Abstract

Fluid inclusion microthermometric analyses and O and C stable isotopic analyses of vein minerals are used to determine the chemistry and trapping conditions of fluids present in the central Appalachian fold-and-thrust belt during the late Paleozoic Alleghanian orogeny. The upper Paleozoic rock section contains three regional hydrostratigraphic systems based on fluid chemistry and temperature. The Ordovician Trenton Formation through the Devonian Helderberg Group was a regional aquitard and was dominated by high-salinity, CH4-saturated, in situ fluids. The Devonian Oriskany Formation through the lower portion of the Chemung Formation was a regional aquifer system and underwent an influx of warm migrating fluids. The upper portion of the Devonian Chemung through Pocono Formations was also a regional aquifer, but it was dominated by an influx of meteoric water that mixed with in situ fluids.

The migrating fluid was a warm (160 to >220 °C) CH4-saturated NaCl-CaCl2 brine that was stratigraphically restricted to the Oriskany Formation through the lower portion of the Chemung Formation, although there is evidence for infiltration into lower stratigraphic units. Two separate fluid migration events are recorded in the rocks. The first event is either late synfolding to postfolding, and the second event is postfolding. Approximately 2–4 km of overburden were removed by erosion between the two migration events. The source of the warm migrating fluids is still unknown. However, the most likely source would be fluids that were tectonically driven through the fold-and-thrust belt by large-scale, out of sequence thrusting in the hinterland. The migrating fluids were transported far into the foreland where they may have been directly responsible for (1) flushing of hydrocarbons from the upper Paleozoic of the Valley and Ridge into the Plateau province and (2) elevated thermal maturation indicators in the Valley and Ridge and Plateau provinces.

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