Analysis of Gas Composition and Nitrogen Sources of Shale Gas Reservoir under Strong Tectonic Events: Evidence from the Complex Tectonic Area in the Yangtze Plate

: Strong tectonic movement brings great risk to exploration of shale gas in southern China, especially in Lower Cambrian shale with complex tectonic backgrounds, which has good hydrocarbon-generation matter but low or no gas content. In this paper, the Lower Cambrian shale from the southeast Chongqing region, located in the Upper Yangtze Platform, and the Xiuwu Basin, located in the Lower Yangtze Platform, were selected as the research objects. First, the gas components in shale gas samples were measured, then analysis of nitrogen isotopic was used to reveal the nitrogen sources. Using regional geological backgrounds, core description, and seismic interpretation, combined with the perpendicular and parallel permeability test and focused ion beam–helium ion microscopy (FIB–HIM) observation, the reasons for high content of nitrogen in the Lower Cambrian shale from the Xiuwu Basin and the Southeast Chongqing region were clarified. The results indicate that the main sources of nitrogen in the Lower Cambrian shale gas at the Southeast Chongqing region is the thermal evolution of organic matter and atmosphere. Nitrogen in the atmosphere is filled into the shale reservoir through migration channels formed by detachment layers at the bottom of the Lower Cambrian, shale stratification planes, and widespread thrust faults. Nitrogen was also produced during the thermal evolution of organic matter. Both are responsible for the low content of hydrocarbon and high content of nitrogen of shale gas in the Southeast Chongqing region. Further, the main sources of nitrogen in the Lower Cambrian shale gas at the Xiuwu Basin is the upper mantle, superdeep crust, and atmosphere. Nitrogen in the atmosphere is also filled into the shale reservoir through migration channels formed by detachment layers at the bottom of the Lower Cambrian, shale stratification planes, and widespread thrust faults. Nitrogen was also produced by volcanism during the Jurassic. Both are the causes of the low content of hydrocarbon and high content of nitrogen in shale gas in the Xiuwu Basin. Finally, destruction models for shale gas reservoirs with complex tectonic backgrounds were summarized. This research aimed to provide a theoretical guidance for shale gas exploration and development in areas with complex tectonic backgrounds.

gas, and that a good shale floor and roof can effectively limit vertical dispersion of hydrocarbon [9]. In this case, the shale reservoir has high porosity, high gas content, and high pressure, which are beneficial to the formation of a high-yield region for shale gas.
Some shale gas blocks located outside the stable basin, such as the Xiuwu Basin and the Southeast Chongqing region, have good hydrocarbon-generation matter. However, commercially valuable shale gas reservoirs have not been found in these blocks, and the exploration wells have shown that the nitrogen content in these blocks is far higher than in other shale gas blocks, illustrating that the characteristics of the Lower Cambrian shale gas reservoir in these blocks are distinct from those of shale gas blocks with high methane content. Based on the geochemical characteristics of the Lower Cambrian shale gas in the typical complex tectonic zone from the Yangtze Plate, such as the Xiuwu Basin and Southeast Chongqing region, this study used nitrogen isotopes to trace the sources of nitrogen and then considered regional geological backgrounds and tectonic deformation characteristics to analyze the reasons for low hydrocarbon and high nitrogen. Finally, destruction models for shale gas reservoirs were established in these blocks, which provide a scientific basis for guiding the exploration and development of highly evolved shale gas in complex tectonic areas.

Tectonic Characteristics
The scope of this research was the whole Yangtze region. The primitive continental crust of southern China was separated into two ancient plates, the Yangtze and Cathaysian, in the early period of the Mesoproterozoic Era [21][22][23]. During the Early Cambrian, the two plates were extended with the occurrence of large-scale transgression, resulting in the deposition of a set of organic-rich shale across almost all of the Yangtze plate. Then, the water body began to shallow. In this case, the lithologic features of the Yangtze plate changed from fine-grained and silty shale to siltstone, sandstone, and other coarse-grained clastic rocks. The collision between the Yangtze and Cathaysian plates in the Ordovician period caused the water body to continue to become shallow, in which the sedimentary system on the Yangtze plate transformed from clastic to carbonate. In the Silurian period, a transgression occurred, causing the sedimentary system to change back to a clastic sedimentary system. During this period, the oceanic basin between the two plates was subjected to gradual subduction and collision toward the Yangtze Plate. By the Late Silurian period, the Yangtze and Cathaysian Plates merged into one, namely the uniform South China Plate. The Xiuwu Basin and the Southeast Chongqing region are representative blocks with complex tectonic backgrounds, both of which are located outside the large stable sedimentary basins. Southeast Chongqing region is located on the southeast side of Sichuan Basin ( Figure 1A,D), and Xiuwu Basin is located on the southeast side of Jianghan Basin ( Figure 1C,F). Southeastern Chongqing borders Sichuan Basin. The distance between Xiuwu Basin and Jianghan Basin is about 60 kilometers. The two blocks are not far from the junction of the two plates and experienced intense tectonic movement against complex tectonic backgrounds.

Sedimentary and Stratum Characteristics
During the Early Cambrian, the sedimentary environments on the Yangtze plate from northwest to southeast were ancient lands, a shallow shelf, a deep shelf, a continental slope, and an ocean basin [24,25], as shown in Figure 1G. The target layer for this research was a set of shale that was widely deposited on the Yangtze plates in the Early Cambrian. Because of the widespread distribution of shale, they are called different names in various regions. In the Upper Yangtze area, they are known as the Qiongzhusi Formation, while in the Sichuan Basin and outside of the Sichuan Basin (such as the Southeast Chongqing region), they are called the Niutitang Formation. In the Lower Yangtze area, they are known as the Wangyinpu and Guanyintang Formations, which are sets of black to deep gray organic-rich siliceous shale deposited early in the Early Cambrian and the key target strata for China's shale gas exploration.

Gas Composition and Nitrogen Isotope Test
The Shimadzu GC-2014 gas chromatograph (JPN) was used for quantitative multicomponent analysis of mixed gases in this study. The stainless-steel chromatographic column was placed in a room with a constant temperature of 60 °C. The six-way valve sample injector used a sample loop with a capacity of 1 mL, and the temperature was kept constant at 100 °C. The thermal conductivity cell detector was kept under a constant temperature of 200 °C. Nitrogen isotope tests were used to determine the source of nitrogen, implemented with an EA IsoLink Plus EA-IRMS device using helium (99.999%) as the carrier gas at a flow rate of 1.3 mL/min. The gas sample feeding was implemented in a split-stream sampling approach (with a split ratio of 20:1) with a gas inlet temperature of 200 °C. Specifically, the temperature was held at 35 °C for 6 min, then increased to 80 °C at a rate of 15 °C/min and 200 °C at a rate of 5 °C/min, and finally held constant at 200 °C for 5 min. The reacting furnace temperature was 940 °C. Eight shale gas samples were taken from the Niutitang Formation in the Youye-1 well and 16 from the Wangyinpu and Guanyintang Formations in Xiuwu Basin. The shale gas sample information can be seen in Table 1. Jiangye-1 Early Cambrian Wangyinpu

Experiment of Permeability Perpendicular and Parallel to Stratification Plane
The traditional steady-state test technology mainly uses Darcy's law to calculate permeability based on the gas flow rate per unit time under the condition of stable pressure difference, which makes the permeability measured higher under low average pressure and lower under high average pressure. Therefore, the Klinkenberg correction of data is necessary in permeability tests [26][27][28]. The non-steady-state pulse attenuation permeability measurement technology can avoid the gas flow measurement and calculate permeability by the pressure difference-time curve of the core front and back to weaken the gas slip effect [29]. The permeability of the shale samples was measured perpendicular and parallel to the stratification planes with a PDP-200 pulse decay permeability analyzer (USA). Since gas slippage is more evident in shales under low pore pressure [28], the permeability of the shale samples was tested at 1000 psi. This study collected 16 shale samples from the Lower Yangtze area. Before the test, the shale samples were made into cylinders with a length of 50 mm and a diameter of 25 mm. We also collected test results for 12 samples from the Upper Yangtze area [30]. The gas sample information is shown in Table 2. Table 2. Experimental numbers, wells, geologic age, and formation names of core samples. The well locations can be seen in Figure 1.

FIB-HIM Experiment
Focused ion beam-helium ion microscopy (FIB-HIM) contains three parts: The cutting function of a focused ion beam (FIB), the imaging function of a helium ion microscope (HIM), and a neon (Ne) ion beam. Compared with a field emission scanning electron microscope (FE-SEM), a scanning helium ion microscope has higher resolution, which makes it easier to distinguish micro-nanopore in shale. Before sample observation, FIB-HIM samples must be ground and Ar-ion polished. In this research, a NanoFab ORION microscope was used to observe an organic-rich shale sample from the Niutitang Formation at 3811 m in the Youye-1 well, and experimental images of organic-rich shale samples were taken from the Longmaxi Formation at 2402 m in the Jiaoye-1 well [31].

Shale Gas Composition Analysis
According to gas composition analysis of eight gas samples from the Niutitang Formation in the Youye-1 well and 16 gas samples from the Wangyinpu and Guanyintang Formations in the Jiangye-1 well, the experimental results show that the gas composition was mainly composed of O2 and N2, as shown in Figure 2A,B. Counter art average volume percentages were 11% and 83%, in the Youye-1 well and 17% and 80% in the Jiangye-1 well. These values are close to the proportions of O2 and N2 in atmosphere (21% and 78%, respectively). The average content of methane was lower than 1% in the two wells.  Table 1.

Nitrogen Isotope Analysis
The results of nitrogen isotope analysis of the Niutitang Formation gas samples from the Youye-1 well and the Wangyinpu and Guanyintang Formation gas samples from the Jiangye-1 well are shown in Figure 3A,B. In the Youye-1 well, the nitrogen isotopes varied from −3‰ to 0‰, while in the Jiangye-1 well, the nitrogen isotopes varied from −1‰ to 0‰.  Table 1.
Due to different origins, nitrogen from various sources is characterized by different isotopic compositions [17,32,33]. Nitrogen sources, origins, and corresponding isotope characteristics are listed in Table 3, which shows that the source of the nitrogen for the gas samples in the Youye-1 well was organic-matter thermal evolution and the atmosphere, while source of the nitrogen for the gas samples in the Jiangye-1 was the upper mantle, the superdeep crust, and the atmosphere. The permeability experiment results and data statistics for the Wangyinpu and Guanyintang Formations shale core from the Jiangye-1 and Jiangye-2 wells and the Longmaxi Formation shale core from the Pengye-1 well are shown in Figures 4 and 5. The experimental results show that due to welldeveloped stratification planes in the shale, the permeability parallel to the stratification plane was more than 1-40 times greater than the permeability perpendicular to the stratification plane, which means that the gas in the shale strata mainly flowed along the direction parallel to stratification plane.   Table 2.
The bedding slip surfaces, known as the detachment layer, often developed microfractures in different directions and higher reflectivity than the shale matrix, resembling a polish from simple shear [34,35]. Through the observation of cores from the Niutitang Formation in the Youye-1 well, the bedding slip surfaces can be found ( Figure 6A,B). Similarly, a bedding slip deformation was observed in cores from the Wangyinpu Formation of the Jiangye-1 and Jiangye-2 wells ( Figure 6C,D), and obvious interlayer slippage and corrugation occurred in the Pukou profile in the northern part of the Xiuwu Basin ( Figure 6E), which indicates that the detachment layers were widely developed at the bottom of the Lower Cambrian in the Xiuwu Basin and Southeast Chongqing region. Because the Xiuwu Basin was subjected to compressive stress in the northeast direction and the Southeast Chongqing region was subjected to compressive stress from the southeast direction and [36][37][38], the relative slide between the hard Sinian siliceous dolomite and the soft Lower Cambrian organic-rich shale resulted in the formation of detachment layers. The stratification plane was the lateral migration channel of shale gas, while the detachment layers greatly accelerated the process of gas diffusion [39]. In this study, both areas were synclinal geological units, and the target strata in the synclinal wings were both exposed to the ground surface, which provided a pathway for the migration of gases (Figures 7 and 8). Hydrocarbon gases migrated along the detachment layers and the stratification planes from the center of the syncline to the flanks. Meanwhile, the nitrogen gas in the air entered the shale reservoir along the detachment layers and the stratification planes, forming high nitrogen content from the atmosphere.

Migration Channels with Perpendicular to Stratification Plane
The Southeast Chongqing region is located in the southeast side of the Sichuan Basin. During the Cretaceous, this region underwent extrusion stress in the southeast direction, which caused the formations to be squeezed and uplifted and produced a large number of thrust faults. According to the results of seismic interpretation, this continuous extrusion was intense, and most of the thrust faults are deep (Figure 7). Based on the analysis of the tectonic evolution history, during the Early-Middle Jurassic, the Xiuwu Basin developed numerous faults and compressed into a syncline due to the collision and extrusion between the North China and South China plates [37]. During the Late Cretaceous-Paleogene, the Xiuwu Basin was subjected to tensile action due to the impact of the collision and subduction of the Pacific plate toward the Eurasian plate [38]. Since the Neogene period, the stress on the thrust faults changed from tension to compression again [31,40]. According to the results of seismic interpretation, there were several deep faults in the study area in the vertical direction ( Figure  8). The development of fractures destroys the seal in the direction perpendicular to the horizontal plane, causing methane and other hydrocarbon gases to escape [41] and the entry of nitrogen from the atmosphere to target layer along the fractures [15]. The fault-opening events happened in the Late Cretaceous-Paleogene in the Xiuwu Basin, and the development of the deep faults in the Southeast Chongqing region accelerated these processes.

Analysis of Nitrogen Produced in Stages of Organic Matter Evolution
Because of the lack of vitrinite in the Lower Paleozoic, maturity evaluation is usually performed using the equivalent vitrinite reflectance (equal-Ro) calculated from asphalt reflectance [42]. According to the hydrocarbon generation history restored by Zhao et al., 2018 (Figure 9), the Niutitang Formation shale in the Southeast Chongqing region entered high mature stage in the Late Ordovician period [43]. In the Middle Silurian, the shale region entered the overmature stage (equal-Ro > 2.0%). Due to strata uplift, the maturity of organic matter no longer increased and equal-Ro of the shale eventually reached 3.13% to 3.49%, falling into the partial graphitization stage [44,45]. Equivalent vitrinite reflectances of shale samples used for FIB observation from the Jiaoye-1 well and Youye-1 well were 2.58% and 3.47% respectively. It was observed that there was nesting of small pyrobitumen pores in large pores in the shale sample taken from the Longmaxi Formation of the Jiaoye-1 well. These pore development features could increase the organic matter reservoir capacity and the specific surface area ( Figure 10A). Only isolated pores were developed in the organic matter pores (OM-pores) in the shale sample taken from the Niutitang Formation of the Youye-1 well, with a small amount and poor reservoir capacity ( Figure 10B). When equal-Ro >3.0%, the graphitization of organic matter appears [45]. During graphitization, the property of organic matter changes, which compromises the reservoir space, mainly embodied as deformation of the organic matter pore structure and dramatic reduction of pores [46][47][48][49][50]. Organic matter in shale samples from Youye-1 well reaches the graphitization stage. Compared with the Jiaoye-1well, organic pores in shale samples from the Youye-1well were poorly developed. Organic matter can produce nitrogen in the mature to high mature stage (equal-Ro = 0.7-2.0%), which was adsorbed in the shale organic matter pores [17]. The Lower Cambrian shale of the Youye-1 well went through the mature to high maturity stage and nitrogen was produced in this stage. Due to changes in the nature of the organic matter, ultra-high thermal evolution of organic matter results in an exhaustion situation of generation potential in shale, and the adsorption capacity of organic matter for methane is reduced [43,46,47]. The result is that the methane in the pores escaped to shallower layers or the atmosphere, and nitrogen with strong adsorption capacity was retained, which finally caused the characteristics of low hydrocarbon and high nitrogen. Krooss et al. (1995) concluded that nitrogen originating from the superdeep zone of crust and upper mantle zone is generally concentrated in volcanic activity zones, while in other regions, the nitrogen content is actually very low [51]. According to the regional structural analysis, the Niutitang Formation in the Youyang-1 well was not directly affected by volcanic activity, while the Xiuwu Basin was influenced by magmatism during the Jurassic, and remnants of Jurassic eruptive rocks are also present on both sides of the basin. The magmatic activity in the Xiuwu Basin led to changes in the ancient heat flow. The values of the ancient heat flow in the Xiuwu Basin have undergone great changes since the Early Cambrian, as shown in Figure 11 and Guanyintang Formation shale reservoir through the fault during the cooling down of magma [53]. Therefore, based on the above analysis, the nitrogen in the Jiangye-1 well was derived from the superdeep zone of crust and the upper mantle zone, while there is no evidence that these were the sources of nitrogen in the Youye-1 well.

Destruction Model for Shale Gas Reservoirs in Areas of Active Plate Movement
From the above analysis, there are three main sources of nitrogen in shale reservoirs under complex tectonic settings: From the atmosphere carried by surface water, produced in mature and high mature stages of organic matter evolution, from the superdeep zone of crust, and from the upper mantle zone. Due to strong tectonic activity, it is easier for the strata to form channels for gas migration. In the lateral direction, stratification planes and detachment layers became the main channels for atmospheric nitrogen into shale reservoirs and methane dissipation, while in the vertical direction, faults are the main channels for entry of nitrogen and methane dissipation, such as the development of deep faults in the Southeast Chongqing region and the opening of the Late Cretaceous-Paleogene faults in the Xiuwu Basin.
Based on nitrogen isotope analysis results, the nitrogen in the southeastern Chongqing region not only comes from the atmosphere, but also from the thermal evolution process of organic matter. On the one hand, when the maturity of organic matter reaches the stage of graphitization, its brittleness changes and the OM-pores collapse under the overlying pressure, resulting in fewer pores, smaller pore size, and bad storage capacity. Then, the strata begin to uplift, causing the development of faults, denudation of the overlying strata, and the escape of hydrocarbon gas. On the other hand, since the adsorption capacity of nitrogen is stronger than methane, part of the nitrogen produced during the thermal evolution of organic matter is adsorbed on the surface of OM-pores and retained in shale reservoirs [54,55]. In summary, detachment layers at the bottom of the Lower Cambrian in the lateral direction, shale stratification planes, vertical deep faults, and thermal evolution of organic matter are the dominant causes of low contents of hydrocarbons and high contents of nitrogen in shale gas from the Southeast Chongqing region ( Figure 12A).
Based on nitrogen isotope analysis results, the nitrogen in the Xiuwu Basin comes not only from the atmosphere, but also from the superdeep zone of crust and upper mantle zone. Combined with previous studies [31], the Xiuwu Basin is not far from the junction of the South China and North China plates, which made the basin easily affected by magmatic activity caused by the collision between the two plates. On the one hand, the magmatism carried nitrogen originating from the upper mantle and superdeep crust into the Lower Cambrian shale reservoir, which caused an increase in nitrogen content. On the other hand, with the heat flow value increasing continuously, abnormally high temperatures not only made the Wangyinpu and Guanyintang Formation shale enter the graphitization stage, but also reduced the adsorption capacity of shale gas. Both of these processes promoted shale gas loss. Therefore, the detachment layers at the bottom of the Lower Cambrian, shale stratification planes which extensively developed faults in the vertical direction, and magmatism during the Jurassic are the reasons for the low contents of hydrocarbons and high contents of nitrogen and in shale gas from the Xiuwu Basin ( Figure 12B). The Xiuwu Basin and Southeast Chongqing region are representative blocks within the complex tectonic backgrounds of the Upper and Lower Yangtze areas. This model can be used to explain the occurrence of low contents of hydrocarbon and high contents of nitrogen phenomena in the Upper and Lower Yangtze areas and in tectonically active regions. Due to the limitation of oil and gas production technology, such shale gas reservoirs cannot achieve effective commercial development. In shale gas exploration, the location of wells in such models should be avoided.

Conclusions
(1) The main sources of nitrogen in the Southeast Chongqing region is the thermal evolution of organic matter and atmosphere. Large-scale tectonic events in the research area caused extensive thrust faults and detachment layers, which destroyed the already-formed shale gas reservoir. The shale stratification planes and partial graphitization of organic matter accelerated the methane dissipation. In the meantime, nitrogen originating from the atmosphere intruded into the shale reservoir and mixed with nitrogen produced by thermal evolution of organic matter in the reservoir.