A tectonic map of the center of the North American continent shows that the western part is geologically part of the Churchill Precambrian province containing at least two and possibly three sedimentary basins (Williston, Kennedy, and Powder River).

The largest tract of territory is underlain by the Williston (Dakota) basin, which would include nine of the state's largest counties. Harding County, the states largest producer, is found in the northwestern area. Oil development in Harding County, from discovery in 1954 to present, has closely paralleled activity in the rest of the Williston basin. The largest and most significant event in Harding County oil history was and continues to be the discovery and development of the Buffalo field. Subsequent to this has been the discovery and development of smaller pools adjacent to or in close proximity. By South Dakota standards, Buffalo field is a large one, with 106 producing wells with a successful, aggressive secondary recovery project (fire-flood) being conducted by Koch Exploration Co. Work also has begun on a water-flood of the Red River formation in the Buffalo field by Apache Corp., although results of the project cannot yet be determined.

While the production in Harding County represents some of the most southerly located in the Williston basin, one would have to travel some 120 miles to the southeast in Dewey County to find the southern limits of oil production in the Williston basin. Here, Lantry field, producing from the Ordovician Red River formation, continues to be an island of producing wells. The 120 miles connecting the Buffalo and Lantry fields is the heart of South Dakota's share of the Williston basin and is virtually undrilled! Two of the largest counties in South Dakota are Corson and Dewey, which are about 5000 sq. miles in area and have a combined total of less than 50 oil tests since 1900. It seems hard to imagine a similar situation existing in another state or province of the Williston basin.

Geologically, South Dakota's share of the Williston basin closely resembles the "pancake" sedimentary section found in other states with a substantial thickness occurring in Harding County (The current depth record of the Red River formation in South Dakota is 9,771 ft. in a well located at 19-23n-63e). All production of oil from the Red River formation is found, as it exists elsewhere, as an alternating deposit of limestone and dolomite with the "b" porosity zone the common drilling objective.

The gravity of the produced oil is in the 30 degree range, with exceptions occurring in the Lantry field (23-25 API), and an average of 22 ft. of pay with average porosity's rated at 16 %. Thus, the South Dakota Red River formation proves itself a reliable target for local producers. Trapping mechanisms include both structural and stratigraphic, with the Cedar Creek anticline generally controlling the geology of the area from the northwest: however, it's importance as a controlling factor seems to differ, depending on the source. No matter how the facts are interpreted, oil can be shown in most tests of the Red River formation from the Buffalo field northward to the North Dakota line. If a well is brought in, it will produce water with the oil with annual water production for Harding County exceeding 2,144,025 bbls. Gas is also produced from the Red River formation although quantity is lacking and no attempt to market the gas has been made. With its history of producing a large part of South Dakota's oil, the Red River formation is understandably the primary target for most operators.

Other Pays.

Potential for oil is not limited to the Red River formation.
The Mississippian Madison formation exhibits as much as 1200 ft. of thickness in western South Dakota. There have been oil shows reported in several Madison formation tests in the Williston basin of South Dakota, though rarely have these rocks been the primary target of explorationists. Reservoir characteristics of the carbonate sequence are excellent. The suggestion has been made by Peterson et al. that perhaps oil sourced or trapped in the Madison formation has been water flushed since water flowing wells are often encountered, and electric log measurements often show a "wet" section. The extensive outcropping of the Madison rocks (Paha Sapa formation) in the nearby Black Hills uplift and their exposure to hydrologic recharge undoubtedly exert hydrologic pressure adverse to oil collection in the Madison formation east of the Black Hills. All of this does exclude the chances of localized traps that may be present in the large area of Madison formation occurrence.

Probably one of the most likely prospects for oil in the Williston basin is in the upper Minnelusa formation. For example, the upper Minnelusa formation is 250 feet thick in eastern Pennington County (Barnes# 1 Nixon, SESW 12N 12E). This upper section is made up of alternating layers of sandstone, carbonates, shales and anhydrite. The lower Minnelusa, containing the Leo sands, which are so productive in the Powder River basin of nearby eastern Wyoming, are also well developed in the Williston basin of South Dakota. Live oil shows have been reported in wells east of the Black Hills uplift as far south as Pennington County,

A potential reservoir, deeper in the South Dakota Paleozoic section, is in the basal sands of the Ordovician Winnipeg formation. In the Williston basin in South Dakota, the basal sands of the Winnipeg formation include that of the Cambrian Deadwood formation. The Deadwood formation thins eastward from the Black Hills and is indefinable from the basal Winnipeg formation. If the Winnipeg shale unit (0-150 ft) can be thought of as a source, this sandy interval would provide excellent reservoir potential. Test penetrating this interval are particularly rare in South Dakota but have borne this fact out. Winnipeg oil reservoirs in North Dakota have, of course, been successfully produced. However, the handful of Winnipeg formation tests in our state has done little to decisively answer that question.

In 1984, a deep test was attempted in the extreme southern Williston basin in Pennington County. That well was drilled on an anticlinal structure and proved unsuccessful (water wet, no hydrocarbon shows in the Winnipeg formation); however, experts believe that potential certainly exists. A paper by Bolyard and Davenport suggests that the Winnipeg shales in the Williston basin of South Dakota are a probable source for hydrocarbons. In addition, they theorized that the basal sandstone's were a logical choices for oil migration and entrapment. A Winnipeg discovery in South Dakota? As with some other geologic units, only further drilling will determine if favorable geological characteristics will yield oil. The prospects are wide open.

While most exploration has been and probably will be conducted for oil, South Dakota's portion of the Williston basin also produces natural gas from two existing fields (West Short Pines and Cady Creek) located in southern Harding County, with a third in Butte County designated but unused. From some fifty plus gas wells in southern Harding County, gas has been produced since 1977 and is marketed at the source for distribution in the Williston Basin pipeline system to points south.

The gas found in shallow Cretaceous age rocks has proved to be attractive to several producers with target depths of less than 2000 feet. Prospects for new discoveries extend farther to the south into Butte County along the area's dominant structure, the Camp Crook anticline (the feature is also known as the Whitewood anticline, as it extends into southern Butte County, and closer to the Black Hills uplift). Indeed, the prospective of shallow gas has been considered throughout the Williston basin in South Dakota. Trends have been identified along structural features stretching from the northwest to Central South Dakota. A common characteristic of the gas reservoirs in all of these areas is it 's existence at shallow depths. With rocks no older than Cretaceous age, depths in all cases range from 1000 to 2000 feet. Certainly, the lower costs associated with shallow gas has been an important factor in South Dakota's gas industry.

Another common characteristic identified recently by geologists, has been the occurrence of gas along identifiable structural features across the state. Specifically, gas has been noted to occur in close proximity to structural lineaments visible on satellite photos (Landsat) of South Dakota. A paper by Bretz and Shurr (1981) published by the South Dakota Geological Survey, outlines these occurrences and the reasons for their existence. These lineaments may represent hingelelines of subsurface basement blocks and exist throughout the northern plains. Gas is shown to be heavily concentrated in the central areas of South Dakota, as well as in currently producing areas. Bretz and Shurr postulated that the lineaments were areas of increased porosity and permeability and were likely conduits for gas generated locally, as well as deeper in the basin center. One result of this investigation was the delineation of potential gas reservoirs along these readily identifiable structural features.

Perhaps the greatest impediment to gas development in western South Dakota has been the serious lack of available pipeline or nearby large markets. While this situation is not expected to change in the near future, the resource continues to be assessed and defined. Gas possibilities are wide ranging, and unlike oil, they probably range eastward across the Missouri River extending into the glaciated terrain.

The South Dakota portion of the Williston basin, by virtue of its history of oil production, and by its geology, has great potential for oil and gas that very probably remains untapped. The positive aspects could be listed as follows :

· Large parts of nine counties containing the thickest stratigraphic sections found in the state.

· Numerous shows of oil and gas throughout the region.

· A logical assumption that "state line faults" have not prevented hydrocarbons from accumulating here as they have in adjacent states.

Southwest South Dakota. Even though it cab be stated that most of South Dakota's oil and gas promise is related to the huge Williston basin, it is also a fact that the southwestern region of the state is fast becoming an area of growing importance. Specifically, Fall River and Custer counties have favorable track records for drilling that have begun to unfold largely since the 1970's. Since that time, a number of producing fields have been developed with several operators taking advantage of a generally favorable drilling and producing environment.

Historically, interest in the southwest region of South Dakota began to heighten in 1955 following the discovery at Barker Dome in Custer County. This discovery marked the successful conclusion to a long running exploration effort on the Barker Dome anticlinal structure and others associated with the Black Hills uplift. But with time, the focus of exploration began to shift away from the easily visible structures of the area and on to the surrounding prairies. These areas resemble the very productive areas of Wyoming and Colorado both topographically and geologically. Similarities are strong enough to have created problems for straigraphers attempting to place the area into a geologic province. To some, the area clearly belonged to the Powder River basin, while others may have extended the Denver basin far enough north to include this area. However, to the successful producers in southwestern South Dakota, the question perhaps is a moot one, and may well have decided to leave the classification question to the academics.

As alluded to earlier, the Southwest South Dakota oil fields offer the explorationist some definite advantages. Drilling costs usually run considerably lower compared with most Williston and Powder River basin tests due to substantially shallower pays. Fall River County pay zones in the Pennsylvanian-Permian Minnelusa formation are from 1.600 to 4,000 ft. below land surface. The area also poses little in the way of difficult drilling horizons or extremely wet zones that could complicate or exacerbate the drilling process. In addition, the southwest benefits from the oil field service industry already built in the Powder River basin to handle the demands of the industry, a fact which minimizes delays in services and assures that service will be available when needed. As new production is found, this close industry proximity allows the producer the same opportunities for operational ease that separates the southwest from the other strictly "wildcat" areas.

Regionally, the southwest represents oil development of a sedimentary basin with similarities to adjacent areas of Wyoming, Nebraska, and nearby Colorado. The depositional development of the area was largely completed by late Cretaceous time. The Black Hills uplift added needed structural features to the sedimentary environment.. Deposition and subsidence followed by uplifting and deformation have made the area a positive one for entrapping petroleum.

The primary target formation to date is in the Leo sands of the Minnelusa formation. These sands have been in the highlight of recent discoveries in the area and the subject of much analysis by explorationists. A general consensus arisen among the authors of Leo sands studies is that they represent largely tidal or near-shore carbonate sequences as well as eolian sand dunes. The Leo sands actually result from mixes of differing depositional environments, including super and supratidal as well as strictly lagoonal and eolian. The resulting sequences are alternating carbonates and sandstones separated by shales that have been suggested as possible source beds. This developmental history resembles the characteristics found in both the Denver and Powder River basins with a few exceptions. One exception would be the lack of porosity and lithologic consistency in the upper Converse sands above the Red Marker shale in the Minnelusa formation. Therefore, the Leo sands become even more important in light of this observation in South Dakota.

South Dakota's Southwest region then begins to emerge as the picture of a sedimentary environment highlighted by the occurrence of numerous isolated reservoirs which serve as stratigraphic-structural traps for locally derived oil.

Primary recovery of oil is on-going in nine small fields in both Custer and Fall River Counties. Recently, one of the area's larger producers, Placid Oil, began an experiment with a water-flood of one of these small reservoirs with very positive results. The success (Placid reports Alum Creek field showed a 100 % increase in oil production) begs the question : Would similar reservoirs of the area respond as well the Alum Creek to the secondary recovery attempts? I would suggest that the answer would be yes, and although the success of such projects depend on many variables, the similarities of the area's other Leo pools to the Alum Creek model are many. First the lithologies show consistency in all the reservoirs found so far. The Minnelusa sedimentary rocks have been stimulated by secondary recovery methods in the Powder River basin and now in the Alum Creek field, thus confidence in such projects in the region is high.

Secondly, the problems of energy dissipation from the reservoirs is typical of the area and is perhaps best addressed by secondary recovery. Because the Leo sands of Fall River County do not outcrop in the nearby Black Hills and because the sands have no known hydrologic connection in the region, the primary source of energy remains in solution within the formation. Thus, projects such as the water-flood in the Alum Creek field are the best methods, including gas repressurization, to recover oil from a declining reservoir in the Leo sands.

Finally, these relatively small oil pools allow for a greater deal of control for such projects. Alum Creek field has seven producers and is the largest field in Fall River County covering 4 1/2 sections on 160 acre spacing. Whether production is primary or secondary in nature, the small Leo sand pools can be effectively drained with a limited number of producing wells.

The success of the Alum Creek water-flood will encourage other operators in the area to consider secondary recovery as a viable option for their own productive acreage. More water-floods will probably be attempted, and this will inevitably lead to increased oil production for the southwest region. Secondary recovery will surely be an important footnote when South Dakota's energy history is written.

Though the counties of the southwest region will probably never rival South Dakota's Williston basin counties in total amounts of oil produced, they do, however, have the potential for making the southwest the most active area in the state in terms of new exploration. Since 1980, the year of the first Fall River County discovery, 10 new producing fields have been established. During the same period, only 146 wildcats were drilled in the two primary counties in the state or only about 18 holes per year to date. Since 1980, wildcat success in the southwest region's Leo sand plays equals 17% or roughly the national average. And while the region will probably not attract the attention of those companies who routinely plan large drilling programs in more established areas, it will continue to draw the independent oil people who have been the backbone of the industry in South Dakota.

West Central South Dakota. The vast area east of the Black Hills uplift and west of the Missouri River is the final area of the state to be reviewed. As with the rest of the state, this region lacks significant exploration history and continues to be an unknown quantity as far as geologic subsurface data is concerned. Recent speculation by experts concerning the economic potential of the region has been of a positive nature, and includes both oil and natural gas.

Since this particular area lacks the benefit of the kind of evaluation that a more intensively explored area might receive, one must look at the known geological facts. These facts represent the minimum criterion by which a given would be assessed for production potential. The criteria must, of course, include an abundance of the following :

· Organic-rich source rocks
· Deeply buried source rocks
· Porous and permeable reservoir rocks
· Mechanisms to transport hydrocarbons to traps

By using these parameters, it is clear that the featured lands could have producing possibilities.

First, organic-rich source rocks are available in abundance throughout the sedimentary section in the form of marine shales. 

Secondly, the stratigraphic section in the west-central area is thick enough to contain buried organic shales at sufficient depth for oil and gas formation (in particular, shales buried in the deep regions of the Williston basin in Corson, Perkins, and Harding Counties could "cook-out" hydrocarbons which then would migrate updip into the west-central counties-Lantry field in Dewey County is a prime example of the phenomenon). Shales as old as the Winnipeg formation are reported as the 101 deg meridian (Shell # 2 Herman, NWNE 3, 1N-29E).

Thirdly, there is an abundance of good reservoir rocks which are in contact with the producing shales and, as in the case of South Dakota's Winnipeg formation, both source rocks and reservoir rocks exist in one single formation or sequence of rocks. Finally, in addition to the folding and faulting common to the region, we can point to some recent information on the presumed existence of widespread structural features (lineaments), which correlate to hydrocarbon occurrences.

A closer examination of the subsurface in the area between the Black Hills and Missouri River reveals a generally thinning of Phanerozoic age sedimentary rock from west to east. The Phanerozoic section is underlain in the west by Precambrian age rocks (as seen in the Black Hills), and in the east by the Precambrian Sioux Quartzite. The Williston and Kennedy basins flank the area under discussion in the north and south, respectively. Beyond these basic facts, little can be said with certainty geologically. However, formations present would include many of the same rocks found in the Williston basin, particularly the Cenozoic and Mesozoic and the upper Paleozoic sections.

Since the basic criteria for oil sourcing and entrapment are met, and the area of discussion includes a thick, deep, and promising stratigraphic section, the question of physical evidence of hydrocarbons must be asked. To this question, the answer is a positive one. Many shows of hydrocarbons have been reported in the areas adjacent to and within the area of interest (Petres, 1988). Possibly one of the more interesting of these shows occurred in Jackson county (City Service # 1 Phillips "A", NWSE 4, 2S-23E). At this location, evidence of a "fossil oil field" was documented. If this interpretation is correct, it would indicate migration and entrapment of oil to a point farther east than previously thought likely. It may also reveal a positive structural influence of the nearby Stanley County High, which occupies parts of four central counties. This confirms the existence of trapping mechanisms as well as the necessary means for subsurface fluid transport.

Perhaps more than oil, natural gas has been of keen interest to explorationists in the state's central region. The question of gas reserves has brought occasional speculation as well as drilling, and the answer remains unanswered due to the lack of subsurface information.

Shows of gas in the rocks of the Cretaceous Niobrara formation are found commonly throughout the west-central. The lithology of the Niobrara is primarily chalk or limy shale, and its area of distribution is extensive. Thickness averages more than 200 feet across the area, and drilling depths are shallow (400-900 foot range). In addition to the Niobrara, the Cretaceous Newcastle sandstone, Greenhorn limestone, and Lakota formation, and the sands in the upper Minnelusa, have all been reported as gas bearing units in tests drilled for oil or from commercial water drilling. It is probable, however, that the highest quality reservoirs in the central region are found in the tight gas sands of the Niobrara or perhaps Greenhorn formations. 

Unfortunately, The strongest impediment for gas exploration of the central region is not the lack of promising prospects but the current status of the gas industry itself. As previously mentioned, the lack of population/markets or pipelines. To date, the only large scale local usage occurred in Pierre at the turn of the century. The gas utilized was from the Dakota formation. 

The counties that make up the center of the state lying between the Missouri River and the Black Hills represent a challenge to explorationists. As all are purely wildcat areas, it will certainly test the will of those who choose to see what lies beneath it. But as I have outlined, there is reason to believe that the historical and geological data suggest opportunity. To the explorationists, it reveals an area where potential pays are not too deep and leases can be obtained for less cost than in other regions.

For these and the reasons already outlined, this section of our state could be the site of some new producing oil fields, the kind that keep the cash flowing, and geologists gainfully employed.