Favorable lithologic combinations for shale oil enrichment in the second submember of Da'anzhai Member, Jurassic, central Sichuan Basin

Abstract

The lithologic combinations in lacustrine shale strata in China are complex, and shale oil content varies significantly across different combinations. Identifying favorable lithologic combinations will assist in locating shale oil-rich zones. This study takes the shale strata of the second submember of the Da'anzhai Member (Da 2 submember) in Jurassic of the central Sichuan Basin as a case study. Using mineral composition analysis and core observation, four primary lithologies were identified: shell limestone, shale with interbedded shell layers, shell-bearing shale, and pure shale. Various types of pores and fractures develop in each lithology, with bedding-parallel microfractures being more developed in shell-bearing shale and pure shale. To further reveal the pore and fracture system in the shale strata, the proportion of oil-wet and water-wet pores and fractures in the rock (pore and fracture configuration) were determined through one-dimensional nuclear magnetic resonance (NMR) experiments. The pore and fracture configuration values of shell limestone and shale with interbedded shell layers exceeded 60%, indicating restricted oil and gas migration. Due to the development of bedding-parallel microfractures, the pore and fracture configuration values in shell-bearing shale and pure shale were less than 60%, indicating that these lithologies provided more favorable migration channels for oil and gas. Furthermore, influenced by the hydrocarbon supply capacity and the resistance in oil and gas migration, rocks with total organic carbon (TOC) content greater than 1.0% as well as pore and fracture configuration values greater than 60% possessed more residual oil and exhibited better mobility than those with pore and fracture configuration values smaller than 60% (S1ranging from 0.86 to 2.19 mg/g, with an average of 1.42 mg/g; OSI values ranging from 65.96 to 123.21 mg/g, with an average of 91.98 mg/g). Finally, based on the characteristics of organic matter enrichment, the favorable lithologic combinations for shale oil enrichment in the Da 2 submember in the study area were clarified.

Keywords

• toc content
• pore and fracture configuration
• shale oil enrichment
• favorable lithologic combinations
• second submember of da'anzhai member
• central sichuan basin