The parts of Minnesota and Iowa included in this study comprise 165 counties which have been divided into two geologic provinces— the Sioux uplift and the Iowa shelf (U.S. Geol. Survey, 1968)—as shown in Figure 52. The Sioux uplift is considered to have negative potential for future petroleum production. The 102 counties comprising 60,000 sq mi (155,400 sq km) assigned to the Iowa shelf can be subdivided further on the basis of oil potential. The counties northeast of a line from the northwest corner of Plymouth County on the west to the northeast corner of Louisa County on the east have little potential for petroleum production. Places where Cambrian oil might be found in this area are limited to the basins which flank the Mid-Continent geophysical high (Henderson et al., 1963a, p. 20), as shown on the basement structure map (Fig. 3). These basins are fairly deep near the mafic lavas of the “high” and are shallower away from the lavas. The potential of these sand-filled basins is impossible to evaluate at this time. All counties southwest of the line described are more attractive for oil prospecting. The future potential of this area lies in the exploration of the Cambrian, post—Lower Ordovician, and basal Middle Devonian rocks. Future production of petroleum from these rocks probably will be less than production from proved petroleum provinces.
There has been only one producing oil well in Iowa—the W. F. Flynn P-l in Washington County—which produced from the lower Middle Ordovician Platteville Formation (Fig. 53). The well is near the crest of the Keota dome, now utilized by Natural Gas Pipeline Company of America for underground storage of natural gas in the St. Peter Sandstone. Total production was slightly more than 400 bbl. The well has been abandoned and now is being used as an observation well.
Figures & Tables
The geology of the entire United States, including the continental shelf and slope, was studied by petroleum geologists to determine its petroleum potential. Prospective areas of the 11 regions were assessed qualitatively and, usually, quantitatively.
The prospective basinal area covers approximately 3.2 million sq mi (statute; 8.3 million sq km) and contains approximately 6 million cu mi (25 million cu km) of sedimentary rock above basement or 30,000 ft (9,144 m). Other less prospective areas are, in the aggregate, large.
The prospective area has not been explored adequately. Many high-potential areas are indicated by the geology and extent of exploration, particularly in parts of Alaska, California, Colorado, Louisiana, Mississippi, Montana, New Mexico, North Dakota, Oklahoma, Texas, Utah, and Wyoming, and in parts of the offshore of Alaska, California, Louisiana, and Texas. The prospective Atlantic, Florida, and Alaska continental shelves, and the entire continental slope, barely have been touched by drilling, and other prospective areas and depths on land and the continental shelf remain largely unexplored.
Estimates of potential crude oil reserves of the basinal area only, exclusive of known reserves, range from 227 to 436 billion bbl of original oil in place. The potential probably exceeds the mean of 332 billion bbl. Approximately 32 percent of the oil in place would be recoverable at known rates of recovery. Ultimately, the rate of recovery may reach 60 percent.
Estimates of potential natural gas reserves exclusive of known reserves range from 595 to 1,227 trillion cu ft of recoverable natural gas. The gas potential also probably exceeds the mean of 911 trillion cu ft.
The ultimate petroleum potential of the United States, including known reserves, may exceed 432 billion bbl of crude oil, 1,543 trillion cu ft of natural gas, and 49 billion bbl of natural gas liquids.
Finding and developing the large petroleum potential will require a great amount of drilling because a significant percentage of the visualized undiscovered crude oil and natural gas is in stratigraphic traps, combination stratigraphic and structural traps, reefs, and complex structural situations. Estimates of future domestic demand call for accelerated exploration. To the extent that policies of industry and government militate against accelerated exploration, particularly drilling, a high percentage of the petroleum resources of the United States will not be reduced to possession.