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



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Source rocks: Black shale and argillaceous limestone in the Middle Ordovician Utica Shale, Antes Shale, Dolly Ridge Formation of the Trenton Group, Trenton Limestone, Martinsburg Formation (lower part), and Chickamauga Limestone are the most likely sources of gas and local oil in the play. The thickness of the black shale and limestone sequences ranges from 200 to 400 ft. Gas and oil generated from these source beds are required to migrate approximately 1,000 – 1,500 ft downsection to reach dolomite reservoirs in the Cambrian and Lower Ordovician sequence. In contrast, between 4,000 and 5,000 ft of upward vertical migration is required for oil and gas from Middle Ordovician source beds to reach Lower and Middle Silurian sandstone reservoirs.

Black shale and argillaceous limestone in the Middle Ordovician Jacksonburg Limestone, Liberty Hall Formation, Paperville Shale, Blockhouse Shale, Sevier Formation, and Athens Shale--now located in far-traveled thrust sheets along the eastern margin of the play area--may be source beds of secondary importance. These shale and limestone sequences are generally thicker and of deeper water origin than those in the western part of play, but hydrocarbons generated from them would have migrated a long distance to charge available reservoirs. Upper Devonian black shale may be a local source rock in southern Virginia and eastern Tennessee where it is overridden by frontal thrust slices of lower Paleozoic carbonate rocks.

TOC values in the Middle Ordovician black shale and argillaceous limestone sequences range from about 0.5 to 3 percent. Organic matter mostly consists of type II kerogen. Based on CAI and Tmax values, the source beds in the Pennsylvania part of the play are overmature with respect to the generation of oil and gas. This region of overmature Middle Ordovician source beds continues northward into adjoining southeast New York and southward into adjoining western Maryland and eastern West Virginia. From northern Virginia and adjoining West Virginia the overmature region begins to narrow southward and follow the eastern limit of the play through southwestern Virginia, eastern Tennessee, northeastern Georgia, and eastern Alabama. The remainder of the play area, from northeastern West Virginia to east-central Alabama, is mainly in the gas generation zone. A narrow region along the western side of the play area in southwestern Virginia, east-central Tennessee, northwesternmost Georgia, and northeastern Alabama is in the oil generation zone. Dry thermal gas and local oil are the expected hydrocarbon types. Because of the generally high thermal maturity of the strata in the play area, high amounts of noncombustible gas may be mixed with the methane gas.

Timing and migration: Peak oil and gas generation from Middle Ordovician shale and argillaceous limestone source beds probably occurred between Late Pennsylvanian and Early Triassic time when these rocks were deeply buried under an eastward-thickening wedge of orogenic sediments and thrust sheets. Ramp anticlines formed during the Alleghenian orogeny (Pennsylvanian-Permian) were available to trap the hydrocarbons. Abnormally high formation pressures achieved in the source beds may have assisted the vertical migration of oil and (or) gas. Middle Ordovician source beds in the interior parts of the fold and thrust belt may have reached maturity at a slightly earlier time when fewer compressional structures were available to trap the oil and gas. Some of these hydrocarbons may have migrated up the adjoining northwest flank of the basin.

Traps: Faulted ramp anticlines and imbricate fault slices formed above zones of bedding plane detachment in Cambrian and (or) Ordovician shale are the major traps. Some of the traps are very complex with structural closure and(or) fracturing commonly occurring at several structural and stratigraphic levels and in different geographic locations. Closure on the traps may cover an area as large as 5,000 acres. The depth range to unexplored structural traps is between 10,000 and 15,000 ft.

Important shale and micrite seals are present in the Middle Ordovician part of the Beekmantown Group; Middle Ordovician Loysburg Formation, Blackford Formation, Lenoir Limestone, Chickamauga Limestone and Antes Shale; Upper Ordovician Reedsville Shale, Martinsburg Formation, Juniata Formation, and Sequatchie Formation; and the Middle Silurian Rose Hill Formation, Rochester Shale, and Mifflintown Formation.



Exploration status: Approximately 100 holes have been drilled to one or more of the objective reservoirs in the play, resulting in the discovery of several small oil and gas fields. The largest oil fields, Rose Hill and Ben Hur, located in Lee County, Virginia, produce oil from fractured limestone in the Middle Ordovician sequence. From 1946 through 1993, about 600,000 BO have been produced from the Rose Hill field. Possibly the production in this field will exceed 1 MMBO. One well in the field produces from fractured dolomite in the Knox Group. Another 1-well oil field was found in 1980 in the Middle Ordovician Stones River Group in Cumberland County, Tennessee.

The largest gas fields found to date produce from the Tuscarora Sandstone in Centre County, Pennsylvania, (discovery dates 1977 – 1982), and in Tucker and Preston Counties, West Virginia, (discovery date 1963). The Devils Elbow field (Centre Co., Pa.) and the Leadmine field (Tucker and Preston Cos., W. Va.,) will each ultimately produce at least 6 BCFG. Additional known gas fields in the play are an unnamed 1-well Knox field in Grainger County, Tennessee, and a 2-well Keefer Sandstone pool in the North Headsville field, Mineral County, West Virginia.



Resource potential: This play has potential for several undiscovered gas fields greater than 6 BCFG. Small oil and gas fields indicate that hydrocarbons are present in all the prospective reservoirs. Several factors may limit the potential of the play: (1) reservoirs may be widely scattered and low in quality, (2) although good source beds are present, they are in less than optimum positions to have charged prospective reservoirs, (3) 10 – 20 dry holes drilled in the 1980's have tested some of the most attractive structures in the play, and (4) post-Triassic uplift and erosion of the fold and thrust belt may have permitted most of the oil and gas to escape

6703. Beekmantown/Knox carbonate oil/gas play

The Beekmantown/Knox Carbonate Oil and Gas Play is defined by oil and gas trapped in Cambrian and Lower to lower Middle Ordovician platform dolomite reservoirs by truncation traps, paleotopographic highs, and low-amplitude basement-controlled anticlines. The play involves Cambrian and Lower Ordovician dolomite approximately between the Allegheny structural front and the western limit of the Appalachian Basin. This play area is northwest of the Valley and Ridge part of the Appalachian Fold and Thrust Belt and contains few, if any, bedding-plane detachment structures in pre-Upper Ordovician rocks. Stratigraphically, the play involves the Upper Cambrian and Lower Ordovician Knox Group and the Lower Ordovician and lower Middle Ordovician Beekmantown Group. Commonly, the Cambrian part of the Knox Dolomite in central Ohio is referred to as the Trempealeau Dolomite. Also included in the central Ohio part of the play are thin quartzose sandstones (for example, Krysik sandstone) intercalated with the Upper Cambrian dolomite.

The play is confirmed and extends across parts of New York, Pennsylvania, Ohio, West Virginia, Kentucky, Virginia, Tennessee, and Alabama. Prospective reservoirs in the play are classified as conventional.

Reservoirs: Dolomite that has vuggy and (or) fracture porosity is the most important reservoir in the play. The vuggy porosity formed by subaerial exposure and karst processes. Commonly, tectonic fracturing is required to improve the quality of the dolomite reservoirs. Most likely, the fracturing resulted from recurrent movement of fault-bounded basement blocks. Some of the fractures may have originated by the undermining and collapse of karst terranes.

The most porous and continuous zones of vuggy porosity are either directly beneath or within several hundred feet of the widespread Middle Ordovician Knox unconformity. Intercrystalline porosity in coarsely crystalline sparry dolomite supplements the vuggy porosity in these reservoirs. Periods of subaerial exposure of shorter duration have formed karst-related porous zones in dolomite far below the Knox unconformity. These zones may be suitable for oil and gas reservoirs, particularly if tectonic fractures and selective dissolution of unstable cement and intraclasts by circulating basin fluids have improved their porosity.



Source rocks: Black shale and argillaceous limestone in the Middle Ordovician Utica Shale, Antes Shale, Trenton Limestone, and Dolly Ridge Formation of the Trenton Group are the most likely sources of oil and gas in the New York, Pennsylvania, Ohio, and West Virginia parts of the play. The thickness of the black shale and limestone sequences ranges from 200 to 400 ft. Oil and gas are required to migrate approximately 1,000–1,500 ft downsection to reach dolomite reservoirs in the Cambrian and Lower Ordovician sequence. In Kentucky and adjoining western West Virginia, shale and argillaceous limestone in the Middle and Upper Cambrian Conasauga Group in the Rome Trough may be the source of oil and gas.

Black shale and argillaceous limestone is the Middle Ordovician Jacksonburg Limestone, Liberty Hall Formation, Paperville Shale, Blockhouse Shale, Sevier Formation, and Athens Shale--outside the play area in far-traveled thrust sheets along the eastern margin of the Valley and Ridge--may be source beds of secondary importance. These shale and limestone sequences are generally thicker and of deeper water origin than those sequences in the play area, but hydrocarbons generated from them would have migrated a long distance to charge available reservoirs. Oil and gas in the Virginia, Tennessee, and Alabama parts of the play probably depend on these source rocks.

TOC values in the Middle Ordovician black shale and argillaceous limestone sequences range from about 0.5 to 3 percent. Organic matter mostly consists of type II kerogen. Based on CAI and Tmax values, Middle Ordovician source beds in the New York, Pennsylvania, eastern Ohio, and West Virginia parts of the play are in the gas generation zone. The gas generation zone continues into Kentucky where Cambrian shale and limestone in the Rome Trough, rather than Middle Ordovician shale and limestone, are probable source beds for the play. Middle Ordovician source beds in a narrow zone that adjoins the Allegheny structural front in Pennsylvania and New York are overmature with respect to oil and gas generation. In most of central and eastern Ohio, Middle Ordovician source beds are in the oil generation zone. Although Middle Ordovician rocks in southern Ohio, Kentucky, Tennessee, and Alabama are in the oil generation zone, they contain no known source rocks. Oil and thermal gas are the expected hydrocarbon types in the play.

Timing and migration: Peak oil and gas generation from Middle Ordovician shale and argillaceous limestone source beds probably occurred between Late Pennsylvanian and Early Triassic time when these rocks were deeply buried under an eastward-thickening wedge of orogenic sediments and thrust sheets. Much of the oil and gas that was generated from Middle Ordovician source beds migrated up the gently dipping northwest flank of the basin along porous carbonate horizons beneath the Knox unconformity. Numerous stratigraphic traps and a modest number of anticlinal traps were available to entrap the migrating hydrocarbons. As the zone of oil and gas generation expanded northward and westward with increasing burial, traps on the homoclinal northwest flank of the basin became charged with locally derived hydrocarbons.

Cambrian source beds in the Rome Trough of Kentucky and adjoining West Virginia probably reached the oil and gas generation zone in latest Ordovician to Late Devonian time. Some of this oil and gas may have migrated vertically upward into Cambrian and Lower Ordovician reservoirs and become trapped in basement-involved fault blocks and anticlines.



Traps: Truncation traps and paleotopographic hills beneath the Knox unconformity are principal traps in the play. Commonly, these traps are combined with small anticlines, anticlinal noses, and faults. These stratigraphic traps cover an area as large as 2,000 acres. Low-amplitude anticlines controlled by basement tectonics also may be important traps in the play. In general, these structural traps are larger than the stratigraphic traps and, because of their history of recurrent growth, may be associated with fracture-enhanced reservoirs.

Important seals in the play are green shale and argillaceous dolomite in the Middle Ordovician Wells Creek Formation (Dolomite), Shadow Lake Formation, and Pamelia Formation and anhydritic dolomicrite and local shale in the lower Middle Ordovician part of the Beekmantown Group. These units directly overlie the Knox unconformity throughout much of the play area. Also, thick dolomicrite units separating karst-related porous zones in the Knox and Beekmantown Group are good seals.



Exploration status: The Morrow County, Ohio, oil boom in the early 1960's led to the discovery of 50 – 75 oil fields trapped in Cambrian dolomite beneath the Knox unconformity. Drilling depths to the reservoir range from 3,000 to 4,000 ft. About 29 of the fields are equal to or greater than 1 MMBO in size, the largest of which has an ultimate recovery of approximately 15 MMBO. Through 1993, about 60 MMBO and 30 BCF of associated gas have been produced from Morrow County and adjoining counties. The largest fields in the trend are Cardington East, ultimate recovery ~15 MMBO; Denmark, ultimate recovery ~7.5 MMBO; and Woodbury South, ultimate recovery ~6.2 MMBO.

Gas and minor oil accumulations were discovered in the Lower Ordovician Beekmantown Group (Dolomite) of eastern Ohio and northwestern Pennsylvania in the late 1960's through the early 1980's. Most of the gas is in 1-well fields at depths between 8,000 and 10,000 ft. Several gas wells in Columbiana County, Ohio, produced briefly at a rate of 10 – 14.5 MMCFGPD. Bakersville, the largest Beekmantown field, is in Coshocton County, Ohio. Discovered in 1980 along the Upper Cambrian(?) Rose Run Sandstone producing trend, this field produces gas and minor condensate from fractured vuggy dolomite at a depth of 6,700 ft. An ultimate recovery of 14 BCFG is expected for the field.

About 14 small gas fields, some with minor associated oil, have been discovered in the Knox Group in Kentucky. The largest gas accumulation is a deeper pool extension of the Mine Fork field, Johnson County, Kentucky, discovered in 1980 by Ashland Oil Company. Located along the Irvine-Paint Creek basement fault, this 3- to 4-well field has yielded an unknown quantity of gas from the Knox Group. A group of subcommercial gas fields (Raccoon Mountain, Fogertown, Trixie, and Chavie), with commingled production from Knox dolomite and Middle Ordovician limestone reservoirs, is along the basement controlled Rockcastle River uplift in Laurel, Clay, and possibly Perry Counties, Kentucky.

Resource potential: This play has potential for a modest number of undiscovered oil and gas fields greater than 1 MMBO or 6 BCFG. Prospective stratigraphic and anticlinal traps are very subtle and could easily have been overlooked in previous seismic surveys and exploration phases. The relatively late discovery (1980) of the largest gas field in the play and recent discoveries of oil and gas in Licking and Pickaway Counties, Ohio, in Cambrian dolomite and sandstone suggest that more fields remain to be found. There are large areas that are sparsely drilled to the Knox and Beekmantown Groups. A limiting factor in the play may be that reservoirs east and south of the Morrow County trend are widely scattered and of low quality.

6704. Rose Run/Gatesburg/Theresa sandstone Gas play

The Rose Run/Gatesburg/Theresa Sandstone Gas Play is defined by gas trapped in Upper Cambrian(?) shelf sandstone reservoirs by truncation traps, paleotopographic highs, and low-amplitude, basement-controlled anticlines. Stratigraphically, the play involves the Upper Cambrian(?) Rose Run Sandstone, lower and upper sandy members of the Gatesburg Formation, lower and upper sandstone members of the Copper Ridge Dolomite, and Theresa Formation.

The play covers western New York, western Pennsylvania, eastern Ohio, westernmost Maryland, western West Virginia, and northeastern Kentucky. In Ohio, the western limit of the play is marked by the northeast-trending subcrop of the Rose Run Sandstone beneath the Knox unconformity. The truncated western limit of Cambrian sandstones in the play continues from Ohio into northwesternmost Pennsylvania and western New York. The western boundary of the play in eastern Kentucky is defined by the depositional limit of the Rose Run Sandstone. Prospective sandstone in the play extends eastward to a poorly defined depositional limit in eastern New York, central Pennsylvania, eastern West Virginia, and southeast Kentucky controlled in part by post-rift subsidence of the Rome Trough. Most of the play area is northwest of the Valley and Ridge part of the Appalachian Fold and Thrust Belt and contains few, if any, bedding plane detachment structures in pre-Upper Ordovician rocks. The play is confirmed, and its prospective reservoirs are classified as conventional.

Reservoirs: The reservoirs in the play are sandstone of shallow-marine shelf and peritidal origin. They are in 50–150-foot-thick sandstone-dominated sequences containing numerous thin- to medium-sized beds of dolomite. Commonly the sandstone beds in the sequences thicken progressively upward at the expense of the dolomite beds. In the Rome Trough of southern Pennsylvania the sandstone-dominated sequences are as thick as 400 ft. Lateral continuity and reservoir quality of the sandstones are diminished by their close depositional and diagenetic association with dolomite lithofacies.

The sandstones consist of quartzarenite, subarkose, and arkose. Secondary intergranular porosity, caused by dissolution of feldspar grains and dolomite cement, is the dominant porosity type. Porosity values in the Ohio and northwestern Pennsylvania part of the play range from <1 to 25 percent (avg ~7), and permeability values range from <0.1 to 240 mD (avg ~5). Drilling depths to reservoirs in this part of the play range from about 6,000–8,000 ft. These reservoirs are among the most porous and continuous in the play because of their location either directly beneath or within several hundred feet of the Knox unconformity. Lower porosity and permeability values are expected in easternmost Ohio, central West Virginia, and west-central Pennsylvania where drilling depths to the Upper Cambrian sandstones exceed 9,000 ft and the dolomite section between the Knox unconformity and the sandstones is thicker. There, tectonic fracturing and extreme dissolution of unstable grains and cement are necessary to increase reservoir quality.



Source rocks: Black shale and argillaceous limestone in the Middle Ordovician Utica Shale, Antes Shale, Trenton Limestone, and Dolly Ridge Formation of the Trenton Group are the most likely sources of gas and local oil in the play. The thickness of the black shale and limestone sequences ranges from 200 to 400 ft. Approximately 1,000 – 1,500 ft of downward migration is required for oil and gas to reach Cambrian sandstones directly beneath the Knox unconformity, whereas 2,000 – 3,000 ft of downward migration is required to reach sandstones separated from the Knox unconformity by dolomite from the Beekmantown Group. Shale and argillaceous limestone in the Middle to Late Cambrian Conasauga Group and equivalent Pleasant Hill Limestone may be a source of gas in the eastern Kentucky part of the play where Middle Ordovician source beds are absent and in central Pennsylvania and central West Virginia where about 3,000 ft of micrite and dolomicrite separate sandstone reservoirs from Middle Ordovician source beds.

TOC values in the Middle Ordovician black shale and limestone sequences range from about 0.5 to 3 percent. Organic matter largely consists of type II kerogen. Based on CAI and Tmax values, Middle Ordovician source beds across most of the play are in the gas generation zone. The gas generation zone extends into eastern Kentucky where Cambrian shale and limestone in the Rome Trough, rather than Middle Ordovician shale and limestone, are probable source beds for the play. Middle Ordovician source beds in a narrow zone that adjoins the Allegheny structural front in Pennsylvania are overmature with respect to oil and gas generation. The oil generation zone for Middle Ordovician source beds crosses most of the Ohio part of the play and extends into northwesternmost Pennsylvania and New York. Although Middle Ordovician rocks in southern Ohio and east-central Kentucky are in the oil generation zone, they contain no known source rocks. Dry and wet thermal gas and local oil and condensate are the expected hydrocarbon types in the play.



Timing and migration: Peak oil and gas generation from the Middle Ordovician shale and argillaceous limestone source beds probably occurred between Late Pennsylvanian and Early Triassic time when these rocks were deeply buried under an eastward thickening wedge of orogenic sediments and thrust sheets. Much of the oil and gas that was generated from Middle Ordovician source beds migrated up the gently dipping northwest flank of the basin along porous carbonate horizons beneath the Knox unconformity. Numerous stratigraphic traps and a modest number of anticlinal traps were available along the Rose Run Sandstone subcrop to entrap migrating hydrocarbons. East of the subcrop belt, some oil and gas may have worked their way downsection through 1,000 ft or more of Beekmantown Group dolomite to the Rose Run Sandstone and became trapped in anticlinal structures.

Cambrian source beds in the Rome Trough of Kentucky, West Virginia, and Pennsylvania probably reached the oil and gas generation zone in latest Ordovician to Late Devonian time. Some of this oil and gas may have migrated vertically upward into Upper Cambrian sandstone reservoirs and become trapped in basement-involved fault blocks and anticlines.



Traps: Truncation traps and paleotopographic hills beneath the Knox unconformity are principal traps in the play. Commonly, these traps are combined with small anticlines, anticlinal noses, and faults. Paleotopographic hills typically cover an area of 80 acres or less but locally may be a large as 2,000 acres. Low-amplitude anticlines controlled by basement tectonics also may be important traps in the play. In general, these structural traps are larger than the stratigraphic traps and, because of their history of recurrent growth, may be associated with fracture-enhanced reservoirs.

Important seals in the play are green shale and argillaceous dolomite in the Middle Ordovician Wells Creek Formation, Shadow Lake Formation, and Pamelia Formation that directly overlie the Rose Run Sandstone subcrop. Also, dolomicrite beds in the Beekmantown Group are good seals for Rose Run Sandstone gas accumulations east of the subcrop belt.



Exploration status: Exploration along the Rose Run Sandstone subcrop trend in eastern Ohio, between the late 1970's and the present, has led to the discovery of about 25 gas fields. Most of the fields are concentrated in Coshocton County, Ohio, and adjoining Holmes and Tuscarawas Counties; however, fields are scattered over at least six additional counties. The trend continues into northwesternmost Pennsylvania and western New York where small gas fields produce from the upper sandy member of the Gatesburg Formation and Theresa Formation, respectively. Drilling depths to the Rose Run Sandstone reservoir in Ohio range from 6,000 to 8,000 ft. Through 1991 about 21.5 BCFG and 0.5 MMBO have been produced from the Rose Run Sandstone. Three fields are equal to or greater than 6 BCFG in size: Baltic, (Holmes Co., Ohio), discovery date 1965, ultimate recovery ~50 BCFG; White Eyes, (Coshocton Co., Ohio), discovery date 1975, ultimate recovery ~8 BCFG; and Bakersville West, (Coshocton Co., Ohio), discovery date 1980, ultimate recovery ~6 BCFG.

The number of gas fields in the Rose Run Sandstone diminishes markedly east of the subcrop belt. Gas was briefly produced from subcommercial one-well fields in Noble County, Ohio, (drilling depth ~9,000 ft), Harrison County, Ohio, (drilling depth ~8,500 ft), and McKean County, Pennsylvania, (Minard Run pool, drilling depth ~10,000 ft). Basement-controlled fault blocks and anticlines have trapped these minor accumulations.


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