Characteristics And Favourable Lithofacies Prediction Of Fine-Grained Sedimentary Rocks In The Lower Part Of Shahejie Formation In Dongying Sag

With the deepening of exploration of shale oil and gas,the fine-grained sedimentary rocks have gradually become a hot topic of sedimentology study.Widely developed thick fine-grained sedimentary rocks in the lower part of Shahejie Formation in Dongying Sag are the main source rocks in the study area,and also a potential exploration field for shale oil.Lithofacies is the basic unit of fine-grained sedimentary rocks study.The mineral petrology characteristics of fine-grained sedimentary rocks in the lower part of Shahejie Formation in Dongying Sag were analyzed,which based on core observation,thin section identification and X-Ray diffraction analysis methods.Combained with the sedimentary structure and mineral composition,the fine-grained sedimentary rocks were divided into six lithofacies types: laminated fine-grained mixed sedimentary rocks,massive fine-grained mixed sedimentary rocks,laminated felsic limestones,massive felsic limestines,laminated limestones and massive limestines.Vertically,the thickness of single lithofacies was small and frequently changes,and the interpretation accuracy of logging data can hardly meet the research needs,so the research was based on lithofacies assemblage.Using the "red mode" method of lithofacies identification,combined with the characteristics of lithofacies,four main lithofacies assemblage were identified: terrigenous mixed sedimentary lithofacies assemblage(assemblage1),terrigenous endogenous interbedding lithofacies assemblage(assemblage 2),endogenous intercalated terrigenous lithofacies assemblage(assemblage 3)and endogenous lithofacies assemblage(assemblage 4).The distribution of lithofacies assemblages is controlled by provenance and climate.Vertically,the lithofacies assemblage changed from endogenous type to terrigenous mixed type,reflecting that the climate gradually became humid.During the Es4 cs sedimentary period,the climate was relatively dry,assemblage 4 developed intensively at the bottom and transited upward to assemblage 3 and 2,while assemblage 1 developed only in thin layers locally,showing the characteristics of short-term alternation of dry and wet in the overall dry environment.During the Es3 x sedimentary period,the climate was humid,assemblage 1 was widely developed,assemblage 2 and 3 were sandwiched in a thin layer,the assemblage 4 was not developed,which showed the characteristics of short-term drought in humid climate.Horizontally,the lithofacies assemblages were distributed in a ring belt,from the edge to the center of the lake basin,the lithofacies assemblages gradually transformed from terrigenous mixed type to the endogenous type.In the northern steep slope of the Dongying Sag,assemblages 1 was relatively developed because of the short terrigenous clastic transport distance and sufficient provenance supply;in the southern gentle slope,the terrigenous clastic transport distance was long,and the provenance supply was reduced,resulting in dominating assemblages 2 and assemblages 3;in the center of the sag,assemblages 3 and assemblages 4were mainly developed,because there was almost no terrigenous clastic input,and the provenance supply was insufficient.The characteristics of organic matter in different lithofacies assemblages of fine-grained sedimentary rocks in the lower part of Shahejie Formation in the Dongying Sag had obvious differences.Lithofacies assemblage 1 with high organic matter abundance and good hydrocarbon generation potential,is a favorable hydrocarbon generation lithofacies assemblage.The heterogeneity of fine-grained sedimentary rock reservoir in the study area was strong,and the reservoir property of different lithofacies assemblages varies.Lithofacies assemblage 3 had various reservoir space types and wide pore size distribution,making it a favorable reservoir lithofacies assemblage with good pore connectivity.Based on the theory of "source reservoir co-control",the shale oil "sweet spot" of Es4cs-Es3 x in the Dongying Sag were evaluated.It is considered that the layer 2 of Es4 cs and the layer 3 of Es3 x are "sweet spot" of shale oil horizon,and the "sweet spot" area is concentrated in the east part of Lijin Sub-sag,the west part of Niuzhuang Sub-sag and the north part of Boxing Sub-sag.