a potential place to seek large cave-fissure oil and gas fields; the underflow layer mainly distributes in the slope of uplift-sag transition, it is another possible place to seek "inside" oil and gas pools.
Advances in research of spatial structures of unconformity
DOI:10.1007/s11442-006-0415-5
URL
[Cited within: 1]
Unconformity generally consists of unconformity interface, overlying rocks and underlying rocks, and is spatially characterized by three stratal structures, consisting of rocks above the unconformity interface, weathered clay below the unconformity interface, and semi-weathered rocks under the weathered clay. Weathered clay is compact under compression of overlying rocks and constitutes good seals. Rocks (mainly including sandstone and conglomerate) above the unconformity interface, are characterized by better porosity and permeability, while semi-weathered rocks (mainly sandstone, conglomerate, carbonate rock, volcanic rock and metamorphic rock) below the unconformity interface, are characterized by development of fractures and solution porosity, form not only good pathways for hydrocarbon migration, but also effective reservoir beds for hydrocarbon accumulation. Due to influence of tectonic position, lithology, and so on, an unconformity spatial structure is very complicated, which renders the law that governs hydrocarbon accumulation and reservoir formation at the unconformity to be complicated too. So, research on unconformity spatial structure is beneficiary to exploration of hydrocarbon reservoirs at such unconformities.
Structure and hydrocarbon accumulation on unconformities
Characteristics of unconformity between the Xishanyao Formation and Toutunhe Formation of Middle Jurassic in Junggar Basin and its significance in petroleum exploration
DOI:10.1007/978-3-540-72108-6_3
URL
[Cited within: 3]
Unconformities are widespread at the base,top and within the Jurassic in Junggar Basin Among these unconformities,the unconformity is quite distinct between the Xishanyao Formation in the lower part of Middle Jurassic and the Toutunhe Formation in the upper part of Middle Jurassic. With an e- roded thickness of 100～340 meters,it is a regional unconformity,and has a lacuna (or hiatus) of 4～10 Ma.On the unconformity,sandstones were deposited and immediately below it were the weathered clay in- terval and the semi-weathered rocks just below the unconformity. Generally,the unconformity shows a succession of sandstone-mudstone-sandstone with a thickness of 70～90 meters.Based on the weathered clay intervals,the sandstones of the Toutunhe Formation formed a series of the stratigraphic onlap traps sealed by the weathered clay intervals at the bottom,while the sandstones of the Xishanyao Formation formed the truncated traps sealed by the weathered clay intervals at the updip.The petroleum exploration has proved that the areas above or below this unconformity are favourable areas for oil and gas accumula- tion.The late stage tilting of the unconformity has played an important role in the oil and gas accumula- tion,and adjustment and reallocation.
Characteristics of composite petroleum systems of superimposed basins in China
The superimposed petroliferous basins in China are characterized by long evolutionary history, multiple stages of tectonic evolution, and multiple depositional sequences. The petroleum systems in these basins present complexities of multiple kitchens with multiple source rocks, multiple phases of petroleum generation-migration-accumulation, multiple oil and gas intervals, and multiple critical moments involved in their formiations. It is difficult to characterize such petroleum systems by applying the concepts proposed by Magoon Dow (1994). Based on the complexities of the formation and features of the petroleum systems in China, this paper proposes the concept of the composite petroleum system, and the methods of the classification and evaluation. The basic types of composite petroleum systems in China include the stacking, variable migration and accumulation, fault connection, common sealing, and modification etc. Each superimposed basin consists of one or more of petroleum systems. The superimposed basins of foreland and cratonic in western and central China are characterized by fault connection composite petroleum system in fold and thrust belts, and by composite petroleum systems migrating and accumulating along unconformities. The superimposed basins of fault sag and depression in eastern China are characterized by stacking composite petroleum system. The superimposed basins of remnant and new born in southern China are characterized by modified composite petroleum system.The features of composite petroleum system in each type basin are much different, and also for its exploration methods. The characterization of composite petroleum system is the basis for choosing the exploration area. The division and evaluation of petroleum migration and accumulation unit are critical to proposing the prospect of petroleum exploration. Integration of modern exploration techniques plays an important role in the promoting efficiency of oil and gas exploration of the composite petroleum systems in China.
Paleo-structural geomorphology and its controls upon distribution of sedimentary facies of Paleozoic Central Uplift Zone, Tarim Basin
Unconformities and oil and gas accumulation in Tarim Basin
Research on the relation of unconformity type with oil and gas migration- accumulation at Zhuengaer Basin
DOI:10.3863/j.issn.1000-2634.2006.02.001
URL
According to the actual unconformity data of Zhuengaer Basin and the relation of lap-erosion on the upper or lower part of unconformity layer, a new kind of unconformity type demarcation project has been put forward. This kind of project has divided the line unconformity as four kinds of types:onlap/erosion truncation type, concordance/erosion truncation type, onlap/concordance type, concordance/concordance type. On the basis of research on the relation of unconformity type with oil - gas migration- accumulation, the upper or lower part of onlap/erosion truncation type are best of all favorableness for accumulation of oil and gas; the upper part of concordance/erosion truncation type are favorableness for migration of oil and gas, the lower part are favorableness for accumulation of oil and gas; the upper part of onlap/concordance type are favorableness for accumulation of oil and gas, the lower part are favorableness for migration of oil and gas; the upper or lower part of concordance/concordance type are favorableness for migration of oil and gas,and favorableness for accumulation of oil and gas if the match of fault and fold.
The unconformity surface in the Permian of Junggar basin and the characters of oil-gas migration and accumulation
The unconformity configuration type and its reservoir forming control in Jiyang Depression
DOI:10.1007/s11442-006-0415-5
URL
Based on an analysis about the geological and geophysical characteristics, the unconformity styles developed in Tertiary in Jiyang depression were studied. The stratigraphic contact relationships, the unconformity relationships and their microscopic constitutions were discussed, and a new classification scheme based on macroscopic style of unconformity was suggested in the paper. In the paper the unconformity in Tertiary in Jiyang depression was divided into 4 macroscopic structural styles, and 16 unconformity configuration types. Based on these research results, the controlling effect of unconformity configuration to oil-gas migration and accumulation was analyzed, the Tertiary unconformity reservoir distribution mode was established, and the exploration and deployment practice of such kind of reservoir were discussed.
Unconformity type and its significance in petroleum geology
Various unconformities can be classified into such seven types of unconformities by conformation and mechanism as parallel, truncation, overlap, fold, faulted-fold, extensional and growth ones. Vertically, rocks near the unconformity appear layers which are in turn semi-weathering rock, weathering clay bed and basal conglomerate. The unconformity plays important roles on the migration and accumulation of hydrocarbons in nature as a pathway for long-distance petroleum migration. And paleo-weathering crust or palaeokarst belt take the part of improvement of reservoir quality of rocks, forming unconformity reservoirs. However, the unconformity may cause caprock damage and affect the maturity of hydrocarbon source rocks.
3D fine geological structure of Langfang-Gu’an fault-sag in Bohaiwan Gulf Basin, China
3D geometry and kinematics of Daxing fault: Its constraints on the origin of Langgu Depression, Bohaiwan Gulf Basin, China
DOI:10.1007/s11783-011-0280-z
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[Cited within: 2]
Daxing Fault is a boundary fault of Langgu Depression which is located in the northwest of Jizhong Depression in Bohaiwan Gulf Basin.It controls the sediment and structural evolution of the whole depression.According to the strike direction,Daxing Fault is divided into three segments which are northeastern segment,middle segment,and southwester segment,and the origin of their linkages has been discussed.This paper shows the triaxial geometric graph of Daxing Fault plane for the first time,and analyzes it.We consider that Daxing Fault plane is a surface which consists of several inclined planes separated by axial planes.According to the strike direction,axial planes are divided into two types,a transversal axial plane whose strike direction is parallel to the fault strike and a vertical axial plane whose strike direction is vertical to the fault strike.Daxing Fault plane is divided into 10 zones by 4 transversal axial planes and 2 vertical axial planes,and each zone has its own configuration and feature.We also have discussed the mechanism of Daxing Fault movement,and consider that Daxing Fault movement in a domino normal pattern at first,and then the fault rotated to form the fault that was flattened from top to bottom.The stratum of the hanging wall rotated with fault at first,and then moved in an inclined shear pattern to form a rollover anticline when the fault was flattened from top to bottom.
Structural genetic types of paleo-buried hill in Jizhong Depression, Bohai Bay Basin
Buried-hill is an important exploration target in Jizhong depression.Analyzing its geologic structure and disclosing its formation mechanism is key to deepen the understanding of its tectonic evolution and the oil and gas exploration fairways.Based on the high-resolution 2-D and 3-D seismic data,and hundreds of deep exploratory wells across the depression,the paper used the structural analysis method to explore the buried-hill stratigraphic strata,fault system,and tectonic evolution as a whole,and put forward a new scheme for buried-hill genetic classification.It is shown that the buried-hill in Jizhong depression underwent a series of processes such as the sedimentary rock formation before Mesozoic,the compression in Late Jurassic,the multi-phase extension during Cretaceous to Paleogene,and the thermal subsiding and burying during Neogene to Quaternary.According to the tectonic location,the structural deformation feature,and the strength of extension,the buried-hill can be divided into 14 kinds of basic genetic mechanism.Due to the long period of tectonic evolution and the combined influences of the inner-and exo-genetic geological processes,the buried-hill has largely of a composite genetics.In map-view,the different genetic buried-hill zones exhibit regularly.It gives 7 kinds of structural association,such as,the tilted block and high bulge,antithetic fault step,synthetic fault step and high bulge,transverse anticline or relay high and inverted slope,high bulge and extension allochthon block and inverted slope,high high-bi-direction rotation-high bulge,tilted block and metamorphic core complex,etc.Among these,such types of buried-hills as the antithetic tilted block,the high bulge,the fault step,the inverted slope,and the allochthon block have favorable petroliferous potential owing to the matching of the source rocks and the reservoir rocks,the timing for trapping and accumulating.The oil and gas resource is much rich in Jizhong depression.The above genetic classification of buried-hill is helpful for addressing the oil and gas accumulation and distribution.
Discovery of Xuanhan- Kaijiang Paleouplift and its significance in the Sichuan Basin, SW China
DOI:10.1016/S1876-3804(16)30115-X
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A large inherited paleouplift from Pre-Sinian to Early Cambrian, named Xuanhan-Kaijiang paleouplift, has been discovered based on multiple geological information in the eastern Sichuan Basin: firstly, onlap deposition of Upper Sinian Dengying Formation and Lower Cambrian is observed from peripheral zone to central Xuanhan-Kaijiang area from the seismic profile; secondly, Dengying Formation thickness distribution map shows that this formation is thinnest in Xuanhan-Kaijiang area, and thickens towards the surrounding areas; thirdly, terrigenous clastic rock is observed in Upper Sinian Dengying Formation on outcrops in Chengkou County and Wuxi County etc., indicating the development of a paleouplift. The analysis of the characteristics, formation and evolution of the paleouplift reveals that the plane distribution of the paleouplift shows an arched structure, stretching in a north-south trend. The area of the paleouplift is around 16 000 km 2 . Vertically, the paleouplift can be divided into three parts: platform in the middle, steep slope in the west and ramp in the east. Originally, the paleouplift was an erosion-type uplift above the water level during the early stage of the Late Sinian; and gradually transformed into deposition-type uplift under water level during the late stage of the Late Sinian to the Early Cambrian. The formation of the paleouplift was controlled by several factors, including basement uplift, intracratonic rift, basement faults and regional uplifting. The paleouplift and its slope areas, favorable for the development of high energy facies and karst weathering reservoirs, are important exploration target areas in the eastern Sichuan Basin in the near future.
The characteristics of Precambrian sedimentary basin and the distribution of deep source rock: A case study of Tarim Basin in Neoproterozoic and source rocks in Early Cambrian, Western China
DOI:10.1016/S1876-3804(16)30116-1
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Based on outcrops of the whole basin, drilling and seismic data, this paper makes a systematic analysis of Neoproterozoic basin stratigraphic distribution and tectonic evolution characteristics in Tarim. Under the control of the aggregation and breakup of Rodinia supercontinent, the extensional sedimentary basin formed with North-South differences in the Neoproterozoic, separated by the middle ancient land. There is a sedimentary basin in the north, east-west banded, with two thickness centers, Aksu region in northwest and Manjiaer-Kuruketage region in northeast and maximum thickness about 2 500 3 500 m. There are two sedimentary basins in the south, north-east banded, with two thickness centers, Yecheng and Hetian with maximum thickness about 1 500 2 000 m. The evolution of the basin can be divided into two stages, rift in Nanhuan (Cryogenian) and depression in Sinian (Ediacaran), the latter stage deposits larger range but with thinner thickness of strata. Its main rock types contain volcanic rock, clastic rock, moraine rock and carbonate rock. The distribution of source rock during Early Cambrian period is mainly controlled by the development characteristics of the Neoproterozoic sedimentary basin.
Structural evolution and hydrocarbon accumulation in Yabulai Basin
Advances and challenge of structural geology of deep sedimentary basins in China
DOI:10.13745/j.esf.2017.03.020
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One of the aim of structural geology is to study the deep sedimentary basins;it applies the theory of modern earth system sciences and utilizes the comprehensive methods of geophysics,geochemistry,and geology to the study of the material composition,geological structures,and processes of the formation and evolution of deep-layers of sedimentary basin,and to the exploration of the structural controls upon its energy resources and mineral deposits distribution.The sedimentary basins in China are predominated by the superimposed basins in nature.As far as the deep-layers of sedimentary basin is concerned,its proto-type,evolution of tectono-paleo-geography,dynamic processes of formation and modification,and controlling of tectonic evolution on the conditions of deep-seated oil and gas pool-formation are the key study areas for petroleum structural geologists.After more than a half century of exploration,researchers in China made lots of important advances in the above-mentioned areas.They recognized four basic types of superimposed basins such as foreland-,depression-,faulted-sag-,and strike-slip-types,restored the proto-type sedimentary basins of different ages,reconstructed the tectono-paleo-geographic evolution,explored the relationships between the basins and the adjacent orogenic belt and its key controls on the structure and framework of the deep basins,built the 3-D structure models by analysis of the multi-period of structural superimposition,and studied the stress-field,the geo-temperature field,the geo-pressure field and their interactive processes.The structural geology on deep basins made progresses in the study thinking,the research methodology and the according technologies and got to a mature science.Looking back on the study history and making a comparison between home and aboard,the structural geology on deep basins in China has a marked lag than western countries on theories and practical needs,and it is needed to explore heavily on the tectonic setting and deep regime for basin development,the processes of basin formation and evolution,and the basin tectonics and structures,and it is hoped to make definite breakthrough on the鈥榤obile'tectono-paleo-geography,3-D structural restoration,rock rheology,and 4-D dynamic basin modeling,etc.Through cultivating lots of researchers and scholars with creativity,choosing the important study area for breakthrough,reforming the technology and tools,and making lots of case studying throughout the world,we hope the structural geology on deep basins will make important advances in the coming years.
Tectonic framework of the northern Junggar Basin Part II: The island arc-basin system of the western Luliang Uplift and its link with the West Junggar terrane
DOI:10.1016/j.gr.2014.08.019
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PMID:2976090
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61Early Carboniferous strata are defined in the western Luliang Uplift.61Northward subduction generated a Carboniferous island arc basin system.61Western Luliang Uplift matches with West Junggar in tectonostratigraphic evolution.61Paleozoic volcanic arc is one of the important components of Junggar basement.
Tectono-depositional setting and its evolution during Permian to Triassic around Mahu sag, Junggar Basin
Relation of unconformity with hydrocarbon accumulation in Luxi area of Junggar Basin
Relation between unconformity surface and hydrocarbon migration and accumulation of Luliang uplift in Junggar Basin
Luliang uplift in Junggar basin is abundant in unconformity surfaces, which are significant to the hydrocarbon migration and accumulation. The unconformity surfaces in the bottom of Permian system and Triassic system are the key factors for hydrocarbon accumulation in the late period of Jurassic. The unconformity surfaces in the bottom of Jurassic system are favorable to the migration of deep hydrocarbon accumulation in Jurassic system. The unconformity surfaces in the bottom of Cretaceous system are the necessary elements for the migration of hydrocarbon from the Jurassic system to the Cretaceous system. There are five types of unconformity surfaces, which played the different roles in the hydrocarbon migration and accumulation in the research section. The distribution of the unconformity surfaces has three characters of discrepancy, succession, and migration which have great influence on the hydrocarbon distribution and reallocation.
History of pool formation of Shinan oil gas field, Junggar Basin
Shinan gas oil field is located on Jidong nose arch, in Luxi region in the centre of Junggar basin. There are oil reservoir in upper and lower part of the section and gas reservoir in the middle parts of Middle Lower Jurassic. Upper oil reservoir is in medium porosity, medium permeability sandstones in Toutun Group, and is a structural lithologic oil trap; middle gas reservoir is in low porosity, low permeability sandstones, and is a lithologic gas trap; and the lower oil reservoir is in a medium porosity, low permeability sandbody in Sangonghe Group, and is a structural oil trap. These oil traps have been considered as secondary oil pools in the past. However, the oil in the pool has not been ed to any alteration. The oil has characteristics of sapropelic origin, and the degree of thermal evolution is rather low. Gas in the reservoir is humic type with rather high thermal evolution. A study of the formation of oil and gas pool indicates that, Permian source rocks in the sag west of well Pen 1 are sapropelic and large amount of liquid hydrocarbons are generated and migrated in Early Cretaceous. Liquid hydrocarbons migrated along the Jurassic tensional faults and entered into sandbodies with rather good petrophysical properties in Toutunhe Group and Sangonghe Group to form primary oil pools. The Upper Permian Urho Group source rocks are humic type and large amount of gaseous hydrocarbons have been generated and migrated along the tensional faults. Most of them were trapped in the sandbodies with inferior petrophysical properties in Sangonghe Group to form primary gas reservoir. Subject
Discovery of intra- cratonic rift in the Upper Yangtze and its coutrol effect on the formation of Anyue giant gas field
DOI:10.1038/aps.2015.144
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PMID:4722986
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APS (Acta Pharmacologica Sinica), the top pharmacology research journal based in China, publishes original articles and reviews on all aspects of pharmacology and the related life sciences
Tectonic evolution from Late Sinian to Early Paleozoic and natural gas exploration in northwestern Sichuan Basin, SW China
DOI:10.1016/S1876-3804(16)30001-5
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Based on the field outcrops, drilling and seismic data of northwestern Sichuan Basin, this paper systematically discusses tectonic evolution characteristics from the Late Sinian to the Early Paleozoic, analyzes the petroleum geological significance, and points out the recent favorable exploration directions in this area. The area experienced mainly three times of tectonic evolution during the Late Sinian to the Early Paleozoic: (1) During the Late Sinian to the early period of Early Cambrian, this area was affected by tension action and formed two sets of nearly north-south trending continental rifts, and two sets of platform margins in the Sinian Dengying Formation. (2) During the middle period of the Early Cambrian to the Middle Ordovician, this area was eroded strongly (stronger in the northwest than in the southeast) because of multi-period uplifting of the northwestern Bikou paleo-land. (3) During the Late Ordovician to the Late Silurian, this area was eroded strongly again because of the compression of the Kwangsian Orogeny, and the erosion was stronger in the southwest than in the northeast. During the Late Sinian to the Early Cambrian, the formation of continental rifts controlled the development of bioherm beach facies reservoirs on the platform margins in the Dengying Formation and the high quality source rock in the Lower Cambrian series. During the middle of the Early Cambrian, the uplifting of the Bikou paleo-land controlled the distribution of the Kongmingdong Formation oolitic beach facies reservoirs around the paleo-land. It is suggested that, vertically, the fourth Member of the Dengying Formation and the Kongmingdong Formation be the main exploration targets, and, horizontally, the Jiulongshan structural trap be the most beneficial exploration belts recently due to its matching with petroleum migration and accumulation.
Hydrocarbon accumulation of Sinian natural gas reservoirs, Leshan- Longnvsi paleohigh, Sichuan Basin, SW China
DOI:10.1016/S1876-3804(16)30023-4
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The old Sinian carbonate reservoir in the Leshan-Longn眉si paleohigh was taken as a research object to study the process of gas accumulation in the Sinian reservoir through analysis of gas reservoir characteristics, gas composition, gas reservoir types, accumulation condition and evolution. The results show that the reservoir lithology and type are almost the same in the six gas pools discovered in the Leshan-Longn眉si paleohigh. All the gas reservoirs are characterized by high temperature, ordinary pressure, and intense heterogeneity. The gas reservoir type in different layers and the gas compositions and carbon isotopes in different locations vary obviously. The gas of Sinian Dengying Formation, originated from oil cracking, is mixed gas mainly from source rocks of Sinian Dengying Formation as well as Cambrian Qiongzhusi Formation. The source and reservoir condition, their combination and fluid transporting conditions are favorable, which can determine the gas accumulation and preservation in Dengying Formation. The Sinian gas reservoirs are believed to have been accumulated by the following processes: paleo-oil accumulation, paleo-oil cracking, and gas reservoir adjustment and finalization. There are three processes of gas accumulation in the reservoir, which are influenced by the formation of paleohigh and differential structural evolution in different positions.
Accumulation conditions and enrichment patterns of natural gas in the Lower Cambrian Longwangmiao Fm. reservoirs of the Leshan-Longnüsi Paleohigh, Sichuan Basin
DOI:10.1016/j.ngib.2014.10.006
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As several major new gas discoveries have been made recently in the Lower Cambrian Longwangmiao Fm reservoirs in the Leshan-Longn菤si Palaeohigh of the Sichuan Basin, a super-huge gas reservoir group with multiple gas pay zones vertically and cluster reservoirs laterally is unfolding in the east segment of the palaeohigh. Study shows that the large-scale enrichment and accumulation of natural gas benefits from the good reservoir-forming conditions, including: (1) multiple sets of source rocks vertically, among which, the high-quality Lower Paleozoic source rocks are widespread, and have a hydrocarbon kitchen at the structural high of the Palaeohigh, providing favorable conditions for gas accumulation near the source; (2) three sets of good-quality reservoirs, namely, the porous-vuggy dolomite reservoirs of mound-shoal facies in the 2nd and 4th members of the Sinian Dengying Fm as well as the porous dolomite reservoirs of arene-shoal facies in the Lower Cambrian Longwangmiao Fm, are thick and wide in distribution; (3) structural, lithological and compound traps developed in the setting of large nose-like uplift provide favorable space for hydrocarbon accumulation. It is concluded that the inheritance development of the Palaeohigh and its favorable timing configuration with source rock evolution are critical factors for the extensive enrichment of gas in the Lower Cambrian Longwangmiao Fm reservoirs. The structural high of the Palaeohigh is the favorable area for gas accumulation. The inherited structural, stratigraphic and lithological traps are the favorable sites for gas enrichment. The areas where present structures and ancient structures overlap are the sweet-spots of gas accumulation.
Role of surface of unconformity in formation of oil or gas reservoirs
The existence of unconformity surface can not only destroy oil or gas reservoirs, but also play an important role in formation of oil or gas reservoirs to some extent. By theoretical analysis and actual examples, this paper discusses the important role of unconformity surface in formation of oil or gas reservoirs. The existence of unconformity surface can make sourcerock generate secondary hydrocarbon, improve space and performance of reservoir of strata that are located in upper and lower part of it. As a lateral migration path of oil or gas, the existence of unconformity surface can lead to oil or gas accumulation from different oil or gas layers. The existence of surface of unconformity provides three kinds overlap traps of buried hill, stratagraphic denudation and stratagraphic for oil or gas to accumulate.
Unconformity and transport hydrocarbon characteristic
This paper studies the unconformity and its upper and lower rock structure and characteristic,dividing them into upper bottom conglomerate,weathered clay and half-weathered rock. It proposes that communicating pores of the bottom conglomerate and the discharge-weathered fracture system of half-weathered rock are main channels of oil and gas transportation by uncomformity. The discharge-weathered fracture system has a stronger transportation capability than the communicating pores of the bottom conglomerate. For the uncomformity formed by non-uniform,brickle and hard formation wearthering,discharge-weathered fracture system and communicating pores of the bottom conglomerate are the main transportation channels.For the uncomformity formed by uniform,fine grained,clay contained and soft formation, discharge-weathered fracture system and communicating pores of the bottom conglomerate are the main transportation channels before the overlapped sedimentation charge reaches a certain amount. After that, only communicating pores of the bottom conglomerate is the main channel.
Petroleum Migration Pathways and Charge Concentration: A Three Dimensional Model
Petroleum migration pathways and charge concentration: A three dimensional model: Discussion
The main factor controlling petroleum migration along unconformity in the Junggar Basin
Many oil and gas pools have been found above or below the unconformity in the Junggar Basin,indicating that the unconformity is a good conduit for petroleum migration.Based on the petroleum-bearing fluid activities in unconformity zones,we suggested that vertical structure of unconformity is the most key factor controlling the petroleum migration.Two layers of unconformity were divided,i.e.bottom conglomerate(upper layer) and weathered crust(lower layer).In addition,if some mudstone were developed on the top of weathered crust,the unconformity was three-layer structured.The weathered crust could be further divided into weathered fractured crust based on volcanic rocks,and weathered solution crust based on detrital rocks.Then,we defined three types of unconformity according to lithostratigraphic features of the bottom conglomerate and the weathered crust.They include overlap-base type,sandstone-conglomerate type,and sandstone-mudstone type,among which the first one is the most favorable for petroleum migration,the second intermediate,and the last could generally not be considered as a conduit.
Characteristics of geologic framework of multicycle superimposed basin in Tarim Basin
The Geologic structures of Tarim Basin nowadays has been resulted from the long geological evolution and the composition andsuperimposition by the different proto-type basins during their different geologic period. It is thus separated vertically by regional unconformitiesand cut horizontally by large-scale fault zones, the outline, as a result, indicates the strong heterogeneities in the geological structures such aslayering vertically and fracturing horizontally.The superimposition of tectonic-sedimentary sequences exhibits such styles as continuous, hiatusand reversion. Basically, there are seven superimposed styles as follows, the superimposition between Cenozoic foreland depressions andMesozoic fault-depressions, the superimposition between Cenozoic foreland depressions and Mesozoic fault-depressions and Paleozoic cratonicpassive margin, the superimposition between Mesozoic marginal depressions and Early Paleozoic marginal depressions, the superimpositionbetween Mesozoic fault-depressions and the hinterland of foreland thrust belts, the superimposition between Cenozoic foreland depressions andPaleozoic intra-cratonic depressions, and the superimposition between Meso-Cenozoic uplifts and Paleozoic intra-cratonic depressions as wellas the superimposition between Cenozoic foreland thrust belts and Paleozoic continental margins.The geologic framework of sedimentary basinhas played an important role in the development of petroleum system and the oil/gas distribution.The late Hercynian and the late Himalayanperiods are the critical moments for oil and gas reservoirs forming in Tarim Basin.The related paleo-uplifts, paleo-slopes, and the foreland thrustbelts has controlled the favorable positions of oil/gas accumulation.
Comparison study on controls of geologic structural framework upon hydrocarbon distribution of marine basins in western China
The marine middle and lower petroliferous associations of Tarim,Sichuan,and Ordos basins in West China are the key areas for oil and gas exploration around the country.To disclose the characteristics of oil and gas distribution is crucial for the on-going exploratory discoveries and the exploration efficiency promotion.Based upon the new discoveries and the new advances in petroleum exploration during the recent years,the paper applied the comparative method to discuss the tectonic controls upon oil and gas occurrence.It is shown that the marine basins,such as Tarim,Sichuan,and Ordos basins,are located in the composite area of the Paleo-Asia Ocean and the Tethys Ocean tectonic domains,and are multi-cycle superimposed basins developed upon the pre-Cambrian crystalline basement.They underwent three extension-compression cycles during the Sinian to Middle Devonian,(Late Devonian-)Middle Carboniferous to Triassic,and Jurassic to Quaternary periods,respectively.Each cycle develops the independent hydrocarbon source-reservoir-cap rock assemblages,characterizing by the excellent source rocks occurred in the differentially subsided areas during extension periods,and the large-scale spreading of the reservoir rocks and the trap associations during compression periods.The basin structures are featured of stratified vertically and faulted blocks horizontally.The basement sub-division played a role in the formation of the above sedimentary proto-type basins.It is demonstrated that the superimposing styles among the proto-type basins controlled the hydrocarbon accumulation strata and accordingly the rich areas and plays.The oil and gas enrichment areas in the basins are formed due to the two constructive tectonic movements such as the Middle Caledonian and the Indo-China,and the two adjustment movements such as the Late Yanshanian(Late Jurassic to Early Cretaceous Period)and the Late Himalayan(Pliocene to Quaternary).It is common for pool-formation in the neo-tectonic period.The marine superimposed basins are entitled of a series of petroliferous beds and intervals,especially of the superimposed boundaries among the proto-type basins proved to be an important area for oil and gas accumulation;and many oil and gas accumulation zones or areas occurring along the uplift belt,the fault zone,the platform marginal zone as well as the litho-facies variation zones.It is concluded that the middle and lower association of the superimposed basins is favorable for the next on-going breakthrough in petroleum exploration.
Kinematics of syn-tectonic unconformities and its implications for tectonic evolution of the Hala'alat Mountain at the NW-margin of Junggar Basin
DOI:10.1016/j.gsf.2014.07.004
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61We construct syn-tectonic unconformities above the Carboniferous in study area.61We depict the characteristics of these syn-tectonic unconformities.61These syn-tectonic unconformities are related to the reactivities of West Junggar.
