SOURCE ROCK PROPERTIES
Hydrocarbon source rocks (> 0.5 weight percent
TOC) are attractive targets for unconventional drilling because their hydrocarbons are indigenous and their hydrocarbon charge does not depend on the fortuitous and inefficient processes of expulsion from a fine-grained source bed, secondary migration through porous and permeable carrier beds, and accumulation in an adequately sealed reservoir.
Source rocks that contain the highest concentrations of organic hydrogen generate the most hydrocarbons. These are typically beds of lacustrine and marine origin that contain Type I and Type II kerogen and generate both oil and gas during thermal maturation.
Oil-to-rock correlation studies document that the Woodford Shale is a prolific oil source, 4-13 and estimates indicate that as much as 85% of the oil produced in central and southern Oklahoma originated in the Woodford. 13 The Woodford Shale contains high concentrations of marine organic matter, 14-19 with mean organic carbon concentrations of 4.9 percent weight for the Permian Basin (Texas and New Mexico), 5.7 percent weight for the Anadarko Basin (Oklahoma and Arkansas) and 5.2 percent weight for both regions combined, Fig. 2. Organic carbon concentrations range from less than 0.1 percent weight in some chert beds 15 to 35 percent weight in black shale, 18 and the organic matter is mostly oil-prone Type II kerogen. 1,14,15,18 Across the region, the Woodford Shale exhibits a wide range of thermal maturities from marginally immature to metamorphic (Ro = 0.37-4.89 %). 15,20
Fig. 2 . TOC concentrations (weight percent) and statistics for geologic provinces in the southern midcontinent. Mean organic carbon concentration exceeds 2.0 weight percent in each of the provinces listed.
STRATIGRAPHY
The Woodford Shale is mostly Late Devonian, but ranges in age from Middle Devonian to Early Mississippian. 21-24 Age-equivalent strata include the Chattanooga Shale, Misener Sandstone, Sylamore Sandstone, the middle division of the Arkansas Novaculite, upper part of the Caballos Novaculite, Houy Formation, Percha Shale and the Sly Gap Formation. 21,24-30 These units were deposited over a major regional unconformity and represent diachronous onlapping sediments. 21,31-35 In the southern midcontinent, these units are the stratigraphic record of worldwide Late Devonian marine transgression. The Woodford is stratigraphically equivalent to several North American Devonian black shales with active and potential unconventional oil and gas production, including the Antrim Shale (Michigan Basin), Ohio Shale (Appalachian Basin), New Albany Shale (Illinois Basin), Bakken Shale (Williston Basin) and Exshaw Formation (Western Canada Basin).
WELL LOG CHARACTERISTICS
The Woodford is identified primarily by high radioactivity on the gamma-ray log and by its stratigraphic position between carbonates, Fig. 3. The Woodford exhibits low sonic velocity, low resistivity and low neutron-induced radiation. Three subdivisions (the lower, middle and upper units) are commonly recognized in the Woodford, and can be correlated regionally based on well log signatures. 36 The lower unit immediately overlies the regional unconformity, has the lowest radioactivity, and contains more carbonate, silt and sand than the other two units. The middle unit has the highest radioactivity, is the most widespread lithofacies, and consists of black shale with high concentrations of organic carbon, abundant pyrite, resinous spores and parallel laminae. The upper unit has intermediate radioactivity and consists of black shale with few resinous spores and mostly parallel laminae.
Fig. 3 . Characteristic well logs for the Permian Basin and Anadarko Basin regions. (A) Permian Basin, Winkler County, Texas.36 (B) Anadarko Basin, Major County, Oklahoma. 37
LITHOLOGY AND FACIES DISTRIBUTION
The most widespread and characteristic Woodford Shale lithology is black shale. Other common lithologies include chert, siltstone, sandstone, dolostone and light-colored shale, with hybrid mixtures between them. 14,15,21-23,38 Optimum reservoir lithologies are siliceous and include the cherts, siltstones, cherty black shales and silty black shales that are dense and brittle and, when fractured, retain open fracture networks. Production potential is greatest where these lithologies are organic-rich, thermally mature and highly fractured. Naturally-fractured Woodford Shale reservoirs, which have produced hydrocarbons for many decades, are completed in organic-rich chert intervals. 1 Figure 4 displays photomicrographs of cherty black shale in a naturally-fractured Woodford reservoir with bitumen-filled fractures from an oil-producing zone. Figure 4A was taken at a depth of 3,056 ft and has 4.5% TOC, and Figure 4B was taken at 3,065 ft and has 7.8% TOC. The association of chert and fractures in producing reservoirs suggests that the best unconventional wells are likely to be completed in the cherty facies.
Fig. 4 . Photomicrographs of core from Texaco No. 1K Drummond, Marshall County, Oklahoma, 11-6S-6E, North Aylesworth field. 1 White elliptical bodies are recrystallized Radiolaria. Photographed in transmitted plane polarized light.
The Woodford facies distribution is the result of Late Devonian paleogeography and depositional processes. During the Late Devonian, the southern midcontinent lay along the western margin of North America in the warm dry tropics near 15° south latitude. 14,39 Woodford deposition began as sea level rose, drowning marine embayments in what are now the deepest parts of the Delaware, Val Verde, Anadarko and Arkoma Basins, and advancing over subaerially eroded, dissected terrane consisting of Ordovician to Middle Devonian carbonate rocks. The broad epeiric sea that formed had irregular bottom topography and scattered, low-relief land masses which supported little vegetation and few rivers.
Oceanic water from an area of coastal upwelling flowed into the expanding epeiric sea and maintained a normal marine biota in the upper levels of the water column. Net evaporation locally produced hypersaline brine, and strong density stratification developed that restricted vertical circulation and resulted in bottom waters depleted in oxygen. Pelagic debris from the thriving biomass settled to the anoxic sea floor where organic- and sulfide-rich mud accumulated. The slow, continuous settling of pelagic debris was interrupted periodically by frequent storms and occasional earthquakes that triggered turbid bottom flows that supplied silt and mud to proximal shelves and basin depocenters, and caused resedimentation throughout the epeiric sea.
This depositional model explains why quartz grains and chert have very different distributions. Quartz grains represent terrigenous detritus transported from exposed older sources. Chert is biogenic and represents siliceous microorganisms (mostly Radiolaria) that bloomed in the nutrient-rich, upwelled water of the ocean and recrystallized after deposition on the sea floor. Detrital quartz is most abundant in areas near land, especially along the northwestern shelf and in the northwestern part of the Anadarko Basin, and in basin depocenters where turbid bottom flows finally converged. Chert beds increase in abundance and thickness toward the open ocean and are common along the continental margin and in distal parts of the major cratonic basins (Delaware, Anadarko, Marietta, Ardmore and Arkoma). The most distal allochthonous beds in the central area and core area of the Ouachita Tectonic Belt are almost pure radiolarian chert. High concentrations of radiolarian chert coincide with high concentrations of organic carbon along distal highs, such as the Central Basin Platform, Pecos Arch and Nemaha Uplift, and along the craton margin in the Arbuckle Mountain Uplift, Marietta and Ardmore Basins, western Arkoma Basin and frontal zone of the Ouachita Tectonic Belt. Where thermally mature, the organic-rich cherts and cherty black shales in these areas are optimum exploration targets.
THERMAL MATURITY
Thermal maturity follows Woodford structure, with the highest maturities in the deep basins and in orogenic belts, and the lowest maturities along structural highs, Fig. 5. 14,15,18,20,40-43 The Woodford Shale reaches its highest thermally maturity in the Anadarko, Delaware and Arkoma Basins where it is most deeply buried, and in the Ouachita Tectonic Belt where stratigraphically equivalent beds have been locally metamorphosed. Intermediate maturities occur in shelf settings, and the lowest maturities occur on structural highs such as the Central Basin Platform, Pecos Arch, Nemaha Uplift, Arbuckle Mountain Uplift and the frontal zone of the Ouachita Tectonic Belt. In deep basins, the Woodford Shale is in the gas generation window, whereas in the shelf and platform settings, the Woodford is in the oil generation window. 14,15
Fig. 5 . Map showing thermal maturity of Woodford Shale and age-equivalent units in (A) Anadarko and (B) Permian Basin regions. 3 Patterns are based on vitrinite reflectance (%Ro).
POTENTIAL PRODUCTION TRENDS
Potential production trends have been qualitatively ranked based on the probability that brittle or naturally fractured, thermally mature organic-rich beds of Woodford Shale are present in the subsurface, Fig. 6. The trends are designated as areas of probable, possible, local and poor success as follows. Probable success areas are those where organic-rich Woodford Shale is in the gas generation stage of thermally maturity and where large volumes of gas are likely to reside. Possible success areas are those where organic-rich Woodford beds are in the oil window and where the formation is shallow enough for economic drilling and for open fracture networks to persist. Local success areas are those in shelf settings where the Woodford Shale is relatively thin, but thermally mature and at a relatively shallow depth. Poor success areas are those where the formation is exposed at the surface or is shallow and unconfined, and where Woodford Shale or equivalent units have been metamorphosed or have very low organic carbon content.
Fig. 6 . Map showing hydrocarbon production potential and estimated volumes of oil-in-place and gas-in-place for Woodford Shale and age-equivalent units in the (A) Anadarko and (B) Permian Basin regions. 3
ESTIMATION OF RESOURCE POTENTIAL
The resource potential estimations assume that oil and gas in the Woodford Shale are indigenous, and were calculated based on organic carbon concentration, organic hydrogen concentration, organic matter type, thermal maturity and facies volumes (thickness times area), Fig. 6. 3 While this is not an assessment of recoverable oil and gas, it does estimate total gas-in-place and oil-in-place through mass balance calculations based on the concentration of organic hydrogen in the source beds. 3 The data suggest that total in-place gas in the Woodford Shale is on the order of 830 Tcf and total in-place oil is on the order of 250 Bbbl in the southern midcontinent. These volumes include 130 Bbbl of oil-in-place in the Anadarko Basin region, and 230 Tcf of gas-in-place and 120 Bbbl of oil-in-place in the Permian Basin region.
In the Anadarko Basin region, the estimated gas potential is 600 Tcf in the area of probable success, an area that includes the Anadarko and Arkoma Basins. The estimated gas potential is 0.24 Tcf and the estimated oil potential is 70 Bbbl in the area of possible success, encompassing the Nemaha Uplift, Marietta and Ardmore Basins, Arbuckle Mountain Uplift, southern flank of the Anadarko Basin, and frontal zone of the Ouachita Tectonic Belt in Oklahoma. About 4.4 Tcf of gas-in-place and 60 Bbbl of oil-in-place are estimated for the area of local success, which includes most of the northern and central Oklahoma Platforms.
In the Permian Basin region, the estimated gas potential is 220 Tcf in the area of probable success, which includes the Delaware and Val Verde Basins. The estimated gas potential is 0.11 Tcf and the estimated oil potential is 35 Bbbl in the area of possible success, encompassing the Central Basin Platform and northern flank of the Pecos Arch. About 9 Tcf of gas-in-place and 84 Bbbl of oil-in-place are estimated for the area of local success, which encompasses much of the shelf and platform provinces and most of the Midland Basin.
Although estimates of the volume of undiscovered hydrocarbons are inherently problematic because of the assumptions that must be made to complete the calculations, the mass balance approach yields orders-of-magnitude for in-place oil and gas, and provide a consistent means to compare and rank different areas of interest as to their hydrocarbon production potential.
CONCLUSIONS
The Woodford Shale is a major unconventional energy resource with the potential for producing significant volumes of both oil and gas. Intuitively, its status as a world-class oil source rock indicates that the formation should contain large residual concentrations of hydrocarbons, and analytical data from numerous studies confirm this inference. The inherent inefficiency of hydrocarbon expulsion is the primary reason why source rocks like the Woodford retain large volumes of oil and gas and are attractive targets for unconventional exploration. Given the ubiquity and magnitude of oil and gas shows, local production from naturally fractured reservoirs, recent unconventional production from the Woodford Shale in Oklahoma, successes in unconventional resource recovery from analogous formations, and current oil and gas prices, the Woodford Shale in the southern midcontinent is a compelling exploration target.
Optimum locations for exploration are where organic-rich beds are currently in the oil or gas generation window. Optimum reservoir facies are those comprising brittle lithologies capable of maintaining open fracture networks. The best reservoirs are likely to be completed in mature organic-rich cherts and cherty black shales but other lithologies, such as sandstone, organic-rich siltstone, and silty black shale, can also be expected to produce locally. Areas having the greatest production potential and most prospective lithologies are the Anadarko Basin in Oklahoma, Marietta and Ardmore Basins in Oklahoma, Arkoma Basin in Oklahoma and Arkansas, frontal zone of the Ouachita Tectonic Belt, Delaware Basin in Texas and New Mexico, Central Basin Platform in Texas and New Mexico and the Val Verde and Midland Basins in Texas.
ACKNOWLEDGEMENTS
The author is indebted to Indiana Geological Survey colleagues Kimberly H. Sowder, Barbara T. Hill and Renee D. Stubenrauch, who drafted the figures and formatted the photographs for this article. Also, IGS staff scientists Margaret V. Ennis, Nancy R. Hasenmueller, Maria D. Mastalerz, and Charles W. Zuppann reviewed the article and offered constructive criticisms. IGS editor Deborah A. DeChurch proofread the manuscript. Publication is authorized by John C. Steinmetz, State Geologist and Director of the Indiana Geological Survey.
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.
THE AUTHOR
John B. Comer is a Senior Scientist at the Indiana Geological Survey with an academic appointment at Indiana University. He earned a BA from Ohio Wesleyan University, an MS from The University of Wisconsin-Milwaukee and a PhD from The University of Texas at Austin, all in geology. During his 36-year career, he worked as a research scientist in the geochemistry group at the Amoco Production Company Research Center in Tulsa, an assistant and associate professor at Tulsa University and the Geochemistry Section Head at the Indiana Geological Survey. Dr. Comer has conducted research in organic, inorganic and environmental geochemistry, clastic sedimentation, sedimentary petrology and the deposition and diagenesis of organic-rich rocks. He is an active member of AAPG, SEPM and GSA and has authored more than 120 scholarly papers and technical reports in geology and geochemistry.
Fig. 1 . Map showing geologic provinces with Woodford Shale in the (A) Anadarko Basin and (B) Permian Basin. 3
