Hunton play in oklahoma (including northeast texas panhandle)

Based on a workshop co-sponsored by the Oklahoma Geological Survey and PTTC's South Midcontinent Region on October 17, 2000 in Norman, Okla.

BOTTOM LINE

The Hunton Play, in spite of many decades of production from numerous Oklahoma basins, has remained too poorly understood to undertake confident exploration programs under adverse conditions, such as in the deep Anadarko Basin or in unfamiliar areas, such as the Carney play on the Cherokee Platform.

PROBLEM ADDRESSED

The stratigraphic, paleogeographic and structural context of a number of Hunton producing fields is compiled in a comprehensive regional study that builds on work done in the past, and adds the new dimensions of sequence stratigraphy to better understand and predict the occurrence of Hunton reservoirs.

KEY WORDS:

Carbonate deposition, Carbonate diagenesis, Hunton, Karstification, Incised valleys, Sequence stratigraphy, Submarine channels

SPEAKERS

Kurt Rottman, Consulting Geologist
Oklahoma City, Oklahoma

Edward A. Beaumont, Consulting Geologist
Tulsa, Oklahoma

Robert A. Northcutt, Independent Petroleum Geologist Oklahoma City, Oklahoma

Zuhair Al-Shaieb, Oklahoma State University
Stillwater, Oklahoma

TECHNOLOGY OVERVIEW

The shallow marine carbonate play in the Silurian Hunton Group in Oklahoma includes most of the Anadarko, Ardmore, Marietta and Arkoma basins and the southwestern part of the Cherokee platform. Production depths range from 465 to 24,928 feet. Cumulative production between 1979 and 1999 totaled over 157 million BO, plus abundant gas. Both structural and stratigraphic traps characterize the play. Structural traps occur mostly on the Cherokee platform and in the deep Anadarko and Ardmore basins. Stratigraphic traps occur mainly on the Anadarko shelf and on the Cherokee platform. Although Hunton reservoirs are present in 48 of Oklahoma's 77 counties, seven oil and gas fields, discovered between 1921 and 1987, were selected in this study as most significant to the development of the Hunton in Oklahoma. In order of discovery, these fields are: Bebee-Southwest Konawa (Pontotoc Co., 1921), Fitts (Pontotoc Co., 1933), West Edmond (Oklahoma Co., 1943), North Custer City (Custer Co., 1959), Aledo (Custer Co., 1967), West Mayfield (Beckham Co., 1972) and Golden Trend (Garvin Co., 1946). Brief exploration and development histories along with geological overviews are presented for each of the seven fields. Detailed reservoir characterization was performed in three additional fields to serve as examples for future exploration and production efforts.

Numerous previous studies contributed to our current knowledge of the Hunton. Perhaps most notable are the 1975, 1980 and 1993 subregional to regional stratigraphic studies performed by Thomas W. Amsden. A regional study sponsored by the Oklahoma Geological Survey and PTTC in 2000 was summarized at the workshop. The goals of this current regional study, which was based on more than 2,000 geophysical logs, were to: (1) project trends and depositional environments from the shelf area into the deep Anadarko, (2) further tie Amsden's paleontologic and lithologic descriptions into the subsurface, (3) discuss the reservoir implications of Hunton karstification, (4) discuss Hunton-Woodford stratigraphic relationships in terms of their effects on reservoir geometries, (5) examine the effects of structure and continued structural movement on Hunton reservoirs, (6) discuss dolomite origins in the Chimneyhill Subgroup and the Henryhouse Formation, and (7) suggest completion techniques for high watercut Hunton reservoirs. Hunton stratigraphy and paleogeography, as revealed by the 2000 study, occupy a lengthy treatment in the publication. Included correlation charts cover the Hunton interval from the Ordovician Sylvan Shale below to the Upper Devonian Woodford Shale above over western Oklahoma and the Texas Panhandle. Because it manifested many features that are addressed in the workshop, the type log for the study was chosen from a well in Canadian County.

The Hunton Group comprises shallow water carbonates deposited on a gently inclined ramp. Facies belts developed more or less parallel to depth contours. Changes in sea level caused migration of facies toward and away from the shoreline and gave rise to numerous disconformities within the Hunton stratigraphic section. The Henryhouse formation of the Hunton Group exhibits three separate stages of dolomitization: dolomites formed simultaneously with or shortly after deposition in shallow to supratidal hypersaline environments, marine and fresh water mixed dolomite, and deep-burial or thermal dolomite. Moldic, vuggy, intercrystalline and fracture porosity are represented. Karst-related reservoirs are common in the Hunton Group. One type characteristically exhibits solution-enlarged conduits and collapse/cavern fill breccias indicative of highly focused flow of meteoric waters. Others formed by more diffuse flow are characterized by abundant interparticle and vuggy porosity and are the best hydrocarbon-producing reservoirs.

The workshop provided a comprehensive overview of the basic principles of carbonate reservoir formation, which is useful information for operators in carbonate rocks in any context of age or location. Carbonate mineralogy and the interplay between low- and high-magnesium calcite, aragonite, and dolomite are the fundamental chemical factors influencing carbonate formation and diagenesis. Carbonate rock types are also viewed, from the perspective of their physical components, as being composed of varying proportions of discrete grains (of various origin and chemical/mineralogical composition), mud-size particles or matrix, and cements. A number of the most commonly applied carbonate rock classification schemes are presented, as are the more common models of large-scale carbonate deposition. The summary presented of the evolution and maturation of models of carbonate diagenesis is particularly informative and applicable to Hunton reservoirs.

The workshop included a detailed reservoir study of the Leedey field in Dewey Co., Oklahoma, in the deeper part of the Anadarko Basin where drilling to Hunton depths has been sparse due to over-pressuring in overlying formations. The Leedey field is a good example of structurally controlled Hunton production. It exemplifies the important relationship between the Hunton and the overlying Woodford Shale. In Middle Devonian time, the Hunton was uplifted, subaerially eroded, and underwent karstification prior to deposition of the Woodford. Considerable topography was developed on the Hunton by the drainage system developed on its surface at this time of exposure. Woodford sediments exhibit typical channel fill facies within these incised valleys. Production was established in the field in 1973 and cumulative production is in excess of 32 BCFG.

A second detailed Hunton reservoir study, also in the Anadarko Basin, addressed the East Arnett field in Ellis Co., Oklahoma. Although still below the over-pressured zone in the basin, this field is located in an area where blowouts and lost circulation are not prevalent problems. This field is also structurally controlled, but distinct periods of structural activity coupled again with pre-Woodford exposure give rise to structural and stratigraphic complexities whose final interpretation is still uncertain. Since discovery in 1965, this reservoir has produced more than 18 BCFG.

The Prairie Gem field in Lincoln Co., Oklahoma is located on the Cherokee Platform. This field, discovered in 1959, was chosen for detailed study and presentation at the workshop because: (1) it produces from rocks originating in depositional environment unique in comparison to typical Hunton sediments and (2) because it is near (within four miles) and analogous to a currently active play developing near Carney, Oklahoma. Facies and stratigraphic relationships are similar to those in the Carney Hunton play. In the Prairie Gem area, Hunton strata are characterized by: (1) repeating sequences of strata with identical geometries and depositional environments, (2) a basal sequence with coarser strata exhibiting good porosity and permeability, (3) sharp contacts between zones, each exhibiting fining- upward sequences, (4) the distal part of zone C exhibits an arcuate shape, (5) a general absence of terrestrial deposits, and (6) old driller's logs referring to the porous zones as Hunton Detrital.

The favored depositional model reflects the influence of a local submarine fan on an otherwise typical shallow marine shelf setting. The shallow submarine fan complex probably extended to the northeast of the Prairie Gem field location and normal shallow shelf deposits extended westward. A ground-water-mixing model of dolomitization probably prevailed in the Prairie Gem area. This led to a close association of dolomite with the previously existing porosity associated with oolite bars and carbonate sands contained in shallow submarine channels at the shelf-break location of the field. The Carney Hunton play is typified by high initial GORs, but very low shows on drillstem tests and low initial-potential flow rates. Oil and gas rates gradualy increase with time, especially after high volume pumps are installed. This "retrograde oil cut," as it is called, is a result of the pumps creating extensive areas of low pressure in fractures and high permeability strata. Water and mobile oil migrate toward the lower pressure regions. Eventually, at the point at which the water drive ceases to have an influence, more conventional production techniques can be applied.

LESSONS LEARNED

New interpretations based on sequence stratigraphic concepts and modern depositional and diagenetic models can yield useful insights for both exploration and development in formations, plays and reservoirs that have been poorly understood for many decades.

SPECIAL PUBLICATION

Hunton Play in Oklahoma (Including Northeast Texas Panhandle), Oklahoma Geological Survey Special Publication 2000-2, order by phone 800-330-3996.

CONNECTIONS:

Kurt Rottman
Consulting Geologist
7113 Northwest 30th Street
Bethany, Oklahoma 73008
Phone: 405-491-0334 

Edward "Ted" Beaumont
Consulting Geologist
1743 East 71st Street
Tulsa, Oklahoma 74119
Phone: 918-496-7777 

Robert A. Northcutt
Independent Petroleum Geologist,
11422 Redrock Road
Oklahoma City, Oklahoma 73120
Phone: 405-755-4783 Fax: 9187-755-0677 

Zuhair Al-Shaieb
Oklahoma State University
105 Noble Research Center
Stillwater, Oklahoma 74078
Phone: 405-744-6358

For information on PTTC’s South Midcontinent Region and its activities contact:
Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-325-7069, E-mail cjmankin@ou.edu

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