Hydrocarbon-charge history of hydrothermally altered reservoirs: The Tailai gas field, eastern Sichuan Basin, China

Hydrothermal dolomite reservoirs near basement strike-slip faults are major gas exploration targets in the Sichuan Basin, China. This work addresses the hydrocarbon generation and accumulation histories of the hydrothermal dolomite reservoirs within the Maokou Formation in the Tailai gas field. Diagenetic veining sequences and the hydrocarbon charge history are investigated through fluid inclusion analysis, geochemical data, and radiometric U-Pb dating of calcite cements. All of these data are integrated by tying the geochemical results to the evolution of the nearby No. 15 fault system and burial/thermal models for key wells. The interpreted seismic data indicate that the No. 15 fault system provided major migration pathways for hydrocarbons. The hydrocarbon charge history of the Tailai gas field can be summarized as follows: (1) activity of the No. 15 fault system led to the migration of oil from the Wufeng-Longmaxi source rocks into hydrothermal dolomite reservoirs from ca. 246.9 to 245.3 Ma; (2) reactivation of the No. 15 fault system fed natural gas from the Wufeng-Longmaxi source rocks into the Tailai gas field at ca. 222.4 Ma, with a small contribution from source rock in the first member of the Maokou Formation; and (3) in situ cracking of oil into gas occurred in the hydrothermal dolomite reservoirs from ca. 143 to 110 Ma. The results in this work show that the oil and gas charge (stages 1 and 2) was controlled by the No. 15 fault system. The combination of fault-derived gas and cracking of oil (stages 2 and 3) fed the prolific Tailai gas field.