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

Abstract

The northern Bogda Mountain is located in the southern margin of the Junggar Basin in northwest China. Based on inorganic geochemistry of oil shale samples of the Middle Permian Lucaogou Formation, the depositional environment, weathering degree, the provenance, and tectonic background of the northern Bogda Mountain were determined. The oil shale in the study area was formed in the deep lake environment. The major and trace element ratios suggest that the oil shale was deposited under semi-humid to humid/warm conditions and a brackish water to salt water setting with strong reduction. The chemical index of alteration values indicates that the oil shale experienced weak to moderate weathering conditions. Geochemical data suggest that the parent rocks of oil shale in the Sangonghe area, the Wujiawan area, and the Dongdalongkou area were siliceous rocks from the North Tianshan Mountain, basalts from the Kelameili Mountain and siliceous rocks mixed with basalts from the North Tianshan Mountain, and the Kelameili Mountain, respectively. According to the parent rock types and the thickness distribution of oil shale, the sediment overspread of ancient rivers from the Kelameili Mountain was larger than that from the North Tianshan Mountains. Combined with the tectonic and sedimentary evolution, it can be inferred that the North Tianshan Mountain area and the Kelameili Mountain area were characterized by a continental collision island arc environment during the Carboniferous period. The differences of weathering and provenance led to the differences of nutrient lead phosphorus (P) between different regions of the ancient lake, which thus led to the lateral heterogeneity of oil shale in its thickness and quality.

Mineralogy and Geochemistry of the Lower Cretaceous Jiufotang Formation, Beipiao Basin, NE China: Implications for Weathering, Provenance, and Tectonic Setting

“…42−44 When the climatic conditions change from warm to dry, the evaporation of water gradually increases, and the elements precipitate in the form of salts, resulting in a relative high content of these elements in the sedimentary rocks. 45,46 The marked depletion in these elements relative to UCC also indicates a semihumid to humid/warm climate in the source area of high quality oil shale layers (Figure 5a). 5.5.…”

“…The results showed that the high quality oil shales mainly occurred in a semihumid to humid/warm climate, whereas low quality oil shales were deposited under a semihumid to semiarid paleoclimate (Figure f). When the climate is warm and humid, the dissolved amounts of Sr, V, Ni, and Ba are relatively large. − When the climatic conditions change from warm to dry, the evaporation of water gradually increases, and the elements precipitate in the form of salts, resulting in a relative high content of these elements in the sedimentary rocks. , The marked depletion in these elements relative to UCC also indicates a semihumid to humid/warm climate in the source area of high quality oil shale layers (Figure a).…”

Factors controlling the distribution of oil shale layers in the Eocene Fushun Basin, NE China

“…Inorganic geochemical indicators such as P, Cu, Cd and Ni are positively correlated with TOC and have been applied to indicate the relative bioproductivity evolution of ancient lakes (Tyrrell, 1999;Li et al, 2020). The P/Ti and Cd/Al ratios have been widely adopted as indices of water bioproductivity, with high ratios indicating high lake bioproductivity (Zeng et al, 2015;Fathy et al, 2018).…”

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

“…In recent years, research on the oil shale in the northern Bogeda Mountains has been documented; this work has determined that the oil shale in the Lucaogou Formation was mainly deposited in an offshore lacustrine environment and that the oil shale development was well to excellently correlated with the continuous subsidence and lake expansion in the northern Bogeda Mountains of Junggar Basin. , …”

“…12,13 In recent years, research on the oil shale in the northern Bogeda Mountains has been documented; this work has determined that the oil shale in the Lucaogou Formation was mainly deposited in an offshore lacustrine environment and that the oil shale development was well to excellently correlated with the continuous subsidence and lake expansion in the northern Bogeda Mountains of Junggar Basin. 14,15 The studies described above were mainly focused on the macroscopic geological settings of oil shale formations. During the depositional period of the Lucaogou Formation, the foreland of the Junggar Basin was in its largest subsidence stage and was experiencing underfilling conditions.…”