Development of the Border-Red Coulee district in Toole County, Montana, and Alberta, Canada, in 1930-1931, marked the fourth discovery of a commercial oil field on the Sweetgrass arch. The proved area in Montana is about 500 acres, and in Alberta about 100 acres. The course of the International Boundary through the north end of the productive area presents an exceptional situation for an oil field. Various regulations of both countries have prohibited drilling in a strip about 525 feet wide. Directional drilling has not been attempted. On January 1, 1940, the total production of the field was 1,078,694 barrels of 31.5° A.P.I. gravity oil with intermediate base. Approximately three-fourths of this came from 21 wells in Montana; and the remainder from 7 wells in Alberta Reserves are estimated to be about 200,000 barrels.

Structurally, the field is on a small, plunging anticlinal nose, which may be the north extension of the axis of Kevin-Sunburst dome. A buried hill on the eroded surface of the Madison limestone appears to have localized its development. Differential compaction occurred over this feature and exerted some influence upon the position and form of the reservoir sands. Deformation took place sometime during the middle or later stages of the Eocene epoch.

Oil is produced from the Vanalta and Cosmos sands at the base of the continental Kootenai formation at an average depth of 2,520 feet. These sands are the equivalent of the Cut Bank sand in the Cut Bank district at the south, whose development followed in 1932-1933. First identification of the stratigraphic horizon of this zone was made in the Border-Red Coulee district. Although the sands are lenticular and stratigraphic traps of several different types are present, migration and accumulation have probably taken place through fractures. The field is, therefore, not a simple or unmodified stratigraphic trap. Such conditions are probably uncommon.

In addition to a thorough review of the regional stratigraphy, structure, and geologic history, this paper describes in detail the sand conditions and the nature of the petroleum and field waters as a background for suggestions about the manner of migration and accumulation of the oil. Brief accounts of production and technology are also included.