Geological characteristics and resource potential of shale gas in Cambrian Wuxun Formation in the southwestern margin of the Xuefeng uplift, China

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

Objective  In 2020, shale gas were first discovered in the Lower Cambrian Wuxun Formation on the southwestern margin of the Xuefeng uplift, but its accumulation conditions and exploration potential are unknown.

Methods Based on field geological survey and drilling data, combined with organic geochemical analysis, a comprehensive study of the Lower Cambrian Wuxun Formation in the southwestern margin of Xuefeng was carried out.

Results (1) The shale of the Wuxun Formation was deposited in the outer ramp subfacies of carbonate facies, which is an different subfacies from the contemporary Longwangmiao Formation in central Sichuan. (2) The organic shale is developed in the middle and lower parts of the Wuxun Formation, and its lithological unit is calcareous carbonaceous shale intercalated with thin limestone, with a thickness of 30–80 m and a long and narrow northeast-southwest distribution. (3) The average TOC content of shale organic matter is 1%, the type of organic matter is sapropelic I-type, and the average Ro is 2.02%, reflecting a moderate-degree thermal evolution. (4) The content of brittle minerals in shale ranges from 43% to 69%, with an average value of 59.2%, and that of clay minerals ranges from 16% to 37%, with an average value of 28.2%. Therefore, the shale is charactered by high siliceous content and high brittleness, which is easy for reservoir stimulation. (5) The shale is a low-porosity and low-permeability reservoir. Organic pores, intercrystalline pores, interlayer pores, and dissolved pores of carbonate mineral provide storage space for shale-gas enrichment, but at least four-stages structural fractures and the shale cleavage domain resulted in tectonism greatly improve the shale storage capacity.

Conclusions Comprehensive evaluation indicators such as geochemistry, reservoir physical properties and preservation conditions suggest that the eastern flank of the Xingren syncline in the southwestern margin of the Xuefeng uplift can be a favorable area for shale-gas exploration.