EVOLUTION AND ORIGIN OF ORGANIC MATTER PORE NETWORK IN UPPER PERMIAN DALONG FORMATION MUDSTONES IN GUANGYUAN AREA, NORTH SICHUAN, CHINA— INSIGHTS FROM LABORATORY THERMAL MATURITY EXPERIMENT

The low-maturity marine mudstone samples with type-II kerogen of Upper Permian Dalong formation are taken to study the OM pore characteristics of shale at different maturity stages , based on the thermal simulation experiments of semi-closed system with water, combined with gas adsorption experiments, high-pressure mercury injection experiments and Scanning Electron Microscope observations. The temperature points set in simulation experiments are 350℃, 400℃, 450℃, 475℃, 500℃ and 550 ℃. The maturity of samples show an exponential growth with the increase of the temperature in simulation experiments (The temperature range is 350～550℃, and the maturity range is 0.71~3.30% Ro ). At the begining of oil-window, mudstones contain few OM pores with low porosity. At the main oil-window, abundant OM macropores with pore size greater than 1000nm are formed, but petroleum is retained in the pores as solid bitumen. At the maturity to high- maturity stage, the number of OM macropores decreases significantly, while the number of OM mesopores with pore size of 20~50nm increase rapidly. At the over-maturity stage, the number of mesopores and micropores continue to increase，specially after entering the dry gas peak stage (500℃). On the whole, with the increase of evolution degree, the OM blocks are gradually disintegrated and become smaller, the OM pore structure becomes more complex , and the color of OM becomes lighter. The thermal maturity evolution of OM promotes the formation of abundant OM pores, and the enrichment of OM pores leads to the collapse of organic matrix, in which process the connectivity, diffusivity and permeability of OM pores are significantly improved. The evolution of OM pores is a complex geological process which is closely related to the formation and expulsion of oil and gas, and the occurrence of OM.