Organic geochemical characteristics and hydrocarbon generation mechanism of marine-continental transitional organic-rich shale: A case study from the Shanxi formation in the eastern margin of the Ordos Basin

Abstract

Previous research and exploration has shown that China has abundant marine-continental transitional shale gas resources. Favourable areas are widely distributed, and are mainly in the Carboniferous-Permian strata. However, the organic geochemical characteristics and hydrocarbon generation mechanism of organic-rich shale remain unclear. This study used organic geochemistry, gold-tube pyrolysis experiments, and basin simulations to evaluate the potential, characteristics, and process of generating hydrocarbon in organic-rich shale from the Shanxi Formation. The results showed that the total organic carbon (“TOC”) content of the downhole and outcrop samples (shale, carbonaceous shale and coal) is high. In addition, the organic matter (“OM”) type is mainly type III kerogen. Further, the Ro distribution of downhole samples ranged from 2.28% to 3.12% (2.62% on average), indicating that OM is in an overmature stage (generating a dry gas stage). Based on the analysis of the characteristics of gold-tube pyrolysis products, the hydrocarbon generation process can be divided into four stages. At a heating rate of 2 °C/h, the highest yields of C6+, C2-5 and C1 were 75.34 mg/g TOC, 16.20 mg/g TOC, and 138.74 mg/g TOC. These results indicate that the Shanxi Formation shale has good gas generation potential and belongs to a typical gas source rock. Under the actual geological conditions, the Shanxi Formation organic-rich shale of wells D5 and D3 almost stopped producing natural gas at approximately 104 MPa. The OM transformation ratio (“TR”) of the Shanxi Formation shale of wells D5 and D3 reached 85.58% and 83.97%, respectively. The gaseous hydrocarbon yields were 66.83 mg HC/g TOC and 32.35 mg HC/g TOC, respectively. This study's results revealed that marine-continental transitional organic-rich shale has a strong gas-generating capacity. This capacity can provide an important gas source for shale gas enrichment, showing a high shale gas resource potential.