Douglas G. Patchen

Silurian Evolution of Central Appalachian Basin

Isopach and lithofacies maps of eight Silurian formations illustrate that during Middle Silurian time a broad shelf emerged from southeast to northwest across southern West Virginia, separating an adjacent basin on the north from the rest of the Appalachian trough in Ohio, Kentucky, and Tennessee. This subdivision of the once-continuous Appalachian trough produced Middle and Late Silurian sedimentation patterns which are quite different from those of the Late Ordovician and Early Silurian. The Lower Silurian Tuscarora Sandstone (as well as the Upper Ordovician Juniata Formation) was deposited throughout an extensive, unbroken area from Tennessee to New York under fluvial to shallow-marine conditions--an onshore-offshore complex from east to west. Likewise, the Middle Silurian Rose Hill Formation was laid down in this same linear trend, with shallow-water ferruginous sandstones confined to the eastern margin. The Rose Hill isopach map indicates the beginning of differential subsidence with a major thickening of the formation in central West Virginia. The broad shelf and adjacent basin developed with sedimentation of the overlying Keefer Sandstone--high-energy sandstone on the shelf surrounding a central region of shale. During deposition of the Middle Silurian McKenzie F rmation and Upper Silurian Williamsport Sandstone, the same basin-shelf relation continued. Distinct members of the McKenzie indicate subaerial to shallow-subtidal conditions on shelves on the east, south, and west; greater subsidence identifies the enclosed basin. Clean Williamsport Sandstone accumulated on the southern shelf while immature sandstone was deposited in the northeast on mud flats associated with the Bloomsburg delta; simultaneously dolomite formed within the basin. The Wills Creek Formation represents uninterrupted deposition of fine clastic and carbonate materials in the basin and on mud flats in the northeast. Restriction of the sea in Late Silurian time is shown by the basin-centered Salina evaporites (anhydrite and halite) and the surrounding-shelf carbonate rocks (Ton loway Limestone). Shelves on three sides of this basin essentially isolated it from a Salina basin in Ohio. Higher energy environments were associated with the Silurian shelves during deposition of both sands and carbonate material, whereas shales, carbonate sediments, and evaporites accumulated in the adjoining basin under low-energy conditions. Known hydrocarbon reservoirs in the Keefer, McKenzie, Williamsport, and Salina are limited to the respective shelves, and future exploration for gas should be concentrated in those areas.

CREATING A GEOLOGIC PLAY BOOK FOR TRENTON-BLACK RIVER APPALACHIAN BASIN EXPLORATION

The Trenton-Black River Appalachian Basin Research Consortium has made significant progress toward their goal of producing a geologic play book for the Trenton-Black River gas play. The final product will include a resource assessment model of Trenton-Black River reservoirs; possible fairways within which to concentrate further studies and seismic programs; and a model for the origin of Trenton-Black River hydrothermal dolomite reservoirs. All seismic data available to the consortium have been examined. Synthetic seismograms constructed for specific wells have enabled researchers to correlate the tops of 15 stratigraphic units determined from well logs to seismic profiles in New York, Pennsylvania, Ohio, West Virginia and Kentucky. In addition, three surfaces for the area have been depth converted, gridded and mapped. A 16-layer velocity model has been developed to help constrain time-to-depth conversions. Considerable progress was made in fault trend delineation and seismic-stratigraphic correlation within the project area. Isopach maps and a network of gamma-ray cross sections supplemented with core descriptions allowed researchers to more clearly define the architecture of the basin during Middle and Late Ordovician time, the control of basin architecture on carbonate and shale deposition and eventually, the location of reservoirs in Trenton Limestone and Black River Group carbonates. The basin architecture itself may be structurally controlled, and this fault-related structural control along platform margins influenced the formation of hydrothermal dolomite reservoirs in original limestone facies deposited in high energy environments. This resulted in productive trends along the northwest margin of the Trenton platform in Ohio. The continuation of this platform margin into New York should provide further areas with good exploration potential. The focus of the petrographic study shifted from cataloging a broad spectrum of carbonate rocks that occur in the Trenton-Black River interval to delineation of regional limestone diagenesis in the basin. A consistent basin-wide pattern of marine and burial diagenesis that resulted in relatively low porosity and permeability in the subtidal facies of these rocks has been documented across the study area. Six diagenetic stages have been recognized: four marine diagenesis stages and two burial diagenesis stages. This dominance of extensive marine and burial diagenesis yielded rocks with low reservoir potential, with the exception of fractured limestone and dolostone reservoirs. Commercial amounts of porosity, permeability and petroleum accumulation appear to be restricted to areas where secondary porosity developed in association with hydrothermal fluid flow along faults and fractures related to basement tectonics. A broad range of geochemical and fluid inclusion analyses have aided in a better understanding of the origin of the dolomites in the Trenton and Black River Groups over the study area. The results of these analyses support a hydrothermal origin for all of the various dolomite types found to date. The fluid inclusion data suggest that all of the dolomite types analyzed formed from hot saline brines. The dolomite is enriched in iron and manganese, which supports a subsurface origin for the dolomitizing brine. Strontium isotope data suggest that the fluids passed through basement rocks or immature siliciclastic rocks prior to forming the dolomites. All of these data suggest a hot, subsurface origin for the dolomites. The project database continued to be redesigned, developed and deployed. Production data are being reformatted for standard relational database management system requirements. Use of the project intranet by industry partners essentially doubled during the reporting period.

Niagaran Bioherms and Interbioherm Deposits of Western West Virginia

Coral-stromatoporoid bioherms are present in the lower part of the Lockport Dolomite (Niagaran Series) of the western West Virginia subsurface. Between these organic buildups is a bedded, argillaceous dolomite with very sparse fauna, and underlying the Lockport is the Keefer Sandstone which served as the firm substrate on which Lockport benthic communites were established. During a transgression of the Middle Silurian sea, areas of thicker Keefer sand stood as submerged topographic highs. The local relief provided optimum sites for the bioherms to develop and protected the fauna from being overwhelmed by incoming terrestrial clays. Conversely, the shaly interbioherm deposits are in areas of thinner Keefer Sandstone. A minor regression followed and is represented by intert dal sediments. Growth of bioherms ceased when the area emerged above the level of low tide. Two southwest-northeast tracts of thicker Lockport Dolomite and Keefer Sandstone mark the trends of Keefer topographic highs and associated Niagaran bioherms. These trends now offer the best potential for gas in the Middle Silurian of western West Virginia.

ENHANCING RESERVOIR MANAGEMENT IN THE APPALACHIAN BASIN BY IDENTIFYING TECHNICAL BARRIER AND PREFERRED PRACTICES

The Preferred Upstream Management Practices (PUMP) project, a two-year study sponsored by the United States Department of Energy (USDOE), had three primary objectives: (1) the identification of problems, problematic issues, potential solutions and preferred practices related to oil production; (2) the creation of an Appalachian Regional Council to oversee and continue this investigation beyond the end of the project; and (3) the dissemination of investigative results to the widest possible audience, primarily by means of an interactive website. Investigation and identification of oil production problems and preferred management practices began with a Problem Identification Workshop in January of 2002. Three general issues were selected by participants for discussion: Data Management; Reservoir Engineering; and Drilling Practices. At the same meeting, the concept of the creation of an oversight organization to evaluate and disseminated preferred management practices (PMPs) after the end of the project was put forth and volunteers were solicited. In-depth interviews were arranged with oil producers to gain more insight into problems and potential solutions. Project members encountered considerable reticence on the part of interviewees when it came to revealing company-specific production problems or company-specific solutions. This was the case even though interviewees were assured that all responses would be held in confidence. Nevertheless, the following production issues were identified and ranked in order of decreasing importance: Water production including brine disposal; Management of production and business data; Oil field power costs; Paraffin accumulation; Production practices including cementing. An number of secondary issues were also noted: Problems associated with Enhanced Oil Recovery (EOR) and Waterflooding; Reservoir characterization; Employee availability, training, and safety; and Sale and Purchase problems. One item was mentioned both in interviews and in the Workshop, as, perhaps, the key issue related to oil production in the Appalachian region - the price of a barrel of oil. Project members sought solutions to production problems from a number of sources. In general, the Petroleum Technology Transfer Council (PTTC) website, both regional and national, proved to be a fertile source of information. Technical issues included water production, paraffin accumulation, production practices, EOR and waterflooding were addressed in a number of SPE papers. Articles on reservoir characterization were found in both the AAPG Bulletin and in SPE papers. Project members extracted topical and keyword information from pertinent articles and websites and combined them in a database that was placed on the PUMP website. Because of difficulties finding potential members with the qualifications, interests, and flexibility of schedule to allow a long-term commitment, it was decided to implement the PMP Regional Council as a subcommittee of the Producer Advisory Group (PAG) sponsored by Appalachian Region PTTC. The advantages of this decision are that the PAG is in already in existence as a volunteer group interested in problem identification and implementation of solutions and that PAG members are unpaid, so no outside funds will be required to sustain the group. The PUMP website became active in October of 2002. The site is designed to evolve; as new information becomes available, it can be readily added to the site or the site can be modified to accommodate it. The site is interactive allowing users to search within the PUMP site, within the Appalachian Region PTTC site, or within the whole internet through the input of user-supplied key words for information on oil production problems and solutions. Since its inception in the Fall of 2002, the PUMP site has experienced a growing number of users of increasingly diverse nature and from an increasing geographic area. This indicates that the site is reaching its target audience in the Appalachian region and beyond. Following up on a commitment to technology transfer, a total of eight focused-technology workshops were sponsored by the Appalachian Region PTTC center at the request of the PUMP project. Five Welltender Operations and Safety seminars were held in Kentucky, West Virginia, Ohio, and Pennsylvania. A two-day Applied Reservoir Characterization seminar and a one-day course on Paraffin, Asphaltene, and Scale problems were held in Pennsylvania. A one-day workshop on Produced Water was held in OH. In addition to workshops and the PUMP website, the project also generated several topical reports available to the public through the website and through USDOE.

Sedimentary Basin Analysis of Middle Ordovician Limestones in Central Appalachians: ABSTRACT

Seven of 28 wells penetrating the Trenton Limestone in West Virginia have reported shows of natural gas, enough to continue industrys interest in this potential reservoir. The formation consists of thin limestones interbedded with shales and bentonites. Sediments were deposited on a ramp that sloped eastward from a shallow platform in northwest Ohio into the foreland basin of Virginia, and the unit forms a wedge-shaped mass that thickens into the basin. Limestones of the upper ramp were deposited on sand shoals (skeletal grainstones) and restricted flats (lime mudstones), which passed downslope into skeletal patches of a deep, muddy environment (packstones and wackestones). Rapid downwarping of the carbonate ramp produced a major transgression, and deeper limestone facie migrated westward during time. Bentonites spread across the region from distant volcanic islands. In West Virginia, a lower bentonite package is present in the Black River Limestone to the southwest, whereas an upper package occurs in the Trenton to the east and north. This distribution End_Page 1444------------------------------ indicates that the site of volcanic activity shifted during time. A growing orogenic source area shed terrigenous sediment into the basin and onto the Trenton ramp. Initially, these muddy influxes came from the north and from the south, but terrigenous mud eventually swamped the entire ramp and carbonate sedimentation then came to an end. Trenton Limestones have only minor intergranular porosity because of abundant mud matrix and cementation. The only significant contribution to porosity is in fractures. Thus, wells with Trenton shows are characterized by high initial potential that decreases rapidly. End_of_Article - Last_Page 1445------------

Computer-Generation of a Devonian Shale Production and Potential Atlas for West Virginia: ABSTRACT

In the final phase of an Eastern Gas Shales Project (EGSP) contract with the U.S. Department of Energy, the West Virginia Geological and Economic Survey compiled an atlas of Devonian shale production and potential. By using both the surveys existing oil and gas data base and a more detailed, computerized data file created during the project, 22 maps were computer-generated for areas of western and southern West Virginia. These multicolored maps show all wells with known gas production from Devonian shales, all wells drilled to the shales that were dry holes, and all other unsuccessful shale wells that produce either from shallower Mississippian or Pennsylvanian units, or from deeper units below the shales (e.g., the Middle and Lower Devonian Huntersville and Oriskany For ations or the Upper Silurian Newburg sand). In addition, a gray screen pattern on the maps indicates wells from which shows of gas were reported from the shales, and isopotential lines contour initial open flows from the shale gas wells. Thus, the atlas can be used to locate further shale wells in areas with known high productivity, as well as areas with potential for dual completions in one or more zones in addition to the shale. This atlas is one of many products that can be generated from high quality, detailed computer files. End_of_Article - Last_Page 1173------------

Subsurface Stratigraphy of Upper Devonian Clastics in Southern West Virginia: ABSTRACT

Studies of upper Devonian shales and siltstones in southern West Virginia have resulted in a refinement of the stratigraphic framework used in characterizing the gas-producing Devonian shales. Gamma-ray log correlation around the periphery of the Appalachian Basin has extended the usage of New York stratigraphic nomenclature for the interval between the base of the Dunkirk shale and the top of the Tully limestone to southern West Virginia. Equivalents of the Dunkirk shale and younger rocks of New York are recognized in southwestern West Virginia and are named according to Ohio usage. Gas production is primarily from the basal black shale member of the Ohio shale. Gas shows from older black shale units (Rhinestreet and Marcellus shales) are recorded from wells east of the major producing trend. Provided suitable stimulation techniques can be developed, these older and deeper black shales may prove to be another potential gas resource.

Silurian-Devonian Embayment into Appalachia: ABSTRACT

Several Silurian and Devonian formations, traced along central Appalachian outcrop belts, undergo anomalous changes in lithology, thickness, and faunal content in the area of Pendleton County, West Virginia. Hematitic sandstones are replaced by nonred sandstones, other sandstones change to shales, shale formations thin and pinch out, and Catskill red beds are replaced by nonred clastics. These changes represent a move from nearshore to more offshore deposition within an embayment along the western shoreline of Appalachia. The principal Silurian sandstones, the Tuscarora, Keefer, and Williamsport, all undergo facies changes in this area, becoming finer grained, argillaceous, and/or calcareous. Hematitic nearshore sandstones of the Rose Hill are replaced by nonred sandstones in the embayment. The Rochester and Rose Hill shales are thickest in, and largely confined to, the embayment; younger shales, the Big Mountain and Mandata, are replaced by sandstones south of the embayment. McKenzie shales contain more interbedded limestones in the embayment, and the fossiliferous middle Tonoloway Limestone contains a different fauna there than in adjacent areas. During Middle Devonian time, thicker, deeper water Needmore Shale beds in the area were flanked by shallower water facies of the Onesquethaw. The Mahantango Formation also is thickest in this area and thins abruptly to the south. Throughout this time two provenances supplied detritus, one east of West Virginias eastern panhandle and the second in central Virginia. The Pendleton embayment was situated between these two regions that occasionally prograded westward, accentuating the embayment. For the final Devonian regression these areas are End_Page 846------------------------------ referred to as the Fulton and Augusta lobes of the Catskill delta, separated by Grant bay. End_of_Article - Last_Page 847------------

Patch Reefs and Interreef Deposits of Silurian McKenzie Formation, West Virginia: ABSTRACT

Coral-stromatoporoid patch reefs are present in the lower part of the McKenzie Formation of the western West Virginia subsurface. Between these organic buildups is a bedded, argillaceous dolomite with very sparse fauna, and underlying the McKenzie is the Keefer Sandstone which served as the firm substrate on which McKenzie benthic communities became established. End_Page 1585------------------------------ During a transgression of the McKenzie sea, areas of thicker Keefer sand stood as submerged topographic highs. The local relief provided optimum sites for the patch reefs to develop and offered better protection to the fauna from being overwhelmed by incoming terrestrial clays. Conversely, the shaly interreef deposits of the McKenzie are present in areas of thinner Keefer Sandstone; they were laid down in turbid, relatively deep water between highs. A minor regression followed as represented by middle McKenzie intertidal sediments, and growth of the patch reefs ceased when the area emerged above the level of low tide. Two southwest-northeast tracts of thicker McKenzie Formation and Keefer Sandstone mark the trends of Keefer topographic highs and associated McKenzie patch reefs. These trends now offer the best potential gas in the Middle Silurian of western West Virginia. End_of_Article - Last_Page 1586------------