Composition Effect on the Pore Structure of Transitional Shale: A Case Study of the Permian Shanxi Formation in the Daning–Jixian Block at the Eastern Margin of the Ordos Basin

Abstract

Marine–continental transitional (hereinafter referred to as transitional) Permian shales are important targets for shale gas in China because of the considerable volumes of shale gas resources present in them. In this study, transitional shale samples from the Permian Shanxi Formation in the Daning–Jixian block along the eastern margin of the Ordos Basin were collected to investigate the effects of organic and inorganic compositions on the development of their pore structures through organic petrographic analysis, X–ray diffraction, scanning electron microscope (SEM) observation, gas (N2 and CO2) adsorption, high-pressure mercury injection (HPMI), and methane adsorption experiments. The organic petrographic analysis reveals that the Permian Shanxi shale comprises Type-II2-III kerogens, and the average vitrinite reflectance (Ro) is 2.3% at the overmature stage or in the dry gas window. The shale interval at the bottom of the lagoon facies is considered the most favorable interval throughout the entire section because of its high total organic carbon (TOC) content (4.19–43.9%; an average of 16.9%) and high brittle mineral content (38.3–73.2%; an average of 55.8%). N2 and CO2 gas adsorption and HPMI tests reveal the pore size distribution characteristics of the shale. The full pore size distribution by the gas adsorption and HPMI test reveals that micropores (<2 nm) and mesopores (2–50 nm) were dominant in the pore system, and the contributions of the two pore sizes were nearly equivalent, accounting for 21.95–55.05% (an average of 42.3%) and 37.94–64.6% (an average of 49.64%) of the total pore volume, respectively. Additionally, the pore characteristics related to different phases (mainly as silicate, clays, and organic matter) are further clarified by SEM observation and correlation analysis of phase content and pore structure parameters. OM contains numerous SEM-invisible micropores, whereas clay minerals mainly develop mesopores and small macropores (50–100 nm). Furthermore, we calculated the contribution of different shale components to shale porosity. The OM pores account for 0.26–44.1% (an average of 18.7%), and clay mineral pores account for 53.8–93.3% (an average of 76.9%) of the shale porosity. In particular, the OM contributes 73.2% to the surface area and 33.5% to the pore volume. This implies that both OM and clay minerals are important for the storage capacity of adsorbed and free gas.

Pore structure and fractal characteristics of transitional shales with different lithofacies from the eastern margin of the Ordos Basin

“…The TOC contents of the Taiyuan and Benxi shale samples are lower, ranging from 0.43%–1.24% (mean 0.88%) and 2.65%–2.96% (mean 2.81%), respectively. The TOC contents of the Shanxi formation samples are higher than those of the Taiyuan and Benxi formation samples, which is basically consistent with previous studies on transitional shale in the Ordos Basin 9,10,29 …”

“…The TOC contents of the Shanxi formation samples are higher than those of the Taiyuan and Benxi formation samples, which is basically consistent with previous studies on transitional shale in the Ordos Basin. 9,10,29 The typical classification scheme is based on the mineral composition of siliceous minerals (quartz and feldspar), carbonate minerals (calcite and dolomite), and clay minerals. Therefore, shales are divided into 4 types: siliceous shale lithofacies (SL), calcareous shale lithofacies (CL), mixed shale lithofacies (ML), and argillaceous shale lithofacies (AL).…”

“…2,7,8 Three sets of transitional shale in the Benxi formation, Taiyuan formation, and Shanxi formation in the Carboniferous-Permian system in the Ordos Basin are largely developed and are favorable for shale gas exploration. 9,10 Shale gas is mainly in a free state or an adsorbed state, and adsorbed gas can account for 20%-85% of the total gas content of shale. 11,12 Therefore, the form and distribution of natural gas are relatively complex and closely related to the pore structure characteristics of shale reservoirs.…”

“…Marine‐continental transitional shale is mainly distributed in the upper Carboniferous Benxi formation, lower Permian Taiyuan formation, and Shanxi formation in the Ordos Basin, Qinshui Basin, and North China Basin, the upper Permian Longtan Formation in the southern Yangtze area, and the Triassic Xujiahe Formation in the Sichuan Basin 2,7,8 . Three sets of transitional shale in the Benxi formation, Taiyuan formation, and Shanxi formation in the Carboniferous‐Permian system in the Ordos Basin are largely developed and are favorable for shale gas exploration 9,10 . Shale gas is mainly in a free state or an adsorbed state, and adsorbed gas can account for 20%–85% of the total gas content of shale 11,12 .…”

Dynamic Production Characteristics of Deep Coalbed Methane: A Case Study of Daning-Jixian Block

“…The structure of the deep coal seam below 2000 m is relatively simple, featuring limited fault development and an overall stratum dip angle ranging from 1 to 3°, indicative of a relatively gentle incline. 23 Regarding the definition of a deep CBM, most industry standards refer to CBM resources with coal seam depths exceeding 1000 m. As exploration and development progress into deeper regions, the term "deep CBM" in the literature becomes somewhat ambiguous. In the United States, a lowpermeability reservoir at depths exceeding 5000 ft (1524 m) is defined as deep CBM, 24 while other definitions utilize critical depth.…”