Breakthrough pressure and in-source/near-source migration of shale oil in saline lacustrine hybrid source rock systems-A case study of the West Qaidam Depression, Qaidam Basin, China

In this study, an innovative breakthrough pressure detection system for shale oil is introduced. Experiments were conducted on source rocks from three main rock types in the upper member of the Lower Ganchaigou Formation in the Western Qaidam depression, in Qinghai, China. The results show that the differences between the breakthrough pressures of laminated calcareous mudstone (LCM), siltstone (SS), and massive mixed mudstone (MMM) in the formation are of several orders of magnitude. In particular, the shale oil breakthrough pressure of laminated calcareous mudstone is more than five times greater in the vertical bedding direction than in the horizontal bedding direction. As black medium shale oil turns into yellow light shale oil, the breakthrough pressures in the same lithology and direction are reduced by two-thirds. In laminated mudstone the horizontal breakthrough pressure is lower than the vertical, while in massive mixed mudstone the vertical breakthrough pressure is lower. A composite migration model for shale oil in hybrid strata-horizontal migration along bedding and vertical migration through micro-fractures-is proposed. The results are of great significance for understanding the accumulation of shale oil and for identifying exploration targets.