Fracturability Of Fine-grained Sedimentary Rocks Influenced By Different Fabric And Stress Directions

Fine-grained sedimentary rocks are rich in oil and gas resources in China.However,due to the complex composition of fine-grained sedimentary rocks and obvious structural differences,it is difficult to explore and develop them.China is now in the rapid development stage of oil and gas exploration and development in fine-grained sedimentary rocks,but the technology is not mature enough.Fracturability evaluation is an urgent problem to be solved.In this paper,the main influencing factors of fracturability are studied by studying the lacustrine fine-grained sedimentary rocks of Mesozoic and Cenozoic in China.The Cenozoic samples mainly come from Huangxian Basin,Fushun Basin and Huadian Basin,and the Mesozoic samples mainly come from Fuxin Basin.The three-terminal mineral components(felsic minerals,carbonate minerals and clay minerals)in Huangxian Basin and Huadian Basin are less than 50%,mainly migmatite,with relatively high TOC content,while felsic minerals are dominant in Fushun Basin and Fuxin Basin,mainly felsic sedimentary rocks,with relatively low TOC content.In this paper,combined with previous studies,a fracturability index which combines static brittleness index,fracture toughness and triaxial compressive strength is proposed,and the control effect of each factor is evaluated by the correlation between the fracturability index and other factors.In terms of mineral composition,the more felsic mineral content,the better fracturability;for plastic and brittle rocks,the role of carbonate minerals is different,but generally speaking,the more carbonate mineral content,the better fracturability;the more clay mineral content,the worse fracturability.In terms of organic matter composition,the more TOC content,the worse the fracturability,and the higher the fracturability index of rocks containing type II kerogen.In terms of grain structure,the larger the number of fine-grained sedimentary rocks,the larger the average particle size,the larger the variance of particle size,the worse the sorting,the lower the roundness,the better the fracturability.In the aspect of laminar structure,the smaller the number of laminae in fine-grained sedimentary rocks,the larger the width of laminae,the greater the variance of laminae width,the better the continuity of laminae and the better the fracturability.As for the stress direction,the stress direction has an effect on the elastic modulus and fracture toughness,but has no relation with Poisson’s ratio,brittleness index and compressive strength.Samples cored in parallel laminar direction are easier to fracturing.Fracturability of fine-grained sedimentary rocks is the result of interaction and mutual influence of various factors.Grey relational analysis is used to analyze the influencing factors of fine-grained sedimentary rocks.It is found that the laminar structure has the greatest effect on controlling the fracturability of fine-grained sedimentary rocks,and the others are felsic mineral content,related parameters of grain structure and other components in turn.