MUCH MOPE GAS REMAINS TO BE DISCOVERED IN MISSISSIPPI'S BLACK WARRIOR BASIN, STUDY SAYS
Konstantinos Zorbalas, Rudy E. Rogers
Mississippi State University
Starkville, Miss.
Since 1952 drilling has been active in the Warrior basin of Alabama and Mississippi.
Concentrating primarily in the relatively shallow Mississippian sandstones, the exploration has resulted in discovery of 58 fields in the Mississippi portion of the basin.
In a study funded by the Mississippi Mineral Resources Institute, an assessment was made of the gas resources in the Mississippi part of the Black Warrior basin in the sands of Pennsylvanian and Mississippian systems in an area outlined in Fig. 1.
The study was to assess the past performance of the basin, but more importantly it was to forecast the level and quality of activity to be expected in the future.
Historical production data were analyzed, an estimate was made of natural gas volumes left to be produced from already producing fields, and an appropriate discovery model was used to forecast the number and size of fields yet to be discovered in the Pennsylvanian and Mississippian sediments of this partially explored basin.
Such a detailed analysis has heretofore not been available, especially to independent companies.
Besides longtime gas production from Mississippian formations, interest in two other la s in the area has also developed this year, namely, coalbed methane in the Pennsylvanian strata as well as natural gas in the deep Knox trend.
The proximity of each to conventional gas production can be seen in Fig. 1, where a recent corehole for coalbed methane was drilled,' and an
ARCO Oil & Gas Co. well targeted for the Knox at a total depth of 20,000 ft is under way.2
The ARCO well is considered a test of the extension of the successes in the Arbuckle of Oklahoma.
CORRELATIONS OF HISTORIC DATA
Workers listed the ultimate reserves of gas in fields discovered chronologically in the Mississippi Warrior basin, beginning with Muldon field in 1952 (Fig. 2).
Corinne field, by far the largest field in the study area, was discovered in 1972. Periods of higher gas prices in the 1970s and early 1980s are evident in the discoveries made in the basin during those times.
A survey of the field sizes of past discoveries in the Mississippi portion of the basin revealed that they may be naturally divided into nine classes, based on ultimate reserves of each field.
Also, an average of all the calculated areal extents of fields of each class-size yielded the average target size for that class (Table 1). These data were used in later calculations.
Decline curve analysis of each of the 256 wells still producing indicated that 121 bcf of natural gas remain to be produced, and 581 bcf of cumulative production has been realized from those wells in the study area. A summary of the analyses of each of the sands is given (Table 2).
Regarding the total number of fields of each class size that have been discovered in the Mississippi Warrior basin (Fig. 3), the distribution skewed toward the larger fields because of economic truncation of many of the smaller fields encountered.
The economic truncation that involves the smaller fields in the basin is evident for class sizes four and smaller.
Another representation of the size variation of past discoveries is the log-normal distribution (Fig. 4).
PREDICTIVE DISCOVERY MODEL
What are future prospects of discovery, in regard to number of fields and field sizes, in this designated area of the Warrior basin?
To answer the question, the discovery model developed by Drew and Schuenemeyer3 was selected as the best for a partially explored basin, verified from applications in two other basins.
Their discovery model derives from Arps and Roberts 4 who in 1958 developed one of the first in the oil industry to forecast the number of future discoveries in an exploratory region.
In the early model the cumulative number of future discoveries declines according to a negative exponential; a discovery efficiency was introduced and related to the expected discovery rates, assuming the same discovery efficiency across class sizes.
In their original study of the Denver basin, Arps and Roberts estimated subjectively the area of the basin where the reservoir sand bodies existed.
Drew and coworkers in 1980 modified this discovery-process model by using the area actually condemned by exploratory wells, with respect to a given size of target pool, as a measure of exploratory effort. In addition, their studies of the Denver basin 3 and the Gulf of Mexico 5 which verified the model's accuracy, proved that each class of deposits has a different discovery efficiency. In Drew's model, the decline of petroleum deposits is a consequence of the progressive crowding of wells, and in the analysis an area of influence of an exploratory hole was introduced.'
For the Warrior basin, an effective basin area, target and class size of the fields, a discovery efficiency, and the area of influence of an exploratory hole were first determined, relying on historic data, and then applied in the Drew/Schuenemeyer model to predict the number and size of fields remaining to be discovered.
A full discussion of the application of the model to the Warrior basin is given in the work of Zorbalas.'
SIZE, NUMBER OF FIELDS
Results of the discovery process model indicate the number of fields of class size six having a midpoint of 1.28-5.12 bcf remaining to be discovered in the partially explored Mississippi Warrior basin for increments of 50 exploratory wells drilled (Fig. 5).
The smaller fields of class size five have a midpoint of .32-1.28 bcf to be discovered with each increment of 50 wildcat wells drilled in the future (Fig. 6). The first 456 exploratory and development wells denote historical data for 17 fields of class size five actually discovered and developed.
The historic trends of the cumulative number of discoveries for the class-sizes five and six are continued by the calculated projections, lending credence to the discovery-process model.
The impact of future discoveries is more readily seen by superposing the predicted discoveries in each class size over the past discoveries (Fig. 7).
Note a significant number of discoveries predicted in class sizes five and six; 27 fields of class five and 12 fields of class six should be eventually discovered.
The fields to be discovered in class-size five and higher are not affected by economic truncation, and they represent the most profitable potential of the basin, providing the economic incentive for future exploration.
There should be five fields of size class seven and one of size class eight yet to be discovered in the study area.
No fields of class size nine, the size of Corinne field of 192 bcf, would remain in the study area, which follows the axiom that large fields tend to be discovered early in the exploration history of a basin.
The small fields of class sizes one and two could not be economically developed with near-term gas prices.
SUMMARY
The Black Warrior basin, including the study area of about 3,500 sq miles in Mississippi, has a variety of targets for explorationists, not the least of which is conventional gas from relatively shallow Mississippian and Pennsylvanian sandstones.
An assessment of past discoveries in North Mississippi provided a field size distribution, cumulative production from the fields, and remaining reserves.
The chronological pattern of their discoveries was established, and the economic truncation of smaller class size fields noted.
The historical data were then used in a predictive discovery model to estimate the number and size of fields left to be discovered in the area.
Forty-five fields in the Mississippi Warrior basin are forecast to be discovered in class sizes five through eight-sizes profitable for development.
The basin remains attractive for exploration.
ACKNOWLEDGMENTS
The authors thank the Mississippi Mineral Resources Institute at the University of Mississippi for sponsoring the study, the Mississippi State University's Department of Petroleum Engineering for support, and J.H. Schuenemeyer at the University of Delaware for his advice.
REFERENCES
- Rogers, R.E., and Carlson, K.W., Corehole to Evaluate Coalbeds in Mississippi, OGJ, Dec. 9, 1991, p. 71.
- ARCO Schedules Rank 20,000 Foot Wildcat for Calhoun County in Northeast Mississippi, Southeastern Oil Review, Jackson, Miss., Mar. 23, 1992.
- Drew, L.J., Schuenemeyer, J.H., Root, D.H., and Attanasi, E.D., Petroleum-Resource Appraisal and Discovery Rate Forecasting in Partially Explored Regions, U.S. Geological Survey Professional Paper 1138-A, B, and C, U.S. Government Printing Office, Washington, D.C., 1980, pp. A-10, B-9, C-19.
- Arps, J.J., and Roberts, R.G., Economics of Drilling for Cretaceous Oil on East Flank of Denver-Julesburg Basin, AAPG Bull., Vol. 42, No. 11, 1958, pp. 2,549-66.
- Drew, L.J., Schuenemeyer, J.H., and Bawiec, W.J., Estimation of the Future Rates of Oil and Gas Discoveries in the Western Gulf of Mexico, U.S. Geological Survey Professional Paper 1252, U.S. Government Printing Office, Washington, D.C., 1982, p. 26.
- Singer, D.A., and Drew, L.J., The Area of Influence of an Exploratory Hole, Economic Geology, Vol. 71, No. 3, 1976, pp. 642-647.
- Zorbalas, K.I., Resource Assessment and Discovery Rate Forecasting for the Mississippi Warrior Basin, M.S. thesis, Department of Petroleum Engineering, Mississippi State University, 1992, p. 68.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.