Geochemical evaluation of the eastern gas shales. Part I

Abstract

Work devoted to assessment of Eastern gas shales is reported. It is noted that although the Late Devonian-age dark shales of the Eastern Interior Basins are thought to be uniformly gassy, organic geochemical studies in the Appalachian and Illinois Basins show that the gas is not uniformly distributed and that most of the gas is probably sourced and largely retained in thin, organic-rich zones that were deposited in restricted marine environments. As the Devonian-age basins filled, the environments of deposition of the Appalachian Basin and Illinois Basin became nonmarine more and more northerly and northwestwardly, respectively. Heavy hydrocarbon-to-organic carbon ratios show that the organic matter associated with the restricted marine environments is different in the two basins. During virtually the entire period in question, the Appalachian Basin had a direct connection; the Illinois Basin was somewhat isolated by the already developing Cincinnati Arch on the east and the Kankakee or Wabash Arch on the north. However, the differences in organic matter noted in this study suggest a northwest connection of the Illinois Basin to a different ocean mass than that which supplied marine waters to the Appalachian Basin. As a fossil fuel resource, certain facies within the dark Devonian-age shale are much richer gas sources than others. The most prolific potential reservoirs (naturally occurring or induced) should be sought or located in the geologic section containing orcontiguous to the richest organic source intervals; i.e., rocks deposited in restricted marine environments. The amount of gas in rocks of each interval depends directly on the amount of detrital organic matter. Virtually all the gas as well as virtually all the liquid hydrocarbons are retained in the rock where they were generated.