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



Download 331.19 Kb.
Page4/10
Date conversion20.02.2016
Size331.19 Kb.
1   2   3   4   5   6   7   8   9   10

Reservoirs: Primary reservoirs in the play are vuggy dolomite formed by karst processes during regional pre-Upper Devonian uplift and erosion. Intercrystalline porosity supplements the vuggy porosity. Highly dolomitic, very fine to fine grained quartzose sandstones of the Big Six sandstone, which has primary intergranular porosity, is important locally. Tectonic fractures, probably resulting from differential movement of fault-bounded blocks, may have improved the reservoir quality of the vuggy, intercrystalline, and intergranular porosity zones. The dolomite and sandstone reservoirs in the play are of very high quality. Porosity ranges from about 4 to 18 percent (avg 9 percent) and permeability ranges from 0.2 to 575 mD (avg 75 mD). Thickness of "Corniferous" limestone reservoirs range from 10 to 50 ft thick whereas that of the Big Six sandstone reservoir ranges from 25 to 50 ft. The drilling depth to the "Corniferous" limestone and Big Six sandstone reservoirs ranges from less than 1000 to 1,500 ft.

Source rocks: The source of oil and gas in the play is the Upper Devonian black shale sequence that rests unconformably on the "Corniferous" limestone. The Upper Devonian black shale sequence in eastern Kentucky is 50–400 ft thick, has TOC values between 3 and 7 percent, and its organic matter consists mostly of type II kerogen. Vitrinite reflectance data suggest that the Upper Devonian black shale sequence in the play area either is at the beginning of the oil generation zone or is immature with respect to oil and gas generation. The low maturation of the Upper Devonian black shale in the play area suggests that oil and gas accumulations resulted from short-range migration from the south and east where the Upper Devonian black shale is thicker and within the zone of oil generation. Oil and wet thermal gas are the expected hydrocarbon types.

Timing and migration: Peak oil and gas generation from the Upper Devonian black shale sequence probably occurred between Late Pennsylvanian and Early Triassic time when it was buried under an eastward-thickening wedge of orogenic sediments. Most of the oil and gas probably migrated into the play from a short distance to the east and south where higher levels of thermal maturation had been achieved. The migrating oil and gas was trapped in a variety stratigraphic, combination, and anticlinal traps.

Traps: Most of the traps in the play are truncation traps, anticlinal traps, and combination traps where truncation has occurred on the flanks of low-amplitude anticlines. Locally, facies changes have trapped oil and gas either alone or in combination with anticlinal flanks and noses. The seal for the truncation traps and many of the anticlinal traps is the Upper Devonian Ohio Shale. The facies-change traps and a few anticlinal traps depend on dolomicrite of the Lockport Dolomite and Salina Formation for their seals.

Exploration status: Most of the oil and gas production in the play was established between 1900 and 1920. The largest fields in the trend are Irvine-Furnace (discovery date 1914) and Big Sinking (discovery date 1918), which have a combined ultimate recovery, including the smaller Pilot field, of about 100 MMBO. The Campton field, discovered in 1903, has an ultimate recovery of about 3 MMBO. Including a few small fields found in the 1980's, between 60 and 70 oil and (or) gas fields have been discovered in the play.

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.

6716. Upper Silurian sandstone gas play

The Upper Silurian Sandstone Gas Play is defined by gas trapped in the Upper Silurian Williamsport Sandstone (Newburg sand of drillers) by facies-change stratigraphic traps and combination traps. The play has an unusual outline: several long, thin lobes are caused by the distribution of the cleanest and thickest sandstone beds in the Williamsburg Sandstone. Most of the play is in west-central and southern West Virginia, but small parts of the play are in northern and north-central West Virginia, western Maryland, southwestern Pennsylvania, and western and northwestern Virginia. The play is confirmed, and the reservoirs are conventional.



Reservoirs: Fine- to coarse-grained quartzose sandstone of littoral and offshore-marine origin constitutes the reservoir in the play. Cementation of commonly well sorted and rounded quartz grains is variable; it ranges from tightly cemented with silica to poorly cemented with dolomite, clay, and minor silica. Primary intergranular porosity, ranging from 6 to 24 percent (avg 12 percent) is the dominant porosity in the reservoir. Permeability of the sandstone reservoir is as high as 183 mD. In the southern and eastern parts of the play, fracture porosity may be needed to improve reservoir quality.

The Williamsport Sandstone ranges in thickness from 10 to 40 ft in most of the play area except in southern West Virginia where it ranges in thickness from 80 to 120 ft. The thickness of the producing part of the Williamsburg Sandstone ranges from 3 to 10 ft. Drilling depths to the Williamsburg Sandstone in known fields are between 4,800 and 6,800 ft. In the deeper parts of the play, the drilling depth to the Williamsport Sandstone is as much as 8,000 ft.



Source rocks: The source of gas in the play is uncertain. The most plausible candidates are (1) shale and argillaceous limestone of the Antes Shale, Trenton Limestone, and Dolly Ridge Formation of the Trenton Group, (2) black shale of the Middle and Upper Devonian sequence, and (3) dispersed organic matter in salt beds of the Salina Group. Only the Middle Ordovician and Middle and Upper Devonian sequences can be evaluated using existing geochemical data. These 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 1–3 percent for both sequences), and have organic matter dominated by type II kerogen; however, gas generated from these source bed sequences is not particularly accessible to the reservoir. For example, between 2,100–3,000 ft of vertical migration, through predominantly shale and siltstone, is required for gas derived from the Middle Ordovician shale-limestone sequence to reach the Williamsport Sandstone. In contrast, between 1,000 and 2,500 ft of downward migration, through at least 500 ft of evaporite and evaporitic dolomite, is required for Devonian shale gas to reach the sandstone reservoir. 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 in the zone of gas generation. Dry thermal gas is the expected hydrocarbon type 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 occurred between Late Pennsylvanian and Early Triassic time when these beds were buried under an eastward-thickening wedge of orogenic sediments. Gas migrated vertically upsection or downsection to the sandstone reservoir depending on which of the two proposed source rock sequences it was generated from. Facies-change stratigraphic traps in combination with anticlinal closure and noses were available to trap the vertically migrated gas.

Traps: Depositional pinchouts of the Williamsport Sandstone in combination with anticlinal flanks and noses represent the major trapping mechanism in the productive part of the play. This trap type may be present in other parts of the play in addition to low-amplitude basement-controlled anticlines and ramp anticlines above a zone of detachment in the lowermost part of the Salina Group or Upper Ordovician shales. Very likely, reservoirs in these types of anticlines must be enhanced by fracture porosity to be gas productive. The seals for the play are evaporites and evaporitic dolomicrite of the Upper Silurian Salina Group.

Exploration status: Most of the nine gas fields in the play were discovered in the late 1960's. The largest of these gas fields Rocky Fork, (Kanawha and Putnam Cos., W. Va.), discovery date 1966, ultimate recovery ~130 BCFG; North Ripley (Jackson Co., W. Va.), discovery date 1970, ultimate recovery ~86 BCFG, and Kanawha Forest (Kanawha and Boone Cos., W. Va.), discovery date 1939, ultimate recovery ~42 BCFG. Although exploration continues for new fields in the play, none have been found since 1970.

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 that more than half of the discovered fields are greater than 6 BCFG in size. Apparently, smaller fields in the population have not yet been found and perhaps several of them are greater than 6 BCFG in size. In addition, a large area in west-central West Virginia only has been sparsely drilled to the Williamsport Sandstone. Limiting factors in the play may be low-quality reservoirs and their poor accessibility to known source rock sequences.

6717. Silurian carbonate gas play

The Silurian Carbonate Gas Play is defined by gas and local oil trapped in Middle and Upper Silurian platform carbonates by facies-change stratigraphic traps including (reefs), low-amplitude basement-controlled anticlines, and combination traps. Stratigraphically, the play involves the Middle Silurian Lockport Dolomite, Middle Silurian Newburg "zone," Middle Silurian McKenzie Formation (Limestone), Upper Silurian Salina Group (Formation), and Upper Silurian Bass Islands Dolomite. The play extends across central and southern New York, northwestern Pennsylvania, eastern Ohio, western West Virginia, eastern Kentucky, and a small part of southwestern Virginia.

The eastern boundary of the play is defined arbitrarily in west-central Pennsylvania, central West Virginia, and southern New York as the eastern limit of stratigraphic traps and basement-involved low-amplitude anticlines in the Middle and Upper Silurian carbonate sequence. Silurian carbonate gas fields east and south of this arbitrary limit are considered to be in the Silurian and Devonian Carbonate Thrust Belt Play (6718). Traps in play 6718 are faulted ramp anticlines, imbricate thrust slices, and fracture zones largely controlled by Upper Silurian Salina Group bedding plane detachment. Because bedding-plane detachment at the Salina level, with overlying thin-skinned structures, extends across western Pennsylvania, most of southern and western New York, and easternmost Ohio, the Silurian and Devonian Carbonate Thrust Belt Play (6718) and the Silurian Carbonate Gas Play (6717) overlap by 100–125 mi.

The northern boundary of the play is defined by the outcrop limit of the Silurian carbonate sequence at the north end of the Appalachian Basin in New York and by the United States-Canada border in the middle of Lake Erie. The western boundary of the play in Ohio is marked by the outcrop limit of the Silurian carbonate sequence, whereas the western boundary of the play in Kentucky is marked by the province boundary and the outline of play 6715. The play is confirmed, and most of its reservoirs are conventional.



Reservoirs: Many reservoirs in the play are dolomitized bioherms and pinnacle reefs that formed on shallow-water carbonate shelves and along margins of deeper water basinal settings. Corals, brachiopods, pelmatozoans, and sponges dominate the diverse fauna of the reef assemblage. Nonreef carbonate facies also are good reservoirs in the play. For example, in Ohio, porous zones at the base and top of the Lockport Dolomite are probably too widespread and continuous to have been caused by bioherm- and reef-building processes alone. The uppermost porous zone, 80–100 ft below the top of the Lockport Dolomite, was named the “Newburg zone” by drillers. Possibly, these porous dolomite zones formed by karst processes that evolved during widespread subaerial exposure of the carbonate shelf. Moderately to highly fractured oolitic and biomicrite facies, containing local dissolution vugs may also be gas reservoirs. Fractures are most common along zones of bedding-plane detachment and near basement-controlled anticlines. Vuggy and moldic porosity are the dominant porosity types in both the reef and nonreef facies. Locally, intercrystalline and fracture porosity are important. The vugs, molds, and fractures have been filled to varying degrees by calcite and dolomite. Porosity in the vuggy zones range from 2 to 37 percent and average about 8–10 percent. Permeability values range from 0.10 to 50 mD. The net thickness of the reservoirs is between 3 and 55 ft. Drilling depths to Silurian carbonate reservoirs vary according to State: Ohio, less than 1,000 to 5,000 ft; New York, less than 1,000 to 6,000 ft; Pennsylvania, 2,000–7,000 ft; West Virginia, 3,500–6,000 ft; and Kentucky, 1,500–2,700 ft.

Source rocks: The source of gas and local oil in the Kentucky, Virginia, and West Virginia parts of the play is the Upper Devonian black shale sequence that either rests unconformably on the Silurian carbonate sequence or overlies it by less than 1,000 ft. The Upper Devonian black shale sequence in eastern Kentucky is 50–400 ft thick and has TOC values between 3 and 7 percent. Vitrinite reflectance data suggest that the Upper Devonian black shale sequence in Kentucky and adjoining Virginia and West Virginia is in the zone of oil generation and the beginning part of the zone of gas generation. Wet thermal gas and local oil are the expected hydrocarbon types. Devonian black shales in northern Kentucky are immature with respect to oil and gas generation. Therefore, gas trapped in Silurian carbonates here resulted from short-range migration from the south and east where the black shale sequence is thicker and within the zone of oil and gas generation.

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, (2) black shale of the Middle and Upper Devonian sequence, and (3) dispersed organic matter in salt beds of the Salina Group. Only the Middle Ordovician and Middle to Upper Devonian sequences can be evaluated using existing geochemical data. These 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 1-10 percent for the Middle and Upper Devonian sequence), and have organic matter dominated by type II kerogen; however, gas and local oil generated from these source bed sequences are not particularly accessible to the carbonate reservoirs. For example, between 2,000 and 3,000 ft of vertical migration, through predominantly shale and siltstone, is required for gas and oil derived from the Middle Ordovician shale-limestone sequence to reach the Silurian carbonates. In contrast, between 800 ft and 2,000 ft of downward migration, through at least 500 ft of evaporite and evaporitic dolomite is required for Devonian gas and oil to reach the carbonate reservoir. A slight preference is given to the Middle Ordovician source beds because upward vertical migration is more plausible than downward migration.



Based on CAI and Tmax data, Middle Ordovician source rocks in the play are in the gas and oil generation zone. Dry and wet thermal gas and local oil are the expected hydrocarbon types derived from Middle Ordovician source rocks. Vitrinite reflectance dates suggest that Middle and Upper Devonian source rocks in the play have achieved several levels of thermal maturity. Devonian source rocks in eastern and central New York, northwestern Pennsylvania, and easternmost Ohio are in the zone of gas generation, whereas those in western New York, northwesternmost Pennsylvania, and eastern Ohio are in the zone of oil generation. Devonian source rocks in east-central Ohio and offshore northern Ohio (Lake Erie) are immature with respect to oil and gas generation. Oil and wet thermal gas are the expected hydrocarbon types derived from the Middle and Upper Devonian source rocks.

Timing and migration: Peak oil and gas generation from the Middle 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. Oil and gas migrated vertically upsection or downsection depending on which of the proposed source rock sequences they were derived from. Oil and gas may have migrated laterally along porous zones in the Lockport Dolomite into east-central Ohio from areas to the east where higher levels of thermal maturity were achieved. A variety of stratigraphic traps, combination traps, and low-amplitude anticlines trapped the gas and local oil.

Traps: Bioherms, pinnacle reefs, low-amplitude basement-controlled anticlines, and combination traps are the major traps in the play. The bioherms and reefs cover an area between 350 and 700 acres, whereas anticlinal closure covers an area as large as 3,500 acres. The largest fields discovered to date are associated with anticlines and combination traps. Combination traps are present where permeability barriers, caused by depositional and diagenetic facies changes and truncation, cross noses and flanks of anticlines. Seals for the traps are Upper Devonian black shale and evaporite beds and evaporitic dolomicrite in the Upper Silurian Salina Group.

Exploration status: Tens of thousands of holes have been drilled through all or part of the Silurian carbonate sequence in eastern Kentucky, Ohio, northwestern Pennsylvania, and western New York. Since the late 1890’s, between 100 and 120 gas and local oil fields have been discovered. Many of the fields consist of 1 or 2 wells and are subcommercial. Moreover, in about a third of the fields, the Silurian carbonate gas production is commingled with gas production from the Clinton sandstone, Oriskany Sandstone, Devonian black shale, and a variety of Mississippian sandstones. The largest gas fields in the play are: Oneida (Clay County, Ky.), discovery date 1928, ultimate recovery 26.6 BCFG; Artemus-Himyar (Knox County, Ky.), discovery date 1931, ultimate recovery 18.8 BCFG; Mayfield (Cuyahoga County, Ohio), discovery date 1938 (Lockport part), ultimate recovery 11.8 BCFG; 4) Green (Summit County, Ohio), discovery date 1928, ultimate recovery 9.8 BCFG; and Henderson dome-Kilgore, (Mercer County, Pa.), discovery date 1966 (Lockport part), ultimate recovery 8.7 BCFG. Exploration drilling continues in the deeper parts of the play in New York, Pennsylvania, Ohio, and West Virginia.

Resource potential: This play has potential for a modest number of undiscovered gas fields greater than 6 BCFG. Most of the undiscovered fields probably are in sparsely drilled deeper parts of the play in western New York, northwestern Pennsylvania, eastern Ohio, and western West Virginia and in undrilled Lake Erie. Prospective reefs and anticlinal traps are very subtle and may have been overlooked in previous exploration phases; however, the small size of the traps may limit the available gas resources in the play.

6718. Silurian and Devonian carbonate thrust belt play

The Silurian and Devonian Carbonate Thrust Belt Play is defined by gas and local oil trapped in Upper Silurian, Lower Devonian, and Middle Devonian shelf carbonates by faulted ramp anticlines, salt anticlines, imbricate fault slices, and fracture zones associated with bedding plane detachment in the Upper Silurian Salina Group. Also, ramp anticlines associated with zones of bedding plane detachment in Cambrian and Ordovician strata may have trapped gas in Middle Silurian carbonates. Stratigraphically, the play involves the Middle Silurian McKenzie Formation; Upper Silurian Salina Group, Bass Islands Dolomite and Tonoloway Limestone; Upper Silurian and Lower Devonian Rondout Limestone and Keyser Limestone; Lower Devonian Helderberg Limestone; Lower and Middle Devonian Bois Blanc Formation; and Middle Devonian Onondaga Limestone. The play extends across south-central New York, northeastern, central, and southwestern Pennsylvania, western Maryland, eastern West Virginia, and narrow pieces of west-central and southwestern Virginia.

The western and northern boundaries of the play are marked by the approximate western and northern limit of high-amplitude salt anticlines, and (or) ramp anticlines formed by bedding-plane detachment and flowage in salt beds of the Salina Group. A narrow zone of imbricate faults and intense fracturing that involves Silurian and Devonian carbonates, called the Bass Islands trend, marks the limit of the play in northwestern New York and adjoining Pennsylvania. The eastern boundary of the play is defined by the eastern limit of Silurian and Devonian carbonate strata in the Appalachian Fold and Thrust Belt. The play is confirmed, and its reservoirs are conventional.

Reservoirs: Fractured dolomite and limestone are the most important reservoirs in the play. Depositional settings of these carbonates include shallow-water carbonate platform, tidal flat, deeper water platform and basin, and restricted basin. Carbonate rocks in the play consist mainly of sparsely fossiliferous micrite and dolomicrite; however, moderately fossiliferous zones, crinoidal calcarenite bars, and coral-stromotoporoid bioherms have been reported locally. The largest bioherms and pinnacle reefs in the Silurian and Devonian sequence are in the Silurian Carbonate Gas Play (6717) and Devonian Carbonate Gas Play (6719). Zones of vuggy porosity may have formed by karst processes that evolved during brief periods of subaerial exposure of the carbonate shelf. Strata with vuggy and (or) moldic porosity probably require fracture porosity to be good reservoirs.

Fractured Upper Silurian and Lower Devonian carbonates in the Bass Islands trend have porosity values ranging from 10 to 15 percent. Drilling depths to the carbonate reservoirs in the trend are between 1,800 and 3,000 ft. In contrast, drilling depths to the carbonate reservoirs over most of the play are between 4,000 and 9,000 ft.



Source rocks: Middle and Upper Devonian black shale is the source of gas and local oil in the play. In most parts of the play, the Devonian black shales either rest directly on the carbonate reservoirs or within 1,000 ft above them. The Middle and Upper Devonian black shale sequence in the play is 50 ft–200 ft thick, and has TOC values between 3 and 5 percent; its organic matter is dominated by type II kerogen. Vitrinite reflectance data suggest that about a third of the Middle and Upper Devonian black shale sequence in the play area is in the zone of gas generation, whereas two-thirds of the sequence is overmature with respect to oil and gas generation. The black shale sequence in the Bass Islands trend is in the zone of oil generation. Dry thermal gas is the expected hydrocarbon type 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 it was buried under an eastward-thickening wedge of orogenic sediments. Gas migrated a short distance downsection into available structural traps.

Traps: Faulted ramp anticlines, salt anticlines, imbricate fault slices and fracture zones associated with bedding plane detachment in the Salina Group are the major traps in the play. Also, highly fractured faulted ramp anticlines and imbricate fault slices associated with older zones (Cambrian and Ordovician) of bedding-plane detachment may be traps in the play. Most traps are very complex in that structural closure and (or) fracturing commonly occurs at several structural and stratigraphic levels and in different geographic localities. The seals for the traps are unfractured to sparsely fractured micrite and dolomicrite and mineralized fractures in the Middle Silurian through Middle Devonian carbonate sequence.
1   2   3   4   5   6   7   8   9   10


The database is protected by copyright ©essaydocs.org 2016
send message

    Main page