Distribution of Retained Oil in Shales from the Lucaogou Formation, Jimusar Sag, Northwest China: Insights from Mineralogy and Pore Structure

Abstract

Shale oil resource is abundant in the world, which has been the dominant incremental oil production in recent years for several countries. A determination of pore structure, as well as the state and distribution of retained oil, not only is critical in identifying the controlling factors for the differential accumulation of shale oil but also affects the exploration and development of potentially favorable intervals in shale-oil systems. Twelve lacustrine shale samples from Lucaogou Formation (P2l) of Junggar Basin were selected to analyze the influence of mineral compositions and organic matter abundance on the pore system development and the retained oil distribution in pore-scale. A series of analyses, including thin section petrography, field emission-scanning electron microscopy (FE-SEM), quantitative evaluation of minerals by SEM, solvent extraction, mercury intrusion porosimetry, and nitrogen physisorption, were carried out to investigate the linkage between pore structure and occurrence of retained oil. The results indicate that the samples from Lucaogou Formation are dominated by slit-shaped, mineral-related pores with a small amount of organic matter-hosted pores. Feldspar has undergone an extensive dissolution, as shown by the development of abundant dissolved inter-particle and intra-particle pores, which significantly improve the reservoir property. The development of quartz overgrowth had a negative impact on the pore structure, and carbonates underwent processes of both dissolution and cementation. The mineral sizes show a positive relationship with pore diameter, and sedimentary microfacies close to the source contained more coarse components such as shore bar and distal bar are more favorable for oil exploration. Retained oil in pores with diameters of 100–1500 nm primarily occur in the free state as a thin oil film, whereas oil in pores with diameters smaller than 100 nm is adsorbed. Moreover, the retained oil volume is affected by organic matter abundance, the storage type, as well as the material source-reservoir configurations.

Influence of Shale Mineral Composition and Proppant Filling Patterns on Stress Sensitivity in Shale Reservoirs

“…Shale reservoirs are renowned for their tightness, with the development of large pores and microfractures serving as the primary natural conduits for oil and gas migration [36]. In this study, permeability stress sensitivity tests were conducted on natural core samples from two lithological shale formations in Jimusaer, Xinjiang under cyclic loading conditions.…”