Dorothy E. Raymond
Geological Survey of Alabama
Tuscaloosa
The Cambrian-Ordovician Knox group of the Black Warrior basin in Alabama and Mississippi has attracted the interest of the oil industry because of recent significant discoveries of oil and gas in the age-equivalent Arbuckle group carbonates of the Arkoma, Ardmore, and Anadarko basins of Oklahoma.
GEOLOGIC SETTING
During the Cambrian and Ordovician a huge carbonate platform covered the craton from Canada to Alabama to West Texas.
Closure of the oceans during the late Paleozoic Ouachita and Appalachian orogenies resulted in the thrusting of a belt of deformed Paleozoic rocks over the southern and eastern margins of the North American craton to form the Ouachita and Appalachian mountains.
On the craton side of these mountains major foreland basins formed in response to thrusting and include the Arkoma basin of Arkansas and Oklahoma and the Black Warrior basin of Mississippi and Alabama (Fig. 1).
Studies of logs and cores in Wilburton field in the Arkoma basin show the Arbuckle reservoir consists largely of vugs and solution-enlarged fractures in a clean dolostone. Some vugs are several inches across.
Permeability is created by a pervasive system of fractures and microfaults. Vertical faulting predominates in the Arbuckle group rather than strike-slip and thrust faulting that characterize the shallow zones in younger rocks.
Some believe the gas in the Arbuckle is self sourcing; others, however, think the gas probably migrated from younger Pennsylvanian shales or Devonian rocks that are in direct fault contact with the Arbuckle in some areas.
One factor that makes the Arbuckle so attractive in the Arkoma basin is the zone's potential thickness. In some areas as much as 2,000 ft of the Arbuckle may be productive.
The Cambrian-Ordovician Knox group in Alabama and Mississippi is age equivalent to the Arbuckle group of Oklahoma and Arkansas.
The Knox is present in the Black Warrior basin and the Appalachian fold and thrust belt. The potential for oil and gas discoveries exists in both the basin and the fold and thrust belt.
The Black Warrior basin is a foreland basin on the southeastern edge of the North American craton in Alabama and Mississippi (Fig. 1).
Paleozoic rocks of the eastern part of the basin crop out in north-central Alabama, but the western part is buried under Mesozoic and Cenozoic sediments of the Gulf Coastal Plain. These unconsolidated Coastal Plain sediments reach a thickness of more than 6,000 ft in Mississippi.1
The basin is triangular shaped, being bounded on the north by the Nashville dome and the Ozark uplift, on the east by the steeply dipping strata of the southern Appalachians, and on the southwest by the buried Ouachita front.
On the west, the Black Warrior basin is separated from the Arkoma basin by a southwestward-plunging Paleozoic arch and fault system, the Mississippi Valley graben.2
Sedimentary rocks of the basin range from Cambrian to Pennsylvanian in age, the Cambrian rocks resting unconformably on Precambrian crystalline basement rocks.1 3 Structurally, the Black Warrior basin is a homocline dipping southwestward (Fig. 2) and locally broken by a system of northwest-trending normal faults. Most faults are down to the southwest.
Thomas (1988) reports that this fault system appears to extend westward into the Arkoma basin. The age of the fault system is uncertain but may be as young as earliest Middle Pennsylvanian. The deepest part of the basin is in eastern Mississippi west of Sumter County, Ala.
The southern extent of the Black Warrior basin occurs at the northernmost extent of the buried Appalachian fold and thrust belt. The fold and thrust belt is characterized by large scale, internally coherent thrust sheets in which structural style is controlled by a thick competent layer of Cambrian-Ordovician carbonate rocks that includes the Knox group.
KNOX GROUP
The Cambrian-Ordovician carbonate sequence represents a period of maximum transgression of the sea, the carbonate being deposited in a shallow marine shelf environment. The sequence includes limestones and dolomites.
Upper Cambrian and Lower Ordovician carbonate rocks are assigned to the Knox group, which is generally unconformably overlain by Middle Ordovician limestones (Fig. 3).
This pre-Middle Ordovician unconformity generally is present throughout the craton and in the Valley and Ridge strata, and underlying rocks may have karstic development. However, in the southern part of the Black Warrior basin this unconformity is not easily recognizable, and the position of the boundary between Lower and Middle Ordovician strata is generally uncertain.
A recent study4 of the subsurface Ordovician of Alabama, Mississippi, and Arkansas indicates that sedimentation may have been continuous in the southern Black Warrior basin and eastern Arkoma basin from the early to middle Ordovician. Therefore, if there is an unconformity, it was of short duration.
The Knox group crops out in Alabama in the Appalachian fold and thrust belt (Valley and Ridge physiographic province) east of the Warrior basin.
In the eastern Valley and Ridge (east of the Helena fault), the group is subdivided, in ascending order, into the Copper Ridge dolomite, Chepultepec dolomite, Longview limestone, and Newala limestone (Fig. 3).
In the western Valley and Ridge, the Knox is either mapped as undifferentiated or is locally subdivided into the Chepultepec and Copper Ridge dolomites.
In the subsurface of the Black Warrior basin, the Knox group is generally undifferentiated, although the Copper Ridge and Chepultepec dolomites have been tentatively identified in some wells.
In outcrop the Copper Ridge dolomite of Cambrian age, at the base of the Knox group, is light-gray thick-bedded medium to coarsely crystalline siliceous dolomite. Chert is characteristically tough, hard, white or yellowish-gray, stromatolitic, and usually jagged.
Thickness ranges from 1,250-1,800 ft. Outcrops of the Copper Ridge are generally highly weathered, and exposures are poor.
The Lower Ordovician Chepultepec dolomite in outcrop generally consists of an upper primarily dolomite unit and a lower limestone unit.
The limestone is light gray, compact, and thick bedded and contains interbedded dolomite. The upper dolomite is dark bluish gray medium to coarsely crystalline thick and thin bedded and yields abundant wormy, cavernous, and fossiliferous chert. Thickness reaches 1,250 ft.
The Lower Ordovician Longview limestone consists of light-gray cherty, locally sandy thick-bedded finely to medium crystalline limestone and to dolomite. The chert is brittle, compact, and fragile and weathers to small fragments. Thickness averages 500 ft.
The Lower Ordovician Newala limestone consists of dark-gray, light-gray, and bluish-gray compact or noncrystalline or textureless, thickbedded micritic or peloidal limestone and some dolomite. Thickness generally ranges from 200-1,000 ft.
At the top of the Newala is the Odenville limestone facies. This dark gray fossiliferous sponge-algal stylonodular argillaceous limestone represents a deeper water facies than most of the Knox. The Odenville is lithologically similar to the overlying Middle Ordovician Lenoir limestone.
In the Black Warrior basin, the top of the Knox has been picked at several different horizons.
Throughout the northern part of the basin in Alabama, the top of the Knox is distinct and is picked at the base of the Pond Spring formation of the overlying Stones River group.
However, to the south deeper in the basin where the Pond Spring is not recognizable and limestone is interbedded with dolomite, there has been some variation in the horizon chosen as the top of the Knox.
In this area there is generally a sequence, going downward, of limestone, dolomite, limestone, and dolomite (Fig. 4). Mellen5 and Kidd1 placed the Knox at the top of the first or upper dolomite. This pick is commonly used by the petroleum industry.
Harris (no date) and Thomas6 on the basis of ostracods placed the top of the Knox in a well in Northeast Mississippi at the base of a sandy carbonate unit and at the top of the lower or second dolomite.
Alberstadt and Repetski4 on the basis of a study of conodonts and lower Ordovician sponges and algae determined the top of the Lower Ordovician to be essentially the base of the first dolomite (within the second limestone) in northeastern Mississippi. They found the limestone to contain an early Ordovician conodont fauna characteristic of the Odenville facies of the Upper Newala limestone.
The subsurface Knox typically consists of light-gray to light-brownish-gray dolomite, dolomitic limestone, and limestone that characteristically contains fine to medium rounded, frosted quartz sand, ghosts of radial ooids, euhedral quartz, algal stromatolites, and chert.4
The Knox dolomite shows characteristics of having formed in a restricted hypersaline environment on a carbonate shelf.
Dolomites dominate the subsurface Knox section in the plateaus from Lawrence County northward. Southward into the basin limestone dominates the section.
The Knox is dominantly dolomite in the eastern part of the basin as well as in the subsurface Appalachian fold and thrust belt penetrated by the ARCO 1 Ethel M. Koch in Hale County and Shell 1 Burke 29-1 in Greene County.
The Koch well penetrated the entire Knox section. In this well it was possible to differentiate the Copper Ridge dolomite, which appears to be entirely dolomite throughout the basin and buried fold and thrust belt.
The Lower Ordovician part of the Knox is dominantly limestone within the southern part of the basin, while the Cambrian Copper Ridge is dolomite. Just to the southeast in the fold and thrust belt the limestone facies changes rapidly to dolomite.
OIL, GAS POTENTIAL
Past exploration for oil and gas in Alabama in the Knox was concentrated in the Appalachian fold and thrust belt.
There were two objectives in the drilling. One objective was shallow upper thrust sheet traps that would contain low pressure gas in fractured reservoirs. The second exploration objective was deep lower traps expected to have high pressure gas and condensate.
The Knox group was the primary target in the deeper prospects. Recently several unpermitted stratigraphic test holes and a few permitted wells were drilled in the Appalachian trend in Alabama, but no wells have been completed in the Knox.
PRODUCTION HISTORY
Knox production has been reported from Alabama and adjoining Tennessee and Mississippi.
In 1983, gas was discovered in the Knox group of the Appalachians in Hancock County, Tenn. The Amoco-Anschutz 1 Reed well produced 1 MMcfd from 4,414-4,624 ft. The well bottomed at 10,569 ft and topped Knox at 3,880 ft.
Production came from a zone near a major thrust fault. Fractures associated with faulting and solution of carbonate strata near the pre-Middle Ordovician unconformity at the top of the Knox are thought to cause the porosity and permeability in the producing zone.7 8 9
The ARCO-Anschutz 1 Alabama Properties well in Shelby County, Ala., was drilled to 17,005 ft and penetrated two complete repetitions of the Knox section.
Driller's mud logs reported a show of gas in the Knox at 6,700-02 ft in the Newala limestone and at 6,858-70 ft in the Longview limestone, probably from fractures.
Examination of cuttings and core show the Newala to be a peloidal, intraclastic, and/or oolitic limestone that is stylolitic and which contains highly fractured zones with calcite-filled fractures.
The Longview is a partly silicified sandy mudstone that is in part peloidal, stromatolitic, dolomitic, and fenestral. Interbeds of peloidal and intraclastic grainstone and packstone are present.
The ARCO/Amoco 1 John Goodson drilled in Bibb County, Ala., had a slight gas show at the base of the Knox above the Helena thrust fault. Underlying Pennsylvanian coal bearing rocks probably were the source of the gas that was trapped in fractures in the overlying Knox.
Knox production has also been reported from the Black Warrior basin in Fairview field in Lamar County, Ala. The 4 S.H. Gilmer has produced about 489 MMcf of gas and 143 bbl of oil from 4,352-4,424 ft since 1976. Although initially reported as Knox production, this gas is actually coming from Middle Ordovician carbonates.
Oil and gas production has also been reported from the Knox in the Black Warrior basin in Mississippi. New Hope field in Monroe County was discovered in 1954 and initially produced 190 b/d of oil.9
Production was from a zone of vuggy dolomite called the "Snow zone" after the discovery well. Subsequent drilling indicated the structure to be a low relief anticline. The well was completed in September 1953 and abandoned in October 1954 after producing 7,813 bbl of oil.
In 1971, the Texaco Inc. 1 Clyde Q. Sheely in Oktibbeha County, Miss., produced gas from this same "Snow zone." The well produced 123.752 MMcf of gas from September 1971 to June 1973.10
Recent studies of conodonts in the Magnolia 1 Pierce well in Monroe County, Miss., indicate that the "Snow zone" dolomite is actually Middle Ordovician in age and is therefore not the top of the Knox group as originally thought.4 The Magnolia 1 Pierce was an offset to the discovery well in New Hope field. Mellen10 reports another vuggy porous zone at 7,130 ft in the Knox. Called the "Pierce zone," this zone is fractured chert, limestone, and dolomite that had shows of oil and gas.
This "Pierce zone" is widespread and has been reported from other wells in Monroe County. Originally thought to be in the Copper Ridge, conodont studies indicate the strata to probably be equivalent to the Longview limestone.
POROSITY, PERMEABILITY
To assess the reservoir potential of the Copper Ridge dolomite in the Appalachians of Alabama, Sternbach9 obtained accurate determinations of porosity and permeability from seven samples of core from shallow coreholes.
Porosity values for the Copper Ridge ranged from 0.8-6.6%. Permeability values were 0.1 md.
These values are conservative estimates of rock-matrix porosity and permeability as they do not take into account subsurface fracturing and local solution development. However, surface and subsurface karst development is present in the Knox in Alabama.
Productive porosity in the Arbuckle of Oklahoma is related in large part to karstification associated with unconformities and tectonic fracturing. The most significant unconformity known to be associated with the Knox group in Alabama is the post-Knox unconformity.
This unconformity has its greatest magnitude in the Western Valley and Ridge (west of the Helena fault), where hundreds of feet of stratigraphic relief developed in the Knox group truncating progressively older units from the northeast to the southwest.11
Paleokarst depressions in the Knox surface are locally filled by the Attalla chert conglomerate member of the Chickamauga limestone of Middle Ordovician age. The magnitude of the unconformity is 30-40 million years, from Upper Cambrian or Lower Ordovician into the Middle Ordovician (Chazyan to Blackriverian).
In the eastern Valley and Ridge southeast of the Helena fault, the unconformity is developed on the Newala limestone of late early Ordovician age. Here, the Odenville limestone member at the top of the Newala is exposed.
The basal Mosheim member of the Lenoir limestone of Middle Ordovician age overlies the Knox. The duration of the unconformity in this area is about 10 million years (Whiterockian).11
To the west in the southern part of the Black Warrior basin conodont studies indicate that the post-Knox unconformity is absent or of limited duration (Whiterockian is present).4
Units of the Knox that are recognized to be particularly susceptible to solution cavity development and sinkhole subsidence in outcrop include the Newala and Long-view limestones and the Chepultepec dolomite.12 13
The Newala is especially notorious for sinkhole development. Similar subsidence problems also occur in outcrops of the Longview, although the problem is not as severe. The Chepultepec dolomite is also susceptible to sinkhole development, especially the lower limestone unit.
SOURCE ROCK POTENTIAL
There have been only a few studies of the source rock potential in the Black Warrior basin, and most of these have been either proprietary or have been limited to strata Devonian or younger in age.
One limited study of Middle Ordovician to Pennsylvanian strata determined that the Mississippian Floyd shale and the Devonian Chattanooga shale have good potential for generating liquid hydrocarbons and that the Floyd shale is probably the principal source of all oil presently produced from Mississippian and younger strata in the basin.
The Floyd and Chattanooga were found to be organic rich, to contain oil-generating kerogen, and to be thermally mature.
Study of the Pottsville and Parkwood formations of Pennsylvanian and Mississippian ages in the Black Warrior basin found that they contain primarily gas generating kerogen and that they are incapable of generating commercial quantities of liquid hydrocarbons.
Other potential source rocks for the Knox in Alabama include the Knox itself and the underlying Conasauga formation, which is comprised of shales, limestone, and occasional dolomite. Unfortunately data are very limited on these two potential sources.
ALABAMA ACTIVITY
Recently five Knox tests have been permitted in Alabama.
The Meridian 1 Weyerhaeuser 2-3 has been permitted to 11,500 ft in northern Greene County, and the Hawkeye 1 Banks 8-1 has been permitted for 5,000 ft in northeastern Greene County. Drilling has not commenced on these two wells.
The ARCO 1 Cypress B-6 Unit 4-3 was drilled last year after an ARCO borehole found shows of free oil in Cambrian-Ordovician carbonates.
The Cypress is a directional hole supposedly drilled along a thrust fault with the Knox group in the hanging wall. The well was drilled to 6,320 ft, and production tubing was set. No other information is available at this time.
Victory Resources drilled the 1 Chandler 34-7 (Permit No. 7,700) just southwest of the ARCO well in Hale County.
The well reportedly perforated two Knox intervals from 4,110-62 ft and 4,185-4,260 ft. Total depth is 4,347 ft. Reportedly the well tested oil initially, and 247 bbl of oil was transported. The well is presently temporarily abandoned.
Meridian drilled the 1 Gulf States Paper 27-15 in Bibb County in 1989-90. The well reached TD of 8,665 ft, penetrating 565 ft of Knox in the fold and thrust belt.
This well had no shows and was converted to a disposal well in the Knox interval at 8,075-8,310 ft.
Bernard/Hickox Inc. is presently reentering the 1 J. Hagerman in Sumter County. The well originally reached a total depth of 10,322 ft in the Knox. Plans call for deepening the well to 14,800 ft.
Most of these wells have targeted Cambrian-Ordovician carbonates in thrust sheets of the fold and thrust belt buried beneath Coastal Plain sediments.
These targets are susceptible to fracturing and are in close proximity to source rocks of Devonian, Mississippian, and Pennsylvanian ages. The Meridian Weyerhaeuser well is in the basin proper and will test structures more like those of the Arkoma basin.
CONCLUSIONS
The Arbuckle group of the Arkoma, Ardmore, and Anadarko basins was essentially untested in 1986.
In these basins, shallower Pennsylvanian reservoirs were easy to reach and more economical to develop. The general consensus was that if a karstic reservoir was not present at the top of the Arbuckle group then there was no potential for oil and gas.
Today the story is different; production zones are being found throughout the Arbuckle group, and drilling has been as deep as 28,000 ft.
The Black Warrior basin is in a similar setting to the Arkoma, it is a foreland basin that has produced from multiple Mississippian and Pennsylvanian horizons at shallow depths. The Knox carbonate is present in a similar structural setting to that of the Arbuckle group at depths generally above 15,000 ft.
In addition, Alabama is even more fortunate in that the buried Appalachian fold and thrust belt along the southern boundary of the basin also provides additional Knox targets with great promise.
In this area Knox dolomites are fractured and folded and are juxtaposed by thrust faulting against Mississippian and Pennsylvanian rocks that are excellent sources of oil and gas.
In conclusion, very little is known at present about the oil and gas potential of the Knox group in the Black Warrior basin of Alabama and Mississippi because few wells have reached the Knox and of these only a handful have penetrated the entire Knox section.
Therefore, the Knox is essentially untested in the Black Warrior basin.
REFERENCES
- Kidd, J.T.. Pre-Mississipian subsurface stratigraphy of the Warrior basin in Alabama: Gulf coast Association of Geological Societies Transactions, Vol. 25, 1975, pp. 20-39.
- The Black Warrior basin, Chapter 16, in Sedimentary cover - North American craton: U.S., Vol. D-2 of The geology of North America: The Geological society of America, 1988, pp. 471-492.
- Neathery, T.L.. and Copeland, C.W., New information on the basement and Lower Paleozoic stratigraphy of north Alabama: Geological Society of America, Southeastern Section, Abstracts with Programs, 1983, p. 98.
- Alberstadt, Leonard, and Repetski, J.E., A Lower Ordovician sponge/algal facies in the southern U.S. and its counterparts elsewhere in North America: Palaios, Vol. 4, 1989, pp. 225-242.
- Mellen, F.F., Regional Paleozoic stratigraphy in Mississippi between Ouachita and Appalachian Mountains: Discussion: AAPG Bull., Vol. 56, 1972, pp. 2,457-58.
- Thomas, W.A.. Regional and Paleozoic stratigraphy in Mississippi between Ouachita and Appalachian Mountains: AAPG Bull., Vol. 56, 1972, pp. 81-106.
- McCaslin, J.C., Amoco scores gas discovery in Tennessee: OGJ, Vol. 81, No. 5, Jan. 31, 1983, pp. 183-184.
- OGJ, Upper Knox yields 100 b/d in Eastern Overthrust Belt: OGJ, Vol. 81, No. 25, June 20, 1983, p. 72.
- Sternbach, L.R., Carbonate facies and diagenesis of the Cambrian-Ordovician shelf and slope-margin (The Knox Group and Conasauga Formation), Appalachian fold belt, Alabama: Rensselaer Polytechnic Institute unpublished master's thesis, 1984, 165 p.
- Possible Ordovician carbonate reservoirs in Mississippi: AAPG Bull., Vol. 58, 1974, pp. 870-876.
- Roberson, K.E., The post-Knox unconformity and its relationship to bounding stratigraphy, Alabama Appalachians: University of Alabama unpublished master's thesis, 1988, 148 p.
- Warren, W.M., Sinkhole occurrence in western Shelby County, Ala.: Alabama Geological Survey Circular 101, 1976, 45 p.
- Osborne, W.E., personal communication.
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