The Characteristics Of Sedimentary Facies And Sandbody Distributionin Chang 8 Reservoir Of Mubo Area

The Mubo area is located in the southwest of Ordos basin,across the Tianhuan depression and Yishan slope,belongs to the braided river delta depositional system.At present,there are more disputes about the types of sedimentary facies and the regularity of sandbody distribution in the study area.Therefore,study of the source direction and the characteristics of sedimentary microfacies and identification of the sandbody can provide some theoretical support for later oil and gas exploration and development.This paper based on the analysis of sedimentary evolution background,through the core and thin section data,the fossil record data and combined with conventional logging data,analyzed the basic data,on the basis of the latest theory about sedimentology,sedimentary petrology,logging geology,Paleontology and Stratigraphy,mineral petrology,and combined with the latest research achievements in the study area,divided and contrast the stratum meticulously in order to define the vertical and horizontal characteristics of the strata.On the basis of source analysis,by statistical the light and heavy mineral,the analysis of sandstone fragmental components to determine the source direction.Based on the description of petrology,clear sedimentary system and divide sedimentary facies groups by combining regional sedimentation and tectonic evolution background.Analyzed the characteristics of sedimentary facies and microfacies from point?single well?to line?well-connecting?to surface?distribution?.Through the fine recognition and characterization of single sand body,compared the vertical and horizontal of composite sandbody,anddescribed the characteristics of the distribution of sandbody,clarified the characteristics on sedimentary facies and sandbody distribution ofChang 8 reservoir in Mubo area.The results show that there is an obvious source area in Chang 8 reservoir of Mubo area,namely the source area of the southwest.The content of zircon and garnet in the north and west is higher,and the content of garnet and tourmaline in the east is higher.The study area of Chang 8 reservoir is braided river delta deposit,divide delta plain and delta front two subfacies,the subfacies of delta plain can be divided into three microfacies of distributary channel,natural dike and distributary depression.The leading edge of the delta can be divided into three microfacies of underwater distributary channel,underwater natural dike,and underwater diversion bay.In the west of the study area,Chang 8₂²layer was the delta plain deposits,the sand body was not far away because of the shortage of westernprovenancesupply and the passage of the lake shoreline.Chang 8₂¹ layer developed delta group,affected by the provenance,the study area in southern and southwestern channel differentiation serious,channel development and width is about 5-8 km,The sandbody extends far away,distributary depression is underdeveloped,distributary channel frequently interacts.In Chang 8₁³layer western sources were strengthened,the northwest fan delta group is widely distributed in north and south and the middle channel is large.In Chang 8₁²layer the extended distance of the western fan delta group is shortened,the underwater distributary channel continues to shrink in the southwest and the sedimentation of distributary channel is enhanced in the southeast.In Chang8₁¹layerwhole water body is deepening,the western fan delta is gradually decreasing,there are only discontinuous residual channels and sheet sand deposits in the middle andthe river depression appeared in the southwest-northeast to spread.The research area of Chang 8₁¹layerdeveloped small-scale sand bodies in the source direction.In the west and southwest of the study area of Chang 8₁²layer,the sandbody thickness is generally large and widely distributed and there is a set of thicker sand bodies in the north of area.In the west and southwest of the study area,the distribution of sandbody in Chang 8₁³ layer is very good,with a large set of sandbody spreading,and the sandbody thickness is generally smaller in northeast,north,southeast and south.The sandbody in the northwest of Chang 8₂¹ is much thicker than other parts,and the thickness of sandbody in the northeast is generally smaller.The Chang 8₂²layerare controlled by micro-facies,such as underwater distributary channel and underwater diversion bay,and mudstone is widely developed in the sandbody.Research suggests that the spatial distribution of sand bodies is controlled by the source direction and sedimentary microfacies.