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

Shaleoil resource is abundant in the world, which has been thedominant incremental oil production in recent years for several countries.A determination of pore structure, as well as the state and distributionof retained oil, not only is critical in identifying the controllingfactors for the differential accumulation of shale oil but also affectsthe exploration and development of potentially favorable intervalsin shale-oil systems. Twelve lacustrine shale samples from LucaogouFormation (P(2)l) of Junggar Basin were selected to analyzethe influence of mineral compositions and organic matter abundanceon the pore system development and the retained oil distribution inpore-scale. A series of analyses, including thin section petrography,field emission-scanning electron microscopy (FE-SEM), quantitativeevaluation of minerals by SEM, solvent extraction, mercury intrusionporosimetry, and nitrogen physisorption, were carried out to investigatethe linkage between pore structure and occurrence of retained oil.The results indicate that the samples from Lucaogou Formation aredominated by slit-shaped, mineral-related pores with a small amountof organic matter-hosted pores. Feldspar has undergone an extensivedissolution, as shown by the development of abundant dissolved inter-particleand intra-particle pores, which significantly improve the reservoirproperty. The development of quartz overgrowth had a negative impacton the pore structure, and carbonates underwent processes of bothdissolution and cementation. The mineral sizes show a positive relationshipwith pore diameter, and sedimentary microfacies close to the sourcecontained more coarse components such as shore bar and distal barare more favorable for oil exploration. Retained oil in pores withdiameters of 100-1500 nm primarily occur in the free stateas a thin oil film, whereas oil in pores with diameters smaller than100 nm is adsorbed. Moreover, the retained oil volume is affectedby organic matter abundance, the storage type, as well as the materialsource-reservoir configurations.