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Fault characteristics and control on hydrocarbon accumulation of middle-shallow layers in the slope zone of Mahu sag, Junggar Basin, NW China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 6, December 2018 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2018, 45(6): 1050–1060.
RESEARCH PAPER
Fault characteristics and control on hydrocarbon accumulation of middle-shallow layers in the slope zone of Mahu sag, Junggar Basin, NW China CHEN Yongbo1,*, CHENG Xiaogan2, ZHANG Han1, LI Chunyang2, MA Yongping1, WANG Guodong1 1. Northwest Branch of Research Institute of Petroleum Exploration & Development, PetroChina, Lanzhou 730020, China; 2. School of Earth Sciences, Zhejiang University, Hangzhou 310027, China
Abstract: The development, evolution and formation mechanism of faults and their control on the migration and accumulation of Mesozoic oil and gas in the middle-shallow layers of the slope zone of Mahu sag were studied by the interpretation of seismic and drilling data. Two types of faults, normal and strike-slip, are developed in the middle-shallow layers of the slope zone of the Mahu sag and they are mostly active in the Yanshanian period. They are divided into four grade faults: The grade I strike-slip faults with NWW to near EW direction are related to the left-lateral transpressive fault zones in the northwest of Junggar Basin since the end of the Triassic. The grade II faults with NE to NNE direction are the normal faults located at the junction of the fault zone and the slope zone, and their formation is related to the extension at the top of the nose-like structures in the fault zone. The grade III faults, which are also the normal faults, are the result of the extension at the top of the lower uplifts in the slope zone and differential compaction. The grade IV faults with NE direction are normal faults, which may be related to the extension environment at the tip of the lower uplifts. Faults not only are the channel for the vertical migration of oil and gas, but also control the oil-gas accumulation. There are two types of oil-gas reservoirs in the middle-shallow layers of slope zone of Mahu sag: fault block reservoirs and fault-lithologic reservoirs. They have large traps and promising exploration potential. Key words: Junggar Basin; slope zone of Mahu sag; middle-shallow layers; fault; genetic mechanism; nose-like structure; lower uplift; hydrocarbon accumulation
Introduction The Mahu sag located in the northwest of the Junggar Basin, is a secondary negative structural unit of the central depression. Since 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 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[12]. 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 middleshallow 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[23]. But there are controversies on the nature, active time, period, forming mechanism of the faults and their effects on hydrocarbon migration and accumulation[2,47]. 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.
Received date: 06 Mar. 2018; Revised date: 16 Sep. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2017ZX05008-001, 2011ZX05003-003). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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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[89]. 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
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.
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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[1012]. 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[1315]. 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[6,1718]. 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 transpres-
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.
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sion 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. 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 2030 m (Fig. 2a). The branching faults at its two sides are normal faults with displacement of 1520 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 2030 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 2030 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 1015 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: 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 2465 km. They often have NW and NE striking branching faults extending 6.08.5 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 lowamplitude bulge top in the slope zone. Trending NWW direction, they are parallel to the long axis direction of the lowamplitude bulge (Figs. 2c, 3, 4 and 6), and 525 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.
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
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At the end of the Permian period, the front of the thrust belt
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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.
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). 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[1920]. 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
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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
Fig. 4. Planar frequency division coherence of the bottom of lower Jurassic Badaowan Formation in Baikouqan region.
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
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
Fig. 5. Seismic interpretation sections through various locations of the slope zone in the Mahu sag (their locations are shown in Fig. 1b). 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.
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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 strikeslip 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). 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.
Fig. 6. Genetic model of middle-shallow faults in the slope zone of the Mahu sag. K—Cretaceous; J—Jurassic; T—Triassic; P— Permian.
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[2425]. 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 en-
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tered 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[2425], 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). (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
Fig. 7. Reservoir model diagram through Wells 446-MH4-AH12 (the profile location is shown in Fig. 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.; P3w—Permian upper Urho Fm.; P2w—Permian lower Urho Fm.; P2x—Permian Xiazijie Fm.; P1f— Permian Fengcheng Fm.; P1j—Permian Jiamuhe Fm.
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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” faultblock 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 311.5 km2, making them efficient exploration blocks in Mesozoic. 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
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.
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formation profiles in delta front facies reaches 25%, considerable fault gouge would be developed, then the faults can play a sealing role basically[2829]. 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.
of fan-controlled large-area lithologic oil reservoirs of Triassic Baikouquan Formation in slope zone of Mahu Depression in Junggar Basin. China Petroleum Exploration, 2014, 19(6): 14–23. [2]
CHEN Yongbo, PAN Jianguo, ZHANG Han, et al. Characteristics of fault evolution in Mahu slope area of Junggar Basin and its implications to the reservior in the Lower Triassic Baikouquan Formation. Natural Gas Geoscience, 2015, 26(S1): 11–24.
Conclusions
[3]
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.
TAO Guoliang, HU Wenxuan, ZHANG Yijie, et al. NW-trending transverse faults and hydrocarbon accumulationin the northwestern margin of Junggar Basin. Acta Petrolei Sinica, 2006, 27(4): 23–28.
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XIAO W J, HAN C M, YUAN C, et al. Middle Cambrian to Permian subduction-related accretionary orogenesis of Northern Xinjiang, NW China: Implications for the tectonic evolution of central Asia. Journal of Asian Earth Sciences, 2008, 32(2/3/4): 102–117.
[10] XIAO W J, SANTOSH M. The western Central Asian Orogenic Belt: A window to accretionary orogenesis and continental growth. Gondwana Research, 2014, 25(4): 1429–1444. [11] HAN Baofu, JI Jianqing, SONG Biao, et al. Late Paleozoic vertical growth of continental crust around the Junggar Basin, Xinjiang, China (Part I): Timing of post-collisional plutonism. Acta Petrologica Sinica, 2006, 22(5): 1077–1086. [12] LI D, HE D F, LIAN Y C, et al. 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, 2017, 59(10): 1234–1255. [13] ZHAO Bai. Formation and evolution of Junggar Basin. Xinji-
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RESEARCH PAPER
Fault characteristics and control on hydrocarbon accumulation of middle-shallow layers in the slope zone of Mahu sag, Junggar Basin, NW China CHEN Yongbo1,*, CHENG Xiaogan2, ZHANG Han1, LI Chunyang2, MA Yongping1, WANG Guodong1 1. Northwest Branch of Research Institute of Petroleum Exploration & Development, PetroChina, Lanzhou 730020, China; 2. School of Earth Sciences, Zhejiang University, Hangzhou 310027, China
Abstract: The development, evolution and formation mechanism of faults and their control on the migration and accumulation of Mesozoic oil and gas in the middle-shallow layers of the slope zone of Mahu sag were studied by the interpretation of seismic and drilling data. Two types of faults, normal and strike-slip, are developed in the middle-shallow layers of the slope zone of the Mahu sag and they are mostly active in the Yanshanian period. They are divided into four grade faults: The grade I strike-slip faults with NWW to near EW direction are related to the left-lateral transpressive fault zones in the northwest of Junggar Basin since the end of the Triassic. The grade II faults with NE to NNE direction are the normal faults located at the junction of the fault zone and the slope zone, and their formation is related to the extension at the top of the nose-like structures in the fault zone. The grade III faults, which are also the normal faults, are the result of the extension at the top of the lower uplifts in the slope zone and differential compaction. The grade IV faults with NE direction are normal faults, which may be related to the extension environment at the tip of the lower uplifts. Faults not only are the channel for the vertical migration of oil and gas, but also control the oil-gas accumulation. There are two types of oil-gas reservoirs in the middle-shallow layers of slope zone of Mahu sag: fault block reservoirs and fault-lithologic reservoirs. They have large traps and promising exploration potential. Key words: Junggar Basin; slope zone of Mahu sag; middle-shallow layers; fault; genetic mechanism; nose-like structure; lower uplift; hydrocarbon accumulation
Introduction The Mahu sag located in the northwest of the Junggar Basin, is a secondary negative structural unit of the central depression. Since 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 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[12]. 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 middleshallow 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[23]. But there are controversies on the nature, active time, period, forming mechanism of the faults and their effects on hydrocarbon migration and accumulation[2,47]. 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.
Received date: 06 Mar. 2018; Revised date: 16 Sep. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2017ZX05008-001, 2011ZX05003-003). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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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[89]. 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
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.
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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[1012]. 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[1315]. 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[6,1718]. 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 transpres-
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.
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sion 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. 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 2030 m (Fig. 2a). The branching faults at its two sides are normal faults with displacement of 1520 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 2030 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 2030 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 1015 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: 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 2465 km. They often have NW and NE striking branching faults extending 6.08.5 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 lowamplitude bulge top in the slope zone. Trending NWW direction, they are parallel to the long axis direction of the lowamplitude bulge (Figs. 2c, 3, 4 and 6), and 525 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.
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
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At the end of the Permian period, the front of the thrust belt
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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.
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). 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[1920]. 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
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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
Fig. 4. Planar frequency division coherence of the bottom of lower Jurassic Badaowan Formation in Baikouqan region.
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
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
Fig. 5. Seismic interpretation sections through various locations of the slope zone in the Mahu sag (their locations are shown in Fig. 1b). 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.
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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 strikeslip 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). 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.
Fig. 6. Genetic model of middle-shallow faults in the slope zone of the Mahu sag. K—Cretaceous; J—Jurassic; T—Triassic; P— Permian.
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[2425]. 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 en-
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tered 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[2425], 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). (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
Fig. 7. Reservoir model diagram through Wells 446-MH4-AH12 (the profile location is shown in Fig. 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.; P3w—Permian upper Urho Fm.; P2w—Permian lower Urho Fm.; P2x—Permian Xiazijie Fm.; P1f— Permian Fengcheng Fm.; P1j—Permian Jiamuhe Fm.
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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” faultblock 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 311.5 km2, making them efficient exploration blocks in Mesozoic. 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
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.
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formation profiles in delta front facies reaches 25%, considerable fault gouge would be developed, then the faults can play a sealing role basically[2829]. 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.
of fan-controlled large-area lithologic oil reservoirs of Triassic Baikouquan Formation in slope zone of Mahu Depression in Junggar Basin. China Petroleum Exploration, 2014, 19(6): 14–23. [2]
CHEN Yongbo, PAN Jianguo, ZHANG Han, et al. Characteristics of fault evolution in Mahu slope area of Junggar Basin and its implications to the reservior in the Lower Triassic Baikouquan Formation. Natural Gas Geoscience, 2015, 26(S1): 11–24.
Conclusions
[3]
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.
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