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Evolutionary sequence of faults and the formation of inversion structural belts in the northern Songliao Basin
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 40, Issue 3, June 2013 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(3): 296–304.
RESEARCH PAPER
Evolutionary sequence of faults and the formation of inversion structural belts in the northern Songliao Basin SUN Yonghe1,*, CHEN Yibo1, LI Xuesong2, SUN Jigang2, FU Xiaofei1 1. College of Geosciences, Northeast Petroleum University, Daqing 163318, China; 2. No.4 Oil Production Company Geological Brigade, Daqing Oil Field Company, Daqing 163111, China
Abstract: On the basis of regional seismic interpretation of the northern Songliao Basin and the analysis of fault geometry characteristics and the formation and evolution of faults, this paper analyses fault deformation mechanisms at different stages, which reveal the formation mechanism of inversion structures and their role for oil and gas migration and accumulation. Studies show that the faults in the northern Songliao Basin have experienced the following stages of evolution: a continued extensional deformation in the faulted phase, continuous transtensional deformation in the depression phase and continuous extrusion inversion tectonic deformation in the inversion phase. The late tectonic inversion and deformation evolved on the basis of huge rifts in the basement of the Songliao Basin, grabens formed in a NW-SE tensile stress field during the faulted period, and multi-orientation fault-dense zones formed in a nearly EW tension stress field during the depression period. The sinistral compresso-shear deformation field, half grabens and huge basal faults jointly control the formation of subprime anticlines and inversion structural belts during the reversal period. The Daqing Placanticline area is controlled by both the NNE half grabens and NNE huge basal faults, so that it has a high reversal degree, which turns NE subprime anticlines into NNE structural inversion belts. Meanwhile, it turns fault dense zones from nearly SN into NW-NNW. Key words: fault evolution; faulted tectonic framework; fault-dense zone; inversion structural belt; formation mechanism; northern Songliao Basin
1
Overview of study area
Songliao Basin, a Meso-Cenozoic terrigenous oil-bearing basin stretching in NE direction [1], has a dual structure of “depression over fault” vertically [2]. The tectonic evolution of the Northern Songliao Basin experienced three stages (rift, depression and reverse) [3−4], and formed three structural units (rift structure, depression structure and inverted structure, from bottom up) respectively. Specifically, the rift structure includes the Lower Cretaceous Huoshiling Formation, Shahezi Formation and Yingcheng Formation; the depression structure includes the Lower Cretaceous Denglouku Formation and Quantou Formation and the Upper Cretaceous Qingshankou Formation, Yaojia Formation and Member 1,2 of Nenjiang Formation; the inverted structure includes the Upper Cretaceous Member 3,4,5 of Nenjiang Formation, Sifangtai Formation, Mingshui Formation and the Cenozoic [5−6]. Previous studies and petroleum exploration shows that the Songliao basin structure was finalized in late tectonic inversion deformation stage [7], which was also the main period of oil-gas generation, migration, accumulation on a large scale in
shallow layers, while deep gas reservoirs formed mainly in the depositional stage of Quantou Formation–Early Qingshankou Formation [8], experienced adjustment and remigration due to late reversal and deformation [9−10], providing conditions for the formation of shallow secondary oil and gas pools [11]. The tectonic inversion deformation during the late period played a key role in oil and gas generation, migration, accumulation and trap formation (Daqing Placanticline in northern Songliao Basin), therefore, studying the formation mechanisms of the inversion structural belts has important guiding significance for the exploration of different layers of oil and gas. On the basis of 3D seismic data, along the main line of fault evolutionary sequence and under the guidance of analytical tectonics, this study looks into the relationship between fault evolutionary sequence and inversion mechanism, then through determination of basin tectonic inversion control factors, the formation mechanism of Daqing Placanticline inversion structural belts was figured out, which has important practical and theoretical significances to further improve Daqing Placanticline exploration potential and enrich fault control
Received date: 26 Jul. 2012; Revised date: 26 Feb. 2013. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41072163) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20112322120002). Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
SUN Yonghe et al. / Petroleum Exploration and Development, 2013, 40(3): 296–304
Fig. 1
Study area and the superimposed map of deep rift structure, basal faults and deep tectonic faults, northern Songliao Basin
hydrocarbon theory [12]. Limited by 3D seismic data interpretation scope, this study area is located in the north-central part of Northern Songliao Basin (Figure 1), with an area of 4 951.7 km2, including the northern part of several shallowmedium tectonic units from west to east, Longhupao-Da’an Terrace, Qijia-Gulong Depression, Daqing Placanticline, Sanzhao Depression and Mingshui Terrace.
2 Fault geometry features and evolutionary sequences 2.1
Fault geometry features
Faults developed in the same basin usually experienced multi-phase superimposed deformation, which resulted in different geometric features of faults in different layers, and the different geometric features reflect the differences in fault deformation nature. Seismic reflection interfaces (T4, T2, T11, T06) represent respectively rift structural layer, depression structural layer and inversion structural layer (Figure 2a),
were selected for statistics of fault typical geometry elements (Figure 2b). The statistical results show that fault strike from bottom up changes in a regular pattern. In the rift structural layer, faults stretch in multiple orientations because of the effect of pre-existing basement fault structures, but mainly in NE direction. In the depression structural layer, the dominant fault strikes are NW-NNW and nearly NS. In the inversion structural layer, the dominant fault strike is NNW. Fault scales gradually diminish bottom-up. In the rift structural layer, fault throw ranges from 10 m to 90 m, extension length ranges from 1 km to 7 km in general. In the depression structural layer, fault throw ranges from 0 m to 30 m, extension length ranges from 0 km to 3 km in general. In the inversion structural layer, fault throw ranges from 0 m to 20 m, extension length ranges from 0 km to 2 km in general. The depression structural layer has the highest fault density. At T2 interface, the fault density is 0.68 pieces/km2. At T11 interface, the fault density reduces significantly to 0.14 pieces/km2. The inversion structure layer fault density is in the second place, 0.11
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Fig. 2
Fault geometric characteristics, rift structure characteristics and fault type classification in northern Songliao Basin
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pieces/km2. The rift structural layer has the lowest fault density of 0.03 pieces/km2. The above change pattern of geometric elements shows that strong fault activities happened during all the three stages and at each stage, fault activities are different in pattern and deformation nature (Figure 2a). The rift structural formations, are wedge-shape and filled in half-graben. The main faults controlled sedimentary filling and became the main formation boundary. Depression structural formations are large in thickness and little in lateral change, where faults developed mainly in T2 reflective interface are small in scale and large in number. Fault deformation mechanism in the inversion structure layer is reflected as the occurrence of anticlines where faults concentrate. These differences in fault geometry also indirectly indicate fault deformation happened in multiple stages and deformation nature is different at each stage. 2.2 Strong fault deformation stages and deformation sequence Relative down throw in longitudinal direction of the two walls in a normal fault forms the vertical throw, while the horizontal separation caused by the two walls of the normal fault is the horizontal throw. So the vertical activity rate of a fault and the extensional ratio of a typical profile can be used to determine the strong deformation period of the fault [13]. In combination with deformation properties during the strong deformation period of the fault, the fault deformation evolution sequence can be restored. The fault vertical activity rate and typical profile extensional ratio (Figure 3) show, faults have been most active since the depositional period of Huoshiling Formation, Shahezi Formation, Yingcheng Formation,
Fig. 3
Qingshankou Formation to Yaojia Formation and the member 3 of Nenjiang Formation to present. In terms of basin evolution, faults kept active during the whole rift stage, including Huoshiling Formation, Shahezi Formation and Yingcheng Formation; part of depression stage, including Qinshankou Formation and Yaojia Formation; part of inversion stage, including late Nengjiang Formation, late Mingshui Formation [5, 14−15], in which the end of Mingshui Formation depositional stage witnessed the most intense inversion deformation, representing the main formation and establishment period of inverted structural belt [7, 16−17]. Based on the above analysis, it is known that the fault deformation sequence in northern Songliao basin can boil down to three stages, namely sustained extensional deformation during the depositional stage of the rift structural layer of the Huoshiling Formation, Shahezi Formation, and Yingcheng Formation; sustained extension during depositional stage of depression structural layer of Qingshankou Formation and Yaojia Formation; sustained inversion during the depositional stage of inverted structural layer of late Nenjiang Formation, late Mingshui Formation and Paleogene. Based on the "three stages, three properties" of deformation, faults can be divided into six types (Figures 2a, 4, 5). They are faults active only during rift stage (type I), faults active only during depression stage (type II), faults active only during inversion stage (type III), faults active during rift stage and depression stage (type I-II), faults active during depression stage and inversion stage (type II-III), faults active during rift stage, depression stage and inversion stage (type I-II-III). These different types of faults played corresponding roles in oil and gas migration and accumulation.
Histogram of faulting intensity distribution in northern Songliao Basin
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3 Fault deformation mechanism and formation of inversion structural belts Faults in Northern Songliao Basin experienced three different deformation stages. The late reverse deformation occurred after the rift and depression. So revealing fault deformation mechanism of each stage is the basis to recognize late tectonic inversion deformation. Research shows that faults occurred in different stages with different deformation nature gave rise to unique geological structures. 3.1 Fault extensional deformation and tectonic framework distribution at rift stage Depositional periods of Huoshiling Formation, Shahezi Formation, Yingcheng Formation belong to rift stage [18]. At this stage, the basin was composed of a series of dustpan shape half-grabens (Figure 1 and Figure 2a) whose main strike is NE-NNE, such as Daqing Placanticline, Rift 3 (corresponding to Longhupao-Da’an terrace), Rift 1 and Rift 2. While rift 4 (corresponding to Sanzhao Depression) strike is NW-NNW. From overlapping relationship of sag structural trend and regional deep faults, it is known that deep faults as pre-exist structures of rift filling have obvious control on the basin framework. Rift 3 in NNE strike is obviously controlled by Keshan-Da’an Fault in NNE direction. Rift 4 in NNW strike, is obviously controlled by Binzhou Fault in NNW direction (Figure 1), and affected by the Binzhou Fault in NNW direction and Hailun-Renmin Fault in NE strike. The strike of controlling faults of Rift 4 also shifts from NNW to NE from south to north. No matter it is rift extending direction or fault strike, only the distribution orientation without pre-exist structure influence can reflect the regional stress field direction. In Rift 1, deep faults didn’t develop. The rift formation and evolution were not affected by deep faults, where half grabens in NNE direction developed. In addition, secondary basement faults strike NE-NNE too and form parallel combination in plan (Figure 1). Therefore, it is speculated that the regional stress field was North West - South East direction during the rift stage (Figure 2b). The stress field is mainly controlled by the deep mantle material arching [19] which is caused by subduction of the Pacific Plate to the Eurasian plate. Under this stress field, shallow crust experienced rifting and extension, giving birth to continental rift basin with dominant direction of NE – NNE, but coupled with the effect of North West - North North West regional deep faults, the final result is a structural framework in which NE – NNE rifts and NNW rifts coexist. This basin structural framework had an important effect on basin evolution, especially the tectonic inversion deformation at later stage. 3.2 Fault tensile shearing deformation and occurrence of fault-dense belts at depression stage Since the initial deposition of the Denglouku Formation,
the basin evolution had entered into depression stage, which lasted until the late depositional period of the Nenjiang Formation [20]. During the deposition of the Qingshankou Formation - Yaojia Formation, strong fault activities happened, giving rise to high-density faults in T2 and T11 (Figure 2a), with T2 interface having the highest fault density. The characteristics of these faults are "nested V-shaped" faults or "V-shaped" faults constituting small graben systems or graben systems in profile (Figure 2b). These faults are distributed in belt-like form in map [21] which is called “fault-dense belt”. In the study area, the T2 reflective interface contains a total of 118 fault dense belts (Figure 4), among them, 37 striking NS, 65 striking NW - NNW, 10 striking NE-NNE and 6 striking EW. Fault-dense belts in Daqing Placanticline and eastern Qijiagulong Depression are most developed, most in NW NNW trend but a few of them in nearly N-S, nearly E-W, and NE trend. In Sanzhao Depression, Mingshui terraces and southern Longhupao-Da’an terraces, there are also a number of fault-dense belts, most of them in north-south trend, but a few in NW, NE and nearly E-W trend. On profile, affected by the horizontal dispatch of lower Quantou plastic mudstone, most fault-dense belts and deep faults are not directly connected. But faults developed during depression stage, as pre-exist structures affected the formation and combination mode of fault-dense belts. In respect of corresponding relationship between fault-dense belts and different fault types in T4 reflect interface (Figure 4), Daqing Placanticline is rich in Type I-II faults and Type I-II-III faults. Because of continuing faulting, fault-dense belts controlled by these faults are highest in development degree. In other regions, there are only sporadic Type I-II sustainable active faults, most faults are type I, they were active in the subsequent late depression period, having little influence on the formation of fault-dense belts, as a result, fault-dense belts are relatively low in development degree. The formation of fault-dense belts in diverse trends was controlled not only by pre-rift structures and lower Quantou plastic mudstone, but also by some particular stress mechanism. In depression stage, mantle plume and volcanism were already in a intense heat decay phase, the upwarping extensional stress significantly weakened, and the basin mainly experienced regional thermal subsidence. During this period, the Pacific Plate and the Eurasian Plate interacted and remotely transferred horizontal stress, as a result, faults went through multiple intense activities, forming the high density fault belts in T2. In Sanzhao Depression, secondary faults in the multi-direction fault-dense belts are mainly in N-S trend during depression stage. This area didn’t have remarkable inversion (Figure 2a, Figure 5), that is to say, fault-dense belts were not reformed strongly. Therefore, it is speculated that the regional extensional stress field direction was nearly EW in the depression stage (Figure 2b). This stress field worked together with pre-rift structures to cause transtensional deformation of faults. In Daqing Placanticline and Qijiagulong
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Fig. 4 Superimposed map of fault-dense belts in reflector T2 and different type faults distribution in reflector T4 in northern Songliao Basin
Fig. 5
Superimposed map of inversion structural layers and different type faults distribution in reflector T06 in northern Songliao Basin
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depression, fault-dense belts and their internal secondary faults should have been mainly in N-S trend under nearly E-W tension stress field. But these fault-dense belts and their internal secondary faults are mainly in NW-SE trend, because the two regions are mainly located in the large inversion tectonic belt (Daqing Placanticline) (Figure 5), the NW – NNW trending fault-dense belts must have experienced strong inversion after their formation. Therefore, in these regions, the strikes of the fault-dense belts changed from initial NW-NNW at depression stage to nearly N-S under stress field of sinistral rotation. So the internal fault-dense belts strikes changed from nearly N-S, E-W and NNE to NE, NW and nearly E-W, respectively. To sum up, the strikes of fault-dense belts and their internal secondary faults are diagonal to stress field direction, indicating different deformation mechanisms. Under the nearly E-W tension stress field, in Sanzhao Depression, Mingshui Terrace and Longhupao-Da’an Terrace, the nearly N-S trending (present) fault-dense belts are stretching belts under extension mechanism; the nearly NE and NW trending fault-dense belts are adjusted belts under oblique extension mechanism; the nearly E-W trending fault-dense belts are tensional-shear belts under strike-slip mechanism; in Daqing Placanticline and Qijiagulong Depression now NW-NNW trending fault-dense belts were formed by extension at depression stage and later reformed by anticlockwise rotation at inversion stage; the nearly N-S and E-W trending fault-dense belts were formed by oblique tensional-shear deformation at depression stage and later anticlockwise rotation at inversion stage; the NE trending fault-dense belts were product of initial strike-slip deformation at depression stage and later anticlockwise rotation at inversion stage. 3.3 Fault sinistral rotation and compression-torsion deformation and the formation of inversion structural belts at inversion stage Since the onset of the Sifangtai Formation depositional period, the basin entered into reversed deformation stage, in which the late MingShui Formation depositional period saw the most intense inversion, ending up to the nearly present structural framework, which features the occurrence of many typical NE and NNE trending reversal anticline belts or inversion structural belts. The study area mainly has four inversion structural belts (Figure 5), namely two NE trending and left-stepped, oblique-arranged inversion structural belts corresponding to the northern part of the Longhupao inversed anticline belt [22]; a NNE trending inversion structural belt corresponding to the northern part of the Daqing Placanticline inversed anticline belt; and a NE trending inversion structural belt in the northern Mingshui inversion structural belt corresponding to the southern part of the An’da anticline belt. Qijiagulong Depression and Sanzhao Depression did not undergo significant inversion deformation. From the reversal tectonic zone distribution pattern and corresponding relation-
ship between half graben formed in rift period and basal deep faults (Figures 1 and 5), where there developed half grabens and deep faults, there developed inversion structural belts above, except Rift 4 corresponding to Sanzhao Depression [23−24]. And the long axes of the inversion structural belts are the same in strike as half grabens and deep faults. Among them, two inversion structural belts at western longhupao-Da’an Terrace side developed only under effect of Rift 1 and Rift 2, the inversion structural belt at eastern MingShui Terrace developed under control of basement deep faults, these tectonic belts inverted slightly (Figure 2a). The inversion structural belts at Daqing Placanticline developed under control of Rift 3 and Keshan-Da’an Fault with the biggest inversion. The study shows that, half grabens and deep faults as pre-existing structures play dominant roles in the formation of inversion structural belts in late return compression deformation process. Besides half grabens and basal deep faults, the formation and evolution of inversion structural belts were also controlled by basin deformation under the regional stress field. In terms of structure elements associated with basin inversion deformation, new faults (Type Ⅲ) mainly strike NW; long axes of secondary anticlines strike mainly NE (Figure 5); most secondary anticlines are left-stepped, oblique-arranged, such as the two inversion structural belts of Longhupao-Da’an Terrace. Therefore, it is speculated that late tectonic inversion deformation in the northern part of Songliao Basin was developed under sinistral rotation and compression-torsion deformation field (Figure 2b). This deformation field came from NNW - SSE trending regional compressive stress field [16]. The North West - South East secondary compressive stress derived by non-collinear simple shear stress, combined with pre-existing structures, controlled the deformation and evolution of North East striking inversion structural belts directly, such as NE trending Rift 1, Rift 2, NE trending Longhupao anticline belts and Anda anticline belts. For Daqing Placanticline, under the unified stress field, NE trending secondary anticline mainly formed at first. Under the effect of Rift 3 and Keshan-Da’an Faults, secondary anticline belts are left-stepped, oblique-arranged along them (Figure 5). Then following the intensified sinistral-rotation and compression-torsion deformation, these belts linked and the Daqing Placanticline formed (Figure 6). While the visible traces today is Daqing Placanticline nose-like structure which is stretching deep into western Qijiagulong Depression, such as western Saertu nose-like structure, western Xingshugang nose-like structure, western Gaotaizi nose-like structure and western Putaohua nose-like structure. Qijiagulong Depression and Sanzhao Depression didn't experience obvious structure inversion. In Qijiagulong Depression, it was mainly because there was no pre-existing structure such as rift or deep faults. While in Sanzhao Depression, it was mainly because the rift trending was the same as the compression direction (NW-NNW). The configuration relationship of half graben orientation and re-
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Fig. 6
Shallow strata deformation process caused by pre-existing strike-slip progressive displacement [25 - 26]
gional stress field was one of the main factors affecting inversion. That is to say, the angle between pre-existing structures trending direction and regional compressive stress field direction, is the key affecting the strength of inversion deformation. The bigger the angle was, the higher the strength of inversion deformation, and vice versa. In addition to inverted anticlines in northern Songliao Basin, the active faults also underwent rotational deformation due to the effect of inversion deformation. From the point view of faults distribution in inversion structure belts, faults mainly strike NW-NNW (Figure 5), in which newborn Type III faults, were formed under control of NE-SW trending extensional stress components of sinistral rotation compression and torsion deformation, Type II - III faults trending nearly N-S at depression stage were boundary faults of T2 interface fault-dense belts. At late evolution stage of the basin, these faults became reactivated and clockwise rotated while secondary inversion anticlines formed in sinistral rotation compression and torsion deformation. Then NW-NNW trending faults formed under NE-SW extensional stress components. Because Daqing Placanticline is large in inversion degree and its inversion deformation affected deeper layers, leading to transformation of the fault-dense belts in T2 interface to North West - North trend (Figure 4).
4
work of depression layers in turn. In addition, inversion tectonic movement provided trapping conditions for hydrocarbon and promoted large-scale hydrocarbon accumulation and formation extensional arch-like cracks to improve the physical properties of reservoirs [9]. So to get a clear idea of the inversion mechanism is crucial for finding out the overall distribution pattern of inversion belts in Songliao Basin, meanwhile it will help reveal oil and gas accumulation pattern in middle-shallow layers and gas adjustment and re-migration pattern in deep layers, it also has practical significance in guiding oil and gas exploration.
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Conclusions
Northern Songliao Basin experienced rift stage, depression stage and inversion stage. Different evolutionary stages were affected by different tectonic stress and pre-existing structures. On the basis of rift-depression restraint evolution which was controlled by multi-phase, multi-nature faulting, under NNW SSE trending compressive stress, affected by rift tectonic framework and basal deep faults distribution, inversion belts formed gradually due to sinistral rotation and compressiontorsion deformation. Rift tectonic framework and basal deep fault distribution had a direct impact on the location and inversion degree of inversion structures, while inversion deformation restricted and reformed original tectonic frame− 303 −
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RESEARCH PAPER
Evolutionary sequence of faults and the formation of inversion structural belts in the northern Songliao Basin SUN Yonghe1,*, CHEN Yibo1, LI Xuesong2, SUN Jigang2, FU Xiaofei1 1. College of Geosciences, Northeast Petroleum University, Daqing 163318, China; 2. No.4 Oil Production Company Geological Brigade, Daqing Oil Field Company, Daqing 163111, China
Abstract: On the basis of regional seismic interpretation of the northern Songliao Basin and the analysis of fault geometry characteristics and the formation and evolution of faults, this paper analyses fault deformation mechanisms at different stages, which reveal the formation mechanism of inversion structures and their role for oil and gas migration and accumulation. Studies show that the faults in the northern Songliao Basin have experienced the following stages of evolution: a continued extensional deformation in the faulted phase, continuous transtensional deformation in the depression phase and continuous extrusion inversion tectonic deformation in the inversion phase. The late tectonic inversion and deformation evolved on the basis of huge rifts in the basement of the Songliao Basin, grabens formed in a NW-SE tensile stress field during the faulted period, and multi-orientation fault-dense zones formed in a nearly EW tension stress field during the depression period. The sinistral compresso-shear deformation field, half grabens and huge basal faults jointly control the formation of subprime anticlines and inversion structural belts during the reversal period. The Daqing Placanticline area is controlled by both the NNE half grabens and NNE huge basal faults, so that it has a high reversal degree, which turns NE subprime anticlines into NNE structural inversion belts. Meanwhile, it turns fault dense zones from nearly SN into NW-NNW. Key words: fault evolution; faulted tectonic framework; fault-dense zone; inversion structural belt; formation mechanism; northern Songliao Basin
1
Overview of study area
Songliao Basin, a Meso-Cenozoic terrigenous oil-bearing basin stretching in NE direction [1], has a dual structure of “depression over fault” vertically [2]. The tectonic evolution of the Northern Songliao Basin experienced three stages (rift, depression and reverse) [3−4], and formed three structural units (rift structure, depression structure and inverted structure, from bottom up) respectively. Specifically, the rift structure includes the Lower Cretaceous Huoshiling Formation, Shahezi Formation and Yingcheng Formation; the depression structure includes the Lower Cretaceous Denglouku Formation and Quantou Formation and the Upper Cretaceous Qingshankou Formation, Yaojia Formation and Member 1,2 of Nenjiang Formation; the inverted structure includes the Upper Cretaceous Member 3,4,5 of Nenjiang Formation, Sifangtai Formation, Mingshui Formation and the Cenozoic [5−6]. Previous studies and petroleum exploration shows that the Songliao basin structure was finalized in late tectonic inversion deformation stage [7], which was also the main period of oil-gas generation, migration, accumulation on a large scale in
shallow layers, while deep gas reservoirs formed mainly in the depositional stage of Quantou Formation–Early Qingshankou Formation [8], experienced adjustment and remigration due to late reversal and deformation [9−10], providing conditions for the formation of shallow secondary oil and gas pools [11]. The tectonic inversion deformation during the late period played a key role in oil and gas generation, migration, accumulation and trap formation (Daqing Placanticline in northern Songliao Basin), therefore, studying the formation mechanisms of the inversion structural belts has important guiding significance for the exploration of different layers of oil and gas. On the basis of 3D seismic data, along the main line of fault evolutionary sequence and under the guidance of analytical tectonics, this study looks into the relationship between fault evolutionary sequence and inversion mechanism, then through determination of basin tectonic inversion control factors, the formation mechanism of Daqing Placanticline inversion structural belts was figured out, which has important practical and theoretical significances to further improve Daqing Placanticline exploration potential and enrich fault control
Received date: 26 Jul. 2012; Revised date: 26 Feb. 2013. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41072163) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20112322120002). Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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Fig. 1
Study area and the superimposed map of deep rift structure, basal faults and deep tectonic faults, northern Songliao Basin
hydrocarbon theory [12]. Limited by 3D seismic data interpretation scope, this study area is located in the north-central part of Northern Songliao Basin (Figure 1), with an area of 4 951.7 km2, including the northern part of several shallowmedium tectonic units from west to east, Longhupao-Da’an Terrace, Qijia-Gulong Depression, Daqing Placanticline, Sanzhao Depression and Mingshui Terrace.
2 Fault geometry features and evolutionary sequences 2.1
Fault geometry features
Faults developed in the same basin usually experienced multi-phase superimposed deformation, which resulted in different geometric features of faults in different layers, and the different geometric features reflect the differences in fault deformation nature. Seismic reflection interfaces (T4, T2, T11, T06) represent respectively rift structural layer, depression structural layer and inversion structural layer (Figure 2a),
were selected for statistics of fault typical geometry elements (Figure 2b). The statistical results show that fault strike from bottom up changes in a regular pattern. In the rift structural layer, faults stretch in multiple orientations because of the effect of pre-existing basement fault structures, but mainly in NE direction. In the depression structural layer, the dominant fault strikes are NW-NNW and nearly NS. In the inversion structural layer, the dominant fault strike is NNW. Fault scales gradually diminish bottom-up. In the rift structural layer, fault throw ranges from 10 m to 90 m, extension length ranges from 1 km to 7 km in general. In the depression structural layer, fault throw ranges from 0 m to 30 m, extension length ranges from 0 km to 3 km in general. In the inversion structural layer, fault throw ranges from 0 m to 20 m, extension length ranges from 0 km to 2 km in general. The depression structural layer has the highest fault density. At T2 interface, the fault density is 0.68 pieces/km2. At T11 interface, the fault density reduces significantly to 0.14 pieces/km2. The inversion structure layer fault density is in the second place, 0.11
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Fig. 2
Fault geometric characteristics, rift structure characteristics and fault type classification in northern Songliao Basin
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pieces/km2. The rift structural layer has the lowest fault density of 0.03 pieces/km2. The above change pattern of geometric elements shows that strong fault activities happened during all the three stages and at each stage, fault activities are different in pattern and deformation nature (Figure 2a). The rift structural formations, are wedge-shape and filled in half-graben. The main faults controlled sedimentary filling and became the main formation boundary. Depression structural formations are large in thickness and little in lateral change, where faults developed mainly in T2 reflective interface are small in scale and large in number. Fault deformation mechanism in the inversion structure layer is reflected as the occurrence of anticlines where faults concentrate. These differences in fault geometry also indirectly indicate fault deformation happened in multiple stages and deformation nature is different at each stage. 2.2 Strong fault deformation stages and deformation sequence Relative down throw in longitudinal direction of the two walls in a normal fault forms the vertical throw, while the horizontal separation caused by the two walls of the normal fault is the horizontal throw. So the vertical activity rate of a fault and the extensional ratio of a typical profile can be used to determine the strong deformation period of the fault [13]. In combination with deformation properties during the strong deformation period of the fault, the fault deformation evolution sequence can be restored. The fault vertical activity rate and typical profile extensional ratio (Figure 3) show, faults have been most active since the depositional period of Huoshiling Formation, Shahezi Formation, Yingcheng Formation,
Fig. 3
Qingshankou Formation to Yaojia Formation and the member 3 of Nenjiang Formation to present. In terms of basin evolution, faults kept active during the whole rift stage, including Huoshiling Formation, Shahezi Formation and Yingcheng Formation; part of depression stage, including Qinshankou Formation and Yaojia Formation; part of inversion stage, including late Nengjiang Formation, late Mingshui Formation [5, 14−15], in which the end of Mingshui Formation depositional stage witnessed the most intense inversion deformation, representing the main formation and establishment period of inverted structural belt [7, 16−17]. Based on the above analysis, it is known that the fault deformation sequence in northern Songliao basin can boil down to three stages, namely sustained extensional deformation during the depositional stage of the rift structural layer of the Huoshiling Formation, Shahezi Formation, and Yingcheng Formation; sustained extension during depositional stage of depression structural layer of Qingshankou Formation and Yaojia Formation; sustained inversion during the depositional stage of inverted structural layer of late Nenjiang Formation, late Mingshui Formation and Paleogene. Based on the "three stages, three properties" of deformation, faults can be divided into six types (Figures 2a, 4, 5). They are faults active only during rift stage (type I), faults active only during depression stage (type II), faults active only during inversion stage (type III), faults active during rift stage and depression stage (type I-II), faults active during depression stage and inversion stage (type II-III), faults active during rift stage, depression stage and inversion stage (type I-II-III). These different types of faults played corresponding roles in oil and gas migration and accumulation.
Histogram of faulting intensity distribution in northern Songliao Basin
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3 Fault deformation mechanism and formation of inversion structural belts Faults in Northern Songliao Basin experienced three different deformation stages. The late reverse deformation occurred after the rift and depression. So revealing fault deformation mechanism of each stage is the basis to recognize late tectonic inversion deformation. Research shows that faults occurred in different stages with different deformation nature gave rise to unique geological structures. 3.1 Fault extensional deformation and tectonic framework distribution at rift stage Depositional periods of Huoshiling Formation, Shahezi Formation, Yingcheng Formation belong to rift stage [18]. At this stage, the basin was composed of a series of dustpan shape half-grabens (Figure 1 and Figure 2a) whose main strike is NE-NNE, such as Daqing Placanticline, Rift 3 (corresponding to Longhupao-Da’an terrace), Rift 1 and Rift 2. While rift 4 (corresponding to Sanzhao Depression) strike is NW-NNW. From overlapping relationship of sag structural trend and regional deep faults, it is known that deep faults as pre-exist structures of rift filling have obvious control on the basin framework. Rift 3 in NNE strike is obviously controlled by Keshan-Da’an Fault in NNE direction. Rift 4 in NNW strike, is obviously controlled by Binzhou Fault in NNW direction (Figure 1), and affected by the Binzhou Fault in NNW direction and Hailun-Renmin Fault in NE strike. The strike of controlling faults of Rift 4 also shifts from NNW to NE from south to north. No matter it is rift extending direction or fault strike, only the distribution orientation without pre-exist structure influence can reflect the regional stress field direction. In Rift 1, deep faults didn’t develop. The rift formation and evolution were not affected by deep faults, where half grabens in NNE direction developed. In addition, secondary basement faults strike NE-NNE too and form parallel combination in plan (Figure 1). Therefore, it is speculated that the regional stress field was North West - South East direction during the rift stage (Figure 2b). The stress field is mainly controlled by the deep mantle material arching [19] which is caused by subduction of the Pacific Plate to the Eurasian plate. Under this stress field, shallow crust experienced rifting and extension, giving birth to continental rift basin with dominant direction of NE – NNE, but coupled with the effect of North West - North North West regional deep faults, the final result is a structural framework in which NE – NNE rifts and NNW rifts coexist. This basin structural framework had an important effect on basin evolution, especially the tectonic inversion deformation at later stage. 3.2 Fault tensile shearing deformation and occurrence of fault-dense belts at depression stage Since the initial deposition of the Denglouku Formation,
the basin evolution had entered into depression stage, which lasted until the late depositional period of the Nenjiang Formation [20]. During the deposition of the Qingshankou Formation - Yaojia Formation, strong fault activities happened, giving rise to high-density faults in T2 and T11 (Figure 2a), with T2 interface having the highest fault density. The characteristics of these faults are "nested V-shaped" faults or "V-shaped" faults constituting small graben systems or graben systems in profile (Figure 2b). These faults are distributed in belt-like form in map [21] which is called “fault-dense belt”. In the study area, the T2 reflective interface contains a total of 118 fault dense belts (Figure 4), among them, 37 striking NS, 65 striking NW - NNW, 10 striking NE-NNE and 6 striking EW. Fault-dense belts in Daqing Placanticline and eastern Qijiagulong Depression are most developed, most in NW NNW trend but a few of them in nearly N-S, nearly E-W, and NE trend. In Sanzhao Depression, Mingshui terraces and southern Longhupao-Da’an terraces, there are also a number of fault-dense belts, most of them in north-south trend, but a few in NW, NE and nearly E-W trend. On profile, affected by the horizontal dispatch of lower Quantou plastic mudstone, most fault-dense belts and deep faults are not directly connected. But faults developed during depression stage, as pre-exist structures affected the formation and combination mode of fault-dense belts. In respect of corresponding relationship between fault-dense belts and different fault types in T4 reflect interface (Figure 4), Daqing Placanticline is rich in Type I-II faults and Type I-II-III faults. Because of continuing faulting, fault-dense belts controlled by these faults are highest in development degree. In other regions, there are only sporadic Type I-II sustainable active faults, most faults are type I, they were active in the subsequent late depression period, having little influence on the formation of fault-dense belts, as a result, fault-dense belts are relatively low in development degree. The formation of fault-dense belts in diverse trends was controlled not only by pre-rift structures and lower Quantou plastic mudstone, but also by some particular stress mechanism. In depression stage, mantle plume and volcanism were already in a intense heat decay phase, the upwarping extensional stress significantly weakened, and the basin mainly experienced regional thermal subsidence. During this period, the Pacific Plate and the Eurasian Plate interacted and remotely transferred horizontal stress, as a result, faults went through multiple intense activities, forming the high density fault belts in T2. In Sanzhao Depression, secondary faults in the multi-direction fault-dense belts are mainly in N-S trend during depression stage. This area didn’t have remarkable inversion (Figure 2a, Figure 5), that is to say, fault-dense belts were not reformed strongly. Therefore, it is speculated that the regional extensional stress field direction was nearly EW in the depression stage (Figure 2b). This stress field worked together with pre-rift structures to cause transtensional deformation of faults. In Daqing Placanticline and Qijiagulong
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Fig. 4 Superimposed map of fault-dense belts in reflector T2 and different type faults distribution in reflector T4 in northern Songliao Basin
Fig. 5
Superimposed map of inversion structural layers and different type faults distribution in reflector T06 in northern Songliao Basin
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depression, fault-dense belts and their internal secondary faults should have been mainly in N-S trend under nearly E-W tension stress field. But these fault-dense belts and their internal secondary faults are mainly in NW-SE trend, because the two regions are mainly located in the large inversion tectonic belt (Daqing Placanticline) (Figure 5), the NW – NNW trending fault-dense belts must have experienced strong inversion after their formation. Therefore, in these regions, the strikes of the fault-dense belts changed from initial NW-NNW at depression stage to nearly N-S under stress field of sinistral rotation. So the internal fault-dense belts strikes changed from nearly N-S, E-W and NNE to NE, NW and nearly E-W, respectively. To sum up, the strikes of fault-dense belts and their internal secondary faults are diagonal to stress field direction, indicating different deformation mechanisms. Under the nearly E-W tension stress field, in Sanzhao Depression, Mingshui Terrace and Longhupao-Da’an Terrace, the nearly N-S trending (present) fault-dense belts are stretching belts under extension mechanism; the nearly NE and NW trending fault-dense belts are adjusted belts under oblique extension mechanism; the nearly E-W trending fault-dense belts are tensional-shear belts under strike-slip mechanism; in Daqing Placanticline and Qijiagulong Depression now NW-NNW trending fault-dense belts were formed by extension at depression stage and later reformed by anticlockwise rotation at inversion stage; the nearly N-S and E-W trending fault-dense belts were formed by oblique tensional-shear deformation at depression stage and later anticlockwise rotation at inversion stage; the NE trending fault-dense belts were product of initial strike-slip deformation at depression stage and later anticlockwise rotation at inversion stage. 3.3 Fault sinistral rotation and compression-torsion deformation and the formation of inversion structural belts at inversion stage Since the onset of the Sifangtai Formation depositional period, the basin entered into reversed deformation stage, in which the late MingShui Formation depositional period saw the most intense inversion, ending up to the nearly present structural framework, which features the occurrence of many typical NE and NNE trending reversal anticline belts or inversion structural belts. The study area mainly has four inversion structural belts (Figure 5), namely two NE trending and left-stepped, oblique-arranged inversion structural belts corresponding to the northern part of the Longhupao inversed anticline belt [22]; a NNE trending inversion structural belt corresponding to the northern part of the Daqing Placanticline inversed anticline belt; and a NE trending inversion structural belt in the northern Mingshui inversion structural belt corresponding to the southern part of the An’da anticline belt. Qijiagulong Depression and Sanzhao Depression did not undergo significant inversion deformation. From the reversal tectonic zone distribution pattern and corresponding relation-
ship between half graben formed in rift period and basal deep faults (Figures 1 and 5), where there developed half grabens and deep faults, there developed inversion structural belts above, except Rift 4 corresponding to Sanzhao Depression [23−24]. And the long axes of the inversion structural belts are the same in strike as half grabens and deep faults. Among them, two inversion structural belts at western longhupao-Da’an Terrace side developed only under effect of Rift 1 and Rift 2, the inversion structural belt at eastern MingShui Terrace developed under control of basement deep faults, these tectonic belts inverted slightly (Figure 2a). The inversion structural belts at Daqing Placanticline developed under control of Rift 3 and Keshan-Da’an Fault with the biggest inversion. The study shows that, half grabens and deep faults as pre-existing structures play dominant roles in the formation of inversion structural belts in late return compression deformation process. Besides half grabens and basal deep faults, the formation and evolution of inversion structural belts were also controlled by basin deformation under the regional stress field. In terms of structure elements associated with basin inversion deformation, new faults (Type Ⅲ) mainly strike NW; long axes of secondary anticlines strike mainly NE (Figure 5); most secondary anticlines are left-stepped, oblique-arranged, such as the two inversion structural belts of Longhupao-Da’an Terrace. Therefore, it is speculated that late tectonic inversion deformation in the northern part of Songliao Basin was developed under sinistral rotation and compression-torsion deformation field (Figure 2b). This deformation field came from NNW - SSE trending regional compressive stress field [16]. The North West - South East secondary compressive stress derived by non-collinear simple shear stress, combined with pre-existing structures, controlled the deformation and evolution of North East striking inversion structural belts directly, such as NE trending Rift 1, Rift 2, NE trending Longhupao anticline belts and Anda anticline belts. For Daqing Placanticline, under the unified stress field, NE trending secondary anticline mainly formed at first. Under the effect of Rift 3 and Keshan-Da’an Faults, secondary anticline belts are left-stepped, oblique-arranged along them (Figure 5). Then following the intensified sinistral-rotation and compression-torsion deformation, these belts linked and the Daqing Placanticline formed (Figure 6). While the visible traces today is Daqing Placanticline nose-like structure which is stretching deep into western Qijiagulong Depression, such as western Saertu nose-like structure, western Xingshugang nose-like structure, western Gaotaizi nose-like structure and western Putaohua nose-like structure. Qijiagulong Depression and Sanzhao Depression didn't experience obvious structure inversion. In Qijiagulong Depression, it was mainly because there was no pre-existing structure such as rift or deep faults. While in Sanzhao Depression, it was mainly because the rift trending was the same as the compression direction (NW-NNW). The configuration relationship of half graben orientation and re-
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Fig. 6
Shallow strata deformation process caused by pre-existing strike-slip progressive displacement [25 - 26]
gional stress field was one of the main factors affecting inversion. That is to say, the angle between pre-existing structures trending direction and regional compressive stress field direction, is the key affecting the strength of inversion deformation. The bigger the angle was, the higher the strength of inversion deformation, and vice versa. In addition to inverted anticlines in northern Songliao Basin, the active faults also underwent rotational deformation due to the effect of inversion deformation. From the point view of faults distribution in inversion structure belts, faults mainly strike NW-NNW (Figure 5), in which newborn Type III faults, were formed under control of NE-SW trending extensional stress components of sinistral rotation compression and torsion deformation, Type II - III faults trending nearly N-S at depression stage were boundary faults of T2 interface fault-dense belts. At late evolution stage of the basin, these faults became reactivated and clockwise rotated while secondary inversion anticlines formed in sinistral rotation compression and torsion deformation. Then NW-NNW trending faults formed under NE-SW extensional stress components. Because Daqing Placanticline is large in inversion degree and its inversion deformation affected deeper layers, leading to transformation of the fault-dense belts in T2 interface to North West - North trend (Figure 4).
4
work of depression layers in turn. In addition, inversion tectonic movement provided trapping conditions for hydrocarbon and promoted large-scale hydrocarbon accumulation and formation extensional arch-like cracks to improve the physical properties of reservoirs [9]. So to get a clear idea of the inversion mechanism is crucial for finding out the overall distribution pattern of inversion belts in Songliao Basin, meanwhile it will help reveal oil and gas accumulation pattern in middle-shallow layers and gas adjustment and re-migration pattern in deep layers, it also has practical significance in guiding oil and gas exploration.
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Conclusions
Northern Songliao Basin experienced rift stage, depression stage and inversion stage. Different evolutionary stages were affected by different tectonic stress and pre-existing structures. On the basis of rift-depression restraint evolution which was controlled by multi-phase, multi-nature faulting, under NNW SSE trending compressive stress, affected by rift tectonic framework and basal deep faults distribution, inversion belts formed gradually due to sinistral rotation and compressiontorsion deformation. Rift tectonic framework and basal deep fault distribution had a direct impact on the location and inversion degree of inversion structures, while inversion deformation restricted and reformed original tectonic frame− 303 −
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