Fluid Inclusion Record Oil Cracking Temperature: Case Study of the Shuijingtuo Formation in the Yichang Area, Hubei

Gas from oil cracking is one of the main sources of marine shale gas in South China. Determination of the temperature of oil cracking into gas is of great significance to the study of the process of shale gas accumulation. Previously, the temperature conditions of oil cracking were mainly based on hydrocarbon simulation experiments and dynamic equations, but the direct geological records are still not sufficient. Through petrographic observation, microthermometry and laser Raman spectroscopy, this study has identified the existence of the bitumen-bearing gas-liquid inclusions, indicative of early oil cracking and restricts the cracking temperature with the homogenization temperatures of the coevally trapped aqueous inclusions. The results show that the early-stage oil inclusions have been trapped later than the precipitation of dolomites and earlier than that of the calcites filling the veins, with the coeval aqueous inclusions homogenizing at 142.1℃, which indicates the lowest cracking temperature of the crude oil. Combined with other information, it is deduced that the dolomite has been precipitating from the late Caledonian to the late Hercynian orogeny, during which point the oil started cracking rapidly (300 Ma). Calcite veins have been formed at the early stage of the Yanshan orogeny (140~120 Ma). The Shuijingtuo formation in the lagoon facies in south Hubei lacks the deposition of Yanjiahe formation, therefore caused by tectonic movements, the migration of early shale oil generated from the Shuijingtuo formation into the dolomite reservoir of the uppermost part of the Dengying formation through dolomite veins, which provides essential constraints to the shale gas accumulation and other natural gas accumulation in the Cambrian strata.