Dominant shale lithofacies and gas generation potential analysis of the WufengLongmaxi Formation in the Yichang slope area, Western Hubei Province

This paper is the result of oil and gas exploration engineering.

Objective The differences in mineral composition, gas content and compressibility of shale lithofacies determine the development potential of shale gas. The division and evaluation of shale lithofacies is important to identify the sweet spot for shale gas exploration and development.

Methods Based on drilling, core analysis, and test data, the comprehensive utilization of geological theory combined with ternary diagram of siliceous mineral, clay minerals, and carbonate mineral content are used to divide the gasbearing shale lithofacies of the Wufeng Formation and the lower part of Longmaxi Formation in Yichang slope of western Hubei province. The dominant lithofacies are also divided combining with the organic carbon content, gas content, and microscopic characteristics. The correlation between the abundance of organic matter, the content of siliceous minerals, the content of clay minerals and the shale gas content are carried out to establish the classification standard of gas-bearing dominant lithofacies of Wufeng-Longmaxi Formation.

Results The dominant lithofacies in the study area is type II. The type II₁ (optimal) dominant lithofacies are mixed siliceous shale facies (S-2) and clay-bearing siliceous shale facies (S-3), located in the first sub-member of Longmaxi Formation and the upper member of Wufeng Formation, with thickness of 6 m. The type II₂ dominant lithofacies (suboptimal) is siliceous shale facies (S), located in the lower member of Wufeng Formation, with thickness of 4 m.

Conclusions The Yichang slope belt and Fuling area have similar vertical evolution sequence of lithofacies, and the dominant lithofacies are both located in the Wufeng Formation and first sub-member of Longmaxi Formation. However, the thickness and quality of same shale lithofacies association are quite different. The subaqueous uplift of western Hunan and Hubei in the Rhuddanian stage is the main reason that caused the thickness and quality of the dominant lithofacies in Yichang area poorer than that in Fuling area.