Coarse Calcite Grains in Hydrocarbon-Rich Tuffaceous Mudstone in a Permian Volcanic Lake - An Indicator for Subaqueous Volcanic Deposition and Related Hydrocarbon Generation
Sediments originated from volcanic-hydrothermal activities show clear relationship with hydrocarbon generation and accumulation in fine-grained sedimentary rocks at a regional scale. However, the fundamental processes and mechanisms of this relationship are still poorly understood at a microscopic scale to hamper efficient hydrocarbon exploration. In addition, coarse grains common in volcanic lacustrine fine-grained sediments have been traditionally interpreted as diagenetic products during early and late burial. Our study, however, suggests that coarse calcite grains are not diagenetic, but formed by subaqueous explosion. The calcite grains occur in lacustrine fine-grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, NW China. They are sand sized, angular, mono- and polycrystalline, and discretely scatter in silt to clay-sized non-calcite pyroclastic matrix. The calcite grains and the mud matrix are interpreted as a pyroclastic debris flow deposit. In-situ 87Sr/86Sr ratios (0.704932-0.705840, and 0.705137 on average) of the calcite grains are similar to those of mantle rocks, indicating an origin closely related to mantle-derived fluids. The in-situ δ13C values (4.24‰ to 5.06‰, and 4.57‰ on average) and extremly negative δ18O values (-21.70‰ to -21.18‰, and - 21.49‰ on average) indicate that extensive hydrothermal alteration and microbial activities influenced the formation of the calcite. Element composition and homogenizaiton temperature of fluid incusions of calcite and biomarkers of the mudstone all suggest a relatively low-temperature hydrothermal condition in a saline and reducing environment. Furthermore, hydrocarbon-rich inclusions are rich in the calcite grains and have two modes of occurrence and maturity. They indicate that the organic matter was generated from algae bloom, which was caused by volcanic-hydrothermal activities. Alternatively, the hydrocarbon may have an inorganic origin originated from a mantle source. Our findings provide an alternative interpretation on the origin of coarse calcite grains in fine-grained sedimentary rocks and demonstrate a microscopic relationship between volcanic-hydrothermal activities and hydrocarbon generation and accumulation in a tectonic active rift basin.
X. Jiao et al., "Coarse Calcite Grains in Hydrocarbon-Rich Tuffaceous Mudstone in a Permian Volcanic Lake - An Indicator for Subaqueous Volcanic Deposition and Related Hydrocarbon Generation," Sedimentary Geology, vol. 436, article no. 106158, Elsevier, Jul 2022.
The definitive version is available at https://doi.org/10.1016/j.sedgeo.2022.106158
Geosciences and Geological and Petroleum Engineering
Keywords and Phrases
Calcite Grains; Fine-Grained Sedimentary Rocks; Hydrocarbon Generation; Permian; Subaqueous Explosion; Volcanic-Hydrothermal Activities
International Standard Serial Number (ISSN)
Article - Journal
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01 Jul 2022