Geological characteristics and exploration potential of shale gas in Cambrian Bianmachong Formation in the southwestern margin of Xuefeng uplift

This paper is the result of Southern China shale gas geological survey engineering.

Multiple sets of the Sinian-Cambrian organic-rich shales developed along the southwestern margin of the Xuefeng Uplift. However, There are still some insufficiencies in researches on other organic-rich shales without the Niutitang Formation. Recent years, shale gas of the Bianmachong Formation was newly discovered from the Well Guidan-1 in the Danzhai area, therefore, it is necessary to systematically evaluate this interval. In this study, we conduct a comprehensively correlative analysis of geological characteristics of the Bianmachong Formation shale across different regions of the southwestern Xuefeng Uplift, in combination with field geological surveys, drilling data, organic geochemistry, scanning electron microscopy (SEM), high-pressure mercury intrusion (MICP), and low-temperature nitrogen adsorption (LTNA) techniques. Some main findings include: (1) Black shales were strictly controlled by deep-water shelf facies, primarily exhibiting a NE-trending distribution of facies belt. In the Cenggong area of the northern margin of the Xuefeng Uplift, the middle Bianmachong Formation contained higher sandy component, and organic-rich shales developed in the upper and lower parts withca. 150–300 m thickness . In contrast, the Bianmachong Formation in the Danzhai area of the northern margin of the Xuefeng Uplift was characterized by a relatively stable suite of gray-black carbonaceous mudstone with a thickness of 50–100 m. (2) TOC of organic-rich shales also show regional variations: TOC in the northern area has an average valueof 1.13%, conversely, TOC in the southern area shows higher values (average = 1.53%). The type of organic matter is I–II1 type sapropelic kerogen, with an average vitrinite reflectance (Ro) of 2.36–2.41%, indicating moderate thermal maturity and gas-generating potential. (3) The shale reservoirs are characterized by low porosity and permeability. Pores such as organic matter pores, intercrystalline pores, interlayer pores, and mineral dissolution pores provided storage space for shale-gas enrichment. pore feature of organic matter is predominantly isolated and poorly developed, however, microfractures and cleavage domains exhibit better storage capacity. The pore size distribution is dominated by mesopores (2–40 nm), with minor micrometer-scale macropores, which form local high-porosity intervals. Integratedly evaluating indices of geochemistry and reservoir property suggest that shale geological conditions are preferable in the southern sector of the southwestern Xuefeng Uplift , and favorable gas-bearing characteristics in certain intervals of the Bianmachong Formation. Therefore, The Bianmachong Formation would be a promising exploration target excluded the Niutitang Formation in the southwestern margin of Xuefeng uplift.