Paleoenvironment, Provenance, and Hydrocarbon Potential of Lower Permian Coal-Bearing Source Rocks in the Southern North China Basin: A Case Study of the Pingdingshan Coalfield

Coal measures, widely developed in the marine-continental transitional environment, have been regarded as an important reservoir of petroleum resources, such as coalbed methane, shale gas, and tight sandstone gas. In this study, petrography, organic geochemistry, and isotope geochemistry were conducted to reveal the organic matter sources, maturation, and paleoenvironment of Permian coal-bearing source rocks in the Pingdingshan coalfield, China. Organic petrography indicates that the organic matter of the Lower Shanxi shales are derived from the terrigenous and marine mixed inputs and those of the coals are mostly sourced from terrigenous input. Thermal parameters suggest that the source rocks of the Lower Shanxi Formation reached the late oil generation stage (similar to 1.06% R-o), and that of the Upper Shanxi Formation entered the early oil generation stage (similar to 0.87% R-o). Geochemical results suggest good hydrocarbon potential of all coal samples but poor potential of most mudstones and shales. The generated hydrocarbons generally originate from the in situ source rocks, and no obvious migrated hydrocarbons exist due to the low production indexes. delta C-13 and delta N-15 values of coals are significantly more negative than those of mudstones and shales, suggesting that the plants flourished with efficient photosynthesis and the coals experienced intense microbial nitrogen fixation during the coal-forming period. Rapid propagation of terrestrial plants leads to the increase of carbon fixation by photosynthesis and the enrichment of C-13 in the atmosphere. The increase of the sea level may lead to the massive burial of terrestrial plants in peat swamps, which decreases the photosynthesis and increases nitrogen fixation, resulting in the depletion of C-13 in the atmosphere and the enrichment of N-15 in organic matter.