Thermal and hydrocarbon generation history of Wufeng and Longmaxi shales in Pengshui area, eastern Sichuan Basin: A well PY1 case study

The erosion thickness and paleogeothermal gradient of the Pengshui area in the eastern Sichuan Basin during various tectonic movements were established by means of vitrinite reflectance forward modeling and basin modeling software (BasinMod 1-D). Since the Late Paleozoic, the paleo heat flow and paleogeothermal gradient in the study area first increased and then decreased. The average paleogeothermal gradients from Silurian to the Middle Permian, from the Middle Permian to the Early Cretaceous, and from the end of the Early Cretaceous until now were 25–30 °C/km, 30–35 °C/km, and 20–25 °C/km, respectively. In the Caledonian and Indosinian periods, the erosion thickness of the study area was less than 500 m; while in the Yanshanian and Himalayan periods, it reached 4 300 m. The Wufeng and Longmaxi shales entered the threshold of oil generation in the Early Devonian, and reached peak generation from the Late Permian to the Late Triassic. Gas generation started in the Early Jurassic, and over-mature dry gas was generated in the Middle Jurassic. In the following Yanshan and Himalayan movements, the study area was uplifted and eroded, which raised challenges for the preservation of shale gas.