OUACHITAS NEED MORE EXPLORATORY DRILLING

Neil H. Suneson, Jock A. Campbell
Oklahoma Geological Survey
Norman, Okla.

The Ouachita Mountains in southeastern Oklahoma and western Arkansas are part of a mostly buried late Paleozoic fold and thrust belt that extends from Alabama to northern Mexico.

The most common rock types in the Oklahoma Ouachita Mountains are sandstone and shale; Mississippian and Pennsylvanian turbidites are as thick as 30,000 ft and dominate the stratigraphic section at the surface. Slightly metamorphosed to unmetamorphosed, mostly pre-Mississippian chert, shale, and minor limestone and sandstone crop out in the Broken Bow uplift, Potato Hills, and Black Knob Ridge areas. Late Mississippian and Early Pennsylvanian shale, limestone, and sandstone crop out along the northern and western borders of the mountains.

In Oklahoma, the Ouachita Mountains can be separated into three belts based on stratigraphy and structural style. From north to south, these are the frontal belt, central belt, and Broken Bow uplift, (Fig. 1). The frontal belt lies between the Choctaw and Windingstair faults and consists of steeply tilted, imbricately thrusted, and tightly folded strata. Shallow-water Morrowan strata are present in the northern part of the frontal belt, and Morrowan basinal strata (turbidites and olistostromes) occur to the south. The Morrowan units in both parts of the frontal belt are overlain by Atokan turbidites. In the extreme western part of the frontal belt, Ordovician to Mississippian strata crop out near Black Knob Ridge. The central belt is characterized by broad, open synclines separated by tight, typically thrust-cord anticlines. Except for the tightly folded pre-Mississippian units in the Potato Hills, the only rocks exposed are Mississippian and Early Pennsylvanian turbidites. The Broken Bow uplift consists of isoclinally folded and thrusted Early Ordovician to Early Mississippian strata of mostly deepwater origin. The boundary between the central belt and the Broken Bow uplift is arbitrarily shown on Figure 1 as the contact between the Arkansas Novaculite and Stanley Group sediments.

North of the Ouachita Mountains in Oklahoma is the Arkoma basin, a foreland basin that formed in response to Ouachita compressional tectonism and tectonic loading. Historically, the geomorphic and structural division between the Arkoma basin and Ouachita Mountains has been the Choctaw fault. Stratigraphically, however, the southern part of the Arkoma basin and the northern part of the Ouachita Mountains frontal belt are indistinguishable, with the exception that Desmoinesian rocks are absent south of the fault. In addition, Ouachita-type structures extend north of the Choctaw fault in the subsurface, although thrust faults have less displacement than to the south, and folds are of lower amplitude. In both areas (southern Arkoma basin and northern Ouachita Mountains), hydrocarbon-exploration efforts have focused on structural traps enhanced by favorable stratigraphy. For these reasons, this paper will include the southern part of the Arkoma basin as part of the Ouachita tectonic province.

The stratigraphy of the Oklahoma part of the Ouachita tectonic province is relatively well-known, (Fig. 2). Prior to the compressional tectonism that resulted in the fold and thrust belt, Early Ordovician to Early Mississippian deep-water sediments ("Ouachita facies") accumulated in a starved basin south of the southern margin of the North American craton while shallow-water clastics and carbonates ("Arbuckle facies" or "foreland facies") accumulated in a shelf environment on the craton. Early and middle Paleozoic shelf strata underlie the southern Arkoma basin and most, if not all, of the Ouachita Mountains. However, (1) the extent of shelf strata beneath the allochthonous units, (2) facies relations in the shelf strata, (3) the location of the early and middle Paleozoic shelf-slope-rise, and (4) transition to "Ouachita-facies" basin strata are presently unknown. An understanding of these relations is critical to continued hydrocarbon-exploration efforts.

Thick Carboniferous turbidites accumulated in a south-to-north-mitigating basin while thinner shallow-water sediments were deposited on the shelf to the north. The facies change from deep- to shallow-water sediments migrated from south to north, as shown by the slightly different stratigraphy in each major frontal belt thrust sheet (Hendricks and others, 1947).

The principal hydrocarbon reservoirs in the Ouachita tectonic province can be subdivided into those that produce gas from shallow-water (including "foreland-facies") units and those that produce oil and/or gas from deep-water (including "Ouachita-facies") units. From oldest to youngest, "foreland-facies" reservoirs are Ordovician Arbuckle group carbonates, early Morrowan Cromwell Sandstone (also called the Union Valley or basal Pennsylvanian sandstone), Morrowan Wapanucka limestone, early Atokan Spiro sandstone (basal Atoka sandstone), and sandstone beds in the Desmoinesian Krebs group. Sandstone beds in the Atoka Formation are generally considered to be deep-water, but are gas-productive in areas dominated by shallow-water reservoirs (Arkoma basin and frontal belt). The Cromwell produces gas from sub-thrust structural positions both north and south of the trace of the Choctaw fault; the Atoka, Wapanucka, and Spiro are productive from thrust and sub-thrust structural positions. "Ouachita-facies" producing units include, from oldest to youngest, the Ordovician Bigfork chert (gas), Devonian Arkansas Novaculite (oil and gas), Mississippian Stanley group sandstones (gas and oil), and Morrowan Jackfork group sandstones (small quantities of oil). All of the "Ouachita-facies" units produce hydrocarbons from thrust sheets south of the trace of the Choctaw fault.

Hydrocarbon reservoirs in the Ouachita tectonic province can also be divided into those that are fractured and those that produce from primary pore spaces or vugs. The Bigfork Chert and Arkansas Novaculite are fractured reservoirs. The Arbuckle group carbonates and Wapanucka Limestone reservoirs are probably fracture-controlled, but vugs and primary pores undoubtedly contribute to production. Reservoir units that are largely (if not entirely) productive because of original primary porosity include the Cromwell, Spiro, Atoka, Krebs, Stanley, and Jackfork sandstones.

HISTORY OF HYDROCARBON
EXPLORATION

The first explorers for hydrocarbons in the Ouachita Mountains of Oklahoma were probably early Native Americans who used solid hydrocarbons ("asphaltenes") as adhesives to bind stone arrowheads to wooden shafts. European explorers in the Ouachitas referred to outcrops of "coal" (probably asphaltites) in their journals. The first development of hydrocarbon resources occurred in the late nineteenth and early twentieth centuries, when some of the larger asphaltite deposits were mined underground.

The first successful oil well in the Ouachita Mountains was drilled in 1913 or 1914 at what later became known as the Redden field. Recorded drilling activity apparently peaked about 1932 as a result of development of the McGee Valley oil fields, in 1962 as a result of Spiro development in the Wilburton field, and in 1984 as a result of Wapanucka development in the Pittsburg field and expansion of the Wilburton field. The last drilling peak also reflected favorable gas prices. Since the discovery of the Redden field, over 800 oil and gas wells have been drilled in the Ouachita tectonic province in Oklahoma. As of the end of 1988, parts or all of 22 oil and gas fields are within the Oklahoma part of the Ouachita tectonic province. Yet, most of the region remains little explored.

Relatively few rank wildcat wells deeper than 10,000 ft deep have been drilled in the Ouachita Mountains frontal belt between the traces of the Choctaw and Windingstair faults, and even fewer have been drilled in the central belt or Broken Bow uplift south of the trace of the Windingstair fault (Fig. 1). Selected significant early exploratory wells and pertinent results are shown in Table 1.

HYDROCARBON PRODUCTION

Hydrocarbon production in the Ouachita tectonic province occurs in three geographic areas that parallel regional structure. The frontal gas belt (including southern Arkoma basin), central oil belt, and central gas belt (Fig. 1) are distinguished by differences in structural setting, reservoir strata, and types of hydrocarbons. The frontal gas belt extends from northern Atoka County to central Latimer County east of Wilburton. Recognized major fields (and cumulative production through 1988) include Panola (20 bcfg), Wilburton (1157 bcfg), SW Haileyville (18 bcfg), S Blanco (24 bcfg), and Pittsburg (25 bcfg). Smaller fields (including one-well fields) within the frontal gas belt are the N Ti, NW Ti, Blanco, Wesley, and W Wesley. The frontal gas belt straddles the trace of the Choctaw fault; production is mostly from sub-thrust Morrowan to Desmoineseian "foreland-facies" and shallow-water reservoirs, although recent Arbuckle discoveries near Wilburton have extended the age of reservoir strata to the Ordovician. The principal reservoir units vary regionally, but locally overlap, throughout the frontal gas belt. Atoka sandstones are productive in the extreme eastern, central, and extreme western parts of the belt in thrusted and sub-thrust structural positions. Local names include (from bottom to top), the Shay, Cecil, Bullard, Brazil, Diamond, Panola, Red Oak, and Fanshawe sandstones, among others. The Spiro sandstone produces gas from near Wilburton west to eastern Pittsburg County, from above and below Ouachita-related thrust faults. The Wapanucka limestone is a reservoir in southern Pittsburg and north Atoka Counties; gas is produced from beneath the Choctaw fault in the Pittsburg field and from multiply thrust-faulted reservoirs in the S Blanco field. The Cromwell Sandstone is sub-thrust reservoir in the western part of the frontal gas belt.

The central oil belt includes the following oil fields (and reported production through 1988): Bald and South Bald (6300 bo), W and SE Daisy (600 bo, 169 MMcf), E Wesley (900 bo), and Potapo Creek (1,200 bo) (now under the water at McGee Creek Reservoir). Actual production may be many times the reported production. Smaller fields, including subcommercial oil occurrences (shown in quotation marks), include "Talihina", "Star", "Oil Well Hol", Redden (now abandoned), "Minnett", and SE Atoka Townsite (abandoned). The central oil belt is mostly south and east of the trace of the Windingstair fault, although locally it extends west of the fault. Production is from shallow traps in Stanley and Jackfork group sandstones and deeper traps in the Stanley and Arkansas Novaculite.

The central gas belt is in that part of the Ouachita Mountains characterized by broad open synclines separated by tight, thrust-cored anticlines. The major field is SW Moyers (1.4 Mcf); other recognized fields include Potato Hills, SE and W Daisy (also produces oil), S Jumbo, and S Farris.

In addition, shallow, subcommercial gas has been produced from near the town of Talihina. Production is from Stanley group sandstones, Bigfork chert, and Arkansas Novaculite.

In addition to oil and gas, solid hydrocarbons have been mined commercially at several places in the Ouachita Mountains, and reported occurrences are scattered throughout the mountains. Commercial deposits (and production) include, from east to west, the Page impsonite mine (2,000 lb of ash, from which vanadium was extracted), Sardis deposit (slightly less than 90,000 tons, include production from Jumbo mine), Jumbo asphaltite mine (also known as Impson Valley or Choctaw mine), and the Pumroy (Moulton) mine (5,000 tons). Many other smaller deposits were used locally to heat homes and in forges.

RESERVOIR UNITS

The youngest reservoir unit in the Ouachita Mountains and southern Arkoma basin (excluding the Krebs group sandstones, which are restricted to the basin) are sandstones in the Atoka formation, (Fig. 2). The Atoka formation is dominantly shale and mudstone, with relatively thin sandstones. It is about 10,000 ft thick in the southern part of the Arkoma basin in Oklahoma, and nearly 15,000 ft thick in a similar position in Arkansas; about 12,000 ft are exposed in the Oklahoma Ouachita Mountains. As exposed in outcrop south of the trace of the Choctaw fault, the thicker sandstones (which are probably similar to productive sandstones in the basin) extend for miles parallel to strike, are tens to hundreds of feet thick, massive to poorly stratified, fine-grained, and composed dominantly of quartz. The best-studied Atoka sandstone is the Red Oak, which Houseknecht (1987) interpreted as an amalgamated turbidite sand that was deposited approximately parallel to east-west striking syndepositional normal faults in channels one to three miles wide. Atoka sandstones show clear gamma-ray and resistivity character on electric logs, are typically 10 ft to 30 ft thick, and are separated by thin shales and siltstones. Density porosity typically ranges from 8% to 12% in the sandstones, although it can extend up to 20%. Gas-productive sandstone exhibit good crossover, with neutron porosity less than density porosity.

The Spiro sandstone underlies the Atoka formation and is separated from it by a widespread shale unit. This is not the same unit as the Spiro sandstone mapped on the surface in the Arkoma basin, which is a member of the Desmoinesian Savanna formation. Some workers consider the Spiro to be the basal sandstone unit of the Atoka formation; others consider it to be part of the underlying Wapanucka formation. In outcrop, the Spiro typically forms a resistant ridge called "Limestone Ridge" on topographic maps. It is a relatively massive, fine- to medium-grained, quartzose sandstone with significant porosity, It is generally light-colored, well-sorted, locally conglomeratic, and unstratified to planar- or cross-stratified. Locally, it is extremely fossiliferous and interbedded with limestone similar to that in the Wapanucka. South of the trace of the Choctaw fault, the Spiro is generally about 300 ft thick; locally it is up to 700 ft thick. Based on an extensive study of cores and subsurface maps patterns, Houseknecht (1987) interpreted the Spiro to consist of fluvial channel, tidally influenced channel, interchannel (shallow subtidal to tidal flat), and some marine shelf deposits (where interbedded with carbonates). The Spiro exhibits a distinctive character on gamma-ray and resistivity logs; there is very little indication of interbedded shales, as is true of outcrops. Density porosity is typically about 6% to 1 0%, but can be up to 20%. The Spiro sandstone exhibits good crossover where potentially gas-productive.

Immediately underlying and locally interbedded with the Spiro sandstone, but locally separated from it by a thin shale, is the Wapanucka limestone. The Wapanucka is poorly exposed, except where it is closely associated where it is closely associated with the Spiro sandstone and/or is spicular. It is mostly a gray, stratified, locally sandy micrite to packstone or bioclastic limestone. South of the trace of the Choctaw fault it is about 750 ft thick. Grayson (1980) measured over 30 exposed sections of Wapanucka Limestone in the Ouachita Mountains frontal belt and concluded that the environment of deposition included shallow-water offshore bars, inter-bar environments, and poorly circulated lagoonal environments. In addition, Grayson (1980) documented that the southernmost Wapanucka Limestone exposures in the higher and more distally derived thrust sheets were deposited in deeper water than those exposed in the more northern thrust sheets. The Wapanucka exhibits a blocky character of gamma-ray and resistivity logs; shale breaks are poorly developed. The electric-log character of the limestone is similar to that of the Spiro sandstone; a mud log is probably the best way to distinguish the two. Some operators use a microlog to detect fracture permeability, with deep resistivity greater than shallow resistivity. Density porosity is typically near zero, but peaks of 4% to 8% are common. The Wapanucka typically is perforated where crossover is observed.

The Cromwell sandstone does not crop out in the Ouachita Mountains, and few published reports have described it in the subsurface of the Arkoma basin; therefore, it will not be discussed in this report. Presently, sandstones in the Jackfork Group are uncommon as a reservoir rock in the Ouachita Mountains. Hydrocarbon production from sandstones in the Stanley group is restricted to areas near or south of the trace of the Windingstair fault. Most of the Stanley group is composed of fissile shale and siltstone; locally, there are persistent siliceous-shale beds that have served as marker beds to workers who have mapped the Stanley. The Stanley locally contains thicker and more numerous sandstone beds than is typical and, in places, is the target for hydrocarbon exploration. The maximum thickness of the Stanley is about 12,000 ft in the southern Ouachita Mountains; it is significantly thinner to the north, where it produces hydrocarbons. In outcrop, Stanley group sandstones are medium- to olive-gray, fine-grained, and poorly sorted, with abundant mud matrix. Beds are typically 1 ft to 2 ft thick but vary from inches to tens of feet thick and are massive to poorly graded. Most workers who have studied the Stanley agree that it was deposited by turbidity currents, and that most of the sediments were derived from the east and south. Stanley sandstones are well-defined on gamma-ray and resistivity logs, but the gamma-ray log does not distinguish potentially gas-productive sandstones from those with abundant mud matrix. The density porosity in single, relatively massive sandstones can range from 6% to over 30%. Operators will not necessarily perforate at high density porosity; rather, they will perforate where the formation exhibits crossover, even if the density porosity is low.

The Arkansas Novaculite underlies the Stanley group and is commonly an objective of exploration programs in the Ouachita Mountains. In outcrop, the novaculite is generally a thin-bedded, blocky, fractured chert, interbedded with very thin beds of siliceous shale. It is about 350 ft thick where exposed on Black Knob Ridge and in the Potato Hills. It is generally interpreted to represent the basinward, deep-water equivalent of the shallow-water Woodford Shale, a source rock for many Oklahoma oils. No analyses of the log character of productive versus nonproductive Arkansas Novaculite have been published.

The Bigford chert is a minor reservoir unit in the Ouachita Mountains. In outcrop, it is grossly similar to the Arkansas Novaculite, but contains more siliceous shale and carbonates. Like the novaculite, the log character of productive versus dry Bigfork chert has not been studied.

Ordovician Arbuckle group carbonates have recently been discovered to be gas-productive in the southern part of the Wilburton field immediately west of the town of Wilburton (Hook, 1989). The trap is reported to be a horst block; however, the detailed geology and its relation to Ouachita tectonics is unknown.

SOURCE ROCKS

One of the historical enigmas of the Ouachita Mountains has been its temperature history. Because of recognized low-grade metamorphism in the Broken Bow uplift, and different theories with regard to the origin of solid bitumens, it was long believed by some that any potential petroleum source rocks had been overheated. However, Curiale (1983) showed that a high-temperature history for most of the Oklahoma Ouachitas is unfounded. That report was based on the examination of 126 samples (11 outcrop and 115 subsurface) of six stratigraphic units for petroleum source-rock potential. Source-rock quality was estimated through analyses for total organic carbon (TOC) and extractable organic matter (EOM). The six units rank as "good" and "fair" source rocks, and all contain more than the 0.5% TOC considered necessary to generate oil and/or gas in substantial quantities (Table 2). Thermal maturity of the rocks was investigated by vitrinite reflectance and by several geochemical methods. All six stratigraphic units are in the lower range of the oil window-however, it is important to recognize that the Stanley group shales and Arkansas Novaculite contain primarily woody plant material (type III kerogen) and are therefore gas-prone source rocks. The two best petroleum source rocks are the Womble Shale (Middle Ordovician) and Stanley group shales (Mississippian) according to Curiale (1983).

The Oklahoma Geological Survey has continued to evaluate potential petroleum source rocks in the area. Unpublished analyses of eight surface samples of Woodford Shale and "Caney" Shale indicate that they have good source potential. In addition, analyses of 29 samples from the Jackfork group, Johns Valley shale, and Atoka formation indicate that these units commonly contain more than 0.5% TOC (range 0.22 to 3.0 wt %). However, EOM tended to be low. (The fact that these are outcrop samples may be the reason for the low EOM.) The maturity of organic matter in these rocks has not been determined.

Crude oils from three oil fields that produce from shallow sandstone reservoirs in the Stanley group were also studied by Curiale (1983). They are low-sulfur crudes and have gravities ranging from 32 to 43 API. Geochemical studies, including those of sulfur isotopes, indicate that the oils have an Ordovician-Silurian source, i.e., Womble, Polk Creek, and/or Missouri Mountain shales. These strata underlie the reservoirs at considerable depth in the same and/or separate thrust sheets. This indicates that significant vertical migration has occurred, probably through fault and fracture systems.

Finally, solid hydrocarbons (bitumens) that occur in the Oklahoma Ouachitas were investigated. Impsonite and the much more common grahamite have a resinous to coaly appearance, and occur as veins and dikes in stratigraphic units from the Bigfork chert to the Wapanucka Limestone. However, most occurrences are in strata of the Stanley and Jackfork groups. Impsonite is the more mature of the two. It has a much lower H/C ratio than grahamite, is infusible, and is virtually insoluble. Like the crude oils, the bitumens originated in Ordovician-Silurian organic-rich strata. It seems probable from Curiale's (1983) study that the solid hydrocarbons are the result of low-temperature degradation of crude oil, i.e., biodegradation, oxidation, and contact with meteroic water. END PART 1 of 2