Abstract
To compare the micropore structure of marine-continental transitional shale with marine shale, organic geochemical, field emission scanning electron microscopy, and low-temperature nitrogen adsorption experiments were conducted on shale samples from the Shanxi Formation in the eastern Ordos Basin and the Longmaxi Formation in the southern Sichuan Basin. The results show that Shanxi Formation shale has a smaller specific surface area and pore volume than Longmaxi Formation shale; therefore, the transitional shales fail to provide sufficient pore spaces for the effective storage and preservation of natural gas. Both the transitional and marine shales are in an overmature stage with high total organic carbon content, but they differ considerably in pore types and development degrees. Inorganic pores and fractures are dominantly developed in transitional shales, such as intragranular pores and clay mineral interlayer fractures, while organic nanopores are rarely developed. In contrast, organic pores are the dominant pore type in the marine shales and inorganic pores are rarely observed. The fractal analysis also shows that pore structure complexity and heterogeneity are quite different. These differences were related to different organic types, i.e., type I of marine shale and type III of transitional shale. Marine Longmaxi shale has experienced liquid hydrocarbon cracking, gas generation, and pore-forming processes, providing good conditions for natural gas to be preserved. However, during the evolution of transitional Shanxi shale, gas cannot be effectively preserved due to the lack of the above evolution processes, leading to the poor gas-bearing property. The detailed comparison of the micropore structure between the transitional and marine shales is of great importance for the future exploitation of marine-continental transitional shale gas in China.
Highlights
After the shale gas successes in North America, China’s shale gas exploration and development has achieved great achievements in marine shales, especially in the southern Sichuan Basin [1,2,3,4]
We investigated the micropore structure characteristics of marine-continental transitional shale from the Shanxi Formation in the eastern Ordos Basin by Field Emission Scanning Electron Microscopy (FE-SEM) and low-temperature nitrogen adsorption (LTNA) tests and FHH fractal analysis
pore size distribution (PSD) curves derived from the BJH adsorption method can be used to determine the PSD of mesopores and some of the macropores
Summary
After the shale gas successes in North America, China’s shale gas exploration and development has achieved great achievements in marine shales, especially in the southern Sichuan Basin [1,2,3,4]. Marine shale gas in China is mainly developed in the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation, where the depositional environment is stable and changes progressively from deep-sea shelf facies to shallow-sea shelf facies, featuring large thickness, vast distribution, and good continuity [5, 6]. Previous studies have covered the depositional sediment, reservoir characteristics, and main constraints for the enrichment and high yield of the marine shale in the Sichuan Basin, especially about its pore development and relating mechanisms [7,8,9]. In the Sichuan Basin, Wufeng-Longmaxi Formation marine shale features high TOC, with the organic maturity generally in the high maturity stage to the overmature stage and the organic type commonly being type I or type II1, promising good hydrocarbon-generating potential [10,11,12]. Shale has good brittleness, which is beneficial to hydrofracturing operation [13, 14]
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