Research On Chang8₁ Reservoir Characteristics And Enrichment Regularity Of Hydrocarbon In Zhenbei Oilfield,Ordos Basin

Chang8₁ oil sublayer of Yanchang Formation?Triassic?is the main production unit in Area M of Zhenbei Oilfield,Ordos Basin.Lack of systematic research on reservoir characteristics and enrichment regularity of hydrocarbon early directly restricts the rolling exploration,reserves and production increase in the later stage.On the basis of reviewing the achievements of predecessors,making full use of the basic geological data and combining with various kinds of texts,a comprehensive and systematic research on the characteristics of Chang8₁ reservoir in Area M is carried out,while the enrichment regularity of hydrocarbon is summarized.The structural pattern of Chang8₁ sublayer in the study area is characterized by a series of nose-like uplifts with strong inheritance in the west-inclined monoclinic background.The Chang8₁ period belongs to the braided river delta front sedimentary environment controlled by southwestern provenance.The sedimentary facies and slope break zone have absolute control over the distribution and scale of sand bodies.Medium-fine-grained and fine-grained lithic feldspar sandstones with medium to low compositional maturity and medium structural maturity are the rock types of Chang8₁reservoir in Area M of Zhenbei Oilfield.Intergranular pore and feldspar dissolved pore constitute the main pore space,and contain a small amount of intercrystalline pore,dissolved pore in lithic fragment and matrix.The average face porosity is 4.7%.The Chang8₁ reservoir in the study area is typical extra-low and ultra-low permeability reservoir.Mechanical compaction,cementation and dissolution determine the direction of physical property evolution of Chang8₁ reservoir.At present,the reservoir is in the late A stage of middle diagenesis.Chlorite cemented residual intergranular pore facies and chlorite,siliceous cemented dissolution pore-residual intergranular pore facies are favorable diagenetic facies belts.Combining the thin-section,scanning electron microscopy?SEM?,pressure-controlled mercury injection porosimetry,rate-controlled mercury injection porosimetry,micro-seepage flow experiment in real sandstone model and measurement of relative permeability curves for a water-oil system to characterize pore structure and seepage flow characteristics in an all-round way,in order to achieve the dual purpose of high accuracy and wide range.Chang 7 source rocks with large thickness,wide distribution and high maturity are the basis for adequate hydrocarbon accumulation.High angle fractures and continuous distribution of composite sand bodies provide channels for hydrocarbon migration.The high abnormal fluid pressure of Chang 7mudstone acts as an important driving force for hydrocarbon migration.Locally high-quality reservoirs are good space for hydrocarbon accumulation.The allocation between sand and mudstone can form lithologic traps that provide effective shelter for oil and gas.The horizontal and vertical distribution characteristics of oil-water are controlled by reservoir physical properties and hydrocarbon differential enrichment in the study area.