Appalachian basin province (067) by R. T. Ryder introduction



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Exploration status: Gas and local oil fields discovered in the Bass Islands trend in the early 1980’s are the largest known accumulations in the play. An ultimate recovery of 30–40 BCFG and 2.5 MMBO is expected from the 10 or 12 fields in the trend. The Gerry field (Chautauqua County, N.Y.), discovery date 1981, ultimate recovery 18 BCFG, 1 MMBO and the North Harmony field (Cattaraugus County, N.Y.), discovery date 1981, ultimate recovery 11 BCFG,
1 MMBO are the largest fields in the Bass Island trend.

Several small subcommercial gas fields have been discovered in the play in north-central Pennsylvania. The Boot Jack pool, discovered in Elk County, Pennsylvania, in 1973 has produced 20 MMCFG through 1986 from the Helderberg Limestone. No current drilling is designed specifically to test for gas in fractured Silurian and Devonian carbonate reservoirs; however, these intervals may be tested as secondary objectives in holes drilled to the Lower Devonian Oriskany Sandstone and Middle Devonian Huntersville Chert.



Resource potential: This play has potential for a modest number of undiscovered gas fields greater than 6 BCFG. The fractured reservoirs required for the play are commonly very subtle and may have been overlooked in previous seismic and exploration programs. Future gas fields in the play probably will be discovered by chance in drilling for more predictable targets in the Lower Devonian Oriskany Sandstone and the Middle Devonian Huntersville Chert. A limiting factor in the play is that many of the prospective fracture zones may be cemented by syn- and post-tectonic episodes of burial and fluid flow.

6719. Devonian carbonate gas play

The Devonian Carbonate Gas Play is defined by gas trapped in pinnacle reefs of the Middle Devonian Onondaga and Columbus Limestones. The play extends across central and southern New York, northwestern Pennsylvania, eastern Ohio, western West Virginia, eastern Kentucky, and a small part of southwestern Virginia.

The eastern boundary of the play is defined in west-central Pennsylvania, central West Virginia, and south-central New York as the eastern limit of pinnacle reefs in the Middle Devonian Onondaga Limestone. Middle Devonian carbonate gas fields east and south of the boundary are considered to be in the Silurian and Devonian Carbonate Thrust Belt Play (6718). Traps in play 6718 are faulted ramp anticlines, imbricate thrust slices, and fracture zones mostly controlled by Upper Silurian Salina Group bedding-plane detachment. Because bedding-plane detachment at the Salina level, with overlying thin-skinned structures, extends across western Pennsylvania, most of southern and western New York, and easternmost Ohio, the Silurian and Devonian Carbonate Thrust Belt Play (6718) and the Devonian Carbonate Play (6719) overlap by 100–125 mi.

The northern boundary of the play is defined by the outcrop limit of the Devonian carbonate sequence at the north end of the Appalachian Basin in New York and by the United States–Canada border in the middle of Lake Erie. The western boundary of the play in Ohio is marked by the outcrop limit of the Columbus Limestone, whereas the western boundary of the play in Kentucky is marked by the subcrop limit of the Onondaga Limestone. The play is confirmed, and its reservoirs are conventional.



Reservoirs: Pinnacle reefs that formed on a shallow-water carbonate shelf and along a transition zone between shelf and deeper water basinal settings are the reservoirs in the play. Reef facies consist of bioclastic limestone as thick as 180 ft that has good to excellent vuggy and interskeletal porosity. The net pay thickness of the reef reservoirs averages about 80–100 ft. Interreef strata are about 50 ft thick and consist of biomicrite and local biosparite. Pinnacle reefs in the play cover an area between 350 and 500 acres. Drilling depths to the Onondaga reef facies across most of the trend range from 3,500 to 5,000 ft.

Source rocks: The source of gas in the play is the Middle and Upper Devonian black shale sequence that overlies the Onondaga and Columbus Limestones by 500 ft or less. The Middle and Upper Devonian black shale sequence in the play area is between 50 and 400 ft thick and has TOC values between 1 and 10 percent. Vitrinite reflectance data indicate that Middle and Upper Devonian source rocks in the play have achieved several levels of thermal maturity. Devonian source rocks in eastern and central New York, northwestern Pennsylvania, and easternmost Ohio are in the zone of gas generation, whereas those in western New York, northwesternmost Pennsylvania, and eastern Ohio are in the zone of oil generation. Devonian source rocks in east-central Ohio and offshore northern Ohio in Lake Erie are immature with respect to oil and gas generation. Wet thermal gas and local oil are the expected hydrocarbon types derived from the Middle and Upper Devonian source rocks.

Timing and migration: Peak oil and gas generation from the Middle and Upper Devonian black shale sequence occurred between Late Pennsylvanian and Early Triassic time when these beds were buried under an eastward-thickening wedge of orogenic sediments. Gas migrated a short distance downsection to the Onondaga reefs. Gas and local oil in the Columbus Limestone, in thermally immature parts of Ohio, probably migrated there from east of the play area where higher levels of thermal maturity were achieved. The Lower Devonian Oriskany Sandstone may have been the carrier bed for laterally migrated gas and oil.

Traps: Pinnacle reefs constitute the trap in the play and surrounding micrite and biomicrite are the seals.

Exploration status: Tens of thousands of holes have been drilled through the Onondaga Limestone in eastern Kentucky, northwest Pennsylvania, and western New York and the Columbus Limestone in Ohio in search of deeper objectives. The first gas fields in the Onondaga Limestone to be recognized as pinnacle reefs were discovered in the late 1960’s and early 1970’s in south-central New York and adjoining Pennsylvania. To date, about 17 fields in New York, 2 fields in Pennsylvania, and about 6 fields in Ohio produce gas and local oil from Onondaga-Columbus Limestone reefs. Several of these fields were discovered in the late 1980’s and early 1990’s. The Adrian Reef field (Steuben County, N. Y., discovery date 1971, ultimate recovery 8.2 BCFG), Thomas Corners field (Steuben County, N. Y., discovery date 1971, ultimate recovery ~6 BCFG), and Cyclone (McKean County, Pa., discovery date 1974, cumulative production through 1990 ~2.3 BCFG) are the largest fields discovered in the play.

Resource potential: This play has potential for a small number of undiscovered gas fields greater than 6 BCFG. Reefs are very subtle features that may have been overlooked in previous seismic and exploration programs. Most of the undiscovered fields are in sparsely drilled deeper parts of the play in western New York, northwestern Pennsylvania, eastern Ohio, and western West Virginia and in undrilled Lake Erie. Some fields will be discovered by chance in search of deeper objectives. A limiting factor in the play may be that the largest reefs are restricted to a 5- or 6-county area in west-central New York and adjoining north-central Pennsylvania.

6720. Oriskany Sandstone gas/faulted anticlines play

The Oriskany Sandstone Gas/Faulted Anticlines Play (6720) and the Oriskany Sandstone Gas Play (6721) are contiguous plays in the Lower Devonian Oriskany Sandstone depositional system. This sandstone system was deposited on a storm-dominated marine shelf. The Oriskany Sandstone Gas/Faulted Anticlines Play, the easternmost of the two plays, is defined by gas trapped in highly faulted ramp anticlines, salt anticlines, imbricate fault slices, and combination traps above bedding plane detachment in the Upper Silurian Salina Group and (or) Cambrian and Ordovician shale sequences. In addition to the Oriskany Sandstone, the play involves gas-bearing fractured chert in the overlying Middle Devonian Huntersville Chert. The play extends across south-central New York, northeastern, central and southwestern Pennsylvania, western Maryland, central and eastern West Virginia, and narrow parts of western Virginia.

The western and northern boundaries of the play are marked by the approximate western and northern limit of high-amplitude salt anticlines and (or) highly faulted ramp anticlines formed by bedding plane detachment and flowage in salt beds of the Salina Group. Bedding-plane detachment at the Salina level continues west of the play area into New York, Pennsylvania, and easternmost Ohio; however, stratigraphic pinch-outs and low-amplitude basement controlled anticlines are the dominant traps in the Oriskany Sandstone here. Oriskany Sandstone reservoirs in these more mildly deformed parts of the basin are included in the Oriskany Sandstone Gas Play (6721). The Oriskany Sandstone in mildly deformed southeastern New York and adjoining northeasternmost Pennsylvania is included with play 6720.

The eastern boundary of the play is defined by the eastern limit of the Oriskany Sandstone in the Appalachian Fold and Thrust Belt. The play is confirmed, and the reservoirs are conventional.



Reservoirs: The primary reservoir in the play is the Oriskany Sandstone, a quartz-rich, calcite- and silica-cemented, fine- to coarse-grained sandstone sequence. Although primary intergranular porosity is important locally, secondary intergranular and fracture porosity are the dominant porosity types in the Oriskany Sandstone. Fracture porosity also is present in the Huntersville Chert, commonly the primary reservoir in the play. Secondary intergranular porosity in the Oriskany Sandstone resulted from the dissolution of calcite cement and unstable grains such as chert and calcareous fossil fragments. Porosity for the sandstone ranges from 3 to 20 percent and averages 8 percent. The thickness of the producing part of the Oriskany Sandstone reservoir ranges from 6 to 150 ft, whereas the thickness of the producing part of the Huntersville Chert ranges from about 60 to 170 ft. Drilling depths to the Oriskany Sandstone reservoir in the play range from about 6,000 to 9,000 ft.

Source rocks: The source of gas in the play is the Middle and Upper Devonian black shale sequence that overlies the Oriskany Sandstone by 500 ft or less. The Middle and Upper Devonian black shale sequence in the play area is between 50 ft and 400 ft thick and has TOC values between 3 and 5 percent. Vitrinite reflectance data indicate that Middle and Upper Devonian source rocks in the play have achieved several levels of thermal maturity. Devonian source rocks in about the northern and western third of the play area are in the zone of gas generation whereas Devonian source rocks in the remainder of the play are overmature with respect to the generation of oil and gas. Dry thermal gas is the expected hydrocarbon type in the play.

Timing and migration: Peak oil and gas generation from the Middle and Upper Devonian black shale sequence occurred between Late Pennsylvanian and Early Triassic time when it was buried under an eastward-thickening wedge of orogenic sediments. Gas migrated a short distance downsection into available structural traps. Local solid hydrocarbon coatings on quartz grains suggest that some oil was originally trapped in the play and later converted to gas as thermal maturation levels increased.

Traps: Highly fractured, faulted ramp anticlines, salt anticlines, and imbricate fault slices above bedding plane detachment at Cambrian, Ordovician, and Silurian stratigraphic levels are the major traps in the play. Depositional and (or) erosional pinchouts of sandstone in combination with anticlinal flanks also are important traps. Most traps are very complex in that structural closure and (or) fracturing commonly occurs at several structural levels and in different geographic localities. The seals for the traps are micrite in the Onondaga Limestone and black shale in the Middle Devonian Marcellus Shale.

Exploration status: Gas was first discovered in the play in the early 1930’s in south-central New York and adjoining Pennsylvania. Drilling depths to the Oriskany Sandstone in this region range from 2,000 to 5,500 ft. By the 1950’s and 1960’s exploration in the play had successfully expanded into central Pennsylvania, southern Pennsylvania, western Maryland, and eastern West Virginia where drilling depths to the Oriskany were 7,000–8,000 ft. Successful exploration continues today for Oriskany-Huntersville gas in generally deeper and most eastward parts of the play. Between 1930 and 1990 approximately 110 gas fields were discovered in the play, and about half of them are greater than 6 BCFG in size. The most recently discovered fields that exceed 6 BCFG are the Jordan Run (Grant County, W. Va., discovery date 1982, ultimate recovery ~25 BCFG) and Stagecoach (Tioga County, N.Y., discovery date 1987, ultimate recovery ~ 8 BCFG).

The largest fields in the play are Driftwood, (Cameron Co., Pa.), discovery date 1951, ultimate recovery ~253 BCFG; Helvetia, (Clearfield Co., Pa.), discovery date 1960, ultimate recovery ~153 BCFG; Dubois, (Clearfield Co., Pa.), discovery date 1960, ultimate recovery ~107 BCFG; and Leidy, (Clinton Co., Pa.), discovery date 1950, ultimate recovery ~94 BCFG.



Resource potential: This play has potential for a modest number of undiscovered gas fields greater than 6 BCFG. Drilling is still active for structurally trapped Oriskany Sandstone gas, and several fields greater than 6 BCFG in size were discovered in the 1980’s. Many sparsely drilled parts of the play having complex structure probably contain undiscovered fields. A limiting factor in the play is that reservoir quality of the Oriskany Sandstone diminishes in the deeper parts of the play. Commonly, fractures and pore space are tightly cemented making porosity very difficult to predict.

6721. Oriskany Sandstone gas

The Oriskany Sandstone Gas Play (6721) and the Oriskany Sandstone Gas/Faulted Anticlines Play (6720) are contiguous plays in the Lower Devonian Oriskany Sandstone depositional system. This sandstone system was deposited on a storm-dominated marine shelf. The Oriskany Sandstone Gas Play, the westernmost of the two plays, is defined by gas and local oil trapped by depositional and erosional pinchouts, combination traps, and low-amplitude anticlines. The play extends across northwestern Pennsylvania, eastern Ohio, offshore Ohio and Pennsylvania in Lake Erie, western West Virginia, and small parts of western New York, eastern Kentucky, and southwestern Virginia. The Oriskany Sandstone is absent in several large areas of the play in Ohio, Pennsylvania, and New York due to erosion and (or) nondeposition.

The eastern boundary of the play is marked by the approximate limit of high-amplitude salt anticlines and (or) ramp anticlines formed by bedding-plane detachment and flowage in salt beds of the Salina Group. Oriskany Sandstone reservoirs in these highly deformed regions of the basin are included in the Oriskany Sandstone Gas/Faulted Anticlines Play (6720). Bedding plane detachment at the Salina level continues into New York, Pennsylvania, and easternmost Ohio parts of the play area; however, stratigraphic pinch-outs and low-amplitude, basement-controlled anticlines are the dominant traps in the Oriskany Sandstone here.

The western and northern boundaries of the play are defined by the depositional and (or) erosional limit of the Oriskany Sandstone. The depositional limit of the Oriskany Sandstone is undefined in Lake Erie and may extend as far north as the United States-Canada border. The play is confirmed, and the sandstone reservoirs are conventional.



Reservoir: The reservoir in the play is the Oriskany Sandstone, a quartz-rich, calcite- and silica-cemented, fine- to coarse-grained sandstone sequence. Although secondary intergranular and fracture porosity are important locally, primary intergranular porosity is the dominant porosity type. Porosity for the sandstone ranges from 6 to 22 percent and averages 12 percent, whereas permeability for the sandstone ranges from 10 to 60 mD and averages 27 mD. The total thickness of the gas producing parts of the Oriskany Sandstone ranges from 5 to 50 ft. Drilling depths to the Oriskany Sandstone in the play are between 2,000 and 6,000 ft.

Source rock: The source of gas in the play is the Middle and Upper Devonian black shale sequence that overlies the Oriskany Sandstone by 500 ft or less. The Middle and Upper Devonian black shale sequence in the play area is between 100 and 400 ft thick and has TOC values between 3 and 5. Vitrinite reflectance data indicate that Middle and Upper Devonian source rocks in the play have achieved several levels of thermal maturity. Devonian source rocks in western New York, northwestern Pennsylvania, easternmost Ohio, southern West Virginia, and southwestern Virginia are in the zone of gas generation, whereas those in northwesternmost Pennsylvania, eastern Ohio, western West Virginia, and eastern Kentucky are in the zone of oil generation. Devonian source rocks in east-central Ohio, offshore northern Ohio in Lake Erie, and northeastern Kentucky are immature with respect to oil and gas generation. Wet thermal gas and oil are the expected hydrocarbon types derived from the Middle and Upper Devonian source rocks.

Timing and migration: Peak oil and gas generation from the Middle and Upper Devonian black shale sequence occurred between Late Pennsylvanian and Early Triassic time when these beds were buried under an eastward-thickening wedge of orogenic sediments. Gas migrated a short distance downsection to the Oriskany Sandstone. Gas and local oil in the Oriskany Sandstone, in thermally immature parts of Ohio, probably migrated there from southeast of the play area where thermal maturity levels were higher.

Traps: Depositional updip pinchouts of the Oriskany Sandstone and combination traps, where the Oriskany Sandstone pinches out along anticlinal flanks or noses, have trapped the largest accumulations in the play. Low-amplitude anticlines controlled by basement fault blocks or minor bedding-plane detachment in the Salina Group also are traps. Seals are micrite in the Middle Devonian Onondaga Limestone.

Exploration status: The first fields in the play were discovered in northeastern Ohio, close to Lake Erie, between 1899 and 1915. By 1935, about 18 gas and local oil fields had been discovered in the Ohio part of the play. Active drilling for Oriskany Sandstone gas had also begun in West Virginia by the mid-1930’s, and shortly afterward the giant Elk Poca field was discovered. Between 1899 and 1986, approximately 100 gas fields were discovered in the play, eleven of them are equal to or greater than 6 BCFG in size. No fields of this size have been discovered since 1967. Tens of thousands of holes have been drilled to or through the Oriskany Sandstone in the play. Several Oriskany Sandstone fields have been discovered in northwestern Pennsylvania in the 1980’s, but they are less than 1 BCFG in size.

The largest fields in the play are Elk-Poca (Sissonville), (Kanawha and Jackson Cos., W.V.), discovery date 1936, ultimate recovery 1.0 TCFG; Blue Creek (Falling Rock), (Kanawha Co., W.V.), discovery date 1944, ultimate recovery 67 BCFG; Campbell Creek-Malden, (Kanawha Co., W.V.), discovery date 1930, ultimate recovery 28 BCFG; and Birds Run, (Guernsey Co., Ohio), discovery date 1942, ultimate recovery ~ 26 BCFG.



Resource potential: This play has potential for a small number of undiscovered gas fields greater than 6 BCFG. Probably the undiscovered fields are located in undrilled Lake Erie. Outside of Lake Erie, this play is exhausted except for very small accumulations.

tuscarora sandstone–clinton/medina sandstone plays

6727. Tuscarora Sandstone gas play

The conventional Tuscarora Sandstone Gas Play (6727), Clinton/Medina Sandstone Gas plays (unconventional continous-type plays 6728, 6729, 6730, 6731), and the conventional Clinton/Medina Sandstone Oil/Gas Play (6732) are contiguous plays that occupy progressively westward parts of the widespread Lower Silurian sandstone depositional system. Although sandstones of fluvial and distributary channel origin are recognized locally, most of the sandstone was deposited in littoral marine, deltaic, and offshore-marine settings. This group of plays extends westward from near the Allegheny structural front in Pennsylvania, West Virginia, and Virginia, where sandstone beds are thickest and have minor shale interbeds (Tuscarora Sandstone), to the depositional limit of the Lower Silurian "Clinton" sandstones in east-central Ohio and eastern Kentucky, where sandstone beds are thinner and intercalated with abundant shale and siltstone. The Lower Silurian sandstone system extends into New York as far north as the outcrop limit of the Lower Silurian Medina Group and as far east as the subcrop of the Tuscarora Sandstone beneath the Middle Silurian Oneida Sandstone.

The Tuscarora Sandstone Gas Play is defined by gas trapped in the Lower Silurian Tuscarora Sandstone by low-amplitude basement-controlled anticlines commonly in combination with diagenetic traps. The most easterly of the Lower Silurian sandstone plays, this play covers large parts of New York, Pennsylvania, and West Virginia; the southwesternmost end of the play includes small parts of Virginia and Kentucky. On the east side, the play is bounded by the approximate western limit of detached anticlines involving Upper Ordovician and older strata, whereas, on the west side, it is bounded by an arbitrary line separating the Tuscarora Sandstone from Clinton–Medina sandstones. The play is confirmed and its prospective reservoirs are conventional.

Reservoirs: Fine- to medium-grained sandstones consisting mostly of quartzarenite constitute the reservoirs in the play. Compaction and burial diagenetic processes have plugged most of the primary intergranular porosity with silica and minor calcite cement. Locally, primary porosity may be preserved by clay coatings that inhibit the formation of quartz overgrowths. The dominant porosity types are: secondary intergranular porosity caused by dissolution of chert grains, rock fragments, and calcite cement and fracture porosity probably caused by tectonic activity. Porosity values are as high as 13 percent but generally average 4 percent or less. Permeability values are less than 1 mD. The thickness of the Tuscarora Sandstone in the play ranges from about 100 to 325 ft, and sandstone to shale ratios generally exceed 3. Drilling depths to the Tuscarora Sandstone range from 5,000 to 11,000 ft.

Source rocks: The source of gas in the play is uncertain. The most plausible candidates are shale and argillaceous limestone of the Middle Ordovician Utica Shale, Antes Shale, and Trenton Limestone or black shale of the Middle and Upper Devonian sequence. Both proposed source rock sequences are relatively thick (200–400 ft for the Middle Ordovician sequence; 50–250 ft for the Middle and Upper Devonian sequence), adequately rich (TOC 0.5-3 percent for the Middle Ordovician sequence; TOC 1-5 percent for the Middle and Upper Devonian sequence), and have organic matter dominated by type II kerogen. However, gas generated from these source-rock sequences is not particularly accessible to the reservoir. For example, between 2,000 and 3,000 ft of vertical migration, through predominantly shale and siltstone, is required for gas derived from the Middle Ordovician shale sequence to reach the Tuscarora Sandstone. In contrast, between 1,500 and 3,000 ft of downward migration, through at least 500 ft of evaporite and evaporitic dolomite, is required for Middle and Upper Devonian shale gas to reach the Tuscarora Sandstone. A slight preference is given to the Middle Ordovician source beds because upward vertical migration is more plausible than downward migration.

Based on CAI and Tmax data for the Middle Ordovician sequence and vitrinite reflectance data for the Middle and Upper Devonian sequence, both proposed source-bed sequences in the play area are in the zone of gas generation. A narrow region that contains Middle Ordovician and Devonian source beds along the west side of the Allegheny structural front in Pennsylvania and part of adjoining West Virginia is overmature with respect to oil and gas generation. Dry thermal gas is the expected hydrocarbon type whether the source is the Middle Ordovician or Middle and Upper Devonian shale sequence.


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