Gas-generation potential of shales in small and medium-sized basins: a case study from the Xuanhua Basin, north China

Abstract

Small- and medium-sized basins are widely distributed, and some contain commercial gas reservoirs demonstrating their gas-generation potential. The Xuanhua Basin, which is a small-sized coal-bearing basin in north China, includes a promising target for shale-gas exploration in the Xiahuayuan Formation. In this study, we used this basin as a case study to assess the critical geochemical features for small or medium-sized basins to form commercial gas reservoirs. Total organic carbon (TOC) analysis, Rock-Eval pyrolysis, microscopic observation of macerals, vitrinite reflectance measurement and kerogen stable carbon isotope analysis were performed to characterise the organic geochemistry of the Xiahuayuan shales. The original total organic carbon (TOCo) content and hydrocarbon-generative potential (S2o) were reconstructed to further evaluate the gas-generation potential of these shales. In addition, geochemical data of shales from other similar-sized basins with gas discoveries were compared. The results showed that the kerogen from the Xiahuayuan Formation is Type III (gas-prone), and macerals are dominated by vitrinite. TOC values showed a strong heterogeneity in the vertical profiles, with most higher than 1.5 wt%. The measured Ro values ranged from 1.4 to 2.0%. However, thermal maturity was not correlated with the present-day burial depth with higher maturity in the wells closest to the diabase intrusion centre. The remaining generation potential (S2) averaged 0.91 mg HC/g rock, equal to 1.4 cm3 CH4/g rock, and the average amount of hydrocarbon generated was 4.33 cm3 CH4/g rock. In small and medium-sized basins, the TOC content of commercially developed gas shales ranged from 0.5 to 2.5 wt%, organic matter was mainly humic (gas-prone), and the burial depth was generally shallow. Biogenic gas reservoirs for commercial exploitation tend to have larger shale thicknesses (120–800 m) than thermogenic gas reservoirs (60–90 m).The Xiahuayuan Formation is a good gas-source rock with gas-prone kerogen type, relatively high TOC values and moderate thermal maturity.The average amount of hydrocarbon generated from the Xiahuayuan shales is about 4.33 cm3 CH4/g rock, indicating a potential to form a shale gas reservoir.Owing to the influence of diabase intrusions, the Xiahuayuan shales have entered the dry gas window at relatively shallow-buried depths.Small- and medium-sized basins have the potential to generate commercial gas reservoirs with the generated volume mainly a product of the thickness and maturity of black shales.