Kelly Carlson
North Dakota Geological Survey
Bismarck, N.D.
Oil development in North Dakota may be divided into two major cycles of activity. Each cycle was influenced by external forces superimposed on exploration success within the state. The first cycle began with the discovery of the Beaver Lodge field by the Amerada Petroleum Corp.-Clarence Iverson in April, 1951. Oil prices in North Dakota during the first cycle were generally in the $2 to $3 per barrel range. They reached $3.59 a barrel near the end of the first cycle, which ended late in 1973 with the Arab oil embargo and its accompanying abrupt, upward price changes.
The second cycle of activity began with "old" oil at $5.25/bbl and "new" oil at $12/bbl. Prices since 1981 have been decontrolled and have followed worldwide prices. In North Dakota, this meant $38/bbl briefly in 1981, followed by a slide to $28, and then to $25/bbl by 1985. The price plunged to $10/bbl in 1986.
PEAK OF ACTION
Activity during the first cycle peaked in 1958 with 454 completions (Fig. 1). In 1959 a total of 255 producing wells were completed. Production reached 27.1 million bbl of oil in North Dakota in 1966. Several pulses of exploration activity occurred during this cycle. The initial surge saw increased activity, through 1954. Steady successes were recorded along the Nesson anticline and initial discoveries in Bottineau County in 1953. Widespread exploration occurred in eastern North Dakota in 1954. This activity which was unsuccessful, refocused exploration priorities to areas where there had been previous success. Increased activity and success in 1957 and 1958 reflected development in the Burke to Bottineau County area. Producers completed between 1953 and 1957 resulted from steady development of Madison reservoirs along the Nesson anticline.
The 1959 peak for producing wells completed reflected full development of Madison reservoirs along the Nesson anticline. After early development, production capacity along the Nesson anticline exceeded the capacity of the Mandan Refinery until November of 1965. Consequently, production in that area was restricted and the production peak for the first cycle was delayed until 1966. The surge in wildcat wells in 1968 and 1969 reflected a widespread search for Cretaceous Newcastle ("Muddy") sandstone reservoirs ... a response to the discovery of the giant Bell Creek field in southwestern Montana.
Success ratios declined for exploration during the first cycle from 15.3% for the first 3 years, to 5.4% for the last 8 years, with an overall success ratio of 8.8% during the first cycle (Fig. 2). Exploration during the first part of the first cycle was concentrated on Mississippian Madison or shallower reservoirs with the average depth of wells decreasing from 8,000 ft in 1952, to less than 6,000 ft by 1965. Exploration and development of Ordovician Red River and Pennsylvanian Tyler reservoirs in southwestern North Dakota during the latter stages of the first cycle resulted in increased depths except for wells drilled as part of the Cretaceous play in 1968 and 1969.
DRILLING PEAK
During the second cycle, drilling peaked at 834 completions and 453 producers in 1981. Production reached about 52.6 million bbl of oil in 1984. Wildcat drilling peaked in 1981, when 267 wells were attempted. Discoveries peaked at 64 in 1982 (Fig. 2). The higher success ratios in the second cycle reflected a number of factors: improved seismic techniques; Red River exploration, which provided multiple uphole possibilities; and higher prices, which allowed smaller reserves to be economic. The average well-depth drilled increased through 1982-a result of the Red River emphasis (Fig. 3)-but as prices decreased, deeper exploration slowed.
A comparison of exploration success during the two cycles shows nominal success ratios of 20.3% during the second cycle, compared to 8.8% during the first cycle. However, if the quality of the discoveries, based on estimated ultimate recovery of hydrocarbons is considered, and the classification of the American Association of Petroleum Geologists is used, then the comparison is not so favorable. Eighty-one of the 151 wildcat producers discovered during the first cycle and 84 of the 358 wildcat producers discovered during the second cycle found at least one million barrels of oil. Three of the 5 Class A pools in North Dakota, 4 of the 5 Class B pools, and 21 of the 34 Class C pools were found during the first cycle and much of that success was during the first decade of exploration, during the 1950's (see table).
STRATIGRAPHIC REVIEW
Six of North Dakota's ten largest oil pools are located along the Nesson anticline. Five of these were found during the first exploration cycle. Eight of the ten largest pools have used water injection to enhance cumulative production (see table).
Most of the paleozoic carbonate and sandstone units in North Dakota have produced oil, but 99% of the oil has been produced from nine horizons (Fig. 4). The Mississippian Madison reservoirs have been primary targets through both cycles and these account for nearly two-thirds of North Dakota's cumulative production. Most of the Madison production has been from the Frobisher-Alida Interval.
Production from the Spearfish formation has been limited to an area in central Bottineau County. In that area, sandstone is present at the base of the Spearfish redbeds, and in some areas this sandstone lies unconformably on the Madison formation at depths of about 3,000 feet. Because many of the wells are perforated in both the Spearfish and Madison formations, some of these pools are listed as Spearfish-Madison pools. The Newburg pool was developed between 1957 and 1959, was unitized in 1967, and has been a successful secondary recovery unit, accounting for about two-thirds of the production credited to these reservoirs.
The Pennsylvanian Tyler formation occurs throughout most of the southwestern North Dakota. Production has been from channel sandstones at the Rocky Ridge field and from offshore bar sandstones from the Medora to Dickinson area. These reservoirs occur at depths of 7,500 to 7,800 feet. The largest of these pools, the Dickinson Pool, was fully developed by 1970, was unitized in 1973, and has been a successful secondary-recovery unit.
The Bakken formation (Devonian-Mississippian) occurs in much of western North Dakota. The initial production and the largest pool has been referred to as the Antelope-Sanish Pool because, in the Antelope area, a sandstone occurs beneath the lower black shale. This sandstone is known as the "Sanish sand." Since its discovery in 1953, development of the Antelope field has resulted in production from the Devonian Three Forks formation, as well as from the Bakken.
Exploration began in the late 1970's near the southwest limit of the Bakken formation. Reservoirs discovered since that time have been marked by high pressures, little or no water, and gradual decline rates. This area has become the focus for evaluation of horizontal drilling techniques since the 1987 completion of a horizontal wellbore in the Elkhorn Ranch field. That well was completed in September, 1987. Through 1988, it produced 109,680 bbl of oil and 258 bbl of water. The upper Bakken shale has been the major target.
Minor quantities of oil have been recovered from sandstones of the Black Island and Deadwood formations. As a result, clastics account for 8 to 12 percent of the production, (depending upon how much of the Spearfish-Madison production is assigned to sandstone of the Spearfish formation).
Development of Madison reservoirs began along the Nesson anticline where these reservoirs range from about 7,800 ft at the north end to 9,500 ft at the south end. The initial development in Bottineau County was from stratigraphic traps at the unconformity with the overlying Mesozoic strata. Subsequent exploration has been largely combination structural-stratigraphic accumulations where facies changes from porous carbonates to updip evaporates or dense carbonates are associated with slight flexures in the area from Bottineau to Burke County. These reservoirs are at depths of 3,000 ft in Bottineau County and progressively deeper westward to 6,000 to 7,000 ft in Burke County.
The major Madison developing in the southwestern area was during the second cycle, highlighted by the discovery of the Class A Little Knife and Big Stick Fields. Reservoirs in this area are at depths of 9,000 to 9,100 feet at the limit of production and about 9,800 feet at Little Knife. These reservoirs also appear to be combination structural-stratigraphic accumulations.
Secondary recovery operations in Madison reservoirs have given mixed results. Generally, units in Burke County and the northern portion of the Nesson Anticline dd not perform as projected and secondary recovery operations were discontinued. Units along the southern portion of the Nesson Anticline and other areas have shown varying degrees of success. A combination of water and nitrogen injection was used in the Clear Creek unit to exceed preliminary projections.
Production from the Duperow has been from numerous small accumulations in the central basin. These reservoirs, which were found primarily during the second cycle, occur at depths ranging from about 8,500 feet in Divide County, to 11,500 ft in Dunn County. Rapid porosity changes indicate that most of these reservoirs are combination structural-stratigraphic traps.
The Beaver Lodge field accounts for more than half of the Duperow production. This field was developed on a 320-acre spacing pattern from 1957 through 1959. It was unitized in 1962. In 1973 an infill-drilling program which added injectors in the central part of the field began.
A primary recovery of 24.1 million bbl of oil was estimated. Current production (December, 1988) of about 3,400 b/d, with cumulative recovery of nearly 50 million bbl, makes it likely that the projected 61.5 million bbl for ultimate recovery will be attained.
Production credited to the Interlake formation is primarily from the upper part of the upper member in reservoirs that occur at depths of 1,000 to 11,500 ft along the Nesson anticline. Early development was in the northern part of the anticline, while the southern part has seen most of its development in the second cycle.
Red River formation production is found throughout the western tier of counties. The first significant production and the largest Red River field, the Cedar Creek field, extended production from these strata in Montana into North Dakota in 1960. Most of the smaller fields on the East flank of the Cedar Creek anticline in Bowman County were found in the late 1960's and early 1970's. Reservoirs in this area are found at depths of about 8,000 feet. Most of the development west of the Nesson anticline was during the second cycle and was concentrated during the time when oil prices exceeded $27/bbl. Depths there range to 14,000 ft.
Cedar Creek field is the largest of the Red River pools. It has been a relatively successful water-injection secondary recovery unit. The Medicine Pole Hills field was unitized in 1985 for a fireflood project. The price decline delayed injection until October, 1987. A productive response began in August, 1988, so it appears that this will be a successful project.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
