Origins of vein-forming fluid and hydrocarbon accumulation in Shiniulan Formation in Dingshan, southeast Sichuan Basin

The Silurian hydrocarbon accumulation research in the southeastern Sichuan Basin lacks evidence of the hydrocarbon accumulation and destruction stages in reservoirs. Bitumen and calcite veins that were developed in the Shiniulan Formation had recorded the information of the fluid evolution in reservoirs and have important implications for fluid activities and hydrocarbon accumulation processes in the Dingshan area, southeast Sichuan Basin. Based on analysis of petrographic, trace element, fluid inclusion and laser Raman spectroscopy of the calcite veins, this research determines the timing and origin of the paleo-fluids formed the calcite veins; recovers the paleo-pressure of the methane inclusions, and reveals the petroleum system evolution and controlling factors of the Shiniulan Formation. The results indicate that two phases of calcite veins were developed in the Shiniulan Formation, and oil charging happened before the formation of the two phases of the calcite veins. The first phase of calcite veins were formed at ~127 Ma under relative reduction conditions, with the forming fluid sourcing from the diagenetic fluid of the same strata, which show brown-red cathode luminescence colors. The second phase calcite veins were formed at ~83 Ma under relative oxidation conditions, showing brown-yellow cathode luminescence colors. Oil generation peak of the Longmaxi source rocks, which were the main source of the Shiniulan reservoirs, was from ~190 Ma to ~150 Ma. Oil to gas cracking happened at ~135 Ma. The trapping pressure of methane inclusions is 51.77-57.46 MPa, and the pore fluid pressure coefficient is 1.26-1.40. Since the Yanshanian period, a series of tectonic movements have taken place in the Dingshan area, which uplifts and lead to erosion and fracturing of the Dingshan area is accounts for the destruction of the Shiniulan gas reservoirs.