Abstract

In recent years (1974-1987) the “Atoka trend”, a stratigraphic trap play in the Midland Basin of West Texas, has been actively pursued by major oil companies and smaller operators alike. The trend as it is presently developed extends over 40 miles NW-SE, from southeastern Andrews County to southeastern Midland County, Texas. Within this trend, Atoka production has been established in a wide variety of field sizes ranging from single well fields to the multimillion barrel Azalea, Desperado, and Bradford Ranch fields. Interestingly, many “Atoka” fields were discovered while drilling on structure for deeper objectives and were discovered upon evaluation of “gas kicks” often as a later recompletion attempt, or for lack of any better “bail out” zone when the primary objectives were dry. Serendipitous field discoveries of this type include Moonlight, Midkiff, Desperado, and Bradford Ranch. Falling oil price, high drilling costs and the high element of risk in exploring for, and developing the subtle and heterogeneous “Atoka” reservoir have virtually squelched all drilling activity in this once highly sought-after play.

The “Atoka” reservoir actually consists of one or more of a series of thin (15-20 ft) silty to bioclastic-rich units within the “Atoka” Shale. (Many explorationists and the U.S. Geological Survey correlate this shale interval in the subsurface as the Upper Mississippian Barnett Shale). No datable endemic fauna has been reported from this stratigraphic interval thus the true chronostratigraphic age of the siliceous to calcareous “Atoka” Shale is equivocal. Hence, some fields such as Lowe and Moonlight are reported as Mississippian (Chester) age, while most are classified as Pennsylvanian (Atokan) age such as Azalea, Bauman, Bradford Ranch and Desperado. Further complicating the issue, Morrowan fusulinids have been reported from the upper few hundred feet of the correlative shale interval (to the west) along the margin of the Central Basin Platform in Ector County, Texas (G.L. Wilde, pers. corresp., 1988). Thus, the “Atoka” Shale may be all or in part Upper Mississippian (Chesterian), or Lower Pennsylvanian (Morrowan or Atokan) in age.

On resistivity logs the silty to bioclastic-rich detrital units typically appear as thin, high resistivity intervals within the upper half of the low resistivity “Atoka” Shale. On gamma ray-compensated neutron/density open-hole logs the detrital units appear as one or more, low porosity, upward-coarsening members (funnel-shaped gamma-ray signature) with an abrupt upper transition into the characteristic high gamma response of the bounding “Atoka” Shale.

The “Atoka” Shale was deposited during the waning stages in the history of the Lower Paleozoic Tobosa Basin. The Tobosa Basin was the predecessor to the Permo-Pennsylvanian age Midland Basin-Delaware Basin complex. During “Atoka” time, the Tobosa Basin was relatively shallow and was rimmed by thick carbonate banks. The rimmed shelves significantly inhibited transport of sediment coarser than the clay size fraction to the deeper basin such that a sediment starved, dysaerobic basin was maintained throughout much of “Atoka” Shale depositional time.

Episodic sea level lowstands in “Atoka” time were conducive to influx of fine grain carbonate detritus (allochems and skeletals), and fine grain siliciclastics (quartz silt and very-fine sand) to the relatively deeper and restricted basin environment. These detrital intervals which vary locally in number and thickness, formed extensive sheetlike units (up to 20 ft thick, locally thicker) which may represent the distal edge of basin-margin clastic wedges, or possibly submarine fan systems. Alternatively, the detrital units may represent broad, low relief channel complexes, analogous to those in the Permian age Delaware Mountain Group. The length and breadth of individual detrital units (up to 40 mi x 10 mi wide) parallels that documented from the Delaware Mountain Group fine sandstone and siltstone channels, and thus may be the product of similar density flow and/or turbidite depositional processes.

The faunal component of the bioclastic members consists of a diverse, poorly sorted assemblage including fenestrate bryozoa, crinoids, fusulinid formanifera, brachipods, molluscs, ostracods, and siliceous sponge spicules. Many of the biotics are endemic to shallow marine environments and are commonly fragmental or exhibit evidence of abrasion. The bioclasts occur within a gray, locally carbonaceous, silty, organic-rich lime mud matrix. These bioclastic wackestones comprise the pay zone in the Moonlight Mississippian Field in northwestern Midland County. Elsewhere, the bioclasts occur with scattered oolites in a very organic-rich, argillaceous lime wackestone to quartz siltstone as in the Bauman, Bradford Ranch, Desperado and Azalea “Atoka” fields in central Midland County.

Comparison of cores from Moonlight, Desperado and Azalea fields reveals the same sedimentological features though these fields occur along a 29 mile swath parallel to depositional dip. The basal contact of the siltstone or wackestone units is commonly ragged and clearly discordant to laminae within the underlying siliceous to calcareous black shale. Isolated occurrences of tabular rip-up clasts of the bounding shale occur along the base of the bioclastic members. Low angle planar and ripple cross-lamination are locally preserved in the detrital units attesting to episodic? tractive current flow along the floor of the Tobosa Basin during detrital deposition. Additionally, the contained bioclasts do not display any high degree of sorting, or grading.

The dysaerobic, basin environment was conducive to preservation of organic matter in the siliceous-calcareous “Atoka” Shale. Total organic carbon and Rock-Eval analyses from core samples from the Desperado Atoka Field reveal the shale to contain sufficient organic carbon to be a source rock (1.1 - 4.7% TOC). The kerogen is most similar to a Type II kerogen but the maturity of the kerogen samples is sufficiently high to make interpretation of kerogen source equivocal. Vitrinite reflectance and Tmax determinations indicate the probable source rock is presently overmature relative to oil generation (Ro = 1.47%, Tmax >450° C) but still lies within the gas generating window.

The produced oil from “Atoka” detrital reservoirs is typically a sweet, high (60-70°) gravity oil with an average GOR >3000. A significant volume of associated high (>1200) BTU gas is characteristic of the overpressured “Atoka” detrital reservoirs. All “Atoka” detrital production lies at or below 10,500 ft drill depths and exhibits an abnormally high pressure gradient of 0.7 psi/ft. Thus initial reservoir pressures of 7500-10,000 psi are commonplace, and require that special precautions be taken during drilling and in casing design to safely control the extreme reservoir pressure. Initial production rates are highly variable with rates to 250 BOPD common, though sustained daily rates of this magnitude are rare. Reservoir characteristics are surprisingly consistent across the trend as it is developed to date. Core measured porosity is characteristically in the 6-8% range, average net pay 15 ft, and average permeability commonly << 0.1 md. Water saturation calculations from logs often indicate very high values of Sw (>60-70 %), yet the reservoirs typically produce water-free. The predominance of micro-intercrystralline porosity over other porosity types observed in core or petrographically, likely is the cause for the high Sw values due to immovable water and the extremely low average permeability.

The “Atoka” Shale which bounds the detrital units is apparently self-sourcing with the detrital units serving as in situ reservoirs for hydrocarbons expelled from the surrounding shales. The trapping mechanism involves both stratigraphic pinchout of the detrital units, as well as loss of porosity (intercrystalline and fracture) and decrease in permeability. In many wells the net pay includes all or a portion of a detrital unit as well as some of the adjoining shale which can be additional reservoir, if fractured. Per well recoveries in excess of 350 MBO have been documented from several fields (Bradford Ranch, Bauman, and Azalea). The producing mechanism is fluid expansion above bubble point, and solution gas drive below bubble point. It is likely an extensive fracture network of tectonic? origin greatly enhances storage capacity, continuity, and fluid transmissibility in these low porosity, low permeability reservoirs. Rapid initial flush production from fractures may explain the high initial rate and subsequent rapid rate decline in many wells, as the reservoir ultimately reverts to the lower yield of matrix permeability.

Because of these reservoir characteristics, it has been common practice to stimulate the “Atoka” reservoir by perforation followed by fracturing with up to 60-70,000 gals of diesel or lease crude (to minimize formation damage by water), and use of 50-100,000 lbs of sand propant. Locally, a simple perforation and acidization treatment has been successfully employed as in Azalea Atoka Field, however this technique has been proven in other areas to be more damaging to reservoir properties than beneficial. Reservoir damage may involve loss of permeability due to water block in the micropores, precipitation of hydrolized ferrous gels after acidization (due to the interaction of the acid and iron-bearing minerals such as chlorite or pyrite), porethroat occluding migrated clays, and/or swelling of water sensitive mixed-layer clays.

The “Atoka” detrital has proven to be an unconventional reservoir in terms of reservoir characteristics, and in terms of the completion technique necessary for a commercially productive well. The storage capacity of the reservoir lies within remnant microporosity and to a larger extent locally, natural fractures. The very low reservoir permeability and high calculated water saturations have led many operators over the years to erroneously conclude that the pay zone was “wet”, and thus dismiss gas shows from this interval as insignificant “shale gas”. It is now clear in retrospect that the “Atoka” detrital represents an unconventional reservoir, and as such, conventional completion techniques are ineffective. The “Atoka” detrital has taught many explorationists the valuable lesson that unconventional reservoirs require: 1) critically petrologic and petrophysical re-evaluation of what constitutes an economically producible reservoir facies (and as a corollary, a viable exploration play); and 2) application of completion techniques proven effective in other reservoirs, may not only be ineffective, but may be damaging to producibility in reservoirs more analogous to the “Atoka” detrital. With this new insight gained from recognition of the subtleties of the “Atoka” detrital pays in the Midland Basin, perhaps additional new and overlooked “unconventional reservoirs” are yet to be identified for their recompletion was well as exploratory potential, in this and many other similarly mature domestic basins.