Geochemical Classification of Terrigenous Sands and Shales from Core or Log Data

TLDR

Geochemical classification of terrigenous sands and shales relies on three ratios—SiO₂/Al₂O₃, Fe₂O₃/K₂O, and Ca content—to distinguish quartzarenites, shales, lithic versus feldspathic sands, Fe‑rich rocks, and calcareous from non‑calcareous units. The method uses geochemical well‑logging data to supply these ratios, allowing continuous, unbiased sandstone classification with depth. The scheme classifies sandstones in agreement with petrographic analysis about 84 % of the time and effectively separates shales from sandstones.

Abstract

ABSTRACT A means of relating geochemical concentrations to existing sandstone classification schemes is based on three chemical parameters: the SiO2/Al2O3 ratio, the Fe2O3/K2O ratio, and the Ca content. In terrigenous sands and shales, the SiO2/Al2O3 ratio separates Si-rich quartzarenites from Al-rich shales, with other sand types showing intermediate values. The ratio of total iron (as Fe2O3) to K2O separates lithic sands (litharenites and sublitharenites) from feldspathic sands (arkoses and subarkoses). In addition, very high Fe2O3/K2O ratios indicate Fe-rich shales (e.g., pyritic, sideritic, hematitic) or Fe-rich sands (e.g., gl uconitic) depending on the silica/alumina ratio. The Ca content is used to differentiate noncalcareous from calcareous sandstones and shales and to separate siliciclastic from carbonate rocks. Sandstones are classified the same by this scheme as by petrographic analysis about 84% of the time, and shales are effectively discriminated from sandstones. The requisite input data can be accurately supplied by geochemical well-logging measurements, enabling unbiased sandstone classification to be displayed on a continuous basis with depth.