Accumulation conditions and prospective areas of shale gas in the Middle Jurassic Dameigou Formation, northern Qaidam Basin, Northwest China

The Qaidam Basin is one of the most resource-rich onshore petroliferous basins in Northwest China. The Middle Jurassic Dameigou Formation crops out along the northern margin of the basin and consists of a sequence of lacustrine sedimentary units, including sets of thick, black shale unit. The shale gas exploration well CY1 had shown significant logging anomalies that have been interpreted to indicate the presence of shale gas. This gas signature well provides valuable shale rock and shale gas parameters that enabled the identification of three areas for future shale gas exploration in this region, namely, to the south-east of Lenghu, in the area around Yuqia, and to the south-east of Huaitoutala. This study presents new data for organic-rich shale samples of the Dameigou Formation that were collected from boreholes and outcrops that enable the determination of some of the conditions of shale gas accumulation. The black shale unit within seventh interval of the Dameigou Formation is about 20 to 80 m thick. Its total organic carbon content is high (average of 2.62%), kerogen types are II1 and II2, and thermal maturity is mid to high (average Ro of 0.89%). It contains low abundances of brittle minerals and high abundances of clay minerals, micrometre to nanometre pore sizes, and high gas contents (average of 1.11 m(3)/t). These shale properties favour both shale gas generation and accumulation within the northern Qaidam Basin and are comparable to the properties of shales within shale gas fields in North America and elsewhere in China.