Abstract: Conodonts for Hydrocarbon Exploration Strategies - The Little Conodont that Could

By

Anita G. Harris

Conodonts are bright, shiny, colorful, apatitic microfossils that are common to abundant in marine rocks of Cambrian through Triassic age throughout the world. In the last decade they have become one of the major biostratigraphic and organic maturation indices throughout most of their geologic range. As a consequence, conodonts now have great utility in the search for oil and gas and mineral deposits in Paleozoic and Triassic rocks. The use of conodonts as metamorphic, paleogeographic, and chronologic indices facilitates interpretation of pre-thrust depositional and burial metamorphic patterns, suggests relative sequence and correlation of thrust sheets, and indicates areas favorable for hydrocarbon exploration. Recent and ongoing published and unpublished studies in the Appalachian Basin, Michigan Basin, central Great Basin, and Arizona provide examples of all these applications in a wide range of geologic settings.

Conodont-based isograd maps for some or all Ordovician through Triassic Systems are now or will soon be available for the Appalachian Basin, Arizona, and the central Great Basin including Nevada, Utah, southeast Idaho, and eastern California. Each of these large terranes with its distinct depositional, thermal, and tectonic history as well as widely dissimilar geologic and conodont data base require different interpretation strategies to assess oil and gas as well as some types of mineralization potential. Conodont-based isograd maps provide a first-cut assessment and target large areas of resource potential in such frontiers as the buried segment of the Valley and Ridge Province beneath the crystalline terrane of the eastern Appalachians, the southern Western Overthrust Belt, and the central Great Basin.

Refinement of conodont zonations has enabled revision and reinterpretation of Ordovician stratigraphy and paleogeography in the Appalachian and Michigan basins. In part of the Appalachian Basin and adjacent craton, the well-documented disconformity between Lower Ordovician dolostones and Middle Ordovician limestones is usually associated with karstification features and porosity horizons. These porosity horizons are often the host or reservoir rocks for stratabound mineral deposits or large volumes of natural gas. Conodont studies now show that in the central Appalachian Basin, 1) a large part of the dolostone sequence is of Middle Ordovician age, 2) nearly continuous deposition occurred across the Lower/Middle Ordovician boundary in a large area centered at the Mason-Dixon Line, and 3) karstification and associated porosity horizons do occur at several levels in this carbonate sequence, but considerably below the dolostone-limestone contact previously taken as the basinwide marker for an unconformity of considerable magnitude. In the Michigan Basin, conodonts from a deep well near the Paleozoic depocenter of the basin prove that over 2300 feet of evaporite-bearing sandy carbonates are of Early through Middle Ordovician age and not Late Cambrian age as widely held. This revision considerably alters the interpretation of basin configuration, paleogeography, and tectonic development.

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