Michael L. Epsman
Consulting Geologist
Tuscaloosa, Ala.
John D. Taylor J.R.
Holland & Associates
Northport, Ala.
Oil and gas exploration has been carried out sporadically in the Appalachian overthrust region of Alabama for years, but recently interest in the play has had a major resurgence.
The Appalachian overthrust region of Alabama is best exposed in the valley and ridge physiographic province in the northeast part of the state (Fig. 1). Resistant ridges of sandstone and chert and valleys of shales and carbonate have been thrust toward the northwest.
Seismic data show that this structural style continues under the Cretaceous overlap. The surface and subsurface expression of the Alabama overthrust extends for more than 4,000 sq miles (Fig. 2).
Oil and gas have been produced for many years from Cambro-Ordovician, Ordovician, Mississippian, and Pennsylvanian rocks in the nearby Black Warrior basin in Alabama and Mississippi and the Cumberland plateau in Tennessee.
The same zones are also potential producing horizons in the Alabama overthrust region.
BACKGROUND
EARLY DRILLING
Although the Appalachian overthrust and adjacent areas of Alabama have been lightly drilled, test wells were attempted in the earliest period of oil and gas exploration in North America.
One of the earliest recorded tests was the 1 Reed's Gap, drilled by Col. J.A. Montgomery in 1988 (Fig. 1). The well was reportedly drilled to 1,935 ft without incident yet later blew out.
The well was located in 28-13s-13w, Blount County, Ala. This location is on the Sequatchie anticline, one of the major structural features of the southern Appalachians.
Later, in 1952, another well, the Ben Stein 1 B.M. Carr, in 5-11s-1e, Blount County, also blew out and according to local sources burned the rig down. The blowout reportedly occurred at 1,120 ft during drilling in the Knox group.
About 100 miles southwest of the Reed's Gap well, Hale Oil & Gas Co.'s 1 Wedgeworth, in 11-2in-3e, Hale County, Ala., blew out oil, gas, and salt water at 1,400 ft in 1922.1 The formation at TD was probably the Knox.
In adjacent Greene County, the Willis & Payne Co.'s 1 R.C. Lett, in 6-21n-2e, blew out oil and gas while drilling in the Tuscaloosa group at 1,208 ft, according to the State Oil & Gas Board of Alabama well file. Numerous shows were reported uphole before the blowout.
Since the Tuscaloosa formation is usually a fresh water aquifer in this part of Alabama, it is reasonable to assume that the source of these hydrocarbons is a deeper, probably Paleozoic, pay zone.
Both wells are in the buried Appalachian trend, beneath the Gulf Coastal Plain.
MILDER SHOWS
Numerous other wells have encountered less spectacular shows of oil and gas within the Appalachian trend in Alabama.
Blount County Limited Partnership drilled the 1 E.J. Martin 30-6, in 30-13s-3w, Blount County. Drilled in 1982 to a TD of 3,000 ft, the well was completed as a gas producer in the Mississippian age Hartselle sandstone. The well never produced commercially and is now plugged and abandoned.
Shenandoah Oil Corp.'s 1 F.W. Smith 26-6, in 26-9s2w, Cullman County, encountered almost a continuous 200 ft of mudlog show start-ing at 6,612 ft in the Conasauga formation.
Similar shows were reported starting at 2,100 ft from Conasauga in the Amoco 1 J.J. Young, in 34-13s-4e, St. Clair County.
These Conasauga shows suggest that it may be an important source rock and where found with sufficient fracture permeability, may also serve as reservoir rock for hydrocarbons.
RECENT DEVELOPMENT
Drilling activity has restarted in the Appalachian overthrust trend of Alabama .2
In April 1990, Victory Resources Inc. ran production pipe on its 1 Chandler 34-7, in 34-23n-4e, Hale County, Ala. (Fig. 1).
TD was 4,347 ft, and mudlog oil shows were reportedly encountered over a wide interval of fractured Knox from about 3,100 ft to near TD. Perforations were made at 4,185-4,260 ft and 4,110-62 ft, and were acidized (Fig. 3). The well is currently being evaluated.
About 11/4 miles southwest of the Chandler well, ARCO Oil & Gas Co. drilled the No. 1 Cypress B-4, Unit 4-6, a directional test with its surface location in 33-23n-4e, and the target interval is be-lieved to be the same fractured Knox encountered in the Chandler.
Bibb County, Ala., has also seen recent activity as Shuler Drilling Co. drilled the No. 1 -A John Goodson 9-2, in 9-22n-9e, a redrill of an earlier ARCO test.
The Arco well reportedly encountered good shows of oil and gas in a Pennsylvanian age sand beneath a thrust fault and a good show of gas in the Mississippian Tuscumbia limestone.
WELLS PENDING
At this time, drilling activity is planned in Greene County, where Hawkeye Oil & Gas has spudded the Banks 8-1, in 8-22n-3e, a 5,000 ft test.
Meridian Oil Inc. has permitted the 1 Weyerhaeuser 23, in 2-24n-2e, an 11,500 ft test.
Bernard/Hickox Inc. has announced plans to reenter and deepen the No.1 Hagerman 9-11, a well drilled in Sumter County by Sonat and Geochemical Surveys in 1960. The well is located in 9-23n-3w.
Although the most recent drilling activity has taken place in west central Alabama, many structures in northeast Alabama remain to be tested. Northeast Alabama is the source of most of the authors' knowledge of structural styles in this play, and this area is closer to already established production in Tennessee.
Thus far, the best well drilled in the Appalachian overthrust of Tennessee is the Amoco 1 Paul Reed in Hancock County (Fig. 4). This well was drilled to a TD of 10,569 ft and perforated in six zones over an interval from 4,414-4,624 ft in the Knox group (Fig. 5).
Initial potential flow rate on a 72 hour test was 1 MMcfd of gas with a flowing tubing pressure of 800 psi, on a 1%4 in. choke. The well is presently shut-in waiting on pipeline connection.
GEOLOGY
STRATIGRAPHY
The Alabama Paleozoic stratigraphic section in the valley and ridge province has been folded, faulted, and thrust into recognizable surface as well as subsurface structural features.
Mississippian and Pennsylvanian age strata are exposed in northeast-southwest trending ridges separated by valleys floored with Cambrian to Devonian age rocks. Resistant sandstone and chert beds commonly form the ridges and shales, and carbonates occupy the valleys.
Precambrian rocks are not recognized in surface exposures in the valley and ridge province of Alabama, and the total Paleozoic section may be more than 23,000 ft thick in places .3 4
In general the Paleozoic stratigraphic section begins with a clastic unit in the lower Cambrian (Rome); followed by carbonate strata in the Cambro-Ordovician (Conasauga, Knox Chickamauga); clastic, hematite beds and carbonates in the Silurian (Red Mountain); black shales, sandstones and siltstones in the Devonian (Chattanooga and Frog Mountain); phosphatic shales (Maury) and carbonates (Fort Payne, Tuscumbia) in the lower Mississippian; clastics (Pride Mountain, Hartselle, Floyd, Parkwood) and carbonate (Bangor) in the middle and upper Mississippian; and clastics and coals (Pottsville) dominate the Pennsylvanian strata.
ROME, CONASAUGA
The Cambrian age Rome formation is primarily clastic with variegated shales and silty sandstones.
Calcareous shales are infrequent in this section.
Geochemical studies of well cuttings have shown this interval to have poor potential as a source rock, and most sandstones are too finegrained to serve as reservoir rocks.
The Cambrian age Conasauga formation contains limestone, dolomite, and shale, and some zones are fossiliferous. Reported thicknesses range from 500-2,600 ft.
Significant gas shows in the Amoco 1 J.J. Young well in St. Clair County, Ala., and in the Shenandoah Oil Corp. No. 1 F.W. Smith well in Cullman County, Ala., suggest that Conasauga may be a source rock.
Where the Conasauga has sufficient fracture permeability, it may be a reservoir for hydrocarbons.
KNOX GROUP
Lithologically, the Knox group consists of dolomite, limestone, and chert. The formations that make up the Knox include, in ascending order (on outcrop) the Copper Ridge dolomite, the Che-pultepec dolomite, the Longview limestone, and the Newala limestone.
The Copper Ridge and Chepultepec dolomites and Longview limestone contain abundant chert.
The Newala limestone does not contain chert, but minor amounts of dolomite may be present.
Reported thicknesses of the Knox group range from 1,476-4,225 ft. In the Black Warrior basin, gas is presently produced at Maben field in Oktibbeha County, Miss., from the Knox.
In Alabama, Knox production is attributed to one well in Fairview field in Lamar County; however, this production may be from the deeper Stones River interval.
CHICKAMAUGA GROUP
The Ordovician Chickamauga group is generally limestone, partly argillaceous, and sometimes fossiliferous.
The formations that make up the Chickamauga group include, in ascending order (on outcrop) the Attalla chert, Stones River limestone, and Nashville limestone.
The Attalla chert conglomerate occurs at the base of the Chickamauga.
The Stones River is an argillaceous and silty, finegrained, medium to dark-gray, thick bedded limestone. Some beds may be fossiliferous, and bentonic shales occur near the top of the Stones River. A dolomitic, sucrosic, greenish-gray limestone can occur in the lower part. Reported thicknesses range from 260-1,100 ft.
The Stones River produces oil and gas in Clay, Fentress, and Pickett counties, Tenn.
The Nashville limestone is a medium gray, medium grained, fossiliferous limestone with minor amounts of gray shale and locally brown dolomitic limestone, greenish dolomite, and brown chert. Thicknesses range from 50250 ft.
Trenton age equivalents of the Nashville are oil productive in Fentress and Pickett Counties, Tenn.
SILURIAN, DEVONIAN ZONES
The Red Mountain formation represents Silurian age rocks in Alabama. The dark-reddish-brown color for the clastic sequence that includes siltstones, sandstones, shale, oolitic hematite layers, and thin, bioclastic limestone layers distinguish this zone from others. Thickness ranges from 95-590 ft.
Gas shows have been reported from the Red Mountain in a number of wells in north Alabama, and a strong flow of oil was encountered in the Woodward Iron Co. No. 3 Mine in Jefferson County.
Devonian age Frog Mountain sandstone in a light gray to brown, very fine to coarse grained, sometimes pebbly sandstone. Fossiliferous, glauconitic limestones and chert may also be found.
The name comes from the type locality on Frog Mountain, Cherokee County, Ala. Thicknesses range from 0213 ft.
The Chattanooga black shale is a recognizable unit in the stratigraphic section that overlies the Frog Mountain (where that unit is present). Its radioactive character also provides a good marker on geophysical well logs. Thicknesses range from 0-82 ft.
Gas is produced commercially from the Chattanooga shale and age-equivalents throughout the Appalachian basin.
MISSISSIPPIAN FORMATIONS
Maury shale represents the lowermost Mississippian unit. This shale is often greenish-gray to grayish-red color and commonly contains phosphate nodules. Thicknesses range from 0-7 ft.
Fort Payne chert overlies the Maury and is primarily a dark-gray to a light-gray, fossiliferous chert with interbedded siliceous limestone. Weathered chert fragments often show the negative molds (impressions) of fossils. Reported thicknesses range from 0-207 ft.
Fort Payne is oil productive in Oneida and Boone Camp fields in Northeast Tennessee and eastern Kentucky.
Tuscumbia limestone is primarily a thick bedded, light gray, bioclastic or micritic, sometimes oolitic limestone. Reported thicknesses range from 0-250 ft.
Tuscumbia commonly has reported oil and gas shows when it is drilled. Some gas production has been reported in Marion and Tuscaloosa counties, northwestern Alabama.
Pride Mountain formation is a clastic unit of sandstones and shales overlying the Tuscumbia. Commonly siderite and rare pyrite nodules are found in the shale. Some calcareous shales contain abun-dant bryozoans and brachiopods.
Thicknesses range from zero to more than 480 ft. Pride Mountain is in part equivalent to Monteagle limestone, which is oil productive in eastern Tennessee.
Subsurface stratigraphic units, the Lewis and Evans, are fine to medium grained sandstones within a few tens of feet above the Tuscumbia limestone. They are significant gas producers in the Black Warrior basin.
Hartselle sandstone overlies Pride Mountain and is composed of fine grained sandstone that is sometimes crossbedded and calcareous. Thicknesses range from 0150 ft.
Hartselle sandstone has produced gas in Bluegut Creek field in Marion County to Jasper field in Walker County in north-central Ala-bama.
Floyd shale in places overlies Hartselle and is primarily a dark-gray clay shale. Thin beds of limestone, chert, and sandstones are locally present. Oil and gas shows are commonly encountered when the formation is drilled, and its is probably a source rock.
Pride Mountain and Hartselle sandstones produce gas in Eldridge field in Walker County, Ala. Floyd thicknesses range from 0-2,000 ft.
Bangor limestone overlies the Floyd and is primarily gray, bioclastic, and oolitic. Calcareous, fossiliferous shales are sometimes interbedded with massive limestones. Thicknesses range from 0-700 ft.
Bangor limestone has produced some gas at Bankston field, in Fayette County, Ala. Oil covered the pit on a well in Walker County while drilling in the Bangor.
PENNSYLVANIAN ZONES
The Parkwood formation is predominantly a shale and sandstone section that transcends the upper Mississippian and lower Pennsylvanian boundary. The Coats, Gilmer, Millerella, Carter, and Sanders sandstones of this interval are major gas and oil producers in the Black Warrior basin in Alabama.
Additional sandstones (Abernathy, Rea) in this interval produce gas and oil on the Mississippi side of the basin. The shales are probably the source rock for the producing reservoirs. The sands are locally crossbedded and ripplemarked. Thicknesses of the Parkwood range from 0-2,500 ft.
The Pottsville formation of Pennsylvanian age is composed primarily of siltstones, fine to coarse grained sandstones, conglomerates, silty shales, and coal. Lower Pottsville is predominately a marine section with shales and sandstones with few marine fossils and erratically distributed coals.
Ten separate sandstones of the lower Pottsville in Woolbank and Sneads Creek fields in Pickens County, Ala., are gas productive in the Black Warrior basin. The upper Pottsville is dominated by the cyclic deposition of coal measures.
The Nason sandstone of the upper Pottsville is gas productive in the Black Warrior basin.5
STRUCTURE
The Appalachian overthrust in Alabama comprises a tectonic fold and thrust belt, herein considered to be the area lying between the Black Warrior foreland basin to the northwest and the Piedmont metamorphic belt to the southeast (Fig. 6).
Regional structural strike is northeast-southwest. Dominant direction of thrusting is to the northwest, although back thrusts have been recognized in both surface exposures and in seismic data.
Neathery and Thomas 4 subdivided the Appalachian fold and thrust belt in Alabama into three areas characterized by distinct structural styles.
The northwestern-most area consists of broad, flatbottomed synclines separated by narrow, asymmetric anticlines. Most anticlines are oversteepened on the northwest limb in association with a ramping thrust.
On the surface in northeast Alabama, the synclines form broad mesas ranging in width from 4-16 miles, and extending along strike for more than 100 miles. The anticlines, where breached, form narrow valleys ranging in width from 2-5 miles. Potential hydrocarbon traps include simple hanging wall closures, imbricate fault panels, and subthrust closures (Fig. 7).
The central part of the fold and thrust belt is characterized by higher amplitude structures. In this area, surface expression of anticlines and synclines is more nearly the same width, about 4-6 miles in the dip direction, and several tens of miles along strike.
Potential hydrocarbon traps include structural duplexes in addition to simple hanging wall closures, imbricate fault panels and subthrust closures (Fig. 8).
The southeastern part of the fold and thrust belt is characterized by a series of thin, imbricate thrust slices on its northwest boundary, This area is called the Coosa deformed belt.
Potential hydrocarbon traps include hanging wall closures, structural duplexes, imbricate fault panels, and subthrust closures (Fig. 9).
The remainder of the fold and thrust belt is characterized by large displacement, low angle thrust faults, mainly in Cambrian age rocks, as far as is known.
These areas have been very sparsely drilled, with no recent wells drilled, but it is believed that thermal maturity of the rocks southeast of the Coosa deformed belt is probably too high for the preservation of hydrocarbons.
ENGINEERING CONSIDERATIONS
DRILLING-COMPLETION-EVALUATION
Perhaps the most difficult aspect of exploring for oil and gas in the southern Appalachian overthrust so far has been the evaluation phase after a well is drilled.
The area is such a frontier, that criteria for production have not been well established. This fact, coupled with the knowledge that most reservoirs are expected to be fractured carbonates, makes evaluation of well logs even more difficult.
This is best illustrated by the compensated neutronformation density logs across the productive intervals in the Amoco 1 Reed (Fig. 5) and the Victory 1 Chandler (Fig. 3). Upon examination of these logs, the pay is not immediately evident in either well.
The authors' experience has shown that at this time, the following procedures insure the best likelihood of obtaining a valid test and evaluation in the southern Appalachian overthrust:
- Use air drilling where possible. This avoids much of the formation damage to fractured formations associated with mud drilling, provides an actual test while drilling, maintains a straighter hole in steeply-dipping rocks, and usually provides a much faster penetration rate.
- Have a wellsite geologist and/or mudlogger rigged up from grass roots to TD. The authors have seen more than one well that could not be evaluated without critical information on lithology, oil shows, fluorescence, mineralogy of fracture fillings, etc.
- The suite of geophysical logs run is critical to successful well evaluation. The usual suite of induction and porosity/density logs is recommended more for correlation purposes than any other reason.
Sonic logs are useful for roughly delineating gas zones.
The new wellbore imaging tools, such as Formation Micro-Scanner, have already proved extremely useful because of their facility in accurately delineating the distribution and orientation of fractures.
- Prior to running production casing, kill the well as gently as possible within safe engineering practice.
- Use lightweight cement, where possible, to cement the production string, in order to minimize formation damage.
- Perforate underbalanced where safe and practical.
REFERENCES
- Semmes, D.R., Oil and gas in Alabama: Geological Survey of Ala-bama, Special Report 15, 1929, 408 p.
- Petzet, G.A., Exploration pace fast in Mississippi, Alabama: OGJ, Mar. 4, 1991, p. 58.
- Thomas, W.A., Southwestern Appalachian structural system beneath the Gulf Coastal Plain: American Journal of Science, Cooper Vol. 273-A, 1973, pp. 372-390.
- Neathery, T.L., and Thomas, W.A., Geodynamics transact of the Appalachian orogen in Alabama: in Profiles of Orogenic Belts, Geodynamic series, Vol. 10, American Geophysical Union, 1983, pp. 301-307.
- Raymond, D.E., Osborne, W.E., Copeland, C.W., and Neathery, T.L., Alabama stratigraphy: Geological Survey of Alabama, Circu-lar 140. 1988, 97 p.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
