Study On Reservoir Chatacteristics And Reservoir-forming Conditions Of Basement Hydrocarbon Reservoir In Kunbei Area Of Qaidam Basin

Oil-gas is widespread accumulated in the weathering crust of bed rock in Qaidam Basin,and some large scale oil-gas reservoirs have been discovered in Kunbei,Yuejin slope,Mabei and Dongping regions.Based on well drilling,seism,logging,analytical and formation testing data,this paper discussed the geological accumulation conditions and mechanisms,and established the hydrocarbon accumulation models of Kunbei area,for providing some theoretical geology basis to the explorations of weathering crusts hydrocarbon accumulations in the future.Well drilling shows that the bed rock of Qaidam Basin includes igneous rock,metamorphic rock and limestone,and thereinto the composition of reservoir layer of weathering crust in Kunbei is mostly alteration granite and few metamorphic rock.In Kunbei area,the reservoir space of weathering crust is mainly composed of corroded pores,corroded and tectonic fractures,the average porosity of the reservoir is 7.5%,and the average permeability is 0.9×10-3?m3,therefore the oil-gas reservoir is formation-lithology reservoir controlled by the degree of the reservoir development and various formational factors.Weathering crust of bed rock is responded obviously in logging due to its characteristics like high gamma value,low sound wave and large resistance.Based on analysis mentioned above,four basic conditions of the hydrocarbon accumulation are concluded: 1.accumulation areas must be close to hydrocarbon sag and have plenty of supplement;2.fault-unconformity and weathering crust reservoir are formed through the uplift,weathering and denudation effects upon bed rock,so they can offer accumulation conditions and lateral migration pathway to hydrocarbon accumulation;3.fractures of oil sources are formed to link up sources and reservoirs and provide the main vertical carrier system;4.formations like anticlines,fault-anticlines,fault-noses and fault-blocks are formed to offer some advantages to bed rock traps.The oil-gas reservoirs of Qaidam Basin basically belong to outward source secondary accumulation,therefore the fracture of oil source is a crucial factor.Based on the dissection of reservoirs,two accumulation models can be established according to the different lateral migration modes: 1.source fracture,unconformity and lateral secondary fracture migration and accumulation model;2.source fracture,unconformity and lateral fracture migration and accumulation model.