Hydrocarbon generation from confined pyrolysis of lower Permian Shanxi Formation coal and coal measure mudstone in the Shenfu area, northeastern Ordos Basin, China

The Ordos Basin is a significant gas-producing region in China. Previous studies have shown that a large amount of the gas discovered in the upper Paleozoic strata originated from Carboniferous-lower Permian coal and its interbedded coal measure mudstone. To compare the hydrocarbon generation characteristics and process, two coal and coal measure mudstone samples from the Shanxi Formation in the Shenfu area were used in confined gold tube pyrolysis experiments. The analytical results revealed that both samples had poor oil generation potential, with maximum C6+ yields of 171.65-175.67 mg/g TOC for the coal and 50.12-51.86 mg/g TOC for the coal measure mudstone at the heating rate of 2 °C/h and 20 °C/h. However, their gas generation potentials are good, and the coal exhibited higher values than the mudstone, with final laboratory C1-C5 yields of 154.71-173.32 mg/g TOC for the coal and 82.52-98.96 mg/g TOC for the coal measure mudstone. Based on a comprehensive analysis of the indicators lnC2/C3, lnC1/C2 and δ13C1-δ13C2, the hydrocarbon generation process can be divided into three stages. The results of extrapolation to geological conditions revealed that the main gas generation stages of both source rocks in the Shanxi Formation began in the Early Cretaceous. Because of the regional tectonic uplift and the decrease in the geothermal gradient in the Late Cretaceous, the C1-C5 volume yields of the coal and the coal measure mudstone reached plateaus of 78.4 ml/g TOC and 32.7 ml/g TOC, respectively, in the Late Cretaceous, and the current transformation ratios of C1-C5 for the coal and the coal measure mudstone are only 32.1% and 22.8%, respectively.