Abstract
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.
Highlights
Shale gas has become an important natural gas resource
Was organic-matter thermal evolution and the atmosphere, while source of the nitrogen for the gas listed in Table 3, which shows that the source of the nitrogen for the gas samples in the Youye-1 well samples in the Jiangye-1 was the upper mantle, the superdeep crust, and the atmosphere
The stratification plane was the lateral migration channel of Chongqing region was subjected to compressive stress from the southeast direction and [36,37,38], the shale gas, while the detachment layers greatly accelerated the process of gas diffusion [39]
Summary
Shale gas has become an important natural gas resource. In recent years, due to the development of geological understanding, organic-rich shale has become a hot target for natural gas exploration and development around the world [1,2,3]. After analyzing the main controlling factors the accumulation of the Fuling shale gas field, Guo et al (2017) believed that the sealing conditions determined by the duration and strength of later tectonic processes were the key geological factors for reservoir formation and the production of shale 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. 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
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