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

This paper is the result of oil and gas geological survey engineering.

Objective Multiple sets of the Sinian-Cambrian organic-rich shales developed along the southwestern Xuefeng uplift. However, the study on the sequences hosting organic-rich shales, except the Niutitang Fm.(Formation), is still inadequate. In recent years, new shale gas was discovered in the Cambrian Bianmachong Fm. of Well Guidan-1 in the Danzhai area, therefore, it is necessary to carry out systematic evaluation to geological condition and exploration potential of this interval.

Methods  In this study, we conduct a comprehensively correlative analysis of geological characteristics of Bianmachong Formation shale across different regions of 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.

Results (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 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 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 value of 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.

Conclusions Study of geochemistry and reservoir property suggest that the southern sector of the southwestern Xuefeng uplift is of favorable geological conditions for shale, with some intervals of the Bianmachong Fm. being of good gas-bearing characteristic and high exploration potential. Therefore, the Bianmachong Fm. will be a promising exploration target besides the Niutitang Fm. in the southwestern Xuefeng uplift.