The Study On Sedimentary Characteristics Of Fine-grained Rocks Of Tenth Rhythmite Of Third Member In Qianjiang Formation,Jianghan Basin,Central China

Qianjiang sag is located in the middle of Jianghan Basin.The tenth rhythmite of the third member in Qianjiang Formation of Qainjiang Sag developed intra-salt fine grained rock(FGR),which is an important target for shale oil exploration in Jianghan Oilfield.In recent years,although some achievements have been made in the study of FGR in this area,due to the complex lithology,the characteristics and control factors of evolutionary process and reservoir are still unclear.In this thesis,based on analysis of drilled cores,thin sections,scanning electron microscopy observation,combined with the logging data and test data,an intensive study of petrological characteristics,sedimentation,lithofacies classification,evolutionary process,and reservoirs characteristics is conducted.And then the major controlling factors of evolutionary process and reservoir are identified,with an overall understanding of the characteristics of distribution of the high quality reservoirs in the area,provide the basis for shale oil exploration in the Qianjiang Sag.The main minerals in the FGR of the study area include calcite,dolomite,clay minerals,glauberite and anhydrite,and the minor minerals are quartz,feldspar and pyrite.Based on the petrological characteristics of FGR,three kinds of sedimentation are identified: 1)bio-chemistry,2)evaporation chemistry,3)gravity flow.Based on the mineral composition and sedimentation of FGR,the classification scheme of FGR is proposed.According to 50% of carbonate minerals,evaporate minerals and argillaceous debris,combined with sedimentary structure and special composition,nine kinds of lithofacies are identified,and then the genesis of each lithofacies is explained.Through the study of “point-line-surface”,combined with the lithofacies characteristics and sedimentation,first the study area is divided into four microfacies types: biochemical event,event biochemistry,evaporation biochemistry and evaporation event.Secondly,the contour map of TOC,carbonate,argillaceous carbonate and carbonate mixed shale lithofacies are drawn in the study area,and then evolutionary process of FGR is analyzed.The main controlling factors of FGR include climate,biology,tectonism and emergency.It provides favorable conditions for the formation of FGR in the study area under the environment of humid climate,microbial eruption and tectonic movement.The intra-salt FGR reservoir mainly develops micron-and nano-scale pores.The reservoir space types include: intergranular pores,organic matter pores,intercrystalline pores,dissolution pores,fractures,etc.Among them,intergranular pores,organic matter pores,dissolution pores and fracture are the relatively high-quality reservoir space types.The quality of reservoir of FGR is mainly controlled by organic matter,mineral composition and lithofacies.Organic matter is an important factor affecting reservoir development,which determines the development of reservoir space and hydrocarbon generation capacity.Mineral components control reservoir physical properties.Among the different lithofacies,carbonate is the main component of three lithofacies: carbonate,argillaceous carbonate,carbonate mixed shale lithofacies have the best reservoir properties.Based on the above characteristics,the reservoir types in the study area are divided into three types,and type I reservoir is considered as the favorable area of the area,with the characteristics of specific sequence,zone,and district.