Thermal Evolution, Hydrocarbon Generation, and Heat Accumulation of a High Geothermal Coalfield: A Case Study of Pingdingshan Coalfield, China

Abstract

As an important energy base in central China, the Pingdingshan coalfield has abundant coal and geothermal resources. The cooperative exploration of coal and geothermal resources is significant for the comprehensive utilization of energy resources. This work collected coalbearing samples from the Pingdingshan coalfield to investigate the tectonothermal evolution of a high geothermal coalfield, especially the present geothermal field and hydrocarbon generation model. The geochemical results show that the Shanxi and Taiyuan source rocks have average R o values of 0.88 and 0.97%, respectively, with an average Rock-Eval T max value of 442 °C. Hydrocarbon generation of source rocks started at ∼205 Ma, with the highest rates at ∼170 Ma, reaching the maximum transformation ratio of 40−50% in the middle of the Early Cretaceous. The age and length of apatite fission tracks (AFTs) indicate that coal-bearing strata underwent significant postdepositional annealing after the Late Permian and suggest an abnormal thermal event that occurred in the Late Mesozoic. Meso-Cenozoic thermal event was mainly caused by the plutonic metamorphism of the Early Jurassic and magmatic thermal metamorphism of the Early Cretaceous, achieving a maximum paleotemperature of ∼140 °C. The magmatic thermal event resulted from the intensive post-orogenic extension of the Qinling-Dabie Orogenic Belt caused by the tectonic transition of the North and South China Plates. The present-day high geotemperature of Pingdingshan Coalfield is dominated by the horst structure caused by the regional extension of the basin-mountain system. The Cambrian limestone with a high thermal conductivity underlying coal measure collects deep heat, forming a heat accumulation center of this horst structure with a heat flow of 74 mW/m 2 and a maximum temperature of ∼50 °C nowadays.