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History of Subtle Trap Exploration, Rocky Mountain Region, U.S.A.

Robert J. Weimer

The Rocky Mountain region is known worldwide for its oil-producing "sheepherder anticlines." Not as well known are the subtle traps for petroleum that were among the first fields discovered, and that now represent the largest future potential.

Oil seeps were first recorded by explorers in Wyoming in 1832 (Dallas dome, Wind River basin) and 1847 (Absaroka thrust, southwest area). Oil, skimmed from springs, was used for medicinal purposes and sold for wagon lubrication. The first commercial well was drilled in 1862 by an oil seep 9 miles north of Florence, Colorado. Subsequent drilling led to the discovery of the Florence field which has now produced more than 15,000,000 barrels of oil. Production at this field is from fractured Cretaceous shale in a tilted graben with a fault trap on the updip side.

Subtle traps are defined as those related to stratigraphic (facies) changes, unconformities, faults or fractures and diagenetic changes. Fluid pressure differential may play a role in each type trap. Early exploration concepts in exploring for subtle traps were to drill near oil seeps, or updip from oil shows in wells.

The search for anticlinal traps dominated exploration prior to 1950, and the few subtle traps found were a fall-out of this effort. With the revolution of concepts in sedimentary geology since 1950, the search for subtle traps became more sophisticated because of better geologic models which incorporated facies changes, tectonics and sedimentation, unconformities identification of source beds, temperature and pressure fields, and development of improved well logging and seismic stratigraphic techniques.

A list of typical examples of subtle traps and concepts related to them is as follows (names of fields are listed in general order of discovery):

Faults and fractures
Florence-Oil Creek area (1862), Canon City graben.
Boulder (1901), Spindle (1971), Denver basin.
Mining of Gilsonite veins (1889), Altamont (1969), Uinta basin.
Verde (1955), San Juan basin.
Antelope (1953), Williston basin.

Shannon-Salt Creek (1889), Hartzog Draw (1975), Powder River basin.
Moorcroft (1888), Osage (1919), Bell Creek (1967), and Recluse (1967), Powder River basin.
Blanco (1926), San Juan basin, Patrick Draw-Desert Springs (1958), Greater Green River basin.

Kevin-Sunburst (1922), Cut Bank (1931), Sweetgrass arch, Montana.

Unconformity and diagenetic
Wattenberg (1970), Denver basin.

AAPG Search and Discovery Article #91028©1989 AAPG History of Petroleum Industry Symposium, September 17-20, 1989, Titusville, Pennsylvania.