Geological characteristics of Wufeng-Longmaxi shale-gas reservoir in the Huangjinba gas field, Zhaotong National Shale Gas Demonstration Area

Following Jiaoshiba and Changning-Weiyuan areas, Huangjinba shale gas field in Zhaotong National Shale Gas Demonstration Area is the place where the shale gas exploration and development have received a good achievement. To systemically demonstrate the reservoir conditions for shale gas accumulation in the Huangjinba shale gas field and provide a reference for future exploration, the authors studied the Wufeng-Longmaxi shale gas reservoir in detail in such aspects as regional geological setting, reservoir petrology, physical properties and geochemistry. The results indicate that the stable regional tectonics and well-developed roof and floor formations are crucial factors for shale gas enrichment in this region. These good preserving conditions help the reservoir maintain a high pressure (pressure coefficient >1). High porosity (mean=4%) and TOC content (target bed >2%) of the reservoir provide good spaces for hydrocarbons, and cause the reservoir rocks to have high gas values (1.35-3.48 cm3/g, >2.50 cm3/g on average). The geochemical data of natural gas show that the predominant composition is CH4(>97%), with minor compositions including C2H6, C3H8 and CO2. The isotopic data of natural gas show that the C isotopic constituents of hydrocarbons have been preserved, indicating that the reservoir is closed or semiclosed. Nevertheless, the pore system of the reservoir is very complicated and heterogeneous, which leads to the low permeability of the reservoir, and such a condition should be carefully treated during stimulation processes. In general, the shale gas development potential of the Huangjingba gas field is good, and the production test shows that the gas production quantity of fractured vertical well is 0.5×104-3.5×104 m3/day/well, and the quantity for fractured horizontal well is 12×104-40×104 m3/day/well.