Lithofacies Characteristics And Sweet Spot Analysis Of Fine-grained Sedimentary Rocks In The 1^st Sub-member Of The 2^nd Member Of The Kongdian Formation In The Cangdong Sag

The 2^nd Member of the Kongdian Formation of the Cangdong Sag in Bohai Bay Basin is a typical lacustrine depression that developed in the continental setting.The fine-grained sedimentary rocks developed in the center of the sag have become an important field of shale oil exploration.They have the characteristics of wide range,large thickness,and high abundance of organic matter.Since 2018,five horizontal wells deployed in the 1^st Sub-member of the 2^nd Member of the Kongdian Formation（Ek2¹）have achieved stable and high production,with daily crude oil production up to 80t.Due to the complex mineral composition and rapid vertical change of fine-grained sedimentary rocks in this member,the development of fine-grained sedimentary rocks is not completely clear,which restricts the evaluation and prediction of shale oil sweet spot.Based on the abundant drill core,well logging and experimental data,this paper studies the characteristics and spatial distribution of organic-rich fine-grained sedimentary rocks in the 1^st Sub-member of the 2^nd Member of the Kongdian Formation in the Cangdong Sag,evaluates the hydrocarbon generation capacity and reservoir property of different lithofacies assemblages,identifies the dominant lithofacies combination types,and predicts the sweet spots and distribution areas of shale oil exploration district.Through a detailed description of the core at centimeter-level,XRD,thin section identification and other methods,according to mineral composition,rock type,sedimentary structure,and other characteristics,the fine-grained sedimentary rocks in the study area can be divided into three major categories and six sub-category lithofacies.Three major categories of lithofacies are fine-grained felsic sedimentary rocks facies,fine-grained mixed sedimentary rocks facies and carbonate facies,and sub-category lithofacies are further divided according to laminar and massive structures class.Because of the fast vertical change of lithofacies,the interpretation accuracy of the logging curve is difficult to meet the needs of the division and correlation of layers in actual production.Based on the relative mineral content of different rocks,combined with the"red model"method of three-porosity logging,five lithofacies assemblages are identified,including terrigenous lithofacies assemblage（assemblage 1）,terrigenous mixed sedimentary lithofacies assemblage（assemblage 2）,mixed sedimentary lithofacies assemblage（assemblage 3）,authigenic mixed sedimentary lithofacies assemblage（assemblage 4）,authigenic lithofacies assemblage（assemblage 5）.The five lithofacies assemblages are distributed stably in the small layers and show the changes of authigenic-terrigenous-authigenic lithofacies assemblages from bottom to top.It shows that the sedimentary period of the 1^st Sub-member of the 2^nd Member of the Kongdian Formation in the Cangdong Sag experienced a complete process of sedimentary evolution from shallow to deep and then shallow.Through the evaluation of hydrocarbon generation ability and reservoir property of different lithofacies assemblage,it is considered that lithofacies assemblage 1 is a favorable lithofacies assemblage with high abundance,good type and moderate maturity of organic matter.Lithofacies assemblage 1and 5 are advantageous reservoirs lithofacies assemblages with the large number of matrix pores and microfractures,good pore connectivity,large total pore volume,total specific surface area,and average pore diameter.Therefore,it is considered that lithofacies assemblage 1 is the advantageous lithofacies assemblage type of the Ek2¹ in the Cangdong Sag,and layer 2 is the sweet spot horizon.Sweet spot area is located on the north and south sides of the Kongdong Fault,north to Fenghuadian and south to Xiaoji,while the thickness is more than 10m.Qilidian and Wangguantun areas have the thickest sweet spot,with the total thickness over 14m.It can be seen as the most favorable areas for the development of sweet spots.