Evaluation of the Shale Oil Reservoir and the Oil Enrichment Model for the First Member of the Lucaogou Formation, Western Jimusaer Depression, Junggar Basin, NW China

The Lucaogou Formation (Fm) in the Jimusaer depression is the first large-scale development of a terrigenous clastic sedimentary shale oil reservoir in China. Nearly one billion tons of shale oil resources have been discovered. However, the current exploration and development is concentrated in the eastern part of the sag. The limited geological understanding in the western area has restricted the prediction and development of “sweet spots” for shale oil. To help rectify this, we have studied the petrology, geochemistry, oil content, and pore properties of the second part of the first member (Mbr) of the Lucaogou Fm (P2l12) in a typical well (Ji-X) in the western part of the sag. The results show that P2l12 in the Jimusaer sag is a mixed fine-grained sedimentary system composed of sandstone, mudstone, and carbonate, which can be divided into seven types: dolomitic mudstone, calcareous mudstone, mudstone, mixed fine-grained rock, argillaceous limestone, sandstone, and argillaceous dolomite. The organic matter type of P2l12 in well Ji-X is dominated by types I and II, and this is the best source rock in the whole exploration area of the Jimusaer sag. The overall oil saturation is relatively high, but the maturity of crude oil is low, and the overall oil quality is heavy, which is mainly controlled by the sedimentary environment and maturity of source rocks. Lithology and reservoir physical properties are the key to control oil content. The high-quality light oil reservoir lithology is argillaceous dolomite and sandstone. The higher the content of macropores and mesopores, the weaker the heterogeneity of the pore structure, and the better the oil content in the reservoir. There are four light oil sweet spots in the upper part (burial depth less than 4366 m), and there are excellent source rocks with high HI and high organic matter content near each sweet spot. This discovery of shale oil enrichment regularity will effectively guide the development of shale oil in continental lacustrine basins in other parts of the world.

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