Structural characteristics of the unconformities on the top of Pre-Tertiary and reservoir types in Jizhong Depression

The unconformities on the top of Pre-Tertiary play an important role in the accumulation of hydrocarbon and the formation of buried hill in Jizhong Depression.By the analyses of seismic data,the unconformities on the top of Pre-Tertiary can be divided into five types:fault-overlap unconformity,truncation-overlap unconformity,fault-fold unconformity,truncation unconformity and parallel unconformity.The fault-overlap unconformities distribute in steep slope belt;the truncation-overlap unconformities distribute in gentle slope belt;the fault-fold unconformities distribute in central uplift belt;the truncation unconformities distribute in uplift region;the parallel unconformities distribute in hydrocarbon generation region.By field investigation,core observation,well-logging response and drilling data,the unconformities on the top of Pre-Tertiary are vertically divided into three parts:basal conglomerate,weathered clay layer and leached zone.There are massive solution pores,holes and fractures in the leached zone,which is a good place where hydrocarbon migrates and accumulates.Currently,it is discovered that there are three types of oil and gas reservoirs related to the unconformities,they are runcation-overlap unconformity reservoirs,corrosion buried-hill reservoirs and fault-fold unconformity reservoirs.