Organic geochemistry of the Upper Triassic T3x5 source rocks and the hydrocarbon generation and expulsion characteristics in Sichuan Basin, central China

The Upper Triassic Xujiahe Formation is a key target layer for development of tight sandstone gas reservoirs in the Sichuan Basin. Most of the previous work focused on the main reservoirs such as T(3)x(2) and T(3)x(4) with lack of research attention on T(3)x(5), an equally important source-reservoir unit. Here we examine the organic matter and source rock distribution in the T(3)x(5) using rock pyrolysis, asphalt extraction, and gas chromatography. The thickness of T(3)x(5) source rocks reduces from west and northwest Sichuan Basin to east and southeast Sichuan Basin. The main sedimentary facies are salt lakes and deltas, of which the organic matter originated from aquatic organisms and a small amount of terrestrial matter. The abundance of total organic carbon (TOG) in T(3)x(5) is relatively high, at values of 0.07%-7.2%. According to the vitrinite reflectance (R-o), pyrolysis, and gas chromatography measurement results, the T(3)x(5) source rocks are at the stage of mature to over-mature, and their hydrocarbon expulsion threshold and hydrocarbon expulsion peak correspond to R-o of 1.05% and 1.23%, respectively. Furthermore, a hydrocarbon generation center and a hydrocarbon expulsion center are formed horizontally, with the maximum hydrocarbon generation intensity of 50 x 10(8) m(3)/km(2) and the maximum hydrocarbon expulsion intensity of 48 x 10(8) m(3)/km(2). The natural gas resource is 3.19-5.36 x 10(12) m(3), indicating great exploration potential.