MRCSP Phase I Geologic Characterization Report - Midwest ...

APPENDIX A: LOWER DEVONIAN ORISKANY SANDSTONE
107
Pennsylvania to southeastern West Virginia) was derived from older
sedimentary deposits to the southeast, in New York it was derived
directly from crystalline rocks in the Adirondacks. Basan and others
(1980) eventually showed that the Oriskany is very different in different
areas. As a result, they suggested three possible source areas
for the Oriskany: 1) the Adirondacks (as per Stow, 1938); 2) an
emergent landmass on the southeastern margin of the basin; and 3)
an emergent landmass in east-central Pennsylvania or New Jersey.
The relative abundance of polycrystalline quartz in eastern Pennsylvania
exposures, derived from a metamorphic source, provides
evidence for this latter possible provenance.
The depositional environments of the Oriskany are varied, but
always fall within the broad category of shallow marine. Swartz
(1913) proposed a high-energy beachface environment for the
Oriskany in the Valley and Ridge province. Other authors have suggested
nearshore, shallow water (Stow, 1938), tidal ridges and submarine
dunes (Basan and others, 1980), shallow to deeper subtidal
(Barrett and Isaacson, 1977), and marine shelf bar (Welsh, 1984;
Bruner, 1988) environments (Figure A11-4). But, as Basan and others
(1980) pointed out, even within a single outcrop or well location
the Oriskany can represent one or more depositional environments.
(i.e., westward and northward) to where the sandstone pinches out
against overlying and underlying impermeable rocks (typically tight
carbonates or shales), creating a stratigraphic trap (Opritza, 1996).
Brine often is trapped between the actual sandstone pinchout and
the zones or belts of gas production. Where the trapping mechanism
is structural, from central-western Pennsylvania and West Virginia
eastward, structural complexity increases from west to east. To the
west and north, anticlinal structures with rifted cores originated
through detachment in incompetent Silurian salt beds. Salt water
typically occurs in the cores of these anticlines. To the east, multiple,
east-dipping thrust sheets (duplexes), resulted from Alleghanian
tectonic thrusting (Flaherty, 1996; Harper and Patchen, 1996).
Combination traps occur in a narrow band across easternmost Ohio
into western Pennsylvania and western West Virginia where moderate
structures enhance trapping in updip porosity pinchout situations
(Patchen and Harper, 1996). Figure A11-2 (see also Figure 6) shows
the areas of structural complexity within the MRCSP study area.
The few faults shown imply far more simplicity and generalization
than actually occurs owing to the scale of the map. Studies of individual
structures and gas fields indicate much more complexity than
can be shown on a map at this scale.
DEPTH AND THICKNESS RANGES
The Oriskany crops out in central New York near its type locality,
as well as within the complex fold belt of central Pennsylvania,
western Maryland, northeastern West Virginia, and western
Virginia. Based on drillers’ records, it ranges from approximately
1,200 feet deep along the shore of Lake Erie in northeastern Ohio
and northwestern Pennsylvania to more than 10,000 feet deep in
Somerset County, Pennsylvania (Figure A11-2). Depths within the
Appalachian Plateau vary greatly as a result of both a general regional
southeastward dip and the occurrence of numerous anticlines
paralleling the regional strike of the Valley and Ridge Province to
the east.
Figure A11-3 illustrates the thickness of the Oriskany Sandstone
throughout the basin. Oriskany thicknesses vary within the Appalachian
Plateau of eastern Ohio, western Pennsylvania, and West
Virginia from 0 to over 300 feet. Adjacent to pinchout areas such
as the “Oriskany no-sand area” in northwestern Pennsylvanian and
along the eastern pinchout in Ohio, the reservoir sandstone typically
averages between 10 and 30 feet thick (Finn, 1949; Abel and Heyman,
1981, Opritza, 1996). At the pinchout, the sandstone forms
a thin wedge between relatively impermeable Lower and Middle
Devonian carbonates and shales. Thicker zones of Oriskany typically
occur in the more structurally complex areas where thrusting
and vertical repetition of beds causes apparent thicknesses much
greater than 60 feet—even as much as 350 feet in western Maryland
(Harper and Patchen, 1996; Patchen and Harper, 1996). The thicknesses
shown in Figure A11-3 are comparable to those previously
published by Diecchio and others (1983) for this unit in the northern
portion of the Appalachian basin.
TRAPS/STRUCTURE
As a natural gas reservoir, the Oriskany is affected by three types
of traps—stratigraphic (i.e., updip permeability pinchout) (Opritza,
1996), structural (Harper and Patchen, 1996), and combination
stratigraphic and structural (Patchen and Harper, 1996). In the areas
of pinchout (Figures A11-2 and A11-3), fluids migrated updip
Figure A11-4.—Interpreted environments of deposition in the Oriskany
Sandstone from a well in Somerset County, Pennsylvania. Diagram based
on gamma-ray log signature and core descriptions (modified from Welsh,
1984).