Accumulation Mechanism of Marine Shale Gas Reservoir in Anticlines: A Case Study of the Southern Sichuan Basin and Xiuwu Basin in the Yangtze Region

State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China Unconventional Natural Gas Institute, China University of Petroleum, Beijing 102249, China Unconventional Petroleum Collaborative Innovation Center, China University of Petroleum, Beijing 102249, China Research Institute of Petroleum Exploration and Development, Beijing 100083, China Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China Research Institute of Unconventional Oil & Gas and Renewable Energy, China University of Petroleum (East China), Qingdao 266580, China Key Laboratory of Strategy Evaluation for Shale Gas, Ministry of Land and Resources, Beijing, China Jiangxi Provincial Natural Gas Company Ltd., Nanchang 330000, China Geoscience Documentation Center, China Geological Survey, Beijing 100083, China Guangzhou Marine Geological Survey, Guangzhou 510075, China School of Earth Sciences and Engineering, Xi’an Shiyou University, Xi’an 710065, China


Introduction
In recent years, due to the transformation of geological thinking and the progress of hydraulic fracturing technology, shale gas has become a popular topic in oil and gas exploration and development at home and abroad [1][2][3][4][5]. In 2010,  Well was drilled in the Weiyuan structure, China, and the industrial breakthrough of shale gas resources in China was realized. Large-scale development has been achieved successfully in Chinese shale gas fields, such as Weiyuan, Changning, Fushun-Yongchuan, and Jiaoshiba in 2017 [6,7]. Although progress has been made in shale gas exploration in China, there are large differences among the shale gas contents in different blocks and there have also been many failures alongside the successes in shale gas exploration. There remains a need for petroleum geologists to further study the shale gas enrichment mechanism [8][9][10][11][12]. The mainland of China is made up of a series of relatively small paleo-plates. Compared to that in North America, the Paleozoic marine shale in southern China has undergone a complex, multistage tectonic evolution, and thermal evolution after deposition. Although its material conditions are similar, such as the TOC content and mineral composition, there are still differences in the gas-bearing properties in different blocks and layers under different preservation conditions. Therefore, it is vital to quickly solve the problem of how to precisely select favorable areas [13][14][15][16][17].
Previous work has been done on the preservation conditions of shale gas. Tang et al. divide the southeast part of the Sichuan Basin into seven tectonic deformation zones based on structural research and systematically reveal geologic factors such as fracture scale, fracture density, lithology and thickness of the roof and floor, burial depth of the target layers, structure style, and fracture spacing index to establish the evaluation index and standard for the effective preservation area of shale gas and the comprehensive evaluation system of preservation conditions for shale gas [18]. Wang et al. use the Niutitang Formation in the Cen'gong block, Guizhou, as an example [19]. Through the detailed interpretation of seismic data and structural evolution analysis, the shale gas preservation conditions are studied in combination with the material basis, the roof and floor, the regional caprock, the natural gas component, the formation water conditions, and the formation pressure. In addition, after studying the Lower Cambrian and the Upper Ordovician-Lower Silurian shale in the Sichuan Basin and its periphery, Wei et al. demonstrated that the roof and floor conditions and the self-seal ability of shale are the key for shale gas retention in the early stages [20]. Additionally, the intensity and duration of tectonic transformation in the late stage have an obvious adjustment effect on the gas abundance of shale gas reservoirs. Furthermore, there is a distinguishable influence of the preservation conditions on gas content, porosity, water saturation, resistivity, and gas composition of shale gas. The gas content, porosity, and resistivity are relatively lower, and the water saturation and N 2 content are relatively higher if the conditions are poor. These factors could be used as indirect indicators to evaluate the preservation conditions of shale gas.
The tectonic setting is an important aspect of the preservation conditions of shale gas [21]. Under the influence of continuous extrusion in the southeast direction, the steeply dipping anticlines and the adjacent wide synclines are usually developed [6,7]. In this paper, the geological factors and their mechanism of shale gas accumulation in anticlines were studied by considering 2 aspects: the angle of limbs and the burial depth, and the enrichment regulation is analyzed under conditions when anticlines and their adjacent synclines both exist. Thus, a shale gas enrichment model in anticlines is proposed.

Geological Settings
2.1. Tectonic Characteristics. The study area in the paper is the Yangtze area ( Figure 1). It has been shown previously that the Yangtze and Cathaysian plates were separated by an oceanic basin [22][23][24]. The Yangtze plate is a cratonic basin. The Yangtze and Cathaysian plates in the Early Cambrian were in the state of stretching and large-scale transgression, and a set of organic-rich shale was deposited almost throughout the Yangtze plate. Then, the water body gradually became shallow and the lithology changed from fine-grained shale and silty shale into siltstone and sandstone. The Ordovician was influenced by the extrusion collision of the Cathaysian plate. The water body continued to shallow, from the clastic sedimentary system to the carbonate sedimentary system. In the Late Ordovician-Early Silurian, transgression occurred, leading to a change into the clastic sedimentary system and to the deposition of a set of organic-rich shale in the deep-water shelf. In the Cambrian to the Silurian, the oceanic basin between the two plates gradually subducted and collided towards the Yangtze plate, and by the end of the Late Silurian, the two plates were formally joined together to become a unified Southern China plate.  [13,15,25,26]. The Lower Cambrian is a stratum widely deposited on the Yangtze plate. Due to its wide area, it is name depends on the location. The Lower Cambrian is named the Qiongzhusi Formation in the Sichuan Basin in the Upper Yangtze region, is the Niutitang Formation outside the Sichuan Basin, and is the Wangyinpu Formation in the Lower Yangtze region. It comprises black or deep-gray organic-rich siliceous shale that was deposited in the Early Cambrian and is one of the exploration targets of shale gas in Southern China. This paper illustrates the stratigraphic characteristics of the Lower Cambrian with the example of Jiangye-1 Well in the Xiuwu Basin, Lower Yangtze. As shown in Figure 2, the Lower Cambrian Wangyinpu Formation consists of a set of black siliceous shales, the underlying strata are the Upper Sinian Piyuancun Formation with gray siliceous dolomite, and the overlying strata are the Lower Cambrian Guanyintang Formation composed of a set of dark-gray siliceous 2 Geofluids

Sedimentary and Stratum
shale. And the Lower Cambrian Wangyinpu Formation is the main target area of shale gas exploration. According to previous studies [27,28], in the Early Cambrian, there were deposits on the ancient land, the shallow-water shelf, the deep-water shelf (intrashelf low), the continental slope, and the ocean basin in the Yangtze plate from the northwest to the southeast.

The Upper Ordovician-Lower Silurian. The Upper
Ordovician-Lower Silurian is another current exploration target in South China. Its name varies from place to place. In the Upper Yangtze region, the Late Ordovician stratum is the Wufeng Formation and the Early Silurian deposit is called the Longmaxi Formation. In the Lower Yangtze region, the Late Ordovician is named the Xinkailing Formation and the Early Silurian deposit is named the Lishuwo Formation. In this study, the research subject in the Upper Yangtze is the Wufeng Formation and the no. Based on previous studies [6,29,30], the Yangtze and Cathaysian plates converged in the Late Ordovician and the Early Silurian. The organic-rich shale formed in the area of the deep-water shelf of the Yangtze plate.

Experiment Section
3.1. Triaxial Unloading Test. The stress-strain process of unloading confining pressure under constant axial compression can be achieved by triaxial unloading testing, which is similar to the stratum loading process during uplift. The tests in this study used 16 samples from the Lower Silurian Longmaxi Formation shale in the Pengshui, Dingshan, and Shizhu blocks of the southeastern margin of the Sichuan Basin, as shown in Table 1. During the test, the axial pressure and confining pressure of the samples in the triaxial cylinder are increased in proper order, to prevent the samples from breaking during the pressurizing process. When the axial stress reaches the predetermined 3 Geofluids value (less than the triaxial compressive strength, but far greater than the uniaxial compressive strength of the rock sample), the axial stress is kept constant and the rock sample is gradually destroyed by only the confining pressure and the unloading rate is set as 0.1 MPa/s. Some of the test data have been compared to the findings of Li et al. [31] and Guo et al. [32].

Permeability Tests
Parallel and Perpendicular to the Stratification Plane. Ten samples were taken from the shale of the Lower Cambrian Wangyinpu Formation in the Lower Yangtze region, among which there are two samples from Jiangye-1 Well and eight samples from Jiangye-2 Well, as Table 1 shows in detail. The permeability tests parallel and perpendicular to the stratification plane are carried out by the overpressure permeability tester Poro PDP-200. The permeability test pressure was 1000 psi and the confining pressure was 1500 psi. In addition, the parallel and perpendicular permeability data are collected from He et al. [33].
The samples, 12 in total, were taken from Pengye-1 Well in the Pengshui block in the southeastern Sichuan Basin. The sample numbers are presented in Table 2.

FIB-HIM Experiment.
Focused ion beam-Helium ion microscopy (FIB-HIM) is a three-beam system which combines the imaging function of high-resolution helium ion microscopy (HIM) and the cutting function of focused ion beam (FIB) and neon ion beam.
In this paper, the imaging of micropores was conducted by the NanoFab ORION microscope made by Carl Zeiss, Jena, Germany. It has higher resolution than field emission scanning electron microscopy (FE-SEM). With imaging resolution of 0.5 nm, the NanoFab ORION can generate new insights with images that have 5 to 10 times greater depth of field when compared to images acquired with FE-SEM. Therefore, for tight rocks such as shale, in which pores with sizes lesser than 10 nm are absolutely predominant in number, the micropores that failed to be distinguished in the       Table 3. A Leica polarizing microscope and a THMSG600 geological heating/freezing system were used for the identification of inclusions in the samples.

Results and Discussion
4.1. In the Anticline 4.1.1. The Angle of the Limbs. According to the limb angles of the fold deformation, the deformation strength of the positive structural style can be represented. The greater the limb angle is, the weaker is the deformation and the fewer are the fractures produced by tectonic extrusion, leading to lower shale gas content. In contrast, the smaller the limb angle is, the stronger is the deformation, which causes considerable development of large fractures in various structural areas and accelerates the dissipation of shale gas [34].
In previous studies, the deformation strength is divided by standards of 70°and 120°. The deformation is weak if the limb angle is 120°-180°, intermediate if the angle is 70°-120°, and strong if the angle is less than 70° [35][36][37]. Another study gathered limb angle data in typical shale gas blocks. The Jiaoshiba and Fushun-Yongchuan blocks are anticlines with limb angles of 120°-180°, which leads to great preservation conditions. The formation pressure coefficient of Jiaoye-1 in Jiaoshiba and Yang-101 in Fushun-Yongchuan is in the range of 1.5-2.2 [38]. The angle of the limbs of the high and steep Huaying Mountain anticline is 70-120°in the eastern Sichuan Basin. Because of the strong extrusion, fractures frequently developed in the Upper Ordovician Wufeng Formation and in the Lower Silurian Longmaxi Formation in the Huadi-1 Well and they control the reservoir properties of the shale along with the organic pores. The shale gas migrates upwards along the fractures and stratifications. Due to the better sealing of the overlaying strata in the Silurian, mixture shale gas reservoir composed of adsorbed gas and free gas is formed through the conventional gas mechanism [38]. The Yuye-1 Well in the southeast Sichuan experienced a stronger extrusion for its location outside the Sichuan Basin. In addition, its limb angle is less than 70°a nd high-angle fractures developed, resulting in a large dissipation of shale gas and the extremely low gas content of the well [39].

Burial Depth
(1) Phenomenon. In addition to the properties of the anticline, burial depth has an important influence on shale gas enrichment. When shale is uplifted as a whole, the horizontal stress is constant and the burial depth gradually decreases. When shale is raised to a certain depth, the shale will break up with massive loss of shale gas, resulting in the pressure coefficient of the formation reduction [31,40,41]. The real-time drilling data shows the same phenomenon. In this paper, six wells, which are all in the Weiyuan anticline in the southern Sichuan Basin, were evaluated; they share a similar material basis and were drilled from the top to the limb, from shallow to deep. As shown in Figure 4, the pressure coefficient of the formation gradually increases with the increase in the burial depth.
(2) The Breaking Depth Calculation of Shale under Uplift. Theoretically, this is the confining pressure in the test constant, which is required to break the same rock in the same horizontal stress field. The breaking confining pressure under different stress fields can be modeled with unloading experiments, and then, an equation of breaking confining pressure and horizontal stress can be established. The breaking   Figure 1 for the well locations. 6 Geofluids confining pressure can be translated into the burial depth according to the following equation.
where H is the burial depth, with the unit of m, and P is the breaking confining pressure, with the unit of MPa. The horizontal stress field is simulated by the axial pressure, and the results of triaxial unloading tests are shown in Table 4. The results show that the confining and axial pressures are positively proportional when the shale fractured. The high axial pressure corresponds to the high breaking confining pressure, indicating that in the high horizontal stress zone, the burial depth is greater when the breaking of shale occurs.
Using statistics of the experimental data, Figure 5 is used to obtain the following mathematical regression equation of the burial depth (H, m) and the horizontal stress (S, MPa) when the rock breaks: H = 15 404S − 754 41 (with a correlation coefficient R 2 = 0 6834). Through the recovery of the regional stress field, the present horizontal stress in one block under the anticline background can be obtained. Through the regression equation, the depth of the large amount of gas loss caused by the shale fracture during the anticlinal uplift can be calculated.

The Combination of the Anticline and Adjacent Syncline.
The tectonic structure formed by the anticline and adjacent synclines easily exists in blocks that are subjected to compression but not large-scale erosion. This structural type is more common in the southern part of the Sichuan Basin [25,26,41]. Here, three aspects of the shale gas migration (dynamic, channel, and evidence) are discussed and the enrichment regulation of shale gas is studied in the structural combination of the anticline and the contiguous syncline.

Migration Dynamics.
In the structural combination, there is an obvious difference in the formation pressure coefficient between the anticline and the adjacent syncline. As shown in Figure 6, there is an anticline and a syncline in the Dingshan block at the same time. In this paper, the burial depth of the drilling and the formation pressure coefficient are measured and the residual formation pressure is calculated. As shown in Table 5, the residual pressure difference of the two wells is 23.81 − 1.67 = 22.14 MPa. In conventional oil and gas reservoirs, the residual pressure difference between the source rock and the reservoir of the overpressure basin is 20-30 MPa, which can cause the efficient migration of natural gas. In shale gas reservoirs, the difference of the pressure coefficient between the anticline and the adjacent syncline can also provide sufficient force for the migration Table 4: The results of the triaxial unloading experiments, including the breaking confining pressure and the burial depth calculated when a certain axial pressure is applied to the rock (namely, the horizontal stress). See Figure 1 for the well locations.   Figures 7 and 8. The result indicates that the permeability parallel to the stratification plane is 1-40 times higher than the permeability perpendicular to the stratification plane, even in different shale strata.
(2) The Connected Organic Pores. In this study, the FIB-HIM experiments were carried out on the organic-rich shale of the Lower Silurian Longmaxi Formation in the Jiaoye-1 well of the Jiaoshiba block. The experimental results are shown in Figure 9. The results show that more connected pores developed in the organic matter of the shale, and the study of Wang et al. (2016aWang et al. ( , 2016b also suggested that the primary connected channel is provided more by the organic pores in the shale than the inorganic pores [44,45]. From the permeability parallel and perpendicular to the stratification plane and from the FIB-HIM images, it is shown that there is a dominant channel in the shale formation, which is composed of organic pores and the stratification plane and the main direction of shale gas migration is parallel to the stratification plane. (3) Quantitative Characterization of Migration Pathway Connectivity. A spontaneous imbibition experiment conducted by the previous studies is used to quantitatively evaluate whether percolation can occur in the migration channel. Spontaneous imbibition refers to the process of spontaneous replacement of a fluid with nonwettability by another fluid with wettability under the capillary force in rock pores [46]. The slope of the spontaneous imbibition curve can be used to quantitatively evaluate pore connectivity in shale reservoirs. Reservoirs with high porosity connectivity (P > 0 28, where P is the average probability of connectivity) have a rate of 0.5. When P is equal to 0.2488 (the percolation critical value), the rate is 0.26 [47]. Huang et al. (2017) sampled the organic-rich shale in the Longmaxi Formation of the Dingshan block at two different depths to conduct the spontaneous imbibition experiment [48]. The samples are cube shaped with a border length of  Figure 1 for the well locations. Table 5: The burial depth, formation pressure coefficient, and residual formation pressure of the target in Dingye-1 and Dingye-2 Wells in the Dingshan block. See Figure 1 for the well locations.  8 Geofluids approximately 1 cm. There are 4 samples in total selected from each direction (parallel or perpendicular to the stratification plane) at each depth, from which the curve rates of the spontaneous imbibition perpendicular to the stratification plane are 0.234 and 0.181, respectively, and parallel rates are 0.441 and 0.275, respectively. This suggests that the shale connectivity is higher in the horizontal direction than in the vertical direction. In addition, the curve rate of the spontaneous imbibition slope in the lateral direction is more than 0.26, which is the percolation critical value, which indicates that   Figure 1 for the well locations. 9 Geofluids the flow of the natural gas can occur in the lateral direction of the shale. However, because the rate is less than 0.5, the curve rate of the highly connected pores has not yet been reached; thus, the percolation migration occurs over a short distance.

Marine Shale Gas Accumulation
Model under the Anticline Backgrounds. These indirect evidences from migration force, migration channel, and migration evidence can prove that in the structural combination formed by syncline and adjacent anticline, the compression and uplift of anticline can form cracks, which lead to the escape of shale gas. Driven by the difference of formation pressure, natural gas in adjacent syncline shale series can migrate from syncline to anticline along the stratigraphic plane and organic holes by short-distance displacement stepped migration, which realizes the large-scale supplement of syncline shale gas to anticline. The migration process is short distance and stair type, which caused the large-scale movement of the shale gas from the syncline to the anticline.
In the structural combination, the anticline burial depth is relatively shallow, making the drilling costs lower, but the formation pressure coefficient is relatively lower as well. While the deeper burial depth in the syncline is accompanied by a higher formation pressure coefficient and higher shale gas productivity, there are higher drilling costs. Thus, the more optimal well position should be arranged in the deeper syncline with an allowance for the drilling cost. An enrichment model of marine shale gas in anticlines is shown in Figure 11.

Conclusions
In this paper, the target layer is the Lower Cambrian and the Upper Ordovician-Lower Silurian marine shale in the Yangtze region, Southern China. The study areas include the Jiaoshiba, Pengshui, Shizhu, Huaying Mountain, Fushun Yongchuan, and Weiyuan blocks in the Sichuan Basin in Upper Yangtze and the Xiuwu Basin in Lower Yangtze. In the two tectonic settings of the anticline and the combination of the anticline and the contiguous syncline, the enrichment regulation of shale gas is analyzed with triaxial unloading tests, the permeability test parallel and perpendicular to the stratification plane, FIB-HIM experiments, and inclusions analysis. Thus, the following conclusions are drawn: (1) The greater the anticline limb angle is, the more favorable are the conditions for shale gas enrichment.
The permeability is much higher in the direction parallel to the stratification plane than in the perpendicular direction. Thus, the migration direction

Geofluids
is primarily along the stratification plane and the stratification plane and the organic pores constitute the migration channel for the shale gas. When the anticline and the adjacent syncline exist at the same time, there will be fractures developed in the anticline that cause the shale gas dissipation. Driven by the difference of the formation pressure coefficient, the short-distance stair-type migration occurs from the syncline to the adjacent anticline. The optimal well positions should be arranged in the deeper syncline with an allowance for the drilling cost

Data Availability
The data used to support the findings of this study are included within the article.

Additional Points
Highlights. (i) The larger the anticline limb angle is, the more favorable is the accumulation for shale gas; (ii) in anticlines, during uplifting, shale is extensively fractured and the fractures are concentrated within a certain depth range and horizontal stress field, causing considerable shale gas losses; and (iii) the bedding plane and organic pores provide the migration pathways of shale gas. Driven by the difference in the formation pressure coefficient, short-distance stair-type migration occurs from the syncline to the adjacent anticline.

Disclosure
Part of this work has been presented as an abstract at GSA Annual Meeting in Indianapolis, Indiana, USA-2018.

Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.