Formation of Unconformity Surface at Cretaceous Bottom and Its Significance for Hydrocarbon Migration in Chepaizi and Its Surrounding Areas of Junggar Basin

The unconformity surface at Cretaceous bottom,which widely develops in Chepaizi area of Junggar Basin,is one of the most important pathways for migration of the Permian and Jurassic oils to the Cretaceous and Neogene reservoirs.The micro-structures and geometric configurations of the unconformity are important for the exploration of petroleum.By analyzing data of cores,logging curves and cuttings logging,three-layer structure of unconformities is identified.Then,further study on the rocks on the unconformity surface,and weathered clay bed and semi-weathered crust of the unconformity surface at Cretaceous bottom is carried out,in which the formation of the unconformity surface and the effects of each layer on hydrocarbon migration are investigated.The formation of the unconformity surface at Cretaceous bottom in the study area is divided into four stages: uplift-weathering period,denudation-transportation period,subsidence-deposition period and diagenetic compaction period.The study shows that the rocks above and below weathered clay bed can be effective hydrocarbon migrating pathways,and the weathered clay bed could be local caprock in the area where faults are rare.Oil and gas migrated along the Cretaceous bottom unconformity from south-east to the uplift of Chepaizi.