Evolution of nanopore structure in lacustrine organic-rich shales during thermal maturation from hydrous pyrolysis, Minhe Basin, Northwest China

Shuang Yang,Na Yin,Lianhua Xue,Guojun Chen,Chengfu Lv,Chao Li,Fei Yang

doi:10.1177/0144598717723647

Abstract

The nanometer-scaled pore systems of gas shale reservoirs have a prominent contribution for gas storage. To obtain information about the characteristics of the nanopore structure within lacustrine organic-rich shales during their thermal evolution, artificial shale samples with different thermal maturities were obtained from a hydrous pyrolysis experiment. Nitrogen adsorption, field emission scanning electron microscopy, and porosity tests were used to investigate the characteristic pore structures of lacustrine shales with different thermal maturities from the Minhe Basin. The results show that the total organic carbon content decreased from 41.89% (unheated) to 27.7% (370℃) and that organic pores, intragranular pores of pyrite, and intergranular pores of clay minerals began to form with an increase in the simulated temperature and pressure. The porosity increased from 3.57% (unheated) to 26.09% (350℃) and then decreased to 20% (370℃) on the whole. The pore sizes were distributed from 1.7 to 500 nm, and the average pore diameter first showed a decreasing trend and then an increasing trend. The cumulative pore volume and cumulative specific surface area both presented a slowly increasing trend from an unheated status to 325℃, exhibited a rapid increase at 350℃, and then showed a slow increase at 370℃. This study could provide a reference for the exploration of shale gas in lacustrine shales with different thermal maturities.

Highlights

Continental sedimentary basins containing thick, black shale strata are widely distributed throughout China, which makes it possible to form shale gases with immense reservoir potential
The degree of shale gas exploration is lower in the Minhe Basin, and a related presentation of the evolution of black shale porosity in the basin is missing from the literature
According to the classification of gas adsorption isotherms defined by the International Union of Pure and Applied Chemistry (IUPAC) (Figure 5(a)) (Brunauer et al, 1940), the samples with unheated temperatures of 250, 300, and 325C are type II, indicating that the samples developed a certain amount of mesopores

Summary

Introduction

Continental sedimentary basins containing thick, black shale strata are widely distributed throughout China, which makes it possible to form shale gases with immense reservoir potential. Based on the limited literature, those studies mainly focused on the pore evolution of shale reservoirs in large-scale deposit basins, such as the Sichuan Basin and Ordos Basin, and ignored the evolution of shale reservoirs in small- to moderate-size basins in the northwestern region of China (Chen and Xiao, 2014; Cui et al, 2013; Tang et al, 2015; Wu et al, 2015; Xue et al, 2015). The Minhe Basin is located in northwestern China and has the character of being small but fat oil reservoir It has an average black shale thickness ranging from 160 to 4000 m and is a shallow shale-favorable area with geological resources and recoverable resources of 2091–8894 Â 108 and 210–890 Â 108 m3, respectively (Su et al, 2011). The degree of shale gas exploration is lower in the Minhe Basin, and a related presentation of the evolution of black shale porosity in the basin is missing from the literature

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Conclusion