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

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 ICPMS analysis. Combining the traditional REE indicator parameters (La/Lu, La/Sm, Gd/Lu, 8Eu, 8Ce, 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 8Eu, 8Ce and GI. The coal-forming environment is judged by a comprehensive combination of GWIVI crossplot, n-ary sumation REY, La/Lu, La/Sm and Gd/Lu. The judgment of provenance characteristics is limited by the diagram of n-ary sumation REY and La/Yb and REE parameters.

Automated summary

Coal facies research helps determine the genetic type and formation environment of coal, while rare earth elements are used to analyze the source and depositional environment characteristics of coal. By analyzing the maceral composition and REE content of coal samples from different periods, and combining traditional REE indicators with coal facies parameters, it is possible to assess the characteristics of coal seams in terms of depositional environment, oxidation-reduction conditions, coal-forming plants, hydrodynamic conditions, and provenance.