Formation of natural fractures and their impact on shale oil accumulation in the Mahu Sag, Junggar Basin, NW China

Abstract

Lacustrine shale oil reservoirs of the Fengcheng Formation in the Mahu Sag, Junggar Bain, NW China contain abundant oil resources, with the highest daily oil production of a single well reaching over 700 barrels. Based on core and thin section observation along with borehole image interpretation, this study integrated paleotectonic stress regime, fluid inclusion analysis, and calcite UPb dating to clarify fracturing periods and discuss the influence of fractures on shale oil accumulation. Results show that natural opening-mode fractures are widespread in the Fengcheng Formation, which play an important role in oil accumulation. Fractures are developed in several stages and filled by three minerals: reedmergnerite, quartz and calcite. Reedmergnerite and quartz fillings show similar fluid inclusions and petrographic features, with numerous hydrocarbon inclusions compared to aqueous ones, yet, such inclusions were less observed in calcite fillings. Fractures in the study area were formed in two separate stages. In the first period, due to the NW-SE compression of the Hercynian movement, the WNW-ESE and NNW-SSE striking fractures were formed in the Late Permian, with Th values of 95–115 °C. The first stage of fractures coincided with hydrocarbon generation, providing migration pathways for the oil. In the second period, due to the nearly SN compression of the Indosinian movement, the NNW-SSE and NNE-SSW striking fractures were formed in the late Triassic, with Th values of 120–140 °C. The second stage of fractures was formed subsequent to major hydrocarbon generation and expulsion period, hence their contribution to hydrocarbon migration and accumulation was found relatively minor. Moreover, fractures with the nearly E-W, ENE-WSW and WNW-ESE strikes intersect with the present-day maximum horizontal principal stress at a small angle, with large apertures and good connectivity, which should have a positive impact on shale oil accumulation.