Special Elemental Sulfur in the Dolomite Reservoirs of the Giant Puguang Gas Field: Proof of Crude Oil Involved Thermochemical Sulfate Reduction

Special elemental sulfurs formed by thermochemical sulfate reduction (TSR) are reported for the first time in the Upper Permian Changxing Formation and the Lower Triassic Feixianguan Formation, Northeastern Sichuan Basin, Southwest China. They occur mainly as single pellets with round morphology, and partly in semi or ¼ pellets of 10 to 100 μm in diameter, or form interpenetrating grains. Most of them precipitate at the inner wall of the vugs, oomoldic or inter-granular pores of the dolomitic reservoirs. Petrogenetic sequence analysis demonstrates that they postdate the dolomite formation and cementation, while are prior to the late calcite and quartz cementation The sulfur develops restrictively in the section where pyrobitumen is found, namely in the paleo-oil window, which strongly supports that the crude oil/liquid hydrocarbon had been involved in the TSR.