Pore evolution of organic maceral in Shahezi Formation shale of Changling fault depression, Songliao Basin

As an important part of shale reservoir space, organic pores possess strong heterogeneity, which hinders the correct understanding and evaluation of shale reservoir quality. The heterogeneity is essentially affected by organic macerals and their pore evolution during hydrocarbon generation process. Through located observation based on the field emission scanning electron microscopy and optical microscopy for pore development of the specific macerals, combined with Image J digital process technique, the pore evolution law of different organic macerals was summarized by quantitative statistics of macerals at different evolutionary stages. The results show that the porosity with solid bitumen is first increased and then decreased with the increase of maturity. The solid bitumen porosity is the highest when SBR_O rangs from 1.6% to 2.0%, while the porosity begins to decrease when SBR_O exceeds 2.0%. The pore development models of vitrinite and inertinite are similar. The both porosity decreases first and then increases slightly as maturity rise. In the oil window stage, the porosity of vitrinite and and inertinite is the lowest, because the filling of inorganic minerals and solid bitumen in the primary cell lumen make the loss of pores more than 90%. In the high mature stage, pore development of solid bitumen makes the original cellular pore getting a certain degree of recovery, becoming the main contribution and accounting for 56.73% and 100% of residual pores of vitrinite and inertinite respectively. It can be seen that solid bitumen pores are of importantance to the shale reservoir. Combined with sedimentary diagenesis and hydrocarbon generation, the pores of shale reservoir are the most developed in the immature stage and the late stage of high maturity. The former is dominated by primitive cell pores, while the latter is dominated by solid bitumen pores. Clarification of the pore evolution pattern of organic macerals can provide a significant reference for favorable shale reservoir prediction and reconstruction of shale gas development.