COREHOLE TO EVALUATE COALBEDS IN MISSISSIPPI
Rudy E. Rogers, Karl W. Carlson
Mississippi State University
Starkville
The coalbed methane industry in the Warrior basin of Alabama has been a mainstay for the state's economy the past 10 years.
By yearend 1990 around 1,770 wells were producing, and 130 bcf of methane had been produced commercially in the area since 1980.1 The growth in number of permits to drill wells illustrates the development history (Fig. 1).
Much of the research and development that allowed the proliferation of the new process in other basins began in Alabama, where data were initially available because of surface and underground coal mining in the area the past 100 years.
But the Warrior basin activity has remained confined to the Alabama portion, although roughly one half of the basin is in northern Mississippi.
A corehole is to be spudded this month to evaluate coals near West Point, Miss., which are not believed to be the same seams as coals to the northeast in Monroe County.
MISSISSIPPI WARRIOR COALS
Coals have been known to exist in the Mississippi portion of the basin, but very little data exist for companies to use as a basis for committing investment capital for coalbed methane ventures there.
An active exploration and drilling program for conventional gas has existed in the Warrior basin of northeastern Mississippi since 1952.
These developments are evidenced by the 58 gas fields now producing, the 256 wells now producing, and the cumulative 460 bcf of gas produced before January 1990 from Pennsylvanian and Mississippian age sands in the state's portion of the basin. 2
As occurred in the San Juan basin for many years, drilling of these wells for conventional gas did not focus on the coals that were penetrated. Primarily, targets were seven sands of Mississippian age: Carter, Lewis, Sanders, Evans, Abernethy, Millerella, and Rea.
The Carter sand has been most prolific.
In fall 1989, the Mississippi Bureau of Geology published a report3 pinpointing coals in Clay County (Fig. 2). This area is 26 miles west of the Alabama line and 56 miles west of active Alabama coalbed methane projects.
The bureau's findings were based mostly on the study of well logs and some sample drill cuttings.
COAL LOG ANALYSIS
Supported by a grant from the Mississippi Mineral Resources Institute, the subject study was recently undertaken in which 48 old well logs from the Mississippi Warrior basin were evaluated with particular emphasis on Monroe County, adjacent to Alabama and 45 miles northwest of coalbed methane production.
Density, compensated neutron, caliper, and gamma ray logs were available for all of the wells.
The analysis indicated coal seams from Splunge field in Monroe County near the Alabama line and continuing 15 miles westward to Four Mile Creek field at the Tombigbee River across Township 12 South.
Maps of the extremities of the Black Creek and Mary Lee Coal groups in Alabama, which were truncated at the state line, suggest these two seams as present in Monroe County, Miss.'
The logs indicated multiple seams in northern Monroe County from the state line to the Tombigbee; the interval from 1,000-2,000 ft was generally logged in the conventional gas wells of the area.
Maximum thickness of an individual seam in any one well was approximately 11 ft; maximum cumulative thickness in any one well over the 1,000 ft interval was 30 ft. A distribution of individual seam thicknesses is given in Fig. 4.
The most prevalent depth of the seams was 1,600-1,800 ft.
The depth distribution is presented (Fig. 4).
GAS RESERVOIRS ADJACENT
Unlike the coalfields of Alabama but similar to other basins (San Juan, Powder River, Southeast Kansas), gas reservoirs below the coals were indicated in Mississippi, suggesting the coal as a source rock.
The two representative logs that illustrate the occurrence of conventional gas reservoirs near coal seams in Monroe County are shown (Figs. 5, 6).
Some substantiation is offered by a 1979 report on the evaluation of drilling cuttings from 37 wells in Monroe and Itawamba counties.5 The most abundant coal shows from the cuttings, described as blocky coals, were reported across Monroe County in Townships 12 South and 13 South.
A corehole is to be spudded in December 1991 in Clay County, Miss., financed with matching funds by the Mississippi Energy and Transportation Division of the Department of Economic and Community Development to further investigate the coals near West Point reported by the Bureau of Geology.
It is expected that the Monroe County coals are not the same seams and that the West Point data do not apply to the coals to the northeast; a major fault, the West Point fault, extends between the two.
SUMMARY
Evaluation of the half of the Warrior basin in Mississippi for coalbed methane is only beginning; very little data exist.
Numerous successful, conventional gas fields in the area provide logs that indicate the presence of coal seams.
Study of logs in Monroe County shows the likelihood of seams, possibly an extension of those in Alabama, at propitious depths and near gas reservoirs probably charged from the coals.
Individual seam thicknesses and cumulative seam thicknesses warrant further study, particularly coreholes to evaluate gas content and permeability.
ACKNOWLEDGMENTS
The authors thank the Mississippi Mineral Resources Institute for sponsoring the study. Appreciation goes to L. Smart for her assistance in collecting and analyzing logs as well as for the support of the Mississippi State University's Department of Petroleum Engineering.
REFERENCES
- Quarterly Review of Methane from Coal Seams Technology, Gas Research Institute, Vol. 8, No. 4, July 1991, p. 7.
- Rogers, R.E., and Zorbalas, K., Modeling Mississippi Oil Production, MMRI Grant 91-8F & USBM Grant G1104128, Final Report, Aug. 27, 1991, p. 30.
- Henderson, K.S., and Gazzier, C.A., Preliminary Evaluation of Coal and Coalbed Gas Resource Potential of Western Clay County, Miss., Mississippi Department of Natural Resources, Bureau of Geology, Report of Investigations 1, 1989, p. 31.
- Wicks, D.E., McFall, K.S., and Zuber, M.D., An Analysis of the Coalbed Methane Resources of the Warrior Basin, Alabama, Proceedings of the 1987 Coalbed Methane Symposium, Tuscaloosa, Ala., Nov. 16-19, 1987, p. 139.
- Bicker, A.R. Jr., Identification of Coal Seams in the Subsurface of Itawamba and Monroe Counties, Report of Investigations 79-5, MMRI Grant 79-8F, Final Report, 1979, p. 9.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.