National Drillers Buyers Guide, November 1996
(Richard B. Wells)
The recent discovery of prolific limestone mounds in the Lodgepole Formation beneath Dickinson, North Dakota, has started one of the most exciting domestic exploration plays in several years. These features, called Waulsortian Mounds can be found on the surface in central Montana and in the subsurface of Alberta, Saskatchewan, and Manitoba. They may also be present elsewhere around the flanks of the basin.
The Williston Basin is one of nine major petroleum provinces in the northern Rocky Mountains-Great Plains area. It has four known giant oil fields, at Beaver Lodge, Pine, Little Knife, and Billings Nose, each of which contains over a hundred million barrels of oil. The basin has produced more than 1.2 billion barrels of oil since Amerada drilled the No. 1 Clarence Iverson Well near Minot, North Dakota in 1951. The U.S. Geological Survey predicts that another 700 million barrels remain to be found.
One reason the basin is so prolific is that it has a nearly complete stratigraphic section, nearly 16,000 feet thick in the center of the basin. There are formations from every time period since the Pre-Cambrian, including several thick carbonaceous shale units that provide the source for the oil. Fifteen out of some twenty-six mapped Paleozoic Formations in the basin produce oil, and two of them also produce gas. The basin is heavily biased towards oil rather than gas, and most of its reservoirs are in limestone and dolomite.
The new Stark County limestone play began when Conoco drilled a deep structure beneath their shallow Dickinson Field, hoping to find production in the Red River or Interlake formations. The oil they found in the Lodgepole was a real surprise, as this formation is generally not very porous and had only sporadic production from fractured reservoirs on the Nesson Anticline some seventy miles farther north.
Traditionally, the main objectives in this area have been the Pennsylvanian Tyler Sandstone, the Mississippian Madison (Mission Canyon) Limestone and the deeper Ordovician Red River Reservoirs. The Conoco discovery found more than 129 feet of limestone porosity and came in flowing 2,045 BOPD. Since then, some 40 Lodgepole wells have been drilled in a fairway that is only 9 miles wide by 15 miles long. At least 23 of these wells are productive, a success ration of 55% - excellent results by any standard. Oil reserves range from 500,000 to 3 million barrels of oil per well.
The porosity in the Lodgepole limestone is found in an accumulation of fossil seashells, crinoids, and skeletal fragments from other marine organisms that lie on the flanks of ancient mounds which existed on the Mississippian seafloor. They are called Waulsortian mounds, named for a town in Belgium where some of them are exposed on the surface. The Waulsortian mound, basically a reef that's not a reef, is a carbonate build-up with a dome of knoll shape. They are typically a half to one mile wide and rise about 250 to 325 feet above the surrounding strata.
Duck Creek and Hiline fields are single dome structures, Dickinson and Versippi fields are more like knolls, with two (or more) summits; Eland, the biggest field found so far, is more like a tabular bank formed by the aggregation of several mounds.
Source rocks for this petroleum system are organic shales in the underlying Bakken Formation; the seal is provided by the overlying Mission Canyon and Charles Formation, which contain thick intervals of anhydrite and salt. Such rocks tend to flow under the high temperature and pressure of deep burial, and they can close any fractures or solution channels that might otherwise allow the trapped oil to escape.
Prospecting for Waulsortian mounds in the Lodgepole depends very heavily on seismic surveys, particularly 3D surveys. By using the more expensive 3D seismic, the subtle porosity differences that may indicate an oil-bearing carbonate reef can be found and precisely located. The mound could still be water-bearing - only drilling and testing can answer that question - but the porosity should be there.
Seismic interpretation is complicated in this area because of the velocity effects of multiple layers of rock salt higher in the section and by lateral changes in the thickness of the salt beds. They have been dissolved away in some places, but not everywhere. The small size of the mounds also makes them hard to find. Seismic reflections bounce off surfaces between formations with different density, which gives them different acoustic velocity. The velocity contrast between these reservoirs and the adjacent limestone is rather small, and, as one Nor'Dakota wildcatter put it, "It's like looking for hay in a needlestack."
There has been an interesting coincidence of a surface geochemical survey being run in the same area where conventional seismic surveys indicated possible carbonate mounds in the Lodgepole Formation. Soil gas samples were taken, and the concentration of inorganic iodine in the soil was mapped. High concentrations - from 2 to 20 times the normal background values - coincide with some very good Lodgepole Wells. Several untested Lodgepole seismic anomalies also lie within areas of high iodine concentration; it will be interesting to see how many of these indicate productive carbonate mounds as drilling progresses. It's not foolproof, however, as some excellent Lodgepole wells have already been drilled where there is no iodine buildup in the soil.
It is unlikely that Waulsortian mounds are unique to Stark County, and explorers are casting their eyes on prospective areas with comparable geology around the flanks of the basin. The Williston Basin extends well into Montana and Canada, and a potential exploration area more than 150 miles long and 15 or 20 miles wide can be projected around the central part of the basin. The Lodgepole Formation and its correlative beds extend around the entire basin, and geologists are busy mapping the potential mound trends northward.
This writer has long been fascinated with the Williston Basin, ever since one cold winter night in 1964 when I was the geologist on the No. 1 Katie Reilly near Medora, just a short way outside of Dickinson in Billings County. We knew we had found oil in the Tyler Sandstone at 8,000 feet, but we weren't prepared for the high pressure oil and gas in the Madison limestone at 9,000 feet. We burned down the rig that night, but we found a couple of great little oil fields, one right on top of the other. Medora Field is still producing.