Understanding the Trenton-Black River Reservoir

Based on a workshop co-sponsored by PTTC's Appalachian Region, The Appalachian Oil and Natural Gas Research Consortium, and West Virginia Geological & Economic Survey, held on June 7, 2004 in Washington, PA

BOTTOM LINE

A Trenton-Black River exploration model, offered by Taury Smith of New York State Museum, has evolved. First, look for appropriate tectonic settings: basement-rooted intra-platform wrench faults and fault intersections, fault-controlled margins, and the first carbonates deposited on newly-rifted/heavily-faulted continental basement. Second, look for evidence of fault movement soon after deposition: much of the alteration takes place in the first kilometer of burial, so faults with minor vertical offset at the time of alteration may be in the best locations. Third, breccias may be either karst or hydraulic, so look for saddle dolomite-cemented breccias. These are probably hydraulic in origin. And finally, look for petrographic evidence of hydrothermal alteration in cores and cuttings. Other speakers addressed the geological environment and activity in other geographic areas, including western Newfoundland, Kentucky and Tennessee. Insights from similar reservoirs developed in Michigan were shared. Schlumberger shared examples of using FMI and DSI Dipole Shear Sonic Imaging logging tools for characterizing reservoirs.

PROBLEM ADDRESSED

The Trenton-Black River Play remains an active exploration play, which PTTC has supported with several prior workshops. Goals for this workshop were to learn more about the Trenton-Black River reservoirs from those who are active in the play and to learn more about other plays in rocks of the same age in different parts of the eastern U.S. and Canada. These other areas included western Newfoundland, eastern Tennessee and the Michigan Basin. During the afternoon, focus was on the Trenton-Black River in the Appalachian Basin play, and at some new tools that are gaining wider acceptance to characterize and visualize the reservoir.

KEY WORDS:

Burial/Thermal History Setting, Exploration Model, Geographic Areas-Kentucky, Michigan, Newfoundland, Tennessee, Hydrothermal Alteration, Logging Tools-FMI and DSI Dipole Shear Sonic Imager, Trenton-Black River

SPEAKERS

The Cambro-Ordovician Carbonate Platform Offshore Western Newfoundland: A New Exploration Frontier in the Northern Reaches of Appalachia,
Paul Patey, Ptarmigan Resources

Tennessee, Early Stages of An Oil Province,
Gary Bible, Miller Petroleum

Trenton-Black River Oil and Gas Reservoirs in Michigan,
Bill Harrison and Michael Grammer, Western Michigan University

FMI Interpretation,
Elliott Wiltse, Schlumberger Oilfield Services

DSI Dipole Shear Sonic Imager,
John Hubbard, Schlumberger Oilfield Services

Burial and Thermal History Setting for the Trenton-Black River,
John Repetski, U.S. Geological Survey

Outcrop Analogs and Subsurface Examples of Tectonic Dolomite in Kentucky,
Dave Harris, Kentucky Geological Survey,

Hydrothermal Alteration of Carbonate Reservoirs: How Common Is It?
L. Taury Smith, New York State Museum Institute

TECHNOLOGY OVERVIEW

Trenton Black River in Newfoundland

Ptarmigan Resources in Novia Scotia is attempting to develop the Trenton-Black River play in fractured and faulted Middle and Lower Ordovician carbonates in a foreland basin west of a regional fault line associated with a paleomargin. All previous drilling has been east of the fault line, but the new play area is west of the paleomargin, updip from good source beds and is capped by a good seal. The prospect area is in a structural position that is similar to a Beekmantown play in the St. Lawrence valley north of Vermont, and to the Knox play in Ohio. The initial discovery well flowed 1,500 barrels per day from a thin (2 to 3 meter) pay zone, but this flow could not be sustained.

Ptarmigan has identified four prospects in what they call the North Platform play, and two prospects in their North Allochthon play. Using a standard industry risk analysis technique, Ptarmigan estimated oil resources in these areas to be in excess of 500 million barrels and gas resources in excess of 975 Bcf. Ptarmigan is seeking partners for these expensive ($16 MM for a dry hole, $30 MM for a producer on shore, up to $80 MM off shore) wells.

Trenton Black River in Tennessee

Gary Bible from Miller Petroleum reviewed the early-stage Trenton-Stones River play in eastern Tennessee. The status of this strongly oil-prone play is approximately equivalent to the Trenton-Black River play in New York five years ago. Using a structural map of eastern Tennessee, he defined and discussed three different structural plays and several of the better wells that have been drilled in them. Wells in Morgan County typically produce a waxy crude, yielding an operational problem that has been solved by allowing produced drip gas or naptha purchased from a refinery to drip down the annulus to prevent paraffin buildup. Another typical technique developed by his company is to run an acoustic noise log to determine gas entry into the wellbore, and then hit that zone with acid. This completion technique has resulted in flows of 5,000 bopd.

Miller Petroleum is also interested in large structures in the Eastern Overthrust area and in the Swan Creek field. The Swan Creek field was discovered by Amoco in the early 1980s, but Amoco chose not to develop the field. Instead, smaller independents have become involved and Miller took a 9-well farmout in the field. Miller Petroleum has not seen massive dolomite bodies in their wells, but they have seen fine vugular porosity lined with dolomite crystals. Usually the section drilled by the second drill rod below the first occurrence of fine, vugular porosity yields oil. Operators still have not determined the full extent of the field.

Trenton Black River in Michigan

Bill Harrison, who directs PTTC activities in the Michigan Basin, presented a comprehensive overview of Trenton-Black River oil and gas reservoirs there. To illustrate points, Harrison laid out 160 feet of Black River core in the meeting room and brought posters that summarized the research of two graduate students who had worked on the core.

Although Michigan fields are developed in dolomite reservoirs encased in otherwise tight limestones, fractures at various scales are the key to field locations and production. Most of the nearly 20 named fields are small, relative to the Albion-Scipio trend, but collectively they have produced 140 million barrels of oil and 260 Bcf. The Albion-Scipio field has been densely drilled and is characterized by low porosity (< 5%) with a wide range in permeability that averages 84.5 mD.

Harrison went on to discuss structural models for several key fields and showed production histories from those fields, before describing the reservoir rock by showing a series of core photos. The core intervals chosen illustrated the types and diversity of fractures observed, the type of fracture filling, the extent of dolomitization and the original fabrics and depositional environments.

The shelf carbonates in the Michigan Basin are very productive of oil and gas, but only because fracturing and dolomitization have created reservoirs in otherwise non-porous and impermeable limestone. These fracture systems are related to basement faults that were reactivated by shear tectonic movement during times of plate collisions along the eastern continental margin. Although matrix porosity is low, fractures, vugs and caverns provide adequate storage, and fractures enhance permeability significantly. Variations in original depositional fabric may have exerted some degree of control over later fracturing and dolomitization that began in the bottom of the formation and proceeded toward the top.

Logging Tools for Reservoir Characterization

John Wiltse, Schlumberger Oilfield Services, discussed the advantages of running a Formation MicroImager (FMI) tool in Trenton-Black River wells. John Hubbard, also of Schlumberger Oilfield Services, discussed an alternative tool-the DSI Dipole Shear Sonic Imager. In general, the FMI tool is more expensive to run than the DSI Dipole tool, and for both tools, approximately 60% of the cost is to run the tool and 40% is for interpretation. The FMI tool is superior to the DSI Dipole tool for Trenton-Black River exploration, but it can be linked to the DSI Dipole tool. The DSI dipole tool can obtain compressional and shear measurements in either open or cased holes, but the degree of cement bonding is a factor in cased holes. Its primary uses are to determine porosity and lithology, but it can be used to determine mechanical properties necessary to design a frac job, and to determine stress direction, location of gas zones, open natural fractures and a permeability index.

Originally, the FMI tool was developed for structural, fault and fracture analyses. Now the tool and interpretation of the resulting images has evolved to a point where it also can be used for stress and stratigraphic analyses. Workshop participants were shown a series of slides illustrating bedding in limestones and dolomite sections of the Trenton and Black River, fractures in these units, and vugular intervals that comprise target zones. Other slides illustrated induced fractures, borehole breakout and porosity from formation damage.

A more recent use of the FMI tool is to analyze bedding and interpret depositional environments. One local example, not in the Trenton or Black River, showed two intervals in close vertical proximity, one with beds dipping northeast, the other with beds dipping west-northwest. Both were interpreted as channel sandstones, but the challenge was to locate two offset wells that would be in the thickest portion of each channel trend. In this case, which was in West Virginia, the interpreter successfully located the two offset wells.

Burial and Thermal History Settings

John Repetski presented the results of a Trenton-Black River thermal maturation mapping and modeling study conducted by four geologists at the U.S. Geological Survey. The study involved a stratigraphic analysis, including conodont zonation, and the determination of the maturation of samples (conodont alteration index) collected from these formations throughout the study area. The thermal maturation values were then compared to the locations of major structural features and provinces and oil and gas fields that produce from Ordovician and Silurian rocks, and to maps of vitrinite reflectance values.

At the conclusion of the presentation, burial history plots of several deep wells located on a cross section from northwestern Ohio to the Rome Trough area in western West Virginia were shown. At the eastern end of the cross section, in the Rome Trough, the Utica Shale, a potential source bed, "jumped" rapidly through the oil window at the end of the Devonian, whereas to the west, on the margin of the Rome Trough, it remained in the oil generation window much longer. Still farther west, near the Findlay arch, the Utica may still be near the top of the oil window. Thus, at the eastern end, over the Rome Trough, the Trenton-Black River section is over-mature but still above the limit of gas production, and as one moves west, the Trenton-Black River section is mature.

Outcrop Analogs in Kentucky

Dave Harris, a geologist at the Kentucky Geological Survey who is involved in the current Trenton-Black River play book project, discussed the results of petrographic and geochemical studies of outcrop samples in Kentucky. He plans to take cores and shoot seismic to determine the subsurface extent of these tectonic dolomites. The final results of this study, funded by the U.S. Department of Energy, New York State Energy Research and Development Authority, and Triana, will be available in October 2004.

Hydrothermal Alteration of Carbonate Reservoirs

Taury Smith, a geologist with the New York State Museum Institute who also is part of the Trenton-Black River play book research team, presented a paper on hydrothermal alteration of carbonate reservoirs that posed the question: How common is this? His answer seemed to be-quite common and very important.

Dolomitization and leaching of carbonates occurs in many diagenetic environments. One that has historically been under-appreciated is the fault-controlled hydrothermal regime. In this regime, hydrothermal fluids that result in dolomitization follow the same migration pathways and are trapped by the same seals as other fluids that result in leaching or diagenesis, or emplace hydrocarbons. Furthermore, these hydrothermal fluids move up the faults and fractures at times when the faults are active until they reach an overlying seal. At that point, the fluids are forced laterally into permeable zones. Thus, facies and fabrics matter - fluids are preferentially displaced into these more porous and permeable rocks.

Not all of this is positive. These migrating fluids also can bring with them dissolved evaporate minerals that are deposited in the voids created by dolomitization, thus reducing the new porosity and permeability. Other later-stage, porosity-reducing minerals found associated with hydrothermal dolomites are sulfides, calcite, chert, chalcedony, kaolinite and other clays. Many of these have been observed in the Trenton-Black River reservoirs, along with breccias, vugs, cemented fractures, celestite, saddle dolomite and matrix dolomite. Porosity in these reservoirs is mainly vuggy, breccia and fracture porosity.

How big is the impact of hydrothermal alteration on these two limestone formations? "Without fault-controlled hydrothermal alteration, there would be no Trenton-Black River play in New York," Smith stated emphatically. He also cautioned that whereas this mechanism can enhance a reservoir, in other cases fault-derived mineral cements are precipitated in what otherwise would have been a good reservoir rock.

Seismic data show that the controlling faults commonly die out just above the altered rock, indicating that this mechanism was active early in the burial history of the rocks and occurred at shallow depths. These faults could be reactivated at later times, resulting in subsequent dolomitization. Smith suggested an exploration model where one would look for carbonates with appropriate faults that die out within or just above them, with evidence of movement in the first kilometer of burial.

The geochemical evidence found in the Trenton-Black River section indicates that these units were dolomitized by high temperature saline brines that rose from underlying basement rocks and/or siliciclastics prior to precipitating dolomite. Much of the dolomitization occurred in rocks that were never very hot, so the dolomite in them must be hydrothermal in origin.

Hydrothermal dolomites can form in various structural settings, but the key, regardless of the type of fault, is to have had minimal vertical offset to protect the overlying seal. They commonly are expressed on seismic as subtle sags or anticlines, but can be sub-seismic. A second exploration model offered by Smith is to look for subtle sags and anticlines associated with basement-rooted faults on seismic in a limestone host. Structural closure is not necessary.

Smith stated that many, perhaps most, carbonate platform margins are set up by basement-rooted normal, strike-slip or wrench faults. The same fault systems that create the platform margins can act as conduits for mineralizing and leaching fluids and as later migration pathways for hydrocarbons. This also is true for many reefs and carbonate mud mounds. Therefore, another exploration model is to look for fault-controlled carbonate margins similar to the western Newfoundland example discussed earlier. Smith then showed an example of this in northwestern Ohio, and showed a series of maps developed to aid in the search for evidence of a similar margin in the subsurface of New York State.

Breccias were also discussed with a focus on their origin - karst or hydraulic? In Smith's view, most brecciated reservoir rock in the Beekmantown-Knox-Ellenburger is hydraulic, not karst. Much of it formed under shallow burial conditions, but in rock that had never been exposed to erosion at the surface of the earth.

In summary, Smith offered this exploration model. First, look for appropriate tectonic settings: basement-rooted intra-platform wrench faults and fault intersections, fault-controlled margins, and the first carbonates deposited on newly-rifted/heavily-faulted continental basement. Second, look for evidence of fault movement soon after deposition: much of the alteration takes place in the first kilometer of burial, so faults with minor vertical offset at the time of alteration may be in the best locations. Third, breccias may be either karst or hydraulic, so look for saddle dolomite-cemented breccias. These are probably hydraulic in origin. And finally, look for petrographic evidence of hydrothermal alteration in cores and cuttings. With this final statement, Smith supported others in noting that there is no substitute for looking at the rocks.

CONNECTIONS:

Gary Bible
Miller Petroleum
3651 Baker Highway
Huntsville, TN 37756
Phone: 423-663-9457
E-mail: tamia@nxs.net

G. Michael Grammer
Western Michigan University
Department of Geology
1127 Rood Hall
Kalamazoo, MI 49008
Phone: 269-387-3667
E-mail: michael.grammer@wmich.edu

David C. Harris
Kentucky Geological Survey
228 Mining & Min Res Bldg
University of Kentucky
Lexington, KY 40506-0107
Phone: 659-257-5500 Ext 173
E-mail: dcharris@uky.edu

William B. Harrison III
Western Michigan University
1137 Rood Hall
Department of Geology
Kalamazoo, MI 49008
Phone: 269-387-5488
E-mail: william.harrison_iii@wmich.edu

John Hubbard
Schlumberger Oilfield Services
6601 N. Broadway Ext., Suite 200
Oklahoma City, OK 73116
Phone: 405-840-2781
E-mail: hubbard1@slb.com

Paul Patey
67A Hillside Avenue
Lower Sackville, Nova Scotia B4C 1W6
Phone: 902-864-8661
E-mail: maxis@eastlink.ca

For information on PTTC’s Appalachian Region and its activities contact:

Douglas G. Patchen, Program Director
West Virginia University, Appalachian Basin Regional Lead Organization
P.O. Box 6064, Evansdale Drive, Morgantown, WV 26506-6064
Voice: (304) 293-2867 ext. 5443; Fax: (304) 293-7822
Email: dpatch@wvunrcce.nrcce.wvu.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

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toll-free 1-888-THE-PTTC; fax 281-921-1723; Email hq@pttc.org; web www.pttc.org