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

Abstract

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.

Geochemistry of the Permian Oil Shale in the Northern Bogda Mountain, Junggar Basin, Northwest China: Implications for Weathering, Provenance, and Tectonic Setting

“…The oil shales of the Lucaogou Formation in the study area are mainly dark grey, grey black, and black, which were formed in the deep lake semi deep lake environment. ,,, The U/Th values of the oil shales in the Sangonghe area were 0.38–0.94 (average 0.76), the V/(V + Ni) values were 0.65–0.94 (average 0.78), and the Cu/Zn values were 0.28–2.20 (average 0.61) (Figure ). The U/Th, V/(V + Ni) and Cu/Zn values of the samples in the Dongdalongkou area were 0.64–0.92, 0.41–0.76, and 0.51–1.17, respectively, with the average values being 0.80, 0.65, and 0.91.…”

“…33−38 The oil shales of the Lucaogou Formation in the study area are mainly dark grey, grey black, and black, which were formed in the deep lake semi deep lake environment. 14,19,20,39 The U/Th values of the oil shales in the Sangonghe area were 0.38−0.94 (average 0.76), the V/(V + Ni) values were 0.65−0.94 (average samples in the Dongdalongkou area were 0.64−0.92, 0.41−0.76, and 0.51−1.17, respectively, with the average values being 0.80, 0.65, and 0.91. In addition, the samples in the Wujiawan area were corresponding to each other.…”

Main Controlling Factors and Development Model of the Oil Shale Deposits in the Late Permian Lucaogou Formation, Junggar Basin (NW China)

“…The main succession exposed in the northern region of Bogurda Mountain comprises Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary successions from the bottom to the top. , Oil shales occur in the Permian Lucaogou Formation, and the oil mine lies east of Urumqi and extends from Demon Mountain to Huangshanjie, including the districts of Shuimogou, Dongshan, Lucaogou, Dadonggou, and Sangonghe. Sangonghe was the depocenter of this region when oil shale was deposited. ,, The author and related investigators explored the oil shale mine in Sangonghe and in other districts with outcropping sections and performed systematic sampling. The Songonghe section has a length of 1635 m, a vertical accumulative thickness of approximately 700 m, and a maximum oil shale thickness up to 205 m. The oil shale is well-developed in the study area reflecting the significant influence of the oil shale formation mechanism and ancient lake environment.…”

Characteristics of Upper Permian oil shale and analysis of the resource development potential in the Lucaogou Formation in the Junggar Basin, northern Bogda Mountains, Northwest China

“…The total organic carbon (TOC) of oil shale in the northern Bogda Mountains is 5-34.75 wt%, in the western region it is 5-19.7 wt% (average 9.4 wt%) and in the central and eastern regions respectively 5.86-32.23 wt% (average 11.80 wt%) and 5.78-34.75 wt% (average 9.8 wt%). The vitrinite reflectance (Ro) of 0.6-1.13% and T max of 433-454 °C indicate that oil shale in the study area is low-maturity to mature [22]. [20] showing the kerogen types in oil shale of the Lucaogou Formation.…”