Volcanic ash content of Permian lucagou shale in the Jimsar sag, Junggar Basin: Evidence from comprehensive analysis of mercury (Hg), major elements, strontium isotope, and thin section

Volcanic ash deposits are commonly associated with shale oil enrichment worldwide, and their composition potentially playing a significant role in the origin of organic-rich shale and shale oil exploration potential. Volcanic activity occurred frequently during the depositional period of the Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin. Microscopic examination shows the development of volcanic ash in the strata, but alteration of volcanic ash may affect the volcanic ash content recovered by the microscopic identification method. Through analyzing 49 rock samples, semi-quantitative restoration of volcanic ash content was conducted combined using Hg, major elements, strontium isotopes and thin section analysis. The restored volcanic ash content, measured via Hg and elemental methods, represents the stratigraphic unit's weight percentage, while the microscopic identification method denotes the stratigraphic unit's volume percentage. Although they are different ratios, they exhibit good correlation. Research suggests that volcanic ash is commonly present in the Lucaogou Formation of the Jimsar Sag. The volcanic ash content varies greatly, from 11% to 96%, mainly within the 10%–40% range. Only a few samples exhibited volcanic ash content exceeding 60%. The nutrients brought by the volcanic ash promoted organic productivity. This, in turn, increased the organic matter content in the water column and facilitated the formation of a reducing environment. The abundance and type of organic matter are crucial factors in determining the potential for shale oil. Thus, volcanic ash influences the enrichment of organic matter and the shale oil potential of the Lucaogou Formation.