Permian geothermal units in the Sichuan Basin: Implications for the thermal effect of the Emeishan mantle plume

Abstract

The Emeishan mantle plume is supposed to play an important role in the Permian thermal regime and source rock hydrocarbon generation evolution in the Sichuan Basin, Southwest China. In this paper, we provide new valuable insights into the spatial thermal effect of the Emeishan mantle plume to the Sichuan basin and maturity evolution of the Paleozoic source rocks. Cambrian core samples from four wells were selected for zircon fission track and zircon (U–Th)/He analysis in order to determine their thermal histories. Then the thermal histories of typical wells throughout the basin were obtained based on the above result and previous studies. According to the peak heat flow value (q), four Permian geothermal units were classified: a stratospheric heat flow unit (q ≥ 90 mW/m2), a high heat flow unit (80 ≤ q < 90 mW/m2), a medium heat flow unit (70 ≤ q < 80 mW/m2), and a normal heat flow unit (q < 70 mW/m2). The interpretation of the abnormal Permian heat flow indicates that the enormous heat flow anomalies with values of >20 mW/m2 occurred in the stratospheric and high heat flow units was mainly caused by the mantle plume, while the weak Permian thermal perturbation with heat flow anomalies of <15 mW/m2 in other two units was more likely associated with crustal thinning. The thermal effect of the Emeishan mantle plume may be contained within the stratospheric and high heat flow units. By comparing the maturation histories of the Paleozoic source rock, we identified the thermal effect on the maturity evolution. The thermal perturbation had no obvious impact on the maturity evolution of the Lower Permian and Lower Silurian Longmaxi Formation. Only the Lower Cambrian Qiongzhusi Formation within high geothermal unit suffered significant thermal effect of the mantle plume and matured rapidly during the end of the Early Permian.