M.G. Whitmire
Bandera Petroleum Inc.
Tulsa
Recent events in the Arkoma basin bring to mind the old hunting story where the veteran hunter lures the new would-be hunter out at 4:30 AM in subfreezing temperatures, and after a few hours of keen excitement but extreme discomfort the new hunter asks, "Are we having fun yet?"
Many newcomers to the basin who have been caught up in the excitement of the hunt, and spent a great deal of time and money, but have not yet enjoyed its rich rewards, are asking the same question, "Are we having fun yet?"
Unfortunately most exploratory plays move painfully slow. The Arkoma play, while actually moving rapidly, seems to be moving slowly, and factual information on, "How well we are doing," is difficult to obtain. Guards are posted at lease gates where wildcat wells are being drilled to keep critical information away from scouts, lease-hounds, speculators and competitors of any kind. Thus, information (or misinformation) is pieced together from the conversations of rig hands and service people at the local coffee shops and spread by a network of information-starved basin players. This information and the deluge of material published about the play is often confusing, particularly to those who are relatively new to the area. While no secrets will be revealed here, perhaps the groundwork for a basic understanding of the play can be laid.
One reason for the confusion and apparent contradictory information is that there are three distinctly different, current gas plays in the basin. Each play differs from the others in depth, structural style, reservoir properties, sourcing relationships, reserve potential, geologic age, areal distribution, and method of exploration. The following is a brief attempt to clarify some of the confusion surrounding what is broadly considered "the hottest play in the United States."
The three plays are (1) Pennsylvanian sands, (2) thrusted Spiro-Wapanucka and (3) Arbuckle.
PENNSYLVANIAN SANDS
This is the oldest and most mature play but still an active, viable play.
A little known fact is that many wells in this play have produced over 15 bcf from depths less than 7,000 ft, and deeper wells commonly produce in excess of 30 bcf. Over 100 bcf has been produced in a few of the lower Atoka and Morrow wells (Red Oak and in-place Spiro, Wapanucka and Cromwell). This production ranges from 1,000 feet to 14,000 ft in depth, is usually stratigraphic but with some strong structural influence. Reservoirs result from clean sands laid down in a myriad of depositional modes (i.e., channel sands, marine bars, deltaic fans, etc.) They are probably sourced and sealed by the shales in which they are encased. Areal distribution of current production includes most of the northern shelf on the Oklahoma side of the basin, and a goodly portion of the northern shelf on the Arkansas side. There has been a slow but steady march of development to the east along the shelf and to the south into deeper, more expensive drilling areas. A strong analogy for this, play can be made between the Arkoma basin today and the Anadarko basin in 1975. If one compares production maps of the Anadarko basin, year by year, from 1975 to 1985, one would see a strong, steady march to the west along the shelf, and to the south into the deeper portion of the basin. Comparing the same series of maps for the Arkoma basin reveals much the same pattern, but in a much earlier stage of development.
There are several reasons why the Arkoma basin has lagged behind the Anadarko basin. First, it is entirely gas prone, no significant liquids have been produced and gas was unpopular (and unprofitable) for many years. When gas fields were finally developed only one pipeline was available for many years (the basin still has limited capacity). The lack of competition in marketing created abnormally low prices, harsh contract terms, curtailments, and slow pipeline hookups, further diminishing interest in the area. The third problem was that, until a few years ago, Arkansas law provided force integration (force pooling) for development wells but not wildcats. Much of Arkansas land is divided into very small tracts with complex title problems, and force pooling is usually required to control all of a producing unit. Hence, stepout wildcats were seldom drilled and development of the trend progressed very slowly, one location at a time. Arkansas installed force pooling of wildcats in the mid-eighties but by then weak gas prices, slow hookups and low takes had stalled the play. This play will spring to new life, particularly on the Arkansas side when gas prices improve and pipeline capacity expands to meet the need. Both of these things are expected to happen within a year or two. The pipelines that are now under construction, planned and proposed, should eliminate the transportation problems and create a more competitive market. Increased demand and the shortfall in reserve replacement should bring price improvements within the next two years.
A portion of this play is simply not economical at $1.50/Mcf gas, but $2.00/Mcf gas (year around average price) will bring an abundance of highly profitable, low risk drilling back to this play. The current producing area and expected future exploration are shown by Fig. 1.
THRUSTED SPIRO-WAPANUCKA
This is a relatively new play located along the frontal zone of the Ouachita thrust system, generally between the Choctaw and Ti-Valley faults, where thin sheets of Spiro and Wapanucka are shoved northward by the Ouachita movement until each sheet breaks, slides upon the preceding segment, with the cycle repeating itself until several sheets (possibly 10 or more) are piled atop each other like shingles on a roof.
Occasionally one or more sheets will "stub their toe" and roll over into recumbent folds thereby increasing the complexity of the Spiro-Wapanucka trap. The thrusted Spiro-Wapanucka (not to be confused with in-place Spiro-Wapanucka) ranges in depth from 4,000 ft. to 18,000 ft, reservoirs have some primary porosity but generally need fracturing for reservoir enhancement and sourcing is probably from lower Atokan and Morrowan shales. Areal extent of established production is within a band 6 to 8 ml wide and 35 ml long and future exploration will probably occupy a similar band eastward along the mobil front into western Arkansas (Fig. 2).
Scientific prospecting in this trend requires high quality seismic data acquired with carefully designed field techniques and sophisticated processing routines; and an interpretation by a veteran seismic interpreter (thoroughly experienced in the Spiro-Wapanucka trend). Three-dimensional seismic surveys would be ideal for evaluating prospects in this trend, but economics, terrain, and surface restrictions, plus widely divided lease ownership curtails the use of this option. The interpreter must go beyond just mapping the highest portion of the "pile." He must attempt to map the structural attitude of the individual sheets and the thrust planes as well. Some sheets will be too thin to provide sufficient time separation of seismic events, and thus will be masked to the interpreter; however, other sheets above and below should be mappable. Drilling through the target zone 6 to 1 0 times in a single borehole provides a statistical advantage to the explorer and, hopefully, one or more sheets will contain reservoir properties in a trap. There is insufficient production data at this point to make reserve projections for this play, but the better wells can reasonably be expected to produce in the 10-50 bcf range.
ARBUCKLE
This is the newest, the most exciting and probably has the highest potential of the plays.
It is also probably the most difficult due to the fickle nature of carbonate reservoirs. Much less is known about this play than the other two.
There is only one prototype within the basin (Wilburton field), and not much is known about the reservoir basin-wide, including how it is created, or how it is sourced. Since we only have one prototype there is a strong tendency to conclude that all prospects must look like the prototype. This is understandable, but short sighted. It is almost a certainty that significant Arbuckle accumulations will be found in other types of traps before the play is over.
Drilling depths in this play will range from 6,000 ft to 20,000 ft, or the economic depth limit (technological advancement and improved gas prices will carry the economic limit deeper). The most popular structural style is the horst block, but fault traps (both upthrown and downthrown), faulted anticlines, simple anticlines and stratigraphic traps (porosity pinchouts) are capable of holding accumulations. For analogies other than Wilburton, one can look to the prolific Ellenberger in West Texas (age equivalent), the Wichita Mountain Front of Western Oklahoma, or, perhaps, even to the massive Mississippian and Hunton limes in the Anadarko basin. These reservoirs tend to be compartmentalized (i.e., not necessarily connected either horizontally or vertically). The massive Arbuckle formation ranges in thickness up to 6,000 ft, and numerous isolated reservoirs separated by impermeable zones may occur randomly through this thick section. The actual limit of the trap may be porosity and permeability barriers, and these may not parallel structural boundaries. Karsting, or subaerial erosion may play an important role in reservoir development in these rocks, thus the explorationist must, as usual, be concerned with paleo structure as well as present day structure. Dolomitization of the limestone is an important ingredient as is fracturing to enhance reservoir development. Folding as well as faulting should increase the amount of fracturing in this massive carbonate section.
The most controversial issue surrounding Arbuckle production is how it is sourced. Even the most renowned experts cannot agree on how hydrocarbons are generated and how reservoirs are sourced, particularly in the massive Cambro-Ordivician carbonates. Excellent arguments can be made for internal sourcing (i.e., organic rich evaporates and thin interbedded shales). Most explorationists are more comfortable with external sourcing, but internal sourcing and the possibility of sourcing from rocks other than dark shales should not be ignored.
Distance of migration is important. How far can oil and gas migrate? The answer is unknown, but if gas can migrate as much as 10 miles one can find an external source for most Arbuckle prospects in the Arkoma basin. Thus, timing becomes important. When did peak generation occur and migration begin, and did the structural barrier or porosity barrier occur before, during, or after migration occurred? Good quality seismic data and geological information provides usable data on structural timing, and time of peak migration can usually be placed within reasonable time boundaries, thereby reducing the risk associated with timing.
Sufficient data to accurately project Arbuckle reserves is not yet available, but arguments for 50-1 00 bcf per well have some support. The areal extent of the play is approximately 300 ml east-west, and 50 ml north-south. It extends from Atoka, Oklahoma to Search, Arkansas and from the shallow basin shelf on the north to the Ouachita thrust system (or the economic depth limit) on the south (Fig. 3).
While the Hunton lime and Simpson sands are not discussed here, they will provide good uphole objectives in some Arbuckle prospects.
To summarize the Arbuckle play, one must conclude that while a great deal is known about the Arbuckle play, there is quite a lot more that is unknown. The most dangerous mistake one can make is to presume to know more than is actually known. For instance, there is a great deal of prejudice in the industry against downthrown fault blocks and low relief, simple anticlines; but if reservoirs are compartmentalized (vertically separated) as suspected, a 100 ft fault, either upthrown or downthrown, (or a 100 ft closure for that matter) could trap several reservoir compartments with 100 ft of gas column each in the same borehole. It is noteworthy that the now famous Cottonwood Creek Arbuckle field appears to be in a downthrown block. The industry must be open minded to all ideas and a wide variety of sound ideas must be thoroughly tested in order to realize the full potential of this play.
The Arkoma basin with its three different plays is ripe with opportunity. With the pipeline system expansions currently planned or under construction, expected increase in demand, the current shortfall in reserve replacement, and the environmental push of clean fuels, the problems that have impeded both exploration and development in the Arkoma basin should be relieved, and all three of the above plays should see even greater acceleration. Prospects and development wells that will not pass the economic filter at $1.50/Mcf, poor takes, and long pipeline hookup delays will suddenly work quite well at the $2.00/Mcf, good takes, and quick pipeline hookups; and $3.00/Mcf would vastly expand the range of economically viable prospects. The deep Arbuckle gas below 20,000 ft will probably have to wait for $5.00/Mcf gas price.
Experts believe that the "gas bubble" was created by the dramatic increase in drilling from 1980 to 1985. Certainly an equal and opposite event has occurred in the precipitous decline in drilling from 1985 to 1990, giving credence to the belief that the "gas bubble" is rapidly disappearing. If this is so, the Arkoma basin will be one of the prime benefactors.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.