Evaluation of shale gas potential in Ordovician Wufeng-Silurian Longmaxi formations, Mugan syncline, northeastern Yunnan

Shale gas potential was evaluated according to wells Xd2 and Ydd3 in the Ordovician Wufeng-Silurian Longmaxi formations in the Mugan syncline of the northeastern Yunnan, using core test results, data from the neighboring area, black shale lithology, distribution, organic geochemistry, reservoir and gas bearing characteristics. The Wufeng Formation to the first member of Longmaxi Formation is mainly composed of deep-water shelf black carbonaceous shale and silty mudstones. The first member of Longmaxi Formation can be divided into the 1st, 2nd and 3rd submembers. The high-quality shale section with a total organic carbon content greater than 2.0% is 40-70 m thick, and the average organic carbon content is 3.00%-3.61%. The kerogen carbon isotope is -30.21‰ to -27.01‰. The higher the organic matter content, the better organic matter type is. The equivalent vitrinite reflectance (VR_o) is 2.73%-2.79%. The shale has limited quartz and feldspar, but more carbonate and pyrite. From bottom to top, quartz, carbonate and pyrite contents decrease, while feldspar and clay contents increase. The shale porosity is 2.38%-8.53%, and the transverse permeability is (0.000 01-15.0)×10^-3 μm², indicating medium porosity and low to extra low permeability. Mesopores and micropores formed by organic matter evolution are dominant. The Wufeng-Longmaxi shale is 1 500-2 300 m deep in the Mugan syncline. The gas-bearing area is 125.55 km², and the resource is 100×10⁹ m³. The main gas-bearing interval is located in the upper member of Wufeng Formation to the first member of Longmaxi Formation, with a thickness of 38-43 m and an average total gas content of 2.95-4.31 m³/t. The Wufeng-Longmaxi shale in the Mugan syncline has a large thickness, a continuous distribution in the horizontal and vertical directions, and a moderate burial depth, which show a great resource potential.