Lithofacies Characteristics And Spatial Distribution Of Sedimentary Facies In The Tight Oil-bearing Strata Of The Chang 7 Oil-bearing Formation In The Ordos Basin

This paper takes the Chang 7 oil-bearing formation in the Ordos Basin as the research object.The lithofacies,depositional environment and sedimentary facies of the Chang 7 oil-bearing formation are analyzed based on predecessors' studies and multi-disciplinary theory.And a spatial combination mode of lithofacies and sedimentary facies is established.Based on the analysis of core and logging data,the facies characteristics of the Chang 7 oil-bearing formation are identified for the first time systematically.There are mainly 9 facies including massive silty sandstone facies,fine sandstone facies,Bouma sequence fine siltstone facies,horizontal silt facies,massive silt facies,horizontal bedding shale facies,argillaceous siltstone-silte mudstone facies,tuff facies,and interlaced bedding fine sandstones.The lithofacies planimetric distribution maps of every single layer are also systematically prepared on the basis of lithofacies and logging characteristics.The sedimentary geochemical method was combined with sedimentary markers and sporopollen assemblage analysis to analyze the paleoenvironmental conditions of the Chang 7 oil-bearing formation in the study area.The Sr/Ba ratio indicates that the water salinity is a terrestrial freshwater environment and the partial area is brackish water deposition environment;Sr/Cu ratio and sporopollen combination analysis showed that the paleoclimate climate was warm and humid;V/Ni ratio indicated that the redox condition was anoxic reduction environment.And the depth of the ancient water body is between 40 and 150 m,combined with the quantitative calculation of cobalt(Co)element content by sedimentary characteristics,which indicating that the water body of the lake basin is a semi-deep lake-deep lake environment.The sedimentary facies types,characteristics and evolution of the Chang 7 oil-bearing formation are studied systematically.It is reckoned that the Chang 7 oil-bearing formation mainly develops channel-type gravity flow sediments and lake basin delta deposits,which mainly develop main near source channel,the far source water channel,the embankment deposit,and the former edge of the body,the semi-deep lake-the deep lake.And the delta front mainly develops underwater distributary channels and estuary dams,and the turbidity sediments and deep lake in situ sediments are developed in the deep lake area.According to the analysis of single well and the comparison of well-connected sedimentary facies,combined with the lithofacies planimetric distribution maps,the sedimentary facies planimetric maps of the Chang 7 oil-bearing formation are drawn,and the spatial combination mode of lithofacies and sedimentary facies of Chang7 formation was established.