Introduction
The Mahu sag located in the northwest of the Junggar Basin, is a secondary negative structural unit of the central depression. Since the 1950s, the “Karamay-Urho oil province of 100 kilometers long” with reserve magnitude of one billion tons has been found in Jurassic, Triassic and Permian in the fault belt in northwestern margin of the west part of this sag. Since the 1990s, with the transition of exploration focus from the fault belt to the slope zone, Mabei oilfield and the oil reservoir in the lower Triassic Baikouquan Formation in the Ma 6 block have been found in the slope zone of the Mahu sag[1]. Since 2010, successive breakthroughs have been made in the lower Triassic Baikouquan Formation in the slope zone, unveiling the extensive hydrocarbon accumulation there gradually[1,2]. Since 2015, several wells have obtained high-productivity commercial oil and gas flows in the middle Triassic Karamay Formation and the lower Jurassic Badaowan Formation of middle-shallow depth. These breakthroughs in the middle-shallow layers have opened up new oil-bearing strata and efficient shallow exploration areas in the slope zone of the Mahu sag with great economic benefit.
Exploration practice shows that oil reservoirs in the middle-shallow layers of the slope zone in the Mahu sag are controlled by both faults and lithology. Hereinto, faults not only are pathways for oil and gas migration in vertical direction, but also control hydrocarbon accumulation. Hence, it is of great significance to figure out the nature, forming mechanism and reservoir controlling feature of faults. Some study on the characteristics of deep thrust faults in this area have been done before, which revealed that there developed three groups of faults (NE, NW and near EW trending) in this area[2,3]. But there are controversies on the nature, active time, period, forming mechanism of the faults and their effects on hydrocarbon migration and accumulation[2,4-7]. Moreover, there are fewer reports about the nature and forming mechanism of faults in the middle-shallow layers and their effects on hydrocarbon accumulation in the slope zone of the Mahu sag.
Based on fine interpretation of high-quality 3D and 2D seismic data, the nature, distribution directions and forming mechanisms of faults in the middle-shallow layers have been sorted out and the relationship between faults and hydrocarbon accumulation have been analyzed in this study, in the hope to guide oil and gas exploration directions in the middle-shallow layers of this area.
1. Regional geology
The Junggar Basin is situated at the junction of the Siberia plate, the European plate and the Tarim plate (Fig. 1a), and is an important part of the Central Asian Orogenic Belt[8,9]. The Junggar Basin and its periphery started the formation and subduction of oceanic basins, and terrain amalgamation and accretion inside ocean during early Paleozoic Era. Late Paleozoic Era was the era when accretion transformed to collision, and complex tectonic framework with coexistent extrusion, expansion and strike-slip, violent magmatic activity, and frequent material exchange between crust and mantle occurred[10,11,12]. During Triassic-Paleogene, the Junggar Basin experienced overall subsidence. From Neogene to Quaternary, the south margin of the Junggar Basin bended and subsided, forming a near EW-striking intracontinental foreland basin[13,14,15].
Fig. 1.
Geotectonic location map (a), structural unit map (b) and cross section (c) of Mahu sag and its periphery. Cz—Cenozoic; K—Cretaceous; J—Jurassic; T—Triassic; P2w—Permian Urho Fm.; P2x—Permian Xiazijie Fm.; P1f—Permian Fengcheng Fm.; P1j—Permian Jiamuhe Fm.; C—Carboniferous.
The Mahu sag borders with the Kebai fault belt (Maxi) to the west, the Wuxia fault belt (Mabei) to the north, the Zhongguai bulge (Manan) to the south, and the Basong bulge and Xiayan bulge (Madong) to the east (Fig. 1b). Between the sag and the periphery uplifted fault belts and bulges are the extensive slope zones (Fig. 1c). According to the previous study results, the most important feature of the northwestern margin of the Junggar Basin is the development of a group of large thrust structural belts distributed in arching style[7,16]. The structural belt consists of three fault belts with quite different structural patterns from north to south: Wuxia fault belt, Kebai fault belt, and Hongche fault belt[16], and the fault belts and different structures in the same fault belt are separated by transverse adjusting faults[3]. Recently, some researchers proposed that there developed large strike-slip structures related to the rotation of the Junggar massif or the strike-slip motion of the Daerbute fault at the northwestern margin of the Junggar Basin[6,17-18]. The west and north of the slope zone, situated at the lower wall of the northwestern margin fault belt, with weaker deformation, appear as a broad SE-dipping slope on the whole. There are some nose structures and low-amplitude bulges in local parts there[1], forming several nose uplifts (such as Urho nose uplift, Baikouquan nose uplift, Karamay nose uplift) intersecting with the fault belt obliquely in en echelon arrangement[2].
There deposit quite continuous strata from upper Paleozoic to Neozoic in Mahu sag (Fig. 1c), mainly including Paleozoic Permian Jiamuhe Formation (P1j), Fengcheng Formation (P1f), Xiazijie Formation (P2x), lower Urho Formation (P2w) and upper Urho Formation (P3w); Mesozoic Triassic Baikouquan Formation (T1b), Karamay Formation (T2k) and Baijiantan Formation (T3b); middle-lower Jurassic Badaowan Formation (J1b), Sangonghe Formation (J1s), Xishanyao Formation (J2x) and Toutunhe Formation (J2t); Cretaceous Tugulu Group (K1tg) and Alice Lake Formation (K2a), and Cenozoic.
2. Features of faults in the middle-shallow layers of the slope zone
Different from the major part of the thrust belt in the northwestern margin with mainly NNE-NE striking boundary thrust faults (or transpression faults), the slope zone of the Mahu sag has dense and small throw normal faults and normal faults derived from strike-slip faults in the shallow strata (Fig. 2b, 2c) besides thrust faults or transpression faults in deep formations. The faults in the deep formations break the strata from Permian to Triassic Baikouquan Formation (T1b), and are a part of the thrust system in the northwestern margin of the Junggar Basin, which are attributed to the strong thrust of the northwestern thrust belt in late Permian and the transpression in Triassic-Jurassic. In contrast, the faults in middle-shallow formations above Baikouquan Formation (T1b) are normal faults with smaller fault displacement, some formed by extension, some derived from strike-slip. The former cut through Baikouquan Formation to Cretaceous Tugulu Group (K1tg), while the later composed of several faults converging downward or 1-2 faults with steep planes extend down to the basement. Smaller in displacement (λ/8-λ/4), the faults in the shallow formations have continuous reflections on seismic sections[2]. Moreover, the strike-slip faults and their associated faults have complicated combination patterns, so we adopted interactive interpretation methods on the plane and profile. The generalized Hilbert transform technique on the profile was taken to process the normal and strike-slip faults, and slicing technique, frequency division and stratum coherence techniques etc on the plane were used to predict the lateral distribution of the faults.
Fig. 2.
Section features of faults in slope zone of the Mahu sag (the locations of the sections are shown in Figs. 1 and 3). Cz—Cenozoic; K2a—Cretaceous Ailikehu Fm.; K1tg—Cretaceous Tugulu Group; J3q—Jurassic Qigu Fm.; J2t—Jurassic Toutunhe Fm.; J2x—Jurassic Xishanyao Fm.; J1s—Jurassic Sangonghe Fm.; J1b—Jurassic Badaowan Fm.; T3b—Triassic Baijiantan Fm.; T2k—Triassic Karamay Fm.; T1b—Triassic Baikouquan Fm.; P2w—Permian Urho Fm.; P2x—Permian Xiazijie Fm.; P1f—Permian Fengcheng Fm.; P1j—Permian Jiamuhe Fm.; C—Carboniferous.
2.1. Fault types and features on section
As mentioned above, according to broken strata and combination patterns, faults in the middle-shallow layers of the slope zone can be divided into two types, strike-slip faults and normal faults. They have different features on section:
Strike-slip faults: mainly occurring in the slope zone, they come in two patterns, negative flower and single branching according to interpretation results of seismic sections in different positions. Cutting the strata from Carboniferous to Cretaceous, the major fault in a negative flower pattern has steep-straight fault plane on section (with a dip angle near 90°), apparently breaking of seismic events on two sides of it, and fault displacement of 20-30 m (Fig. 2a). The branching faults at its two sides are normal faults with displacement of 15-20 m, opposite in dipping directions, constituting a graben. They converge downwards at Permian, and make up a negative flower structure with the major fault. The single branching pattern only has a major fault, with no or few branching faults. They appear as high-steep narrow normal faults, generally with dip angle of fault plane more than 80° on section, and also break the Carboniferous-Cretaceous. The fault displacements of the Triassic-Jurassic systems are 20-30 m, and less than 20 m at two sides in deep Permian System (Fig. 2b). The most famous representative of such faults in the Mahu sag is the Dazhuluogou strike-slip fault[5]. They are usually deemed to play a regulation role between different nose uplifts or thrust belts[3].
Normal faults: they appear in various tectonic positions of the slope zone.
(1) At the junction of the fault belt and slope zone in northwestern margin: Normal faults are mainly located at the top of the inherited nose uplift in the borderland between the Kebai fault belt, Baibai fault belt and the slope zone. They are composed of several normal faults with identical (or opposite) dip direction (s). These normal faults break Triassic, Jurassic and Cretaceous systems, and have displacement of 20-30 m and dip angle of 60°-70° (Fig. 2b). The strata underlying them are the frontal zone and its nose uplift of the northwestern margin fault belt.
(2) Slope zone: Normal faults mainly occur in the slope zone of the downdip direction of the Urho nose uplift and the Baikouquan nose uplift. The displacement of the normal faults gradually decrease to the downdip direction (generally 10-15 m). The local tectonic background is low-amplitude bulge. They break Triassic, Jurassic and Cretaceous systems, and have dip angle of 50°-60° (Fig. 2c).
2.2. Plane distribution features of faults
Through interactive interpretation between plane and section by using slice technique, frequency division and stratum coherence technique etc., the orders of faults developed in middle-shallow layers in the slope zone of the Mahu sag were classified according to the profile types and plane extension of faults and their effects on hydrocarbon accumulation (Figs. 3, 4 and 6). The detailed classification is as follows:
Fig. 3.
Distribution of faults in middle-shallow layers of the slope zone in the Mahu sag. The base map is the structural map of the top of the Karamay Formation.
First order faults: They are strike-skip faults trending NWW and near EW on the plane, including Dazhuluogou strike-slip fault, Baikouquan south strike-slip fault, Baikouquan strike-slip fault and Huangyangquan strike-slip fault, etc from south to north. They are all dextral strike-slipping, with extending length between 24-65 km. They often have NW and NE striking branching faults extending 6.0-8.5 km in pinnate shape on two sides of the major strike-slip fault (Figs. 2a, 3, 4 and 6). The first order strike skip faults and deep thrust faults compose the major oil-source faults.
Second order faults: They are the normal faults at the borderland between the Kebai fault belt and Baibai fault belt and the slope zone. Trending NE-NNE on the plane, they are basically parallel to the extending direction of northwestern margin fault belt, 7-21 km in extending lengths, and mostly in parallel and en echelon arrangement. The second order faults and deep thrust faults combine into “Y” shape, and can form stepwise hydrocarbon migration pathways (Figs. 2b, 3, 4 and 6).
Third order faults: They are the normal faults at the low- amplitude bulge top in the slope zone. Trending NWW direction, they are parallel to the long axis direction of the low- amplitude bulge (Figs. 2c, 3, 4 and 6), and 5-25 km in extending length.
Fourth order faults: They are the secondary normal faults trending NE direction, with extending lengths between 2-8 km. They are perpendicular to or intersect at high angles with the third order faults (Fig. 4). The third order normal faults connecting with fourth order normal faults (Figs. 3, 4 and 6) act as important pathways for hydrocarbon migration in vertical direction.
Fig. 4.
Planar frequency division coherence of the bottom of lower Jurassic Badaowan Formation in Baikouqan region.
3. Characteristics of fault activity and forming mechanism of the faults in middle-shallow layers since late Permian
3.1. Characteristics of fault activity in Mahu sag and its periphery since late Permian
At the end of the Permian period, the front of the thrust belt in the northwestern margin of the Junggar Basin basically reached the present position [17]. During this period, the thrust belt in the northwestern margin violently thrusted and deformed, leading to strong deformation, uplift and erosion of the early-middle Permian strata and the development of the regional angle disconformity between Triassic and Permian (Fig. 5).
Fig. 5.
Seismic interpretation sections through various locations of the slope zone in the Mahu sag (their locations are shown in
At the end of the Triassic period, the preexisting late Permian thrust faults in the northwestern margin were successively active, but faults in different strikes exhibited different properties: (1) The NE-NEE striking Wuxia fault belt showed violent compressional deformation, leading to development of low-angle and imbricated thrust faults at the lower sides of some boundary faults (Baikouquan fault and Wulanlinge fault etc.) and several anticlines that were diagonal to the boundary faults (such as Xiazijie anticline and Urho anticline). The Triassic system at the tops of these anticlines was violently eroded (Fig. 5a), showing transpression overall. (2) The NNE-near SN striking Kebai fault belt and Hongche fault belt show strong strike-slip features, with steeper fault planes and weaker deformation in the lower wall, and strong uplifting and erosion of the Triassic system in the upper wall (Fig. 5b). Hence, the northwestern margin fault belt had strike-slip behavior on the whole in this period (Fig. 1c).
During early Jurassic period, the Junggar Basin was in an intraplate extension environment[19,20]. During late Jurassic period, the Junggar Basin experienced tectonic inversion, changing from an extensional basin to a compressional depression basin. The faults in the northwestern margin of this basin inherited the transpression nature at the end of the Triassic period, where the Jurassic system at the upper wall of the fault belt suffered uplifting and erosion, giving rise to low-amplitude angle disconformity between middle Jurassic and lower Cretaceous (Fig. 5).
During Cretaceous period, this basin exhibited overall depression deposition. However, the whole western region of China saw a violent tectonic event during late Cretaceous, causing intense uplift of the periphery of the Junggar Basin. The northwestern margin of the Junggar Basin witnessed strong compressional thrusting with strike-slip, as a result, the Mahu sag titled entirely toward the orogenic zone direction, and the Cretaceous system suffered massive erosion (Fig. 1c), to this point, the slope zone of the Mahu sag was fixed in shape basically.
Affected by multiphasic inherited transpression of the boundary thrust faults during the end of Triassic period, late Jurassic period and late Cretaceous period, a series of strike-slip faults, inherited nose uplifts, low-amplitude bulges and associated normal faults came about in the middle-shallow layers of the slope zone in the Mahu sag.
Since Cenozoic, the southern margin of the Junggar Basin saw extensive deflection and subsidence, giving rise to a near EW-striking regenerated foreland basin[15]. The northwestern margin of the Junggar Basin mainly appeared as a monocline dipping southeast, with weaker fault activity and slight tectonic deformation (Fig. 1c).
3.2. Forming mechanism of faults in middle-shallow formations of the slope zone
By testing fluid inclusions from the fillers in the Dazhuluogou strike-slip fault, Wu Kongyou et al.[5] inferred that the Dazhuluogou strike-slip fault was formed at the end of Traissic period, and was violently active during the Yanshan epoch. The active time of other first order strike-slip faults was also this period. Moreover, on the basis of analyzing the stata cut by the second to fourth order normal faults, their major active time should also be the Yanshanian period (Fig. 2).
As mentioned above, the northwestern margin of the Junggar Basin was affected by mutiphasic episodic tectonic movements since the end of Permian period, so faults of various natures have been formed in the study area and its periphery. In the transpressional deformation at the end of Triassic period, late Jurassic and the end of Cretaceous period, NWW and near EW striking adjusting (strike-slip) faults were likely to occur at the borderland of boundary faults and bending positions of faults. Meanwhile, in the Hoxtolgay Basin in the northwestern margin of the Junggar Basin, above the Carboniferous base, upper Triassic series began to deposit locally; the maximum residual thickness of the middle-lower Jurassic series was up to 3 000 m thick[21], which was much bigger than that in Mahu sag (Fig. 1c). Hence, the Hoxtolgay Basin and the Mahu sag did not belong to the same basin system during late Triassic-Jurassic period, and it is speculated that the basin is a pull-apart basin formed by the sinistral strike slipping of the Daerbute Fault and Xiemisitai Fault. Therefore, the Daerbute Fault began to change from dextral to sinistral strike slipping since the end of Triassic period or at latest since Jurassic period[5, 22]. According to the Sylvester simple shear model, the NWW and near EW striking dextral strike- slip faults might be derived R’ dextral tensional shear surfaces of the Daerbute Fault[2, 5]. Therefore, the NWW and near EW striking first order strike-slip faults might be the result of the joint effects of above two mechanisms. The Dazhuluogou strike-slip fault and Baikouquan strike-slip fault etc. are typical representatives of this type of faults (Figs. 3 and 4).
Second order faults: Striking NE-NNE, basically parallel to the northwestern margin fault belt (Figs. 3 and 4), they are usually composed of several normal faults with identical (opposite) dip direction and located at the tops of the NE-NNE striking inherited nose uplifts (Figs. 2b and 3). According to their development and distribution features, we inferred that their formation was related to the inherited nose uplifts formed by multiphasic thrusting processes in the thrust belt of the northwestern margin since the end of Triassic period, and are the extending result of the tops of these nose uplifts (Fig. 6). These faults are usually cut by the first order faults striking NWW and near EW (Figs. 3, 4 and 6).
Fig. 6.
Genetic model of middle-shallow faults in the slope zone of the Mahu sag. K—Cretaceous; J—Jurassic; T—Triassic; P—Permian.
Third order faults: They strike NWW, basically identical to the long axis direction of the inherited low-amplitude bulges in the slope zone, thus we inferred that they were generated by the extension of the bulge tops (Fig. 6). In addition, because of the differential compaction caused by sharp increase of overlying strata thickness, the third order faults can also be formed from the Kebai fault belt to the slope zone. They might be also generated by the NW-NWW striking extensional planes derived from the sinistral strike slipping of the fault belt in the northwestern margin.
Fourth order faults: Striking NE, they might be transverse tensional normal faults developed in extensional environment at the pitching end of the low-amplitude bulges (Fig. 6).
4. Controlling effect of faults on hydrocarbon
migration and accumulation in Mesozoic
Multiple orders of faults formed in the slope zone of the Mahu sag since Mesozoic Era make up several sets of hydrocarbon-bearing layers with several reservoir-caprock assemblages in vertical direction. Exploration results indicate that most oil and gas discovered in the slope zone (about 94%) were from the deep Permian source rock of the Mahu sag[23]. The evolution of these faults is closely linked with hydrocarbon migration and accumulation. They not only act as pathways for oil and gas migration in vertical direction, but also control hydrocarbon accumulation.
Based on the features that the Mesozoic in the slope zone of the Mahu sag is far away from source rock in vertical direction and hydrocarbon accumulation mainly occurred during late period, we established the hydrocarbon accumulation model for the slope zone: deep oil-source faults + “sweet spot” reservoirs + adjusting faults.
4.1. Fault activity and hydrocarbon accumulation process in the Mahu sag
The Permian source rock experienced multiphasic tectonic movements (Hercynian, Indo-Chinese, and Yanshan epochs). Faults played a critical role during the process of hydrocarbon migration and accumulation.
Middle-late Permian was the period when the Junggar Basin subsided quickest. With the rapid increase of overlying formation thickness, the source rock in the Jiamuhe Formation became mature quickly[24] and reached hydrocarbon generation threshold. The oil and gas entered the reservoirs of the Permian Jiamuhe Formation and Xiazijie Formation etc. in the slope zone along the northwestern margin fault belt and unconformity surface inside the Permian system.
During late Triassic-early Jurassic, the preexisting thrust faults in the Mahu sag were successively active, and some first order strike-slip faults (Dazhuluogou fault and Huangyangquan fault etc.) began to be formed. The major source rock in the lower Permian Fengcheng Formation in the Mahu sag became mature and expelled massive hydrocarbons[24,25]. Oil and gas migrated from the source rock along oil-source faults (deep thrust fautls and first order strike-slip faults), and then were adjusted by faults in vertical direction; after lateral transport through unconformities and permeable carrier beds, oil and gas accumulated in various reservoirs in Permian-Triassic systems.
During late Jurassic-early Cretaceous, oil-source faults were successively active, and many normal faults striking different directions developed in Triassic-lower Cretaceous (i.e., the second to fourth order normal faults mentioned above). Then the source rock in the Jiamuhe Formation entered gas generation stage, and the source rock in the middle Permian Urho Formation also reached the peak of hydrocarbon generation and expulsion. Additionally, the tectonic activities during this period destroyed the oil and gas reservoirs formed during early period[24,25], thus oil and gas migrated and accumulated again into new reservoirs.
After Cretaceous, the northwestern margin of the Junggar Basin was at relatively stable tectonic environment, with no new faults developing, which is favorable for preserving the reservoirs[25].
4.2. Oil and gas transporting models related to faults in the slope zone
According to their effects during hydrocarbon accumulation process, the faults can be divided into deep oil-source faults and shallow reservoir faults. The deep oil-source faults (including first order strike-slip faults and deep thrust faults) connect the deep source rocks, and provide pathways for vertical migration of oil and gas generated by the source rocks. The shallow reservoir faults are the second order to fourth order faults. They adjust oil and gas distribution in vertical direction, and provide sealing condition for hydrocarbon adjustment and accumulation. The slope zone mainly has three kinds of oil and gas transporting models related to faults:
(1) “Flower-like” transporting model: Mainly occurring near first order strike-slip faults, this kind of transporting model has strike-slip faults formed at the end of Indosinian period-Yanshan epoch and the Mesozoic “sweet spot” reservoirs[26] combining into migration pathways. The first order strike-slip faults cut deeper strata in vertical direction, with long extension on plane and steep fault planes. As they were formed just when the major Permian source rock was in massive hydrocarbon generation window, they became favorable pathways for vertical hydrocarbon migration. After entering the strike-slip faults, hydrocarbons preferentially accumulated in “sweet spot” reservoirs along fault belt under buoyancy and pressure difference. When these reservoirs were filled up, oil and gas continuously migrated along major faults and branching faults in diverging style, and accumulated in several strata on section and multiple traps on the plane, hence came the stereoscopic hydrocarbon accumulation model in vertical direction (such as the reservoir model in Well MH4) (Fig. 7).
Fig. 7.
Reservoir model diagram through Wells 446-MH4-AH12 (the profile location is shown in
(2) “Y”-type transporting model: Mainly found in second order faults at the tops of the Urho nose uplift and Baikouquan nose uplift, in which the second order faults and underlying deep thrust faults combine into “Y”-shape. The deep thrust faults are oil-source faults. The planes of the second order faults have big dip angles, high possibility of opening, and late active period. They could form stepwise migration pathways with the deep thrust faults, for example, the reservoir model in Well 446) (Fig. 7).
(3) Relay-type transporting model: Largely found in the slope zone, the model features deep thrust faults “connected” with the third and fourth order normal faults. The staircase switching association of normal and reverse faults is an efficient hydrocarbon accumulation mode. Fault-lithology reservoirs with higher abundance have been found in the slope zone[27], for example, the reservoir model in Well AH12 (Fig. 7).
4.3. Types of the reservoir in shallow-middle layers of the slope zone
Faults are the sealing condition constituting fault-block traps and fault-lithology traps in the slope zone, thus forming two reservoir types: fault-block reservoir and fault-lithology reservoir.
Fault-block reservoirs can be divided into triangle “graben-like” fault-block reservoir and “wall corner-like” fault- block reservoir according to fault order and trap pattern.
(1) Triangle “graben-like” fault-block reservoir (Figs. 8a and 7): Triangle “graben-like” fault-block traps are formed by intersection of the secondary faults on two sides of the major strike-slip belt generated by the first order strike-slip faults. They were local “extensional” stress concentration regions. After the stress was released, they became low stress regions, and hydrocarbons entered the strike-slip faults and then preferentially accumulated in “sweet spot” reservoirs along fault belt. When these reservoirs were filled up, hydrocarbons would continuously migrate along major faults and branching faults in divergent style, and accumulate in several strata on the section and multiple traps on the plane. After hydrocarbon faults stopped action, under fault plane stress, muddy filling and smearing, faults gradually closed, forming fault sealing or shielding. In the slope zone of the Mahu sag, Well MH1, Well MH4 and Well MA27 in such fault blocks all obtained commercial oil flows from sand bodies in the second member of the Triassic Baikouquan Formation and the first member of the Jurassic Badaowan Formation.
(2) Fault-block reservoirs developed at the uplifting end of inherited nose uplifts (Fig. 8b): “Wall corner-like” fault-block traps are formed by intersection of second order faults and NWW-striking third order faults at high angles. They appear as NEE-striking fault block groups on the plane, with inherited nose structure as background. The faults and deep oil-source fault come together in “Y” shape (Fig. 7), acting as favorable migration pathways for oil and gas in vertical direction pointing to the fault block groups. Well M26, Well 446 and Well M27 in this type of reservoir all obtained high-productivity commercial oil flows in the Triassic Karamay Formation. Such type of fault block groups in Kebai region covers an area of up to 410 km2, and individual fault-block traps have an area between 3-11.5 km2, making them efficient exploration blocks in Mesozoic.
Fig. 8.
Reservoir types in middle-shallow layers of the slope zone, Mahu sag. J—Jurassic; T3b—Triassic Baijiantan Fm.; T2k—Triassic Karamay Fm.; T1b—Triassic Baikouquan Fm.; P—Permian.
Fault-lithology reservoirs (Figs. 8c and 7) mainly occur in the slope zone too. Oil and gas in the drilled wells AH12 and AH15 in the slope zone chiefly exist at the top of the first member of the Jurassic Badaowan Formation and the top of the upper Triassic Karamay Formation. The oil layers are 5-8 m thick each, with lithologic association of “thinner sandstone in between thicker mudstone”. The overlying stable mudstone about 100 m thick is a regional caprock. The sedimentary facies is NE-striking underwater distributary channel microfacies in fan delta front, with “sweet spot” reservoirs. The NE-striking “sweet spot” reservoirs are cut by NWW-near EW striking first order faults or third order faults to form fault-lithology trap group higher in the north and lower in the south, in which the oil layer thickness is less than the fault displacements of first order faults or third order faults. According to previous study results, if the muddy content on formation profiles in delta front facies reaches 25%, considerable fault gouge would be developed, then the faults can play a sealing role basically[28,29]. As these faults have good sealing property, fault-lithology reservoirs in which sandstone is enveloped in mudstone can be formed. Currently, the fault-lithology trap group in the Kebai slope zone has an area of 525 km2, but there are few prospecting wells, thus the area has huge exploration potential.
5. Conclusions
The slope zone in the Mahu sag has two fault systems in deep and shallow strata respectively. The deep fault system is mainly composed of NE-NNE striking thrust faults or transpression faults; while the middle-shallow fault system is made up of normal faults, strike-slip faults and their associated normal faults with smaller displacements in different striking directions.
The middle-shallow faults in the slope zone of the Mahu sag can be divided into four orders: (1) The NWW-near EW striking first order strike-slip faults formed by the sinistral transpression process of the northwestern margin fault belt in the Junggar Basin since Triassic period. (2) The NE-NNE striking second order faults that are normal faults along the borderland between the fault belt and slope zone in northwestern margin, whose formation was related to the extensional movement of the inherited nose uplift top in the fault belt. (3) The NWW-striking third order faults are normal faults at the low-amplitude uplift top in the slope zone formed by the extensional movement of the low-amplitude uplift top; moreover, the third order faults could also be formed by differential compaction as the result of sharp increase of overlying strata thickness from the Kebai fault belt to the slope zone; they could also be generated by the NW-NWW striking extensional planes derived from the sinistral strike-slipping of the northwestern margin. (4) The NE-striking fourth order faults are transversely extensional normal faults related to the extensional environment at the pitching end of the low-amplitude uplift.
Faults are pathways for oil and gas migration in vertical direction, controlling hydrocarbon migration direction and accumulation. According to their effect during hydrocarbon accumulation process, the faults can be classified into deep oil-source faults and shallow reservoir faults. Meanwhile, faults provide sealing condition for fault-block traps and fault-lithology traps in the slope zone, thus forming two reservoir types (fault-block reservoir and fault-lithology reservoir) in the slope zone of the Mahu sag.
The middle-shallow layers in the slope zone have various types of reservoirs, big trap area and lower exploration degree, thus they have huge exploration potential on the whole.
Reference
Exploration of fan-controlled large-area lithologic oil reservoirs of Triassic Baikouquan Formation in slope zone of Mahu Depression in Junggar Basin
Characteristics of fault evolution in Mahu slope area of Junggar Basin and its implications to the reservior in the Lower Triassic Baikouquan Formation
NW-trending transverse faults and hydrocarbon accumulationin the northwestern margin of Junggar Basin
Characteristics of tectonic evolution and prospect of petroleum exploration in northwestern Junggar Basin
Based upon an analysis on the evolution of the paleo-plates,that of the basin as well as the thrust nappe,the characteristics of tectonic evolution and the hydrocarbon accumulation in Hongshanzui—Xiazijie area,northwest- ern Junggar basin,are discussed in this paper.It has the characteristics of multisource,multi-cyclothem,and multiple oil bearing zones,thus it has a very optimistic prospect.
Strike-slip characteristics, forming mechanisms and controlling reservoirs of Dazhuluogou fault in Junggar Basin
DOI:10.3969/j.issn.1673-5005.2014.05.006
URL
[Cited within: 4]
Using the shear structural theory and physical simulation experiments,this study focuses on the strike-slip characteristics,forming mechanisms and controlling reservoirs of Dazhuluogou fault in the northwestern margin of Junggar Basin.Comprehensive analyses of the field outcrops are combined with high precision 3D seismic data in the framework of simple shear model. The results show that the Dazhuluogou fault which was formed in a twist-compression environment is a dextral strike-slip fault,which belongs to the derived structure of Daerbute fault. It is suggested from the stratigraphic distribution relationship and fluid inclusion information that the Dazhuluogou fault was initialized during the Indosinian movement and was active in the Yanshan movement. The development of the fault was along the R' shearing plane in the simple shear model of Sylvester,where secondary structures were evolved before the strike-slip fault system was formed. Rocks in the Dazhuluogou fault zone were severely ruptured and broken,and some evolved into mylonites and schists. The closure in the main and secondary faults in strike-slip fault system is strong,resulting in the forming of a series of faulted block traps. The shear structures which developed in the northwestern margin of Junggar Basin are relevant to the oil and gas accumulation obviously.
Strike-slip structures and oil-gas exploration in the NW margin of the Junggar Basin, China
New hydrocarbon exploration areas in footwall covered structuresin northwestern margin of Junggar Basin
DOI:10.1016/S1876-3804(08)60004-X
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The fold-and-thrust belt in the northwestern margin of the Junggar Basin is composed of the northern monocline belt, the middle syncline belt, the Ke-Wu thrust belt, and the frontal slope belt, and is divided into the upper and lower structural layers in vertical. The upper structural layer precedes the lower one in terms of thrust sequence. Inferred from the restoration of unconformities, the motion of thrust faults started in late Triassic, activated in succession in Jurassic, and stopped in Cretaceous. The footwall venture exploration should aim at “cap brim” traps, especially in the triangular region, where the Dazhuluogou tear fault and Ke-Wu thrust belt intersect and the exploration horizon imbeds moderately, and then aim at the deep anticline traps in duplex structures when obtaining the essential data of deep geology and the velocity data.
Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia
DOI:10.1038/364299a0 URL [Cited within: 1]
Middle Cambrian to Permian subduction-related accretionary orogenesis of Northern Xinjiang, NW China: Implications for the tectonic evolution of central Asia
DOI:10.1016/j.jseaes.2007.10.008
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Middle-Cambrian to Permian subduction-related records are widely distributed in Northern Xinjiang which can be grouped into the Chinese Altay–East Junggar–Eastern Tien Shan, West Junggar, Yili, and Tarim domains. By integrating paleogeographic and geological data, we suppose that the Chinese Altay–East Junggar–Eastern Tien Shan domain was more closely located to Siberia, while the West Junggar and Yili domains occupied an intermediate position near the Kazakhstan block in the early Paleozoic Paleoasian Ocean. Distribution of Andean-type magmatic arcs, island arcs, accretionary wedges, ophiolitic slices, and/or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences. The Tarim domain was on the opposite side of the early Paleozoic Paleoasian Ocean remaining passive margin. These tectonic units drifted northwards and approached the southern active margin of the Siberian craton in the late Paleozoic, leading to termination of the Paleoasian Ocean and formation of a complicated orogenic collage between Siberian craton and the Tarim block between the end-Permian and Triassic. These multiple accretion processes significantly contributed to the lateral growth of central Asia.
The western Central Asian Orogenic Belt: A window to accretionary orogenesis and continental growth
DOI:10.1016/j.gr.2014.01.008
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61Japan-, Mariana-, and Alaska–Aleutian-type arc systems existed in western CAOB.61Circum-microcontinent and parallel accretion and oroclinal bending occurred.61Trapped basins played a significant role in orogenesis.61Multiple-subduction and long continuous accretion feature continent growth.
Late Paleozoic vertical growth of continental crust around the Junggar Basin, Xinjiang, China (Part I): Timing of post-collisional plutonism
Structural evolution and late Carboniferous magmatism of the Zhongguai arc in the western Junggar Basin, Northwest China: Implications for tectonic evolution of the Junggar Ocean
DOI:10.1080/00206814.2016.1160801
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(2016). Structural evolution and late Carboniferous magmatism of the Zhongguai arc in the western Junggar Basin, Northwest China: implications for tectonic evolution of the Junggar Ocean. International Geology Review. Ahead of Print. doi: 10.1080/00206814.2016.1160801
Formation and evolution of Junggar Basin
The embryo of graben - horst - like tectonic framework of Junggar Basin was originaly formed by an uplift ofupper mantle during the time of the early- middle Carboniferous followed by a tension fault of the transitional base-ment for Devonian and Carboniferous systems. Its Permian, the stage of preliminary development of the basin, wasdown-faulted developing period. A period of fault/seg and depression occurred in Mesozoic with a continuous subsi-denoe. Finally, after contractive uplift in Miocene,Present Junggar Basin was formed in the end.
Types and tectonic evolution of Junggar Basin
Junggar basin is a large continental interior basin in western China, which has been developing since Early Permian Controversy focuses on the type of basin in Early Permian and its origin Also the attributions of the basin from Triassic to Early Tertiary is unknown Evidences about basin fill architecture, tectonic subsidence, geo thermal histories, petrological and geochemical characteristics of Early Permo carboniferous volcanic rocks can provide the following new recognition (1) The basin was initially rifted in Early Permian followed by thermal cooling subsidence during Late Permian, thence went through the cratonic basin stage from Triassic to Paleogene, and finally through the foreland basin developing (2)A large number of calc alkaline suite volcanic rocks erupted in early Permo Carboniferous accompanied by type A granites intrusion suggest that the rift in Early Permian was related to postorogenic extension collapse Furthermore soft collision was a tectonic background for postorogenic extension collapse during Paleozoic in Northern Xinjiang (3) The driving mechanism of postorogenic extension collapse was a mantle thermal convection
Further researches on the tectonic evolution and dynamic setting of the Junggar Basin
Characteristics of structural segmentation of foreland thrust belts: A case study of the fault belts in the northwestern margin of Junggar Basin
Tectono-depositional setting and its evolution during Permian to Triassic around Mahu sag, Junggar Basin
Mesozoic reactivated transpressional structures and multi-stage tectonicdeformation along the Hong-Che fault zone in the northwestern Junggar Basin, NW China
DOI:10.1016/j.tecto.2016.04.039
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61We demonstrate the Hong-Che fault zone in NW Junggar basin is a buried paleo-strike-slip fault.61The dextral strike-slip faulting prevailed in the Junggar Basin during the Middle Triassic to Late Jurassic.61The Early Permian-Late Jurassic multi-phase deformation along the NW margin of Junggar basin is demonstrated.
A new insight into the thrust structures at the northwestern margin of Junggar Basin
The uppermost tectonic feature of the northwestern margin of Junggar Basin is a group of thrusts there.Disputes over these thrust structures are mainly about their formation time and attributes,and the relevant debate of whether the Junggar Basin is a faulted depression or a foreland basin.At the northwestern margin,granite and dyke swarms developed,suggesting that the studied area underwent an extension from 314 Ma to 255 Ma.The facts that the sedimentary scope of Jurassic is wider than that of Triassic and the alkali basalt developed in the early Jurassic indicate that the Junggar Basin underwent an extension and kept expansion in Jurassic.The late Triassic strata unconformably overlying the granite indicates that the formation time of these thrust structures might be from the late Permian to the early Triassic,no later than the Late Triassic.The seismic profile of northwestern margin shows gentle and stable Permian strata in thrust fronts.As to Permian sedimentation,its major characteristics include volcanic development,positive cycle fining up in the long term,higher depositional rate,a steep-to-gentle pattern of subsidence curve,as well as higher paleo-geothermal gradient and heat flow values.The main hydrocarbon traps are non-structural traps,like litho-stratigraphic traps,while the tectono-lithological traps are the minority.Based on the analysis above,we believe that the thrust structures are short term inversion structures occurring between 314-255 Ma extension and Jurassic extension,rather than large scale thrust belts associated with orogeny.
Discovery of lisa basalt northwestern edge of Junggar Basin and it’s geological significance
The discovery of the Lias basalt at Wenzigou,Kelamay provided the evidence for answering the long pendent question about Mesozoic magmatic events of Northern part of Xinjiang.The Lias basalt consists of gray-green basalt and dark gray peridotite-basalt.According to petrochemistry of the basalt,TiO2,FeO and P2O5 are highly concentrated,as well as K2O and Na2O.it shows the characters of continental basalt;Nd(t) value is between 4.4 and 4.9 and 87Sr/86Sr value is between 0.70541 and 0.70543,it indicates that the lava came from mantle;lanthanide La/Yb varies between 1.21 and 6.65 and the average is 3.04,it shows the concentration of light lanthanide and depletion of heavy lanthanide;Generally,the abundance of trace elements are 20-30 times higher than they are in common basalts,As well as the abundance of Rb,Ba and Th.The abundance of Sr,Zr and Yb are low.By 40Ar/39Ar method,the age of(192.7卤1.3) Ma(2 )of the basalt was obtained.That is Lias.The eruption of basic lava happened in early Lisa is the symbolic event which indicate Jungaer basin began to transform from extrusion setting to stretch setting.It also shows the large scale thrust activity which formed between late Paleozoic and early Mesozoic ended.Therefore,the northwestern edge of Junggar began to transform from foreland basin to continental sag basin.It shows great significance to the formation,travel and concentration of oil and gas in Junggar basin.
Analysis of tectonic evolution and oil-gas reservoir formation condition of Heshituoluogai Basin in Northwest Xinjiang.
DOI:10.3863/j.issn.1674-5086.2011.05.011
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Heshituoluogai Basin is an Early Mesozoic of the northwest thrusting folded zone of Junggar in Xinjiang,China.The basin rudiment formed because of extrusion and over thrusting during Indo-Chinese epoch;regional extensionmade basin deposit thick early and medium Jurassic formation,basically forming the pattem with steep slopeand down warping central region;during late Yanshan epoch and Himalayan epoch,the South border of basin overthrust Junggar Basin and was completely separated.As lower exploration degree,oil-gas reservoir formation conditionlack systematic and thoroughgoing research.Based on the oil-gas geological theory,the basic element of oil-gasreservoir,including hydrocarbon source rock condition,reservoir condition and cap rock condition,trap conditionand migration condition,etc.,are analyzed in this paper.Heshituoluogai Basin has fairly good hydrocarbon sourcerock,many reservoir-seal assemblages,different types trap and compound passage system,the basic element ofoil-gas reservoir formation,and Good prospects for the gas and oil exploration.
Strike-slip thrust composite structures and its relationships to hydrocarbon in Halaalate mountains, northern Xinjiang
Mechanism of fault-control oil and gas in Junggar Basin in Xinjiang
Evolution of petroleum migration and accumulation in the northwestern margin of the Junggar Basin: Fluid inclusion geochemistry
DOI:10.1007/s11442-006-0415-5
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Petroleum migration and accumulation in the northwestern margin of the Junggar Basin is a typical case of oil and gas formation in tectonically complex basins. Based on biomarker correlation of reservoirs and inclusion oil components in sandstone, micro-petrographic feature of inclusions, fluid inclusion homogenization temperature measurement, and analysis on FTIR of oil inclusions, it is suggested that oil and gas generated from three Permian source rocks (Lower Permian Jiamuhe and Fengcheng formations, and Middle Permian Wuerhe Formation) migrated mainly during three phases (Late Permian, Late Triassic-Early Jurassic, Early Cretaceous) with different maturities. Then, combined with basins tectonic evolution and source rocks generating history, the evolution of petroleum migration and accumulation was further reconstructed. During the Middle to Late Permian, oil and gas sourced from the Jiamuhe Formation migrated into Carboniferous and Permian traps in the slope area. The Late Triassic to Early Jurassic was the peak for oil and gas accumulation sourced from the Fengcheng Formation in almost every Permian and Triassic reservoir rock. From the Early Cretaceous on, overmature gas derived from the Jiamuhe Formation, with the mature oil and gas sourced from the Lower Wuerhe Formation, propelled the pre-existing hydrocarbons to migrate toward the up-dip areas.
Tectonic evolution characteristics of Hala’alate mountains and their influence on hydrocarbon accumulation in northern Xinjiang.
The area of Hala’alate mountains is located in the foreland thrust belt,north segment of the Junggar Basin,the tectonic evolution characteristics of which was complicated.Using Basin dynamics theory,basin-range coupling idea and balanced cross section technique,this paper shows the whole structural interpretation and resumes the tectonic evolution of the area of Hala’alate mountains.We analyze and discuss about the factors of oil and gas migration and accumulation which are controlledandinfluencedbytectonicevolutionofthearea.Onthisbasis,weconcludethat:(1)tectonicevolutionoftheareaof Hala’alate mountains is closely related to the northwest Junggar Basin,and the entire development can be divided into 4 stages: impact extrusion period(C—P),back-arc extrusion period(P2—T),extended fault period(J1—J 12)and regenerated foreland basin(J 22 —Q);(2)bitumen and heavy oil existence in Hala’alate mountains and the surrounding areas illustrate that oil and gas migration and accumulation had been occurred in this area;(3)there are two kinds of source supply system which are local source and Mahu sag distal source in the area of Hala’alate mountains.The formation of a critical period of oil and gas were the Triassic and Jurassic.The reservoir formation models may be briefly summarized as follows:earlier formation model of far source and side transportation through sand-unconformity,earlier formation model of near source and vertical transportation through sand-fault,and later formation model of near source and ladder transportation;(4)the anticlinal structures in the lower plate of the Daerbute fault should be favorable direction for oil and gas exploration.
Prediction method of the low permeability sandy-conglomerate “Sweet Point” reservoirs and its application: A case study of Mahu Depression northern slope area in the Junggar Basin
Features of oil-gas reservoir generated by fault- lithologic body
DOI:10.1016/S1872-5813(07)60034-6
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Fault is a chief factor for affecting oil-gas reservoir formation.The lithologic bodies such as alluvial fan,fan-delta,reef and bio-bank are the main oil-gas reservoir bodies.When the fault is connected with the lithologic body,the fault-lithologic body oil-gas reservoir will be generated.The study on Junggar Basin showed that the fault and lithologic body became the trap elements and resulted in the dominant type of oil-gas pools in Junggar Basin.The fault activities controlled the basic features of pool formation in the basin.The fault combination style moderately influenced the migration,accumulation,distribution and abundance of oil and gas.The faulting phases strongly influenced the oil-gas migration and accumulation period and the style of pool formation.The Chepaizi-Mosuowan paleo-uplift in the interior of the basin provided the elementary environment for the development of the stratigraphic traps and the fault-lithologic traps to a large extent.The two flank slopes of the paleo-uplift are the favorable area for generating the lithologic and stratigraphic oil reservoirs.The southeast flank slope is the best area.
Main geological factors controlling the accumulation and distribution of oil and gas in a complicated fault-block region
More than 20 years have passed since the beginning of full field development of Dongxin complicated fault block in Dongying seg, Bohai bay basin in early 70's. Proven oil and gas reserves in this area has been doubled by detailed geological studies and progressive exploration by the end of the period compared to the proven oil and gas reserves at the beginning of this period, and new oil and gas reserves have been discovered in recent years through progressive exploration and development. This shows that there are enough potentialities in such complicated fault block areas for further exploration. The authors of this paper analyzed those main geological factors controlling the accumulation and distribution of oil and gas in a complicated fault block area by taking Dongxin area as an example. It is recognized that: ①Superior regional structural background controls the formation of hydrocarbon accumulation belt in a complicated fault block area; ②Sealing property and degree of completeness of the hydrocarbon traps are important factors controlling the difference in the degree of abundance of hydrocarbon accumulation in various fault block areas, fluids migrated and accumulated along the hydrocarbon source bed faults from deep beds toward the shallow ones, multiple activities of these faults caused the accumulation or dispersion for many times, the alteration of the sealing and opening of these faults caused the accumulation of hydrocarbons in various formations forming groups of comb like oil and gas traps; ③Stable distributed mudstone cap rocks controlled the hydrocarbon accumulation vertically and also gave an important lateral sealing of hydrocarbon migrated.
