Coupled alteration of hydrothermal fluids and thermal sulfate reduction (TSR) in ancient dolomite reservoirs – An example from Sinian Dengying Formation in Sichuan Basin, southern China

• Hydrothermal activity and TSR co-exist in the Sinian Dengying Formation reservoir.
• Hydrothermal fluids provide necessary high temperature condition for TSR to proceed.
• Coupled alterations of hydrothermal activity and TSR enhanced porosity development and preservation.

To better understand the fluid activities and their roles in the formation of petroleum reservoirs, dolomite was thoroughly studied in the ancient Sinian Dengying Formation in Sichuan Basin, southern China. The high homogenization temperatures (above 190 °C) of fluid inclusions, positive Eu anomaly, and O and Sr isotopic values of coarse crystalline pore-filling dolomite (FD) in dissolution pore spaces have suggested that the FDs are of hydrothermal origin. There are also other minerals (pyrite), organic compounds (pyrobitumen) and gas components (H2S, CO2 and CH4) in pore spaces. The sulfur isotope compositions of pyrite and H2S, along with the carbon isotopes of CO2 have shown the involvement of thermochemical sulfate reduction (TSR). Hydrothermal activities during burial diagenesis of the Dengying Formation provided high temperature environment necessary for TSR to proceed. The presence of CO2 and H2S in both processes enhanced fluid acidities and consequently porosity and permeability of the Dengying Formation. Emplacement of petroleum and acidic fluids helped to maintain pre-existing reservoir spaces during deep burial stage. Overall, the coupled hydrothermal activities and TSR provided favorable environments to allow for maintaining high quality reservoirs.