Shale gas enrichment conditions in complex geological structure areas: A case study in the front margin of Daba Mountain

A series of investigations on both outcrops and drilling cores from the front edge of Daba Mountain were carried out. Organic-rich mudstones collected from the Wufeng-Longmaxi formations in Fengjie-Kaixian-Baiquan-Manyue areas were sampled and analyzed. Information was obtained from thin sections and Scanning Electron Microscopy (SEM), and some relevant laboratory tests including organic geochemistry, X-ray diffraction, He-porosity and permeability and Specific Surface Area were made to summarize the basic geological conditions of Wufeng-Longmaxi formations in the study area. The results show that the organic-rich mudstones in the front edge of Daba Mountain were probably formed in a shallow to deep shelf during the Late Ordovician-Early Silurian. The thicknesses of these mudstones range 40-70 m, and the average total organic carbon (TOC) content is more than 2%. The kerogen mainly shows characteristics of type Ⅰ. The thermal maturity of mudstones ranges 0.82%-3.04%, indicating a high to over mature stage. Additionally, the features of low permeability and low porosity as well as several types of pore microstructures were observed in these mudstones. The analyses of typical wells indicate that the preservation of shale gas is a key element for hydrocarbon accumulation. The well-preserved box-shaped anticlines are most favorable for shale gas preservation.