Main sedimentary structure types of marine fine-grained terrigenous clastic rocks and their significance for shale gas
Sedimentary structures can not only provide insights into the formation environment and hydrodynamics of marine fine-grained terrigenous clastic rocks,but also significantly influence the permea-bility and fracturability of shale gas reservoirs.Based on comprehensive analysis of relevant materials at home and abroad,the types and origins of marine fine-grained terrigenous clastic rocks were systematically summarized.The rocks exhibit physical,chemical and biological structures.The physical structures mainly include flow structures and soft-sediment deformation structures.Flow structures include cross lamination,horizontal bedding or lamination,massive structure,graded lamination,and composite lamination.Soft-sediment deformation structures include slumps and slides,load structures,flame structures,ball-and-pillow structures,convolute lamination,contorted lamination,disrupted lamination,and dropstones.Chemical structures comprise carbonate concretes and pyrites,with biogenic structures primarily consisting of trace fossils and bioturbation.The three major types of sedimentary structures are mainly the result of fine-grained turbidites,contourites,and pelagites-hemipelagites,with a few arising from sediment de-formation.Variations in sedimentary structures directly influence the permeability of shale gas reservoirs and affect the generation and propagation direction of hydraulic fractures.
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