Evaluating Shale Gas Sweet Spots of the Jurassic Da'anzhai Member in the Yuanba Area, Sichuan Basin

Terrestrial shale strata generally has frequent and strong heterogeneity characteristics, thus affecting shale gas enrichment and target selection. In this study, samples from the Da’anzhai Member of Well A in Yuanba area have performed a series of experiments including total organic carbon (TOC) content measurement, rock pyrolysis, organic petrology, vitrinite reflectance (Ro), whole-rock mineral composition, thin section analysis, focused ion beam scanning electron microscope, high-pressure Hg injection, nitrogen (N2) adsorption, and methane (CH4) isothermal adsorption. The results show that: (1) Da'anzhai (J1da) Member is a set of mixed sediments with various lithology, which consists of black shale and well-developed siltstone and carbonate rocks interlayers. The second section of the Da'anzhai Member (J1da2) has relatively greater organic matter abundance (1.24%TOC), better organic matter type dominated by type III and II, as well as higher maturity (1.67%Ro), which is the most favorable section of J1da. (2) J1da2 shale strata can be divided into three shale lithofacies types, two interlayer lithofacies types, and three macro-lithofacies assemblage types. Among them, mudstone–limestone lithofacies assemblage (type AB-I) has preferable hydrocarbon generation conditions, better reservoir properties, and more developed shell limestone interlayers with better fracturing performance, is thought the most favorable interval. (3) The optimal interval and horizontal target of J1da2 are further selected based on the source–reservoir coupling relationship, corresponding to the depth is 3909–3914 m, which imply the optimal horizontal target window for Well A. This study has application values not only for the identification of “sweet intervals” and well location deployment of J1da but also supports the later systematic studies of shale gas accumulation mechanism and enrichment regularity in Yuanba area.