Introduction
The rifted basin is one important type of petroliferous basins in China, and the Bohai Bay Basin is a typical representative of this kind of basin. The basin is characterized by single-rift or dual-rift slope structures, and the middle and low parts of the slope and the deep rifted areas (generally corresponding to the trough during sedimentation) account for more than 60% of the slope. After more than 50 years of large-scale exploration and exploitation, China’s continental oil and gas fields have all entered middle-late stage of exploration, naturally, it is increasingly difficult to find large scale oil and gas fields in the shallow positive structural belts there, and the exploration is gradually turning to negative structures such as troughs deeply buried, thus, the trough has become a hot spot of hydrocarbon exploration and research[1,2,3,4,5,6,7,8]. The trough refers to a secondary negative structural unit separated by local bulges within a sedimentary sag and is the secondary depocenter within the sag. Based on summarizing the hydro-carbon accumulation conditions and enrichment rules in troughs of the Jizhong depression and the Erlian basin, the hydrocarbon accumulation rules of “multi-factors controlling sand body distribution, dominant hydrocarbon accumulation, three-principal factors controlling hydrocarbon enrichment, and symbiotic complementarity of multi-type oil and gas reservoirs” in the trough have been put forward[4,5], which have played a guiding role in discovering hundreds of millions of tons of reserves in the trough areas of the rifted basin in the Huabei Oilfield. Statistics shows that in the past 10 years, of the controlled and predicted reserves newly added annually, reserves in stratigraphic-lithologic oil and gas reservoirs account for more than 60% in the oil-rich troughs of the Bohai Bay Basin. CNPC has found hundreds of millions of tons of reserves in the slope belts in the troughs such as Qibei, Chengbei, Wen'an, etc., and SINOPEC has found 10 oil and gas fields in the northern steep slope of the Dongying sag, with proven reserves of 11.2×108 t. Exploration of beach-bar sandstone reservoirs in gentle slopes is fruitful too, with reserves of 2.25×108 t proved cumulatively to date. Taking the second member of the Paleogene Kongdian Formation of the Cangdong sag, the Bohai Bay Basin (hereinafter referred to as Kong-2 Member, Ek2) as an example, this study has probed further into the distribution and enrichment pattern of oil and gas reservoirs in the trough areas of rifted basins, in the hope to provide guidance for hydrocarbon exploration.
1. Formation and evolution of Cangdong sag
Located in the middle of the Bohai Bay Basin, the Cangdong sag has an exploration area of 1 800 km2 and is one of “small and high production” oil-rich sags in the Bohai Bay Basin. After more than 40 years of exploration, 9 sets of hydrocarbon-bearing strata in the Paleozoic, Mesozoic and Cenozoic have been discovered, with cumulative proven oil reserves of 4.08×108 t and proven gas reserves of 10.3×108 m3. During sedimentation of Ek2, the lake basin of Cangdong sag covered a large area, and the trough accounted for more than 85% of the sag (Fig. 1).
Fig. 1.
Fig. 1.
Location of the Cangdong sag and paleo-geomorphology of Ek2.
The pro-type trough of Ek2 has undergone differential transformation in the late stage. Before Ek2 sedimentation, the Cangdong sag was in depression stage with an original lake basin area of 2 000 km2, when nearly 1 000 m thick stratum of the third member of the Kongdian Formation (hereinafter referred to as Kong-3 Member, Ek3) and Ek2 deposited. Since the sedimentation of the first member of the Kongdian Formation (hereinafter referred to as Kong-1 Member, Ek1), the lake basin evolution transited from depression to rift stage, when the fault activities in the northern and southern parts of the trough were quite different[9,10]. During the superposition of the lake basin in two stages in the northern part of the Cangdong sag, the early lake basin edge was denuded, and the basin center was inversed to form the Kongdian structural belt, experiencing strong reconstruction. Thus, during sedimentation of Ek2, the remaining lake basin area was only 60% of the prototype basin, the trough had features of inverse slope structure (A-A' profile of Fig. 2), which is called the Kongxi slope on the west side and the Kongdong slope on the east side. The superposition of the lake basins in the two stages was relatively stable in the southern part of the Cangdong sag. After sedimentation of Ek2, the slope inherited the shape before, showing features of inherited slope (see B-B' profile of Fig. 2), which is called the Nanpi slope.
Fig. 2.
Fig. 2.
Main structure types of troughs in the rifted basin (location of the section is shown in
Both profiles of inversed and inherited slopes are composed of high, middle and low belts[7] (Fig. 2). The high slope belt, shallow in burial depth, is generally located at the edge or the internal uplift area of the trough, where the source rock is in an immature stage, and the reservoirs are mostly structural or stratigraphic ones. The middle slope is in the middle ring zone, where the organic matter is in the mature stage, and the reservoirs are largely lithologic or tight oil ones. The low slope is located in the trough center with deep burial depth, where organic matter has entered massive oil generation stage and produced some gas, so it is a favorable area for searching tight oil and shale oil.
2. Hydrocarbon accumulation characteristics in the trough of the Cangdong sag
Controlled by Ek2 trough, an oil-bearing system with Ek2 as the source rock and multiple sets of Kongdian Formation as reservoirs has formed.
2.1. Sedimentation and reservoir differentiation characteristics
Previous studies[11,12] show that the Kongdian Formation is the sediment of the Paleogene lake basin during early rift and is a complete second-order sedimentary sequence, which can be subdivided into 4 third-order sequences. Among them, Ek2 is the sediment during lake expansion and is a complete third-order sequence, which can be subdivided into 4 fourth- order sequences (SQEk24-SQEk21) from bottom-up. In this sedimentary period, the Cangdong sag was a closed lake basin, where 10 sub-provenance corresponding to 10 delta lobe bodies developed around the periphery of the lake basin, due to the influence of four sediment provenances, the Kongdian bulge, the Cangxian uplift, the Dongguang bulge and the Xuhei bulge[12]. Because of the difference in the trough structure and the range of different delta lobe bodies, the sedimentary facies vary regularly from the lake basin edge to the lake basin center, forming inner, outer and middle zones of sediments and reservoirs (Fig. 3).
Fig. 3.
Fig. 3.
Sedimentary system of Ek2 in the Cangdong sag.
In the outer ring of the trough (zone A), the medium-fine sand of delta plain and delta front subfacies deposited, with an average sandstone-formation ratio of more than 30%. The sandstone is dominated by thick layers, with the average single layer thickness of more than 2 m. The mudstone is mostly purple-grey and has no hydrocarbon generation conditions.
The middle ring (B zone) in the trough area is the transition zone of conventional-tight reservoirs dominated by siltstone- fine sandstone-silty mudstone and calcitic dolomite, with an average sandstone-formation ratio of 5%-30%. The sandstone is dominated by thin interbeds, with an average single layer thickness of less than 2 m. The mudstone is largely gray-dark gray of delta front, gravity flow and calcific dolomite bay facies. The dark mudstone has medium thickness and TOC of 0.5%- 2.0%, showing favorable hydrocarbon generation conditions.
The inner ring (zone C) in the trough area is dominated by fore-delta and semi-deep lake facies, and silty mudstone, limy dolomite, muddy limy dolomite, limy dolomitic mudstone, and mudstone-shale etc. mixed fine-grain rock, with an average sandstone-formation ratio of less than 5%. There are few sandstone layers, and the sand clasts come mixing with others. The dark mudstone has large thickness, TOC of more than 2%, and the excellent hydrocarbon generation conditions. The middle and inner zones constitute the fine-grained sedimentary facies of Ek2[12,13,14,15]. The correspondence between zoning of the trough and the slope shows that the inherited slope is mostly controlled by the convex boundary, and the high, middle and low slopes correspond to the outer, middle and inner zone of the trough. The reformed slope is controlled by the boundary fault. Due to the later structural inversion, the original outer, middle and inner rings become low, middle and high slope belts. These differences cause the differences in hydrocarbon accumulation patterns and reservoir distribution sequences in different slope belts.
2.2. Distribution and evolution characteristics of
hydrocarbon source rocks
During Ek2 sedimentation, the Cangdong sag was a closed lake basin in a salinizing and reducing sedimentary environment. A high-abundance high-quality source rock dominated by fine-grained sedimentary rock deposited during the maximum lake flood period. The source rock is dominated by dark gray block mudstone and oil shale with abundant lamellation, which is widely distributed in the sedimentary trough with a maximum thickness more than 400 m. Affected by the late transformation of the lake basin, the source rock at the edge of the trough is denuded, showing a trend of thinning along the paleo-trough rings. The terrigenous nutrients input led to flourish of organisms, and booming of the ancient productivity of the lake basin, giving rise to organic matter of largely type I and II1. In addition, the humic kerogen accounts for about 20%, reflecting that the terrigenous organic matter is also an important part of the parent material source[16]. The Ek2 source rocks of different structural types have different organic matter abundance. The blocky dark mudstone has TOC generally above 2%, on average 3.07%, and the oil shale with abundant lamellation has TOC of more than 3%, on average 4.87% (Table 1). The source rock in the trough center has the highest organic matter abundance, higher soluble hydrocarbon content, and (S1+S2) above 30 mg/g, meeting the standard of high quality source rock. Drilling data reveals that Ro of Ek2 source rock is generally 0.50%-0.92%. Basin simulation shows that the source rock in the deepest buried area is more than 1.3% in Ro at present, produces oil primarily. It can be seen from the kerogen swelling heat simulation experiment that when Ro is less than 0.5%, a small amount of low-maturity oil is generated, most of which is adsorbed or dissolved in the kerogen network. As the kerogen adsorption reaches the saturation state, the excess hydrocarbon separates from the kerogen and moves to the adjacent clay micro-cracks and intergranular pores of mineral particles and forms the main part of shale oil[17,18]. Therefore, the Ek2 source rock has the characteristics of concentrated distribution, highly-efficient evolution and large hydrocarbon generation amount.
Table 1 Organic matter abundance of Ek2 source rocks in the Cangdong sag.
| Lithology | TOC/% | (S1+S2)/(mg•g-1) | Bitumen “A”/% | Total hydrocarbon/10-6 | Comprehensive evaluation |
|---|---|---|---|---|---|
| Blocky dark mudstone | 0.14-8.41 3.07(195) | 0.03-69.91 19.46(135) | 0.003-1.410 0.350(93) | 14.23-10 332.44 2 106.44(89) | High quality source rock |
| Oil shale | 2.32-9.23 4.87(28) | 1.23-77.55 36.59(27) | 0.05-2.79 0.63(14) | 396.36-14 278.81 3 448.43(13) | High quality source rock |
Note: The number in the numerator is the minimum-maximum, the denominator is the average, and the value in parentheses is the number of samples.
2.3. Characteristics of oil accumulation in the trough of the Cangdong sag
Influenced by the late tectonic movement in the Bohai Bay Basin, the prototype trough in the Cangdong sag settled differentially, and the shale in the central uplift belt is uplifted, and ended with lower maturity. The source rock in the trough subsiding late has deeper burial depth, high thermal evolution, Ro generally between 0.7%-1.1%, and is in massive oil generation stage. The oil and gas generated by Ek2 retain in situ or migrate, and most of them is retained in the source rock in the form of residual oil and gas. Calculation of the evolution degree and hydrocarbon generation and discharge shows that the residual hydrocarbon in the entire trough area accounts for more than 48% of the total hydrocarbon generation amount, forming shale oil and tight oil. The discharged hydrocarbon migrates in two ways: some hydrocarbon migrated laterally along the interconnected sand bodies at the edge of the trough, forming stratigraphic-lithologic reservoirs and tectonic reservoirs at the edge of the trough. Some hydrocarbon moved along the oil-origin faults, forming structural reservoirs in the upper part of the trough region. In addition to the migration mode, the difference in the transport capacity of different transport systems plays an important role in controlling the hydrocarbon migration characteristics. Taking the fault transport system as an example, the main oil-source faults in the Cangdong sag are distributed on the second-order structural belt, with a fault growth index (the ratio of the downthrown thickness of the fault to the upthrow thickness, the greater the value is, the stronger the fault activity) of above 1.5[19], which is conducive to the upward migration of oil and gas into the conventional oil accumulation zone in the upper part of the trough area. For example, the Kongdian structural belt with the largest reservoir scale discovered is in the area with the highest fracture activity intensity. In contrast, in the central trough, the fault growth index is small, and the fault activity is weak, which is good for the preservation of shale oil.
3. Orderly accumulation of conventional and
unconventional oil and gas in the trough
Regarding classification of oil and gas reservoirs, most researchers tend to divide oil and gas reservoirs into two categories: conventional and unconventional. The conventional oil and gas reservoirs are divided into structural and lithologic ones. The unconventional oil and gas in a narrow sense includes tight oil and shale oil, etc.[20,21,22,23,24] According to the hydrocarbon generation and expulsion characteristics and hydrocarbon migration and accumulation laws of the source rocks in the Cangdong sag, the prototype troughs (including slopes) as complete petroleum systems, have a series of oil and gas reservoirs in orderly distribution, i.e. the complete sequence of tectonic, stratigraphic-lithologic, tight oil, shale oil reservoirs from the outer-source to the inner-source (Fig. 4). There are some differences in the accumulation sequence of the inner ring zone of the reformed trough. In the late stage, the original trough center was inversed and uplifted, so the source rock there has low evolution degree and no hydrocarbon generation capacity. The hydrocarbon generation center is transferred to the middle ring of sedimentary trough center, so the inner ring of the reformed trough has relative under developed tight oil and shale reservoirs.
Fig. 4.
Fig. 4.
Orderly accumulation model of conventional-unconventional oil and gas in the Kongdian Formation of Cangdong sag (The location of the section is shown in
3.1. Formation of complex fault block reservoirs above the source rock
The northern part of the prototype trough was transformed and uplifted, forming the Kongdian structural belt. The main faults and overall basic structural pattern in this area took shape during fault depression period when the upper Ek1- Shahejie Formation deposited, and this period is also the time that traps in the target strata came into being. After sedimentation of the Neogene Guantao Formation and Minghuazhen Formation, the Ek2 source rocks gradually matured and started to generate hydrocarbons massively. The oil and gas migrated along main faults formed in the early stage and accumulated in the reverse drag anticlines, extrusion anticlines, slump anticlines, faulted noses and fault block traps in contact with the oil source faults, forming five types of structural reservoirs (Fig. 5). Therefore, the configuration between large faults and favorable sand bodies controls oil and gas enrichment. Taking the Wangguantun oilfield as an example, there are a series of faulted-nose or reverse drag anticlines along the Xuxi fault and its branch faults. The lower Ek2 has high-quality source rock in the stage of massive hydrocarbon generation and expulsion, and the Xuxi fault with stronger activity intensity acts as the main channel for upward migration of oil and gas, and the oil and gas accumulate in the reverse drag anticlines or faulted noses of upper Ek1.
Fig. 5.
Fig. 5.
Types of complex fault block reservoirs in Cangdong sag.
3.2. Stratigraphic-lithologic hydrocarbon accumulation model beside the source rock
During sedimentation of Ek2, the sediment material was transported into the basin at high intensity from the provenances outside the basin, forming a sedimentary system from delta plain to delta front. Thus the sand body shows a thinning trend from the basin edge to the basin center, and the middle ring zone is the site lithologic pinchout occurs. The outer ring zone of the inherited trough, i.e. the high slope is outside the range of effective source rock of Ek2 and has poor accumulation conditions on the whole. The middle slope is in the main sand body area of braided river delta front, with abundant underwater branch channel and mouth bar sand bodies and good reservoir properties. Here, the multi-provenance sand bodies of Ek2 are interbedded with oil source rocks, favorable for self-generation and self-accumulation and near source rock charging of hydrocarbon. Moreover, the middle slope is close to Ek2 lake basin center and is on the direction of hydrocarbon migration, so it is the favorable site for forming stratigraphic-lithologic reservoirs. In addition, in the northern part of the Cangdong sag, the inner ring zone of the sedimentary trough formed the Kongdian structural belt at the end of Ek2 sedimentation, where the downward pinchout sand body is reversed into the upward pinchout sand body, thus upward pinchout lithologic traps could exist there, making it possible to form lithologic reservoirs in the original inner ring (Fig. 6).
Fig. 6.
Fig. 6.
Stratigraphic-lithologic hydrocarbon accumulation model beside the source rock in the inversion trough of the Cangdong sag (The location of the section is shown in
3.3. Near source-inside source lithologic-tight oil accumulation model
The middle ring of the sedimentary trough is a transitional facies belt, where the dark shale, gray dolomite and sandstone interbed with each other. The carbonate and sandstone reservoirs here are relatively tight and have the porosity generally between 4% and 7% and fairly abundant fractures, and have certain oil and gas storage capacity. The dark shale of the middle ring, with high organic matter abundance, has generally entered the stage of massive hydrocarbon generation and expulsion. The generated oil and gas may migrate short distance to the closely contacted tight reservoirs, thus forming tight sandstone oil or tight gray dolomite oil reservoirs[25,26,27,28,29,30]. According to source-reservoir combinations, the reservoirs can be divided into layered cake, interbed and sandwich types (Fig. 7).
Fig. 7.
Fig. 7.
Schematic of main types of tight oil reservoirs in the Cangdong sag.
3.4. Shale oil enrichment model inside the source rock
The inner ring of the sedimentary trough is dominated by fine-grained sedimentary rocks and is characterized by shale lithology. According to rock composition analysis and microscopic identification, the fine-grained sedimentary shale can be subdivided into thick layered gray dolomite, thick layered felsic shale, lamellar mixed sedimentary rock and thin layered gray dolomite, which all have good hydrocarbon generation capacity and certain storage capacity, so typical source-reservoir- in-one shale oil pool can be formed there. Through normalized evaluation of TOC (total organic carbon content), S1 (free hydrocarbon content), porosity, permeability, brittle mineral content, oil test results and semi- quantitative parameters such as stratigraphic structure and fluorescence display level, it can be seen that the laminar mixed sedimentary rock model with the most developed organic matter laminae is the most favorable source-reservoir combination model, followed by the thin layered gray dolomite model. The shale oil enrichment is closely related to the evolution degree of source rock, so the favorable sweet spot zone is further divided into Type I and Type II sweet spots with the organic maturity of 0.8% as the bound. It should be noted that due to late elevation, the north part of the inner ring has low evolution degree and poor hydrocarbon generation capacity, so the area favorable for shale oil enrichment takes on “C” shape, with an area of 640 km2 (Fig. 8).
Fig. 8.
Fig. 8.
Evaluation of favorable shale oil area in the fine-grained zone of the inner ring of Ek2 in the Cangdong sag.
3.5. Hydrocarbon enrichment rules in the trough
The effective source rocks of Ek2 in the Cangdong sag are distributed around the deep troughs, and the hydrocarbons might transport laterally and vertically from the source rocks to the reservoirs in circle-layered pattern, forming a circle-layered oil and gas accumulation model. On the plane, the shale oil, tight oil, stratigraphic-lithologic reservoirs and structural reservoirs are distributed orderly in ring shape in Ek2[31]. The inner ring is the area of source rock in conventional sense, with few sandbodies, where a large amount of oil is retained in the high-quality source rocks, especially the shale with Ro of 0.5%-1.3%. The middle ring of Ek2 is the favorable site for lithologic oil reservoirs and tight oil reservoirs. The high-quality source rock and upward pinchout lithologic traps combine to form lithologic reservoirs, and the high-quality source rocks and tight reservoirs make up tight oil reservoirs. The lithologic oil reservoirs and tight oil reservoirs are “complementary”. The outer ring mainly has stratigraphic lithologic reservoirs and structural reservoirs, this ring was denuded in later stage, so stratigraphic traps or the faulted nose structures against large faults may come about. As the outer ring is far away from the source rocks, only the traps on oil migration pathways can become pools. The reservoirs discovered so far are distributed in the Shenüsi faulted-nose belt and the high part of the Kongdong slope.
Vertically, structural reservoirs, lithologic reservoirs and shale oil reservoirs (tight oil reservoirs) are distributed orderly in the trough. The structural reservoirs such as faulted-noses, fault blocks, etc. along oil source fault occur in the upper part of Ek1 in the trough. The shale oil occurs mostly within the trough. The tight sandstone oil reservoirs occur in the Ek22 and Ek24 oil groups in the transition zone. The tight oil reservoirs dominated by gray dolomite occur in the lower part of Ek23 and the Ek21 oil groups. The conventional structural-lithologic oil reservoirs exist in the middle and high parts of the trough.
4. Practices of conventional-unconventional oil and gas exploration
In Cangdong sag, a set of high-quality source rock develops around the Ek2 sedimentary trough. Controlled by the thermal evolution of the source rock and the combination of conventional and unconventional reservoirs, structural reservoirs, stratigraphic-lithologic oil reservoirs and tight oil/shale oil are distributed orderly spatially. Therefore, the exploration of different reservoir types in different parts around the hydrocarbon generation trough is an inevitable choice to the success of exploration.
4.1. Exploration of structural oil and gas reservoirs above the source rock
The upper part of the trough in the Cangdong sag is dominated by Ek1. Most structural belts are composed of multiple fault blocks intersected by faults. In addition, there are several reservoir layers and the reservoirs change fast laterally, so the oil and gas enrichment is controlled by faults connecting with source rock and favorable sand bodies. There are three sets of fault systems in the central structural belt, connecting with source rock and experiencing strong activity, they controlled the migration of oil and gas. The reservoirs controlled by the fault include structural oil reservoirs, lithologic oil reservoirs, and stratigraphic-lithologic reservoirs. To date, Ek1 has proven oil geological reserves of 2.64×108 t, with the proven rate of over 64%. Through fine characterization of the structure and sand body, and overall evaluation, the proven oil reserves of 2061.2×104 t were found recently in Xiaoji, and 80 new wells are to be drilled, with the expected productivity of 25.05×104 t. Xiaoji area becomes the block with the highest newly added production in Dagang Oilfield. Similarly, breakthroughs have been made in the exploration of the Qikou depression, validating that the structural belts above source rocks in oil-rich troughs still have large exploration potentials.
4.2. Exploration of stratigraphic-lithologic reservoirs
beside the source rocks
In recent years, the superimposed braided river delta sand bodies of multiple-sedimentary stages with continuous distribution and favorable reservoir properties in the east, west and south provenance directions of Ek2 have been identified in the Nanpi slope. The accumulation model of large-scale continuous oil-bearing lithologic reservoirs within the effective source rock range has been established. Based on this, the fine sand body characterization and reservoir forecast were carried out for sand bodies of different provenances in the Nanpi slope. The eastern and western sandbody belts were identified, which have a favorable sand body area of 196 km2 and resources of 13 225×104 t combined, making them important targets for increasing reserves. In the past two years, 10 exploration wells have been drilled, of which 8 have obtained industrial oil production, adding oil geological reserves of 5507×104 t, demonstrating large exploration potential.
4.3. Exploration of lithologic-tight oil reservoirs near and within the source rock
Based on the continuous coring and laboratory analysis of the whole section of Ek2, the criterion of identifying the tight sandstone sweet spot has been established. Four wells in the Guanxi area were drilled, targeting at Ek22 and Ek24 tight sandstones, and all of them has tapped industrial oil flow. The “sweet spot” has a depth of 3 300-3 800 m and a favorable area of 120 km2. Tight sandstone of 3 224.2-3 237.2 m in Ek22 top in Well G1608 was tested, with 5 mm nozzle, the daily oil production of 21.3 t and cumulative oil production of 80.96 t was obtained, showing promising exploration prospects of Ek2 sand tight oil reservoirs.
4.4. Resource potential and exploration prospects of shale oil within the source rock
The fine-grained sedimentary rocks are distributed stably and continuously in the main area of Ek2 lake basin of the Cangdong sag. Through the evaluation and prediction of sweet spot distribution, Guandong and Gaunxi shale oil sweet spot areas have been identified, and seven sweet spot intervals have been identified vertically. Among them, four lithologic combination models have been identified in Ek21 fine-grained sedimentary rocks. The models of the thick felsic shale and lamellar mixed rock are most conducive to shale oil enrichment. In order to increase and effectively produce the reserves of shale oil in this area, two horizontal wells were drilled in Guandong area, and fractured with 40 000 m3 liquid and 1 300 m3 proppant [32]. So far, they have flown back for about 140 days, with flowback rate of above 25%, and the industrial oil flow of nearly 40 tons/d each. According to the preliminary analysis with the mass oil content method, the predicted resources of the shale oil in the Ek2 of the Cangdong sag are about 6.8×108t, the total technological recoverable reserves are more than 5000×104t, and the controlled and predicted reserves of the two wells reaches 100×104 t. In addition, in recent years, 13 vertical wells has tapped industrial oil flow in this layer, confirming that Ek2 shale oil (a small part is the tight oil accumulated after short-distance migration) is an important replacement resource for increasing production and reserves in the Cangdong sag.
5. Conclusions
In the continental rifted basin, the trough (including slope) controls the generation and distribution of oil and gas, and the oil and gas distribution has obvious zoning characteristics vertically and laterally. The oil and gas exploration around the main body of the trough is the key to achieving sustainable benefit in the mature exploration area. The trough of the Cangdong sag has orderly distribution of conventional and unconventional oil and gas reservoirs. Structural oil and gas reservoirs occur in the upper part of the pro-type trough. The delta front of outer ring zone is the favorable area for forming stratigraphic-lithologic reservoirs. The middle ring is tight oil sweet spot zone, and the inner ring with fine sedimentary facies is the sweet spot for shale oil exploration. The reformed trough features center reversal, so the inner ring zone in this kind of trough has no tight oil/shale oil sequence, but may has large- scale structural reservoirs. The shale oil, tight oil, lithologic reservoirs and structural oil and gas reservoirs around the troughs in the Cangdong sag are explored to different degrees. Among them, the structural oil and gas reservoirs are highly explored, and the shale and tight oil are explored at low degree. The structural oil and gas reservoirs along main faults are the main areas of increasing production and reserves. Breakthroughs have been made in exploring tight and shale oil in the main trough, so the main trough will be the major replacement area in the next step exploration.
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The conventional petroleum geology theory is suitable for the reservoirs with high permeability and fluid flows obeying Darcy law with buoyancy effect mechanism,and oil and gas are situated at the high position of structure and sand body,and they are always structural reservoirs.But this theory does not apply to the reservoirs with low and ultra-low permeability or tight reservoirs where fluid flows obey non-Darcy law,and oil and gas cannot be separated by gravitation.On the condition of overpressure expulsion,flow primarily migrates as episodic expulsion manner,the oil droplets gradually deform and pass through the pore throats as non-Darcy flow.This low speed migration must lead to the stagnant mechanism that oil stagnates and accumulates in the lower area and form continuous reservoirs.This stagnation accumulation reservoirs will cause the phenomenon that oil accumulates in anticline area,or oil accumulates at low position of slope but water accumulates at high position,which is the oil-gas inversion.The stagnation accumulation reservoirs which have no structure,no lithology traps,no unified temperature-pressure systems and oil-water contacts have changed the traditional trap exploration method,expanded the concept of reservoir,and formed the fundamental theory for the research field such as reservoirs with low and ultra-low permeability and unconventional and tight reservoirs.Theory of petroleum accumulation in syncline is an important supplement to traditional petroleum geology.This theory expands the exploration area,changes the exploration target from high position of structure to low position,extremely increases the petroleum source and is regarded as important guiding principle for the petroleum exploration and development.
Hydrocarbon enrichment theory and exploration practice in the slope of fault lake basin: A case study of Paleogene in Huanghua depression
Slope is a major structural unit of fault lake basin in eastern China, and also an important zone in the Bohai Bay Basin for increasing oil and gas reserves. Through years of comprehensive studies and exploration practices in slopes, it is newly proposed that the slopes(or large gentle slopes) show a high-moderate-low differentiation. To be specific, the geologic differentiation of high, moderate and low slopes in subsidence rate, sedimentary sequence, reservoir physical property, hydrocarbon generation and evolution, formation pressure, fluid property and reservoir type results in the theory of hydrocarbon enrichment in dominant facies belts in slope of fault lake basin. Essentially, in high slope, channel/trough controls sand, and oil and gas enrich in dominant migration and preservation facies. In moderate slope, slope break controls sand, and oil and gas enrich in dominant structural-lithologic facies. In low slope, distal fan controls sand, and oil and gas enrich in dominant source-reservoir coupling facies. According to this theory, fine exploration and drilling operations have been carried out in the slopes. As a result, remarkable achievements were made in stratigraphic-lithologic reservoirs in the slope zone. During the past five years, additional 3P oil reserves exceeded 3 10~8t, and four 10 million-ton class profitable productivity construction blocks and three 100 million-ton class blocks with the potential of significant reserves increased. Thus, it is demonstrated that the theory of hydrocarbon enrichment in dominant facies belts in slope plays an important role in directing the large-scale profitable reserves increase and high efficient productivity construction.
Geological characteristics of shale rock system and shale oil exploration in a lacustrine basin: A case study from the Paleogene 1st sub-member of Kong 2 Member in Cangdong sag, Bohai Bay Basin, China
DOI:10.1016/S1876-3804(18)30043-0
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A deep understanding of the basic geologic characteristics of the fine-grained shale layers in the Paleogene 1 st sub-member of Kong 2 Member(Ek_2~1) in Cangdong sag,Bohai Bay Basin,is achieved through observation of 140 m continuous cores and systematic analysis of over 1 000 core samples from two wells.Basic geological conditions for shale oil accumulation are proposed based on the unconventional geological theory of oil and gas.The shale rock system mainly developed interbedded formation of felsic shale,limy and dolomite shale and carbonates; high quality hydrocarbon source rock formed in the stable and closed environment is the material base for shale oil enrichment; intergranular pores in analcite,intercrystalline pores in dolomite and interlayer micro-fractures make tight carbonate,limy and dolomite shale and felsic shale effective reservoirs,with brittle mineral content of more than 70%; high abundance laminated shale rock in the lower section of Ek_2~1 is rich in shale oil,with a total thickness of 70 m,burial depth between 2 800 to 4 200 m,an average oil saturation of 50%,a sweet spot area of 260 km~2 and predicted resources of over 5 10~8 t.Therefore,this area is a key replacement domain for oil exploration in the Kongdian Formation of the Cangdong sag.At present,the KN9 vertical well has a daily oil production of 29.6 t after fracturing with a 2 mm choke.A breakthrough of continental shale oil exploration in a lacustrine basin is expected to be achieved by volume fracturing in horizontal wells.
Conventional and unconventional petroleum “orderly accumulation”: Concept and practical significance
DOI:10.1016/S1876-3804(14)60002-1
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Based on the latest global conventional-unconventional petroleum development situation and the conclusion of petroleum geology theory and technology innovation in recent 10 years, the connotation of conventional and unconventional petroleum “orderly accumulation” connotation is formulated. This concept indicates that, unconventional petroleum occurs in the hydrocarbon supply direction of conventional petroleum, and conventional petroleum may appear in the outer space of unconventional petroleum. Proper evaluation methods and engineering technology are important to push the conventional-unconventional petroleum co-development, and the petroleum finding thought from outer-source into inner-source. Unconventional petroleum evaluation focuses on source rocks characteristics, lithology, physical property, brittleness, oil-gas possibility and stress anisotropy. Taking shale gas for examples, in China, these six properties areTOC>2%, laminated silicious calcareous shale or calcareous silicious shale, porosity 3%-8%, brittle minerals content 50%-80%, gas content 2.3-4.1 m3/t, pressure coefficient 1.0-2.3, natural fractures; in north America, these six properties areTOC>4%, silicious shale or calcareous shale or marl, porosity 4%-9%, brittle minerals content 40%-70%, gas content 2.8-9.9 m3/t, pressure coefficient 1.3-1.85, natural fractures. “Sweet spot area” assessment, “factory-like” operation pattern and other core evaluation methods and technologies are discussed. And 8 key elements of unconventional “sweet spot area” are proposed, 3 of them areTOC>2% (for shale oilS1>2 mg/g), higher porosity (for tight oil & gas >10%, shale oil & gas >3%), and microfractures. Multiple wells “factory-like” operation pattern is elaborated, and its implementation needs 4 elements, i.e. batch well spacing, standard design, flow process, and reutilization. Through horizontal well volume fractures in directions, “man-made reservoirs” with large-scale fracture systems can be formed underground. For “shale oil revolution” in future, non-water “gas in critical state” and etc. fracturing fluid and matching technology should be stressed to be industrially tested and encouraged to be low cost developed.
Tectonic-sedimentary evolution of the first and second members of the Kongdian Formation in southern Kongdian, Huanghua Depression
The sedimentary facies analysis and Palaeogeographic reconstruction have disclosed that the Kongdian uplift-Kongdian tectonic zone was initiated during the early stage of the deposition of the first member of the Kongdian Formation,and came into being during the late stage of the deposition of the first member of the Kongdian Formation.The southern part of Kongdian was once a down-warped basin during the deposition of the second member of the Kongdian Formation,whereas during the deposition of the first member of the Kongdian Formation,this area was in the subsidence stage and the Kongdian uplift-Kongdian tectonic zone was a slowly subsiding zone rather than a relatively uplifted area.
Geologic features of fine-grained facies sedimentation and tight oil exploration: A case from the second Member of Paleogene Kongdian Formation of Cangdong sag, Bohai Bay Basin
DOI:10.1016/S1876-3804(16)30003-9
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Based on analysis of successive and whole cores over six hundred meters from the second Member of Kongdian Formation (Kong 2 Member) of fine-grained facies zone in central lake basin of Cangdong sag, ten thousands of systematic and joint analysis data, and the matching logging data, the sedimentary characteristics in the fine-grained facies zone is examined by means of traditional petroleum geology, sedimentology, and new theory and method of tight oil and gas generation. The Kong 2 Member is the sedimentation during the maximum lake flooding in the Kongdian Formation. Fine-grained sedimentation of half-deep lake to deep lake are widely distributed, the fine-grained feldspar-quartz contained sedimentary rock, fine-grained mixed sedimentary rock, and dolomite are developed, and the clay rock is poorly developed. The fine-grained rocks have these features as follows: more rock compositions, less advantageous minerals; more debris minerals, less clay minerals; more analcites, less pyrites; more brittle minerals, less quartz; more rock types, less oil shale; more tight reservoirs with less non-cracked reservoirs; more high-quality source rocks, less non-source rocks; more high-frequency cycle sediments, less single sediments. These understandings updated the former view that only mud shale source rock was dominantly developed in the closed continental lake. With the help of geologic research and matched engineering technologies, several exploration wells have produced industrial oil flows, showing a good prospect of tight oil exploration in fine-grained facies zones of continental lacustrine basins.
Sedimentary characteristics and hydrocarbon exploration of Ek2 of Nanpi slope area, Cangdong sag, Huanghua depression, North China.
DOI:10.3969/j.issn.1671-9727.2015.05.04
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Using sedimentology and petroleum geology,based on the analysis of sedimentary background,the core observation,the sand distribution and by building the hydrocarbon accumulation patterns,this paper studies the sedimentary characteristics and the models of controlling sand and oil reservoirs in Member 2of Paleogene Kongdian Formation(Ek2)of the Nanpi slope area,Cangdong sag of the Huanghua depression.The researches show that the Nanpi slope is a successively developing gentle slope.Because the influence of the provenance from the Dongguang salient,the Cangxiang uplift and the Xuhei salient,the sand bodies are well-developed in Ek2.In the pattern that the sand bodies are supported forward from the salient,transported in erosion grooves,and controlled by the single slope-break,there mainly develops the braided river delta-off shore subaqueous fan.There mainly develop the bodies of delta front in the middle slope,where the sand bodies matching thesource rocks are buried in a moderate depth,have better physical properties,at the same time,have good trap conditions,and are located in the favorable directional zone of hydrocarbon migration.So the middle slope is the important exploration area of lithostratigraphy reservoir.In the low slope,there develop the superior source rocks which are intercalated with the sand bodies formed by far delta front-off shore subaqueous fan and the thin dolomite.So the low slope is the favorable exploration area of tight oil.
Provenance system analysis at Ek2 in Nanpi sag based on well to seismic integrated studies
Lithologic characteristics and geological implication of fine-grained sedimentation in Ek2 high stand system tract of Cangdong sag, Huanghua Depression
Lithologic character is one of the main controlling factors for development of tight oil sweet spot.The Ek2 layer of Cangdong sag,Huanghua depression,is rich in tight oil resource.To clarify the lithologic development characteristics of fine-grained Ek2 sedimentation,highresolution sequential stratigraphy and tight oil and gas abundance geological theory are used to make high-resolution sequence stratigraphic division of the Ek2 layer and analyze its sedimentary characteristics on the basis of coring,testing,logging and well-log data.The study indicates that the Ek2 layer in Cangdong sag is a set of intact three-order sequences,which can be further divided into four-order sequences(SQEk21-SQEk24).Fine-grained sedimentation in SQEk21 high stand system tract is complicatedly composited without favorable minerals development.It is characterized in extensive distribution of brittle minerals,such as muddy felsites debris and analcite.Four main types of rock developed in the area 鈥 shale,dolomite,sandstone and mixed sedimentary rock,of which mixed sedimentary rock developed best.Tight sandstone and dolomite are the main reservoirs.The research results refresh the previous views that shale developed as source rock in finegrained sedimentation of Ek2 layer in Cangdong sag but no reservoirs developed there,effectively expand the oil and gas exploration field in Cangdong sag.
Several issues in studies on fine-grained sedimentary rocks
DOI:10.3969/j.issn.1673-8926.2016.01.002
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There are huge oil and gas resources in fine-grained sedimentary rocks. Researchers pay less attention to it compared to coarser sediments. Classification, lithofacies and brittleness evaluation of fine-grained sedimentary rocks remain a relatively weak research field. Based on the X-ray diffraction data, fine-grained sedimentary rocks were divided into 12 types of rock in four main categories by means of a three-unit division system. The three units are carbonate minerals, clay minerals and felsic minerals. Meanwhile, fine-grained sedimentary rocks were named according to the concrete content of special mineral. The study of lithofacies is mainly focused on the differences in components, texture, structure, reservoir physical properties, source rock character and oil-bearing property. A great emphasis should be placed on sedimentary environment, sedimentary progress and dynamical mechanism of different lithofacies to identify advantaged lithofacies for oil and gas exploration. Brittleness of fine-grained sedimentary rocks is of vital significance to the fracturing effect. Due to the demerits of elastic parameters method and mineral constituent method which are commonly used to evaluate the brittleness of fine-grained sedimentary rocks, a new accurate and rational method should be proposed.
Relationship between conventional oil and tight oil In the second member of Kongdian Formation, Cangdong sag
Sand-gathering and reservoir-controlling mechanisms of Ek2 slope in Cangdong sag
Hydrocarbon forming environment of source rocks of the second member of Kongdian formation in Cangdong sag, Baohai Bay Basin
Take the second section of Kongdian formation in Cangdong sag as an example,use of trace elements,geochemical analysis to analysis of the distribution and chemical characteristics of hydrocarbon source rocks,study the hydrocarbon forming environment of source rocks,established a sedimentary mode of lacustrine source rocks,discusses its relationship between conventional and unconventional oil distribution.The results show that the thickness of source rocks is 50~450m,the average w(TOC)is3.07%,Rois 0.5%~1.1%,the type of organic matter is-1,the distribution area of high-quality hydrocarbon source rock is large;basin evolution,ancient climate and geography,paleontology,ancient productivity and preservation conditions control the hydrocarbon source rocks;the wide/deep of basin is big,warm and humid climate,water is fresh to brackish water,water exchange,relatively frequent,development temperature stratification is not stable,high-quality hydrocarbon source rocks are mainly developed in the center of the basin;the center of the basin is dominated by tight oil exploration,and in the edge of the basin mainly conventional oil exploration.Research on hydrocarbon environment and source rocks mode is significant for the exploration of conventional and unconventional oil.
Geochemical characteristics of organic-rich shales of the second member of Kongdian Formation in Cangdong sag, Huanghua Depression
The organic-rich shales of the second member of Kongdian Formation(E k2) are developed very well in Cangdong Sag, Huanghua Depression. Research on geochemical characteristics of the shales is highly significant for the shale oil resource evaluation. Based on collecting and sorting the research results of predecessors, additional core samples were taken to carry out tests and analysis of organic carbon, rock pyrolysis, chloroform bitumen "A", kerogen maceral and vitrinite reflectance. Single well numerical simulation of burial and thermal history and geochemical evolution sections were also studied. Organic-rich shales of Ek21, E k22 and E k23 were all evaluated as the best source rocks. The types of organic matter are mainly of ⅠandⅡ1, with small amount of Ⅱ2and Ⅲ. Immature to low mature shales are mainly distributed in central uplift zone, while the organic-rich shales in slope zones are at low maturity to maturity stage. The organic-rich shales of E k2 began to generate and expulse immature to low mature hydrocarbon in early Eocene and reached the peak in the late sedimentary period of Guantao Formation. Then with being continuously buried deeply, the source rocks could entered into low mature to mature hydrocarbon generation stage and the conversion rates of hydrocarbon generation is as high as 60%.
Trend and progress in global oil and gas exploration
DOI:10.1016/S1876-3804(13)60052-X
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4D microgravity measurement is used to study the underground medium, especially underground fluid variation state and process, using repeated measurements of gravity data at different periods. A model of microgravity variation calculation for the Guantao Formation in the Du 84 block of the Liaohe Oilfield was established according to the practical condition of the steam assisted gravity drainage (SAGD) of super heavy oil reservoir. The distribution of residual gravity that reflects the density change of the steam injection layer and heavy oil layer can be obtained using the target area 4D microgravity measured at different periods through wavelet transformation. With 20 Gal as the main anormaly, the variation of steam chamber shape in the steam assisted gravity drainage can be quantitatively described by analyzing the distribution of steam chamber, and drawing the steam chamber overlapping map of two observations. Numerical simulation and well temperature test verified the feasibility of the technology. The result from the technology can be used to adjust steam injection scheme timely according to the morphological changes in the steam chamber, for example, the Gh12 well has got good results, with oil production going up from 65.4 t/d in May 2010 to 82.8 t/d in May 2012 after adjustment of its injection scheme.
Geological concepts, characteristics, resource potential and key techniques of unconventional hydrocarbon: On unconventional petroleum geology
Types, characteristics, genesis and prospects of conventional and unconventional hydrocarbon accumulations: Taking tight oil and tight gas in China as an instance
DOI:10.1038/aps.2011.203
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PMID:4010334
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Obesity and obesity-related metabolic diseases represent a growing socioeconomic problem throughout the world. Great emphasis has been put on establishing treatments for this condition, including pharmacological intervention. However, there are many obstacles and pitfalls in the development process from pre-clinical research to the pharmacy counter, and there is no certainty that what has been observed pre-clinically will translate into an improvement in human health. Hence, it is important to test potential new drugs in a valid translational model early in their development. In the current mini-review, a number of monogenetic and polygenic models of obesity will be discussed in view of their translational character.
Characteristics and exploration prospect of unconventional shale gas reservoirs in China
Characteristics of tight oil in Triassic Yanchang Formation, Ordos Basin
DOI:10.1016/S1876-3804(13)60018-X
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Based on the data from multiple sample analysis and tests and exploration practice, key factors controlling gas accumulation, enrichment and high production in the continental Triassic Xujiahe–Jurassic Ziliujing formations in Yuanba, Tongnanba and other areas, northern Sichuan Basin, were discussed. Natural gas in continental strata in this part of the basin are derived from the source rocks in the same strata, which are good – very good source rocks with high abundance of organic matter (mostly type III) and generally in high mature – overmature gas generating stage. Depending on provenance, multi-period (fan) delta systems are developed in the research area, where the main fluvial channel sands are superimposed in multi periods and distributed extensively, and reservoirs and source rocks form the “lower generation and upper storage” and “inter-bedded” assemblages. Five typical high-yield wells in the Jiulongshan, Malubei and Yuanba areas are investigated and an overall concept for exploration and research in the area is proposed: sedimentary source controls rock types, cementation types and sedimentary microfacies; source rocks control the size and location of gas accumulation; structural types control the magnitude and location of fractures; the combination of fracture and reservoir determines the level and retention duration of gas production. According to this model, the following areas have enriched gas and high production: Xu-3 and Xu-4 members of Xujiahe Formation in the western Yuanba and Jiange, Xu-4 member and Ziliujing Formation in the mid-eastern Yuanba, Zhenzhuchong and Xu-2 members in Malubei and Hebachang areas in the Tongnanba structure.
Assessment criteria, main types, basic features and resource prospects of the tight oil in China
DOI:10.1016/0031-9384(73)90235-7
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The tight oil refers to an oil that accumulates in source rocks in a free or adsorbed state or in tight sandstones and carbonates interbedded with or adjacent to source rocks.Generally,this oil accumulation has not yet experienced a large-scale,long-distance migration.Based on the clarity of the tight oil concept and connotation,we proposed ten key indices to evaluate the tight oil.Tight oil reservoirs can be generally divided into 3 different types based on porosity and permeability,while the tight oil itself can be also classified into 3 types depending on the genetic relationship of a close contact between tight reservoirs and source rocks,i.e.①tight oil in lacustrine carbonate rocks;②tight oil in deep-lake gravity flow sandstones and ③tight oil in deep-lake delta sandstones.The tight oil is widely distributed in China,currently,a number of important exploration discoveries of the tight oil have been achieved in the Triassic Chang-6 and Chang-7 sections of Yanchang Formation in the Ordos Basin,the Permian Lucaogou Formation in the Junggar Basin,the Middle-Lower Jurassic of the Sichuan Basin and the Cretaceous Qingshankou-Quantou Formation of the Songliao Basin.To expect the tight oil prospect in China,we preliminary forecast that the tight oil geological resources in China are about(106.7~111.5) ×108t.Combined with the analysis of future prospects of petroleum development,we can come to a conclusion that the tight oil in China should be a realistic replacement resource of the conventional oil.
A new method for lithology identification of fine grained deposits and reservoir sweet spot analysis: A case study of Kong 2 Member in Cangdong sag, Bohai Bay Basin, China
DOI:10.1016/S1876-3804(17)30061-7
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Based on systematic coring of 500 m of Kong 2 Member of the Paleogene Kongdian Formation in Cangdong sag of Bohai Bay Basin, identification and XRD(X-Ray Diffraction) analysis of over 1000 thin sections, a simplified method to quantitatively calculate contents of fine grained minerals with conventional logging data such as acoustic travel time(AC) and density log(DEN) has been proposed, and a quick lithologic identification "green mode" has been worked out in this study. By fitting the relationship between normalization of logging curves and mineral content measured by XRD, the mineral contents of sections or wells not cored can be calculated to identify lithology. With this method, several dolomite sweet spot intervals and one sandstone sweet spot interval have been found in the Kong 2 Member of Cangdong sag, where high production oil and gas flows have been tapped from drilled wells. The study shows that the dolomite is in band distribution and enriched in local parts of the study area. This method is applicable to lithologic identification of fine grained deposits in front delta-lake basin center, especially lithologic identification of mud and dolomite dominated fine grained deposits with low sand content of semi-deep, deep lake facies.
Reservoir characterization and factors influencing reservoir characteristics of the lower Shaximiao formation in Xinchang gas field, Western Sichuan, China
DOI:10.1360/972009-1380
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In contrast to the laterally continuous blanket-like reservoirs in the Upper Shaximiao Formation(Js1+2),the Lower Shaximiao(Js3) lacustrine-delta ribbon sandstones with the thickness of 20 m~ 35 m in the Xinchang gas field exhibit limited lateral continuity and were deposited with dominant paleo-currents from north to south.The Lower Shaximiao reservoirs are characterized by low-medium porosity(av.10.7%),very low permeability(av.0.15 mD),and strong heterogeneity with reservoir characteristics better than the upper Shaximiao sandstones and with less developed fractures.Reservoirs are composed of lithic arkoses and feldspathic litharenites,and the dominant types of porosity include secondary porosity(intergranular and intragranular),and primary intergranular porosity.Micro-facies played a critical role in the formation of the Lower Shaximiao sandstone reservoirs and high-quality reservoirs are mainly found in channel sandstones(especially in the main body of the channel).The diagenesis and the occurrence of micro-fractures also make great impact on reservoir quality.Medium-grained fluvial channel sandstones with high feldspar content,low lithoclast,carbonate cement,and clay content are of highest porosity and permeability.
Formation conditions and exploration potential of tight oil in the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China
DOI:10.1016/S1876-3804(12)60095-0
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The features and exploration potential of the Permian tight oil in the Junggar Basin were analyzed and evaluated using outcrop, core and geochemical data etc. The Junggar Basin in the Early-Mid Permian is a saline lacustrine basin after the residual sea is closed, a set of hybrid sedimentation of deep-lake dark mudstone and dolomitic rock is developed, and the high-quality mudstone source rocks and the dolomite mudstone are alternated. High quality source rocks in mature stage are next to tight dolomitic rock reservoirs closely and provide good conditions for tight oil accumulation of proximal source type. Tight oil reservoirs are mainly distributed in the centre and slope region of the lake basin, and two types sweetspots of “dissolved pore” and “fracture-pore” exist locally. The enrichment of tight oil is controlled by the distribution of effective source rocks and dolomitic rocks, and the tight oil occurs in the entire strata vertically and spreads across large continuous areas horizontally. The Junggar Basin has four Permian tight oil distribution areas, Fencheng Formation in the Mahu sag, Lucaogou Formation in the Jimusaer sag, Pidiquan Formation in the Shazhang-Shishugou sag, and Lucaogou Formation in the Bogeda piedmont. A number of wells obtained oil flow in these areas, suggesting great resource potential and favorable targets for future exploration.
Tight gas in China and its significance in exploration and exploitation
DOI:10.1016/S1876-3804(12)60043-3
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Tight gas reservoirs refer to the tight sandstone fields or traps accumulating natural gas of commercial values. According to reservoir characteristics, reserves, and structural height, they can be divided into two types, continuous-type and trap-type: the former are located at the lower part of the structure and have indistinct trap boundaries, inconsistent gas-water boundaries and reversal of gas and water, and their reservoirs are the same as or near the source; the latter are located at the higher part of the structure, with gas above water in traps, low reserves, and relatively high production. Tight gas in China is all coal-derived, dominantly alkane gases (C1 4), in which the amount of methane is greatest and the alkane gases have positive carbon isotopic series. The content of non-hydrocarbon gases (mainly CO2 and N2) is low. At the end of 2010, the reserves and annual production of tight gas in China accounted for 39.2% and 24.6% of the total natural gas, respectively, and the proportions are expected to increase. Compared to the shale gas and coalbed gas, tight gas should be considered in priority in the exploration and exploitation of unconventional gas in China.
The sedimentary structure and petroleum geologic significance of the ring belt of the closed Lake Basin: An integrated interpretation of well and seismic data of the Kong2 Member in Cangdong sag, Central Bohai Bay Basin, China
DOI:10.1190/INT-2017-0041.1 URL [Cited within: 1]
Controlling factors of hydraulic fractures extending into network in shale formations.
Fracturingpracticeshowsthathydraulicfracturesinshalereservoirpresentcomplexextendingmodelsofnon-planar and multiple branches,which is essentially different from traditional symmetry plane bi-wing fractures in forming mechanism.Based on the research results of indoor experiments,field fracturing practice,theory analysis and numerical simulation etc,this paper has systematically analyzed the controlling factors of hydraulic fractures extending into network in shale formations.Research shows the extending shape of fractures in shale reservoir is influenced by both geological factors and engineering factors:according to reservoir geological attributes,higher brittle mineral contents of rock,stronger elastic characteristic of rock mechanical properties,smaller horizontal differential stress and better developed natural fractures will be constructive to better extension and propagation for hydraulic fractures extending into network;according to engineering conditions of fracturing operations,higher operation net pressure,lower fluid viscosity and larger fracturing scale will be more helpful to the formation of fully propagated fracture-network.This study has important theoretical value and practical significance to the understanding of shale reservoir fracture-network propagation mechanism and can improve the science of shale reservoir fracturing design.
