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Significant new progress was made during the last year with regard to linking the evolution of hydrothermal dolomite (HTD) reservoirs in the Trenton-Black River stratigraphic  interval with fluids derived from the Utica Shale.  Fluid inclusion and stable isotope data from fractured basement rock underlying the Paleozoic sedimentary sequence in the northern Appalachian Basin demonstrate that the highly saline (halite-saturated) fluids were available for up-migration along fault systems in the basin.  Work done by Colgate undergraduates Nicole MacDonald and Kristen Meisner in the summer of 2009 documented the alteration of basement rock in the Mohawk Valley region by these highly saline fluids.  In addition, work during the summer and fall of 2009 by Bruce Selleck and Nicole MacDonald provided evidence that vein systems in the Utica Shale, which overlies the Trenton-Black River HTD, were conduits fpr voluminous low-salinity fluids derived from compaction and clay dehydration of the Utica.  Vein development in the Utica was accompanied by fluid-induced sand injection.  Sand was derived internally from volcanic ash beds in the Utica. 

These observations led to the development of a new model for HTD development in the Trenton-Black River.  This model, presented at EAAPG in October, 2009 (Selleck, Meisner, and MacDonald, 2009), invokes seismic pumping along faults to transport saline fluid upward from the basement and overlying basal sandstone aquifer, into the Trenton-Black River interval.  The same fault systems, in dilatent mode, pump fluid downward from the dewatering Utica interval into the zone of dolomitization in Trenton-Black River interval.  The mixing of these fluid types promoted the development of HTD reservoirs.   This fluid mixing model explains the variation of fluid compositions within HTD reservoirs in the subsurface of the Appalachian Basin (e.g. Smith and Davies, 2006).  This model is consistent with the results of our monazite chemical age-dating work, which yields Late Ordovician (ca. 450 ma) monazite overgrowth ages that date fluid flow in the basal sandstone aquifer (Selleck, Williams and Jercinovic, 2008).  Fluid activity was thus triggered by Taconic Orogeny compression and faulting in the foreland, with seismic pumping and fluid flow occurring during or shortly after deposition of the Utica Formation.

Given this new model that links fluid evolution in the Utica Shale with faulting and seismic pumping to promote fluid mixing and HTD reservoir formation in the Trenton-Black River, field work during the summer of 2010 was devoted to detailed mapping of fracture and vein systems in the Utica and overlying upper Ordovician and lower Silurian units in the Mohawk Valley region.  This work, carried out by Bruce Selleck and undergraduates Julian Michaels and Jacqueline Colborne, has documented the role of E-W oriented Mode 2 (strike-slip) fractures in vein evolution in the lower Utica Shale (=Flat Creek Member). Dilatent jogs and en echelon veins in this fracture system host aromatic hydrocarbon-stained calcite; hydrocarbon GC analysis of this vein calcite demonstrates that the gas system operating at this time was generating hydrocarbon mixtures similar to those found in Utica-source gas reservoirs in the basin to the south.  This Mode 2 fracture system is rarely present in the overlying Indian Castle Member of the Utica or in the succeeding Upper Ordovician Frankfort Formation clastics.  N-S oriented Mode 1 tensile fractures are also present in the lower Utica, and these systems host sulfide/dolomite mineralization and sand injectite dikes, as well as coarse calcite stained by hydrocarbon. The mineralogy of the sand injectite suggest derivation of sand from underlying Paleozoic rocks, mineral and rock clasts from faulted Proterozoic basement, and sand grains winnowed from volcanic ash within the lower Utica. Fracture types and orientations suggest that the lower Utica (=Flat Creek Member) interval in the central and eastern Mohawk Valley was fractured and partially dewatered during early hydrocarbon evolution, likely during active Taconic deformation and warping of the foreland basin.  Later burial of the Utica under Silurian strata promoted additional late, dry gas generation, providing a gas source for Silurian sandstone reservoirs.  The results of this phase of the research will be presented at AAPG in April, 2010.

Selleck, B. , Meisner, K. and MacDonald, N. (2009) Fluids, veins and sand injectites, Ordovician Utica Shale, Mohawk Valley, NY. Eastern Section AAPG Abstracts, October, 2009

Selleck, B.W., Williams, M., and Jercinovic, M. (2008) In situ U-Th-Pb microprobe dating of authigenic monazite and xenotime in the Potsdam Sandstone, eastern New York: A new approach to dating hydrothermal fluid flow and dolomitization:  Eastern Section AAPG Abstracts, October, 2008

Smith, L.T. (2006) Origin and reservoir characteristics of Upper Ordovician Trenton–Black River hydrothermal dolomite reservoirs in New York; AAPG Bulletin, V. 90, #11, p. 1691-1711