Identification of organic matter components and organic pore characteristics of marine shale: A case study of Wufeng-Longmaxi shale in southern Sichuan Basin, China

Abstract

Abstract With the global success of the commercial exploration and development of shale oil and gas, research interest in the structure of shale pore, the gas and oil storage carrier, is increasing. However, current studies on shale organic pore are insufficient. Based on this, the basic parameters of Wufeng-Longmaxi shale in Sichuan Basin were tested and shale samples were observed using optical and electron microscopes. In the Changning and Weiyuan areas, CO2, N2 adsorption and mercury intrusion porosimetry (MIP) experiments were conducted on shale samples with different total organic carbon (TOC) content. In this study, we termed the organic matter (OM) with fixed form as formed OM, and the OM without fixed form as amorphous OM, rather than the amorphous OM in the conventional context, which only exists when the maturity of shale is less than 1.0%. The results showed that: (1) formed and amorphous OM can be distinguished under optical and electron microscopes. The formed OM was mainly graptolite, while the amorphous OM could be subdivided into bitumen and micrinite. Under reflected light, bitumen appeared gray-black, while micrinite exhibited certain fluorescence. Under the electron microscope, bitumen showed larger particles than micrinite; (2) there were abundant organic pores in graptolite and bitumen. The organic pores in graptolites were significantly smaller than those in bitumen. Due to the limitation associated with the scanning electron microscope (SEM) resolution, we did not observe organic pores in micrinite, which indicated that the organic pore size in micrinite is smaller than that in bitumen; (3) there were obvious differences in the development of organic pore. In different areas, the relationship between shale porosity and TOC content varied greatly, which was related to the shale gas preservation conditions. Microscopically, there were also several reasons for the developmental differences of organic pore. Among them, inorganic minerals contribute to the preservation of organic pore by filling these pores or forming rigid spatial skeletons, which is an important reason for the micro-difference of organic pore development; (4) The SEM image processing results showed that the organic pore of Wufeng-Longmaxi shale in the Sichuan Basin mainly occurs in the range of mesopore. By comparing the pore size distribution curves of shale samples in the Changning and Weiyuan areas, it is concluded that the organic pore size of shale in the Changning area is primarily 4–10\u202fnm and that in the Weiyuan area it is primarily 2–20\u202fnm.