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Home , Dakota Formation, Graneros Shale, Greenhorn Limestone

PROC. S. D. ACAD. SCI. XXXVII (1958) 161

THE IN CENTRAL

John Paul Gries South Dakota School of Mines and Technology, Rapid City

INTRODUCTION Historical summary. The and clays which make up the type section of the Dakota formation were described in 1853, and formally named Dakota in 1861 (1:419). The section in the type area of northeastern was described as consisting of about 400 feet of , clays, and beds of impure , separated from the overlying Greenhorn by about 100 feet of Fort Benton (Graneros) . The entire sequence was identified as upper in age.

Darton (2:20) correlated the upper sandstone of the Inyan Kara around the with the type Dakota, and applied the name Dakota to the Hills area. He considered it to be upper Cretaceous, whereas he be- lieved the underlying Fuson and Lakota formations of the same group to be of lower Cretaceous age.

These correlations were based largely upon paleobotanical evidence. A determination that the flora of the Dakota of the Black Hills area was lower Cretaceous led Russell (3:402) to restrict the name Dakota to the - stone of the type area, and to suggest the name Fall River for the upper member of the Inyan Kara Group around the Black Hills. He considered the true Dakota to be a near shore deposit of a sea which encroached east- ward over the old Sioux highland, forming a series of successively higher and younger sandstone lenses to the east. He stated that the sandstones of the type Dakota fingered out westward into the upper Cretaceous Graneros (4:10, 5:10), but this suggestion was largely ignored by subsequent geologists.

Plants collected from the Newcastle sandstone around the Black Hills were identified by E. W. Berry (6) as Dakota forms. The significance of this identification was not immediately appreciated. Tester, in 1931 (7:284), after extensive study of the Dakota formation in and adjacent areas, con- cluded that the type Dakota was lower Cretaceous. An excellent summary of work done on the Dakota to that date is included in his paper. Subse- quently, in the Black Hills area, the lower-upper Cretaceous boundary was raised to the top of the Skull Creek shale by Reeside (8), and more recently to the top of the by Cobban and Reeside (9:1892-3).

Recent work. The clue to the proper correlation in central South Da- kota was suggested when the Kadoka town water well was drilled in 1950. Howard Brady, consulting geologist, brought the electric log to the writer, 162 PROC. S. D. ACAD. SCI. XXXVII (1958)

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MB Z —J -CA u_ u ) RE u_ 0 0 a_ c:1 1 Z lit o Q w O cc 1 1 1 1

. x W cn (1) us x w cn (.1) w FM U Cr _J le < < V) M I W WWIX i u") 0 u) _J Cr W ORN W I— 0 > (1) —J CO w a .- E < _I 1 8 __Ji _1z 0 1-- 0 _I Cf) D _1 cn GREENH PROC. S. D. ACAD. SCI. XXXVII (1958) 163

with the comment that the resistivity curve of the waterbearing Dakota sandstone section closely resembled that of the Mowry shale section in western Nebraska.

Sample studies, correlated with electric logs, soon showed that the shale below the Dakota sandstone in central South Dakota, then generally called Morrison, was really Skull Creek, and that the sandstones below that shale, which were being called Morrison, Sundance or even , were, in part at least, the equivalent of the Inyan Kara sandstones of the Black Hills outcrop area. The correlation indicated in Figure 1 was subsequently proposed.

The Dakota formation, as the term is used in eastern and central South Dakota consists of a great blanket of and clays, underlain west of the dashed line on Figure 2 by the Skull Creek shale, and overlain by a variable amount of Belle Fourche (Upper Graneros) shale. East of the dashed line, the Skull Creek disappearb by thinning and by sanding up, so that there is recognizable only one thick sand and sequence, from the top of the Dakota sands to the base of the Inyan Kara Group. Even older sands may be included.

Ii/most of the central part of the state, the top of the Dakota formation

Figure 2. Isopach Map, Showing Thickness of Dakota Formation in Central South Dakota. Dashed Line Indicates Eastern Limit of Recogniz- able Skull Creek Shale. Area of Dakota Gas Outlined by Dotted Line 164 PROC. S. D. ACAD. SCI. XXXVII (1958) lies about 340 feet below the top of the , but eastward, sands build up higher in the section. Electric log correlations by students at the School of Mines show that a conspicuous near the base of the Greenhorn formation (as defined by the U. S. Geological Survey), lies below the highest sandstones of the Dakota formation in eastern South Dakota. ECONOMIC CONSIDERATIONS i The concept of the Dakota sandstone as a huge delta deposit, separated from the Inyan Kara sandstones over most of the central part of the state by the Skull Creek shale, justifies a re-evaluation of the water, gas and oil potentials of the formation. Artesian water. In South Dakota, the first artesian well to the Dakota sandstone was drilled at Yankton in 1881. Literally thousands of wells were drilled east of the within the next few decades. Yields of several hundred gallons per minute were not uncommon, and in some areas the pressure was so great that the flow was used as a source of power. Darton completed an extensive study of the Dakota artesian system in 1909 (2). He believed the Dakota sandstone to be continuous from the Black Hills outcrop area to the exposures in southeastern South Dakota. His belief was supported by pressure studies at that time which showed that the piezometric surface declined from an elevation of 2000 feet along a north- south line through the central part of the state to about 1100-1200 feet along the eastern edge. His indicated heads of 3000 feet or more around the Black Hills fit his overall picture, though it is now recognized that the source of that water was the stratigraphically lower Fall River sandstone. Russell (6), restudying Darton's data in light of his own geological interpretation of the Dakota formation, concluded that the water within the Dakota sandstones was largely connate, and that the artesian pressure was due entirely to compaction by overlying sediments. His suggestion as to the source of the artesian pressure was promptly criticized by Piper (11), Terzaghi (12), and Thompson (13). Under the present concept of the Dakota as a series of deltaic sands fingering out into shale to the west, the artesian system is certainly no longer the textbook example envisaged by Darton. He visualized the water entering at high elevation outcrops around the Black Hills, its eastward movement accompanied by a drop in pressure caused by friction of the water through the sandstone. But since the Dakota does not crop out around the Hills, except for the thin, fine sandstones which make up the Newcastle and higher "dike sandstones," the source of the artesian water must be less direct than Darton thought. It is still likely that the Black Hills form the intake area, but if so, the water must enter the sandstones of the Inyan Kara Group, move eastward through them, then find its way up into the higher sand- stones within the Dakota formation. Two possible routes are suggested. It may migrate eastward to where PROC. S. D. ACAD. SCI. XXXVII (1958) 165 the Skull Creek shale disappears, and all sandstones coalesce; then migrate back westward in the upper sandstones. Or it may move upward through fractures in the Skull Creek shale. If the former is true, the Dakota piezo- metric surface should drop westward from the feather edge of the Skull Creek shale (dashed line, Figure 2). It does not do so. If the second sugges- tion is valid, the Skull Creek shale, with a thickness of 100 to 200 feet, must be much more permeable than would normally be assumed. If neither of these is acceptable, the only obvious alternative is that the water is not moving, and that the pressure is due to weight of overlying sediments as suggested by Russell. The writer hesitates to accept this hypothesis as the principal source of artesian pressure within the Dakota sandstone.

The first sandstones encountered beneath the Skull Creek shale in central South Dakota are usually fine-grained. Below them, or interbedded with the lower ones, are often some very coarse sands with grains in the granule and small pebble size ranges. The writer believes that these are basal Cretaceous, essentially continuous with the sands of the Inyan Kara Group around the Hills. They are commonly called "Sundance" sands. Some of these coarse sands have very high permeability, and contain water under very high hydrostatic pressures. These pressures must also be explained either by rather free communication with the Black Hills outcrops or by compaction. There are many earlier examples, but two recent instances point out the problems encountered when drilling into these high pressure permeable sandstones. The Hunt No. 1 State School Lands oil test in Sec. 24, T. 116 N., R. 73 W., Hyde County, was drilled in 1951. This hole was lost at 1940 feet, and was plugged successfully only because prompt and adequate engineering measures were applied. The other example is the municipal well drilled at Onida, Sully County, in 1954. Despite several abortive at- tempts to control it, the flow is not under complete control as this paper is written. The spotty nature of these high pressure areas appears to be due as much to changes in permeability as to variations in the distribution of the sand and granule zone. In numerous wells in the central part of the state, the coarse zone is present, but the interstitial space is filled with white clay, believed by the writer to be altered volcanic ash. Where the clay is present, there is little porosity or permeability, and little or no water is produced.

Natural gas. Gas is associated with Dakota water in a large area in central South Dakota (Figure 2). The area indicated is from data compiled by Ihli (14).

Although gas had been encountered earlier in glacial drift and Niobrara chalk, the first gas to be noticed in the artesian water was from the Pierre Indian School well in 1894. Soon a well was drilled for natural gas at the Locke Hotel in Pierre. Several wells were subsequently developed in the Pierre-Fort Pierre area primarily for gas; in other cases ranchers trapped sufficient gas from domestic wells to heat their buildings over a period of many years. 166 PROC. S. D. ACAD. SCI. XXXVII (1958)

The gas is essentially a dry gas. Analyses at hand show a range of from 74 to 94 percent methane (CH.), and from a trace to 13 percent of higher or "wet" hydrocarbons. A trace of helium is reported from the old Lacy well in Stanley County. There are not enough reliable analyses avail- able to indicate whether the variation in composition is real or apparent. Most of the samples have been collected by inexperienced persons, and many of the early analysts used non-standard techniques. The gas is apparently dissolved in the water. There is no evidence of a gas-water contact, and no well has produced gas without water. The source may be from either of the enclosing shales, or from vegetal material within the deltaic deposits of the Dakota formation. The State Geological Survey has studied the distribution of the gas, and pointed out that it occurs on the Eureka-Mission Terrace which lies on the southeast flank of the Williston Basin (15:19). Superimposing the gas- producing area on the isopach map of the Dakota sandstone (Figure 2) brings out two more points whose significance is not clear. First, the gas lies al- most entirely in that area where the Dakota sandstone is underlain by Skull Creek shale, and secondly, no gas is reported where the Dakota formation has a thickness of less than 100 feet. It should be pointed out that the outline of the gas-bearing area is not accurate. In the western portion of the area, the excessive depth to the Dakota has limited the number of wells drilled. Oil tests within the area have been drilled with rotary tools, and such small quantities of dissolved gas would not be noticed. Gas is found in the Newcastle sandstone in the vicinity of Ardmore, Fall River County. At the present time, there is no suggestion of a genetic relationship between the gas found in the two areas. No gas is known from the Fall River sandstone in western South Da- kota, and the writer can find no report of gas from the sandstones below the Skull Creek shale in the central part of the state. The origin and areal distribution of the gas should be reconsidered in light of the present interpretation of the geology of the Dakota sandstone.

Petroleum. Early accounts mention numerous rainbows of oil, or even "slugs" of viscous oil on stock tanks, shortly after "drilling in" Dakota sand- stone artesian wells. The writer has not seen evidence of oil around any of the old wells which are still flowing, and has been unable to substantiate reports of oil shows in wells drilled within the last 20 years. He consequently does not know how much credence to place in reported oil shows from the Dakota formation of central South Dakota. Successive discoveries of oil in the Newcastle sandstone west of the Hills, and in the "D", "G", and "J" sands in western Nebraska, led the writer to point out in 1953 (10:452) that the tongues of Dakota sandstone, pinching PROC. S. D. ACAD. SCI. XXXVII (1958) 167

out to the west, on the east-clipping flank of the Black Hills uplift, offered ideal conditions for stratigraphic traps. Over 100 test holes have been drilled through the Newcastle and higher sandstones since that time, but there has been only one good show of oil to the writer's knowledge. The geologic picture has been verified, but the lack of oil shows has been disappointing. Much further study of the hydrodynamics of the prob- lem is warranted, based on concepts suggested by M. King Hubbert (16).

CONCLUSION The Dakota sandstone, which lies in the form of a huge wedge in the central part of South Dakota, contains artesian water of greatest value to the state, limited quantities of natural gas within a rather definitely pre- scribed area in the central counties of the state, and to date, no oil. New de- tailed studies of the hydrology and geology of the Dakota formation are warranted.

BIBLIOGRAPHY 1. Meek, F. B., and Hayden, F. V., Descriptions of New Lower (Primordial), , Cretaceous, and Tertiary Fossils, Collected in Nebraska Territory, with Some Remarks on the Rocks from Which They Were Obtained, Proc. Phila. Acad. Nat. Sci., 13, 417-432 (1861). 2. Darton, N. H., Geology and Underground Waters of South Dakota, U. S. Geol. Survey, Water Supply Paper 227 (1909). 3. Russell, W. L., Origin of Sandstone Dikes of the Black Hills, Amer. Jour. Sci., Ser. 5, 14, 402 (1928). 4. Russell, W. L., The Possibilities of Oil in Western Corson County, South Dakota, S. Dak. Geol. and Nat. Hist. Survey, Circ. 27 (1926). 5. Russell, W. L., The Possibilities of Oil in Western Ziebach County, South Dakota, S. Dak. Geol. and Nat. Hist. Survey, Circ. 20 (1925). 6. Russell, W. L., The Origin of Artesian Pressure, Econ. (1928). Geol., 23, 132-155 7. Tester, A. C., The Dakota Stage at the Type Locality, Iowa Geol. Survey, 35, 195-337 (1931). 8. Reeside, J. B., Jr., Thickness and General Character of the Cretaceous Deposits in the Western Interior of the , U. S. Geol. Survey, Oil and Gas Inves., Prelim. Map 10 (1944). 9. Cobban, W. A., and Reeside, J. B., Jr., Lower Cretaceous Ammonites in , , and , Bull. Amer. Assoc. Petrol. Geol., 1892-1893 (1951). 35, 10. Cries, John Paul, Cretaceous Rocks of Williston Basin, Bull. Amer. Assoc. Petrol. Geol., 38, 443453 (1954). 11. Piper, A. M., The Origin of Artesian Pressure, Econ. Geol., (1928). 23, 683-696 12. Terzaghi, Chas., The Origin of Artesian Pressure, Econ. Geol., (1929). 24, 94-100 13. Thompson, D. G., The Origin of Artesian Pressure, Econ. Geol., 771 (1929). 24, 758- 168 PROC. S. D. ACAD. SCI. XXXVII (1958)

14. Ihli, J. A., "Natural Gas in Central South Dakota," S. D. School of Mines and Tech., unpublished bachelor's thesis (1955). 15. Wing, M. E., A Structural Survey of the Pierre Gas Field, S. Dak. Geol. Survey, Rept. Inves. 29 (1938). 16. Hubbert, M. K., Entrapment of Petroleum under Hydrodynamic Con- ditions, Bull. Amer. Assoc. Petrol. Geol., 37, 1954-2026 (1953).