The Characteristics Of Chang 8 Reservoir And The Enrichment Regularity Of Hydrocarbon In The Heshui Area Of Ordos Basin

Heshui Area is located in the southeast of Longdong area in Ordos Basin,and Chang 8 of Yanchang Formation of Upper Triassic is one of the main oil producing strata in this area.Due to the lack of systematic understanding of the reservoir characteristics and the enrichment regularity of hydrocarbon in the early stage of the study area,it directly restricts the work of increasing reserves and production and rolling edge expansion.Under the guidance of sedimentology,petrology,sequence stratigraphy,petroleum geology and logging,through field profile,core observation,drilling,logging data and a variety of experimental testing methods,this study carried out a comprehensive and systematic study on the characteristics of Chang 8 reservoir and the distribution of oil and gas in Heshui area,so as to clarify the combination relationship between source reservoir and cap rock.The main achievements are as follows:(1)the structure of Chang 8 sublayer is gentle as a whole,and It is monoclinic that show an increasing trend on the whole from West to East.The thickness of each sublayer is stable on the plane,with an average of 26.2m.(2)The lithology of Chang 8 series is mainly lithic feldspathic sandstone and feldspathic sandstone,with low petrochemical maturity and medium preference for physical maturity.The porosity is 7.77%and the permeability is 0.19×10^-3?m²,mainly intergranular pore dissolution.It is a low porosity and ultra low permeability reservoir with high interstitial content and complex pore throat structure.(3)Chang 8 oil-bearing formation is a typical braided river delta front deposit.Vertically,sandbodies are overlapped frequently,and underwater distributary channel with multi-stage superimposition is the main reservoir.On the plane,the channel sandbodies are banded and continuously distributed in southwest and northeast directions.(4)The diagenesis of the reservoir is mainly compaction and pressure solution,cementation and dissolution.Dissolution and chlorite film in diagenesis are conducive to the improvement of the physical properties of the reservoir.Compaction and cementation are the main reasons for the low porosity and low permeability of the reservoir.(5)Chang 7 Series is located in a large area of organic rich source rocks in the semi deep lake and deep lake sedimentary facies belt.The thickness of source rocks gradually thinned from northeast to southwest,and the thickness is mainly distributed in 20-30m.(6)Chang 8 reservoir can be divided into four types.The physical properties of type I to type IV reservoir become worse in turn,and type IV reservoir is the main type.(7)Oil and gas enrichment is affected by sedimentary microfacies,and generally accumulates in turbidite channel and underwater distributary channel microfacies.Under the control of diagenesis,the favorable diagenetic facies zone is more conducive to hydrocarbon accumulation.Vertically,the residual pressure of Chang 7₃formation is more than 15MPa,and the oil and gas produced by Chang 7₃source rock migrate along the micro fracture to Chang 8 in low pressure area.On the plane,oil and gas are mainly concentrated in the area with relatively low residual pressure,and the migration of oil and gas is characterized by high pressure to low pressure.