Shale gas potential of Lower Permian marine-continental transitional black shales in the Southern North China Basin, central China: Characterization of organic geochemistry

• The first well to investigate transitional shale gas potential in SNCB.
• Organic geochemical characterization of transitional shales in SNCB is revealed.
• Original TOC content and S2 are restored for thermally over-mature organic matter.
• The volume of gas generated during thermal maturation is calculated.
• The SNCB would not be considered as a promising shale gas play to explore.

With the purpose of characterizing the organic geochemistry and investigating the shale gas potential, we applied multiple methods, including quantifying organic matter richness, polarizing microscope observations, maceral composition analysis, kerogen stable carbon isotope analysis, vitrinite reflectance analysis, and Rock-Eval pyrolysis were performed on black shale samples from the Lower Permian Shanxi and Taiyuan formations in the Mouye-1 well of the Southern North China Basin. The results indicate that the average total organic carbon (TOCpd) content at the present time is 1.73% and 2.41% for Shanxi and Taiyuan shale samples, respectively. The microscopic distribution of organic matter includes scattered types and local enrichment types, as well as layered enrichment types. These types were revealed by polarizing microscope observations and exhibit an excellent correlation with the TOC content of shale samples. Based on maceral compositions and kerogen stable carbon isotopes, organic matter in Shanxi and Taiyuan shales is characterized by gas-prone, inertinite-dominated type III kerogen. The thermal maturity, as indicated by measuring vitrinite reflectance (3.2–3.6%Ro), suggests that shale samples from Shanxi and Taiyuan formation in the Mouye-1 well have evolved far into the metagenesis stage, and no significant amounts of hydrocarbons can be generated. The remaining hydrocarbon generative potential, S2 (0.02–0.77 mg HC/g Rock), which was determined by Rock-Eval pyrolysis, also supports this conclusion. Additionally, the original total organic carbon (TOCo) content and hydrocarbon generative potential (S2o) were reconstructed based on Jarvie's equations for these thermally over-mature shales; this reconstruction indicated poor to fair original source rock potentials based on the correlations of TOCo and S2o. Furthermore, the total volume of gas generated during thermal maturation was calculated using a conversion formula based on molar mass and resulted in a yield of 5.69 cm3/g Rock and 3.54 cm3/g Rock for Shanxi and Taiyuan shale samples, respectively. Overall, inertinite-dominated maceral compositions of kerogen with extremely high thermal maturity can have a negative effect on the gas potential of shale in the Southern North China Basin.