Identification of Oil Produced from Shale and Tight Reservoirs in the Permian Lucaogou Shale Sequence, Jimsar Sag, Junggar Basin, NW China

The Permian Lucaogou Formation of the Jimsar Sag in the Junggar Basin, NW China, is one of the largest areas of oil exploration and exploitation in the lacustrine shale sequence in China. Oil is commercially extracted from this markedly heterogeneous formation, which is characterized by frequently interbedded shale and tight reservoirs, although producible intervals within the Lucaogou Formation remain unresolved. This study focused on the Jimsar Sag reservoirs to investigate petrological and mineralogical characteristics, source rock and reservoir physical properties, and the molecular biogeochemistry of core extracts and crude oils. The i-C-18/n-C-18 and Pr/n-C-19 ratios of two-step ultrasonic extracts were applied to infer whether the oil is produced from shale or tight reservoirs, taking into account solvent polarity, molecular characteristics of n-alkanes and isoprenoids, and physical properties of the reservoir. The experimental results indicated that the shallower pay zone is mainly produced from tight reservoirs, while in deeper zone with organic-matter maturity above 1.0%, some of the oil is produced from shale. The reservoir properties in organic-rich shale with vitrinite reflectance (Ro) exceeding 1.0% are improved by pore interconnectivity, and oil mobility is enhanced by high gas/oil ratios, which favors production of free-phase hydrocarbons. Such zones may become major prospects for shale oil exploration and production. The results of the present study can potentially apply to sweet-spot identification and development optimization for the Lucaogou shale and other lacustrine shale sequences.

Automated summary

The Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin, NW China, is a significant area for oil exploration and production in lacustrine shale sequences. Commercially extractable oil is found in this heterogeneous formation, with different reservoir properties influencing oil production from shale or tight reservoirs. The study focuses on petrological and mineralogical characteristics, source rock properties, and molecular biogeochemistry of core extracts and crude oils to identify potential sweet spots for shale oil exploration and production.