A means of relating geochemical concentrations to existing sandstone classification schemes is based on three chemical parameters: the SiO 2 /Al 2 O 3 ratio, the Fe 2 O 3 /K 2 O ratio, and the Ca content. In terrigenous sands and shales, the SiO 2 /Al 2 O 3 ratio separates Si-rich quartzarenites from Al-rich shales, with other sand types showing intermediate values. The ratio of total iron (as Fe 2 O 3 ) to K 2 O separates lithic sands (litharenites and sublitharenites) from feldspathic sands (arkoses and subarkoses). In addition, very high Fe 2 O 3 /K 2 O ratios indicate Fe-rich shales (e.g., pyritic, sideritic, hematitic) or Fe-rich sands (e.g., glauconitic) 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.

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