Ordovician/Cambrian strata of Central Tennessee- a new play in a mature area

Steven A. Tedesco
Atoka Geochemical Services Corp.
Englewood, Colo.

Recent interest in the Rome formation, Cambrian age stratum of the Appalachian basin as reflected by Harris and Drahovzal¹ suggests a similar play along the flanks of the Nashville dome in Central Tennessee (Fig. 1 [107435 bytes]).

In this area the Upper Cambrian strata pinch out, forming updip stratigraphic traps. Petroleum that charged the Middle Ordovician breccias located along the southern Cincinnati arch in Kentucky and Tennessee used these same beds as migration pathways extending out of the Appalachian and Illinois basins.

In the area where the Cambrian strata pinch out, the present petroleum pro- vince encompassing Central Tennessee and South-Central Kentucky almost exclusively produces from Middle and Lower Ordovician breccias (Fig. 1 [107435 bytes]). There are fewer than a dozen basement tests in this area.

Geology

The Upper Cambrian strata in Central Tennessee are represented by the Con- asauga and Knox Groups (Fig. 2 [25919 bytes]).

The strata are comprised of interbedded carbonates and shales with minor sandstones and conglomerates. The Nashville dome was a positive feature from Lower to the end of Upper Cambrian time. The Conasauga and Lower Knox groups were deposited as transgressive onlapping sequences around the dome.

The dome provided clastic material for deposition in adjacent basins but was inundated in either late Upper Cambrian or Lower Ordovician time. The Lower Knox and Conasauga strata vary in depth from 3,500-5,500 ft and are 2,000-3,000 ft thick.

The upper part of the Knox Group, which is Lower Ordovician age, submerged the Nashville dome and is composed of dense lithographic carbonates except where brecciated and altered by secondary dolomitization. The breccias, which yield both petroleum and zinc, were formed by reactivation of basement faults, rubblization of localized strata, subject to karsting and finally secondary dolomitization. This was followed by petroleum accumulations and sphalerite deposition.

Little is known about the Cambrian and Lower Ordovician rocks in most of Tennessee. Few wells penetrate below the middle of the Knox Group in the area shown in Fig. 1.

Seismic

A recent exploration program using seismic and surface geochemistry over a known petroleum charged Upper Knox mineralized breccia yielded the first clear indication of Upper Cambrian strata pinching out along the Nashville dome (Fig. 3 [107250 bytes]).

The original target, the breccia and the part of it that is petroleum charged in the Lower and Middle Ordovician strata, is easily definable with seismic and surface geochemistry. The Upper Ordovician strata pinch out from about 50-900 ft in less than 1/2 mile in the updip direction.

An additional line was shot updip, outside the surface geochemical anomaly and area of brecciation as indicated by mineral tests (Fig. 4 [116865 bytes]). The western termination of the breccia coincides with the pinchout. The Upper Cambrian strata on this line are either thin or nonexistent, and there is no indication of brecciation.

Fig. 5 [37235 bytes] is a structure map on top of the Knox formation that clearly indicates the zone of collapse associated with the breccia. Fig. 6 [34716 bytes] is an isopach of the Lower Ordovician and Upper Cambrian section that clearly indicates a thinning of the strata. Overlain on Figs. 6 and 7 is a surface geochemical anomaly map whose outline closely matches the location of the breccia and the Cambrian pinchout.

Model

The Trempealeau in southern Ohio is equivalent to the Rome of Copper Ridge dolomite formation.

The Copper Ridge is a member of the Conasauga Group in Tennessee. The Trempealeau is one model for petroleum exploration in Central Tennessee in which erosional remnants lie in structural lows that pinch out updip.

The porous carbonates and sandstones would provide a reservoir for any trapped hydrocarbons with the dense overlying carbonates of Lower Ordovician age providing an effective seal.

Petroleum source

Numerous breccias in the Cincinnati arch area contain producible petroleum derived from Devonian shales and which migrated from the Illinois and Appalachian basins.

Several of the mineralized breccia deposits in Central Tennessee and South-Central Kentucky contain significant amounts of petroleum. The Elmwood zinc mine in Smith County, for example, is noted for the petroleum present in the deposit.²

Petroleum is considered a major factor in the formation of the breccias and sphalerite by providing sulfur to bond with the zinc.³

Petroleum migration had to have occurred through the porous beds of the Middle and Upper Cambrian strata in order for the breccias to be charged, and because of that, may indicate the presence of hydrocarbon accumulations that are deeper.

Conclusions

The Middle and Upper Cambrian strata provided a petroleum pathway for Devonian hydrocarbons migrating out of adjacent basins into the Cincinnati arch area.

The Lower and Middle Ordovician breccias clearly have been charged by this migration from basement fault and fracture systems extending across the Cambrian strata basement. The petroleum migration continued right to termination edge of the Cambrian strata along the Nashville dome. This is supported by the presence of mineralized breccia containing significant hydrocarbons in this area.

The depth to the Cambrian section is less than 5,500 ft with potential reservoir thickness of greater than 100 ft. The area represents a new and exciting play in a mature area.

References

1. Harris, D.C., and Drahovzal, J.A., Cambrian potential indicated in Kentucky Rome trough, OGJ, Feb. 19, 1996, pp. 52-57.

2. Anderson, W.H., Mineralization and hydrocarbons emplacement in the Cambrian Ordovician Mascot dolomite of the Knox Group in South-Central Kentucky, Kentucky Geological Survey, FI 4, 1991, 31 p.

3. Tedesco, S.A., Integrated exploration locates Cincinnati arch dolomite breccias, OGJ, Nov. 28, 1994, pp. 86-90.

Bibliography

Miller, R.A., The geologic history of Tennessee, Department of Conservation, Division of Geology, Bull. 74, 1974, 63 p.

Ohio Department of Natural Resources, Cincinnati Arch Consortium-The geology and geophysics of the East Continent rift basin, Ohio Geological Survey, 1992.

Promap, Cincinnati arch map, Englewood, Colo., 1995.

Sargent, M.L., Sauk sequence: Cambrian system through Lower Ordovician series, in Interior Cratonic Basins, AAPG, 1992.

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