Abstract
Similar to North America, China has abundant shale resources. Significant progress has been made in the exploration and exploitation of shale gas in China since 2009. As the geological theory of unconventional oil and gas was proposed, scientists have started researching conditions for shale gas preservation. The shale roof and floor sealing and the shale self-sealing are the critical objects of such research, which, however, are still in the initial stage. This article studies the formation mechanism of shale roof and floor sealing and shale self-sealing by taking marine shales from Member I of the upper Ordovician Wufeng Formation–lower Longmaxi Formation in the upper Yangtze region as the research object. Analyses were performed on the TOC content, mineral composition, and porosity, as well as the FIB-SEM, FIB-HIM, and gas permeability experiments on the core samples collected from the marine shales mentioned above. The conclusions are as follows: for the sealings of shale roof and floor, the regional cap rocks, roof, and floor provide sealing for shales due to physical property differences. For the self-sealing of shales, the second and third sub-members of Member I of the Wufeng Formation–Longmaxi Formation mainly develop clay mineral pores which are dominated by macropores with poor connectivity, while the first sub-member of Member I of the Wufeng Formation–Longmaxi Formation mainly develops organic-matter pores, which are dominated by micropores and mesopores with good connectivity. Owing to the connectivity difference, the second and third sub-members provide sealing for the first sub-member, while the methane adsorption effect of shales can inhibit large-scale shale gas migration as it decreases the gas permeability; thus, the organic-rich shales from the first sub-member of Member I of the Wufeng Formation–Longmaxi Formation provides sealing for itself.
Highlights
As the economy in China has been developing fast in recent years, the demand for natural gas has increased vastly nationwide, while the domestic output of such a resource is far from sufficient for the demand
The largest difference between shale gas and conventional gas in preservation conditions lies in the roof and floor conditions, as the roof and floor may feature the lithology of mudstone, shale, tight sandstone, or carbonatite, and its quality depends on the physical properties of the rock (Zou et al, 2015; Guo et al, 2016; Zou et al, 2016)
The lithology of the roof and floor is critical to the preservation conditions of shale gas because the roof and floor with quality lithology in combination with a gas-preserving shale bed can form a fluid compartment, which effectively slows the outward migration of shale gas and makes it well preserved (Zou et al, 2017; He et al, 2019; Zou et al, 2020)
Summary
Silurian Longmaxi Formation in the Yangtze Region. Ziya Zhang 1,2,3, Kun Zhang 4,5,6,7*, Yan Song 1,2, Zhenxue Jiang 1,2, Shu Jiang 6,7, Pengfei Wang 8, Yong Li 9, Xiangdong Yin 1,4,5, Zhiyuan Chen 10, Zhengwei Li 11, Xuejiao Yuan 4,5, Pei Liu 4,5, Fengli Han 4,5, Liangyi Tang 4,5, Yiming Yang 4,5 and Yao Zeng 4,5. The shale roof and floor sealing and the shale self-sealing are the critical objects of such research, which, are still in the initial stage. This article studies the formation mechanism of shale roof and floor sealing and shale self-sealing by taking marine shales from Member I of the upper Ordovician Wufeng Formation–lower Longmaxi Formation in the upper Yangtze region as the research object. Owing to the connectivity difference, the second and third sub-members provide sealing for the first sub-member, while the methane adsorption effect of shales can inhibit large-scale shale gas migration as it decreases the. Formation Mechanism of Shale Seal gas permeability; the organic-rich shales from the first sub-member of Member I of the Wufeng Formation–Longmaxi Formation provides sealing for itself
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