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



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Resource potential: This play has potential for a small number of undiscovered oil and gas fields greater than 1 MMBO or 6 BCFG. The undiscovered fields are in undrilled Lake Erie. Outside of Lake Erie, this play is exhausted except for very small oil and gas accumulations.

6737. Upper Devonian sandstone oil/gas play

The Upper Devonian Sandstone Oil/Gas Play (6737) and the unconventional Upper Devonian Sandstone Gas plays (6733, 6734, 6735, 6736) are contiguous plays in the sandstone depositional system of the Upper Devonian Catskill Delta. Most of the sandstone was deposited in littoral-marine, deltaic, offshore shallow-marine, and offshore deeper water marine (turbidite) settings. The Upper Devonian Sandstone Oil/Gas Play is defined by oil and gas trapped in Upper Devonian sandstone and local siltstone by facies-change stratigraphic traps in combination with diagenetic traps or fractured reservoir 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 structural control on accumulations. Because of the high drilling density, the play is most conveniently described, however, as conventional, and its resources are assessed as reserve growth. The play covers western New York, northwestern Pennsylvania, southeastern Ohio, and north-central West Virginia.

Stratigraphically, the play involves numerous formal and informal sandstone units deposited during multiple regressive events. In Pennsylvania, Upper Devonian sandstones are organized into four major sequences, in ascending order, the Braillier Formation, Elk Group, Bradford Group, and Venango Group. In West Virginia, Upper Devonian sandstones are organized, in ascending order, into the Braillier Formation, Greenland Gap Formation, Venango Formation, and Oswayo Formation. Farther to the westward in West Virginia, the Greenland Gap Formation changes facies and is replaced by shale and siltstone of the Braillier Formation. Pennsylvania and West Virginia nomenclature correlate approximately as follows: (1) the lower part of the Elk Group and underlying Braillier Formation in Pennsylvania correlate with the lower part of the Braillier Formation in West Virginia; (2) the upper part of the Elk Group, the Bradford Group, and the lower part of the Venango Group in Pennsylvania correlate with the Greenland Gap Formation in West Virginia; and (3) the middle and upper parts of the Venango Group in Pennsylvania correlate with the Venango Formation and Oswayo Formation in West Virginia.

The eastern boundary of the play is marked by the approximate limit of oil production in the Upper Devonian sandstone sequence, whereas the western boundary of the play is the approximate depositional limit of the Upper Devonian sandstone and siltstone sequence.



Reservoirs: Coarse siltstone, very fine to fine grained sandstone, medium-grained sandstone, and local pebbly sandstone, classified as sublitharenite, subarkose, quartzarenite, and quartzwacke, constitute the reservoirs in the play. Primary intergranular porosity, although reduced somewhat by silica and calcite cement and authigenic clay minerals, is the dominant porosity type in the play. Porosity types of lesser importance are (1) secondary intergranular porosity caused by dissolution of detrital grains (feldspar, metamorphic rock fragments) and silica and (or) calcite cement, (2) fracture porosity caused by movement between basement-involved fault blocks, and (3) moldic porosity caused by fossil fragment dissolution. Porosity values for reservoirs in the play range from 2 to 26 percent and average about 14 percent. Permeability values range from 0.07 mD to several thousands of millidarcies and average about 10 mD.

A variety of sandstone bodies differing in depositional setting, orientation to paleoshoreline, and degree of lateral continuity are recognized in the play. Among the most common types are (1) sheetlike sandstone bodies deposited along deltaic and interdeltaic shorelines, (2) long, narrow pebbly sandstone bodies deposited as offshore bars subparallel to the shoreline, and (3) discontinuous sandstone bodies deposited normal to the shoreline representing reworked delta distributary channels, distributary mouth bars, and turbidites. The thickness of the producing part of the sandstone reservoirs ranges from about 10 to 40 ft and averages about 10 ft. Commonly, more than one sandstone reservoir is productive in a given well. Drilling depths to the sandstone reservoirs are from less than 1,000 to 5,000 ft.



Source rock: The sources of oil and gas in the play are the Middle Devonian Marcellus Shale that underlies the Upper Devonian sandstone sequence and the Upper Devonian black shales that are located west of and intertongue with the Upper Devonian sandstone sequence. The Middle and Upper Devonian black shale sequence in the play is between 100 and 500 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 western New York and northwestern Pennsylvania are in the zone of oil generation, whereas Devonian source rocks in west-central Pennsylvania, southeastern Ohio, and northern and north-central West Virginia are in the zone of gas generation. The southeastern part of the play in southwestern Pennsylvania and adjoining West Virginia may be overmature with respect to the generation of oil and gas. Oil and wet thermal gas are the expected hydrocarbon types in the play.

Timing: Peak oil and gas generation from the Middle and Upper Devonian black shale sequence occurred between Late Pennsylvanian and Early Triassic time when the sequence was buried under an eastward-thickening wedge of orogenic sediments. Oil and gas migrated short distances laterally and upsection to the sandstone reservoirs. A variety of facies-change stratigraphic traps, diagenetic traps, and fractured-reservoir traps were available to trap the oil and gas.

Traps: Facies-change stratigraphic traps, commonly in combination with anticlinal flanks, anticlinal noses, diagenetic traps and fractured-reservoir traps, are the most important traps in the play. In general, anticlinal closure has exerted only subtle influence, if any, on oil and gas accumulations in the play. Seals for the stratigraphic traps are shale and fine-grained siltstone intercalated with the Upper Devonian sandstone sequence.

Exploration status: Oil and associated gas in the play was discovered in the Drake well, Venango County, Pennsylvania, in 1859. This oil discovery marked the beginning of a prolific oil and gas trend in Upper Devonian sandstones that stretches from western New York, across northwestern Pennsylvania, to northern West Virginia. Most oil fields in the trend were identified by the early 1920's, and many of the larger fields were exploited using secondary recovery techniques, mainly water flooding, in the 1930's. Gas fields containing minor associated oil continued to be discovered along the eastern margin of the play in the 1950's and 1960's. Approximately 800 oil and (or) gas fields have been discovered in the play since 1859, of which about 50 have been converted to gas storage facilities. The exploration phase of the play is now over, and current drilling consists of infill wells that add small amounts of oil and gas to existing fields. Many of the sandstone reservoirs in the eastern part of the play have tight formation status.

Among the largest oil fields in the play are Bradford (McKean Co., Pa. and Allegheny Co., N.Y.), discovery date 1871, ultimate recovery 680 MMBO; Saxonburg (Butler Co., Pa.), discovery date 1866, ultimate recovery 59 MMBO; Salem-Wallace-Zinnia (Harrison and Doddridge Cos., W. Va.), discovery date 1899, ultimate recovery 41 MMBO; Mannington (Marion Co., W. Va.), discovery date 1899, ultimate recovery 32 MMBO; and Bullion-Clintonville (Venango Co., Pa.), discovery date 1876, ultimate recovery 29 MMBO.



Resource potential: This play has no potential for undiscovered oil and gas fields greater than 1 MMBO or 6 BCFG. The shallow depth of the reservoirs and the high drilling density in the play area suggest that the play is exhausted except for very small accumulations.

upper devonian sandstone gas plays
6733. UPPER DEVONIAN SANDSTONE GAS HIGH POTENTIAL

6734. UPPER DEVONIAN SANDSTONE GAS MEDIUM POTENTIAL (HYPOTHETICAL)

6735. UPPER DEVONIAN SANDSTONE GAS MEDIUM-LOW POTENTIAL (HYPOTHETICAL)

6736. UPPER DEVONIAN SANDSTONE GAS LOW POTENTIAL (HYPOTHETICAL)

The unconventional Upper Devonian Sandstone Gas plays (6733, 6734, 6735, 6736) and the conventional Upper Devonian Sandstone Oil/Gas Play (6737) are contiguous plays in the sandstone depositional system of the Upper Devonian Catskill Delta. Most of the sandstone was deposited in littoral-marine, deltaic, offshore shallow-marine, and offshore deeper-water marine (turbidite) settings. The unconventional Upper Devonian Sandstone Gas plays (6733, 6734, 6735, 6736) located east of the conventional play (6737) are defined as a continuous-type gas accumulation because of (1) low-permeability reservoirs, (2) abnormally low formation pressure, (3) coalesced gas fields, (4) gas shows or production in most holes drilled, and (5) general lack of control by anticlinal closure on gas accumulations.

Four Upper Devonian sandstone gas plays are recognized. The Upper Devonian Sandstone Gas High Potential Play (6733), the largest of the plays, is down-dip of the conventional Upper Devonian Sandstone Oil/Gas Play (6737). It extends across most of central Pennsylvania and part of north-central West Virginia. The relatively small Upper Devonian Sandstone Gas Medium Potential Play (6734) consists of two parts: a northeastern Pennsylvania part attached to the northeast end of play 6733 and a south-central Pennsylvania, western Maryland, and eastern West Virginia part attached to the down-dip side of play 6733. The Upper Devonian Sandstone Gas Medium-Low Potential Play (6735) also consists of two parts. The southern part of the Upper Devonian Sandstone Gas Medium-Low Potential Play (6735) occupies most of southeastern West Virginia and small pieces of adjoining Virginia and is attached at its north end to plays 6733, 6734, and 6737. The northern part of the Upper Devonian Sandstone Gas Medium-Low Potential Play (6735) occupies small parts of northeastern Pennsylvania and adjoining south-central New York and is attached along its west side to plays 6734 and 6737. The Upper Devonian Sandstone Gas Low Potential Play (6736) wraps around the northern parts of plays 6734 and 6735 in a large part of southeastern New York and northeastern Pennsylvania.

Stratigraphically, the play involves numerous formal and informal sandstone and siltstone units deposited during multiple regressive events. In Pennsylvania, Upper Devonian sandstones and siltstones are organized into four major sequences, in ascending order, the Braillier Formation, Elk Group, Bradford Group, and Venango Group. Westward in Pennsylvania the Elk, Bradford, and Venango Groups change facies and are replaced by siltstone and shale of the Braillier Formation, whereas eastward and southward the Elk, Bradford, and Venango Groups are replaced, respectively, by the Scherr, Foreknobs, and Catskill Formations. In West Virginia, Upper Devonian sandstones are organized, in ascending order, into the Braillier Formation, Greenland Gap Formation, Hampshire Group, and Price Formation (part). Farther to the west in West Virginia, the Hampshire Group and Price Formation (part) are replaced, respectively, by the Venango and Oswago Formations, and the Greenland Gap Formation changes facies and is replaced by shale and siltstone of the Braillier Formation. Pennsylvania and West Virginia nomenclature correlates approximately as follows: (1) the lower part of the Elk Group and underlying Braillier Formation in Pennsylvania correlate with the lower part of the Braillier Formation in West Virginia; (2) the upper part of the Elk Group, the Bradford Group, and the lower part of the Venango Group in Pennsylvania correlate with the Greenland Gap Formation in West Virginia; and (3) the middle and upper parts of the Venango Group in Pennsylvania correlate with the Hampshire Group and lower part of the Price Formation in West Virginia.

The eastern boundary of the play in West Virginia, Virginia, Maryland, central and northeastern Pennsylvania, and southeastern New York and the northern boundary of the play in east-central New York are marked by the erosional limit of the Upper Devonian sandstone sequence. The western boundary of the play in west-central New York, west-central Pennsylvania, and north-central West Virginia is marked by the approximate limit of oil production in the Upper Devonian sandstone sequence. The western limit of the play in southern West Virginia is the depositional limit of the Upper Devonian sandstone and siltstone sequence.

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



Reservoirs: Coarse siltstone and very fine to fine grained sandstone, classified as sublitharenite, quartzwacke, subarkose, and quartzarenite constitute the reservoirs in the play. Compaction and burial diagenesis have greatly reduced the primary intergranular porosity of the reservoir. Silica and calcite cement and authigenic clay minerals are the primary pore-filling materials. Secondary intergranular porosity, caused by dissolution of detrital grains (feldspar, metamorphic rock fragments), and silica and (or) calcite cement, and fracture porosity, caused by movement between basement-involved fault blocks, are the important porosity types in the play. Porosity types of secondary importance are moldic porosity caused by fossil fragment dissolution and reduced primary intergranular porosity in medium-grained quartzarenites. Porosity for reservoirs in the play ranges from 0.5 to 15 percent and averages 5 percent. Permeability is as high as 2 mD, but generally averages less than 0.01 mD. Porosity and permeability values are higher than average along the crests of anticlines.

Lateral continuity of the sandstone reservoirs ranges from discontinuous lens-shaped sandstone bodies to continuous sheet like sandstone bodies. The net thickness of the sandstone reservoirs ranges from about 10 to 25 ft. Commonly, more than one sandstone reservoir is productive in a given well. Drilling depths to the sandstone reservoirs are between 2,000 and 6,000 ft.



Source rocks: The sources of gas in the play are the Middle Devonian Marcellus Shale that underlies the Upper Devonian sandstone sequence and the Upper Devonian black shales that are west of and intertongue with the Upper Devonian sandstone sequence. The Middle and Upper Devonian black shale sequence in the play area is between 50 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 the northern and southern parts of the play area are in the zone of gas generation, whereas Devonian source rocks in the central and eastern parts of the play are overmature with respect to the generation of oil and gas. Dry and wet thermal gas are the expected hydrocarbon types in the play.

Timing and migration: Peak gas generation from the Middle and Upper Devonian black shale sequence occurred between Late Pennsylvanian and Early Triassic time when the sequence was buried under an eastward-thickening wedge of orogenic sediments. Gas migrated a short distance laterally and upsection to the sandstone reservoirs. A variety of facies-change stratigraphic traps, diagenetic traps, and fractured-reservoir traps were available to trap the gas.

Traps: Facies-change stratigraphic traps, commonly in combination with anticlinal flanks, anticlinal noses, diagenetic traps, and fractured-reservoir traps, are the most important traps in the play. In general, anticlinal closure has exerted only subtle influence, if any, on gas accumulations in the play. Water block, a suggested trapping mechanism for the Clinton/Medina Sandstone Gas Plays, also may apply to the Upper Devonian Sandstone Gas plays. 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 this play, it would imply the presence of a basin-center gas accumulation.

Seals for the stratigraphic traps are shale and fine-grained siltstone intercalated with the Upper Devonian sandstone sequence.



Exploration status: The first 15–20 gas fields in the play were discovered from 1865 through 1900 in Armstrong, Cambrian, Elk, Indiana, Jefferson, and Westmoreland Counties, Pennsylvania. These early fields, whose drilling depths to Upper Devonian sandstone reservoirs range from 1,500 to 3,000 ft, are along the eastern perimeter of the prolific Upper Devonian sandstone oil and gas trend (play 6737) discovered in 1859. For the next 60 years, new fields were discovered in Pennsylvania along the margins of the Upper Devonian oil and gas trend at the approximate rate of one field per year. In the 1960's, gas field discoveries in the play showed a modest increase, and 11 new fields were discovered in Pennsylvania (drilling depth ~3,000 to 5,000 ft). This modest increase signaled a shift in exploration to deeper parts of the play where gas is trapped in low-permeability sandstone reservoirs. An abrupt increase in the amount of gas discovered in low-permeability sandstone reservoirs occurred in the 1970's and 1980's when approximately 70 new fields were discovered in Pennsylvania and 2 new fields were discovered in West Virginia. The number of gas fields discovered in West Virginia in the 1970's and 1980's is probably under represented here because Upper Devonian sandstone gas discoveries were reported as deeper extensions of existing fields rather than new fields. Successful exploration continues today in west-central Pennsylvania and east-central West Virginia where drilling depths range from about 3,000 to 7,000 ft. Most sandstone reservoirs in the plays have tight formation status.

An important factor that insured exploration success in low-permeability sandstone reservoirs in the plays 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 a regional-type accumulation with additional drilling. For example, three or four Upper Devonian sandstone fields that were discovered in Centre and Clinton Counties, Pennsylvania, in the early 1980's have now merged into a single accumulation having an ultimate recovery of about 250 BCFG.



Resource potential: This group of plays has potential for a large quantity of undiscovered gas in a proposed 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 Upper Devonian Sandstone Gas High Potential Play (6733) range from 7 (F95) to 197 (F5) MMCFG and have a median (F50) of about 54 MMCFG. Slightly smaller EUR values are estimated for producing wells in the Upper Devonian Sandstone Gas Medium Potential Play (6734); they range from 5 (F95) to 200 (F5) MMCFG and have a median (F50) of about 50 MMCFG. Low EUR values, high risk, and a low success ratio are assigned to the Upper Devonian Sandstone Gas Medium-Low Potential (6735) and Upper Devonian Sandstone Gas Low Potential (6736) Plays. A well spacing of 40 acres was selected for all plays.



Devonian Black Shale Gas Plays

by R.C. Milici
6740. DEVONIAN BLACK SHALE–GREATER BIG SANDY play

6741. DEVONIAN BLACK SHALE–GREATER SILTSTONE CONTENT

6742. DEVONIAN BLACK SHALE–LOWER THERMAL MATURITY (HYPOTHETICAL)

6743. DEVONIAN BLACK SHALE–UNDEVELOPED NE OHIO AND WESTERN
PENNSYLVANIA play
(HYPOTHETICAL)

Black gas-producing shale of Devonian and Mississippian age is present in much of the Appalachian Basin, in an area that extends from New York generally southwestward through Pennsylvania, Maryland, Ohio, West Virginia, and eastern Kentucky, into Tennessee. In general, the shale was deposited in a foreland basin along the distal margins of the Acadian Catskill delta. Sediment input into this basin was from the northeast, from erosion of Acadian highlands that were elevated by the collision of ancestral North America with European sialic crust (Perroud and others, 1984).

Throughout much of their occurrence, the Devonian and Mississippian black shales are inclined gently to the east or southeast into the Appalachian Basin, away from the crest of the Cincinnati Arch. The western boundary of the play is along the outcrop of the Devonian shale along the western margin of the basin, where it is exposed on the eastern flanks of the Nashville and Jessamine Domes in Tennessee and Kentucky, and along the trace of the shale outcrop in central Ohio. The southern boundary of the play is in the Appalachian Plateau regions of Tennessee, where the entire black shale sequence thins to 50 ft or less. At the northern end of the Appalachian Basin, Devonian-Mississippian gas shales are exposed in western and central New York, where they trend easterly and dip generally to the south. The eastern margin of the play in general is within the Appalachian Plateau, where the black-shale-dominated deltaic sequence gives way eastward to coarser grained siltstone and gray shale that contain relatively less organic matter.

In general, the plays are a combination conventional-unconventional continuous-type accumulations. The black shales serve both as source and reservoir for the gas and, thus, are autogenic (Milici, 1993). Production depends upon the coincidence of several factors, including relatively abundant organic matter at suitable thermal maturity and a reservoir that is significantly enhanced by a naturally occurring fracture system.



Reservoirs: In its productive area, the Devonian-Mississippian Catskill delta sequence consists of interbedded black shale facies and gray shale and siltstone facies (de Witt and others, 1993; Milici, 1993). Devonian black shales include the Marcellus Shale, Rhinestreet Shale Member of the West Falls Formation, Pipe Creek Shale Member of the Java Formation, lower and upper parts of the Huron Member of the Ohio Shale, and Cleveland Member of the Ohio Shale. The black Sunbury Shale is the only gas shale of Mississippian age in this sequence. A Mississippian heterogeneous, multifacies sandstone, the Berea Sandstone, is beneath the Sunbury throughout much of the area and is a significant reservoir within the shale sequence. The greatest thickness of combined black-shale units is in the depocenter in central Pennsylvania, where together the units are about 1,400 ft thick and constitute about 15 percent of the deltaic sequence (Milici, 1993). In the productive area in southwestern Virginia, black shale beds constitute about 40 percent of the section, although the entire sequence is only about 400 ft thick.

Gas shale reservoir quality depends to a large degree on the occurrence of an integrated natural fracture system within the shale. In the southern part of the play area, in southwestern Virginia, eastern Kentucky, southern Ohio, and West Virginia, fracturing within the shale probably is related to subhorizontal decollement (Schumaker, 1980; Milici, 1993). In the Lake Shore fields of northern Ohio and northwestern Pennsylvania, fracture porosity in the shale may be related to deformation caused by glacial loading and differential isostatic rebound (White, 1992).


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