Reservoir characteristics and oil properties of a lacustrine shale system: Early Jurassic black shale from the Sichuan Basin, SW China

• Published in: Journal of Asian earth sciences Vol. 242; p. 105491
• Main Authors: Wang, Enze, Guo, Tonglou, Li, Maowen, Xiong, Liang, Dong, Xiaoxia, Wang, Tong, Ouyang, Jiasui
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2023
• Subjects: Da’anzhai shale; Favorable exploration lithofacies; Lacustrine shale; Oil mobility; Shale oil reservoir; Favorable exploration lithofacies; Lacustrine shale; Shale oil reservoir; Oil mobility; Da’anzhai shale
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2022.105491

[Display omitted] •Nanoscale clay mineral-related pores dominate the pore system of the Da’anzhai shale.•TOC content, kerogen type, clay mineral content, and porosity are key controlling factors for oil content in shales.•Real (S1-1/TOC) and maximum ((S1-1 + S1-2)/TOC) movable oil index derived from multistep pyrolysis can be used to evaluate oil mobility.•Oil mobility is mainly influenced by the thermal maturity, physical properties, mineral composition, and oil occurrence state. Early Jurassic lacustrine shales in the Sichuan Basin contain significant resources potential and are one of the most important exploration targets in China. However, studies of these shales are limited and controlling factors on oil contents are poorly known despite the importance of such studies in evaluating favorable exploration targets and risks. In addition, effectively evaluating oil mobility is challenging in lacustrine shale exploration. Herein, lacustrine shales of the Lower Jurassic Da’anzhai Member from the Sichuan Basin are investigated through a series of experiments, including thin sections, field emission scanning electron microscopy, total organic carbon (TOC), X-ray diffraction, nitrogen adsorption, mercury intrusion capillary pressure, routine and multistep pyrolysis, and organic petrographic analysis. Our results show that four lithofacies can be identified in the Da’anzhai shale: organic-lean mixed shale with bioclastics (LM), organic-lean argillaceous shale with siliceous mineral concretions (LCM), laminated organic-rich mixed shale with bioclastics (RM), and laminated organic-rich argillaceous shale (RCM). The pore system is dominated by clay mineral-related (CM) pores, with significant contribution from nanopores. The TOC content is the primary factor that influences oil content, with kerogen type, clay mineral content, and porosity also having important effects. The influences of these factors on oil content are mainly controlled by the oil occurrence states and pore structures. The internal and surficial parts of the organic matter and the widely distributed nanoscale CM pores are the main storage spaces for oil. Oil mobility in the four shale lithofacies varies considerably: the oil mobility of the RCM shale is the greatest, and differences in oil mobility are mainly due to thermal maturity, physical properties, mineral composition, and oil occurrence state. Overall, RCM shale is the most favorable lithofacies for exploration in the Da’anzhai shale. The real movable oil index (S1-1/TOC×100) and the maximum movable oil index ((S1-1 + S1-2)/TOC×100) derived from multistep pyrolysis data can be used to evaluate oil mobility. Our findings may provide new insights into the evaluation of oil resource potential and the exploration of lacustrine shale.