Hydrocarbon accumulation characteristic and exploration on large marine carbonate condensate field in Tazhong Uplift

Plenty of resource evaluation and exploration work in Tazhong Uplift have confirmed its superior accumulation condition and abundant oil-gas resources in marine carbonate rocks,dominantly occurring in the reef-bank body of Upper Ordovician and weathered crust of Lower Ordovician.In this area,large condensate reservoirs' distribution,which is remarkably characterized by multi-superimpositions in vertical and layer-like features in horizon,is mainly controlled by relative cave-fracture systems.Researches from both multi-discipline and combination of static and dynamic analysis show that large condensate reservoirs in Tazhong Uplift,mostly the carbonate reservoirs of marine face,are primarily feed by two suite of carbonate rocks of Cambrian and Ordovician,whose raw oil is obviously of mixed sources feature;comparatively,the gas mainly comes from the highly matured pyrolysis gas of the Middle and Lower Cambrian;moreover,the Ordovician condensate reservoirs developed in the Himalayan period by the intrusion of Cambrian pyrolysis gas on the palaeoreservoirs,which has experienced three periods of accumulation:oil injected during Late Caledonianand-Late Hercynian period,while gas injected during Himalayan period.Hydrocarbons from two sets of source rocks are the material basis of forming large condensate gas fields of Tazhong Ordovician marine carbonate;tectonics and karstification are the dominant factors that form multiple sets of high-quality carbonate reservoirs in Tazhong Ordovician,additionally,mesh-conduct systems of oil and gas constructed by faults,unconformities and fracture-cave development belts are the important guarantee of accumulation mechanism of Tazhong marine carbonate,which displays complex accumulation and mingled source rocks.Interestingly,Ordovician marine carbonate has a potential reserve of 1 billion bbl hydrocarbon.