Abstract

Siluro-Devonian carbonates from both the Mid-Continent and West Texas contain prolific oil and gas reservoirs. A comparison between these carbonates indicate that the rocks are stratigraphically similar and often reservoir development is analogous. The purpose of this paper is not to compare reservoir to reservoir, but instead to discuss the types of subtle traps within the Hunton Group of the Anadarko and Arkoma basins for perhaps a better understanding of the reservoir potential in the Siluro-Devonian of the Permian Basin.

In both regions the majority of Siluro-Devonian production to date is associated with rather well defined structural and/or truncation-style traps. Yet the trapping mechanism in these settings depends to a large extent on the development of particular depositional facies within the carbonates.

The Silurian was a time of widespread carbonate deposition. The transgression responsible for the expansive sea was probably as extensive as that of the Ordovician. Marine waters covered the Trans-Continental Arch and most of the Canadian Shield (Fig. 1), where a relatively thin veneer of carbonate was deposited. Extensive reefs are known at the cratonic edges in Nevada, arctic Canada, Franklin Trough of Baffin Island, and Greenland. The typical basinal extracratonic sediments are dark graptolitic shales.

Correlations based on paleontology and lithologic similarities indicate that the Chimneyhill Subgroup and Henryhouse Formation of the Anadarko and Arkoma basins are roughly equivalent to the Fusselman Formation and “Upper Silurian” (Wristen) rock unit, respectively, of the Permian Basin (Fig. 2).

Some of the Silurian was removed by erosion prior to the Devonian. In both Anadarko and West Texas basins the Late Silurian evaporites of Michigan and Hudson Bay basins are missing. The oldest Devonian strata are much more restricted in their distribution than the Silurian. Early Devonian was also a time of widespread carbonate deposition—but only east of the Trans-Continental Arch. Remnants of these Lower Devonian units as young as Onondaga of New York (Coblenzian) exist in the West Texas Basin, Oklahoma, Tennessee, Kentucky (southern part of Illinois Basin), Cincinnati Arch, and the Michigan Basin. Similar and distinctive lithologic units may be traced widely over these areas. For example, a white chert and limestone (Camden, Penters, Stribling, and Sallisaw) in the upper Hunton and high in the Lower Devonian as well as the older, well sorted (Oriskany-type) quartz sand, may be traced from Tennessee to far West Texas.

Increasingly careful study of Lower—Middle Silurian and Lower Devonian strata in Oklahoma indicates upward-shoaling shelf cycles which end in tidal flat or restricted marine units. Approximately four transgressions and regressions occurred in the Llandoverian, and one main transgression-regression in the Wenlockian. More than five major evaporite cycles are represented in the Upper Silurian Salina evaporites of the Michigan basin. These are equivalent in part to cyclic and reefy Ludlovian carbonates on the Indiana shelf. These cycles offer a tool for future detailed correlation from basin to basin.

Although units of Hunton strata thicken southward into the Anadarko basin, they seemingly do not demonstrate the extensive downwarping which might be expected by comparison with other basinal areas. The stratigraphic succession in the Hunton is almost identical to that of the West Texas (Tobosa) basin. The Chimneyhill, Henryhouse, and Devonian units have direct lithologic analogs, but total thickness in West Texas is greater than that in the Anadarko basin.

In the Mid-Continent accurate correlation and subdivision of the Hunton require an understanding of the overall depositional environment and history. The depositional model for the Silurian Chimneyhill and Henryhouse formations, and the Devonian Haragan and Bois d’Arc formations, is a carbonate ramp. Both aggradational and progradational sequences formed, ad did several unconformities during periods of erosion and non-deposition. The Frisco Formation, on the other hand, was deposited on submerged paleohighs, probably partly as a mud-mound type of deposit.

Diagnetic overprints are also key to understanding Hunton porosity. Karstification is one of the primary processes in the development or destruction of Hunton reservoirs. In many cases karst profiles can be recognized by log character and then correlated and mapped. The Sallisaw (Penters) reservoir in the Arkoma basin is a mappable collapse karst breccia formation. Probably the Ives breccia of the Llano uplift is a remnant of this unit.

Using the above depositional and diagenetic models as a guide, subdivisions of the Mid-Continent Hunton, based on regional markers related to changes in sea level between progradational episodes, are recognizable and correlatable throughout the Anadarko-Arkoma region. Comparisons between core data and log signatures, along with application of depositional cycles, permit more detailed correlations as their component facies are recognized by log character. Reservoir-prone facies within the carbonate cycles, then are identifiable, correlatable, and mappable.

The Cheyenne Valley Field in Major County, Oklahoma, West Edmond Field in Oklahoma County, Oklahoma and Bonanza Field in Sebastian County, Arkansas, represent excellent examples of the relationship between facies and reservoir development which can be delineated by correlation from an environmental and/or diagenetic perspective. They represent exploration models for subtle traps in Siluro-Devonian carbonates.

In West Texas the thick Siluro-Devonian section produces in complex structural and stratigraphic situations. Firstly, anticlines and fault blocks associated with Permo-Pennsylvanian orogeny provide structural traps. The overlying Woodford black shale furnishes source beds for much oil associated with the Siluro-Devonian section and most such reservoirs are overlain directly by Woodford or exist close to the Woodford subcropping edgeline. The Lower Silurian Fusselman Formation is a widespread carbonate, often porous where dolomitized. It consists of two facies separated by an internal unconformity: a lower oolite with some primary porosity and an upper crinoidal and dolomitic facies. The top of the Fusselman under the higher Middle Silurian Wristen Formation is also unconformable. The Silurian consists of several transgressive-regressive cycles like the Hunton of Oklahoma. In the northern and central Midland basin where these units subcrop under the Woodford, extensive karst breccias with paleosoils occur at the top of the eroded Lower Silurian units (e.g., Wells Field in Dawson County). In Winkler County at the Flying W field, the unconformable top of the Fusselman has a tripolitic residual reservoir for 100 feet below the surface.

The complex facies within higher Middle Silurian strata change from thin gray-green shale in the southern basin to massive 2000 foot Niagaran shelf carbonate, and grade to dolomitized porous sections of reservoir rock in the northern Central Basin Platform and adjoining areas in Lea County, New Mexico and northern Midland basin (e.g., Breedlove Field, Martin County, Texas). Individual reefy patches, possibly rimming a Silurian carbonate platform developed to the north, also afford reservoirs in these areas (e.g., University Lands Block 9 Field, Andrews County, Texas). The overlying thick Lower Devonian siliceous limestone, the Thirty-one Formation, is very cherty, very competent, and highly fractured, adding to reservoir potential (e.g., Bedford Field, Andrews County, Texas). Also internal facies changes within the Devonian section result in porous layers traceable across fields (e.g., Thirty-one pool, Crane County, Texas).

The subcrop of Siluro-Devonian strata beneath Woodford or Permo-Pennsylvanian shale provides opportunity for extensive truncation and karstification of both reefy and platform carbonate and Devonian bioclastic beds (e.g., Fullerton area of northern Andrews County and Dollarhide Field of southwest Andrews County, Texas). In TXL Field, tripolitic chert at the top of the Lower Devonian under a Permian cover forms the reservoir. Under these unconformities breccia and vugs and tripolite, due to meteoric leaching as well as dolomitization, offer important contributions to karstic porosity and permeability. The relative importance of these two major unconformities (pre-Woodford and pre-Permian) to karst development needs to be evaluated.