Analysis Of Deep Water Deposition Mechanism Of Chang 7 Oil Reservoir In The Southern Triassic Ordos Basin

For a long time,lithofacies analysis of sedimentary structures and lithology has become the main means of interpretation of sandstone genesis.In deep water sedimentation,apart from the sandstones that may have multiple beddings,massive sandstones with no obvious beddings are also widely developed.In order to reduce the multi-solutions of sandstone genetic interpretation based on lithofacies analysis and solve the problem of genetic identification of massive sandstone with no obvious beddings,this paper focuses on the Upper Triassic Chang 7 oil reservoir in the outcrop of the southern Ordos Basin.Based on the analysis of field profile and sedimentary structure,the use of rock flake analysis and laser particle size analysis of high-density rock samples is used to judge the deep-water sandstone genesis mechanism by studying the particle size distribution,their vertical variation and the statistical results of the intersection.The research shows that the study area develops the two deep-water sedimentation about the sliding gravity flow and the flood-type hyperpycnal flow.The former is caused by the sediments of the shallow water environment,such as delta and shore,which are triggered by earthquakes.The latter is caused by the high-density rivers directly injected into the deep water area.And there are differences between the two genesis mechanisms.Firstly,the common landslide deformation,the development of mudstone tearing debris,and the lack of red-brown boulder outside the basin is the features of the deep-water sandstones caused by the collapse.And the parallel bedding,large interlaced bedding,rich land-source oxidized color gravel and carbon scraps and the imbricate structure gravel of the bottom bed load is common in the deep-water sandstones caused by the flood-type hyperpycnal flow.Secondly,the grain size distribution of the sandstones caused by the collapse has a unimodal characteristic,and the suspended component is dominant;however,the grain size distribution of the sandstones caused by the flood-type hyperpycnal flow has twin peaks,the part of the bottom bed load is increased,and the suspended load is it's roughly the same.In the particle size parameters,the sandstones of the collapse are moderate sorted,and the coarse fraction is poorly sorted.Their Sahu index is close to 9.8433.But,the sandstones of the flood-type hyperpycnal flow are poorly sorted,and the coarse fraction is better sorted.And their Sahu index is about 7.On the cross-section map of particle size parameters,there are obvious differences between the sandstones of different genesis and the delta sandstone.The results of this study confirm that the analysis of the sandstones microstructure improves the genetic interpretation of deep-water sandstones based on lithofacies analysis,and also provides a scientific identification of deep-water sandstones genesis,and deepens the understanding of deep-water sandstones sedimentary model about the collapse and the flood-type.