abstracts: geology 443 



GEOLOGY. — Oil and gas geology of the Birch Creek-Sun River area, 

 Northwestern Montana. Eugene Stebinger. U. S. Geol. Survey- 

 Bull. 691-E. Pp. 149-184, pi. I, figs. 3. 1918. 

 The purpose of this paper is limited to a presentation of the field 

 evidence having a bearing on the oil and gas prospects of the area, 

 including a description of the broader features of the geology and 

 more detailed accounts of local structural features that seemed to be 

 possible sources of oil and gas. 



All the formations in the Birch Creek-Sun River area are of sedi- 

 mentary origin. Exclusive of the surficial rocks the formations pre- 

 sent range in age from Carboniferous to probably lower Tertiary. Oil 

 and gas possibilities of promise seem to be confined to the Cretaceous 

 rocks. Earth stresses of intensity great enough to fold and tilt the 

 rocks appreciably from their originally horizontal attitude were de- 

 veloped in this region during Paleozoic or later time only after the end 

 of the Cretaceous period. 



Summary descriptions of the geologic formations, arranged in their 

 proper sequence, are given. 



It is believed that the sandstones in at least the upper part of the 

 Kootenai formation, closely associated with the overlying petroliferous 

 rocks in the lower part of the Colorado, offer a possibility of being pro- 

 ductive of oil or gas. The Colorado shale seems to be of first impor- 

 tance as a possible source of oil and gas in northwestern Montana and 

 according to present information is the only promising source known. 

 In the area here discussed the lower part of this shale was found to be 

 petroliferous in every extensive exposure in a belt extending about 35 

 miles from north to south across the area between Deep Creek and Sun 

 River. It contains bituminous shale, which yields oil on distillation, 

 and soft maltha or natural tar in crevices of fractured limestone, which 

 very probably is a residuunr from the evaporation of petroleum. The 

 evidence seems fairly conclusive that this shale offers a source for petro- 

 leum that may have accumulated in commercially valuable quanti- 

 ties either in the Colorado or in the sandy portions of the Kootenai 

 and Virgelle sandstone in immediate contact with it. 



The area described in this report can be readily divided into two large 

 structural units which differ greatly in the degree to which the strata 

 have been deformed. A slightly curving line, in general parallel to 

 the mountain front, extending northward a distance of nearly 60 miles, 

 would mark a sharp transition from an area on the east in which the 



