ARCO-DAVIS WILDCAT SEEKS ANSWERS TO BLACK WARRIOR'S DEEP MYSTERIES

Rick L. Ericksen
Office of Geology
Jackson, Miss.

The Paleozoic Black Warrior basin is located in northeastern Mississippi and northwestern Alabama. At the southwestern edge of the basin are the locations of two of the more significant, deep wells.

Located in 9-22n-10e is the ARCO 1 Bobbie-Ruth-Aycock-Senter, Calhoun County, Miss. This well was a rank wildcat drilled to test the potential of the Paleozoic section, and in particular the deep Cambro-Ordovician strata that are equivalent in age and lithology to the Arbuckle.

The ARCO well was an attempt to extend the successes of the prolific Arbuckle at Wilburton field in the Arkoma basin of Oklahoma. Studies that have been conducted on Arbuckle conventional cores and logs in Wilburton field show that the Arbuckle reservoir is generally comprised of vugs, some of which are several inches across, and solution-enlarged fractures in a clean dolostone.1

The development of vugular and solution-enlarged porosity in the Arbuckle of Oklahoma is related in large part to the karstification associated with unconformities and tectonic fracturing.

MISSISSIPPI EXPLORATION

The ARCO 1 Bobbie et al. well, originally projected for a total depth of 20,000 ft, was spudded Apr. 14, 1992, and reached a total depth of 19,200 ft. Total depth was reached in a sidetracked hole, with 13 3/8 in. casing set at 2,706 ft, 9 5/8 in. casing set at 11,237 ft, and 7 in. liner run to 19,198 ft.,

Perforations at 18,120-19,068 ft overall reportedly tested noncommercial. The well was next perforated at 15,267-17,940 ft overall with no details released.

The well was then block squeezed at 13,200-202 ft and 13,650-652 ft and perforated for additional testing at 13,406-521 ft, also with no details released afterward.

Unconfirmed reports indicate the well had shows in the Devonian, the Mississippian age Lewis sandstone, and in an undifferentiated Pennsylvanian sandstone, all of which were not reported to have been tested. The well was later plugged and abandoned on Feb. 9, 1993.

Operations for the second ARCO proposed rank wildcat have been assumed by Davis Petroleum Corp New Orleans. That test well, the 1 ARCO-Williams 9-6, is located in 9-19n-15e, Oktibbeha County, Miss. (Fig. 1).

The Williams 9-6, spudded July 12, 1993, is proposed as a 17,000 ft Lower Knox test.

OKTIBBEHA DRILLING

ARCO at the Mississippi Oil & Gas Board's July meeting presented a structure map contoured on Cambro-Ordovician dolomite (Fig. 2). The location of the 1 ARCO-Williams 9-6 test well is on the upthrown side of a faulted, anticlinal feature.

Per ARCO's exhibit, the trapping fault has approximately 4,200 ft of vertical displacement (down to the northeast), as well as an undisclosed amount of left lateral, strike-slip movement.

Significant gas shows were encountered in the downdip McAlester Fuel Co. A-1 W. P. Suddeth well. That well, spudded in 1958, is in 6-19n-15e, Oktibbeha County, about 2 1/3 miles west-northwest of the Davis 1 ARCO-Williams 9-6.

The Suddeth well encountered and tested oil and gas shows throughout the Pennsylvanian, Mississippian, Devonian, Silurian, and Cambro-Ordovician sections. After extensive testing and subsequent failure to establish commercial production, McAlester plugged the well.

H. Richey reentered the well in October 1963, and Cleary Petroleum Corp. reentered it in October 1977. Both failed to establish commercial production but did test gas from numerous zones in Pennsylvanian and upper portion of the Mississippian.

GEOLOGIC SETTING

The Black Warrior basin has been described as a "triangular-shaped basin," bounded to the north by the Nashville dome and Ozark uplift, to the east-southeast by the southwest plunging Appalachian Mountains, and to the southwest by the buried Ouachita front (Fig. 1).

On the northwest, the basin is separated from the Arkoma basin by a southwestward plunging, Paleozoic arch and fault system termed the Mississippi Valley graben.3

Simply put, the Black Warrior basin is classified as a foreland basin with relatively undisturbed, southwest regionally dipping Paleozoic strata.

Faulting trends generally northwest-southeast, with the larger displacement, normal faults being down-to-the-southwest, with smaller displacement, normal, down-to-the-northeast antithetic faults. The age of the faulting is not known, but some may be as young as earliest Middle Pennsylvanian.1

It is along the down-to-the-northeast faults where most of the hydrocarbon production, which is Mississippian age, has been found to date in the Black Warrior. The Paleozoic strata in Mississippi are truncated by a pre-Mesozoic unconformity that regionally dips gently to the southwest.

Unconformably overlying the Paleozoic beds are strata of Cretaceous age. As the result, all that is known of the Paleozoic strata has been derived from sparse control derived from geophysical means, including seismic and petrophysical well logs, as well as drillers logs and well sample cuttings.

The Paleozoic rocks of the Black Warrior basin range in age from Cambrian to Pennsylvanian and form a southwesterly thickening wedge of sediments in excess of 18,000 ft thick. The Cambrian rocks rest unconformably on the Precambrian crystalline basement rocks.4 5

Henderson6 noted that the greatest known thickness of Cambro-Ordovician strata penetrated in the subsurface was 7,950 ft, which was encountered by the Pruet & Hughes 1 Dunlap in 18 7s 1w, Lafayette County. It appears that the Cambro-Ordovician strata consist predominantly of shallow, open marine, and shelf carbonates deposited along the southern margin of the proto North American continent.6 These shallow, marine carbonates were deposited in essentially stable areas. However, very small changes in sea level effected widespread transgressions and regressions on these stable, carbonate shelves.

During the periods of repeated regressions there were extensive periods of karstification, which have been interpreted to be represented by zones of high porosity in the strata.7 Only two wells have penetrated the entire Cambro-Ordovician section of Mississippi,6 and therefore the amount of subsurface control is very limited.

HYDROCARBON POTENTIAL

Over the years the potentially productive strata of the Cambrian, Ordovician, Silurian, and Devonian have been marginally explored. With noted exceptions, production of commercially significant volumes of hydrocarbons from these strata has yet to occur.

Most of the commercially significant oil and gas production has been produced from Mississippian sands. Overall, the bulk of hydrocarbon production is natural gas, and most of that production has been from the Carter sandstone. Of less productive significance are the Sanders, Lewis, Millerella, Evans, and Rea sandstones, and the Pennsylvanian age Pottsville formation sands.

There have been several areas from which the Devonian has produced both oil and gas. However, production was not sustainable nor has it been in large enough quantities to be considered commercially significant.

Several attempts at Devonian completions in both Alabama and Mississippi (e.g. McKinley Creek field, Monroe County, Miss., and Mt. Zion field, Lamar County, Ala.) have produced oil and gas in noncommercial quantities.

The Silurian strata have had numerous shows. The first reported production from the Silurian was in 1953 when L.E. Salmon drilled the 1 Patterson in 21-11s-1e, southwestern Pontotoc County, Miss.8

The Patterson well tested a sandstone interval of the same name that flowed a small volume of natural gas, but the well was deemed noncommercial and was plugged and abandoned. The Patterson sandstone has been described as a medium-gray to brownish calcareous sandstone.

In addition to the Patterson sandstone facies, the Silurian also contains some coarsely crystalline, crinoidal limestone beds. Their presence suggests that "reefal" reservoirs may eventually be encountered that would be prone to hydrocarbon accumulations.

KNOX HYDROCARBONS

Cambro-Ordovician Knox production was first established in Mississippi in 1953 from the Magnolia Petroleum Co. RA J.B. Snow well in 27-13s-7e, Monroe County.

This well, the discovery well for New Hope field, produced 7,834 bbl of 35 gravity oil before going to water and being abandoned on Oct. 23, 1954. Interestingly, more than 5,000 bbl of this oil were produced within the first two to three weeks after the well was placed on production.

Several offset wells have been drilled in the New Hope field area, which is a low-relief anticline, but none was completed as a commercial producer. All of the production from the Knox (Stones River dolostone of Raymond9) has been from a vuggy dolomite named the "Snow zone" after the fee name of the discovery well at New Hope.

In 1971, Texaco Inc. drilled the 1 Clyde Q. Sheely, discovery well for Maben field. The Steely well, in 28 19n-12e, Oktibbeha County, Miss., has produced gas from the equivalent to the "Snow zone."

The Sheely was completed from a series of perforations at 14,690-15,036 ft and had an initial potential of 1.959 MMcfd of gas. The cumulative production from the well from September 1971 to January 1993 was 925.563 MMcf.

It should be noted that Maben field is about 2 1/2 miles south of the current Davis ARCO wildcat.

Significantly, the Cambro-Ordovician section has numerous oil and gas shows, reservoir quality strata, and the apparent existence of large, undrilled structures to serve as traps for hydrocarbon accumulation.

To date more than 30 test wells in 19 counties in the Black Warrior basin of Mississippi have had documented oil and gas shows.6

CURRENT EXPLORATION

ARCO's initial attempt at establishing commercial production from the Cambro-Ordovician section with its 1 Bobbie et al. test well resulted in shows at least significant enough to run production casing and test.

As discussed, unconfirmed reports indicate that shows were encountered in the Cambro-Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian.

In regard to the petrophysical interpretation of the Mississippian and Pennsylvanian strata, it would be informative to note that based on this writer's experience the following petrophysical log parameters must generally be met in reference to a potentially productive reservoir:

Cross-plotted primary porosities of 8% or greater;
Separation between the compensated neutron and litho-density tool readings, i.e. "gas effect," of approximately three log chart divisions (3% per division), assuming gas production, and
Positive, low resistivity (1-40 ohms) microlog separation, which qualitatively infers sufficient permeability (i.e. mudcake buildup) for commercial production. 10

If these conditions are not met, even though zones may carry mud log shows, it is historically unlikely that any sustained, commercial production will be realized.

'LONGBRANCH FIELD'

As previously referenced, Fig. 2 is a copy of the structure map contoured on top of the Cambro-Ordovician dolomite that ARCO supplied to the board during the July 1993 hearing on the Davis 1 ARCO-Williams well.

ARCO, in an unprecedented move, petitioned the board to establish the field name, geographical field limits, well spacing/production unit size, and definition of productive intervals within the area of the Davis 1 ARCO-Williams well before the completion of a well capable of commercial production has been drilled and successfully completed.

The board approved the petition, which also included reformation of the previously spudded 1 ARCO-Williams well from a 640 acre drilling/production unit to a 1,280 acre unit. The unit consists of all of 4- and 9-19n-15w and integrates all interests in the proposed reformed unit.

At this writing in mid-August well details were being held confidential. It is anticipated that total depth may be reached as early as October.

Based on the downdip oil and gas shows and the structural interpretation ARCO presented, this rank wildcat holds great promise for establishing commercial production from the Cambro-Ordovician section as well as from the shallower, more traditional producing horizons. This is particularly true for the Mississippian and Pennsylvanian sections that tested noncommercial quantities of gas from multiple zones in the McAlester well.

REFERENCES

Raymond, D., The Knox Play, in Cambro-Ordovician "Knox" of Mississippi and Alabama, 1991, pp. 6-26.
Southeastern Oil Review, July 5, 1993, Vol. 56, No. 15, p. 8.
Thomas, W.A., The Black Warrior Basin, Chapter 16, in Sedimentary cover-North American craton: U.S., Vol. D-2 of The geology of North America: Geological Society of America, 1988, pp. 471-492.
Kidd, J.T., Pre-Mississippian subsurface stratigraphy of the Warrior basin in Alabama: GCAGS Transactions, Vol. 25, 1975, pp. 20-39.
Neathry, T.L., and Copeland, C.W., New information on the basement and Lower Paleozoic stratigraphy of north Alabama, GSA Southeastern Section, abstracts with programs, 1983, p. 98.
Henderson, K.S., Cambro-Ordovician subsurface stratigraphy of the Black Warrior Basin in Mississippi, Mississippi Office of Geology, Report of Investigations 2, 1991, pp. 1-51.
Henderson, K.S., and Knox, S.C., Hydrocarbon potential and subsurface stratigraphy of the Cambro-Ordovician sequence, Black Warrior basin of Mississippi, in Cambro-Ordovician "Knox" of Mississippi and Alabama, 1991, pp. 27-30.
Melen, F.F., Patterson Sandstone, Ordovician or Silurian?, AAPG Bull., Vol. 58, No. 1, 1974, pp. 143-148.
Raymond, D.E., and Osborne, W.E., Stratigraphy of the Knox Group in Alabama in Cambro-Ordovician "Knox" of Mississippi and Alabama, 1991, pp. 1-5.
Streets, E., Schlumberger senior sales representative, personal communication, 1993.