Abstract
The Lower Permian Shanxi Formation in the Eastern Ordos Basin is a set of transitional facies shale, and it is also a key target for shale gas exploration in China. Based on lithofacies classification by X-ray diffraction and kerogen type identification, nanoscale reservoir space, pore volume, pore size distribution, surface area, and fractal characterization were studied using comprehensive methods including N2 and CO2 adsorption, mercury injection capillary pressure, field emission-scanning electron microscopy (FE-SEM), and nuclear magnetic resonance. The results indicate that Shanxi Formation shale can be subdivided into five types of lithofacies: clayey shale (lithofacies I), siliceous clayey shale (lithofacies II), siliceous shale (lithofacies IV), calcareous siliceous shale (lithofacies V), and siliceous calcareous shale (lithofacies VI). Lithofacies V and lithofacies VI are the best lithofacies in terms of organic pore morphology, connectivity, and development degree, followed by lithofacies II. Inorganic pores and microfractures are well developed in all lithofacies. The majority of pores in lithofacies I comprise organic mesopores, but pore volume is contributed by a few inorganic macropores. The pore types and pore volume contributors of lithofacies II are organic macropores. The pore size distribution of lithofacies IV is very similar to that of lithofacies I. The pore size distribution of lithofacies V shows typical bimodal characteristics. It is suggested that the inorganic pores of lithofacies V are mainly macropores, which have the greatest contribution to pore volume, followed by organic mesopores. Total organic carbon (TOC) and thermal maturity do not present obvious controls on pore structure. Vitrinite is the main kerogen type in lithofacies II and IV, and this is associated with disfavored morphology, low connectivity, and poor development degree of organic pores. In contrast, sapropelinite is observed in other shale lithofacies, and it is suggested to be an effective kerogen type that contributes to better development of organic pores.
Highlights
Marine–continental transitional facies shale is an important field of unconventional oil and gas exploration in China, which accounts for 25% of shale gas resources in China (Kuang et al, 2020)
The lithofacies is divided according to the following steps: 1) clay >75%, divided into clayey shale using contents of clay, carbonate, and siliceous minerals as three end members; 2) according to RQC (Jiang et al, 2016), the lithofacies with clay mineral content
Combined with field emission-scanning electron microscopy (FE-scanning electron microscopy (SEM)) observation (Figures 4G, H), it is suggested that the inorganic pores of lithofacies V are mainly macropores, which have the greatest contribution to PV, followed by organic mesopores (Figure 10D)
Summary
Marine–continental transitional facies shale is an important field of unconventional oil and gas exploration in China, which accounts for 25% of shale gas resources in China (Kuang et al, 2020). Many study cases of marine shale suggested organic matter abundance is the main factor controlling nanoscale pore structure (Huo et al, 2020), while the mineral composition and diagenetic intensity are indicated to play a secondarily important role in determining the pore structure (Wang et al, 2019; Jia et al, 2020). It is of great significance for exploration evaluation and development plan-making to clarify different shale lithofacies types and pore structure characteristics. The pore diameter is generally less than 100 nm, mainly distributed in the range of 2–50 nm. Wu et al (2021) proposed that Type II2 kerogen is the key factor leading to better nanopore structure of favorable lithofacies than other lithofacies in Shanxi Formation transitional shale
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