In recent years, a large number of large-scale strike-slip faults with long stretching distance, have been discovered in the middle of the craton basins in China^[1], including the Ordovician strike-slip faults in Tazhong and Tabei areas of Tarim Basin^[2,3,4], and the Triassic Yanchang Formation strike-slip faults in Ordos Basin^[5]. These faults have played a significant role in controlling reservoir formation. As the exploration of deep oil and gas in Sichuan Basin goes on, a large number of high- angle faults with strike-slip characteristics have been identified in the stable area, especially in the Gaoshiti-Moxi area in the middle of Sichuan Basin.

Some research work on this has been conducted. Li Wenke, et al.^[6] and Ding Bozhao, et al.^[7] reported that collapsed-paleocave systems were developed in Central Sichuan, and their distribution was mainly controlled by continuously active strike-slip faults. Yang Ping et al.^[8] considered that the columnar pull-down anomalies on the Sinian-Permian seismic section of Central Sichuan were hydrothermal fluid channels formed on the basis of grabens caused by strike-slip faults. Yin Jifeng et al.^[9] depicted the distribution of deep and large strike-slip faults in Central Sichuan. Li et al.^[10] analyzed the characteristics and evolution of Sinian-Permian faults and their control on the formation of carbonate reservoirs in Gaoshiti-Moxi area, Central Sichuan. It was confirmed by previous researches that strike-slip faults were developed in Gaoshiti-Moxi area, but their geometry, kinematics, formation and evolution and significance to hydrocarbon accumulation need further study. The structural analysis of deep strike-slip faults in Gaoshiti-Moxi area is of guiding significance for the development of Sinian-Cambrian Anyue giant gas field, exploration and production of natural gas from the Permian Qixia Formation-Maokou Formation, and is also of great scientific significance for finding out the development characteristics of the strike-slip faults and their controlling on hydrocarbon accumulation in stable parts of cratonic basins.

Based on the latest high-precision three dimensional seismic data and drilling data of Gaoshiti-Moxi area, by using fault structure analysis method, the geometry, kinematics, evolution process and significance for gas accumulation of strike-slip faults have been analyzed systematically through high-precision coherent extraction and fine fault interpretation.

Central Sichuan covers Yanting-Nanchong-Hechuan area in the middle of Sichuan Basin. Tectonically, it belongs to the Central Sichuan gentle structural zone and is the most stable remaining block of the Upper Yangtze area^[11] (Fig. 1). Gaoshiti-Moxi area is located in the south of Central Sichuan. The 3D seismic data of this area is merged pre-stack time migration data of recent years with a coverage area of 6600 km², bin size of 20 m×20 m, and deep dominant frequency of 28 Hz, which meet the requirements of fine fault characterization.

Gaoshiti-Moxi area mainly develops the Sinian, Cambrian, Ordovician, Silurian, Carboniferous and Permian strata and lacks the Devonian strata. The Ordovician-Carboniferous strata only remain at the edge of the study area, and the Permian overlies the Cambrian directly in the main part. The Cambrian is a set of relatively stable marine carbonate deposits with clastic rocks interbed, including black shale of Qiongzhusi Formation at the bottom, two sets of red clastic rocks of the Canglangpu Formation and Gaotai Formation in the middle and upper parts, and two sets of thick dolostone of the Longwangmiao Formation and Xixiangchi Formation. The Upper Permian includes the Longtan Formation and Changxing Formation, and the Lower Permian includes the Liangshan Formation, Qixia Formation and Maokou Formation (Fig. 1)^[11].

The study area underwent multiple stages of tectonic movements. Controlled by Xingkai taphrogeny in the Sinian- Cambrian, Deyang-Anyue intracratonic rift was formed^[12,13]. In the late Caledonian, Leshan-Longnüsi paleo-uplift was generated under intense compression, which constitutes the main deep structural form in Central Sichuan currently. In Hercynian, Indosinian and Yanshanian, the paleo-uplift developed successively, with the structural axis shifting slightly, and fixed in shape after Himalayan movement^[14].

2.1.1. Section

According to the difference in combination pattern of faults on section, three main structural styles are identified in the study area: steep and erect faults, flower structure, "Y" and reversed "Y" type faults (Fig. 2).

(1) Steep and erect faults are the most common in strike-slip faults of the study area. The majority of them are transtensional faults^[10], nearly vertical on section with a dip angle of more than 80°. They extend up into the Cambrian or Permian, and cut through the underlying Sinian to pre-Sinian rift. Their planes are steeper and steeper with the increase of depth (Fig. 2a).

(2) Flower structure is the vertical configuration style of main faults and branch faults of strike-slip fault, and is one of important signs for strike-slip fault^[15,16]. The strike-slip faults in Gaoshiti-Moxi area mainly take on negative flower structure (Fig. 2b) and half-flower structure (Fig. 2c) composed of a series of concave-up normal faults distributed in the Sinian-Permian strata. Due to the superposition of multi-stage structural deformations, multiple flower structures develop in local area, forming superposition of flower structures on section. The extensive development of negative flower structures suggests that strike-slip faults in the study area are developed in a transtensional tectonic environment.

(3) "Y" and reversed "Y" types are two kinds of fault styles relatively rich in the study area, and appear as small grabens composed of two normal faults with opposite inclination (main fault and branch fault). According to the relative position of the main fault and branch fault, "Y" type (Fig. 2d) and reversed "Y" (Fig. 2e) type faults are classified.

2.1.2. Plane characteristics

In this paper, the high-precision coherence slices and corresponding fault distribution maps of Cambrian bottom (Fig. 3), and Permian Longtan Formation bottom (Fig. 4) in Gaoshiti-Moxi area are selected to illustrate the plane distribution patterns of strike-slip faults. The nearly S-N trending F₀ fault in Fig. 3 is the boundary fault of Deyang-Anyue intracratonic rift. According to previous study on the formation and evolution of Deyang-Anyue intracratonic rift, F₀ was formed in the deposition period of Dengying Formation during the Late Sinian^[12,13], and died in the late depositional period of Qiongzhusi Formation during the Early Cambrian. Most strike-slip faults in the study area cut through the overlying Cambrian strata and cut F₀ fault obviously. Therefore, strike-slip faults in this area were formed later than F₀. In this study, F₀ was regarded as a mark of regional boundary and a reference for analyzing strike-slip fault displacement, and wasn’t discussed in detail.

There are three groups of strike-slip faults in the Cambrian in near EW, NW and NE strike respectively (Fig. 3). They are in linear distribution. The near EW-trending faults (such as F₃, F₄, and F₆) are mainly distributed in Moxi-Longnüsi area in the north, with extension of 110 km along. The whole fault is composed of multiple secondary faults, which are commonly overlap and divergent from west to east. The NE-trending faults (such as F₈, F₉, and F₁₀) are mainly distributed in the southwest of the study area, with poor continuity. These faults cut through the underlying Sinian strata and generally only reach the bottom of Cambrian Longwangmiao Formation upward. The NW-trending faults (such as F₅, F₁₅, and F₁₇) are widespread across the study area and small in single fault length, but often multiple such faults distributed in en-echelon arrangement constitute a large fault (Fig. 3). Faults F₁ and F₂ developed in the middle of the study area are the boundary faults of Gaoshiti-Moxi structure, fault F₁ is the southern boundary of Moxi structure, and fault F₂ is the northern boundary of Gaoshiti structure. These faults start from the Sinian, and extend into the Permian-Triassic, and most of them are multi-stage active faults. On the seismic section, faults F₁ and F₂ show as reversed faults with the folding and uplifting of the strata, which indicate a structural inversion during the compression and uplifting of Gaoshiti-Moxi structure (Fig. 5).

There develop a few faults in the Permian, which are mainly distributed along the Cambrian main fault, reflecting the inheritance of the main fault. On the plane, the Permian faults are mainly distributed in the middle-east of the study area, and more in the north than the south. The ones in the north are mostly near EW and NW trending, intermittent and long in extension on the whole, while those in the south are largely NW trending and short in extension (Fig. 4). The faults in Moxi-Longnüsi area to the north are much denser than those in Gaoshiti area to the south. There are two NW-trending fault belts consisting of normal faults in en-echelon arrangement in the southeast of the study area, which are generally distributed along Cambrian faults, reflecting the existence of transtensional stress field in this area.

The geometric analysis shows there develop transtensional strike-slip faults in the Paleozoic in the study area, which appear in steep and erect, negative flower or half flower structures, "Y" and reversed "Y" type faults on the section. On the plane, there are 3 groups of faults in near EW, NW and NE strike respectively. They are composed of multiple secondary faults distributed in en-echelon or oblique arrangement.

2.2.1. Determination of movement direction

Due to small horizontal displacement, the movement direction of faults in the study area is difficult to tell. In this work, the movement direction was judged according to the cutting of fault F₀ and fold by strike-slip faults and the en-echelon arrangement of secondary faults in the main fault^[17,18].

As mentioned above, the fault F₀ was formed earlier than the strike-slip faults in the study area, therefore, the movement direction of strike-slip fault can be judged according to the faulted direction of F₀ by the strike-slip fault. The 2 360 ms time slice of faults F₃ and F₄ trending near EW show dextral offset of F₀ by F₃ and F₄, which indicates F₃ and F₄ are dextral strike-slip faults (Fig. 6a). The fine coherence map shows that secondary faults within NW and nearly EW trending faults are distributed in left-step en-echelon arrangement (Fig. 7a), which further confirms that the NW and near EW trending faults are dextral strike-slip faults. The 2 290 ms time slice of F₁ shows the dextral offset of F₀ by F₁, which confirms F₁ is a dextral strike-slip fault (Fig. 6b). The secondary faults within F₁ are arranged in left-step en-echelon (Fig. 7b), also proving that F₁ is a dextral strike-slip fault. From the time slice of 2498 ms of NE-trending F₇ and time slice of 2412 ms of NE-trending F₉ (Fig. 6c, 6d), it can be seen that the NE-trending faults are sinistral strike-slip faults, within which the secondary faults are distributed in right-step arrangement (Fig. 7c), which also suggests the NE-trending faults are sinistral strike-slip faults.

Based on the above analysis, it is concluded that the strike-slip faults in near EW and NW strikes in Gaoshiti-Moxi area are dextral transtensional strike-slip faults, while those in NE strike are sinistral transtensional strike-slip faults (Fig. 3).

2.2.2. Displacement

By fault displacement analysis, the intensity of faulting activity can be reflected quantitatively. In this work, the horizontal and vertical displacements of the Paleozoic strike- slip faults in the study area were counted, in which the horizontal displacement was used to analyze the strike-slip strength, and the vertical throw was used to identify the faulting strength.

The comparison of structural features, such as folds, faults and channel systems, across the fault is the most effective method to measure horizontal displacement of strike-slip fault^[19]. In this work, the fault F₀ and folds in the south were taken as reference, and their faulted distances by strike-slip fault were measured to calculate the horizontal displacement of transtensional strike-slip fault. Chinese and foreign researchers believe that the horizontal displacement of a strike- slip fault is positively correlated to length of the fault^[20,21].Thus, 5 large faults cutting the references were selected to calculate the horizontal displacement, and the results represent the maximum horizontal displacement in the study area. According to the measurement, the dextral displacements of fault F₀ by faults F₁, F₃ and F₄ are 490 m, 335 m and 550 m respectively, so the horizontal displacements of faults F₁, F₃ and F₄ are 490 m, 335 m and 550 m respectively (Fig. 6a, 6b). Fig. 6c and 6d shows that the horizontal displacements of NE-trending F₇ and F₉ are 110 m and 290 m respectively. Comparing the horizontal displacements of the 5 faults shows the NE-trending faults F₇, small in size, is also small in horizontal displacement (with a minimum displacement of 110 m). The fault F₄ trending near EW, the largest size, also has the largest horizontal displacement of 550 m.

The vertical fault throw is an important index^[22] for quantitative analysis of fault activity strength. On the seismic section, the faults have larger vertical throw and obvious discontinuity of seismic events in Cambrian, while the faults have smaller vertical throw and no obvious disconnection in Permian, and sometimes only twisted events, such as F₆ (Fig. 5). By measuring the vertical throw of the same fault in different horizons, it is found that the vertical throw is the largest at the Cambrian bottom, followed by that at the Longwangmiao Formation bottom, while the vertical throw at the Permian Longtan Formation bottom is much smaller than the former two (Fig. 8a, 8b). The Permian throw shows a cliff-like drop rather than a gradual one compared with the Cambrian throw, such as fault F₁ (Fig. 8c). This suggests that the variation of vertical throw of the Paleozoic fault in the study area is resulted from the faulting intensity difference in different stages, rather than gradual reduction upward caused by late faulting.

In summary, the strike-slip strength of the nearly EW trending faults is the strongest, with the maximum horizontal displacement of about 550 m, while that of the NE trending faults is the weakest. The Cambrian faults are much stronger in activity strength than the Permian ones.

Sichuan Basin is a superimposed intracratonic basin^[23] experiencing multiple extensional-compressional tectonic cycles, so there develop multi-stages of faults. In order to study the formation and evolution of strike-slip faults in Central Sichuan, first the active stages of faults were defined, and then the formation and evolution process of strike-slip faults in the study area were described based on regional tectonic background.

Defining fault active period has always been a difficult problem in the field of structural geology^[17]. Based on the investigation of tectonic evolution background of Central Sichuan area, the active periods of strike-slip faults in the study area were determined by using the differences of fault structural styles in different strata and horizon cut through by fault jointly.

3.1.1. Differences of fault structural styles in different strata

Faults active in multiple stages would give rise to different structural styles in different stages^[2,3]. For strike-slip faults, an activity is a process of tectonic stress release, giving birth to flower or half-flower structure. Strike-slip faults active in multiple stages would form flower structures stacking over each other, or “flower-over-flower” structure, by which the active stages of the fault can be inferred^[24]. For example, according to the superposition of half-flower structures of 2 stages identified on the F₁₄ section (Fig. 9a), the lower one below the Permian bottom and the upper one mainly below the Permian Longtan Formation, it can be inferred that the faults experienced two evolution stages, before the Permian and before deposition of the Upper Permian Longtan Formation, respectively.

3.1.2. Horizons cut through by faults

The strike-slip faults in the study area mainly cut through 2 important unconformity surfaces: the Permian bottom and Longtan Formation bottom.

The Permian bottom is a truncated unconformity formed by denudation after folding and uplifting of Gaoshiti-Moxi structure, and the Permian overlies the Cambrian directly. Some strike-slip faults (such as F₁₅, F₁₈, and F₂₀) in the study area cut across the Cambrian and end at the Permian bottom (Fig. 9a), so it is speculated that the strike-slip faults developed before the deposition of the Permian. To determine the exact formation time of faults, the boundary fault of Moxi structure F₁, a fault active in multiple stages, was analyzed in this work. The western segment of F₁ is a transtensional strike-slip fault, and the eastern segment is the southern boundary of Moxi structure, which appears as a steep and erect reverse fault on the seismic section (Fig. 9b). But transtensional faults similar to those in the western segment are found in the upper part of F₁ in the eastern segment, indicating that the eastern segment of F₁ fault was a tensional fault in the early stage, and transformed into a reverse fault with uplifting of Gaoshiti-Moxi structure by compressional process in the late stage. The northern boundary fault F₂ of Gaoshiti structure also has similar characteristics (Fig. 5). Therefore, it is confirmed that the strike-slip faults in the study area were formed earlier than folding uplift of Gaoshiti-Moxi structure. Previous studies showed that the Gaoshiti-Moxi structure was formed in the late Caledonian period^{[11, 14]}, so the transtensional strike-slip faults in the study area were formed before the late Caledonian and after deposition of the Cambrian. Combined with the regional tectonic evolution background, the strike-slip faults are deemed to be formed in the early Caledonian.

The Longtan Formation bottom of the Upper Permian is an unconformity formed by the Dongwu movement. Although no obvious truncation is observed, eroded channels and valleys are identified at the top of the Maokou Formation on the seismic section. Some strike-slip faults (e.g. F₂ and F₁₉) extend across the Sinian-Cambrian (Fig. 9a) and end at the bottom of the Longtan Formation, indicating that strike-slip faults were formed before deposition of the Upper Permian, i.e. the Late Hercynian.

Based on the above two methods, it is concluded that the active stages of strike-slip faults in Gaoshiti-Moxi area were mainly early Caledonian and late Hercynian periods.

The strike-slip faults in Gaoshiti-Moxi area are transtensional strike-slip faults generated by oblique tension of the pre-existing weakness zones in Xingkai and Emei Taphrogenesis. Specifically, in the early Caledonian, influenced by Xingkai taphrogenesis, Gaoshiti-Moxi area was in intense extension geodynamic background^[25,26], where the principal compressive stress (σ₁) was nearly erect, the intermediate principal stress (σ₂) and the minimum principal stress (σ₃) were horizontal. As Deyang-Anyue intracratonic rift trended near SN during the Early Cambrian, it is inferred that in the Early Caledonian, Sichuan Basin was in NEE-SWW extension with the minimum principal stress σ₃ along NEE-SWW direction. There exists pre-existing basement weakness zones in the study area, such as Pre-Sinian rift^[27,28] and basement faults^{[10, 29-30]}. According to previous research, the early basement weakness zones trend NE in the south of the study area, and near EW, NW in the north (Fig. 10a). Under the effect of NEE-SWW oblique tension, these basement weakness zones generated transtensional strike-slip faults in oblique arrangement in the Sinian-Cambrian strata, trending near EW, NW and NE (Fig. 10b, 10c), which are similar to those formed in the late stage of Reykjanes rift and Gulf rift described in foreign literatures^[31,32].

In the late Hercynian period, influenced by Emei taphrogenesis, Central Sichuan was in a tension stress field again^[33,34]. Based on the NW-SE distribution pattern of Yanting-Tongnan trough in Central Sichuan^[35], it is inferred that Central Sichuan was in a NE-SW tension stress field during the late Hercynian period, where the minimum principal stress was along the NE-SW direction (Fig. 10d). Under the oblique tension, the pre-existing strike-slip fault (the early Caledonian main fault) was re-activated, giving rise to some normal faults with small horizontal displacements in the Permian strata (Fig. 10e). These faults mainly trend near EW, and NW, but no faults trend NE (Fig. 10f). Moreover, three pairs of NW-SE trending normal faults were newly generated in the northern part of the study area, which appear as a graben held by two normal faults on the section. These newly-generated faults are the direct results of NE-SW tension during the late Hercynian period.

The development of strike-slip faults in Gaoshiti-Moxi area in central Sichuan is of great significance for gas accumulation in this area. It can be explained from the following two aspects.

(1) The development of faults is helpful to improve physical properties of reservoir. The faulting-related fractures can not only improve the reservoir permeability, but also facilitate the formation of dissolved pores by leaching of meteoric fresh water. Previous studies have revealed that the dissolved pore type reservoir in the Longwangmiao Formation in Gaoshiti- Moxi area was formed when exposed after uplifting of paleo-uplift during the Carboniferous - Early Permian period^[36], and the strike-slip faults developed in the Early Caledonian period was conductive to the leaching of meteoric freshwater into the Longwangmiao Formation covered by the 20-30 m thick Xixiangchi Formation, and thus the creation of dissolved pores. In addition, high-angle fractures trending NW and near EW were developed in the Longwangmiao Formation, especially near the strike-slip faults. These fractures improved the overall percolation capacity of reservoir and increased the porosity and permeability^[37]. The permeability of beach bodies in the Longwangmiao Formation near the strike- slip faults is (3.246-13.000)×10^-3 μm², while the permeability of beach bodies far from strike-slip faults is generally lower than 1×10^-3 μm² .

The development of karst reservoirs in the Permian Maokou Formation in Gaoshiti-Moxi area also reflects the controlling effect of strike-slip faults on reservoir development. The karst reservoir in Maokou Formation is characterized by low amplitude and low frequency in seismic data, so the distribution of karst reservoirs in the Maokou Formation can be accurately predicted according to the root mean-square amplitude attribute^[38]. The Maokou Formation reservoirs are distributed consistently with strike-slip faults (Fig. 11), mainly in Moxi area to the north, and reservoirs are more developed near faults, while karst reservoirs are not developed in Gaoshiti area to the south.

(2) As faults are the main pathways of oil and gas migration, the strike-slip faults active in multiple periods resulted in multi-layer gas bearing situation in the study area. There are two sets of high-quality source rocks in Central Sichuan^[39], mudstone in Member 3 of Sinian Dengying Formation and shale in the Qiongzhusi Formation of Lower Cambiran. The strike-slip faults widely developed in the area connected multiple reservoir-cap assemblages, including the Sinian Dengying Formation, Cambrian Longwangmiao Formation, Qixia Formation and Maokou Formation of Permian, forming multi- layer gas bearing situation in the Sinian-Palaeozoic. It is also confirmed by the current drilling data. Many wells in Gaoshiti-Moxi area obtained favorable gas logging shows and even high-yield gas flows from the Permian Qixia Formation-Maokou Formation and also high oil and gas flows from Sinian-Cambrian, the distribution of these gas-producing wells are closely related to strike-slip faults. A total of 6 wells (Fig. 11) in the study area obtained high-yield gas flow from the Qixia Formation-Maokou Formation, of which 5 wells are near faults. Gas reservoir is identified in the Qixia Formation-Maokou Formation of 8 wells (Fig. 11) based on logging data, and except for Well G23 in the south, the rest are all 2 km from strike-slip faults. The above evidences all prove that the development of Paleozoic transtensional strike-slip faults caused the multiple layer gas-bearing situation in Gaoshiti- Moxi area.

To sum up, there are multiple gas-bearing layers in the Sinian-Permian strata in Gaoshiti-Moxi area, which lays the foundation for multi-layer stereoscopic exploration. Because of development of a large number of strike-slip faults and superior source-reservoir configuration in the area north of Moxi, more oil and gas resources may be discovered from the Sinian-Cambrian there.

Transtensional strike-slip faults developed during the Paleozoic period in Gaoshiti-Moxi area have the following geometric characteristics: (1) They are mostly transtensional faults on the section, in 3 structural styles: steep and erect, flower structure, "Y" and reversed "Y". (2) On the plane, there develop strike-slip faults extending linearly in near EW, NW and NE strikes in Cambrian; the whole fault is composed of multiple secondary faults distributed in en-echelon arrangement. Faults in Permian are largely in near EW and NW strike, and mainly distributed in the middle and east of the study area, along the underlying Cambrian faults. And they are more in the north than in the south.

The strike-slip faults in the study area have the following kinematic characteristics. (1) The near EW and NW trending faults are dextral strike-slip faults, and the NE trending faults are sinistral strike-slip faults. (2) The strike-slip strength of near EW trending faults is the strongest, with the maximum horizontal displacement of 550 m, while the strike-slip strength of the NE trending fault is the weakest. (3) The activity of the Cambrian faults is stronger than the Permian faults.

The transtensional strike-slip faults are generated by pre- existing tectonic weakness zones under oblique extension in Xingkai and Emei taphrogenesis, and mainly experienced the early Caledonian period and the late Hercynian period tectonic movements. In the early Caledonian period, under extension setting of Xingkai taphrogenesis, transtensional strike-slip faults were formed in the Sinian-Cambrian when tectonic weakness zones of the basement underwent the NEE-SWW oblique tension. In the late Hercynian period, under extension setting of Emei taphrogenesis, some main faults developed in the early Caledonian period were reactivated, generating strike-slip faults in the Permian distributed along the underlying Cambrian faults.

The strike-slip faults and the associated fractures enhanced the porosity and permeability of the reservoirs in Longwangmiao Formation and controlled the distribution of karst reservoirs in the Permian Maokou Formation. As the main pathways of oil and gas migration, multi-stage activity of strike-slip faults results in multi-layer gas bearing situation in the study area.