Determination of Organic-Matter Content of Appalachian Devonian Shales from Gamma-Ray Logs

Abstract

The organic-matter content of the Devonian shale of the Appalachian basin is important for assessing the natural-gas resources of these rocks, and patterns of organic-matter distribution convey information on sedimentary processes and depositional environment. In most of the western part of the Appalachian basin the organic-matter content of the Devonian shale can be estimated from gamma-ray wire-line logs using the equation: ^phgro = (^ggrB - ^ggr)/1.378A, where ^phgro is the organic-matter content of the shale (fractional volume), ^ggr the gamma-ray intensity (API units), ^ggrB the gamma-ray intensity if no organic matter is present (API units), and A the slope of the crossplot of gamma-ray intensity and formation density (API units/(g/cm3)). The quantities A and ^ggrB vary regionally and are mapped using data from gamma-ray and formation-density wire-line logs. Organic-matter contents estimated using this equation are compared with organic-matter contents determined from direct laboratory analyses of organic carbon for 74 intervals of varying thickness from 12 widely separated wells. The organic-matter content of these intervals ranges from near zero to about 20% by volume. Excluding the Cleveland Member of the Ohio Shale and the lower part of the Olentangy Shale, the distribution of the differences between volume-percent organic-matter content determined from core samples and estimated from gamma-ray logs has a mean of 0.44% and a standard deviation of 1.98%, which indicates that the accuracy of the gamma-ray method is adequate for most geologic applications. The gamma-ray intensity of the Cleveland Member of the Ohio Shale and the lower pa t of the Olentangy Shale is anomalously low compared to other Devonian shales of similar richness, so that organic-matter content computed for each of these units from gamma-ray logs is likely to be too low. Wire-line methods for estimating organic-matter content have the advantages of economy, readily available sources of data, and continuous sampling of the vertically heterogenous shale section. The gamma-ray log, in particular, is commonly run in the Devonian shale, its response characteristics are well known, and the cumulative pool of gamma-ray logs forms a large and geographically broad data base. The quantitative computation of organic-matter content from gamma-ray logs should be of practical value in studies of the Appalachian Devonian shale. The general approach described here may also be applicable to other formations with physical and geochemical characteristics similar to the Appalachian Devonian shale.