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=Herkimer Formation=

Introduction
The Herkimer Formation (also referred to as the Herkimer Sandstone) is a Late Silurian geologic formation composed of sedimentary rocks deposited in central and south-eastern New York. Herkimer lithologies were first described in 1828 by Amos Eaton who divided the unit into the "Gray Band" of eastern outcrops (orthoquartzite) and the "Ferriferous Sandrock" of western outcrops (dolomitic sandstones). G.H Chadwick (1918) later combined the two units into the greater Herkimer Formation, named for Herkimer County, NY.

The Herkimer Formation is the uppermost stratigraphic unit of the Clinton Group. It is underlain by the Willowvale Formation and overlain disconformibly by the Ilion Formation of the Lockport Group. The eastern facies of the Herkimer Formation (Richfield and Winfield quadrangles) is composed of a fairly uniform fine-grained, light grained orthoquartzitic sandstone with occasional shale beds. The western facies (Oneida, Rome and Utica quadrangles) is composed of interbedded, pyritiferous gray sandy dolomite, dolomitic sandstone and shale. Zenger noted "the shale-sandstone ratio ranges from 1:3 to 1:1, the higher ratios being in the west." At the base of the Herkimer lies a ~1 meter thick bed of hematitic dolostones (often referred to as the Kirkland hematatic limestone). The Herkimer Formation sandstones and dolostones are fairly resistant, forming several cascades and gorges, most notably the Ilion Gorge. . Subsurface mapping indicates an east to west depositional thickening. Similar mapping also suggests east-west extensional structural features.

The Herkimer Formation outcrop belt runs east-west from southern Oswego County, NY, through central Oneida County, NY and continues through the Mohawk Valley towards Albany, NY. The Herkimer Formation outcrop belt reappears in southeastern, NY, spanning through Ulster, Sullivan, and Orange County, NY.

Depositional Environment
Fossil and lithologic evidence indicate a coastal/beach depositional environment. Donald H. Zenger (1972) proposed the Herkimer Formation was deposited in a shallow subtidal environment based on the abundant marine fossils. Zenger also proposed that dolomization, though primarily occurring as a diagenic process, may have occurred in a supratidal evaporite settings with high Mg/Ca brines. The presence of the ostracode Drepanellina clarki within the uppermost Joslin Hill Member correlates the Herkimer Formation to equivalent units in the southern regions of the Appalachian Basin.

Economic Value
The Herkimer Formation is an important natural gas bearing unit. The probable hydrocarbon source for the Herkimer Formation is the underlying organic-rich Utica Formation (Utica Shale). Structural models indicate normal faulting (extension) within the Herkimer Formation, implying the formation has been significantly fractured. Such fracturing, in addition to diagenic alteration, increases the porosity of the Herkimer Formation to the productive gas-bearing range of 7-14%. The potential drilling fairway for the Herkimer Sandstone forms a crescent spanning from southern Onondaga County, NY to western Orange County, NY. A significan reservoir of natural gas is present within the Sangerfield Pool of southern Madison County, NY. The Sangerfield Pool is a "tilted hydrodynamic dome" that has trapped upwelling natural gas. Several exploratory drilling operations in southern Madison County, NY (Lebanon Field) have estimated a regional gas productivity of 4.72 BCF.