Lloyd E. Gatewood
Consulting Geologist
Oklahoma City
Robert O. Fay
Oklahoma Geological Survey
Norman, Okla.
Recent events bring to mind how two large but different Arbuckle discoveries could cause such excitement as to spark interest along a 535 mile trend of the Arkoma basin in Southeast Oklahoma, Arkansas, Mississippi, and Alabama.
One was an oil discovery, the other the first commercial gas discovery in this formation in the Arkoma basin.
In fact, the Arbuckle, Knox, and Ellenburger group reservoirs constitute a major play along a 1,500 mile trend extending to West Texas and Mexico.
One of the richest oil and gas producing areas in the U.S. is a 200 mile wide, 850 "le long belt from the Central Kansas uplift across Oklahoma, North and West Texas, to the Central Basin platform and the Delaware basin of West Texas.
More than 90% of all oil and gas produced from Ordovician-Cambrian rocks has come from this area and accumulations from grassroots to depths in excess of 26,000 ft in thick, environmentally complex Arbuckle-Ellenburger carbonates, mostly dolomite.
ARKOMA'S FIRST ARBUCKLE
ARCO Oil & Gas Co. drilled its 2 Yourman Arbuckle deeper pool gas discovery well in Wilburton field of Latimer County, Okla., in 1987 (Fig. 1).
The well traversed 2,329 ft of Arbuckle dolomite before penetrating granite at 15,376 ft. The Cool Creek member of Arbuckle flowed 9.309 MMcfd of gas from 14,259-500 ft, and the West Spring Creek member flowed 17.849 MMcfd from 13,066-531 ft. This well, with 1,434 ft of heterogeneous fractured Arbuckle dolomite pay, was followed by 13 additional good gas wells. Wilburton Arbuckle has yielded more than 200 bcf of gas.
Surely this thick, fractured, and vugular Arbuckle dolomite reservoir is not unique considering the analogous geological setting still available nearby in Oklahoma, Arkansas, Mississippi, and Texas.
COTTONWOOD CREEK OIL
CNG Producing Inc. made a 2,595 b/d Arbuckle oil discovery in December 1987 on the down side of the Lone Grove fault in Carter County, Okla., in the Ardmore basin. Fourteen wells were drilled, and 15 million bbl of oil are estimated recoverable from a heterogeneous karstic dolomite. Production is from the West Spring Creek "Brown zone" 1,000 ft below the top of Arbuckle.
ARBUCKLE CHARACTER
Wilburton Arbuckle and Cottonwood Creek are distinct in many ways. Wilburton field is a thrust faulted trap, and Cottonwood Creek field is trapped on the down side of a normal fault. Despite these differences, good Arbuckle fields have four things in common:
- The Arbuckle producing zone is a dolomite.
- The producing zone almost invariably has some sort of induced (secondary) porosity.
- The Arbuckle production is related to a structure of early growth or origin.
- The fields have had both lateral and vertical heterogeneity characteristics, which signifies that their Arbuckle facies have lithologic, physical, and porosity characteristics that vary within the formations depending on the environment of deposition and depths.
Periodic 10, 20, and 30 year cycles of Arbuckle exploration have produced some highly productive, complex, and unusual fields. The Arbuckle carbonate still remains the greatest mystery of any sedimentary rock unit in the geologic column. By recent count, 298 Arbuckle fields have produced in 42 of Oklahoma's 77 counties. Many good producing fields, found since Cushing 79 years ago, have been surprises, and many were serendipitous byproducts of drilling for sandstone reservoirs like Oklahoma City field.
NECESSARY INGREDIENTS
Is it any wonder that the Arbuckle has been such a mystery? Almost every Arbuckle producing field has been a surprise. This 8,000 ft thick carbonate is complex, both tectonically and lithologically, offering a diversity of traps and a variety of facies and heterogeneous porosities that greatly enhance the potential for stratigraphic accumulation.
Information, data, and statistics developed by the authors and the accompanying discussion of the history of the environment of deposition of Arbuckle and Ellenburger oil and gas fields show clearly the necessary factors for commercial oil and gas production.
These necessary ingredients are dolomite reservoirs, porosity, heterogeneity for migration, and timely trapping mechanisms. These have in the past been the most important ingredients for finding commercial Arbuckle and Ellenburger oil and gas production.
OKLAHOMA FIELDS
Most important of Oklahoma's Arbuckle fields, in terms of heterogeneous multizone porosity and cumulative Arbuckle production, are:
- Cushing, with Arbuckle production 300 ft below the formation top.
- Oklahoma City, 200-700 ft below top.
- Mayfield, West Spring Creek 1,200 ft below top.
- Southeast Hoover, West Spring Creek-Kindblade 1,300-1,400 ft below top and Butterly 4,687 ft below top.
- Springer, West Spring Creek gas 812 ft below top, and Coot Creek 3,782 ft below top.
- Healdton, West Spring Creek 200-350 ft below top, and Brown zone 1,000 ft below top.
- Cottonwood Creek, West Spring Creek-Brown zone average 1,000 ft below top, and Cool Creek 2,000 ft below top, and McKenzie Hill 3,000 ft below top.
- Wilburton, West Spring Creek gas 466 ft below top, and Cool Creek gas 1,435 ft below top.
- Southeast Apache, West Spring Creek gas 42-292 ft below top.
Ten Texas panhandle Ellenburger producing fields include Mills Ranch, with the deepest commercial gas production in the world from 23,418-26,518 ft. This production was 3,100 ft below the top of Ellenburger.
OTHER AREAS
Hardly any area in Oklahoma is exempt from Arbuckle potential, even the Ouachita mountains.
This province is a proverbial dumping ground of thick organic rich and carbon rich sediments, especially for the Jackfork, Stanley, Womble, and Mazarn black shales. There is some gas and even oil production in almost every well drilled in these formations, but few penetrations have been drilled into the Ordovician-Cambrian Arbuckle. It is an untested province.
The unusual, 26 well Isom Springs field, in Marshall County, Okla., in 7s- and 8s-5e near the surface trace of the Choctaw fault, is also in the deepest part of the Ardmore basin where the Arbuckle subsea top is about minus 25,000 ft. Cumulative production is 3,002,430 bbl through March 1991.
ARBUCKLE STUDY
The authors spent nearly 4 man years interpreting and evaluating the Arbuckle geology and its potential in Oklahoma, Arkansas, and Mississippi.
The objective has been to use all of the tools and methods available to make the current subsurface interpretation valid, accurate, and fitting from every discipline used, i.e., seismic, gravity, surface, isopachs, well data, stratigraphic facies, and porosity control.
The maps include a new interpretation of the structure and fault pattern of the Arbuckle group in the southern Arkoma basin and Ouachita mountains of Oklahoma and Arkansas.
It delineates the hidden anticlines and horst blocks along the downdip tract of the Ti Valley fault, which appears to be about 4 miles south or southeast of the town of Bengal, Okla.
This submarine horst block has been named Bengalia, or ancestral Ouachita mountains, and the fault face has been named Bengalia fault (Fig. 2). It also shows the basement detachment, which must be close to the interface boundary of the Arkoma shelf facies and the Ouachita basin facies at the basement detachment near Potato Hills in southern Latimer County.
The subsurface structural edge of the shelf rocks of the Arbuckle-Timbered Hills group of the southern Arkoma basin are mainly dolomite and sandstones, about 2,200-4,500 ft thick, dipping basinward to the south or southeast at the surface trace of the overthrust Ouachita Mountain facies.
Conventional wisdom has dictated that the marine Arbuckle carbonate shelf disappeared somewhere south underneath the surface trace of the Choctaw fault and intersects a southern basin of black highly organic graptolitic shales of the Ouachita facies (Fig. 3).
By seismic, Bengalia and the basement detachment has been extended eastward across Arkansas and north of Little Rock, and then it bears southeastward into North Central Mississippi and on trend with the Central Mississippi uplift.
OKLAHOMA PRODUCTION
Data from 4,000 Oklahoma wells were used to construct structure, subcrop, and isopach maps covering the 11,016 sq miles of the Arkoma basin and Ouachita mountains of Oklahoma.
More specifically, 356 wells penetrated to Arbuckle, and 42 penetrated the entire Arbuckle section to granite basement. Arbuckle producing areas are shown on the maps and listed here:
Latimer County, Okla. ARCO found Wilburton field in 1987 in 5n-18e. It produces from Arbuckle at 13,000-16,000 ft.
Sequoyah County, Okla. Oxley Petroleum Co., Tulsa, found Hunton, Simpson, and Arbuckle gas production at Paw Paw field in the 1 Pates Farm well, in 25-11n-26e, in 1988. Arbuckle produced gas from open hole at 6,528-6,812 ft, and 397 ft of Arbuckle dolomite was drilled to TD 6,812 ft.
ARKANSAS PRODUCTION
Data from 2,000 Arkansas wells were used to provide the same map coverage for the 15,264 sq miles of the Arkoma basin. Only 73 wells have drilled to Arbuckle, and 19 wells penetrated the entire Arbuckle to granite.
In Arkansas the scarcity of Arbuckle and basement well data coverage for such a large area dictated that in order to construct valid Arbuckle structure and isopach maps there must be more data, even it if were to be contrived and estimated from Arbuckle data.
A cue was taken from the adjacent Oklahoma Arkoma basin, where a technique had been tested, proved, and used wherein the interval from the top of Morrow-Wapanucka limestone was added to the interval down to the top of the Arbuckle dolomite and it has proved an accurate Arbuckle datum.
The equivalent shallow Morrow datum in Arkansas is the Kessler limestone.
By using the Kessler, in these more numerous shallow wells and adding the interval down to the Arbuckle in actual Arbuckle wells either along strike or dip a close interval isopach map was constructed from which Arbuckle datums can be extrapolated with reasonable accuracy. This isopach interval, when converted to time, is a good tool for geophysicists to locate the Arbuckle reflection on seismic cross sections.
The Arbuckle structure and fault pattern is, of course, the most visible and useful prospecting map, but it took several other maps such as the isopach, gravity, and seismic maps, to help develop and refine the structure and fault interpretation as it is submitted.
White County, Ark. The only Arbuckle production in Arkansas was found in 1981 at the Moran Exploration Co. 1 Reaper in 12-8n-8w, TD 8,000 ft, which produced for only a few months.
MISSISSIPPI WELLS
Well control in Mississippi's northern part, which includes mainly the Warrior basin, amounts to 153 wells.
Of these 119 are Ordovician tests, 40 are Copper Ridge (Knox) tests, and four are basement wells, according to data provided by Vaughan Watkins, Jackson.
Monroe County, Miss, Magnolia Petroleum Co. completed the 1 A.J. Snow well, in 27-13s-7e, in 1953. Knox TD was 4,769 ft, and perforations were at 4,753-56 ft in Upper Knox dolomite. Initial flow was 190 b/d of 35 gravity oil, and the well made 7,813 bbl of oil and 23,513 bbl of salt water in 1 year. Other zones produced in the field named New Hope, but no more Knox production was found.
Oktibbeha County, Miss. Texaco's 1 Sheely, in 28-19n-12e, TD 17,422 ft, made 123.752 MMcf of gas and no oil from Upper Knox perforations at 14 690-784 ft and 14,904-15,036 ft during September 1971-June 1973.
Oktibbeha County, Miss, Exxon's 1 Fulgham, in 33-19n-12e, 1 mile south of the Texaco well went to TD 21,376 ft in granite. Knox was nearly 7,000 ft thick.
DETACHMENT BOUNDARY
The Ti Valley detachment boundary must be close to the interface boundary between the shelf facies and basin facies.
The Bigfork chert, at the surface above the Ti Valley fault, contains much gray limestone and was first described as the Viola limestone, being a hybrid Viola facies. The interface is probably a gradual interbedded black shale and chert facies and probably occurs over many miles. The Arbuckle-Timbered Hills rocks could possibly have some interbedded black shales near the Ti Valley detachment.
Only three wells have been drilled into the Bengalia, but none to basement, and these were not tested, except one in Pennsylvanian rocks, which had gas and some oil. Simpson and Arbuckle had gas and some oil but were not tested. Downside the Bengalia has not been drilled, nor have the adjacent horst blocks.
Hydrocarbons could have moved into these Arbuckle facies rocks at many times from Cambrian time to post-Atoka time, with excellent source beds in the Ouachita basin facies, excellent fractured reservoirs in the shelf facies, thick sandstones in the hybrid facies, tectonic dolomite formed at depth, and illite-bearing shales that allowed movement of fluids with dilation of this live basin and shelf.
It is impossible to calculate reserves in these hidden structures, especially in Arkansas, where no wells have been drilled even close to the structures. The Bengalia has been spotted in Arkansas, on the Cocorp line, at about 30,000 ft, with about 10,000 ft of displacement, being located north of Mount Ida, south of the Ouachita River in northern Montgomery County on the northern edge of the Benton anticlinorium.
It is also apparent on seisline A-90-18 (N-5) in White and Lonoke counties, Ark., east of Little Rock (Fig. 4).
Using Wilburton Arbuckle or Texas Gomez, Puckett, Grey Ranch, and Brown Bassett fields with 13 tcf of gas, Bengalia-Ti Valley detachment area reserves could be quadrillions of cubic feet.
Oil could be found down to 15,000 ft in Simpson, McLish, and Oil Creek sandstones. No reserves can be estimated in the Ouachita facies rocks.
REFUGE AND TRAPS
The Arbuckle dolomite provided a refuge for oil and gas, and 500 million years of subsequent tectonism has added needed fracturing, vugs, and karstification as porosity for accumulations and traps along the way.
This area represents the modern era. The Arbuckle in itself can give tremendous new reserves. Astute use of modern geophysical techniques and interpretations seems a necessity in the highly faulted and fractured Arbuckle carbonates.
Studies of paleogeology of Arbuckle deposition localize dolomite for reservoir and source rocks; shales and interbedded evaporates with faults for entrapment, fracturing, vugs, and karstification intensity for recoverability in large volumes are all requisites for solving the mysteries of the Arbuckle.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
