The Middle Silurian McKenzie Formation at the type section near McKenzie Station, Maryland, is composed of thinly interbedded, ripple-marked mud-cracked shale and limestone with minor amounts of siltstone. These same rock types are predominant in cuttings from wells drilled farther west over a distance of 50 mi. Southwestward from the eastern panhandle of West Virginia, however, the McKenzie contains coarser clastics. In Pocahontas County, Travis divided the formation into three units: an upper limestone and shale, a middle sandstone, and a lower limestone and shale. Westward in the subsurface, the upper limestone and shale facies is replaced by dolomite, but thin shale beds still are present near the top of the formation. The middle sandstone persists as a distinct unit s far as Roane, Kanawha, and Boone Counties, a distance of approximately 100 mi. Farther west in the state, sandy dolomite is present at this level. The lower limestone and shale unit of outcrop areas in Pocahontas County becomes more dolomitic westward and still is recognized as a sandy, dolomitic limestone as far west as Ohio and Kentucky.

The McKenzie Formation in the eastern panhandle represents accumulation in a marine environment along a low-lying coastal plain where rivers supplied clay and silt. Minor fluctuations of sea level and intermittent turbulent and calm conditions created alternating open-marine to intertidal environments. In the southwest in Pocahontas County, Travis interpreted the rocks of the lower unit as having been deposited under conditions ranging from normal marine to intertidal. The sandstone unit was formed as a beach deposit during a minor regression; it is overlain by a shallow-marine limestone and shale facies. An increasing quartz-sand content in the top signaled the coming of another regression and the beach deposit of the overlying Williamsport Formation.

Environments of deposition for the McKenzie in the western subsurface have been interpreted at a detailed level from a complete core in Wayne County. The paleoenvironment of the lowest unit was a bryozoan-stromatoporoidal bioherm. A middle sandy dolomite represents intertidal deposition. An overlying oolitic facies is considered to be of a littoral environment, and stromatolitic dolomite in the uppermost unit is interpreted as intertidal and supratidal algal mats. A core from another well on the northeast, in Mercer County, Pennsylvania, contains similar lithologies, also indicative of intertidal and supratidal deposition.

Most small gas shows and pays previously referred to as production from the Keefer or Big Six sand are actually from the biohermal facies of the lower McKenzie. Gas has been encountered in this facies in 75 percent of the wells drilled through the McKenzie in Wayne County. Gas shows also have been found in this zone in Cabell and Mingo Counties. Farther north, in Jackson, Mason, and Roane Counties, gas shows are recorded higher in the McKenzie at a level equivalent to or immediately above the middle sandy unit. This porous zone has been productive in Meigs County, Ohio, where it is called the "Ohio Newburg."

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