Study On Hydrocarbon Preservation Condition Of Marin Carbonate Rock In Jianghan Plain

Jianghan Plain is located in the eastern of middle Yangtze region, It has undergone multi-phase basin evolution and tectonic rebuilding processes. The superposition of multiple different types of antetype basines have afford crucial material basis for hydrocarbon accumulation. Studies indicate that preservation condition are the the key controlling factors of oil and gas exploration. The traditional static evaluation for marin carbonate rocks suggested their initial preservation conditions was better, but it failed to explore all the time.。the multi-phase tectonic deformations in superimposed basin had initial preservation condition differently improved or destroyed, so the way of traditional static preservation is not suitable for evaluation of real preservation condition. In this article, a new way of preservation condition evaluation was introduced. Isotope geochemistry, fluid inclusion and geohydrochemistry are employed to trace the source of fluid and determin whether there were fluids from paleo-oil or paleo-gas reservoir in reservoir overlying seal, In that, paleo preservation condition for different tectonic units can be suggested. Based on the comparison of preservation condition of different tectonic units, better tectonic units with better preservation condition can be determined.Research indicated that there were two fluid systems in Dangyang synclinorial zone. Upper fluid system developed in Ja-lingjiang Formation and came from itself. Lower fluid system developed in Daye Formation and Permian, two stage injection of fluid can be indentified,the first stage's fluid was from lower Cambrian and the second one was from lower Triassic or Permian. All the facts suggested that preservation condition for middle-upper association is better during later stage's fluid injection.Research based on samples from drilling wells and outcrop indicated that the fluid in lower association had migrated vertically across and the preservation condition had been worse in Chenhu-Tuditang synclinorial zone, the former hydrocarbon moved upward to middle-upper association and reaccumulated. The preservation condition in southeast is relatively better than that in northwest and paleo-oil and gas in northwest was destroyedStudies based on hydrogeochemistry suggested that preservation condition in south was better than that in north because the connate water in south was more enclosed than that in north of Lexiangguan-Qianjiang synclinorial zone. Nappe footwall located at Bahong thrust anticline and Chongyang-Tongshan thrust anticline are supposed to be enclosed, so they may be better for hydrocarbon conservation.Comprehensive comparison for preservation condition in different structural belts shows that preservation condition in most of lower association was destroyed and hydrocarbon was reaccumlated in middle-upper association. In general,the preservation condition in synclinorium is better than that in anticlinorium located in the middle-upper association.The preserving conditions in Chenhu-Tuditang synclinorium is better than that in Dangyang synclinorium, The preservation condition in Lexiangguan-Qianjiang anticlinorium is better than that in Bahong thrust syncline and Chongyang-Tongshan thrust syncline.