Characteristics Of Oil And Gas Migration And Accumulation In Tight Migmatite Reservoirs In The West Of Yingxiongling Area,Qaidam Basin

The underground Ganchaigou reservoir in Qaidam Basin is the main oil producing reservoir in the west of yingxiongling area of Qaidam Basin.The xiaganchaigou formation is a set of mixed sediments developed in the Saline Lakes.Its mineral species are diverse,strong heterogeneity,pore structure is complex,and the characteristics of oil migration and accumulation are unknown,which seriously restricts the large-scale exploration and development of tight oil.In this paper,the samples of dense migmatite in the upper member of xiaganchaigou formation in the west of yingxiongling area are selected.Based on the rock thin section observation,X-ray diffraction tests,high-pressure mercury intrusion(HPMI)and micron CT scanning,this paper carries out the physical simulation experiment of oil accumulation,studies the migration and accumulation characteristics of tight mixed rock reservoir,and discusses the oil migration and accumulation model of tight mixed rock reservoir.The results show that:The upper member of xiaganchaigou formation in the west of yingxiongling area is a typical migmatite reservoir,which is composed of terrigenous clastic minerals,carbonate minerals,clay minerals and sulfate minerals.The lithology classification scheme is determined according to the mineral characteristics of the reservoir,and the lithology of the reservoir is divided into four types: migmatite,limestone,sandstone and gypsum.The average porosity and permeability of tight migmatite reservoir in the study area are 4.11±2.88% and 0.0418±2.88 m D respectively,and micro fractures are developed.Porosity and permeability are mainly affected by lithology,micro pore structure and micro fractures.The pore radius of the reservoir is concentrated in2.5～10μm.The throat radius is concentrated in 0.5～7.5μm.There are mainly four pore structure types: microporous full throat type,microporous fine throat type,full pore fine throat type and full pore full throat type.Among them,the physical properties and micro pore structure parameters of limestone reservoir are better than those of other reservoirs.The oil migration in tight migmatite reservoir is characterized by non Darcy seepage,there is starting pressure,the seepage curve is concave,and the oil saturation growth curve is gradually stable after rapid increase.The oil migration and accumulation are jointly affected by lithology,physical properties,pore structure and micro fractures.Oil migration and accumulation patterns can be divided into three types: the Ⅰ type is high minimum migration pressure gradient low oil saturation slow growth type;TheⅡ type is the medium minimum migration pressure gradient-medium oil saturation-steady growth type;The Ⅲ type is low minimum migration pressure gradient-high oil saturation-rapid growth type.Tight migmatite reservoirs with different pore structures have different oil migration and accumulation mechanisms.The more carbonate minerals in the reservoir,the higher the permeability,and the more developed macropores,large throats and micro fractures are,the more conducive to the migration and accumulation of oil and gas in tight migmatite reservoirs.Full pore full throat and microporous full throat tight migmatite reservoirs are favorable reservoirs.