On the internal oil migration in shale systems and implications for shale oil accumulation: A combined petrological and geochemical investigation in the Eocene Nanxiang Basin, China

Internal oil migration within shale systems is of significance to shale oil accumulation as indicating the retention, expulsion, and accumulation of shale oils. However, conventional geochemical proxies to study this internal oil migration are commonly overlapped, and thus the migration is still well understood. Here we conduct a case study using the methodology of combined petrology and geochemistry, rather than the geochemistry alone in previous studies which are mostly bulk and thus cannot reveal the complex migration in different scales including macro (millimeter), micro (micron), and nano. The study area is in the lacustrine Eocene Biyang sag, Nanxiang Basin of eastern China. Results show that three types can be identified according to the petrological characteristics of the internal oil migration. Their geochemistry was further clarified. In detail, according to the correlation between residual hydrocarbons (S-1) and total organic carbon content (TOC): Type A, inward migrated oils from external sources within the shale system; Type B, weak expulsed oils to external space; and Type C, strong expulsed oils to external space. The amount of hydrocarbon expelled per unit rock mass (Q(expulsion)) for migration types A, B and C are < 0, 0-1.5 mg/g rock, and > 1.5 mg/g rock, respectively. They have good bulk and molecular geochemical responses. Typically, Type A samples have the highest saturates/aromatics and n-C-19/Sigma MP ratios, with Type C having the lowest ratios and Type B having intermediate values. Stable carbonisotope compositions indicate the level of internal oil migration too. Our data suggest that combined petrological and geochemical methods can effectively characterize internal oil migration in shale oil systems. A good combination of source and reservoir in shale systems, i.e., interbedded or juxtaposed Type A and Type C sequences is favorable for shale oil accumulation. This is universal and obviously provides useful data for regional exploration.