Reservoir space characteristics and exploration of shale oil mobility of the Jurassic Lianggaoshan Formation shale in the northeastern Sichuan Basin

The Jurassic Lianggaoshan Formation shale is a key exploration interval in the Sichuan Basin, but its pore structure and shale oil mobility are still unclear. In order to reveal the reservoir space characteristics and the mobility of shale oil in the Lianggaoshan Formation, this paper divided lithofacies types according to sedimentary structure and mineral composition. Basic geochemical characteristics were obtained by total organic carbon determination, rock pyrolysis and vitrinite reflectance experiments. The porosity and pore structure were characterized and evaluated by means of field emission scanning electron microscopy, nuclear magnetic resonance, low temperature nitrogen adsorption and high pressure mercury injection. The difference in reservoir space characteristics between different rock phases was also compared. With a centrifugation time of 3 h and centrifugation speed of 11 000 r/min, quantitative evaluation of the mobility of shale oil with different lithofacies was carried out by NMR centrifugation and the influential factors are clearly defined. The evaluation model of movable oil quantity logging was established and. the favorable rock facies was selected. The results show that: (1) The TOC of Lianggaoshan Formation shale is mainly between 0.15%~2.95%, the R_o is between 1.06%~1.68%, and the shale is in the mature-high mature stage. After the recovery of light hydrocarbons, the change range of S₁ was 0.03 mg/g ~3.32 mg/g. The mineral types are mainly clay minerals and quartz. The developed lithofacies are lamellar clay shale facies, lamellar felsic shale facies, lamellar mixed shale facies and massive silty mudstone facies. (2) Shale reservoir space types are mainly clay mineral intergranular pores, organic matter pores, in addition to quartz dissolution pores, interparticle pores, pyrite intergranular pores and microfractures. The porosity is between 1.15% and 4.71%. The shale has a wide pore size distribution. The pore volume is mainly contributed by mesopores and macropores smaller than 200 nm. Laminated clay shale has the best physical properties (3) the movable oil content of the Lianggaoshan Formation shale ranges from 0.25 mg/g to 3.26 mg/g, and the movable oil rate ranges from 5.13% to 44.8%. Lamellar clay shale has the best mobility, and massive silty mudstone has the worst mobility. TOC, clay mineral content and porosity are the key factors controlling movable oil content in the Lianggaoshan Formation. Based on these three factors, a mobile oil quantity prediction model is established and verified. Laminated clayey shale is preferred as the key exploration object of the Lianggaoshan Formation, which is indicative of the exploration and development of shale oil in the Sichuan Basin.