Swan Creek field: Potential giant develops in East Tennessee

John A. Clendening, Michael W. McCown,
Tengasco Inc.
Knoxville

The lineup of wells in Swan Creek field, Tennessee [83,172 bytes]
Tengasco Inc. completed its most recent successful well in March 1999 in the company's Swan Creek field of Hancock County, northeastern Tennessee, on the Appalachian Overthrust Belt.

The 1 Sutton Heirs well was drilled to a TD of 5,000 ft, encountering 30 ft of natural gas pay in the Upper Ordovician Trenton formation and 79 ft of gas pay in the Lower Ordovician Knox Group. The well flowed without treatment at the rate of 5.3 MMcfd of gas with 940 psi pressure on a 1/2 in. choke.

Coburn Engineering, Tulsa, calculated recoverable reserves for the well, on approximately 160 acre spacing, at 7.1 bcf of natural gas.

This is the latest well to confirm the importance of understanding and applying organic petrology and thus thermal maturation studies to prospective hydrocarbon provinces, particularly in "frontier" areas. This meaning of frontier applies to depths below current production as well as laterally into unexplored geography.

Drilling at Swan Creek continues, and the field area has been the site of 3D seismic acquisition since December 1998 (Fig. 1) [362,781 bytes].

Recent completions Tengasco has encountered oil in several formations including the Upper Ordovician Trenton, Stones River, and Murfreesboro formations as well as in the Lower Ordovician Knox Group (Fig. 2) [183,060 bytes].

The well symbols on the stratigraphic column mark the zones in which Tengasco's wells have encountered commercial occurrences of hydrocarbons.

An example of the oil potential in the Swan Creek field area is the company's 2 Paul Reed well, which encountered 34 ft of pay in the Stones River formation at 2,950-84 ft in late 1998. The well tested 220 b/d of oil and showed no decline in the first 3 months of production.

The 1 Steven Lawson well encountered 40 ft of oil pay in the top of the Trenton at 2,442 ft. Initial potential of this well was 60 b/d, natural, along with 243 Mcfd of gas. Coburn Engineering has estimated gross reserves for these two wells at a combined 694,910 bbl of oil.

Swan Creek field

The mapped structure of Swan Creek field (Fig. 1) represents our interpretation of the top of the Knox Group prior to acquisition of 3D seismic data.

The Knox structure portrayed on Fig. 1 is a subthrust structure beneath the Clinchport thrust. Knox initial potential flow rates range from 740 Mcfd in the downdip 1 Helton well to 6.8 MMcfd of gas in the 2 Gary Patton. The normal fault portrayed on the structure map may be nothing more than very steep dip, certainly not unknown in the area.

The structure and trapping mechanisms are known to be quite complex as demonstrated by the hydrocarbon occurrences in the 1 and 2 Paul Reed wells.

The 1 Paul Reed had no shows in the Stones River, but just 1,800 ft to the northeast Stones River was encountered 229 ft high to the 1 Reed. That well, 2 Paul Reed, flowed oil at the rate of 220 b/d.

The 1 Steven Lawson, significantly downdip from the Reed wells, flowed 60 b/d, natural, from the top of the Trenton, stratigraphically well above the Stones River (Fig. 2) [183,060 bytes].

The significance of the T.J. Harrison 1 oil well, insetted in the northeast corner of Fig. 1, is that the well and a 2D seismic line prove the existence of another large structure to the northeast of Swan Creek field and on Tengasco's lease position.

Geology, reserves

The Knox strata are themselves highly variable. To date it appears that a high concentration of calcarenites in the Knox samples, indicative of a shoreline-nearshore shallow water depositional environment, are the primary gas bearing strata, i.e., the reservoir.

Primary porosity ranges from 6-18%, and fracture porosity contributes to the reservoir.

(Fig. 3) [228,404 bytes] is a portion of the log from the 2 Warren Reed. As stated, this well encountered 103 ft of net pay and tested 4.4 MMcfd, natural. If a liner (41/2 in.) were set, perforated, and treatment applied, this well should easily exceed the 6.8 MMcfd calculated for the 2 Gary Patton well.

Swan Creek field is attributed more than 14 bcf of gas and 300,000 bbl of proved developed reserves and 26 bcf of gas and 480,000 bbl of proved undeveloped reserves as of Dec. 31, 1998, despite limited production history. Tengasco has more than 60,000 acres under lease and is conducting an active leasing program.

The question of areal extent arises for Swan Creek field. We believe, although it is yet unproven, that the structure upon which the field exists encompasses many thousands of acres. The area of interest might prove to be several distinct features that could ultimately prove to be separate fields.

Maturation studies

The first author of this article undertook an extensive investigation for Amoco Production Co. along the entire trend of the Appalachian thrust belt and Appalachian front in the 1970s.

This work was designed to ascertain if an area could be identified that could be determined to be in either the peak oil or peak gas generation windows. Either, if present in a significantly large area, would have been of interest to the company for exploration.

Earlier employed by the West Virginia Geological and Economic Survey as coal geologist-palynologist, the author was familiar with coal rank and the effect of increasing thermal exposure upon palynomorphs and other organic debris including vitrinite. Organic petrology studies of vitrinite are used widely by the coal industry to determine rank (Fig. 4) [188,951 bytes].

Coal rank and the accompanying effects upon palynomorphs are equally important to the oil and gas industry insofar as determining where hydrocarbons may be generated and reservoired and in what form (oil, oil and gas, gas and condensate, dry gas, etc.).

Also of utmost importance are what changes or alterations are occurring within the potential reservoir rocks at the various thermal stages. It quite fortunately happens that organic acid dissolution occurs just in time to create porosity to receive and provide storage facilities (reservoirs) for oil and gas as they are being generated and migrated from the source rock (Fig. 4).

Effects of high temperature

The organic acid dissolution process apparently occurs in a very limited thermal window, from about 0.4 to 0.6 on the vitrinite reflectance (% Ro) scale in the temperature range of perhaps slightly above 127° F. (53° C.) to about 139° F. (59° C.).

Above this temperature, porosity destruction can and will commence. This process rapidly becomes critical as thermal exposure increases.

By the time an Ro value of 1.2 is reached (315° F., 157° C.), significant changes will have occurred in the reservoir rock, and all existing oil will have been cracked to gas and condensate. Thereafter, wet gas and condensate have a narrow window between Ro of 1.2 and 1.4 (315° F., 157° C. to 377° F., 192° C.).

Beyond an Ro of 1.4 only dry gas will remain, and the reservoir will be undergoing significant changes, i.e., reduction of porosity. When an Ro of 2.1 is reached, reservoirs have been essentially destroyed and hydrocarbons (gas) will have been largely driven off to other environs.

The point of this is that we should not want to spend exploration dollars in areas where we have little or no expectation of finding a commercially productive reservoir.

Favorable thermal setting

Two principal reasons for high thermal exposure are (1) great depth of burial in the basin of deposition, and (2) high thermal gradient.

The authors know of no significant variation in the thermal gradient of the Appalachian basin of deposition.

What we do have is a huge difference from northeast to southwest in the amount of sediment that was deposited in the basin. This difference is tens of thousands of feet, being greatest to the northeast and easily adding 300-400° F. to the Knox Group equivalents in the northeast area relative to the Knox thermal exposure in northeastern Tennessee.

Our study of palynomorphs and organic debris, including vitrinite, clearly defined an area in eastern Tennessee in peak oil generation stage. The area of Tengasco's Swan Creek field is situated in this favorable thermal setting.

The presence of oil in several of Tengasco's wells is absolute confirmation of the 1970s thermal study, even if one should doubt the validity of the organic maturation studies generally.

Exploration efforts by many operators in numerous areas of high thermal maturity attest that the petroleum industry in general doubts or ignores the validity of organic maturation studies. This includes most of the Appalachian thrust belt, where operators have persisted in the belief that oil and/or gas, but especially gas, will be found in the huge structures simply because they are huge.

Some structures have thousands of feet of closure. However, even a cursory examination of coals and other organic materials would have alerted operators to the fact that they were spudding wells in strata at the semi-anthracite and anthracite thermal levels with only the potential for higher thermal maturity with each foot of depth.

Effects on exploration

The Ouachita Province of Arkansas is another example where false hopes persist and drilling has continued in recent years even though Ro values may exceed 4.0 at the surface (Fig. 4).

Regrettably, this activity is not restricted to small operators with limited technical staff. Major companies with highly competent organic petrologists, geochemists, etc., are lured by the giant structures and proceed with drilling, indifferent to this part of the scientific evidence.

The poor results of drilling in areas such as discussed above also have a chilling effect on the thought processes of many explorationists.

For example, there is a widely held belief that the entire Appalachian thrust belt has no commercial potential. Many, including geologists, commonly hold that no large fields are to be found in the thrust belt. It is generally acknowledged that some small gas fields may be found but with limited upside potential.

We suggest that this concept may be in error. The key to finding hydrocarbons in the thrust belt is to know the thermal history. This concept differs little from drilling to great depths in Texas, Oklahoma, and other areas with very deep basins. The same rules apply regarding thermal maturity; i.e., the preservation of oil and gas along with the reservoirs is dependent upon the thermal exposure.

Field delineation outlook

We continue to believe, based on our overall geologic knowledge, that Swan Creek field will ultimately develop into a giant field with as much as 2-3 tcf of recoverable gas reserves from the Knox and as much as 100 million bbl of oil in the Trenton, Stones River, and Murfreesboro.

Drilling, due to very hard strata and very steep dips, requires a substantial learning experience for most drillers. Diamond bits on air hammers are used. Torque must be controlled carefully, and directional surveys are helpful to keep the hole in proper alignment.

Nevertheless, the gas pay zone in Swan Creek field is only 4,000-6,000 ft deep. The cost to drill and complete averages $250,000/well. On-structure wells in Swan Creek field exceed 4 bcf/well of estimated ultimate recovery per 160 acres.

The company has constructed a 28 mile pipeline from Swan Creek field to Rogersville, Tenn. Additional pipeline to connect the producing field to an East Tennessee Natural Gas Co. pipeline is in the planning stage and is to be completed in 1999 (Fig. 5) [99,970 bytes].

To accelerate and improve the Tennessee exploration effort, Tengasco has been conducting a 3D seismic survey in late 1998-early 1999 that is expected to be of major importance in defining structures and the most promising targets for oil and gas (Fig. 6) [195,438 bytes]. The 3D data are in hand over more than 10,000 acres as of mid-March 1999, with imaging as deep as 16,000 ft.

References

Clendening, John A., Calibration scale of temperatures from vitrinite reflectance values, unpublished, 1977.
Hayes, J.B., Porosity evaluation of sandstone related to vitrinite reflectance, Organic Geochemistry, Vol. 17, No. 2, 1991, pp. 117-129.

The Authors

John A. Clendening, PhD, is an independent oil exploration and production geological consultant and since late 1998 a director of Tengasco Inc. He operates as Laird Exploration Co. in Glasgow, Ky. He was a senior geologist with Amoco Production Co. from 1971-92 and was instrumental in Amoco's discovery of Swan Creek field.
Michael McCown is chief geologist with Tengasco Inc. Also an engineer, he has 28 years of industry experience. He has served with Schlumberger Well Services and several independent operators. He has expertise in satellite imagery, geomorphology, structural tectonics, and geophysics. He holds a BS in geology from the University of Toledo.