WOODFORD SHALE IN THE ANADARKO BASIN: COULD IT BE ANOTHER 'BAKKEN TYPE' HORIZONTAL TARGET?

Dec. 3, 1990
Timothy C. Hester James W. Schmoker Howard L. Sahl U.S. Geological Survey Denver The Woodford shale is one of several organic rich "black" shales of late Devonian and early Mississippian age present in basins of the North American craton. Other examples of similar age include the Antrim shale of the Michigan basin, the new Albany shale of the Illinois basin, the lower and upper members of the Bakken formation of the Williston basin, the Exshaw formation of the Alberta basin, and the "Devonian"
Timothy C. Hester
James W. Schmoker
Howard L. Sahl

U.S. Geological Survey
Denver

The Woodford shale is one of several organic rich "black" shales of late Devonian and early Mississippian age present in basins of the North American craton.

Other examples of similar age include the Antrim shale of the Michigan basin, the new Albany shale of the Illinois basin, the lower and upper members of the Bakken formation of the Williston basin, the Exshaw formation of the Alberta basin, and the "Devonian" shales of the Appalachian basin.

Where thermally mature, these black shales are economically important as hydrocarbon source rocks.

The Woodford shale is widely regarded as a major source rock in the Anadarko basin. This report describes regional depositional trends and organic carbon content of the Woodford shale as evidenced by wire line logs in the Oklahoma portion of the Anadarko basin (Fig. 1).

The relation between depositional patterns and organic carbon content is discussed, and the mass and distribution of organic carbon ,in thermally immature and mature areas of the Woodford shale is estimated.

To better document variations and regional trends within the Woodford shale, the formation is considered here in terms of three informal members analogous to those described by Ellison for the Woodford shale in the Permian basin of West Texas and Southeast New Mexico.

GEOLOGIC SETTING

The Woodford shale is present throughout most of the Anadarko basin of Oklahoma and is thought to be a major source of the basin's hydrocarbons.

The Woodford shale is usually treated as a single depositional package in the sense that its physical and chemical properties are not considered with respect to vertical stratigraphy.

As a result, changes in depositional and environmental factors during Woodford time that affected the source rock characteristics of the formation have been largely unexplored. In this study, the Woodford shale is subdivided into lower, middle, and upper members on the basis of log character. Differing kerogen concentrations account for the distinctive character of the three members and are the physical basis for this subdivision.

Isopachs of the three members of the Woodford reveal a positive structural feature, parallel with and about 75 miles (120 km) north of the Amarillo-Wichita uplift, that divided the Woodford into northeast and southwest depocenters and was a hinge line separating areas of regional basement flexure during Woodford time.

Lower and middle members of the Woodford shale thicken to the southwest into the now eroded central trough of the southern Oklahoma aulacogen.

The upper member thickens to the northeast towards the Sedgwick basin of south-central Kansas.

In the southwest region, present day thermal maturity of the Woodford is generally greater than that of peak oil generation, whereas in the northeast region, the Woodford is generally immature to marginally mature. As a result of these regional depositional and thermal maturity trends, most hydrocarbons sourced by the Woodford shale of the study area were generated from the lower and middle members.

The regional tripartite zonation of the Woodford shale reveals depositional patterns and variation in organic matter content that are obscured if the formation is treated as a single unit.

Subdivision of the Woodford into three informal members, as suggested here, provides a framework for the planning and integration of detailed source rock studies of the formation.

GEOLOGIC SETTING

The late Devonian and early Mississippian age Woodford shale is present in most of the Oklahoma portion of the Anadarko basin.

The Woodford is deposited on a major regional unconformity developed in the late Devonian (Amsden, 1975) and conformably overlain by shales and limestones of early Mississippian (Kinderhookian) age.

Total thickness ranges from near zero to about 125 ft (40 m) on the extensive northern shelf areas and increases to more than 900 ft (270 m) in limited parts of the deep Anadarko basin (Amsden, 1975). Maximum thickness in the area of the study (Fig. 1) is about 300 ft (90 m).

The Woodford is a highly radioactive, carbonaceous and siliceous, dark gray to black shale. In most areas of the basin, total organic content (TOC) of the Woodford shale exceeds a commonly accepted shale source rock minimum of 0.5 wt % (Tissot and Welte, 1984).

Organic matter in the Woodford is thought to be a mixture of Type 11 and Type III kerogens preserved as a result of deposition in anoxic (but not necessarily extremely deep) waters (Heckel, 1972; Cluff, 1981). Large areas of the Woodford shale has attained the maturation levels required for hydrocarbon generation (Schmoker, 1986).

In terms of wire line character, the Woodford shale can be generally described as two similar shales separated by a less dense, more radioactive, and often more resistive middle member (Hester and others, 1988). For this reason, the Woodford is considered here to consist of three informal stratigraphic units: the lower, middle, and upper members of the Woodford shale.

The typical log character of these three members is illustrated (Fig. 2). The Woodford and its members were identified in the 99 wells of this study (Fig. 1) by Hester and others (1 988).

The Anadarko basin is a Paleozoic basin that formed in two stages. The Woodford shale was deposited upon the predominantly carbonate sediments of the first stage southern Oklahoma aulacogen (Feinstein, 1981).

The present day configuration of the basin, reflected by structure on top of the Woodford, developed primarily in a post-Woodford (Pennsylvanian and Permian), second stage foreland style basin.

The Woodford shale dips into the Anadarko basin from subsea depths of roughly 4,000 ft (1,200 m) near the Kansas-Oklahoma border to more than 25,000 ft (7,600 m) near the Amarillo-Wichita uplift and thus encompasses an unusually broad range of thermal maturities.

Vitrinite reflectance (R,) ranges from slightly less than 0.5% on the northern shelf to well over 2% in the deep basin (Fig. 3). Thermal maturity trends are generally similar to structural trends, although vitrinite reflectance contours cut those of present day structure in parts of the central and northeastern study area.

DATA SET

Data of this report were collected from 99 wells distributed throughout the study area (Fig. 1).

In each well, depth, thickness, and average formation density are taken directly from wire line logs. Total organic carbon is calculated from average formation density (equation 1), and the mass of organic carbon per unit surface area is calculated using equation 2.

Vitrinite reflectance at each well location is picked from contoured data of Cardott and Lambert (1985) and B.J. Cardott (Oklahoma Geological Survey, written communication, 1987).

Because the specific gravity of kerogen is low relative to that of shale matrix minerals (Smith and Young, 1964; Kinghorn and Rahman, 1983), variations in the formation density of organic rich, low porosity shales such as the Woodford can be equated to changes in organic matter content (Schmoker, 1979, 1980; Hasmy and others, 1982).

TOC (weight percent) is calculated here according to the equation (Schmoker and Hester, 1982, equation 7):

TOC = (A/pb) - B (1)

where Pb is average formation density (g/cm3), A = 156.956, and B = 58.272.

A and B are empirically derived constants incorporating shale and organic matter properties and interactions. These constants are determined from the least squares fit to the crossplot of 148 pairs of laboratory TOC analyses and log derived formation densities.

The calibration data represent four organic rich black shales of late Devonian and early Mississippian age of which the Woodford is a typical example,

TOC is independent of thickness and thus does not reflect the total amount of kerogen in a formation. To better characterize source rock properties of the Woodford shale, the mass of organic carbon per unit surface area (OC/cm2) is introduced:

OC/cm2 =

(TOC/100) (pb) (DZ) (2)

where DZ is formation or member thickness (cm) and OC is in grams.

DEPOSITIONAL PATTERNS

The total thickness of the Woodford shale is less than 125 ft (40 m) over most of the study area but increases rapidly to the south near the Amarillo-Wichita uplift.

A Woodford isopach map depicts the northern shelf area of the southern Oklahoma aulacogen prior to the rapid subsidence of Pennsylvanian and Permian time.

The heavy arrow of Fig, 3 marks the axis of a structural element that divides the study area into southwest and northeast regions.

This positive feature influenced deposition throughout Woodford time and was a hinge line separating areas of regional basement flexure and downwarping.

Local thickness variations of the lower member of the Woodford shale (Fig. 4) reflect the eroded and karsted surface on which it was unconformably deposited. The structural element marked in Fig. 3 separates the lower member of the Woodford into two distinct depocenters (Fig. 4).

Maximum thickness in the northeast depocenter is only about 25 ft (8 m). Isopachs of the southwest depocenter do not close in the study area but show thickening into the (now eroded) deep axis of the southern Oklahoma aulacogen.

During deposition of the lower member of the Woodford the northeast region of the study area was quite stable, but the central trough of the southern Oklahoma aulacogen was actively subsiding.

Because the lower member tended to smooth and bury pre-Woodford topography, the thickness of the middle member of the Woodford shale (Fig. 4) varies more uniformly than that of the lower member,

Like the lower member, maximum thickness in the northeast depocenter is about 25 ft (8 m). In the southwest depocenter, the middle member gradually thickens into the deep axis of the southern Oklahoma aulacogen (Fig. 4).

Fig. 4 indicates continued stability of the northeast depocenter and moderate subsidence of the central trough of the southern Oklahoma aulacogen during middle Woodford time.

In contrast to the lower and middle members, isopachs of the upper member of the Woodford shale do not close in the northeast depocenter, but show thickening to the northeast into Kansas (Fig. 4).

Also in contrast to the lower and middle members, the upper member does not thicken appreciably to the southwest near the Amarillo-Wichita uplift.

Fig. 4 indicates a shift in depositional pattern during upper Woodford time due to both initial development of the Sedgwick basin of south-central Kansas (Kelly and Merriam, 1964) and marked slowing of subsidence along the axis of the southern Oklahoma aulacogen.

Average thickness of both the lower and upper members of the Woodford shale is about 30 ft (9 m), but thickness distributions of these two members appear quite different. The thickness distribution of the lower member reflects deposition on an irregular, eroded and karsted surface that had greater topographic relief in the southwest region than in the northeast region.

The bimodal thickness distribution of the upper member reflects a significantly greater accumulation of sediment in the northeast region than in the southwest region.

The middle member has an average thickness of only 20 ft (6 m).

In the southwest region, the thickness distribution of the middle member resembles that of the lower member, but with no outliers. In the northeast region, the middle member is thin and approximates a uniform blanket of sediment.

Depositional relationships between the three members of the Woodford shale suggested are more easily visualized in the smoothed regional cross sections of Figs. 5 and 6.

The middle member generally onlaps the lower member.

The upper member tends to offlap the middle member in the southwest region and to onlap the middle member in the northeast region, reflecting the shift of deposition to the northeast during late Woodford time.

The series of basement movements indicated by Figs. 5 and 6 hinged along the northwest-trending axis identified in Fig. 3.

Woodford depositional patterns (Figs. 5 and 6) depict the transition between the first-stage southern Oklahoma aulacogen and the second-stage Pennsylvanian and Permian foreland downwarping that together shaped the present-day Anadarko basin.

TOTAL ORGANIC CARBON

TOC of the lower, middle, and upper members of the Woodford shale averages 3.2, 5.5, and 2.7 wt %, respectively, and is rarely less than a commonly accepted shale source rock minimum of 0.5 wt % (Tissot and Welte, 1984).

As calculated from density logs, the middle member is significantly richer in organic carbon than the lower and upper members.

Higher kerogen content accounts for the lower density, higher gamma-ray intensity, and moderately higher resistivity typical of the middle member (Fig. 2) and is the physical basis for the subdivision of the Woodford shale into three members according to log character.

TOC of the Woodford shale's three members is mapped in Fig. 7. Local variations in TOC probably relate to the accumulation and preservation of kerogen early in the depositional history of the formation.

The general grain of the contours, however, tends to parallel present-day structure and thermal maturity (Fig. 3) and probably reflects regional processes of kerogen maturation during burial.

The lowest values of TOC occur along the western edge of the lower and upper members and are perhaps indicative of poor organic-matter preservation at the time of deposition. Other occurrences of thinning or pinchout are not characterized by unusually low TOC values (Fig. 7).

TOC does not decrease as thickness decreases to near zero, showing that organic matter was in general not anomalously degraded near depositional edges. At the other extreme, TOC does not decrease as thickness increases, showing that a fixed supply of organic matter was not diluted by varying rates of clastic sedimentation.

Thermal maturity of the northeast region of the study area is significantly lower than that of the southwest region. TOC of each member of the Woodford shale tends to be higher in the northeast region than in the southwest region.

Lower TOC associated with higher thermal maturity is interpreted here to result from the progressive depletion of organic carbon by the conversion of kerogen to oil and gas and the subsequent expulsion of these hydrocarbons from the formation.

The middle member, which is a better source rock in terms of TOC than the lower and upper members, also appears to be a better source rock in terms of generation potential and expulsion efficiency.

An alternative but less likely explanation for the differences between northeast and southwest regions is that the original, preserved quantity of organic matter decreased regionally from northeast to southwest.

Crossplots of TOC versus burial depth show that maximum values of TOC decrease by 3-4 wt % in each member of the Woodford shale as depth increases to about 18,000 ft (5,500 m) or as vitrinite reflectance increases to about 2%.

This decrease of maximum TOC with depth, like the decrease of average TOC from northeast to southwest regions, is attributed here to the progressive loss of organic carbon from the formation as hydrocarbons are generated and expelled.

At depths greater than 18,000 ft (5,500 m), maximum values of TOC no longer decline, suggesting that the potential for generation of additional hydrocarbons has been largely exhausted.

About 40-50% of the organic carbon originally preserved in the Woodford shale remains in the formation at high thermal maturities in the form of inert kerogen or unexpelled hydrocarbons.

MASS OF ORGANIC CARBON

The mass of organic carbon per unit surface area (OC/cm2), calculated using equation 2, combines thickness and TOC and can be thought of as the grams of organic carbon that would be recovered in a 1 cm2 core through the formation.

Maps of OC/cm2 for the Woodford shale and its three members are characterized by relatively smooth regional trends.

These trends correlate with those of corresponding isopach maps (Fig. 4), reflecting the fact that percent changes in thickness tend to exceed those of TOC or formation density.

For the Woodford as a whole, OC/cm2 in the northeast region ranges as high as 400 g/cm2; OC/cm2 in the southwest region is as high as 600 g/cm2 at the extreme southern edge of the mapped area.

These values are of the same magnitude as those determined in similar fashion for the Bakken formation of the Williston basin, in which OC/cm2 is as high as 350 g/cm2 in the lower shale member and 150 g/cm2 in the upper shale member (Schmoker and Hester, 1983).

The distributions of OC/cm 2 for the lower, middle, and upper members of the Woodford shale have nearly identical means of 77, 76, and 70 g/cm2 , respectively, and are similar in general appearance for values of OC/cm 2 greater than the means.

However, the mode of the lower- and upper-member distributions is 0-25 g/cm2, whereas only 5% of middle-member values fall in this range. The mode of the middle member distribution is 50-75 g/cm2.

The thermal maturity of the Woodford shale in the southwest region is generally greater than that of peak oil generation, whereas the Woodford in the northeast region is generally immature to marginally mature. Distributions of OC/cm2 for the northeast depocenter reflect kerogen that has retained its potential to generate hydrocarbons, and distributions of OC/CM2 for the southwest depocenter reflect kerogen whose generative potential has been significantly depleted.

The total mass of organic carbon in each member of the Woodford shale in the mapped area is measured in the tens of trillions of kilograms.

Somewhat surprisingly, the total amount of organic carbon in the Woodford shale is evenly divided between its three members.

Of the 73 trillion km of organic carbon in the Woodford shale of the study area, some 54 trillion km are in thermally mature areas characterized by Ro 0.6%, and 38 trillion km are in areas where Ro is 1.3%. As a point of comparison, the two shale members of the Bakken formation in North Dakota and Montana contain 126 trillion kg of organic carbon, of which 102 trillion kg are in thermally mature areas (Schmoker and Hester, 1983).

Because the upper member is generally thicker in the low maturity northeast region than the lower and middle members combined (Fig. 5), geochemical analyses to establish baseline values for immature Woodford source rocks are likely to be analyses of the upper member.

Table 1 shows, however, that most hydrocarbons sourced by the Woodford shale of the study area were generated from the lower and middle members, in that these two members contain 74% of the thermally mature organic carbon. As illustrated by this example, geochemical data should be considered in view of the three members of the Woodford shale.

REFERENCES

  1. Amsden, T.W., Hunton Group (Late Ordovician, Silurian, and Early Devonian) in the Anadarko basin of Oklahoma: Oklahoma Geological Survey Bulletin 121, 1975, 214 p.

  2. Cardott, B.J., and Lambert, M.W., Thermal maturation by vitrinite reflectance of Woodford Shale, Anadarko basin, Oklahoma: AAPG Bull., Vol. 69. No. 11, 1985, pp. 1,982-1,998.

  3. Cluff, R.M., Mudrock fabrics and their significance-reply: Journal of Sedimentary Petrology, Vol. 51, No. 3, 1981, pp. 1,029-31.

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  7. Heckel, P.H., Recognition of ancient shallow marine environments, in Rigby, J.K., and Hamblin, W.K., eds., Recognition of ancient sedimentary environments: Society of Economic Paleontologists and Mineralogists Special Publication 16, 1972, pp. 226-286.

  8. Hester, T.C., Sahl, H.L., and Schmoker, W., Cross sections based on gamma-ray, density, and resistivity logs showing stratigraphic units of the Woodford Shale. Anadarko basin, Oklahoma: USGS Miscellaneous Field Studies Map MP-2054. 1988, 2 plates.

  9. Hester, T.C., and Schmoker, J.W., Determination of organic content from formation-density logs, Devonian-Mississippian Woodford Shale, Anadarko basin, Oklahoma: U.S. Geological Survey Open-File Report 87-20, 1987, 11 p.

  10. Kelly, T.E., and Merriam, D.F., Structural development of the Sedgwick basin, south-central Kansas: Transactions of the Kansas Academy of Science, Vol. 67, No. 1, 1964, pp. 111-125.

  11. Kinghorn, R.R.F., and Rahman, M., Specific gravity as a kerogen type and maturation indicator with special reference to amorphous kerogens: Journal of Petroleum Geology, Vol. 6, 1983, pp. 179-194.

  12. Schmoker, J.W., Determination of organic content of Appalachian Devonian shales from formation-density logs: AAPG Bull., Vol. 63, No. 9. 1979, pp. 1,504-09.

  13. --, Organic content of Devonian shale in western Appalachian basin: AAPG Bull., Vol. 64, No. 12, 1980, pp. 2,156-65.

  14. --, Oil generation in the Anadarko basin, Oklahoma and Tex-as: modeling using Lopatin's method: Oklahoma Geological Survey Special Publication 86-3, 1986, 40 p.

  15. --, Schmoker, J.W., and Hester, T.C., Organic carbon in Bakken Formation, U.S. portion of Williston basin: AAPG Bull., Vol. 67, No. 12, 1983, pp. 2,165-74.

  16. Smith, J.W., and Young, N.B., Specific-gravity to oil-yield relationships for black shales of Kentucky's New Albany Formation: U.S. Bureau of Mines Report of Investigations 6531, 1964, 13 p.

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