Lake Level Controls on Oil Shale Distribution in the Lucaogou Formation, Wujiawan Area, Junggar Basin, Northwest China

摘要

In a thick section of the upper Permian, high-quality oil shale is present in the Lucaogou Formation on the northern flanks of Bogda Mountain, which is located in the southern Junggar Basin, northwest China. However, the distribution of these oil shales has yet to be studied. Based on 56 boreholes and associated oil yield analytical data in the area of Wujiawan in the south Junggar Basin, we investigated the distribution and formation of these oil shales. The oil shales in the Lucaogou Formation have an average oil yield of 6.09 wt %, and the maximum is 22.27 wt %. According to the borehole and oil yield data, the oil shale formed in a third-order sequence and was deposited mainly in the transgressive system (TST) and high stand tracts (HST). The oil shale has an average TOC content of 12.12 wt %, and the average petroleum potential is 66.54 mg/g. The TST oil shale mainly contains type II kerogen, which was derived from terrestrial sources and lake algae. The oil shale deposited during the early stages of the TST occurs locally distributed and occurs as lens-shaped bodies. As the lake level rose, the area over which the oil shale was deposited gradually increased, along with the thickness, extent, and oil yield. The oil shale layers increase in the average thickness from 6.69 to 11.36 m, from the base to top of the TST, and the average oil yield increases from 5.66 to 6.90 wt %. In the HST, the oil shale has an average TOC content of 20.56 wt %. Also, the average petroleum generation potential (S-1 + S-2) is up to 130.41 mg/g. Additionally, the organic matter type is largely type I kerogen and dominantly lake algae. The HST oil shale is thick (up to 130 m), layered, and continuously and widely distributed. Several parasequences can be identified in a single layer of oil shale, and the average thickness and oil yield of each parasequence are 10.13-12.47 m and 5.27-7.07 wt %, respectively. The curve change trends of V/(V + Ni), Ni/Co, and Cu/Zn are divided into three stages: phase I, rise (TST); phase II, high-Value zone (HST); and phase III, stable zone (HST). A similar trend of change was found in comparisons between the central and eastern regions, which showed that with the rise of a lake plane, the lake changes from an oxygen-poor to oxygen-deficient condition, which increases the space and is conducive to the formation of oil shale.