Well data, remote sensing point to potential in Ouachitas

John H. Gray
Firebird Inc.
El Dorado, Ark.
Arthur J. Pyron
Pyron Consulting
Pottstown, Pa.

The Ouachita Mountains are the exposed section of a larger thrust regime that extends from Mississippi to the Marathon basin of Texas (Fig. 1).

The Ouachitas in Arkan- sas and Oklahoma form a geographical and structural break between the older (i.e., Paleozoic) rocks of the Midcontinent and the younger rocks of the Gulf Coast (Mesozoic and Cenozoic).

Analogs to production

Significant hydrocarbon production is found on both sides of this break, yet very little production has been found in the thrust trend itself. This is in part due to drilling difficulty that results from complex geology.

Numerous seeps and shows of hydrocarbons (grahamite, asphaltite, gilsonite, etc.) have been documented by workers in surface geology since the turn of the century. In Oklahoma, particularly, these shows have been tested by the drill or by mining adits with mixed results.

The average depth per well is 1,700 ft, but this figure is skewed because of three deep fields that have production from reservoirs at depths in excess of 3,000 ft; omitting those three, the average depth of reservoir per field is less than 1,000 ft.

Pays in this area include the Jackfork sandstone members of the Stanley shale and fractures in the Bigfork chert and Arkansas novaculite. The average production is in the low tens of thousands of barrels per well, biased by Isom Springs field production. The existence of production in Ouachita facies rocks is very encouraging.

To date no production has been found in the Arkansas portion of the Ouachitas, but this is in part due to the limited number of viable test wells and the even smaller number of wells that have tested the pre-Mississippian strata. This is the importance of the OXY well, because it is considered by most observers to be the first significant test of the pre-Mississippian section.

Isom Springs field (Fig. 2 [71846 bytes]) is the most significant productive field in the Ouachita facies. Morrison¹⁰ noted estimates of primary reserves in 1980 of 285,000-450,000 bbl per well per zone. He estimated ultimate recovery would be 15 million bbl; however, cumulative production since discovery in 1977 was about 4.2 million bbl through Dec. 31, 1994. Fractured Arkansas novaculite produces at 2,500-3,000 ft. Initial potentials ranged between 200-400 b/d.

While most workers associate Isom Springs field with the Ardmore basin, the authors believe the subsurface character of the field is typical of what future oil and natural gas fields in the Ouachita province will look like.

The discovery of Potato Hills field established the first significant production of natural gas in Ouachita facies rocks. Production is found at a depth of 3,000 ft from fractured Bigfork chert. Average production was 300 Mcfd/ well. Two wells produced 1.2 bcf of gas during 1960-80.

The significance of the field is that production is associated with the Bigfork chert and the Arkansas novaculite, a formation with no primary porosity. The presence of hydrocarbons in fractured chert suggests that secondary recrystallization and tectonic readjustment of reservoir rocks did not affect hydrocarbon accumulation. Potato Hills field is representative of natural gas reservoirs in the Ouachita Province.

The Marathon basin (or more properly, the Marathon-Ouachita thrust) of Pecos County, Tex., is correlative to the Ouachita Province. Subsurface mapping suggests that the thrust belt extends from the Black Warrior basin of Alabama to Pecos County, Tex. In recent years operators in Texas have been successful in establishing production in the Lower Ordovician Ellenburger formation. The Ellenburger reservoir is age equivalent to the Arbuckle group and the Knox group.

Trabelsi¹¹ indicates that porosity in the productive Ellenburger consists of interstitial porosity in dolomites (on average 7%) and fracture porosity in dolomites associated with tectonic activity. Permeability in reservoir rocks has been enhanced by microfracturing associated with the tectonic activity. In addition, subsurface rocks have indications of paleokarst development in the Ellenburger, which contributes to porosity formation, especially as associated with brecciation.

Finally, throughout the region production has been established on trend in the Knox group (Lower Ordovician) in Kentucky and Alabama and in the Arbuckle in Kansas and Oklahoma. The Knox is correlative to the Arbuckle and may be another indication of the potential of Arbuckle equivalent rocks in the Ouachita Province.

Petrophysical analysis

Well log analysis for the OXY Danville well reveals some enticing potential in the pre-Mississippian section.

While complete analysis of the well log involves information not released publicly, some generalizations can be derived from a cursory petrophysical evaluation. In this regard, the authors have chosen to discuss an interval (Fig. 3 [27776 bytes]) near the top of the Everton (Arbuckle equivalent).

In the example, the Everton is located at about 18,500 ft. Immediately beneath the picked top there is a large gas cross-over event that is suggestive of a natural gas reservoir. Gas cross-over occurs when neutron porosity, which normally reads higher porosity, is lower than density porosity, which normally reads lower porosity. This phenomenon occurs when open pore space in the formation (reservoir) is filled with natural gas instead of formation water or liquid hydrocarbons. Gas cross-over occurs throughout the pre-Mississippian section, especially in Ordovician age rocks.

Review of the caliper log suggests rugosity in the borehole above norm. Given that the formation is a competent limestone or dolomite, this may be suggestive of fractured or brecciated rock. Karstification of Arbuckle age rocks (i.e., Ellenburger) mentioned above would manifest itself in the upper 100 ft of a formation, because this is the section that would be exposed to surface erosional events. The released minilog for this section shows small, intense peaks throughout the interval, which are suggestive of fractures. Therefore this interval is probably a gas filled fractured carbonate interval.

Petrophysical evaluation of fractured intervals is difficult without core data or mud log gas chromatographic support. The gas cross-over effect in combination with the fractured interval makes porosity determination almost impossible. Traditional methods of interpretation of porosity fall short because the media are essentially nonporous except for the fractures themselves. The average density porosity based on the density log is 7.5%, and this value may be typical for the interval.

Calculation of water saturation is also difficult because there are no published formation water resistivity values for the Everton in this province. If the analogy to the Ellenburger of Pecos County, Tex., is correct, then published formation water resistivity for this formation ranges between 0.1 and 0.3 ohm-m at 75° F. Given this information, the authors calculated water saturation ranging between 11-40% for the interval. This water saturation is consistent with a gas filled interval. At any rate, the interval would be prospective, especially for additional testing. Unfortunately, pressure testing of the interval was not published, so no gross estimation of reserves could be made.

References

1. Pyron, A.J., and Gray, J.H., The initial data indicate good chance for Ouachita production, World Oil, September 1986, pp. 62-65.

2. Lille, R.J., Nelson, K.D., deVoogd, B., Brewer, J.A., Oliver, J.E., Brown, L.D., Kaufman, S., and Viele, G.W., Crustal structure of the Ouachita mountains, Ark.: A model based on integration of Cocorp reflection profiles and regional geophysical data, AAPG Bull., Vol. 67, 1983, pp. 901-931.

3. Keller, G.R., Braille, L.W., McMechan, G.A., Thomas, W.A., Harder, S.H., Chang, W.F., and Jardine, W.G., Crustal structure of the Ouachita orogenic belt determined by a Passcal wide angle reflection/refraction experiment, preprint paper, 1988, 17 p.

4. Pyron, A.J., and Feder, A.M., Remote sensing for the petroleum industry, Houston Geol. Soc. Bull., November 1987.

5. Pyron, A.J., and Feder, A.M., Practical application of Landsat imagery in hydrocarbon exploration, Houston Geol. Soc. Bull., December 1987.6. Pyron, A.J., Landsat indications of hydrocarbons in the Arkansas Ouachitas: Analogs on trend with productive areas in Oklahoma, Sipes National Convention, Shreveport, La., 1987.

7. Gray, J.H., and Pyron, A.J., Oil and gas prospects in the Arkansas Ouachitas, abstract and poster session, SEPM Foreland Basin Symposium, Fribourg, Switzerland, 1985.

8. Davidson, M.J., Toward a general theory of vertical migration, OGJ, June 21, 1982, p. 288.

9. Suneson, N.H., Brown, D.P., and Mycek-Memoli, A.C., Update on Ouachita mountains frontal belt exploration and development, Oklahoma Geology Notes, Oklahoma Geological Survey, Vol. 51, No. 3, 1991, pp. 84-97.

10. Morrison, L.S., Oil in the fascinating Ouachitas, OGJ, May 11, 1981, p. 170.

11. Trabelsi, A.S., Source, reservoir promise seen in Marathon-Ouachita Overthrust, OGJ, Sept. 26, 1994, pp. 109-112.

Bibliography

Bennison, A.P., and Johnson, N.L., Figure on the Potato Hills area, in Ouachita Symposium, Dallas and Ardmore Geol. Socs., 1959.

Collins, D.S., and Bohm, R.A., Description of insoluble residues from the T.P. Russell 1 Strake drill hole and other drill holes in Southeast Missouri, USGS Open File Report 93-291, 1993.

Fay, R.O., Oil in the Ouachita Mountains of Oklahoma, Guidebook to the 1976 field trip, GCAGS, 1976, pp. 91-105.

Gatewood, L.E., and Fay, R.O., Untapped potential from Mexico to Mississippi: Surprises of the Ellenburger-Arbuckle-Knox trend, OGJ, Oct. 19, 1992, pp. 93-96.

Gray, J.H., and Pyron, A.J., Oil and gas prospects in the Arkansas Ouachitas, AAPG Bull. abstract, Vol. 68, No. 9, 1984.

The Ouachita System: Oil and gas developments along the Overthrust trend, Petroleum Frontiers, Petroleum Information Corp., Spring 1985.

Pyron, A.J., Lowering the cost of exploration for independents: How remotely sensed data aid in the search for oil and gas, ERIM International Conference, Houston, May 15, 1988.

Pyron, A.J., Use of space based detectors as a strategic tool to evaluate the hydrocarbon potential of the U.S., NASA visiting investigator program, John C. Stennis Space Center, Miss., 1993.

Suneson, N.H., and Campbell, J.A., Ouachitas need more exploratory drilling, Parts 1 and 2, OGJ, Apr. 9-16, 1990.

Suneson, N.H., and Grasmick, M.K., Oil and gas wells, Ouachita mountains, Okla., Oklahoma Geology Notes, Oklahoma Geological Survey, Vol. 49, No. 5, 1989, pp. 152-183.

The Authors

John G. Gray is principal of Fire Bird Inc., a multinational oil and gas company, and Information Galore Inc., a company that provides information access and education. He has 48 years' domestic and international experience with multinational oil companies. He was involved with the United Nations as director of Unitar in Rome. He is an Arkansas Geological Commissioner and a special commissioner of the Arkansas Oil & Gas Commission. He has been associated with discovery and development of a number of oil fields, including Diamond M Canyon reef field. He has a BS in geology from Texas School of Mines.
Arthur J. Pyron is sole proprietor of Pyron Consulting. He has 17 years of professional experience in oil and gas exploration and appraisals, minerals investigations, environmental investigations and site remediation. He has an MS in geology from the University of Texas at El Paso.