COMPARTMENTATION, FLUID PRESSURE IMPORTANT IN ANADARKO EXPLORATION

Zuhair Al-Shaieb
Oklahoma State University
Stillwater, Okla.

Basin compartmentation, introduced by Bradley1 and Powley,2 is an important concept in the exploration and production of hydrocarbons in deep sedimentary basins.

Compartmentation can arise in a number of distinct ways through the interplay of sedimentological, mechanical, and chemical factors. These compartments are most easily recognized by their abnormal fluid pressures.

These three dimensionally isolated compartments are essentially sealed for an appreciable duration of geologic time. The seal is an encasing zone of very low permeability to all fluids-internal or external to the compartments.

Integration of geologic and pressure data in the Anadarko basin (Fig. 1) strongly suggests the presence of a basin-wide, overpressured, and completely sealed compartment called the megacompartment complex (MCC).

This overpressured domain is controlled by depositional, tectonic, and mechano-chemical digenetic processes. These processes were responsible for the formation and preservation of completely sealed overpressured reservoirs.

The MCC in the Anadarko basin contains rock intervals representing the Pennsylvanian, Mississippian, and Devonian systems (Fig. 2). The internal volume of the MCC consists of a network of totally isolated, smaller, nested compartments.

GEOLOGICAL SETTING

The Anadarko basin is an elongated, west-northwest trending basin.

The basin's cross-sectional asymmetry is due to a complex fault zone on its southern margin that separates the basin from the Wichita Mountains uplift (Fig. 2). Structural displacement between the uplift and basin floor exceeds 30,000 ft.'

The Anadarko basin is one of several west-northwest trending tectonic features in southern Oklahoma and Texas. Other basins include the Marietta and Ardmore.

Positive structural features include the Amarillo-Wichita-Criner uplift, the Muenster-Waurika arch, and the Arbuckle-Tishomingo uplift (Fig. 1).

Southern Oklahoma was initially described as an aulacogen by Schatski .4

Subsidence rate curves for the Anadarko basin infer relatively rapid rates of sedimentation in Cambro-Ordovician times. This was followed by relatively slow sedimentation rates in Silurian, Devonian, and early Mississippian times.

Extremely rapid rates of sedimentation for the Late Mississippian to Pennsylvanian age coincide with tectonic development of the Anadarko basin and the Wichita Mountain uplift.

By early Pennsylvanian, the Anadarko basin received 10,000 ft of Springer-Morrowan and Atokan sediments. Pennsylvanian-age deformation within the aulacogen was dominated by displacements along major high-angle fault zones.

As time progressed, flooding of the craton occurred, followed by a slowing in subsidence rate, regression of the sea, and filling of the basin from east to west.

The stratigraphy of the Anadarko basin is dominated by carbonates and sandstones in the Lower Paleozoic, while the middle to late Paleozoic sequences are primarily sandstone and shales with minor carbonates.

Hydrocarbon production in the basin is mostly from Pennsylvanian sandstone and Lower Paleozoic carbonate reservoirs. The pressure domains of these reservoirs are the focus of this study.

PROCEDURE

Pressure compartments can be identified on pressure-depth profiles (PDPS) as deviations from a "normal" gradient (0.465 psi/ft for a standard 80,000 ppm formation brine).

Locations of sealing zones associated with these compartments are also indicated on PDPs at the top and/or bottom of anomalous zones .2 Pressure-depth profiles were constructed for various geographical areas of the Anadarko basin.

Pressure data used in these profiles were obtained from several sources, including drill stem tests, recorded bottom hole pressures in production records, and calculated bottom hole pressures from static initial wellhead shut-in pressures,

During the course of the study, 28,452 pressure data values were analyzed. From this group, 4,439 bottomhole (reservoir) pressure data points were entered into a computer data base.

These consisted of 2,579 reservoir pressure data points calculated from static initial wellhead shut-in pressures,5 1,787 data points from the shut-in bottom hole pressures of drill stem tests 6 and 73 recorded bottom hole pressures from P/Z plots in production records.5

The integration of stratigraphic and pressure data established the following general relationships in the Anadarko basin.

Normal to sub-normal gradients are observed in all stratigraphic horizons down to approximately 7,50010,000 ft below the surface.
Overpressuring is observed in all reservoirs between 7,500-10,000 ft deep and the Mississippian or Woodford intervals.
A return to normal and near-normal pressure gradients is observed in the Hunton and older Paleozoic reservoirs. This pattern in PDP curves is observed in all areas of the Anadarko basin that lie within the limits of the MCC.

Once this pattern of overpressuring was recognized, a systematic approach to mapping pressure gradients in the basin was formulated.

Gradient maps were constructed for the Missourian/Virgilian, Desmoinesian, Morrowan, and Hunton intervals. Potentiometric surface maps were then constructed from the bottom hole pressure data for the same intervals in accordance with the procedure discussed by Dahlberg.7

MCC GEOMETRICAL CONFIGURATION

Pressure gradient and potentiometric surface maps were used to study the vertical and lateral configuration of the pressure regimes in the basin,

The mapped horizons were selected on the basis of stratigraphic position, pressure regime, and availability of data.

The MCC is an elongated body of overpressured rocks that is approximately 150 miles long, 70 miles wide, and has a maximum thickness of 16,000 ft. The myriad of nested compartments that subdivide the MCC display varied overpressured domains.

Sedimentary sequences of the Missourian-Virgilian systems exhibit relatively near-normal pressure gradient values across the Anadarko basin (0.3-0.47 psi/ft). Maximum gradients observed are 0.5-0.65 psi/ft in the Pennsylvanian Marchand sandstone and Granite Wash sequences found below 10,000 ft.

The corresponding potentiometric head values indicate subnormal to normal fluid pressure distribution within most Missourian-Virgilian reservoirs. These values are commonly less than or near the average surface elevation.

The Red Fork reservoirs, which are stratigraphically below the Missourian-Virgilian strata, show a distinct change in pressure gradient and potentiometric head values.

On the shelf area to the north, these reservoirs are underpressured and/or normally pressured (0.4-0.5 psi/ft), while they are overpressured (0.6-0.8 psi/ft) in the deeper parts of the basin to the south and southwest.

The transition from underpressured to overpressured rocks is generally observed at a depth of 10,000 ft. However, along the eastern fringe of the basin in the proximity of the Nemaha ridge, this transition is observed at 7,500 ft deep.

The Morrowan reservoirs exhibit a similar pressure pattern to the overlying Red Fork sandstones. Again, the transition from normal pressures to overpressures occurs at approximately 10,000 ft deep, except in eastern Anadarko basin, where the change occurs at 8,000 ft. In the deep basin, Morrowan rocks show pressure gradients as high as 0.98 psi/ft (more than 20,000 ft of potentiometric head).

Crossing the Mississippian/Devonian Woodford shale, which constitutes the basal seal, into the Devonian to Ordovician/Hunton group, the pressure regime returns to a normal domain.

Pressure gradients within these reservoirs generally range from 0.3-0.5 psi/ft. An exception is indicated by an abnormally overpressured value of 0.74 psi/ft. in Custer County, Okla. This anomaly is interpreted as an isolated Hunton compartment outside the MCC.

The integration of stratigraphic, pressure, and hydrodynamic data illustrate very clearly the geometrical configuration of the MCC.

PDPs over the northern platform and outside the MCC indicate normal to subnormal pressures prevailing in Permian through Lower Paleozoic sequences (Fig. 3). However, basinward, the PDPs show deviations from a normal gradient of 0.465 psi/ft, which occurs within the Lower Missourian rocks.

Maximal deviation is attained in Middle Pennsylvanian through Upper Mississippian sequences. The pressure gradients return to near normal in the Hunton group (Fig. 4).

Three dimensional diagrams of potentiometric surface values of the Morrowan and Hunton rocks, respectively, show the marked difference in the geometry and pressure values (expressed as the peak height) of the Morrowan surfaces as compared to those of the deeper Hunton group (Figs. 5, 6).

SEALS AND SEALING MECHANISMS

The isolation of the MCC as a basin-scale overpressured domain is maintained via encasement by low permeability zones.

The top seal is relatively flat and dips slightly to the west-southwest. It is usually encountered at 7,500-10,000 ft below the surface and is largely controlled by diagenetic processes.

The top seal evolved during the Lower to Middle Pennsylvanian orogenic episode when the basin was subsiding very rapidly. Banded structures induced by diagenetic enhancement of existing lithologies are characteristic features of the top seal.8 9

Dewers and Ortoleva 10 attributed the genesis of the banded structure of seals to mechano-chemical processes that prevailed during the subsidence of the basin.

The MCC is sealed laterally to the south by a zone of low permeability associated with the Wichita Mountains fault zone. However, the MCC is sealed to the northeast, north, and northwest by the convergence of the top and basal seals.

The Woodford shale and, to a certain extent, the Caney shale appear to represent the basal seal of the MCC. This seal is basically stratigraphic, and shale constitutes the major lithology.

IMPLICATIONS FOR EXPLORATION

Understanding basin compartmentation and pressure regimes will lead to more focused exploration and development strategies.

Due to their hydraulically isolated nature, nested compartments may provide drilling prospects that are not constrained by relative structural position to existing reservoirs,

Discovery of these compartments could significantly enhance natural gas reserves estimates. Additional drilling may be justified in existing fields for deeper or laterally positioned nested compartments.

Prediction of compartment and seal geometries and internal reservoir quality should improve drilling success ratios in compartmented sedimentary basins. In addition, it should diminish the hazards associated with drilling abnormally pressured rock sequences.

ACKNOWLEDGMENTS

The author gratefully acknowledges the Gas Research Institute for funding this research through Contract No. 5089-26-1805. The contributions and support of James Puckette, Patrick Ely, and Azhari Abdalla were very valuable. The author also thanks David Powley and John Bradley of Amoco Production Co. who gave helpful suggestions concerning this study. Stimulating discussions with Peter Ortoleva have contributed to the refinement of the concept. Amoco, Dwights Energydata Inc., and Kopco Inc. provided drill stem test, completion, and production data essential to completion of this study.

REFERENCES

Bradley, J.S., Abnormal Formation
Pressure: AAPG Bull., Vol. 59, 1975, pp. 957-973.
Powley, D.E., Abnormal Pressure Seals, Gas Research Institute; Gas Sands Workshop, Chicago, 1987.
Donovan, R.N., Marchini, W.R.D., McConnell, D.A., Beauchamp, W., and Sanderson, D.J., Structural imprint on the Slick Hills, Southern Oklahoma: Okla. Geol. Sur. Circular 90, 1989, pp. 78-84.
Schatski, N.S., The Great Donets Basin and Wichita System. Comparative Tectonics of Ancient Platforms: U.S.S.R., Akad. Navk. Izv. Geolo. Serial, No. 1, 1946, pp. 5-62.
Dwights Energydata Inc., Natural Gas Well Production Histories: Richardson, Tex.
Amoco Production Co., Drill stem test data for various counties in Oklahoma, 1989.
Dahlberg, E.C., Applied Hydrodynamics in Petroleum Exploration: Springer-Verlag, New York, 1982, 161 p.
Tigert, V., and Al-Shaieb, Z.. Pressure Seals: their diagenetic banding patterns: Earth Science Re- view, 29, 1990. pp. 227-40.
Al-Shaieb, Z., Ely, P., Puckette, J., and Abdalla, A., Annual Report prepared for the Gas Research Institute, 1990, Chicago.
Dewers, T., and Ortoleva, P., A coupled reaction/transport/mechanical model for intergranular pressure solution, stylolites, and differential compaction and cementation in clean sandstones: Geochim. Cosmochim. Acta, 1990, in press.