Philip L. Cook Jr.
Cook Energy Inc.
Jackson, Miss.
Robert D. Schneeflock
John D. Bush
John C. Marble
Paramount Petroleum Co.
Jackson, Miss.
The Upper Cretaceous Eutaw formation of Mississippi has produced almost 2 tcf of gas since its initial discovery at Gwinville field in 1944.
Prior to Trimble field, the last major Eutaw gas discovery in the state was Maxie-Pistol Ridge field in 1951.
Consequently, the Trimble discovery is the most important shallow gas find in the Interior Salt basin in nearly 40 years. Trimble field will likely develop into at least 20 wells with combined reserves in excess of 100 bcf of gas and a production rate of 40 MMcfd.
The Trimble discovery was made on a faulted structural closure drilled twice before. Both operators drilled excellent locations but failed to detect low resistivity pay.
The discovery of gas on the third attempt by Cobra Oil & Gas Corp. and Paramount Petroleum Co. was facilitated by the 1987 accidental discovery of Eutaw gas at Gitano field, where a flow test of 2 MMcfd from an Environmental Protection Agency permitted salt water disposal well proved a new gas pay zone for the field.
A petrophysical examination of the Gitano discovery and its implications to the "dry holes" at Trimble has initiated intensive industry activity in a renewed Eutaw play in Mississippi.
FIELD HISTORY
Gas was discovered at Trimble in December 1988 with the completion of the Cobra and Paramount 1 Purser, 17-10n-16w, Smith County, Miss. The 1 Purser encountered 51 ft of net pay and tested 2.759 MMcfd of gas, 16.8 b/d of condensate, and 2 b/d of water from the Stanley sand member of the Eutaw formation.
This completion initiated a low resistivity, bypassed gas play that has operators hastily re-examining other plugged wells throughout the trend for production potential.
Trimble field, at a shallow depth of minus 7,000 ft, presently consists of 17 producing wells with a proven productive area of 4,200 acres. Net pay ranges from 16 ft to 51 ft with a gas column estimated at about 250 ft. To date the field is 7 miles long and averages 1 mile wide.
The Stanley sand of the Eutaw formation of east-central Mississippi is a blanket marine deposit averaging 100 ft in thickness throughout the trend. Interpretation of subsurface and seismic data indicates the trap at Trimble field to be a faulted structural closure that is related to an underlying deep-seated salt anticline.
The upthrown Eutaw reservoir is laterally sealed by impervious Selma chalk in the downthrown block. Lower Cretaceous production associated with the same fault is present at Magee field 3 miles to the northwest.
The Trimble field structure was drilled and abandoned twice prior to its discovery in 1988.
The fact that the productive zone went unrecognized in both wells is attributed to the lack of core shows and the use of conventional log analysis techniques that were later proven to be inadequate by the accidental discovery of gas at Gitano field, 20 miles to the east.
EXPLORATION, DEVELOPMENT
The Upper Cretaceous Eutaw play of east-central Mississippi includes parts of Rankin, Smith, Jasper, Jones, Covington, and Simpson counties.
Trimble field is centrally located along the play in 10n-15w, 16w, and 17w, and encompasses parts of Smith and Simpson counties (Fig. 1).
Initial exploration of the Eutaw trend in Mississippi dates to the mid-1940s and led to the discovery of four major fields by the end of 1951.
The first, Gwinville field, was discovered in 1944 and has proven to be an impressive Eutaw find with cumulative production approaching 1.2 tcf. This was followed by the discovery of Soso field in 1945 (118 bcf), Sharon field in 1949 (31 bcf), and Maxie-Pistol Ridge field in 1951 (181 bcf).
To date, the Eutaw formation in Mississippi has produced more than 1.82 tcf of gas. Prominent Mississippi Eutaw oil fields such as Heidelburg, Eucutta, and Yellow Creek have already produced in excess of 224 million bbl of oil.
The early prospects in the Eutaw trend were classic fault traps and were developed primarily through the use of gravity information and single fold seismic data. However, reservoir evaluations were difficult due to low resistivity readings and the lack of conventional and sidewall core shows in gas bearing intervals.
At Soso field, the Stanley sand was not recognized as a producing interval until five wells had been completed in the underlying Christmas sand.
Research indicated that the low resistivity in the Stanley sand was attributable to its high glauconite content. Glauconite acted as a clay that decreased the resistivity curve to such a degree that normally recognizable gas pays were overlooked.
The Christmas sand was lower in glauconite content and exhibited a classic resistivity pay signature. By the time Sharon field was discovered in 1949, low resistivity pays were anticipated, and the Eutaw reservoir was apparent.
Following discovery of Sharon field, successful activity in the Eutaw play declined. Interest was revived in 1987 when Eutaw gas was unexpectedly discovered at Gitano field during the recompletion of an EPA permitted salt water disposal well.
The Trimble field structure, 20 miles west of Gitano field, was drilled twice for Cretaceous objectives below the Eutaw before the 1988 discovery of shallow bypassed gas.
The first well, drilled by Love Petroleum in 1963, was bottomed in the Lower Tuscaloosa at a total depth of 8,220 ft with a questionable show of gas indicated by sidewall core in the Stanley sand. No tests were run, and the well was plugged and abandoned.
A second well, drilled by Pruet Oil Co. in 1984 to a total depth of 11,127 ft in Lower Cretaceous, was also plugged with no tests, although there were good mud log gas shows in the Eutaw Stanley sand.
Plugging decisions for both wells were probably based on conventional log analysis techniques that indicated the Stanley sand to be nonproductive.
In 1988, the discovery well at Triryible, the Cobra and Paramount 1 Purser, encountered 51 ft of net pay and tested 2.759 MMcfd of gas, 16.8 b/d of condensate, and 2 b/d of water from the Stanley sand.
The first step-out to this well, the Cobra and Paramount 1 Amason, came in 50 ft high to the Purser. This well encountered 40 ft of net pay and tested 2.570 MMcfd.
Pruet Production Co. drilled the third well for the field, the 1 Rayvon Amason, 5 miles to the southeast in 25-10n-16w and encountered 16 ft of net pay.
Field development during the next 15 months continued east and west of the discovery well and occurred as systematic step-outs along the seismically defined fault.
A suspected gas-water contact found by the Pruet Production 1 Rayvon Amason was confirmed by the Cobra and Paramount 1 Robinson, in 21-10n-16w, in December 1989. The 1 Robinson encountered 26 ft of net pay and tested 1.324 MMcfd of gas.
Net pay throughout Trimble ranges from 16 ft to 51 ft, and the gas column is estimated at approximately 250 ft. To date, Trimble field has 16 productive wells and boasts an impressive 100% development well success rate.
Trimble field is 7 miles long and averages 1 mile in width. Final limits of the field are currently undefined by well control.
A total productive area of 4,200 acres is estimated based on seismic data, subsurface information and the lowest known gas-water contact at -7,171 in the Cobra and Paramount 1 Robinson well. Field development is continuing, and final definition is expected to take approximately 2 more years.
STRATIGRAPHY
The Eutaw formation of east-central Mississippi includes those sedimentary rocks of Late Cretaceous age that are underlain by sedimentary rocks of the Tuscaloosa group and overlain by the Selma chalk (Dinkins, 1971, p. 184).
The Eutaw formation strikes west-northwest through the study area, and regional dip is to the south-southwest. The thickness of the Eutaw formation varies from 400-600 ft throughout the trend with thickness variations resulting from local structural conditions (Moore, 1963, p. 66).
Studies indicate Eutaw sediments were laid down in a relatively stable, shallow transgressive marine environment where depositional rates were comparatively slow (Garrett, 1956, p. 31).
The Eutaw of central Mississippi consists of very fine to fine-grained, glauconitic sandstones alternating with gray to dark-gray shales that are compact to fissile, carbonaceous, slightly glauconitic, finely micaceous and light-gray, waxy, and bentonitic in areas (Garrett, 1956, p. 19).
Sandstones of this group carry various nomenclature throughout the state. A type log is provided (Fig. 2).
The Eutaw Stanley sand at Trimble field is a very fine to fine-grained, greenish-gray, shaly, glauconitic sandstone that exhibits fair to good porosity and permeability. It averages about 100 ft of gross thickness in the Trimble area.
STRUCTURE
Trimble field is located on a faulted structural closure on the south flank of a northwest-southeast trending salt anticline. Fig. 3 is a structure map on the Eutaw Stanley sand that shows the trap to be positioned on the upthrown block of a large down to the northeast fault.
Fig. 4 is a north-south structural cross section of Trimble field that shows the hydrocarbon trap to be formed by the juxtaposition of upthrown Eutaw sands against downthrown Selma chalk.
Interpretation of subsurface and seismic data indicates that faulting at Trimble field is related to an underlying deep-seated salt anticline. The fault reaches a maximum throw of 270 ft at the apex of the Eutaw structure.
SEISMIC
Conventional seismic techniques are adequate to analyze the structural components of the trap at Trimble field.
To date, the authors' work in processing for direct hydrocarbon indicators such as amplitude versus offset and frequency absorption analysis has not proven effective for the Eutaw trend in east-central Mississippi.
A migrated version of seismic line MD-1, which traverses the field in a north-south direction (Fig. 3), is shown in Fig. 5. The line is 24 fold with far geophone group offsets of about 10,000 ft.
The data were recorded in March 1983 by GFS Co. for Pruet Oil using a dynamite source.
Line MD-1 shows reflectors for the Selma chalk and the Eutaw at 1.5 sec and 1.7 sec, respectively, in the upthrown block.
Faulting in relation to the underlying salt anticline and the resultant lateral seal provided by the downthrown Selma chalk is clearly indicated. The fault originates at the apex of the large salt anticline shown below 3.0 sec, and growth of the fault at the Ferry Lake level is obvious.
ANALYSIS, EVALUATION TECHNIQUES
A review of the logs from the two early attempts at Trimble field indicates that the resistivity of the Stanley sand in both wells fell within what are now known to be the productive limits for the field.
The fact that the productive zone went unrecognized in both wells can be attributed to erroneous log analysis and the lack of sidewall core shows. The difficulty in log analysis arises from the low resistivity found in gas productive intervals in the Stanley sand.
Water wet resistivity, Ro, for the Stanley was determined to be 0.4 ohmm from perforations in the Pruet Production 1 Rayvon Amason well. Production with a water cut was seen at 0.55 ohm-m, and water-free production occurred at 0.6 ohm-m.
This low resistivity production is attributed to the presence of conductive minerals, predominately glauconite, in the pore lining of the rock matrix that have a short circuiting effect on the electrical measurements from resistivity tools.
Two variables that are influenced by this effect have been measured from Trimble field using core and log information. The electric current path through a rock matrix is measured as the cementation exponents M in the formation resistivity factor equation and for most homogeneous sediments has a value of 2.
As the current path becomes less tortuous, M decreases.
M values for the reservoir rock in Trimble field were derived from conventional core measurements in the Cobra and Paramount 1 Amason (Fig. 6) and from the Schlumberger electromagnetic propagation tool with excellent agreement between the two. Core derived M values ranged from 1.80 to 1.91, while log derived M values are approximately 1.8.
Another calculation variable that was found to differ from the norm was the water saturation exponent, N, which is related to the influence that insulating fluids such as hydrocarbons have on the shape and continuity of conductivity solutions within the pore space.
N usually has a value of 2, but the measured value of N from the Amason conventional core ranged from 1.72 to 1.84 with an average of 1.77.
Prior to having measured values for these variables, log derived water saturations in the 1 Purser and 1 Amason calculated in the low 60s to low 80s. These values were approximately the same as water saturations derived from sidewall cores and slightly higher than those derived from conventional cores.
After adjustment of the M and N values, log derived water saturations were in the low 50s.
When an Ro value was determined at the reservoir's most downdip location in the Pruet Production 1 Rayvon Amason well, the Archie resistivity formula
[SEE FORMULA]
where Sw = water saturation, Ro = resistivity when Sw = 100%, and Rt = true resistivity, was applied to the crestal wells.
Water saturations now calculated in the high 40s corresponding to the irreducible water saturation for the reservoir as calculated by capillary pressure measurements from the Cobra & Paramount 1 Amason.
Further application of this resistivity ratio method has worked well with continued field development due to a fairly constant porosity and lithology of the objective Stanley sandstone.
Well evaluation techniques applied to Trimble field include gas detection equipment on mud logging units and electric logging suites consisting of a dual induction spherically focused log, a lithodensity-compensated neutron-gamma ray log, and the electromagnetic propagation tool.
Sidewall cores have not been useful as direct hydrocarbon indicators, since the production at Trimble field is primarily dry gas.
PRODUCTION
As of the end of February 1990, Trimble field had produced 3.3 bcf of natural gas and 22,374 bbl of condensate.
Production during the same month averaged approximately 26 MMcfd of gas and 182 b/d of condensate from 11 wells, and three additional wells were ready for pipeline connection by the end of March.
Gas production for 1990 is expected to be 10 bcf/year, with a projection of 13 bcf for 1991.
CONCLUSIONS
Additional prospective structures analogous to Trimble field should be present in the Eutaw trend.
Log analysis techniques developed at Trimble field facilitate the recognition of the low resistivity nature of Eutaw gas production and should be considered in future exploration.
Trimble field is an excellent example of an anomalous set of occurrences in a trend that should offer explorationists many new opportunities.
ACKNOWLEDGMENTS
The authors express their gratitude to Cobra Oil & Gas Corp., Wichita Falls, Tex.,and to the partners in Trimble field: Pruet Oil Co., Jackson, Miss; M.B. "Duke" Rudman, Dallas; Kurt J. Wiseman, Wentworth Energy Inc., and Southern Mineral Corp., all of Houston; and McFarland Energy Inc., Santa Fe Springs, Calif., for their counsel and recommendations in regard to this paper. In addition, they thank Richard Hobbs for drafting and Rosalie Robinson and Rosie Rhodes for editing and typing the manuscript.
BIBLIOGRAPHY
Dinkins, T.H., Jr., Subsurface stratigraphy of Rankin County, in W.T. Baughman, Rankin County geology and mineral resources: Mississippi Geological, Economic and Topographical Survey Bulletin 115, 1971, pp. 163-194.
Garrett, M.E., A regional study of the Eutaw formation in the subsurface of central and southern Mississippi: unpublished master of science thesis, Mississippi State University, Starkville, Miss., 1956, 39 p. Moore, W.H., Jasper County subsurface stratigraphy, in D.A. DeVries, Jasper County mineral resources: Mississippi Geological, Economic and Topographical Survey Bulletin 95, 1963, pp. 53-68.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.