The Sichuan Basin is a key area for increasing natural gas reserves and production in China. The Triassic Xujiahe Formation (T₃x) in the western Sichuan depression is an important exploration and exploitation object in the continental domain of the Sichuan Basin^[1,2,3], with huge resources but low utilization rate, and it is difficult to upgrade the reserves. By the end of 2019, a total of 7379.8×10⁸ m³ of 3P geological reserves in the Xujiahe Formation of the western Sichuan depression had been submitted, of which a total of 1250.44× 10⁸ m³ of proven reserves, 1104.65×10⁸ m³ of controlled reserves and 77.39× 10⁸ m³ of produced reserves in the 2^nd Member of the Xujiahe Formation (T₃x²) of Xinchang tectonic zone had been submitted, with a relatively low overall utilization rate (only 6.39%). Early exploration practices have proved^[4,5,6] that the degree of fracture development is the main factor controlling high production of gas wells, and the quality of matrix reservoir is the main factor controlling moderate and stable production of wells. The previous researches mainly focused on fractured reservoirs near faults, and a number of wells with high and stable production have been completed. With the deepening of exploration and development, the fracture- pore type reservoirs, which are relatively far away from faults, with more fractures and good quality matrix, have become the focus of our research and production in the next step. And the key to evaluating the fracture-pore type reservoir and availability of reserves in it lies in the understanding on lithofacies.

The concept of lithofacies was first proposed by Miall in 1977^[7]. According to the fluvial facies sedimentary system, he put forward 18 types of lithofacies and 9 kinds of main configuration elements^[8], and supplemented and improved it in 2006 and 2014 respectively^[9,10]. Lithofacies is a kind of rock or rock association formed in a certain sedimentary environment, and the main component of sedimentary facies^{[7, 11-12]}.

Through the study of lithofacies, we can have good understandings on the influences of original sedimentary hydropower, energy and diagenesis etc. on the reservoirs^[13,14]. Previous studies have proved that the main sedimentary microfacies of T₃x² in the Xinchang tectonic zone is underwater distributary channel^[15,16], but different channels and different positions in the same channel differ widely in reservoir quality, so it is necessary to further characterize lithofacies on the basis of sedimentary microfacies. There are several types of fractures in the study area, of which low-angle fractures are mostly controlled by lithofacies and are related to sedimentation. These low-angle fractures are open underground, contributing to the productivity to a certain extent. In order to study this type of geological target, it is necessary to study the lithofacies. In addition, the study area has large geological reserves, low utilization rate and large productivity difference, and sediment lithofacies is the material basis determining productivity difference^[17,18], therefore, it is of great significance to evaluate high-quality reservoirs based on favorable lithofacies. In this work, the lithofacies of T₃x² gas reservoir in Xinchang tectonic zone of the Sichuan Basin has been investigated, the identification standard and evaluation system of favorable lithofacies have been established, the planar distribution law of favorable lithofacies has been clarified, and the main controlling factors of productivity have been sorted out, providing strong support for the effective utilization of T₃x² gas reservoir in Xinchang tectonic zone.

The western Sichuan depression is located in the west of the Sichuan Basin, bounded by Longmenshan thrust belt and Longquanshan uplift belt (Fig. 1a), and is a superimposed basin formed since Late Triassic^[19,20,21]. Xinchang tectonic zone in the study area is located in the central-northern part of Sinopec exploration area in the Western Sichuan Depression, bounded by Mianyang in the north and Deyang in the south, with an exploration area of about 3000 km². So far, 44 wells have penetrated/revealed T₃x² in the study area, of which 30 are coring wells, with quite complete analysis and testing data. And three-dimensional seismic data cover the whole area (Fig. 1b).

The Xinchang tectonic zone is a NEE-SWW trending long axis compound anticline with multiple structural highs. Its major part develops two groups of faults (EW and SN). Of them, the EW-trending faults control the anticline shape and are third order faults; the SN- trending faults control the internal structure amplitudes of the anticline, and are 4th order faults and major production-controlling faults of the gas reservoirs in T₃x² in Xinchang tectonic zone. The formation time of the Xinchang tectonic zone has always been controversial^[22,23]. We think that its major part was formed after Jurassic sedimentation. This is because that during Jurassic, the whole western Sichuan depression had a nearly SN-trending long-axis provenance^[24,25,26], while the Xinchang tectonic zone was in a relatively flat terrain (Fig. 2a), thus the sediments could cross this zone and advance to the middle of the depression.

The T₃x² Member in the Xinchang tectonic zone is the target layer of this study (Fig. 2b). It can be divided into three sub-members from bottom to top: the lower sub-member (T₃x²₃), the central submember (T₃x²₂) and the upper submember (T₃x²₁). According to the theory of high-resolution sequence stratigraphy, its interior can be subdivided into 10 sand groups (Tx₂¹—Tx₂¹⁰), and each sand group corresponds to 1-2 short-term cycles. During the depositional period of the T₃x² Member in the Xinchang tectonic zone, the main sedimentary facies was underwater distributary channel in braided river delta front. The channels of various periods overlap with each other, resulting in sand bodies stacking vertically and piecing together laterally. During the depositional period of the T₃x²₂, the water energy was the strongest, followed by T₃x²₁, and that of the T₃x²₃ was the weakest. The T₃x² Member is mainly composed of medium-thick linen lithic sandstone and arkose with thin black mudstone interlayers. The medium-coarse grained sandstone is mainly distributed in the middle and upper sub-members, which further verifies that the water body energy was stronger in the sedimentary period of the T₃x²₂ and the T₃x²₁ sub-members.

The sand bodies in the T₃x² Member of Xinchang tectonic zone stack over each other and come together into large pieces with large thicknesses. According to the current drilling results, its average cumulative thickness is about 420 m. Several wells were tested in long intervals, with a perforated interval thickness of about 41.7 m on average and 489 m at maximum. It is very important to find out the types and characteristics of the real gas producing reservoirs in thick sand bodies. The previous studies mainly focused on the distribution of sedimentary microfacies and sand bodies in the T₃x² Member, but less on the reservoir configuration units inside the sedimentary microfacies. The study of lithofacies, on the one hand, can give us a more precise understanding on underground geological bodies, and on the other hand, guide the selection of development and evaluation targets and deployment of well locations.

By observing cores of the T₃x² Member from eight wells, we totally identified 12 types of lithofacies (Table 1, Fig. 3), namely, multiple layer medium-coarse grain sandstone lithofacies, parallel bedding medium-coarse grain sandstone lithofacies, massive bedding medium-coarse grain sandstone lithofacies, mud gravel-bearing medium-coarse grain sandstone lithofacies, coaly debris-bearing medium-coarse grain sandstone lithofacies, inclined bedding medium-coarse grain sandstone lithofacies, inclined bedding fine grain sandstone lithofacies, coal bed lithofacies, wavy bedding siltstone lithofacies, and carbonaceous mudstone lithofacies, etc. Different from T₃x³₁, T₃x²₂ is charactrized by development of multiple layer medium-coarse grain sandstone. The classification of lithofacies types is mainly based on the granularity and sedimentary structure etc. of sedimentary rocks (Table 1). The nomenclature of lithofacies refers to Miall's understanding on lithofacies^[7,8]. In addition, considering the influence of diagenesis, the degree of cementation is taken into consideration in classification of the lithofacies types.

Different lithofacies generally appear in different microfacies in a certain combination form (Table 2). In the underwater distributary channel microfacies, generally, coaly debris- bearing/coaly medium-coarse grain sandstone, multiple layer/parallel bedding medium-coarse grain sandstone, inclined bedding medium-coarse grain sandstone, massive bedding medium-coarse grain sandstone, and wavy bedding medium-coarse grain sandstone lithofacies come up from bottom to top, indicating the process of hydrodynamics changing from strong to weak. The interdistributary bay generally occurs in the middle of distributary channels and is mainly composed of wavy bedding siltstone, mudstone, carbonaceous mudstone, thin coal seam and other lithofacies. The mouth bar mainly turns up at the bifurcation of the river channel. After long-term washing by waves, the sediments in mouth bar are generally medium-fine in grain size, and mainly composed of inclined bedding fine sandstone, wavy bedding siltstone and other lithofacies.

2.2.1. Medium-coarse grain lithofacies

The multiple layer medium-coarse grain sandstone lithofacies (Figs. 3a and 4) is mainly composed of grayish white medium-coarse grained sandstone, with high-density (about tens of beddings per meter), low-angle parallel bedding and trough bedding, and is crisp cake-shaped. With higher quartz content, this lithofacies has good reservoir physical properties, with a permeability range of (2.3-33.2)×10^-3 µm² (on average 15.4×10^-3 µm²). Its genesis is still controversial in the academic circle. We think that this type of lithofacies with mainly sedimentary bedding seam, is generally medium-coarse grain sandstone with parallel bedding and trough cross bedding. And this type of fracture is caused by stress change. The parallel bedding medium-coarse grain sandstone lithofacies (Figs. 3b and 4) is mainly grayish white medium-coarse grained sandstone with parallel bedding. Compared with multiple layer medium-coarse grain sandstone lithofacies, it has lower bedding density, and cores of this lithofacies don’t fracture along bedding planes. This lithofacies has quite good physical properties, with an average porosity of 5.6% and the highest porosity of 9.0%. The multiple layer medium-coarse grain sandstone lithofacies and parallel bedding medium-coarse grain sandstone lithofacies occur mostly in the T₃x²₂ sub- member, and the Tx₂⁴ sand group is the most typical one.

The massive bedding medium-coarse grain sandstone lithofacies is dominated by grayish white medium- coarse grained sandstone, massive, with no obvious sedimentary structures (Figs. 3c and 4). It generally occurs in the middle of channel sand body. It has better physical properties, with a porosity of 1%-12% and 5.1% on average. The inclined bedding medium-coarse grain sandstone lithofacies has mainly inclined beddings of different scales (Fig. 3d). Similar to massive beddings medium-coarse grain sandstone lithofacies, inclined bedding medium-coarse grain sandstone lithofacies usually occurs in the middle-lower parts of channel sand body. It has better physical properties, with a porosity of 2.2%-11.8% and 4.0% on average. The coaly debris-bearing medium-coarse grain sandstone lithofacies has thin coaly debris, and usually appears at the bottom of channel sand body (Figs. 3e and 4). It has better physical properties, with an average porosity of 3.8%. The mud gravel-bearing medium-coarse grain sandstone lithofacies contains many mud gravels in tearing style, and generally appears at the bottom of channel sand body (Fig. 3f). It has moderate physical properties with a porosity of 0.4%-6.8% and 2.1% on average. The above four types of lithofacies are seen in both the T₃x²₂ sub-member and T₃x²₁ sub-member, such as the Tx₂², Tx₂⁴and Tx₂⁵ sand groups.

2.2.2. Fine-grained and calcarinate lithofacies

In addition to the medium-coarse grained lithofacies types, lithofacies types with fine grain size or strong calcareous cementation are also identified in the study area. Inclined bedding fine grain sandstone lithofacies (Fig. 3g) is dominated by grayish white fine grained sandstone with inclined beddings. It has poor physical properties, with an average porosity of 2.5% and an average permeability of (0.001-2.800)×10^-3 µm². The wavy bedding siltstone lithofacies (Fig. 3h) is mainly grey black siltstone, with wavy bedding and sand ripple bedding. It has poor physical properties, with an average porosity of 1%. The calcarinate inclined bedding medium-coarse grain sandstone lithofacies (Fig. 3k) is mainly calcarinate interval in the inclined bedding medium-coarse grain sandstone lithofacies, which produces gas bubbles violently when acid is applied. This lithofacies is mainly composed of grayish white medium-coarse grained sandstone with inclined bedding. It has a lower porosity (1.7%-2.5%). The calcarinate massive bedding medium-coarse grain sandstone lithofacies (Fig. 3l) is mainly the calcarinate interval in the massive bedding medium-coarse grain sandstone lithofacies. It has poorer physical properties with a porosity of 0.5%-4.3%. Due to the differences of diagenesis, calcarinate sandstone lithofacies more likely occur in the sand bodies with obvious water-rock reaction, such as the middle and lower parts of a channel near mudstone.

Based on the description of lithofacies by cores and the analysis of test data, the favorable lithofacies in different sand groups were selected. The favorable lithofacies discussed in this paper generally refer to the lithofacies with good physical properties, high gas content and certain contribution to productivity. Based on well logging data, the identification standards of favorable lithofacies in different sand groups have been established, and the spatial distribution characteristics of the favorable lithofacies have been figured out. The identification of favorable lithofacies has certain guiding significance for selecting favorable development target areas and well locations in the T₃x² Member of Xinchang tectonic zone.

Based on the analysis of relationships between the "four properties", lithology, physical property, electrical property and gas-bearing property of different lithofacies in the T₃x² Member of Xinchang tectonic zone, the favorable lithofacies were selected by considering physical properties, gas-bearing property and diagenesis of the reservoirs. The overall analysis results show that the favorable lithofacies of the T₃x² Member in Xinchang tectonic zone have coarser grain size, better physical properties and better-developed beddings indicating strong hydrodynamics. Due to the different hydrodynamic intensities during the deposition of middle sub-member (T₃x²₂) and upper sub-member (T₃x²₁), the favorable lithofacies types in them are also different. The T₃x²₂ is different from the T₃x²₁ in that it has multiple layer/parallel bedding medium-coarse grain sandstone lithofacies.

Taking the T₃x²₂ sub-member (Fig. 5) as an example, it has five types of favorable lithofacies, namely, multiple layer parallel bedding medium-coarse grain sandstone, parallel bedding medium-coarse grain sandstone, massive bedding medium-coarse grain sandstone, inclined bedding medium- coarse grain sandstone, and coaly debris-bearing medium- coarse grain sandstone lithofacies. The reasons are: (1) In terms of physical properties, all these five types of lithofacies have a porosity of more than 4% and permeability of more than 0.1×10^-3 µm², ranking top five among all types of lithofacies. (2) In terms of gas-bearing property, the parallel bedding medium-coarse grain sandstone, massive bedding medium-coarse grain sandstone and inclined bedding medium- coarse grain sandstone lithofacies all have gas contents of more than 25%, ranking top three among all types of lithofacies. (3) The multiple layer parallel bedding medium-coarse grain sandstone, inclined bedding medium-coarse grain sandstone and coaly debris-bearing medium-coarse grain sandstone lithofacies all have higher quartz and feldspar contents. (4) These five types of lithofacies have low values of natural gamma (GR), resistivity (RT) and neutron porosity (CNL) in electrical property, indicating they have better reservoir physical properties and gas-bearing property.

On the basis of core analysis, according to the identification results of grain size and mineral composition from well logging data, we identified favorable lithofacies of the T₃x² Member in Xinchang tectonic zone using well logging data. Taking the T₃x²₂ sub-member as an example, the major favorable lithofacies in it include multiple layer parallel bedding medium-coarse grain sandstone lithofacies, parallel bedding medium-coarse grain sandstone lithofacies, massive bedding medium-coarse grain sandstone lithofacies, inclined bedding medium-coarse grain sandstone lithofacies, and coaly debris-bearing medium-coarse grain sandstone lithofacies. These lithofacies are characterized by good physical properties, high gas content and superior reservoir quality. In this study, GR, CNL, AC and RT curves were used jointly to identify the favorable lithofacies. Previous researchers have done a lot of studies on diagenetic facies based on well logging methods^[27,28,29], which provide good reference for identifying favorable lithofacies by well logging data in this study. Among the favorable lithofacies of the T₃x²₂ sub-member, the multiple layer/parallel bedding medium-coarse grain sandstone lithofacies is the most favorable one, thus we identified it carefully in this research. The identification process is mainly based on the idea of "selecting the best from the good step by step". Firstly, all the silty and fine-grained lithofacies were excluded. Secondly, favorable lithofacies was selected from the medium-coarse grain lithofacies. Finally, the optimal lithofacies was selected from the favorable lithofacies. The detailed process is as follows.

3.2.1. Removing fine and silty lithofacies

By examining the GR and AC crossplot of different lithofacies in the T₃x²₂ sub-member in Xinchang tectonic zone, it is found that fine-grained lithofacies and coarse-grained lithofacies have distinctive differences (Fig. 6a). The two types of lithofacies can be easily distinguished by taking GR value of 77 API as the cut-off point.

In this chart, the favorable lithofacies are characterized by low GR and high AC. Their point groups are mainly distributed at the bottom of the chart, and some data points are distributed in the lower right corner. But when the lithofacies contains mud gravels and a small amount of charcoal, the GR value will increase, for example, the mud gravel-bearing medium-coarse grain sandstone lithofacies and carbonaceous debris- bearing medium-coarse grain sandstone lithofacies. The multiple layer/parallel bedding lithofacies, carbonaceous mudstone lithofacies and carbonaceous debris-bearing medium-coarse grain sandstone lithofacies have higher AC values, which is mainly caused by bedding seam and grain-edge fracture.

3.2.2. Selecting favorable lithofacies

Fine-grained lithofacies can be removed by using GR value of 77 API as the limit value. But even the coarser lithofacies differ considerably in reservoir characteristics. We firstly removed the calcarinate lithofacies (above L₂ and L₃) based on the crossplot of AC and RT (Fig. 6b). Secondly, we identified favorable lithofacies in the middle zone between L₁ and L₂, such as multiple layer bedding, parallel bedding, massive bedding, inclined bedding and coaly debris-bearing medium-coarse grain sandstone lithofacies.

3.2.3. Selecting high-quality lithofacies

Among the favorable lithofacies (the middle zone between L₁ and L₂, Fig. 6b), L₃ and L₄ were overlaid to identify the multiple layer/parallel bedding medium-coarse grain sandstone lithofacies, namely, the parallelogram region defined by L₁, L₂, L₃ and L₄ in Fig. 6b.

On the basis of identification of favorable lithofacies by cores and well logging data, the plane distribution characteristics of favorable lithofacies in the sub-members were investigated.

Taking Tx₂⁴ sand group as an example (Fig. 7), the research results show that: (1) The distribution trend of favorable lithofacies and high-quality lithofacies is consistent with sand body distribution direction, including the northeast and northwest directions, etc. (2) The favorable lithofacies are 10-60 m thick, 33 m thick on average. Well areas of XC12 and X856 have larger thickness of favorable lithofacies (about 30-60 m). (3) Striking northeast and northwest etc., the high-quality lithofacies are 4-25 m thick (11 m on average), and thicker in Well areas X3 and L150 (about 22 m).

By looking at the profile through wells X10-XC8-X601 (Fig. 8) closely, it is found that the favorable lithofacies, largely multiple layer bedding lithofacies in T₃x²₂ sub-member, are mostly distributed in bands, and occur near the bottom of sand body or near mudstone.

In actual production, lithofacies, as the basis of oil and gas reservoirs, has always been a research focus of oil and gas prospectors. For instance, the multiple layer/parallel bedding medium-coarse grain sandstone lithofacies, also called “crisp cake-like” lithofacies by predecessors^{[23, 30]}, is often an important target in exploration and exploitation. However, there are few reports on the genetic mechanisms of these favorable lithofacies. Based on the above research, we think that the favorable lithofacies of the tight sandstone in the T₃x² Member in Xinchang tectonic zone are mainly controlled by sedimentation, diagenetic fluid activity and later tectonic action, etc.

4.1.1. Joint control of sedimentation process, diagenetic fluid and later tectonic action on the formation of favorable lithofacies

Sedimentation (Fig. 9a) determines the spatial distribution of sand bodies. Hence, it is the basis and the most important factor for the formation of favorable lithofacies. In the case with large accommodation space, the single-stage channel is well developed, and thin isolated lenticular channel sand bodies and plastic mudstone layers are interbedded. Generally, the multiple layer/parallel bedding medium-coarse grain sandstone lithofacies occurs at the bottom of the channel sand body, and gradually transits upward to inclined bedding and massive bedding medium-coarse grain sandstone lithofacies, for example, Tx₂⁴ sand group. In the case with small accommodation space, multi-stage channels are superimposed on each other, and at the bottom and top of a set of sand bodies near plastic mudstone are mostly multiple layer/parallel bedding lithofacies, such as Tx₂² sand group. As the burial depth increases, the activity of diagenetic fluid enhances (Fig. 9b). At different positions of the sedimentary system, the epidiagenetic fluids reform differentially the favorable lithofacies. Core observation shows that the multiple layer medium- coarse grain sandstone lithofacies is mostly loose (Fig. 3a), with obvious local gas invasion, which indicates that it has experienced a certain degree of reformation by fluids. The late tectonic activity (Fig. 9c) would cause local bedding sliding of the strata reformed by the early diagenetic fluid, producing fractures in the multiple layer medium-coarse sandstone with certain opening. Because of different compressive strengths of sandstone and mudstone, fractures are more likely to occur in the side near plastic mudstone of a set of sand body under structural compression.

4.1.2. Configuration pattern of sandstone and mudstone determines the development position of favorable lithofacies

Most favorable lithofacies in the gas reservoir of the T₃x² Member in Xinchang tectonic zone are medium-coarse grained. The reservoir with coarser granularity would retain more primary pores during diagenesis, so the lithofacies of medium and coarse granularity are mostly favorable lithofacies. In the same medium-coarse grain lithofacies, in the middle sub-member, the sedimentary water body had stronger hydropower and sedimentary structures such as parallel bedding and inclined bedding; the water-rock interaction was more likely to occur near mudstone (Figs. 4 and 9a)^[31,32,33], and the multiple layer/parallel bedding medium-coarse grain sandstone lithofacies formed by frequent fluid activities is the most favorable. In the upper sub-member, the sedimentary hydropower was weaker, and the sedimentary structure is mainly thick and rapidly cumulative massive bedding; generally, the massive bedding medium-coarse grain sandstone lithofacies in the middle of a set of sand body has better physical properties and is the most favorable lithofacies.

Through the fitting and crossplot analysis of geological, geophysical, engineering reformation and other parameters in the tested intervals with daily gas production and cumulative gas production in the stable production period, it is found that the favorable lithofacies thickness has a strong control on stable production of gas wells. Through comparion, it is found that the reservoirs in the area with abundant favorable lithofacies have better physical properties and are mainly medium-coarse grained sandstone, indicating the geological bodies deposited under stronger hydropower condition.

To further clarify the relationship between favorable lithofacies and productivity, based on the evaluation and prediction results of favorable lithofacies in the T₃x² Member in Xinchang tectonic zone, the thicknesses of favorable lithofacies of various intervals were counted. Combined with the dynamic productivity data of 19 typical wells, the favorable lithofacies thicknesses in the tested intervals were fitted with the cumulative gas production in the stable production period of the gas wells (Fig. 10). The result shows that in the middle and upper sub-members of the T₃x² Member, there is a positive power correlation between the two, with high coincidence and a multiple correlation coefficient of 0.94. The analysis shows that the favorable lithofacies is a key parameter to evaluate the stable production capacity of gas wells in the gas reservoirs of the T₃x² Member in Xinchang tectonic zone. The favorable lithofacies reflects multiple aspects of information, such as reservoir granularity and physical properties to some extent. With the increase of its thickness, the gas wells become stronger in stable production capacity.

Based on the core description, identification and evaluation by well logging data of favorable lithofacies, the development model of favorable lithofacies in the T₃x² Member in Xinchang tectonic zone has been figured out (Fig. 11), which has the following characteristics: (1) The favorable lithofacies can be divided into two types: the typical massive bedding type in the upper sub-member and the typical multiple layer/parallel bedding type in the middle sub-member. (2) The distribution of favorable lithofacies is controlled by sedimentary sand bodies. (3) The Tx₂⁴ sand group mainly has multiple layer/parallel bedding medium-coarse sandstone facies under the background of high-energy channel. (4) The Tx₂² sand group mainly has inclined bedding and massive bedding medium- coarse grain sandstone lithofacies under the background of middle-lower energy channel. (5) The combination of favorable lithofacies (such as multiple layer type) and high-angle fractures near faults is the major controlling factor for high production of gas wells.

The T₃x² Member in Xinchang tectonic zone of the Sichuan Basin has 12 types of lithofacies in total. Among them, the multiple layer parallel bedding medium-coarse grain sandstone, parallel bedding medium-coarse grain sandstone, massive bedding medium-coarse grain sandstone, and inclined bedding medium-coarse grain sandstone are the favorable ones. The favorable lithofacies in the T₃x² Member in Xinchang tectonic zone feature low values of GR, CNL and RT and high value of AC. The distribution of favorable lithofacies is controlled by sedimentary sand bodies. The Tx₂² sandstone group has mainly inclined bedding and massive bedding medium-coarse sandstone under the background of medium-low energy channel, mostly in the middle of the sand body. The Tx₂⁴ sandstone group has mainly multiple layer/parallel bedding medium- coarse grain sandstone lithofacies under the background of high-energy channel, mostly at the sand body bottom or near mudstone. Sedimentation determines the spatial distribution of sand bodies and is the material basis for the formation of favorable lithofacies. Epidiagenetic fluids reform differentially the favorable lithofacies. Structural activity and abnormal pressure in deep formations make the strata slide along the weak surfaces, resulting in fractures in multiple layer medium-coarse grain sandstone and other lithofacies, and thus improving reservoir permeability significantly. Under the current technical conditions, the productivity of gas wells in the T₃x² Member in Xinchang tectonic zone in stable production period is mainly controlled by the thickness of favorable lithofacies.