Regional Unconformities Beneath Source beds and Geologic Constraints on Deep Plays in the Appalachian Basin.

MILICI, R .C. and RYDER, R. T., U. S. Geological Survey, Reston, VA

In the Appalachian basin, regional unconformity-bound Sauk, Tippecanoe, and Kaskaskia sequences are unique in their hydrocarbon-bearing characteristics. This paper describes the hydrocarbon geology of the two "deep" sequences, within the basin.
The Sauk sequence (Cambrian - L. Ordovician) was deposited unconformably upon Grenville-age crystalline rocks over much of the continental margin during and following regional extension and breakup of eastern Laurentia. Transgressive Cambrian sediments accumulated generally in oxidizing subaerial to littoral environments. These basal clastics, even where overthickened in the Rome trough, show little evidence of the high organic productivity and/or anoxia required to produce good source rocks. Siliciclastic sediment sources became more remote as the craton was flooded, and the upper part of the sequence consists of widespread thick, porous dolomitized limestones. These strata recrystallized first during meteoric flow and flushing events that occurred when they were exposed in early Middle Ordovician time (Knox unconformity).
In the eastern part of the basin, the overlying Tippecanoe sequence (M. Ordovician - L. Devonian) may be divided into Taconic flysch and post-Taconic molasse that were deposited during and after the closing of the proto-Atlantic ocean. Middle Ordovician shales on the basin margin intertongue westward with limestones and, where suitably mature, serve as source rocks for Sauk reservoirs. Penecontemporaneous porous carbonate rocks are commonly overlain by a sub-Kaskaskia unconformity and form reservoirs. Widespread post-tectonic (molasse) sandstones contain hydrocarbons derived from below or from overlying Devonian-Mississippian flysch (Kaskaskia sequence). The hydrocarbon potential of deep Tippecanoe strata has not been tested adequately on the eastern side of the basin.

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