Fine-grained sublacustrine fan deposits and their significance in shale oil reservoirs in the Lucaogou Formation in the Jimsar Sag, Junggar Basin

Abstract

Objective The Jimsar Sag of the Junggar Basin is representative of successful shale oil exploration in China. However, the sedimentary facies in the Permian Lucaogou Formation hosting shale oil has long been disputed. Previous studies proposed two contradictory environments of shallow lakes and semideep to deep lakes in the interior lacustrine basin and identified sandstone interlays as deltaic distal sandbar deposition and beach and bar deposition. Methods The study combined core observations with analyses of sandstone granularity, logging facies, and reservoir properties to identify the development of sublacustrine fans in a semi-deep to deep lake background. Results Fine-grained turbidity currents played a significant role in the transportation and deposition of sediment in these fans. The lithofacies observed included lenticular-wave bedded muddy siltstone to siltstone (dominant type), parallel bedded siltstone, graded siltstone, and massive-graded bedded siltstone to fine-grained sandstone. The cumulative probability granularity curves exhibited smooth upper arches with a high suspension population, and the cross-spots on the C-M diagram were located in the grade suspension zone. The sublacustrine fans consisted of sedimentary microfacies of sheet-like lobes and gravity flow channel levees, showing fining- and coarsening-upwards successions, respectively. These fans contained numerous interlayers of siltstone and fine-grained sandstone, as well as terrigenous detrital particles that influenced the mineral content and enhanced shale oil reservoir properties by promoting the growth of solution pores. Conclusion The study concluded that sublacustrine fan deposits are thin-bedded and fine-grained, and they occur frequently. It also determined that desert reservoirs formed in the areas where sublacustrine fans were deposited, which has significant implications for shale oil exploration in the study area.

Keywords

• junggar basin
• jimsar sag
• shale oil
• fine-grained sediment
• fine-grained turbidity current
• sublacustrine fan
• dessert reservoir