A method for judging depositional environment of coal reservoir based on coal facies parameters and rare earth element parameters

Abstract

Coal facies refers to the genetic type and coal rock type of coal formed in a certain depositional environment. Rare earth elements (REE) are not easily disturbed by metamorphism, which is used to determine the provenance characteristics of coal-forming materials and the depositional environment in the coal seam. A total 76 coal samples from Carboniferous, Permian and Jurassic in the southern Ordos Basin were selected to determine the content and distribution characteristics of maceral compositions and REE using microscope observation and ICP-MS analysis. Combining the traditional REE indicator parameters (La/Lu, La/Sm, Gd/Lu, δEu, δCe, and REY) of the sedimentary rocks with the unique facies parameters in coal (Vitrinite content/Inertinite content (V/I), Gelation Index (GI), Texture Preservation Index (TPI), Groundwater Flow Index (GWI), Vegetation Index (VI) and Framework Matrix Ratio (F/M)), we determine the depositional environment, oxidation-reduction condition, coal-forming plants, hydrodynamic conditions and provenance characteristics of coal seams in different periods. The depositional environment is performed with the GI-TPI crossplot. The oxidation-reduction environment is explored by δEu, δCe and GI. The coal-forming environment is judged by a comprehensive combination of GWI-VI crossplot, ∑REY, La/Lu, La/Sm and Gd/Lu. The judgment of provenance characteristics is limited by the diagram of ∑REY and La/Yb and REE parameters. • The petrological features and REE characteristics of Permo-Carboniferous and Jurassic coal seams were explored. • The coal facies parameters suitable for the coal seams were performed. • A method for judging depositional environment of coal seams was established. • The depositional environment and coal-forming environment of coal seams in the southern Ordos Basin were discussed.