Research on paleo-fluid sources and hydrocarbon preservation conditions in marine carbonates inthe central Yangtze,China

87Sr/86Sr,δ18O andδ13C ratios of calcite,dolomite,gypsum filling vugs and fissures in marine carbonates and their host rocks from the Sinian to the middle Triassic,are employed to trace the possible source and migration path of key fluids related to development of hydrocarbon,hydrocarbon preservation condition are then discussed further.Comprehensive research,based on the paleo-fluid,the property of formation water and the deformation intensity etc.,indicates that the preservation conditionsin the Shizhu synclinorium zone and Fangdoushan anticlinorium zone are the most predominant in the central Yangtze.Three sets of fluid systems in the Shizhu synclinorium zone are identified.Little fluid exchange occurs between the upper-middle fluid system and the lower fluid system,so two independent preservation units have developed.Both the Permian and the Triassic in the upper preservation unitsand the Dengying Formation of the Sinian and the lower Cambrian in the lower preservation units havegood preservation conditions.The preservation condition in the lower association(Z-S)located in the Huaguoping synclinorium zone in the western Hubei and Hunan is better than that in other tectonic units,where the fluids in the lower association migrated vertically across strata when the fluid isolating intervalswere destroyed,the regional seals are absent and the conditions for the preservation of hydrocarbon accumulations are totally destroyed.The preservation condition is increasingly more favorable from the western Jiangxi and Hunan to the western Hubei&eastern Chongqing on the whole.