Enrichment laws and regional selection of shale gas at the edge of palaeohigh: A case study on the Lower Cambrian Niutitang Formation on the southwestern margin of Xuefeng Uplift

The study aims to clarify the main controlling factors of shale gas enrichment at the edge of palaeohigh, and guide the evaluation of shale gas effectively in the similar geological conditions, taking the Lower Cambrian Niutitang Formation on the southwestern margin of Xuefeng uplift as an example, combined with the latest exploration data and previous understanding, the enrichment laws of shale gas are discussed further through the comprehensive research on sedimentary facies, evolution history and preservation conditions. The following results were obtained:(1) The shale in lower strata of Niutitang Formation is deposited in deep-water shelf facies. With the participation of hydrothermal activities, the shale possesses high organic matter abundance, Type-I kerogen, rich hydrothermal silicon and high brittle minerals was developed, which has the material basis for shale gas enrichment. (2) The thermal evolution degree has reached an overmature stage, but the organic matter is not carbonification, hence the hydrocarbon generation evolution and the reservoir evolution achieved the best match, which has the prerequisite of shale gas enrichment. (3) The preservation conditions are key factors for shale gas enrichment, which have undergone multiple phases tectonic reworking since the Yanshanian. Based on the above understanding, the criteria for evaluation of shale gas area selection on the southwestern margin of Xuefeng uplift are established. The Cengong-Shibing-Huangping-Majiang-Duyun-Dushan belt has the high-quality shale with great thickness (50~140m), high organic carbon content (TOC ≥ 2.0%), suitable thermal evolution (the average value of R_O is 2.63%), moderate burial depth (1500~4000m), and relatively stable structures, which is proposed favorable zones for Niutitang Formation shale gas exploration.