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



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Resource potential: This play has potential for a modest number of undiscovered gas fields greater than 6 BCFG. Many prospective stratigraphic and anticlinal traps are very subtle and could easily have been overlooked in previous seismic surveys and exploration phases. The recent surge of discoveries in the play and the extension of the play southward into Licking and Pickaway Counties, Ohio, suggest that more fields remain to be found. There are large areas that have been sparsely drilled to the Rose Run Sandstone and equivalent Upper Cambrian reservoirs. Limiting factors for the play east of the subcrop trend may be (1) reservoirs are widely scattered and of low quality and (2) Middle Ordovician source beds are too far removed to charge the reservoirs.

6706. Trenton/Black River carbonate oil/gas play

The Trenton/Black River Carbonate Oil/Gas Play is defined by oil and gas trapped in Middle Ordovician platform limestone reservoirs by facies-change stratigraphic traps, low-amplitude basement-controlled anticlines, dolomitized fracture zones, and natural fractures. The play involves Middle Ordovician limestone 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 Middle Ordovician Trenton Group (Limestone), Black River Group (Limestone), Lexington Limestone (in part Upper Ordovician), High Bridge Group, Stones River Group, Nashville Group (in part Upper Ordovician), and Chickamauga Group (Limestone). Also included in the eastern Kentucky part of the play is quartzose sandstone of the St. Peter Sandstone that rests directly on the Knox unconformity.

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

Reservoirs: Fractured micrite of the Trenton Limestone is an important gas reservoir in north-central New York. Fractures are most common in the transition zone between the Trenton Limestone and overlying Utica Shale where limestone beds are intercalated with thin black shale beds. Recurrent movement of fault-bounded basement blocks and differential stretching of anticlinal flanks account for most of the fracturing. Fractured limestone reservoirs in New York State cover an area as large as 10,000 acres.

Narrow fault-controlled zones of dolomitized limestone, akin to those in the Albian-Scipio field of the Michigan Basin, are recognized in the Trenton and Black River Limestones of central New York and east-central Ohio. Coarsely crystalline sparry dolomite in the dolomitized fracture zones provide high-quality reservoirs with vuggy and intercrystalline porosity.

Another reservoir in the play is bioclastic limestone that has vuggy (biomoldic) porosity. Vugs were formed mostly by the leaching of fossil fragments during brief periods of subaerial exposure. These reservoirs are most commonly present in the Lexington Limestone and High Bridge Group of Kentucky and the Nashville and Stones River Groups in Tennessee.

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 black shale and limestone sequences are between 200 and 400 ft thick and they rest directly on or within several hundred feet of reservoir units. 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 in the Middle Ordovician Jacksonburg Limestone, Liberty Hall Formation, Paperville Shale, Sevier Formation, and Athens Shale--located 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 and contiguous Kentucky, Tennessee, and Alabama are in the oil generation zone, they contain no known source beds. 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 beds 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. Some of this oil and gas was trapped in Trenton and Black River Limestones as it migrated to the Knox unconformity. A modest number of stratigraphic and anticlinal traps were available to trap 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 Middle Ordovician limestone reservoirs and become trapped in basement-involved fault blocks and anticlines.



Traps: A variety of oil and gas traps are present in the play. They consist of stratigraphic traps of the facies-change variety, diagenetic traps characterized by fault-controlled coarsely crystalline dolomite pinching out into nonporous micrite, low-amplitude, basement-controlled anticlines, and highly fractured limestone.

Important seals in the play are black shale of the Middle Ordovician Utica and Antes Shales and thick gray shale of the Upper Ordovician Reedsville Shale, Lorraine Shale, and Clays Ferry Formation. In the southern part of the play the seals consist of shale and argillaceous micrite of the Upper Ordovician part of the Nashville Group and the Upper Ordovician Sequatchie Formation.



Exploration status: Between 1879 and 1933, 11 gas fields were discovered in fractured Trenton Limestone reservoirs in north-central New York. Drilling depths to the reservoir vary from less than 1,000 to about 2,500 ft. Most of the fields are now abandoned. A twelfth field, the Blue Tail Rooster, discovered in 1966, is still producing. The largest fields in the trend are Pulaski, ultimate recovery ~23 BCFG; Baldwinsville, ultimate recovery ~17 BCFG; and Rome, ultimate recovery ~9 BCFG.

Several gas and associated oil accumulations have been discovered in dolomite and dolomitic limestone associated with fracture and(or) fault zones. The Harlem field, discovered in Delaware County, Ohio, in 1964, is the largest of the accumulations, having produced an unknown quantity of gas and oil from 5 or 6 wells. Additional fields of this type are subcommercial 1-well accumulations discovered in the mid-1980's in the Finger Lakes region of north-central New York and in the early 1990's along the Starr fault zone in southeastern Ohio. The depth to the reservoir in these areas is between 6,000 and 9,000 ft.

About 15 small gas fields, some with minor associated oil, have been discovered in the High Bridge Group, Lexington Limestone and St. Peter Sandstone in Kentucky and in the Stones River and Nashville Groups of Tennessee. Most of these accumulations are trapped in basement-controlled fault blocks and anticlines but a few of them may be trapped by facies-change stratigraphic traps.

Resource potential: This play has potential for a modest number of undiscovered gas and oil fields greater than 1 MMBO or 6 BCFG. Middle Ordovician source beds have adequate organic richness and are in an optimum location to charge many of the reservoirs. Prospective structural and diagenetic traps and fractured reservoirs are very subtle and could easily have been overlooked in earlier periods of exploration. There are large areas in the play that have been sparsely drilled to the Middle Ordovician carbonate sequence. A limiting factor in the play may be that reservoirs are widely scattered and of low quality.

6708. Queenston/Bald Eagle Sandstone gas play

The Queenston/Bald Eagle Sandstone Gas Play is defined by gas trapped in the Upper Ordovician Bald Eagle (Oswego) Sandstone and sandy facies of the Upper Ordovician Queenston Formation. Traps consist of low-amplitude anticlines controlled by basement faults and facies-change stratigraphic traps. Fractures play an important role in both types of accumulation. The play area covers most of central Pennsylvania and central New York between the depositional limits of the Bald Eagle (Oswego) Sandstone and sandy facies of the Queenston Formation on the west and the Allegheny structural front in Pennsylvania on the east. The northeast limit of the play is in east-central New York where Upper Ordovician sandstone units are truncated. A narrow piece of the play extends into West Virginia approximately between the western depositional limit of the Oswego Sandstone and the western limit of detached anticlines in Upper Ordovician and older strata. The play is confirmed, and its prospective reservoirs are conventional.



Reservoirs: The reservoirs in the play are sandstone of nearshore shallow-marine and fluvial origin deposited near the distal end of the Upper Ordovician Queenston delta. In central New York the sandstone reservoirs that occur in the upper 300 ft of the 700-to 800-foot-thick Queenston Formation are fine-grained. The sandstone reservoir in central Pennsylvania is fine- to medium-grained and produces gas from a 130-foot-thick zone near the middle part of the approximately 1,000-foot-thick Bald Eagle Sandstone.

The sandstones are mainly litharenites that contain abundant metamorphic fragments and minor feldspar. Primary intergranular porosity supplemented by fracture porosity is the dominant porosity type in the central New York part of the play. Pore-filling clay minerals have reduced the permeability of the reservoirs. Porosity values range from 2 – 13 percent (avg ~4 percent), and permeability values range from <0.1 to 5 mD (avg ~0.2 mD). Drilling depths to the sandstone reservoirs of the Queenston Formation in central New York range from about 1,500 – 5,500 ft. Fracture porosity dominates the porosity of the Bald Eagle Sandstone reservoir in central Pennsylvania where drilling depths to the reservoir range from 10,000 to 13,000 ft.

Fractures in the Queenston Formation of New York formed mostly by recurrent movement of fault-bounded basement blocks. A recent study indicates that many of the fractures in the Bald Eagle Sandstone of central Pennsylvania formed by hydraulically induced stress in regions of high abnormal formation pressure.

Source rocks: Black shale and argillaceous limestone in the Middle Ordovician Utica Shale, Antes Shale, and Trenton Limestone are the most likely sources of gas 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 upward vertical migration is required for gas to reach Upper Ordovician sandstone reservoirs. TOC values in the Middle Ordovician black shale and limestone 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 play are in the gas generation zone. Middle Ordovician source beds in a narrow zone that adjoins the west side of the Allegheny structural front in Pennsylvania are overmature with respect to oil and gas generation. Dry thermal gas is the expected hydrocarbon in the play.



Timing and migration: Peak gas generation from Middle Ordovician shale and argillaceous limestone source beds probably occurred between Late Pennsylvanian and Early Triassic time when these beds were deeply buried under an eastward-thickening wedge of orogenic sediments and thrust sheets. Much of the 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; however, an equal amount or more of the gas migrated vertically upward, through available fracture systems, to the Upper Ordovician sandstone reservoirs. A modest number of stratigraphic and anticlinal traps were available to trap the vertically migrated gas.

Traps: Facies-change stratigraphic traps and low-amplitude basement-controlled anticlines are the major traps in the play. Truncated strata beneath the widespread Taconic unconformity may have trapped some gas in east-central New York. A northward decrease in sandstone grain size and an increase in shale beds in the upper part of the Queenston Formation are responsible for the permeability barrier in many of the stratigraphically trapped gas fields in New York. The seals for these fields are clay-rich siltstone and silty shale of the Lower Silurian Medina Group. Several of the larger fields are crossed by northeast-trending fracture systems that have improved reservoir quality.

The northeast-trending anticline that has trapped gas in the Grugan field in central Pennsylvania is bounded by a down-to-the-northwest basement fault. Major fractures parallel the fold axis. Low-permeability sandstone and mineralized fractures in the Bald Eagle Sandstone provide the seal.



Exploration status: Gas fields in the Queenston Formation of central New York were discovered between 1895 and the mid-1980's. The largest of about 14 fields in the trend are Auburn West, (Cayuga Co., N.Y.), discovery date 1960, ultimate recovery ~60 BCFG; and Fayette-Waterloo, (Seneca Co., N.Y.), discovery date 1962, ultimate recovery ~18 BCFG. The Grugan gas field, (Clinton Co., Penn,), was discovered in 1982. Approximately 7 BCFG has been produced in three wells through 1991. The ultimate recovery of the field is about 10 BCFG.

Exploration still continues for gas fields in the Queenston Formation and Bald Eagle Sandstone. In the mid-1980's several dry holes were drilled to the Bald Eagle Sandstone in central Pennsylvania in search of Gugan-like accumulations. The sandstone reservoirs in the play have tight formation status.



Resource potential: This play has potential for a modest number of undiscovered gas fields greater than 6 BCFG. There are large areas in the play that have been sparsely drilled to Upper Ordovician sandstone reservoirs. A limiting factor in the play may be the lack of pervasive fracture systems that greatly improve reservoir quality and facilitate the 1,500 – 2,000 ft of vertical gas migration from Middle Ordovician source rocks.

6714. Keefer/Big Six sandstone gas play

The Keefer/Big Six Sandstone Gas Play is defined by gas trapped in the Middle Silurian Keefer Sandstone, Big Six sandstone, Oneida Sandstone, and Herkimer Sandstone by facies-change stratigraphic traps, combination traps, and low-amplitude anticlines. The main part of the play covers eastern Kentucky where the Big Six sandstone was named by early drillers, central and western West Virginia where the Keefer Sandstone is present, and southwestern Pennsylvania where the Keefer Sandstone is present. A small part of the play covers northeastern Pennsylvania, near the northern limit of the Keefer Sandstone, and adjoining southeastern New York, where Keefer Sandstone equivalents, the Middle Silurian Oneida and Herkimer Sandstones, are present. The play is confirmed because several gas fields in eastern Kentucky probably exceed 6 BCF of gas in size. The reservoirs in the play are conventional.



Reservoirs: Quartzose sandstone of littoral and offshore marine origin constitutes the reservoir in the play. The grain size of the sandstone is quite variable; along the eastern boundary of the play the sandstone is medium to coarse grained and contains quartz pebbles, whereas along the west side of the play, near the depositional limit of the Keefer, the sandstone is very fine to fine grained. Most commonly, the quartz grains are cemented with dolomite, but locally silica and calcite cement are present. Although porosity of the sandstone reservoirs may be as high as 20 percent, in most localities it averages about 4 to 5 percent. Primary intergranular porosity is the dominant porosity type, but secondary intergranular porosity and fracture porosity may be present locally.

The Keefer Sandstone and Big Six sandstone generally range in thickness from about 25 to 50 ft. Drilling depths to the sandstone reservoir range from about 1,800 to 3,000 ft in Kentucky and southeastern New York and from about 3,700 to 7,000 ft in West Virginia.



Source rocks: The source of gas in the Kentucky and adjoining West Virginia part of the play is the Upper Devonian black shale sequence that rests unconformably on Silurian and Lower and Middle Devonian strata within 100–600 ft of the underlying Big Six sandstone. The Upper Devonian black shale sequence in eastern Kentucky is 50–400 ft thick and has TOC contents between 3 and 7. Vitrinite reflectance data suggest that the Upper Devonian black shale sequence in Kentucky and adjoining West Virginia is in the oil zone of generation and the beginning of the gas zone of generation. Wet thermal gas and local oil are the expected hydrocarbon types.

The source of gas in the remainder of the play is less certain. The most plausible candidates are (1) shale and argillaceous limestone of the Middle Ordovician Utica Shale, Antes Shale, and Trenton Limestone and (2) 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; 50–300 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. Gas generated from these source bed sequences however is not particularly accessible to the sandstone reservoirs. For example, between 1,500 and 2,000 ft of vertical migration through predominantly shale and siltstone is required for gas derived from the Middle Ordovician shale sequence to reach the reservoirs. 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 sandstone reservoirs. A slight preference is given to 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 source bed sequences in the West Virginia, Pennsylvania, and New York part of the play are in the zone of gas generation. A narrow part of the play area in eastern West Virginia, southwestern Pennsylvania, and northeastern Pennsylvania and adjoining southeastern New York that contains Middle Ordovician and Devonian source beds is overmature with respect to oil and gas generation. Wet and dry thermal gas are the expected hydrocarbon types whether the source is the Middle Ordovician or Devonian shale sequence.

Timing and migration: Peak gas generation from the Middle Ordovician and Devonian shale sequences probably occurred between Late Pennsylvanian and Early Triassic time when these beds were deeply buried under an eastward-thickening wedge of orogenic sediments. Gas migrated vertically upsection or downsection to the reservoir depending on which of the two proposed source rock sequences it was generated from. A modest number of facies-change stratigraphic traps, combination traps, and low-amplitude anticlines were available to trap the vertically migrated gas.

Traps: Facies-change stratigraphic traps, combination traps, and low-amplitude anticlines are the major traps in the play. The combination traps formed by facies changes and locally diagenetic changes that occurred on the flanks and noses of gently plunging, low-amplitude anticlines. The seal for the traps consist of dolomicrite of the overlying Middle Silurian Lockport Dolomite.

Exploration status: Gas production from the Big Six sandstone was established in the late 1890's and early 1900's during exploration for oil-bearing zones in the Silurian and Devonian "Corniferous" reservoirs. Most of the 30 or 40 Big Six sandstone gas fields produce from multiple reservoirs that include the Middle Silurian Lockport Dolomite, Middle Silurian Salina Formation, Middle Devonian Boyle Dolomite, and many Mississippian formations. Most of the Big Six sandstone has been explored, but the Keefer Sandstone in many parts of West Virginia remains untested. Moreover, southeastern New York, where several fields were discovered in the late 1950's and early 1960's, is still sparsely drilled. Sandstone reservoirs in parts of the play have tight formation status.

Resource potential: This play has potential for a small number of undiscovered gas fields greater than 6 BCFG. The most attractive aspect of the play is its updip continuity with the prolific oil and gas fields of the "Corniferous" limestone. In addition, large areas in the play have been sparsely drilled to the Keefer Sandstone, and parts of prospective structures remain untested. Limiting factors in the play may be low-quality reservoirs and their poor accessibility to known source rock sequences.

6715. "Corniferous" limestone/Big Six sandstone oil/gas play

The "Corniferous" Limestone/Big Six Sandstone Oil/Gas Play is defined by oil and gas trapped in Middle and Upper Silurian carbonate units, commonly referred to by the driller's term "Corniferous" limestone, and in the underlying Big Six sandstone. Traps consist of truncation traps, facies-change traps, combination traps, and low-amplitude anticlines. Carbonate units included in the "Corniferous" limestone are the Middle Silurian Lockport Dolomite, Middle Silurian Bisher Dolomite, Upper Silurian Salina Formation, and Middle Devonian Boyle Dolomite. The play is confined to a small area in north-central Kentucky where oil and associated gas are produced from the "Corniferous" limestone and Big Six sandstone. The contiguous Keefer/Big Six Sandstone Gas Play (6714) and the Silurian Carbonate Gas Play (6717) are mostly gas plays. The play is confirmed, and the reservoirs are classified as conventional.


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