The Appalachian Basin is a foreland basin containing Paleozoic sedimentary rocks of Early Cambrian through Early Permian age. From north to south, the Appalachian Basin Province crosses New York, Pennsylvania, eastern Ohio, West Virginia, western Maryland, eastern Kentucky, western Virginia, eastern Tennessee, northwestern Georgia, and northeastern Alabama.
In a clockwise direction, starting in northern New York, the Appalachian Basin is bounded by the following provinces: Adirondack Uplift (071), Blue Ridge Thrust Belt (068), Louisiana–Mississippi Salt Basins (049), Black Warrior Basin (065), and Cincinnati Arch (066). The northern end of the Appalachian Basin extends offshore into Lakes Erie and Ontario as far as the United States–Canada border. The northwestern flank of the basin is a broad homocline that dips gently southeastward off the Cincinnati Arch. A complexly thrust faulted and folded terrane (Appalachian Fold and Thrust Belt or Eastern Overthrust Belt), formed at the end of the Paleozoic by the Alleghenian orogeny, characterizes the eastern flank of the basin. Allochthonous metamorphic and igneous rocks of the Blue Ridge Thrust Belt that bounds the eastern part of the Appalachian Basin Province were thrust westward more than 150 mi over lower Paleozoic sedimentary rocks.
The Appalachian Basin province covers an area of about 185,500 sq mi. The province is 1,075 mi long from northeast to southwest and between 20 to 310 mi wide from northwest to southeast.
The Appalachian Basin has had a long history of oil and gas production, and much of it has not been systematically recorded; thus, there are no commercial data bases for field size and production history. An ad hoc field file consisting of about 1,100 fields was used in this assessment. It was compiled from published production records from the States of Pennsylvania, West Virginia, New York, and Tennessee which have been kept from the early 1960's; proprietary oil and gas, Energy Information Administration (EIA), U.S. Department of Energy Integrated Field File (OGIFF); published scientific articles and reports; and unpublished industry reports and records.
Discovery of oil in 1859 in the Drake well, Venango County, northwestern Pennsylvania, marked the beginning of the oil and gas industry in the Appalachian Basin. Oil in the Drake well was produced from an Upper Devonian stray sandstone at a depth of about 70 ft. This discovery well opened a prolific trend of oil and gas fields, producing from Upper Devonian, Mississippian, and Pennsylvanian sandstone reservoirs, that extends from southern New York, across western Pennsylvania, central West Virginia, and eastern Ohio, to eastern Kentucky. From 1859 through 1993, approximately 2.3 BBO and 31 TCFG have been produced from the trend. Primary annual oil production in the trend peaked at about 36.3 MMBO in 1900, whereas oil production from secondary recovery peaked at about 37.6 MMBO in 1937. Exploration in the trend presently is for gas in low-permeability sandstone reservoirs at depths between 4,500 and 6,000 ft.
A second major trend of oil and gas production in the Appalachian Basin began with the discovery in 1885 of oil and gas in Lower Silurian "Clinton" sandstone reservoirs in Knox County, Ohio. By the late 1880's and early 1900's, the trend extended both north and south across east-central Ohio and included several counties in western New York where gas was discovered in Lower Silurian Medina Group sandstones. Hydrofrac techniques introduced in the early 1950's greatly improved oil and gas recovery from "Clinton" and Medina sandstones and thus encouraged exploration to depths greater than 4,000 ft. The Lower Silurian "Clinton" and Medina sandstone trend now extends across most of central and eastern Ohio, northwestern Pennsylvania, western New York, and a small part of northeastern Kentucky. From 1885 through 1993 approximately 345 MMBO and 7.5 TCFG have been produced from the trend. Current exploration in the trend is for gas in low-permeability sandstone reservoirs at depths between 5,500 and 7,000 ft.
About 1900, large oil reserves were discovered in Silurian and Devonian carbonate reservoirs in east-central Kentucky. Truncation traps beneath a widespread pre-Upper Devonian unconformity and low-amplitude anticlines control most of the accumulations. The high-quality limestone and dolomite reservoirs resulted from extensive subaerial exposure and karst processes. Drilling depths to the reservoirs range from about 800 to 2,500 ft. This trend is confined to east-central Kentucky where, between 1900 and the end of 1993, approximately 162.5 MMBO and 205 BCF of associated gas were produced. Now the trend is almost exhausted.
Important gas discoveries from the Lower Devonian Oriskany Sandstone in Cambridge County, Ohio, in 1924, Schuyler County, New York, in 1930, and Kanawha County, West Virginia, in 1936 opened a major gas-producing trend across parts of New York, Pennsylvania, Maryland, Ohio, West Virginia, Kentucky, and Virginia. Early gas exploration was concentrated in the western part of the trend where the gas was trapped in high-porosity sandstone by updip pinchouts, broad anticlines, and combination traps. By the 1950's, exploration for Oriskany gas moved eastward into deeper and structurally more complex parts of the basin where highly compressed, thrust-faulted anticlines are the traps, and fracture porosity has improved the quality of the tightly cemented reservoirs. Fractured Middle Devonian Huntersville Chert, which overlies the Oriskany Sandstone, is an important reservoir in the Oriskany Sandstone gas-producing trend in Pennsylvania, West Virginia, and Maryland. Approximately 2.9 TCFG have been produced from this trend through 1993. Exploration is still active in the trend for gas trapped in faulted anticlines at depths between 6,000 and 9,000 ft.
The most recent drilling boom in the Appalachian Basin occurred in the 1960's in Morrow County, Ohio, where oil was discovered at about 3,000 ft in the Upper Cambrian part of the Knox Dolomite. Paleotopographic highs beneath the widespread Middle Ordovician Knox unconformity provide the traps. The reservoirs consist of vuggy dolomite formed by prolonged subaerial exposure and karst processes. About 60 MMBO and 30 BCF of associated gas have been produced from Morrow County and several adjoining counties through 1993. Exploration continues in Ohio for fields beneath the Knox unconformity, but the activity has shifted eastward where the objective is gas and condensate in the Late Cambrian(?) Rose Run Sandstone and Lower Ordovician part of the Knox Dolomite (Beekmantown Dolomite).
The Appalachian Basin has produced about 3 BBO and 42 TCFG through 1993. Among the largest oil fields in the basin are Bradford (McKean Co., Pa., and Allegheny Co., N.Y.), discovery date 1871, reservoir Upper Devonian Bradford sandstones, ultimate recovery 680 MMBO; East Canton (Stark Co., Ohio), discovery date 1966, reservoir Lower Silurian "Clinton" sandstone, ultimate recovery 100 MMBO; Big Sinking (Estill and Lee Counties, Ky.), discovery date 1918, reservoir Upper Silurian and Middle Devonian "Corniferous" limestone, ultimate recovery 80 MMBO; Fairview-Statler Run-Mt. Morris, (Monongalia County, W.Va.), discovery date 1890, reservoir Lower Mississippian Big Injun sandstone, ultimate recovery 32 MMBO. The largest gas fields include Big Sandy area, eastern Kentucky and adjoining West Virginia, discovery date 1892 (major development of shale gas began in 1920's), reservoir(s) Upper Devonian black shale and Mississippian and Pennsylvanian sandstone, ultimate recovery >3 TCFG; Elk-Poca, Kanawha County, W. Va., discovery date 1936, reservoir Lower Devonian Oriskany Sandstone, ultimate recovery 1 TCFG; Lakeshore, Chautauqua County, N.Y., discovery date 1904, reservoir Lower Silurian Medina Group sandstones, ultimate recovery 650 BCFG; Driftwood, Cameron County, Pa., discovery date 1951, reservoir Lower Devonian Oriskany Sandstone, ultimate recovery 260 BCFG.
Deep drilling in the late 1970's and early 1980's for gas in the Appalachian Fold and Thrust Belt east of the Allegheny structural front has been unsuccessful. Potential reservoirs in the Upper Cambrian and Lower Ordovician Knox Group (dolomite), Middle Ordovician Trenton Group (limestone), and Tuscarora Sandstone were primary objectives. In addition to two small oil fields discovered in southwesternmost Virginia in 1949 and 1963, this part of the Appalachian Basin has yielded only five or six small gas fields.
Equally disappointing were the results of deep drilling in the 1970's for gas in the Rome Trough of Pennsylvania, West Virginia, and Kentucky. This Middle Cambrian rift basin, with prospective Middle Cambrian sandstone and carbonate reservoirs, has been tested by about 20 drill holes that end in Middle Proterozoic basement rock. Several of these drill holes in west-central West Virginia exceed depths of 19,000 ft. Discoveries in the Rome Trough have been limited to one small oil field and several good gas shows.
Exploration in the last 10 years has focused primarily on the following objectives: gas in low-permeability sandstone reservoirs of the Lower Silurian "Clinton" sandstone and Medina Group in eastern Ohio, northwestern Pennsylvania, and western New York; gas in low-permeability sandstone reservoirs of the Upper Devonian sequence in central Pennsylvania and east-central West Virginia; gas in the Lower Devonian Oriskany Sandstone and overlying Middle Devonian Huntersville Chert trapped by thrust-faulted anticlines (New York, Pennsylvania, and West Virginia); gas in fractured Middle and Upper Devonian black shale in New York, Pennsylvania, Ohio, West Virginia, Kentucky, and Virginia; and gas in the Upper Cambrian(?) Rose Run Sandstone and Lower Ordovician part of the Knox Dolomite (Beekmantown Dolomite) trapped beneath the Knox unconformity in eastern Ohio.
Nineteen conventional plays and 16 unconventional plays are recognized in this province for this assessment. Of the 16 unconventional plays, 12 are in continuous-type accumulations and 4 are coalbed gas plays. R.C. Milici has described those continuous-type plays in the Devonian black shale. Dudley D. Rice and Thomas M. Finn have described the coalbed gas plays; further discussion of and references for coalbed gas plays may be found in Rice's chapter "Geologic framework and description of coalbed gas plays" elsewhere in this CD-ROM.
The plays are listed below numerically; unconventional plays are shown in italics. In the text, the conventional plays will be described and followed, in general, by the unconventional plays. One exception is the Tuscarora Sandstone and Clinton/Medina Sandstone group of plays 6727 through 6732 which are discussed together; this group includes both conventional and unconventional plays; the narrative for the group begins with the conventional Tuscarora Sandstone Gas Play (6727). A similar exception is the group of Upper Devonian sandstone plays 6733 through 6737 which are also discussed together, beginning with unconventional play 6733, Upper Devonian Sandstone Gas High Potential Play, and also includes both conventional and unconventional plays.
Upper Cambrian, Ordovician, and Lower/Middle Silurian Thrust Belt
Beekmantown/Knox Carbonate Oil/Gas
Rose Run/Gatesburg/Theresa Sandstone Gas
Trenton/Black River Carbonate Oil/Gas
Queenston/Bald Eagle Sandstone Gas
Keefer/Big Six Sandstone Gas
"Corniferous Limestone" /Big Six Sandstone Oil/Gas
Upper Silurian Sandstone Gas
Silurian Carbonate Gas
Silurian and Devonian Carbonate Thrust Belt
Devonian Carbonate Gas
Oriskany Sandstone Gas/Faulted Anticlines
Oriskany Sandstone Gas
Mississippian and Pennsylvanian Sandstone/Carbonate
Devonian Black Shale-Undeveloped NE Ohio and Western Pennsylvania
Northern Appalachian Basin–Anticline
Northern Appalachian Basin–Syncline
Central Appalachian Basin–Central Basin
Cahaba Coal Field
Scientists affiliated with the American Association of Petroleum Geologists and from various State geological surveys contributed significantly to play concepts and definitions. Their contributions are gratefully acknowledged.
conventional and unconventional PLAYS
6701. Rome trough play (HYPOTHETICAL)
The Rome Trough is a narrow northeast-trending rift basin that underlies the Appalachian Basin in Pennsylvania, West Virginia, and Kentucky. Approximately 6,000 – 9,000 ft of Lower and Middle Cambrian shallow-marine to peritidal carbonate, sandstone, and shale fill the rift. The Rome Trough Play is defined by gas trapped in sandstone and carbonate reservoirs within the rift by basement-controlled fault blocks and anticlines. Stratigraphically, the play involves the Lower and Middle Cambrian Rome Formation and the Middle and Upper Cambrian Conasauga Group. The play is hypothetical and is confined mostly to the Kentucky and West Virginia part of the Rome Trough. The sandstone and carbonate reservoirs are classified as conventional.
Reservoirs:Sandstone in axial and basin-margin locations with respect to the rift system are important reservoirs in the play. Axial sandstone deposits generally are more quartzose in composition and better sorted than the basin margin sandstones, and they contain abundant carbonate grains. Locally, the axial sandstone deposits are so calcareous that they are more appropriately identified as sandy limestone. Both the axial and basin-margin sandstone facies contain feldspar, but the latter facies generally contains the most because of its closer proximity to granitic terranes. Deep burial diagenesis has occluded much of the primary porosity in the axial and basin-margin sandstones with silica and calcite cement. Most intergranular porosity that remains is secondary in nature, having resulted from the dissolution of feldspar and carbonate grains and carbonate cement.
Vuggy limestone and dolomite, created by brief subaerial exposure and karst processes, are potential reservoirs. Limestone reservoirs having oomoldic and biomoldic porosity and secondary intergranular porosity may be present.
Very likely, tectonic fractures are required to improve the quality of the sandstone and limestone reservoirs. In some cases, fractures may be the sole cause of a reservoir's porosity and permeability. Fractures are abundant in the rift-fill sequence as a result of multiple periods of reactivation along fault-bounded basement blocks, but there is some question as to whether or not they are still open at depths greater than 10,000 ft.
Source rocks:Dark-gray to black shale and argillaceous limestone in the Conasauga Group and upper part of the Rome Formation are the likely sources of gas and local oil in the play. Individual shale beds are no thicker than about 1 ft, but their cumulative thickness may be several tens of feet. Argillaceous limestone sequences may be as thick as 150 – 200 ft. TOC values of the dark-gray to black shale and limestone in the Conasauga Group and Rome Formation range from 0.05 to 0.59 percent (avg 0.27, n=22). Because of their high level of thermal maturation, the type of organic matter in the shale and limestone could not be determined.
Scattered CAI and Tmax values indicate that the rift-fill sequence in the northern West Virginia and Pennsylvania parts of the Rome Trough is overmature with respect to the generation of oil and gas. In central and southern West Virginia the Rome Trough is in the late stages of the gas generation zone, whereas in eastern Kentucky the Rome Trough is in the late stages of the oil generation zone. Dry thermal gas is the expected hydrocarbon type in most of the Rome Trough. In the Kentucky part of the Rome Trough local oil is expected to be associated with the dry gas.
Timing and migration:Oil and gas generation from the Conasauga Group and Rome Formation in the Rome Trough probably occurred from latest Ordovician to Late Devonian time, depending on local thermal and burial conditions. Most of the oil and gas generated at this time was trapped in nearby structures formed by extensional and(or) transtensional tectonics. Many of the original traps were modified by later compressional events such as the late Paleozoic Alleghenian orogeny, and, thus, oil and gas might have been redistributed to younger traps or lost.
Traps: Faulted rollover anticlines, tilted fault blocks, and positive flower structures are the major traps expected in the play. Closure on these structural traps may cover an area as large as 10,000 acres. The traps are most likely to be present between the depths of 8,000 and 15,000 ft. Seals for the structural traps are micrite and argillaceous micrite of the Conasauga Group and shale and siltstone of the Rome Formation. Abnormally high formation pressure measured in parts of the Rome Trough indicates that the seals are very efficient.
Exploration status:No more than 50 holes have been drilled through all or part of the rift-sequence in the Rome Trough. Several good gas and oil shows have been reported, the best of which was an initial potential flow of between 5 and 9 MMCFGPD from the Exxon No. 1 McCoy well in Jackson County, West Virginia. The well produced briefly before it was abandoned in 1975. The Mavity oil field, discovered in 1967 in Boyd County, Kentucky, produced about 10,000 BO in 1-1/2 years from a sandstone in the upper part of the Conasauga Group.
Resource potential:This play has potential for several undiscovered gas fields greater than 6 BCFG. A small oil field and good gas and oil shows indicate that hydrocarbons are present in the rift sequence. Moreover, numerous structures in the Rome Trough have not been tested. Several factors may limit the potential of the play: (1) reservoirs may be widely scattered and of low quality, (2) source beds may be very local and lean, and (3) numerous episodes of basement-fault reactivation may have permitted much of the gas to escape.
6702. Upper Cambrian, Ordovician, and Lower/Middle Silurian thrust belt play
The Upper Cambrian, Ordovician, and Lower/Middle Silurian Thrust Belt Play is defined by gas trapped in anticlines of the Appalachian Fold and Thrust Belt between, approximately, the Allegheny structural front on the west and the Blue Ridge structural front and thrust fault on the east. Dolomite, limestone, and sandstone reservoirs are prospective. Stratigraphically, the play involves the Upper Cambrian and Lower Ordovician Knox Group, Upper Cambrian Gatesburg Formation, Lower Ordovician and lower Middle Ordovician Beekmantown Group, Middle Ordovician Black River, Trenton, and Chickamauga Limestones, Upper Ordovician Bald Eagle (Oswego) Sandstone, Lower Silurian Tuscarora (Clinch) Sandstone, and Middle Silurian Keefer Sandstone. The play is confirmed and extends from New York to Alabama. The prospective reservoirs are classified as conventional.
Reservoirs: Platform dolomite and limestone that have vuggy porosity are important reservoirs in the play. The vuggy porosity was caused largely by karst processes and (or) by migrating deep basin fluids. Although these zones of vuggy porosity are capable of producing high-quality reservoirs in general, they probably are very discontinuous and heterogeneous in the play area. Among the most extensive zones of vuggy porosity are those formed beneath the Middle Ordovician Knox unconformity as a result of widespread subaerial exposure and karst activity. Limestone containing zones of oomoldic and biomoldic porosity, formed by one or more dissolution mechanisms, represents reservoirs of secondary importance.
Sandstone of shallow-marine, fluvial, and deltaic origin constitute a second group of important reservoirs. The composition of the sandstones range from quartzarenite to litharenite, and their feldspar content ranges from several percent to as much as 10 percent. Burial diagenesis has plugged much of the primary porosity in the sandstones with silica and calcite cement. Secondary intergranular porosity caused by the dissolution of feldspar, rock fragments, and cement is the dominant porosity type in the sandstone reservoirs. Locally, primary intergranular porosity has been preserved by unusual circumstances such as grain coatings of iron-rich cement.
Tectonic fractures greatly improve the quality of the carbonate and sandstone reservoirs and, in some cases, may be the sole cause of the reservoir. Fractures are commonly associated with faulted ramp anticlines, imbricate fault slices, and decollement zones, but many of them are cemented by syn- and post-tectonic episodes of burial and fluid flow.