ABSTRACT: Unconformity Related Traps and Production, Lower Cretaceous Through Mississippian Strata, Central and Northern Rocky Mountains

John Dolson

Unconformities provide a useful means of equating stratigraphic traps between basins. Systematic mapping can define new concepts through analogy, often from geographically separate areas. Lower Cretaceous through Mississippian surfaces in the central and northern Rockies provide examples.

Late Mississippian and Early Pennsylvanian surfaces formed at least four paleodrainage basins separated by the Transcontinental arch. Tyler Formation valley fills (Montana, North Dakota) have produced more than 100 million BOE. Analogous targets in Utah remain untested, but the Mid-Continent Morrow trend continues to yield new reserves.

Permian and Triassic paleodrainages filled primarily with seals and form regional traps. A breached Madison trap (Mississippian, Colorado), more than 350 million BOE (Permian Minnelusa, Wyoming), more than 8 billion BOE from the White Rim Sandstone tar deposits (Permian, Utah), and eastern Williston basin (Mississippian) are examples. Minor basal valley fill trapping also occurs.

Transgressive carbonate facies changes have trapped more than 40 million BOE (Permian Phosphoria Formation, Wyoming). Additional deep gas potential exists.

Jurassic unconformities control seal distribution over Nugget Sandstone (Jurassic) reservoirs and partially control Mississippian porosity on the Sweetgrass arch (Montana). Minor paleohill trapping also occurs.

Lower Cretaceous surfaces have trapped nearly 2 billion BOE hydrocarbons in 10 paleodrainage networks. Undrilled paleodrainage basins remain deep gas targets.

The systematic examination of Rocky Mountain unconformities has been understudied. New exploration concepts and reserve additions await the creative interpreter.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990