Abstract

Anthraxolite, a high-rank solid petroleum residue, occurs in vugs, fractures, and disseminations throughout the Upper Cambrian and Lower Ordovician dolomite-limestones of the Mohawk Valley, upper New York State. Anthraxolite reflectance values (2.55%–3.35%) confirm other thermal maturity parameter values that the region has experienced paleotemperatures of ∼200 °C. Euhedral to subhedral, transparent crystals of quartz precipitated in association with the anthraxolite.

Paragenetic relationships and fluid inclusion studies allow deduction of a succession of fluid types: (1) a cool aqueous fluid that deposited quartz cement; (2) a warm brine that deposited calcite-sulfide veins; (3) a mixed aqueous-petroleum fluid from which the quartz crystals were precipitated, and (4) later aqueous and methane-rich fluids in cross-cutting planes. The first two stages (phase I) represent fluids that predate hydrocarbon generation and migration. The third and fourth stages (phase II) indicate addition of hydrocarbon fluids to basinal brines.

The lack of hydrocarbons in the warm brines at oil window temperatures (phase I), and the presence of oil in later fluids that exceeded oil window temperatures (phase II) suggest transient heating due to hot fluid flow rather than prolonged heating due to burial. This is consistent with models of regional heating and fluid movement due to orogenic activity, probably the Alleghenian orogeny. The traces of sulfide in the Mohawk Valley may be a weak expression of a regional fluid migration event that caused widespread base metal mineralization in the Cambrian–Ordovician rocks of the Appalachians.

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