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



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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 and thrust sheets. 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 anticlinal and combination traps were available to trap the vertically migrated gas.

Traps: Basement-controlled anticlines and combination traps are the major traps in the play. The combination traps are formed by diagenetically produced permeability barriers that cross the flanks and noses of gently plunging, low-amplitude anticlines. The seal for the traps consists of red shale and silty shale of the Middle Silurian Rose Hill Formation.

Exploration status: Gas production from the Tuscarora Sandstone was established in the mid-1970's by deeper drilling in the Campbell Creek-Malden and Hernshaw-Bull Creek oil fields in central West Virginia. The Indian Creek field, largest of three fields in Kanawha County, West Virginia, has an ultimate recovery of about 32 BCFG. Several subcommercial 1-well fields have been discovered in northern West Virginia and southern Pennsylvania along the trend of the Chestnut Ridge anticline. One of these accumulations, the Heyn pool, Fayette County, Pennsylvania, produces gas from fractured Tuscarora Sandstone probably caused by minor bedding-plane detachment in underlying Upper Ordovician shale. Additional 1-well, Tuscarora Sandstone fields are scattered around Pennsylvania and West Virginia and many of them are associated with basement-controlled anticlines. Exploration for gas fields in the Tuscarora Sandstone continues in Pennsylvania and West Virginia, at the rate of several drill holes per year.

Resource potential: This play has potential for a modest number of undiscovered gas fields greater than 6 BCFG. The most attractive aspect of the play is its lateral updip continuity with the prolific oil and gas fields of the Clinton and Medina sandstones. In addition, there are large areas in the play that have been sparsely drilled to the Tuscarora Sandstone, and some parts of prospective structures remain untested. Limiting factors in the play may be (1) low-quality reservoirs, (2) poor accessibility to known source rock sequences, and (3) a high percentage of noncombustible gas mixed with the methane gas.

Clinton/Medina Sandstone Gas Plays
6728. CLINTON/MEDINA Sandstone gas high potential

6729. CLINTON/MEDINA Sandstone gas medium potential (HYPOTHETICAL)

6730. CLINTON/MEDINA Sandstone gas medium-low potential (HYPOTHETICAL)

6731. CLINTON/MEDINA Sandstone gas plays low potential (HYPOTHETICAL)

The Tuscarora Sandstone Gas Play (6727), Clinton/Medina Sandstone Gas Plays (6728, 6729, 6730, 6731), and 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" sandstone 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 Clinton/Medina Sandstone Gas plays are defined as parts of a continuous-type gas accumulation. These plays are characterized as continuous-type accumulations because of low-permeability reservoirs, abnormally low formation pressure, coalesced gas fields, gas shows or production in most holes drilled, and general lack of control by anticlinal closure on gas accumulations. Four Clinton/Medina Sandstone Gas plays are recognized according to their estimated potential for undiscovered gas resources. The Clinton/Medina Sandstone Gas High Potential Play (6728), the largest of the plays, is down-dip and along strike of the Clinton/Medina Sandstone Oil and Gas play (6732). It extends across western New York, northwestern Pennsylvania, and eastern Ohio and includes the offshore Lake Erie and Lake Ontario part of these states. The relatively small Clinton/Medina Sandstone Gas Medium Potential Play (6729) consists of two parts: an east-central New York part attached to the north end of Clinton/Medina Sandstone Gas High Potential Play 6728 and a southern Ohio, northeastern Kentucky, and westernmost West Virginia part attached to the south end of play 6728. The Clinton/Medina Sandstone Gas Medium-Low Potential Play (6730) consists of three parts: (1) a long narrow piece of northern and western West Virginia and adjoining southwestern Pennsylvania attached to the downdip side of plays 6728 and 6729, (2) a narrow piece of north-central New York between the outcrop and plays 6728 and 6729, and (3) a small piece of south-central New York attached to the down dip side of play 6729. The two parts of the Clinton/Medina Sandstone Gas Low Potential Play (6731) mark the extreme northern and southern ends of the proposed continuous-type accumulation. The northern part of the play is in east-central New York, whereas the southern part is in southeastern Kentucky and adjoining Virginia.

Stratigraphically, the plays involve the "Clinton" sandstone and its informal driller's subunits--the red, white, and stray Clinton sands--in Ohio and the Whirlpool Sandstone, stray sandstones in the Cabot Head Shale, and Grimsby Sandstone of the Medina Group in New York and Pennsylvania. The "Clinton" sandstone (or sands) in Ohio correlates with the Lower Silurian Medina Group of New York and not with the Middle Silurian Clinton Group of New York as originally believed; however, the term is so popular in the petroleum industry that its usage continues.

Flanked by the Tuscarora Sandstone Play (6727) on the east and the Clinton/Medina Sandstone Oil/Gas Play (6732) on the west, the plays cover large parts of New York, Pennsylvania, Ohio, and West Virginia; the southern end of the plays includes small parts of Kentucky and Virginia. On the east side the plays are bounded by an arbitrary line separating the Tuscarora Sandstone from Clinton/Medina sandstones, whereas, on the west side, they are bounded by the approximate limit of oil production in the Clinton/Medina sandstones. The United States–Canada border in the middle of Lake Erie marks the northern limit of the play.

The plays are confirmed, and their prospective reservoirs are classified as unconventional because of the probable continuous nature of the gas accumulations.



Reservoirs: Very fine- to fine-grained sandstone consisting of quartzarenite and local sublitharenite and subarkose constitutes the reservoir in the plays. Compaction and burial diagenesis have greatly reduced the primary intergranular porosity of the reservoir. Silica cement and authigenic clay minerals are the primary pore-filling materials. Locally, calcite, dolomite, anhydrite, and hematite cement may be abundant. Secondary intergranular porosity, caused by the dissolution of rock fragments, feldspar grains, and cement, and fracture porosity, caused by movement between basement-involved fault blocks, are the important porosity types in the play. Porosity for the reservoirs ranges from 3 to 11 percent and average 5 percent. Permeability is as high as 0.2 to 0.6 mD but it generally averages less than 0.01 mD. The thickness of the Clinton sandstone sequence and the Medina Group in the plays ranges from 120 to 210 ft, and sandstone to shale ratios vary from 0.6 to 1. The net reservoir thickness ranges from 2 to 90 ft and averages about 25 ft. Drilling depths to the sandstone reservoirs in eastern Ohio and northwestern Pennsylvania range from 4,000 to 6,300 ft and in southwestern Pennsylvania they may be as much as 10,000 ft. In New York the drilling depths to the Medina Group are between 1,000 and 4,000 ft, whereas in southern Ohio and adjoining eastern Kentucky the drilling depths to the "Clinton" sandstone are between 2,000 and 3,000 ft.

Source rocks: The source of the gas in the plays is uncertain. 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. However, gas generated from these source bed sequences is not particularly accessible to the reservoir sandstones. 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 Clinton/Medina sandstone 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, Middle Ordovician source rocks in the plays are in the gas generation zone. Although Middle Ordovician rocks in southernmost Ohio and eastern Kentucky are in the zone of gas generation, they contain no known source beds. Dry and wet thermal gas are the expected hydrocarbon types derived from Middle Ordovician source beds. Vitrinite reflectance data suggest that Middle and Upper Devonian source rocks in the plays have achieved several levels of thermal maturity. Devonian source rocks in western Pennsylvania, eastern Ohio, and northern West Virginia are mainly in the zone of gas generation, whereas those source rocks in western New York and eastern Ohio are in the zone of oil generation. Devonian source rocks in offshore northern Ohio (Lake Erie) and southern Ohio probably 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 gas generation from the Middle Ordovician and Devonian shale sequences 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. Gas migrated vertically upsection or downsection to the sandstone reservoirs depending on which of the two proposed source rock sequences it was generated from. Numerous facies-change stratigraphic traps, some in combination with subtle anticlinal closure and noses, were available to trap the vertically migrated gas.

Traps: Facies-change stratigraphic traps are the most important traps in the play; however, commonly the permeability barriers produced by facies changes, and to a lesser extent by diagenetic changes, are in combination with subtle anticlinal flanks and noses. In general, anticlinal closure has exerted only subtle influence, if any, on gas accumulations in the plays. Water block has been suggested by several petroleum geologists as a possible trapping mechanism for gas in the Clinton/Medina sandstones. This type of trap is created by low-permeability sandstone sequences whose permeability to water is much greater than their permeability to gas. If the model is applicable to these plays, it would imply the presence of a basin-center gas accumulation.

Seals for the stratigraphic traps are shale beds within the Clinton/Medina interval and Middle Silurian shale and micrite beds of the Packer Shell limestone, Dayton Limestone, Rochester Shale, and Clinton Group.



Exploration status: Gas was first discovered in the play in the mid to late 1880's in western New York and in the late 1890's in southern Ohio. Drilling depths to the Clinton and Medina sandstones in these regions range from less than 1,000 to about 2,000 ft; however, because of the low permeability of the Lower Silurian sandstones, most of the exploration was concentrated in east-central Ohio where reservoir quality was higher and oil was produced as well as gas. By the late 1950's and early 1960's, exploration activity had successfully expanded into eastern Ohio, northwestern Pennsylvania, and western New York where lower quality sandstone reservoirs occur, drilling depths commonly exceed 4,000 ft, and gas is the dominant hydrocarbon type. Successful exploration for gas continues today in sparsely drilled parts of the play such as southern and east-central New York, northwestern Pennsylvania, and eastern Ohio. In Pennsylvania and Ohio, drilling depths to the Clinton and Medina sandstones exceed 6,000 ft. An offshore gas well drilled in 1958 in Pennsylvania and numerous offshore wells drilled since 1913 in Canada indicate that gas in the play extends into Lake Erie at least as far north as the United States–Canada border. Most sandstone reservoirs in the play have tight formation status.

The most important factor that insured exploration success in low-permeability sandstone reservoirs was the development of hydraulic fracturing techniques. Wells that originally had a very low natural flow of gas were transformed by hydraulic stimulation into wells producing as much as 1–2 MMCFGPD. Field size has little meaning in the plays because older fields tend to merge together into continuous-type accumulations with additional drilling. For example, the three or four Medina gas fields that were discovered in Chautauqua County, western New York, in the 1960's have now merged into the giant Lakeshore field, which has an ultimate recovery of about 650 BCFG.



Resource potential: This group of plays has the potential for yielding a large volume of undiscovered gas in a continuous-type accumulation. Undiscovered gas in the plays is assessed using a continuous-type unconventional resource model. This model incorporates: (1) estimated ultimate recovery (EUR) per well probability distributions, (2) optimum area that a well can drain (spacing), (3) number of untested drill sites having the appropriate spacing area, (4) success ratio of previously drilled holes, and (5) risk.

EUR values for producing wells in the Clinton/Medina Sandstone Gas High Potential Play (6728) range from 5 (F95) to 330 (F5) MMCFG and have a median (F50) of about 70 MMCFG. Smaller EUR values are recorded for producing wells in the Clinton/Medina Sandstone Gas Medium Potential Play (6729); they range from 5 to 150 MMCFG and have a median (F50) of about 50 MMCFG. Low EUR values, high risk, and a low success ratio are assigned to the Clinton/Medina Sandstone Gas Medium-Low Potential (6730) and Clinton/Medina Sandstone Gas Low Potential Plays (6731). A well spacing of 40 acres is selected for all plays.



6732. CLINTON/MEDINA sandstone oil/gas play

The Tuscarora Sandstone Gas Play (6727), Clinton/Medina Sandstone Gas Plays (6728, 6729, 6730, 6731), and Clinton/Medina Sandstone Oil/Gas Play (6732) are contiguous plays that occupy progressively western 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 Clinton/Medina Sandstone Oil and Gas Play (6732) is defined by oil and gas trapped in the Lower Silurian "Clinton" sandstones and Medina Group sandstones by facies-change stratigraphic traps and combination traps. The play is confirmed, and it has many aspects of continuous-type accumulations such as abnormally low formation pressure, coalesced oil and gas fields, and a general lack of control by anticlinal closure on accumulations; however, because of the high drilling density of the play, it is most conveniently described as conventional and its resources--except for those in Lake Erie--assessed as reserve growth.

Stratigraphically, the play involves the "Clinton" sandstone and its informal driller's subunits, the red, white, and stray Clinton sands, in Ohio and the Whirlpool Sandstone, stray sandstones in the Cabot Head Shale, and Grimsby Sandstone of the Medina Group in New York and Pennsylvania. The "Clinton" sandstone (or sands) in Ohio correlates with the Lower Silurian Medina Group of New York and not with the Middle Silurian Clinton Group of New York as originally believed, however, the term is so popular in the petroleum industry that its usage continues. The most westerly of the three Lower Silurian sandstone plays, this play covers most of east-central Ohio and a very small part of northwestern Pennsylvania. On the east side the play is bounded by an arbitrary line that marks the approximate eastern limit of oil production in the Clinton/Medina sandstones, whereas on the west side it is bounded by the approximate depositional limit of the Clinton/Medina sandstones. The northern part of the play extends into Lake Erie.



Reservoirs: Very fine to fine grained sandstone consisting of quartzarenite and local sublitharenite and subarkose constitute the reservoir in the play. Compaction and burial diagenesis have reduced the primary intergranular porosity of the sandstone reservoirs but not to as extreme a degree as in the sandstone reservoirs of the adjoining Clinton/Medina Sandstone Gas Plays. Porosity for reservoirs in the play ranges from 8 to 15 percent and averages 12 percent. Most of this porosity is caused by primary intergranular porosity, but fracture porosity, caused by movement between basement-involved fault blocks, may be important locally. Permeability ranges from <0.1 to 75 mD and averages 5 mD. The thickness of the Clinton sandstone sequence and the Medina Group in the play ranges from 100 to 200 ft and sandstone to shale ratios range from 0.2 to 1.0. The net reservoir thickness ranges from 6 to 105 ft and averages about 40 ft. Drilling depths to the sandstone reservoirs in the play range from 1,000 to 5,000 ft.

Source rocks: The source of the oil and gas in the play is uncertain. 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. However, oil and gas generated from these source bed sequences are not particularly accessible to the reservoir sandstones. For example, between 1,500 and 2,000 ft of vertical migration, through predominantly shale and siltstone, is required for oil and gas derived from the Middle Ordovician shale sequence to reach the Clinton/Medina sandstone 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-derived oil and 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 are located in the zone of oil generation; however, Devonian source rocks in about half the play are immature with respect to oil and gas generation. Oil and wet thermal gas are the expected hydrocarbon types whether the source is the Middle Ordovician or Devonian shale sequence.



Timing and migration: Peak oil and gas generation from the Middle Ordovician and Devonian shale sequences occurred between Late Pennsylvanian and Early Triassic time when these beds were buried under an eastward-thickening wedge of orogenic sediments and thrust sheets. Most of the oil and gas was generated east of the play, probably in the vicinity of the Clinton/Medina Sandstone Gas plays and the Tuscarora Sandstone Gas Play (6727). Oil and gas migrated vertically upsection or downsection to Lower Silurian sandstones, depending on which of the two proposed source rock sequences they were generated from, and then migrated up the gently dipping northwest flank of the basin to east-central Ohio. Numerous facies-change stratigraphic traps, some in combination with subtle anticlinal flanks and noses, were available to trap the laterally migrated oil and gas.

Traps: Facies-change stratigraphic traps are the most important traps in the play; however, commonly, permeability barriers produced by facies changes, and to a lesser extent by diagenetic changes, are in combination with subtle anticlinal flanks and noses. Seals for the stratigraphic traps are shale beds within the Clinton/Medina interval and Middle Silurian shale and micrite beds of the Packer Shell limestone, Dayton Limestone, Rochester Shale, and Clinton Group.

Exploration status: Oil and gas was first discovered in the play in the mid- to late-1880's in Knox, Licking, Fairfield, and Perry Counties in east-central Ohio. Clinton sandstones are relatively shallow in this region and range from depths of 2,000 to 2,800 ft. In a short time, the trend had spread northward across all of east-central Ohio to the southern shore of Lake Erie. For about 70 years this region has yielded large quantities of oil and gas from reservoirs of good to moderate quality. Most of the oil and gas fields found at this time began to coalesce with later infill drilling. By the late 1950's and early 1960's, exploration for oil and gas in the Lower Silurian sandstones had successfully expanded into eastern Ohio where drilling depths were commonly greater than 4,000 ft and reservoir quality had diminished.

The most important factor that insured exploration success along the eastern part of the play was the development of hydraulic fracturing techniques. Wells that originally had very low natural flows of oil and gas were transformed by hydraulic stimulation into wells producing as much as 150 BOPD and 1 to 2 MMCFGPD. The largest oil field in the play is East Canton (Stark Co., Ohio, discovery date 1966, ultimate recovery ~100 MMBO). The exploration phase of the play is now over and sandstone reservoirs in its eastern parts have tight formation status.


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