Information Exchange Helps Producers Understand Trenton-Black River

(Tech Connections Column, August 2004, American Oil and Gas Reporter)

The search to develop new reserves from mature basins involves sharing information on complex geological formations. Production is accelerated through applying a variety of technologies and discussing lessons learned. The Trenton-Black River extends across many states and into Canada. The Petroleum Technology Transfer Council is witnessing the value of facilitating information exchange to move industry along the play’s learning curve. A summary of exploration insights is available at www.pttc.org.

Ptarmigan Resources is looking in Novia Scotia at fractured and faulted Middle and Lower Ordovician carbonates in a foreland basin west of a regional fault line associated with a paleomargin. The play area west of the paleomargin is updip from good source beds, and is capped by a good seal. It is in a structural position similar to a Beekmantown play in the St. Lawrence Valley north of Vermont, and to the Knox play in Ohio.

In Tennessee, the strongly oil-prone Trenton-Stones River play is at a stage similar to New York activity five years ago. Miller Petroleum has not seen massive dolomite bodies in its Swan Creek Field, according to Gary Bible, but rather 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. Miller runs an acoustic noise log to determine gas entry into the well bore, and then acidizes those zones.

Bill Harrison from PTTC’s program at Western Michigan University says shelf carbonates in Michigan’s more mature Trenton-Black River fields produce only because of fracturing and dolomitization. Matrix porosity is low, but vugs and caverns provide storage while fractures enhance permeability. Using core examples, Harrison points out the types and diversity of fractures, the type of fracture filling, the extent of dolomitization, and the original fabrics and depositional environments. These fracture systems are related to basement faults that were reactivated. Variations in original depositional fabric may have exerted some degree of control over later fracturing and dolomitization.

Dave Harris, a geologist with the Kentucky Geological Survey involved in the Trenton-Black River play book project, discusses the results of petrographic and geochemical studies of outcrop samples in Kentucky, and plans to take cores and shoot seismic. The 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.

A U.S. Geological Survey study presented by John Repetski compares thermal maturation values based on conodont color and vitrinite reflectance values to the locations of major structural features, and oil and gas fields producing from Ordovician to Devonian rocks. Burial history plots, based on maturation data from Pennsylvanian to Ordovician rocks in several deep wells are shown. At the eastern end, over the Rome Trough, the Trenton-Black River section is over-mature, but still within the limit of gas preservation. As it moves west, the Trenton-Black River section is mature for both gas and oil.

“Without fault-controlled hydrothermal alteration, there would be no Trenton-Black River play in New York,” emphatically states Taury Smith of the New York State Museum Institute.

Seismic data show that the controlling faults commonly die 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 suggests looking for carbonates with appropriate faults that die within or just above them, with evidence of movement in the first kilometer of burial.

Regardless of the type of fault, the key to hydrothermal dolomites is minimal vertical offset to protect the overlying seal. Hydrothermal dolomites commonly are expressed on seismic as subtle sags or anticlines, but can be subseismic. Smith also notes that 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.

Smith says to 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. Finally, look for petrographic evidence of hydrothermal alteration.

Those involved in this play are encouraged to watch PTTC’s calendar because workshops focusing on hydrothermal dolomites are being organized in the Appalachian and Rocky Mountain regions.