Controlling Factors And Evaluation Of Storage Capacity In Marine Shale And Transitional Shale Of South China

Sweet spots prediction needs 2 demands: deep analysis of different sedimentary evolutionary backgrounds and precise exploration of main controlling factors of shale reservoirs.Large-scale commercial exploitation of shale gas has been achieved in marine shale of Lower Silurian Formation inside and at the margin of Sichuan basin,South China.However,the breakthroughs of gas exploration are rare in both marine shale of Lower Cambrian Formation and transitional shale of Upper Permian Formation.The above 3shale series were selected in this paper.Based on the differences of sedimentary tectonic settings,geochemical attributes,petrological features and pore structure characteristics,main controlling factors of storage capacity has been explored.In addition,evaluation model is established to rank storage capacity of targeted shales in South China.Shale reservoirs in different geological backgrounds have various material bases and evolutionary degrees.Many factors are exploited in lithofacies classification,in terms of geochemical attributes(macerals,Ro and TOC)and minerals(components,originals and fabrics).Pore types and structural characteristics are various in either different formation or different lithofacies.For Lower Cambrian shale,pinhole OM-hosted pores and isolated dissolved pores are the main types.Also,OM-hosted pores are extremely small and mineral-related pores are mostly filled causing low pore volume(PV)and high pore specific surface area(PSA).Typical lithofacies of Lower Cambrian shale is I type organic-rich siliceous shale with at over thermal maturity(ORI-S).For Lower Cambrian shale,cellular OM-hosted pore is the main type with large diameters and complicated surfaces,leading to high PV and high PSA.Typical lithofacies of Lower Silurian shale is I type organic-rich siliceous shale with at high thermal maturity(HRI-S).For Upper Permian shale,pore associated with clay flakes is the main types,while OM-hosted pore is rare,in result of high PV and low PSA.Typical lithofacies of Upper Permian shale is III type organic-medium argillaceous shale with at high thermal maturity(HMIII-CM).Influence factors of Shale reservoir capacity can be listed as OM type,thermal maturity,OM abundance and mineral fabric.OM-hosted pore is the main type in marine shales since OM can migrate with I type,while pore associated with clay flakes is the main type in transitional shale as OM is featured by inertness with III type.A large number of thermogenic pores can develop in type I OM,while a few sedimentary inherited pores are exist in type III OM.At similar thermal degree(2.6%Ro),PV(PSA)of isolated type I OM are approximately 6 times(8 times)higher than that of isolated type III OM.moreover,with same OM type(I),isolated OM at high thermal degree(2.6%Ro)are about 2.2 times(1.4 times)higher than that of isolated OM at over thermal degree(>3.0%Ro).For Lower Cambrian shale,pores of graphitized OM are minute or disappear and over thermal degree limit the storage capacity.For Lower Silurian shale with appropriated thermal degree,OM abundance controls the storage capacity.For Upper Permian shale,feasible mineral fabric(Si/Al=2/3)is the favorable condition for the protection of pore associated with clay flakes.Based on the analysis of influence factors,analytical approach of controlling factors is utilized to establish the evaluation mode of storage capacity.Storage capacities of both free gas and adsorbed gas are conducted by the evaluation mode.Target areas with marine shales and transitional shale in South China are ranked in according with literature research and data collation.The storage capacity of Lower Cambrian shale is restricted by the degree of thermal evolution,so exploration should focus on regions around the paleo-tectonic uplift with relatively shallow paleoburial depth and relatively low maturity.The storage capacity of Lower Silurian shale is controlled by organic matter abundance,so exploration should focus on the deep-water shelf center with high TOC and high thickness.The storage capacity of Upper Permian shale is controlled by mineral fabric,so silty argillaceous lithofacies in coastal-marsh and tidal-lagoon sedimentary area with high thickness and continuous distribution are favorable for exploration.