Distribution And Controlling Factors On Oil Reservoir Of Jurassic Yan 9 In The Ordos Basin

Recently,the exporation of Jurassic reservoir in the Ordos Basin has shown a good potential.In plane view the Jurassic reservoirs were distributed in pairs of spots.In order to explor the enrichment regularity of Juraccis resorevoirs and its controlling factors,the Yan 9 member in Tianciwan area was taken as an example.This paper is aimed at fine classification in strata,great depiction on low amplitude structures,better understanding of sedimentary and reservoirs and more discussion on occurrence and controlling factors of petroleum by fundamental theories and methods of petroleum geology associated with the data of drilling,well logging,mud logging and well tests.Yan 9 member was divided into Yan 91 and Yan 92;and Yan 92 was divided into Yan 921,Yan and Yan 923.Four layers were steadily distributed in the studied area.The small-scale low-amplitude structure can not effectively identified by seismic method.Therefore,large scale mapping method was used to describe the structural characteristics of each layers according to well logging data in this study.In terms of tectonic structures,oil reservoir in Yan 9 member developed 4 groups of northwest-southeast low amplitude nose-like structures under the background of monocline,and showed up as higher amplitude in the southeast and otherwise in the northwest.These low amplitude structers were resulted from differential compaction and regional tectonic stress.According to sedimentary analysis,oil reservoir in Yan 9 member belongs to fluvial deposit,where river channels,flood lands,natural levees and flood fans can be found;and horizontal distribution of sand bodies is mainly controlled by sedimentary facies and appears like fluvial sand bodies.The study area situates outside the source sag of the Ordos Basin,which led the oil reservoir spotlike distributed and complicated oil-water relationship.Oil enrichment regions are coincident with the low-amplitude structures which indicates low-amplitude structures have an important influence on the degree of hydrocarbon enrichment.Besides,thin sand layers and thick sand layers with strong heterogeneity generally have good oil-bearing capacity.Conversely,thick sand layers with weak heterogeneity have poor oil-bearing capacity.This phenomenon indicates barrier condition is an important controlling factor of accumulation in the sandstones far from source rock.Based on the special data,the critical oil column height for accumulation was in the range of 4.16-12.48 m by physical principles.The limit values of porosity and permeability for the effective reservoirs were determined as 15%and 9 mD,respectively,using distribution curve method,which showed that porosity over 15%is favorable for oil enrichment.The results of oil-source correlation revealed that oil in Yan 9 member may be derived from the souce rocks in Chang 7 member,and the connection between them relies on vertical superimposed sand bodies,fractures and unconformity surface on Chang 1 member.Finally,the reservoirs forming mode of Yan'an formation which is far away from source rock was established.And the suggestions of exploration for various types of reservoirs were also provided.