Relationship between gas reservoir distribution and structural system of upper Triassic Xujiahe Fm in the Sichuan Basin

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ScienceDirect Natural Gas Industry B 6 (2019) 220e235 www.elsevier.com/locate/ngib

Research article

Relationship between gas reservoir distribution and structural system of upper Triassic Xujiahe Fm in the Sichuan Basin*,** Liu Shu a,*, Ren Xingguo b, Yao Shengxian b, Liu Ziping b, Ning Meng c, Wang Xin a & Huang Xiaohui b b

a Sinopec Southwest Oil & Gas Company, Chengdu, Sichuan, 610213, China CNPC Chuanqing Drilling Engineering Co., Ltd., Chengdu, Sichuan, 610056, China c School of Earth and Space Sciences, Peking University, Beijing, 100871, China

Received 21 September 2018; accepted 11 November 2018 Available online 21 May 2019

Abstract The gas reservoir of Upper Triassic Xujiahe Fm in the Sichuan Basin is characterized by “accumulation in the early stage, entrapment in the middle stage and activation in the late stage”. In order to provide guidance for the prediction of hydrocarbon enrichment zone of Xujiahe Fm, we first plotted the regional structure map based on the 2D and 3D seismic merging data of the basin. Then, we displayed and described the structural characteristics by means of the 3D visualization technology of low-angle vertical backlight irradiation. In addition, the structural system was classified according to the dynamic direction of regional structure and the structural interrelationship, and the formation stages of the structure were confirmed. Finally, based on drilling and testing data, the hydrocarbon enrichment zone of Xujiahe Fm was predicted. And the following research results were obtained. First, five structural systems are developed in Xujiahe Fm in the Sichuan Basin, including EW-oriented arc structure, NEoriented linear structure, NE-oriented arc structure, SN-oriented to NW-oriented brush structure and NW-oriented arc structure. Second, the EWoriented arc structure is formed due to the uplift and extrusion of the northern section of Longmenshan thrust belt in the Indosinian period, successively developed in the Yanshanian and stabilized in the Himalayan period. And it is widely distributed in the basin. Third, the NE-oriented linear structure in the north of Western Sichuan Depression is resulted from the Anxian movement of Longmenshan thrust belt in the Indosinian period. Fourth, the NW-oriented arc structure before the Dabashan and the NE-oriented linear structure before the Huaying Mountain are stabilized in the Yanshanian and successively developed in the Himalayan period. Fifth, the SN-oriented structure in the Western Sichuan Depression is formed due to the extrusion of the southern section of Longmenshan thrust belt in the Himalayan period. Sixth, the combination of SN structure to the east of Longquanshan fold belt and NW-oriented linear structure in the basin is a brush structure belt with Jiangyou paleo-uplift as the mainstay, which is resulted from the extrusion of the southern section of Longmenshan thrust belt in the Himalayan period. Seventh, the high-yield wells are usually distributed in the arc anticlines of IndosinianeYanshanian superimposed with the faults of Himalayan period. The structures of the Himalayan period are usually dry traps, and their fracture development zones are mostly water producing layers. The Indosinian synclines are also ineffective traps even though they are uplifted to anticlines in the Himalayan period. In conclusion, the confirmation of structural system stages can provide a technical support for the prediction and description of hydrocarbon enrichment zone of Xujiahe Fm in the Sichuan Basin. © 2019 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Sichuan Basin; Western Sichuan depression; Late Triassic; Xujiahe Fm; Structural characteristics; Structural system; Formation stage; Prediction of hydrocarbon enrichment zone

*

Supported by the Sinopec Science and Technology Research Project “Main control factors for hydrocarbon accumulation and target evaluation of the Xujiahe Formation in the Sichuan Basin” (No.: P11088). ** This is the English version of the originally published article in Natural Gas Industry (in Chinese), which can be found at https://doi.org/10.3787/j.issn.10000976.2018.11.001. * Corresponding author. E-mail address: [email protected] (Liu S.). Peer review under responsibility of Sichuan Petroleum Administration. https://doi.org/10.1016/j.ngib.2018.11.014 2352-8540/© 2019 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. 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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0. Introduction The Upper Triassic Xujiahe Formation (T3x) in the Sichuan Basin is a set of sandstone and mudstone strata with coal seams. The whole basin contains natural gas and high-yield commercial gas wells were drilled in most of the structures. However, its development has been quite difficult, that is, the success rate of exploration wells is high and that of development wells is low, showing poor economic benefits generally. In the XiaoquaneXinchangeHexingchang structural belt (“XX-H structural belt”) in the Western Sichuan Depression, Sinopec Southwest Oil & Gas Company has totally drilled 41 wells targeting the second member of the Xujiahe Formation (“Xu 2 Member” or “T3x2”). By the end of 2017, there had been only 7 wells with cumulative gas production of more than 1.0
108 m3, with many low-yield and dry wells. Due to poor exploration and development effects, no new wells have been deployed since 2010. The resources of the Xujiahe Formation are huge [1,2]. Since the 1980s, the research on the distribution law of the Xujiahe gas reservoir has continued and deepened. Wang Jinqi [3] proposed the “early accumulation and late activation” model. Yang Keming et al. [4] believed that the primary gas reservoirs were formed in the Late Indosinian to the MiddleeLate Yanshanian, the reservoirs became tighter in the Late Cretaceous to “entrap” the hydrocarbons, and the early reservoirs were “entrapped” due to activation of numerous fractures along with the Himalayan movement. Yang Keming et al. [1,5] systematically summarized the accumulation system as accumulation in the early stage (hydrocarbons were accumulated in the paleo-structures before J3), entrapment in the middle stage (the reservoirs were tightened to entrap the gas pool during J3eK2), and activation in the late stage (fractures of Himalayan were activated after K2). Chen Dongxia et al. [6] reconfirmed that conventional gas reservoirs were formed in the period from the end of Triassic to the end of Middle Jurassic, tight gas reservoirs were developed from the end of Middle Jurassic to the end of Late Jurassic, and the superimposed continuous tight reservoirs were formed and reworked since the Early Cretaceous to present. The Xujiahe Formation in the centralenorthern Sichuan Basin is shallowly buried, and evolved to hydrocarbon pool late; the sandstone reservoirs remain tight, and the accumulation mechanism is basically identical [7]; the paleostructure and paleo-uplift are prospects, showing the characteristics of multiple hydrocarbon sources, accumulation near source, accumulation controlled by lithology, and reworking by fractures in the late stage [8]. Zhao Wenzhi et al. [9] believed that the Xujiahe Formation in central Sichuan Basin presents the large-scale plaque-type accumulation of natural gas, the overall uplifting since the end of the Cretaceous is an important event of hydrocarbon unloading, expelling and accumulation, and the gas source kitchen, major sandbodies, structural setting and fractures control the accumulation and enrichment of natural gas in the Xujiahe Formation. These research results show that the paleo-structure controls the early accumulation of oil and gas in the Xujiahe Formation, and the

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effective and high-yield zone is formed by the fracture reworking in the late stage. The prediction of paleo-structure and description of late fractures are key technologies for the prediction of oil and gas enrichment zones in the Xujiahe Formation. However, there are many limitations in the methods to study paleo-structure [10], and the prediction of fractures is more difficult. Stratigraphic thickness map is a commonly-used method for paleostructure research. During the 8th Five-Year Plan period, the Xiaoquan-Xinchang-Hexingchang (X-X-H) structural belt was proposed as a Yanshanian paleo-uplift based on the Jurassic thickness map [11], and this recognition has been continuously deepened [1,5]. However, this method can only exhibit the paleo-structural trend and is inapplicable in the areas with hiatus of stratum. Cao Lie et al. [12] used the stripping technique and decompaction correction to compile the stratigraphic thickness map to describe the local tectonic evolution history of the Hexingchang, revealing unsatisfactory application results. The structural unionecomposite method is also an important means to analyze the structural period. Deng Kangling and Li Zhiwu et al. [13,14] described the structural period of the X-X-H structural belt by analyzing the superposition relationship of structures in different directions, but their study only involved a small coverage and provided unclear conclusions. The amplitude of all structures in the basin is small. As a result of superposition and reworking in multiple phases, the neo-structures of the Himalayan inundated the paleo-structures to varying degrees, rendering difficulties in the study on structural periods. According to the viewpoint of geo-mechanics, the structural belts of different shapes, properties, grades and sequences produced by one tectonic stress field and the blocks they hold are combined into one structural system. As more and more data have been acquired, the 3D data in the central part of the Western Sichuan Depression are merged, and most areas in the basin are covered by 2D survey. Based on the latest regional structure map, the 3D visualization technology of low-angle vertical backlight irradiation was used to display the structural traces in different directions in the Sichuan Basin. According to the regional structural dynamic direction and structural relationship, the structural systems were divided, and the structural systems in different directions were clarified. Then, by using the well test data, the oil and gas enrichment zones of the Xujiahe Fm in the basin were predicted. The study results of the central part of the Western Sichuan Depression are expected to extend to other areas of the basin. 1. Activity sequence of orogenic belts around the Sichuan Basin The Sichuan Basin is located in the western part of the Yangtze Block. It is bounded by the Mianlue suture zone and bordered by the Qinling orogenic belt in the north; by the Longmenshan thrust belt and connected with the SongpaneGanzi fold belt in the west; by the Qiyaoshan fault in the east; and gradually transits to the SichuaneYunnan

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uplift belt without clear boundary [5,15] (Fig. 1). The basin, with an area of about 19.1
104 km2, exits in a diamond shape with NE-oriented axis. The Longmenshan thrust belt and its foreland depression are segmented [5,15] to the northern segment (to the north of Anxian) and the southern segment (to the south of Dujiangyan), between which is the central part of the Western Sichuan Depression. Since the Late Triassic, it has experienced three tectonic cycles in Indosinian, Yanshanian and Himalayan (Fig. 1, Table 1). The surrounding orogenic belts compressed and overthrusted the basin in different directions in multiple periods, leading to the composition and reworking of structural systems in multiple periods and different directions within the basin [15e19]. (1) Indosinian (T2eJ1) The northern segment of the Longmenshan thrust belt was mainly active in this period, recording the most important tectonic movement in the basin. The extrusion direction is generally NeS, and NWeSE (Fig. 1). There are four episodes. Episode I (T2eT3) This is the major episode of the Indosinian movement [5,15], which was called the Xinchang movement by Yang

Keming [20]. At the end of T2, the Yangtze Plate rotated clockwise [21] and collided with the North China Plate. The South Qinling thrusted southward. As a result, the tectonic setting transformed from early extension to strikeeslip extrusion. This means the end of the passive continental margin and the onset of the evolution stage of continental basin. Episode II (T3teT3x2) The Bikou Massif collided with the Yangtze Plate, the northern segment of the Longmenshan thrust belt was uplifted, and the Longmenshan thrust belt rotated in a sinistral direction [14,21]. The Bikou Massif and the Micangshan Block were extruded from north to south, resulting in nearly SeN compressional stress in the basin [5,20] (Fig. 1). As a result, a series of nearly EeW structural belts were formed [20]. Episode III (T3x3eT3x4) This stage of tectonic movement was most prominent in Anxian, called the Anxian movement [22]. The movement divided the Xujiahe Formation into lower Xujiahe basin (T3x1 and T3x2) and upper Xujiahe basin (T3x4 and T3x5). Its dynamic direction is northwest to southeast extrusion (Fig. 1).

Fig. 1. Structural diagram of the Sichuan Basin, the tectonic dynamic direction of each period, and location of main gas reservoirs of Xujiahe Formation (according to Refs. [14,18]).

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Table 1 Main tectonic movements since the Late Trassic. Erathem

System

Cenozoic

Quaternary Neogene Paleogene

Paleozoic Cretaceous Jurassic

Series Pliocene Miocene Oligocene Eocene Paleocene Upper Lower Upper Middle Lower

Mesozoic Triassic (T) Upper

Middle Lower

Formation (member and code)

Tectonic movement

Stress direction and event

Q N2 N1 Lushan Fm (E3l ) Mingshan Fm (E1-2 m)

Late Himalayan Episode II Early Himalayan Episode II Himalayan Episode I Yanshanian Episode III Yanshanian Episode II Yanshanian Episode I Indosinian Episode IV Anxian Movement Indosinian Episode II Indosinian Episode I

The southern segment of the Longmenshan thrust belt was uplifted, and suffered a nearly EW-oriented extrusion. The Micangshan eDabashan belt was finally shaped in the Himalayan. The Huayingshan fault and Eastern Sichuan high-steep structural belt were finalized. The Dabashan and Eastern Sichuan highsteep structures were formed in the Middle Yanshanian. The Longmenshan thrust belt was combined and unified. Nearly SN-oriented and NWeES-oriented extrusions occurred. The Yanshanian structural prototype of Micangshan eDabashan was initially shaped. The NS-oriented and NWeES-oriented extrusions appeared due to the uplifting of the northern segment of the Longmenshan thrust belt. The Dabashan Mountain was uplifted.

Guankou Fm (K2g)-Jiaguan Fm (K2j ) Jianmenguan Fm (K1j ) Penglaizhen Fm (J3p) Suining Fm (J3sn) Shaximiao Fm (J2s) Qianfoya Fm (J2q) Baitianba Fm (J1b)/Ziliujing Fm（J1z） Xujiahe Fm (T3x)

Xu Xu Xu Xu

6 5 4 3

Member Member Member Member

(T3x6) (T3x5) (T3x4) (T3x3)

Xu 2 Member (T3x2) Xiaotangzi Fm (T3t) Xu 1 Member (T3x1) Ma'antang Fm (T3m) Leikoupo Fm (T2l ) Jialingjiang Fm (T1j ) Feixianguan Fm (T1f )

Episode IV (T3eJ1) The northern segment of the Longmenshan thrust belt was uplifted intensively, and overthrusted from northwest to southeast within the basin; the central and northern parts of the Western Sichuan Depression became denudation zones [15]. The Jurassic was in unconformable contact with the underlying strata [5,15], with the Lower Jurassic Baitianba Formation/Xu 3 Member in the northern part, the Middle Jurassic Qianfoya Formation/Xu 4 and 5 Members in the central part, and conformable contact in the south. Generally, it shows an uplifting process of the Longmenshan thrust belt from north to south [14]. (2) Yanshanian (J2eK1) The Dabashan was strongly overthrusted. In the Early and Middle Jurassic, the Yangtze Plate continued to advance northward (Fig. 1), and the Qinling orogenic belt was thrusted and compressed southward [17,23], producing a nearly SeN extrusion. The nearly EeW “paleo-structure and paleo-uplift” of Indosinian presented an inherited growth. The remote dynamical action of the subduction of the continental margin in the east was prominent [16], and the uplifting of the Xuefengshan basement was activated, forming an NE-oriented trougheblock high steep belt in eastern Sichuan Basin, which was combined with the Dabashan arc structural belt. The Longmenshan thrust belt was relatively quiet [15,17], and the central segment was uplifted slightly in the Late Yanshanian.

The 40Ar/39Ar dating results [19] show that the MaoweneWenchuan shear zone (120e130 Ma) and the Pengguan complex (110 Ma) had a cooling event, which resulted in a sinistral strikeeslip shearing. In the central segment, the Qianfoya Formation and the Xu 5 Member was unconformably contacted, and the Baitianba Formation was absent, indicating that there was a paleo-uplift in the Yanshanian. The uplifting and extrusion in the central segment of the Longmenshan thrust belt produced a compressional stress orienting NWeSE in the centralesouthern part of the Western Sichuan Depression [24], forming a series of NE-oriented structures. (3) Himalayan (K2eN2) At the end of the Late Cretaceous, the southern segment of the Longmenshan thrust belt became active, and the Longmenshan thrust belt was in sinistral motion, with a new front formed at its leading edge [15]. The basin was generally under the background of compressional stress orienting SWeNE or nearly EW [14,17,18] (Fig. 1). The structure formed in the basin is the Longquanshan fault fold belt, and a row of NSoriented new structures were formed in the south of the Western Sichuan Depression. In summary, the main activity periods of the orogenic belts in the basin are different [18]. As for the Longmenshan thrust belt, the northern segment was strongly thrusted in the Indosinian, relatively quiet in the Yanshanian, and greatly uplifted in the

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Himalayan; the central segment was uplifted to a certain extent at the end of the Yanshanian; the southern segment was greatly thrusted and uplifted in the Himalayan. As for the Dabashan orogenic belt, there was orogeny by collision in the Indosinian, and it was strongly thrusted in the Middle Yanshanian and uplifted in the Himalayan (typically in the Miocene). As for the XuefengshaneHuayingshan thrust belt, thrusting and uplifting occurred in the Yanshanian; the main structure was formed before the Late Cretaceous and the structure was superimposed and altered again in the Himalayan. The periphery belts in different periods and directions were thrusted to the basin, and the structural systems formed in the basin were superimposed. Due to the large structural amplitude in the Himalayan, the early structures were strongly modified, making the identification of paleo-structure a major problem. 2. Relationship between structural system and hydrocarbon enrichment in Xujiahe Fm in the central and southern parts of Western Sichuan Depression 2.1. Structural system The central part of Western Sichuan Depression is the largest depression of the Xujiahe Fm, with the largest quantity of resources. Its structural characteristics have been studied in detail: the shallow and deep structures are similar, but the deep structures are more complex and have obvious inheritance [1,5]. According to Yang Keming's structural unit division [1,5], the structural system at the bottom boundary of the Xujiahe Fm in the central part of Western Sichuan Depression is divided into three structural units, i.e., the Longmenshan piedmont NEoriented structural belt with front extensional deformation, the nearly EW-oriented X-X-H structural belt, and the nearly NS-oriented Longquanshan fault fold belt (Fig. 2). (1) The Longmenshan piedmont NE-oriented structural belt with front extensional deformation It is a row of anticlines developed at the font of Longquanshan thrust belt and includes the structures such as Yazihe and Dayi in the central part of Western Sichuan Depression (Figs. 1 and 2). Controlled by the Longmenshan nappe structural system, the structures usually appear in the form of semi-anticlines. Typically, the Yazihe structure is the largest in the piedmont belt, and developed on the east side of the Pengguan complex (Fig. 1). At the bottom boundary of the Xujiahe Fm, the structural relief is more than 1000 m and the trap area exceeds 300 km2. (2) The nearly EW-oriented X-X-H structural belt It is the largest uplift in the depression, and generally trends in NEE. The local tectonic morphology is relatively complete, and the deep and shallow structures are basically the same. There are some anticlines like Xiaoquan, Xinchang and Hexingchang. Some large- and medium-sized gas fields have been discovered, such as Xinchang, Xiaoquan and Hexingchang,

where the gas layers are mainly in the Xu 2 Member, while the Xu 4 and Xu 5 Members are gas-bearing layers. (3) The nearly NS-oriented Longquanshan fault fold belt It is the dominant structural belt. There are also nearly NSoriented dense small fault belts. 2.2. Periods of structural system Previous studies only presented qualitative description of structural characteristics, but rarely dealt with the relationship between structures in different directions. According to structural trace analysis (Fig. 2), the structures on the belt are superimposed in multiple directions. Based on the abovementioned regional structure dynamic direction and sequence [14,16e18,24], together with the relationship among the structural traces, the structural systems in different directions and their periods can be clarified (Fig. 2). (1) Nearly EW-oriented arc structural system The structural system consists of more than five arc anticlines that are basically uniform in trending. For the convenience of description, they are named according to the toponym or drilling wells on the Zitong Arc, 851 Arc (Well 851, revealing commercial gas reservoir), 100 Arc (Well 100, revealing commercial gas reservoir), 137 Arc (Well 137, commercial gas well), 566 Arc (Well 566, commercial gas well with a low and stable yield). The X-X-H structural belt was strongly modified, but the Zitong Arc and 566 Arc still maintain their relatively clear original shapes. According to the direction of dynamics, it can be confirmed as an IndosinianeYanshanian structure. During the IndosinianeYanshanian, the northern segment of the Longmenshan thrust belt was uplifted and strikeeslipped from north to south [14,18,24]. Its dynamic direction is from north to south and from northwest to southeast (Fig. 1). A series of EW-oriented structural systems were initially shaped in the Indosinian and inherited to grow in the Yanshanian [20]. Additionally, according to the restriction and intersection between the structural traces, it can be inferred that the EWoriented structural system is the earliest structure (Fig. 2). As shown in Fig. 2, all NE-oriented linear structures are constrained by EW-oriented structures (e.g., Zitong Arc); the EW-oriented structures are uniformly intersected and reworked by NNEeSNeoriented dense small fault belts (two yellow dashed lines in Fig. 2); the NS-oriented Longquanshan fault fold belt divides the X-X-H structural belt into two parts. Clearly, the EW-oriented arc structure is the earliest one. (2) NE-oriented linear structural system It is developed in the Anxian area, with 4e6 strips parallel to the Anxian piedmont thrust belt. It is restricted by the EWoriented arc structure, and is intersected by the NNE-oriented small fault belt. Therefore, the NE-oriented linear structure

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Fig. 2. Bottom structural traces and drilling distribution map of the second member in the central part of the Western Sichuan Depression (the backlight in the sharp northern direction, not including the Dayi Structure).

was formed later than the NE-oriented arc structure and earlier than the NNE-oriented small fault belt. From the analysis of the dynamic direction, the structural system is parallel to the Anxian piedmont nappe belt in the northern segment of the Longmenshan thrust belt (Fig. 1), indicative of a NE-oriented linear structural system formed by the Anxian movement due to the NWeSE compressional stress in the Anxian area as a result of the Bikou Massif's extrusion from north to south in the middle episode of Indosinian [24]. 3) NS-oriented structural system On the X-X-H structural belt, there is a dense small fault arc belt (Fig. 2), which trends generally in nearly SN. Another large structural belt is the SN-oriented Longquanshan fault fold belt, whose major structure was formed in the Himalayan but was active in the Yanshanian [13,15]. This structural system clearly intersected the EW-oriented arc lines and NEoriented linear structures, indicating that they were the latest structures. 4) NE-oriented arc structural system The structural system includes the Yazihe structure in the central depression, partial structure of the Longquanshan fault

fold belt, and the Pingluoba structure in the southern depression. According to the trend and formation mechanism, all these structures belong to the same type of structural system. This structural system is a relatively early structure, which experienced the formation in the Indosinian, the inherited development in the Yanshanian, and the large-scale uplifting in the Himalayan [1,5]. A section along the flattened bottom boundary of the Upper Jurassic was used to qualitatively describe the Yazihe structure, namely, the structural evolution of the middle segment of the Longmenshan thrust belt. Three situations can be derived from Fig. 3. First, the Upper Jurassic (J3) and the Cretaceous (K) vary slightly in thickness, and generally display continuous and parallel reflections, indicating that the Longmenshan thrust belt was quiet in the Yanshanian [15]. The section can roughly characterize the paleo-structures of the underlying strata before the Himalayan structural deformation. Second, the Middle and Lower Jurassic strata are apparently thinner in the Yazihe area, and the Lower Jurassic Baitianba Fm (J1b) is absent. This indicates that, in the early Yanshanian, the central segment of the Longmenshan thrust belt was uplifted [5]. Third, there is an anticlinal structure in the Xu 2 Member, indicating that the Yazihe structure was initially shaped in the IndosinianeYanshanian, with a small amplitude and occurrence in the sub-sag. Therefore, in the front extensional deformation zone, the

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Fig. 3. Characteristics of the IndosinianeYanshanian paleo-structures of the Yazihe structure before the Himalayan.

NE-oriented arc structural system was developed as a prototype in the IndosinianeYanshanian, and intensively uplifted in the Himalayan. This is basically consistent with the zircon fission track dating. The Pengguan complex was cooled at about 110 Ma [19,24], and experienced a sinistral strikeeslip in the Yanshanian Episode II, but the structural deformation was revealed on the section to be the Yanshanian Episode I, which may be the error of dating. Because the Pengguan complex is the hardest, its uplifting and extrusion could produce NWeSE compressional stress, thereby forming the NEoriented arc structural system. Its structural dynamics might be the major contributor to the formation of the NE-oriented arc structural systems in the central and southern segments of the Longmenshan thrust belt. It is noted that the Yazihe area had been the depression center before the Yanshanian, prior to the Himalayan uplifting, although the Yazihe structure existed somewhat during the IndosinianeYanshanian [5,25]. This is different from the locations of the Yanshanian paleo-uplifts in Dayi and Pingluoba gas fields [14,25]. The abovementioned is the method that the period of structural system is determined by the peripheral dynamic direction of the basin and the intersection relationship. By comparing the deep structure with the shallow structure, the structural period can be directly and clearly described (Figs. 2 and 4). The Upper Jurassic (J3) only experienced the middle to late episode of Yanshanian, and the Himalayan, but no structural trace in the Indosinian and the early Yanshanian. By correlating the structures at the bottom of the Xujiahe Fm (Fig. 2) and the bottom of the Upper Jurassic (Fig. 4), the structural periods can be further visually confirmed.

(1) The NE-oriented linear structural system in the Anxian piedmont There is no NE-oriented structure at the bottom of the Upper Jurassic (J3). This structural system was formed in the Indosinian. (2) The EW-oriented arc structural system There are arc structures in the Penglaizhen Fm, which are smaller than the Xujiahe Fm in amplitude and different in morphology. This indicates that the EW-oriented arc structure presented as a prototype in the Indosinian and inherited growth in the Yanshanian. (3) The NE-oriented arc structural system The Yazihe structure is uniformly developed at the bottom of the Xujiahe Fm and the bottom of the Upper Jurassic. According to Fig. 3, the Yazihe structure presented as a prototype in the Indosinian, an inherited and limited development in the Yanshanian, and intensively uplifted in the Himalayan. (4) The nearly NS-oriented structural system The Longquanshan fault fold belt is a typical example, with larger magnitude in shallow strata than deep strata, indicative of a late structure. As to the formation of this nearly SNoriented structural system, structural sequence can be used to describe the extrusion process in the southern segment of the Longmenshan thrust belt. As shown in Fig. 4, the SN-

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Fig. 4. Structural traces of the Upper Jurassic in the Western Sichuan Depression (Northeast backlighting, indicating the Late YanshanianeHimalayan structural deformation process).

oriented structural system began to develop from the Dayi area, with more than eight structures (SN1eSN8) from west to east, which present uplifting from west to east and decrease in amplitude successively. The tectonic deformation and uplifting direction are from west to east, indicating that during the Himalayan, the regional dynamics was the WeE compression (Fig. 1). Thus, the corresponding deep structures were formed. For example, the dense nearly SN-oriented fault belt at the bottom of the Xujiahe Fm is located between SN5 and SN6; the large Longquanshan fault fold belt is located between SN6 and SN7, whose structural amplitude is speculated to be related to basement blocking [5]. The nearly NS-oriented structure in the Western Sichuan Depression and the NW-oriented structure in the central part of the basin were both formed by the extrusion in the southern segment of the Longmenshan thrust belt. They can be assigned to the same type of structural system, which will be detailed later. In summary, in the central part of the Western Sichuan Depression, the nearly EW-oriented arc structural system was formed in the Indosinian and successively developed in the Yanshanian; the NE-oriented linear structure in the piedmont of Anxian was formed due to the Anxian movement in the Indosinian; the NE-oriented arc structure in the piedmont presented as a prototype in the IndosinianeYanshanian and was intensively uplifted in the Himalayan; the nearly SN-

oriented structure was mainly formed by the compression in the southern segment of the Longmenshan thrust belt in the Himalayan, forming a series of nearly SN-oriented structures in westeeast trending. The Longquanshan fault fold belt is the largest structural belt. 2.3. Relationship between the structural traces and the oil and gas enrichment zone in the central part of the Western Sichuan Depression There is a large number of wells on the X-X-H structural belt, which can be used to analyze the relationship between the structural system and the oil and gas enrichment zone. According to the gas shows of existing wells, it is found that high-yield wells, dry wells, and water-producing wells coexist in the same structural highs and the same contour line (Fig. 2). The gas reservoir in the Xu 2 Member was formed by “early accumulation, middle storage, and late activation”. According to the abovementioned structural system period, the oil and gas enrichment zone in the Xu 2 Member is regular. 1) The nearly EW-oriented arc anticline and NE-oriented linear anticline are paleo-structures in the IndosinianeYanshanians and they are the early hydrocarbon migration destinations. They are superimposed with the Himalayan fracture zone to form an activated

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oil and gas enrichment zone. Typical structures are Arc 851, Arc 100, and Arc 137 (Fig. 2). Amongst them, Arc 851 anticline was formed in the Indosinian and inherited from Yanshanian, and the oil and gas are transported and accumulated in the anticline during the peak generation period [5,6]. The southern segment of the Longmenshan thrust belt was uplifted and compressed in the Himalayan [1,5,14]. Due to the presence of a set of rigid dolomite sedimentary strata in the Middle Triassic Leikoupo Fm in the Xiaoquan area, a nearly SN-oriented dense small fault belt [26] and fracture development zone were developed in front of it. The early EWoriented structural anticline and the late nearly SNoriented dense fault belt intersect to form a high-yield oil and gas enrichment zone. Commercial oil and gas wells such as 3, 851, 150 and 561 are deployed in the belt where the structures of two periods and in two directions are intersected. Arc 100 is also an anticline which is rich in oil and gas, and located in the zone where the EW-oriented arc anticline and the Longquanshan fault fold belt are intersected. Arc 137 is similar to Arc 100, but a with smaller scale. The lowand stable-yield Well 566 is located at the intersection of Arc 566 anticline and the secondary fault of the Longquanshan fault. 2) The nearly SN-oriented structure in the Himalayan is usually an effective trap. The typical structure is the Longquanshan fault fold belt formed in the Himalayan, where multiple wells (e.g., 173) are dry wells (Fig. 2). Another example is the Xiaoquan structure, which is the largest structural trap on the X-X-H structural belt. There are many wells such as 93, 12 and 13; Wells 12 and 13, both dry wells, were deployed after high-yield commercial gas wells such as 851 (Fig. 2). 3) The NE-oriented arc structure is related to the paleostructural position. The Yazihe structure shows the largest structural amplitude and trap area in the Western Sichuan Depression (Figs. 2 and 3). It was intensively uplifted in the Himalayan and remained in the depression before the Yanshanian (Fig. 3). In this structure, 95 wells were deployed from the 1980se1990s, all of which were dry wells. Well Y3 drilled in 2010 was still a dry well. From 2016 to 2017, additional 4 wells were deployed targeting the gas reservoir in the Leikoupo Fm, as well as the Xujiahe Fm, but there is no good show in the Xujiahe Fm. This indicates that the structure has a poor enrichment of natural gas. In contrast, the Dayi structure in the IndosinianeYanshanian slope [25] were deployed with several wells showing commercial gas, indicative of an effective structure. 4) The anticlinal structure formed due to the later uplifting of IndosinianeYanshanian syncline is ineffective trap. In the IndosinianeYanshanian syncline, existing wells are dry or produce water, such as Well 560 and Well 565 (Fig. 2). Well 560 is located at a higher position than Well 851, in a structure which was a syncline during the IndosinianeYanshanian and uplifted in the late

YanshanianeHimalayan to form an anticline; it is a heavily water-producing well. Well 565, also a heavily water-producing well, is also located in the Indosinian anticline. Other dry wells such as G3 and G4, and lowyield wells such as X6 are in the Indosinian anticline. In summary, on the same structural belt and the roughly consistent sedimentary facies belt, the IndosinianeYanshanian paleo-structure forms oil and gas enrichment zone, and the fracture zone associated with the Himalayan fault forms highyield zone. In areas with a large burial depth, such as the Yazihe structure, the IndosinianeYanshanian structure may be less effective. 2.4. Relationship between the structural period and oil and gas enrichment zone of the Xujiahe Fm in the southern part of the Western Sichuan Depression In the southern part of the Western Sichuan Depression, the structural traps in piedmont of the Longmenshan Mountain are distributed in rows and belts (Figs. 1 and 5). So far, the Xujiahe Fm gas fields have been established in the structures of Pingluoba, Qiongxi, Lianhuashan, and Zhangjiaping. The gas accumulation mechanism is roughly the same as that of the central part, namely, near-source accumulation [27]. The source rocks became highly mature to overmature in the Late Jurassic, which is the main stage for the formation of gas reservoirs in the Xu 2 Member. Hydrocarbons were substantially charged from the end of Yanshanian to the Early Himalayan, and adjusted in the Late Himalayan [28]. The Pingluoba and Qiongxi structures are the Yanshanian paleouplifts [29], which has been highly valued by scholars [1,5,14]. Multi-stage fracture systems form effective reservoirs, with well-developed nearly SN-oriented fractures; early (Indosinian and Yanshanians) fractures are filled, and late (Himalayan) fractures are effective [31]. The reservoirs have strong heterogeneity [32], greatly variable gas productivity among wells, and poor connectivity between structural units. Moreover, the accumulation time of natural gas is different. Therefore, fine description of paleo-structure and structural period is the key to the exploration of gas reservoirs. The Himalayan structure is characterized by strong intensity and multiple faults, and the early structure was strongly reworked, for which the description of paleo-structure based on thickness map is not accurate enough [14,29]. In the southern part of the depression, there are many dry wells and water-producing wells, possibly because the Himalayan structure was treated as an effective structure. Attempts were made to analyze paleo-structural characteristics based on the period of structural system (Fig. 5). However, no detailed information was collected in the study, so the structural period can only be described simply, instead of using 3D visualization. As mentioned above (Figs. 2 and 3), the SN-oriented structure was formed by the extrusion in the southern segment of the Longmenshan thrust belt in the Himalayan, and the NE-oriented arc structure was formed in the IndosinianeYanshanian. Since the southern segment is far away

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Fig. 5. Relationship between the structural period and the enrichment zone in the southern part of the Western Sichuan Depression (the direction of stress field extrusion is based on Chen Yingli et al. [30]).

from the northern segment of the Longmenshan thrust belt, its effect is relatively small, but the Yanshanian structuralism should be dominant. The structures and faults formed by the same structure and having the roughly same direction are confirmed to be the same structural system. Thus, the structural system in the southern part of the depression is actually simple (Fig. 5), and the two-directional structural system is mainly developed. 1) From west to east, more than 5 rows of NE-oriented arc structures are developed, including Hetaoping, ZhangjiapingePingluobaeQiongxieSangyuaneGuankou, Lianhuashan, Hanwangbei, and Xiongpo. Some structures are relatively complete anticlines, such as Hanwangbei, and Pingluoba. Most of them are fault anticlines, such as the Qiongxi structure and the Xiongpo fault anticline. The structural system and its trend are similar to those of the Yazihe and Dayi structures (Fig. 3). The dynamics may come from the NWeSE compression of the Pengguan complex. The Baoxing complex might also play a role [33]. They are classified as the same structural system. Due to data limitations, the characteristics of paleo-structure in the Indosinian are difficult to analyze. According to Refs.

[1,5,11] and the regional stress field [30], this system is believed to be formed in the Yanshanian. 2) Multiple rows of SN-oriented structures are developed. The SN-oriented structures intersect all NE-oriented structures, indicating apparently a structural system of the last period. The SN1 to SN8 structural system in the central part of the depression is an extension of the SN structure in the south, indicative of one structural system that was formed in the Late Himalayan. The IndosinianeYanshanian paleo-structure þ Himalayan fault and associated fractures form an effective enrichment zone. High-yield commercial wells such as PL2 and QX4 are located in this zone. The Lianhuashan and Zhangjiaping structures present a superimposition of the NE-oriented arc structure and the SN-oriented fault (Fig. 5), forming effective enrichment zones, where multiple commercial gas wells have been obtained. It is important to note that in some parts of the southern depression, the NE-oriented arc structure and the SNoriented structure form a coaxial superposition. Due to the large structural amplitude in the Himalayan, a large number of the current structures were formed in the Himalayan. Typical structures include Hanwangchang structure, Laojunshan structure, and the northern section of the Hetaoping structure.

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The Pingluoba and Zhangjiashan structures also have the structural deformation components in the Himalayan. Due to the data and text limitations, the relationship between the oil and gas enrichment extent and the structural system in the southern part of the Western Sichuan Depression remains to be further analyzed. 3. Oil and gas enrichment zone of Xujiahe Fm in the Sichuan Basin In addition to the Western Sichuan Depression, Xujiahe gas reservoirs are widely distributed in the rest parts of the basin, such as the Zhongba gas field, the Penglaizhen gas field, and the Guang'an gas field (Fig. 1), where the complex gasewater relationship is prominent. The gas reservoirs are deeply buried and formed relatively early in the Western Sichuan Depression, but shallowly buried and formed relatively late with the same accumulation mechanism in the easternecentral Sichuan Basin. Studies on the chronology and diagenetic evolution [9,34] indicate that hydrocarbon discharge, migration and accumulation in the Xujiahe Fm started at the end of the Late Jurassic and terminated in the Late Cretaceous. The sandstone reservoir became tight between the Early Cretaceous and the Paleogene. Hydrocarbon accumulation was adjusted since the Himalayan, and the late fracture network is key to the formation of gas reservoirs [35]. In the entire Sichuan Basin, paleo-structure identification and prediction of Himalayan fractures are all critical to describing the oil and gas enrichment zone of the Xujiahe Fm. In the northern and eastern parts of the Sichuan Basin, due to the combination and composition of three orogenic belts (Longmenshan, MicangshaneDabashan, and eastern Sichuan high-steep structural belt) [17,18], the structural superposition is more complicated than that in the Western Sichuan Depression [16e18,36]. In the Middle JurassiceLate Jurassic, the Dabashan was uplifted sharply and thrusted to the basin. At the same time, the Xuefengshan thrust belt was also squeezed from the southeast to the northwest due to the extrusion of the South China Plate. In the Late Yanshanian, the Dabashan was relatively quiet, and the Xuefengshan thrust belt continued to be active. The basin was still squeezed in the southeast direction. This period was the main formation period of the Eastern Sichuan high-steep structural belt, forming the NE-oriented structure in the basin. In the Himalayan, the Dabashan structural belt and the Xuefengshan thrust belt were also strongly uplifted [16]. The Longmenshan thrust belt was uplifted in the northern part in the Indosinian, relatively quiet in the Yanshanian, and greatly uplifted in the southern part in the Himalayan. Because of different orientation, the structure in the Western Sichuan Depression is mainly slanted and restricted, showing a relatively clear relationship (Fig. 2). However, the extrusion direction of the Dabashan structural belt and the eastern Sichuan high-steep structural belt did not experience any large deflection, and the resulted structure was mainly inherited and developed, making the identification of paleostructure more difficult.

3.1. Structural trace characteristics and structural periods A set of NW-oriented structural system developed in the basin is associated with oil and gas enrichment. Wang Zecheng et al. [37] made an in-depth discussion, and concluded that in the Late YanshaneHimalayan, the Huayingshan fault experienced a sinistral strikeeslip to form the NW-oriented structure in the central Sichuan Basin, which superimposed with the NE-oriented structure to create a favorable zone. By using its structural map, the fine structures in the Western Sichuan Depression were spliced and displayed by vertical backlighting and low-angle 3D visualization, to describe the structural system and period of the entire basin (Fig. 6, excluding the southern part of the Western Sichuan Depression). Since the structures are characterized by a small amplitude, multiple directions, and a complex structural system, the light was casted from the northwest (Fig. 6-a) and the northeast (Fig. 6-b) respectively to clearly describe the structure perpendicular to the backlight direction. According to the dynamic extrusion direction and time sequence of the basin periphery and the relationship among structural traces, the structures with large amplitudes and the same genetic relationship in the basin are classified into the same structural system. The structural traces of the Xujiahe Fm in the Sichuan Basin can be divided into five structural systems by direction (Fig. 6). 1) Nearly EW-oriented structural system (Fig. 6-a) It is widely distributed in the basin, represented by the arcshaped structures, such as Arc 566, Arc 851, and Arc WC in the Western Sichuan Depression, Arc Z1, Arc Z2, and Z3 in the northern part and Zhongtai in the central Sichuan uplift belt, and P1eP4 in the Penglaizhen gas fieldeHechuan gas field in the central Sichuan uplift belt. In the Indosinian, the northern segment of the Longmenshan thrust belt was extruded from north to south and from northwest to southeast; in the Yanshanian, the DabashaneMicangshan was extruded in a nearly SeN direction [17,18] (Fig. 1). The EWoriented arc structure was generated in the Indosinian and inherited in the Yanshanian, with roughly the same direction and same genetic relationship, belonging to the same structural system. 2) NE-oriented Zhongba B-series linear structural system in the northern part of the Western Sichuan Depression (Fig. 6-a & b) From the Zhongba gas field, it gradually progresses to the basin and extends to the central Sichuan area. There are four structures (B1 to B4). The NE-oriented linear structure in the central part of the Western Sichuan Depression is an extension (Fig. 2). This structural system was formed along with the Anxian movement in the Indosinian due to the NWeSE extrusion of the northern segment of the Longmenshan thrust belt.

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Fig. 6. Structural traces and gas reservoir distribution at the bottom of Xujiahe Fm in the Sichuan Basin (excluding the southern segment of the Western Sichuan Depression).

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3) NE-oriented Huayingshan H-series linear structural system to the west of the Huayingshan fault belt in the eastern Sichuan Basin (Fig. 6-a) There are six structures (H1 to H6). The eastern Sichuan high-steep structural belteHuayingshan fault belt was shaped in the Yanshanian and was uplifted and squeezed into the basin the Himalayan [17,18]. The NE-oriented H-series linear structural system on the west side was also initially shaped in the Yanshanian and finalized in the Himalayan. Wang Linqi et al. [36] proposed that the NE-oriented structure was a product of superposition of multi-stage coaxial deformations, and comprises a large structural deformation component of the Himalayan. It is difficult to distinguish the structural deformation components of the Yanshanian and Himalayan using available data. According to the analysis of intersection relationship, the Huayingshan H-series structures clearly cut the Dabashan D-series structures (Yingshan and Guang'an gas fields, in the ellipse of Fig. 6), indicative of a later stage, which may be the larger component of deformation in the Himalayan. 4) NW-oriented Dabashan D-series arc structural system formed by the compression of the Dabashan structural belt (Fig. 6-a & b) There are five structures (D1 to D5). The structural system is consistent with the striking of the Dabashan structural system, and it advances progressively from the Dabashan to the basin until the Guang'an gas field. It belongs to the same structural system as the Dabashan arc structure. The Dabashan structure was mainly formed in the Yanshanian and developed in the Himalayan [16,17,23]. Therefore, the five arcs were initially formed in the Yanshanian and finalized in the Himalayan, with larger structural components in the Himalayan [16,23]. 5) SN-oriented to NW-oriented braided structural system (Fig. 6-b) Here, the nearly SN-oriented structure in the Western Sichuan Depression and the NW-oriented structure in the basin are assigned to one structural system, that is, a braided torsional structural system with the Jiangyou paleo-uplift as the mast. On the one hand, in the early Himalaya, the Indosinian Plate collided with the Chinese Plate to produce a nearly EW extrusion, and the southern segment of the Longmenshan thrust belt was greatly uplifted [11,15,18], with significantly stronger intensity and dynamics than other orogenic belts [11,18]. Thus, the activity intensity in the basin is inevitably larger than other orogenic belts. On the other hand, in terms of structural combination morphology (Fig. 6b), the nearly SN-oriented SN6 and SN7 structures in the Western Sichuan Depression (Fig. 4) and the SN8 and NW1eNW3 structures to the east of the Longquanshan fault fold belt (Fig. 6-b) are all converged in the Jiangyou paleouplift in the north. Therefore, with the Jiangyou paleo-uplift

as a mast, the SN-oriented structure and the NW-oriented structure in the basin form a braided torsional structural system. Wang Zecheng et al. [37] concluded that the NW-oriented structure in the central Sichuan Basin was controlled by the late activation of the NW-oriented basement fault, which was the result of the dextral strike slip of the Huayingshan fault in the Late YanshanianeHimalayan (Fig. 6-b). The southern segment of the Longmenshan thrust belt was squeezed to form a dextral strikeeslip action, which does not conflict with the tectonic movement of the abovementioned braided structure. In summary, there are five structural systems of Xujiahe Fm in the Sichuan Basin: (1) the nearly EW-oriented arc structure, which was formed in the IndosinianeYanshanian and was altered in the Himalayan, and shows an extensive distribution within the basin; (2) the NE-oriented B-series linear structural system in the northern segment of the Western Sichuan Depression, which was formed along with the Anxian Movement in the Indosinian; (3) the NW-oriented D-series arc structural system in the piedmont of Dabashan, which was finalized in the Late Yanshanian and developed in the Himalayan; (4) the NE-oriented H-series linear structural system to the west of the Huayingshan fault belt, which might have a larger deformation component in the Himalayan; and (5) the SN-oriented and SN-oriented to NW-oriented braided structural system in the Western Sichuan Depression, belonging to the same structural system, was formed by the extrusion of the Longmenshan thrust belt from west to east in the Himalayan. 3.2. Relationship between structural periods and oil and gas enrichment zone As mentioned above, the model of high-yield and enrichment zones in the Western Sichuan Depression is the IndosinianeYanshanian paleo-structure þ the Himalayan fractures. This model is also applicable to other areas of the basin. 1) The paleo-structure initially shaped in the Indosinian, inherited in the Yanshanian, and superimposed with the Himalayan fractures is the main oil and gas enrichment zone (Fig. 6-a & b). The most typical one in the Western Sichuan Depression is the Xinchang gas field on Arch 851. The Penglaizhen gas field in the Central Sichuan uplift belt corresponds to the convergence zone of the EW-oriented P1eP2 arc structures and NW1eNW2 linear structures as well as H6 linear structure (Fig. 6-a), the Anyue gas field is located in the convergence zone of P3 arc structure, NW2 linear structure, and H4eH5 linear structures, and the Hechuan gas field corresponds to the convergence zone of P4 and H2. There is the Zhongba gas field in the B1 anticlinal structure in the northern part of the Western Sichuan Depression, the Weicheng gas field (commercial well WC1) in the convergence zone of B2 structure and NW-oriented structures, the Zhebachang gas field at which B3 structure and NW5 are converged, the Jianmen gas field in

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the B3 structure, and the industrial well Zhongtai 1 in the convergence zone of B4 structure and NW5. The Himalayan NW-oriented faults and fractures activated the oil and gas enrichment zone of the YanshanianeIndosinian paleo-structure, resulting in high yield. It is important to note that the IndosinianeYanshanian paleo-structure was strongly modified by the Himalayan structure. In the same structural system, some structural highs were actually formed during the Himalayan. This may be the main reason for the proximity of commercial gas wells and dry wells. For example, dry well PL1 in the Penglaizhen gas field is both on the P2 arc structure and NW1. Similar case is witnessed for dry well 5. Because the Himalayan NW-oriented structures have a small effect on oil and gas enrichment, the effect of fractures is great. If the structural highs of NWoriented structures were taken as favorable zones, drilling failure might occur. Even if there are fractures, the well may produce water. 2) The Yanshanian structure is an important oil and gas enrichment zone. Typical gas fields are the Pingluoba gas field in the southern part of the Western Sichuan Depression (Fig. 5) and the Guang'an gas field in the central Sichuan Basin (Fig. 6). The Guang'an gas field is located in the convergence zone of the D5 arc structural belt and the H-series linear structures (within the ellipse in Fig. 6-a). The D-series structural system formed by the Dabashan structural belt and the H-series structural system formed by the Huayingshan structural belt were all formed in the Yanshanian, and greatly reworked in the Himalayan, which is favorable for the early enrichment and later activation, suggesting the possibility of an effective enrichment zone. 3) In the Huayingshan H-series linear structural system formed in the Eastern Sichuan high-steep structural belt, not all structural highs are important oil and gas enrichment zones. As mentioned above, the H-series structural system was formed relatively late, and it may have a larger Himalayan structural component. Guo Zhengwu et al. [15] considered that the Eastern Sichuan high-steep structural belt and the Huayingshan fault suffered strong uplifting and thrusting in the Miocene. He Dengfa et al. [16] indicated that in the Late CretaceousePaleocene, the Neo-Tethys and the Pacific tectonic domain were combined to cause strong folding and faulting again, and recombined on the Yanshanian structure. The Himalayan structures are large in amplitude and inherited, which may be one of the reasons for the failure of gas reservoir development in the Eastern Sichuan Basin (Fig. 6-b). In the Yingshan structural belt (circled in Fig. 6-b), which was formed by the vertical intersecting/intercepting of the D4 arc structure with H2 and H3, the exploration effect is not ideal e Wells 24 and 104 are dry, Well 110 produces water, Well 103 is a non-commercial low-yield gas well, and only Well 23 is

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a low-yield commercial gas well (with a gas production rate of 2.2
104 m3/d). A similar situation exists in the Guang'an gas field. The Guang'an structure is a product of the vertical interception and superposition of the D5 structural system and the H-series structural system. Well 101 is a water-producing well and Wells 13 and 102 are dry. The Guang'an gas field, with the proved gas reserves of 3049.78
108 m3 in the Xujiahe Fm, has not been developed satisfactorily [34]. It should be noted that the D-series structural system also displays some Himalayan structural components. The abovementioned analysis reveals that it is difficult to distinguish the Yanshanian paleo-structures from the Himalayan neo-tectonics in the same structural system on the basis of existing data, but the sequence relationship between structural systems is relatively clear. The structures interested must be formed relatively early. For instance, the EW-oriented arc structures are almost intersected by all structures (Fig. 2), and the Dabashan D-series structures are obviously intersected by the Huayingshan H-series structures (Fig. 6). Accurate identification of the Himalayan structure and the Yanshanian structure is a key issue in the future development of the Penglaizhen gas field and the Guang'an gas field. 4. Conclusions (1) Multiple stages of the Xujiahe Fm structures in the Sichuan Basin are superimposed in multiple directions. The structural traces with genetic relationships are assigned to the same structural system. This method is helpful in the analysis of the structural periods. The formation sequence of the structural system can be confirmed according to dynamic direction and period of the basin margin, and intersection and restriction between structural systems. (2) There are five structural systems of Xujiahe Fm in the Sichuan Basin: 1) the EW-oriented arc structure of the IndosinianeYanshanian, which was formed by the NeS extrusion of the northern segment of the Longmenshan thrust belt; 2) the NE-oriented B-series linear structural system of Anxian in the Indosinian, which was formed by the NWeSE extrusion of the northern segment of the Longmenshan thrust belt; 3) the NE-oriented arc structural system in the centralesouthern part of the Western Sichuan Depression, which was formed in the process in which the central segment of the Longmenshan thrust belt was initially shaped in the Yanshanian and intensively uplifted in the Himalayan; 4) the NW-oriented Dseries arc structural system, which was predominantly developed in the Yanshanian and reworked by superimposition again in the Himalayan; 5) the NE-oriented H-series linear structural system, which was formed later than the D-series arc structural system and has a larger deformation component in the Himalayan; and 6) the SN-oriented and SN-oriented to NW-oriented braided structural system, which was formed by the

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WeE extrusion of the southern segment of the Longmenshan thrust belt in the late Himalayan with the Jingyou paleo-uplift as the mast. (3) The IndosinianeYanshanian paleo-structure, which controls the early enrichment of oil and gas, and the Himalayan faults and associated fractures, which form an effective reservoir development zone, are superimposed to form high-yield enrichment zones. (4) The Xujiahe structures formed in the Himalayan are mostly dry traps. If not superimposed by the early structures, the fracture development zone may usually produce water. The syncline in the Indosinian, which was even uplifted to be an anticline in the Himalayan, usually is an ineffective trap.

Conflicts of interest The authors declare that there is no conflicts of interest. References [1] Yang Keming & Pang Xiongqi. The formation mechanism and prediction of tight sandstone gas reservoirs: a case study from Western Sichuan Depression. Beijing: Science Press; 2012. [2] Li Guohui, Li Nan, Xie Jirong, Yang Jiajing & Tang Dahai. Basic features of large gas play fairways in the upper Triassic Xujiahe Formation of the Sichuan Foreland Basin evaluation of favorable exploration zones. Nat Gas Ind 2012;32(3):15e21. [3] Wang Jinqi. Early accumulation and late seal-the basic character of gas reservoirs in West Sichuan Depression. Nat Gas Ind 2001;21(1):5e12. [4] Yang Keming, Ye Jun & Lu¨ Zhengxiang. Characteristics of gas distribution and reservoiring in upper triassic Xujiahe formation in Western Sichuan depression. Oil Gas Geol 2004;25(5):501e5. [5] Yang Keming, Zhu Hongquan, Ye Jun, Zhang Keyin & Ke Guangming. The Geological characteristics of tight sandstone gas reservoirs in Western Sichuan Basin. Beijing: Science Press; 2012. [6] Chen Dongxia, Pang Xiongqi, Yang Keming, Zhu Weiping & Yan Qingxia. Genetic mechanism and formation of superimposed continuous tight sandstone reservoir in deep Xujiahe Formation in Western Sichuan Depression. J Jilin Univ (Earth Sci Ed) 2016;46(6):1611e23. [7] Chen Taotao, Jia Ailin, He Dongbo, Ji Lidan & Shao Hui. Gas and water distribution regularity for tight sandstone reservoirs of Xujiahe formation in Central Sichuan Basin. Geol Sci Technol Inf 2014;33(4):66e71. [8] Xu Zhangyou, Song Li, Wu Xinsong & Chen Ce. Typical gas reservoirs and main controlling factors of reservoir-forming of upper triassic Xujiahe Formation in central Sichuan Basin. Lithologic Reservoirs 2009;21(2):7e11. [9] Zhao Wenzhi, Wang Hongjun, Xu Chunchun, Bian Congsheng, Wang Zecheng & Gao Xiaohui. Reservoir-forming mechanism and enrichment conditions of the extensive Xujiahe gas reservoirs, Central Sichuan Basin. Petrol Explor Dev 2010;37(2):146e57. [10] Jiu Kai, Ding Wenlong, Li Chunyan & Zeng Weite. Advances of paleostructure restoration methods for petroliferous basin. Lithologic Reservoirs 2012;24(1):13e9. [11] Southwest Bureau of Petroleum Geology MGMR. Petroleum geological atlas of clastic rocks in Sichuan Basin. Chengdu: Sichuan Science Press; 1996. [12] Cao Lie, An Fengshan & Wang Xin. Relationship between palaeostructure and gas reservoirs in Xujiahe formation in Western Sichuan depression. Oil Gas Geol 2005;26(2):224e9. [13] Deng Kangling & Yu Fulin. Compound structures and hydrocarbons in Western Sichuan depression. Oil Gas Geol 2005;26(2):214e9.

[14] Li Zhiwu, Liu Shugen, Chen Hongde, Sun Dong, Lin Jie & Tang Cong. Structural superimposition and conjunction and its effects on hydrocarbon accumulation in the Western Sichuan depression. Petrol Explor Dev 2011;38(5):538e51. [15] Guo Zhengwu, Deng Kangling & Han Yonghui. The formation and development of Sichuan Basin. Beijing: Geological Publishing House; 1996. [16] He Dengfa, Li Desheng, Zhang Guowei, Zhao Luzi, Fan Chun, Lu Renqi, et al. Formation and evolution of multi-cycle superposed Sichuan Basin, China. Chin J Geol 2011;46(3):589e606. [17] Shen Chuanbo, Mei Lianfu, Xu Zhenping & Tang Jiguang. Architecture and tectonic evolution of composite basinemountain system in Sichuan Basin and its adjacent areas. Geotect Metallogenia 2007;31(3):288e99. [18] Deng Bin. MesoeCenozoic architecture of basinemountain system in the Sichuan Basin and its gas distribution. Chengdu: Chengdu University of Technology; 2013. [19] Liu Shugen, Zhao Xikui, Luo Zhili, Xu Guosheng, Wang Guozhi, Wilson CJL, et al. Study on the tectonic events in the system of the Longmen mountaineＷest Sichuan foreland Basin, China. J Chengdu Univ Technol (Sci Technol Ed) 2001;28(3):221e30. [20] Yang Keming. Characteristics of Xinchang movement in Sichuan Basin and its geological significance. Petrol Geol Exp 2014;36(4):391e7. [21] Wang Erqi, Meng Qingren, Chen Zhiliang & Chen Liangzhong. Early Mesozoic left-lateral movement along the Longmenshan fault belt and its tectonic implications. Earth Sci Front 2001;8(2):375e84. [22] Wang Jinqi. Recognition on the main episode of IndoeChina movement in the longmen Mountains: a review on the Anxian tectonic movement. Acta Geol Sichuan 2003;2(2):65e9. [23] Liu Shugen, Li Zhiwu, Liu Shun, Luo Yuhong, Xu Guoqiang, Dai Guohan, et al. Formation and evolution of the foreland basin of the Dabashan thrust belt[M]. Beijing: Geological Publishing House; 2006. [24] Liu Shugen. The formation and evolution of Longmenshan thrust zone and Western Sichuan Foreland Basin, Sichuan, China. Chengdu: University of Electronic Science and Technology of China Press; 1993. [25] Su Jinyi & Liu Shu. Seismic facies of gas reservoir within the second member of Xujiahe formation in West Sichuan depression. Geophys Prospect Pet 2008;47(2):167e71. [26] Ning Meng, Liu Shu & Gong Wenping. Prediction of dolomite reservoir distribution on tap of triassic Leikoupo formation in Chuanxi depression. China Petrol Explor 2015;20(3):30e7. [27] Fan Ranxue, Zhou Hongzhong & Cai Kaiping. Carbon isotopic geochemistry and origin of natural gases in the southern part of the Western Sichuan depression. Acta Geosci Sin 2005;26(2):157e62. [28] Gao Gang & Huang Zhilong. Research on organic inclusion in reservoir and gas accumulation process in Pingluoba gas field. Acta Sedimentol Sin 2002;20(1):156e9. [29] Lan Daqiao & Qiu Zongtian. The research on the formation of upper triassic Xu 2 gas reservoir of Pingluoba gasfield in Chuanxi depression. Petrol Explor Dev 2002;29(3):8e10. [30] Chen Yingli, Gu Yang, Chen Guming, Jiang Desheng & Du Qiang. Research on paleo-structure stress of Qiongxi structure in West Sichuan depression. China Petrol Explor 2008;13(3):10e7. [31] Chen Yingli, Gu Yang, Chen Guming & Jiang Desheng. Characteristics and stages of fractures in the second member of Xujiahe Formation in Qiongxi structure, western sichuan depression. Nat Gas Ind 2008;28(2):46e50. [32] Wang Shunyu, Ming Shuang, Liang Cong & Zhang Tingshuai. Geochemical characteristics of natural gas and reservoir continuity of the second member of Xujiahe Formation in the Pingluoba Structure, Sichuan Basin. Natural Gas Geoscience 2014;25(3):388e93. [33] Lin Maobing. A first discussion on the tectonic emplacement of Baoxing anticlinorium, Sichuan. J Chengdu Univ Technol (Sci Technol Ed) 1995;22(2):22e6. [34] Zeng Qinggao, Gong Changming, Li Junliang, Che Guoqiong & Lin Jianping. Exploration achievements and potential analysis of gas reservoirs in the Xujiahe Formation, central Sichuan Basin. Nat Gas Ind 2009;29(6):13e8.

Liu S. et al. / Natural Gas Industry B 6 (2019) 220e235 [35] Wang Wei & Lin Liangbiao. High-yield mechanism of the second member of upper triassic Xujiahe reservoir in the Malubei area, NE Sichuan Basin. Nat Gas Ind 2017;37(12):11e7. [36] Wang Linqi, Fan Cunhui, Fan Zenghui & Jiang Bo. Presumption of the tectonic evolution and regional sedimentation of the Sichuan Basin based on seismic exploration technology. Nat Gas Ind 2016;36(7):18e26.

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[37] Wang Zecheng, Li Zongyin, Li Zhirong, Fan Bin, Zhang Haijie, Li Ling, et al. Genesis of structural deformation of the Xujiahe Formation in the central Sichuan Basin and its significance to petroleum exploration. Nat Gas Ind 2012;32(4):13e8.