Study On Petroleum Accumulation Conditions Of The Yiliping Area,Qaidam Basin

Located in the central Qaidam Basin,the Yiliping area has been thought as one of the most important exploration areas by the Qinghai Oilfield of Petrochina in recent years.Based on the formers'achievements,this research evaluated hydrocarbon source rocks,analyzed petroleum accumulation conditions and process,and built petroleum enrichment models for the Yiliping area through interpreting seismic,drilling and logging data,observing drilled cores and testing shale and sandstone samples.The thickness of Neogene in the study area is large,and the sedimentary facies are mainly deep lake and semi deep lake.The Neogene dark mudstone,which is widely developed in this area,is the main source rock.The organic matter types of source rocks are mainly type II₂ and type III.The average organic carbon content is about 0.4%,and the maturity of organic matter is low,with average Ro of 0.56%.The Jurassic below in the study area could also be regard as source rocks.In the study area,there are huge thick mudstone deposits,and the vertical mudstone and sandstone occur alternately,forming a good reservoir cap combination.Structural traps,including anticline and faulted anticline,are mainly developed at the study area.Caused by the late Himalayan tectonic movement,faults are relatively developed in the study area,which provide vertical and lateral carriers for oil and gas.Combined with unconformities between the Jurassic and Paleogene,widely distributed siltstone and argillaceous siltstone of the Neogene can serve as horizontal carriers for oil and gas migration.Every set of source rocks has entered the corresponding hydrocarbon generation stage at the late Pliocene,and the time and space distribution of source rocks and traps is relatively good.In a comprehensive view,the three enrichment models were established,i.e.,“coal gas accumulation model controlled by deep-large faults and unconformity”,“oil type gas accumulation model in primary reservoir”,and“oil type gas-biogas mixing accumulation model”.According to the comprehensive analysis of the above conditions and gas test data,three favorable areas are predicted:the Yikeyawuru favorable area,the Jianshishan-Luoyanshan favorable area and Eboliang favorable area.