ABSTRACT: Unconformities in Exploration, Examples from Lower Cretaceous through Mississippian Strata, Central and Northern Rocky Mountains

John Dolson

Recognition of unconformities and the character of their intervening sequences provides a useful tool for contrasting stratigraphic traps both spatially and temporally between basins. Lower Cretaceous through Mississippian unconformity surfaces in the central and northern Rockies provide several diverse examples.

During Late Mississippian to Early Pennsylvanian time, several erosional unconformity surfaces were created. Within the western United States, four paleodrainage basins, separated by the Transcontinental arch, were formed. Sand-rich valley fill sequences within the Tyler Formation (Montana, North Dakota) have produced in excess of 100 MMBOE. Similar sequences within the midcontinent Morrow trend continue to yield new reserves. Unexplored traps of a similar nature and age remain untested in Utah.

In contrast, Permian and Triassic paleodrainages are filled primarily with low permeability rocks which help to form regional traps and seals. Subcrop traps capped by Permian and Triassic rocks include the Permian Minnelusa Formation of Wyoming (350+ MMBOE), Permian White Rim Sandstone tar deposits of Utah (8+ BBOE), some of the eastern Williston basin Mississippian subcrop traps, and a breached Mississippian Madison trap in northwestern Colorado.

Jurassic unconformities control seal distribution over Nugget Sandstone (Jurassic) reservoirs. Where permeable, clastic-rich facies overlie the Nugget, production has not been established. Mississippian dolomite porosity on the Sweetgrass arch of Montana has been enhanced by dissolution of framework grains beneath the Jurassic surface. Jurassic paleotopography has also created minor Mississippian paleohill traps in Montana.

Lower Cretaceous erosional surfaces have trapped nearly 2 BBOE in ten paleodrainage networks. Traps are in valley fills, buried hills, and onlapping marine sandstones. Deep gas targets are viable in the deeper portions of basins along extensions of these drainage networks.

Systematic examination of unconformities within the Rocky Mountain basins has been underutilized. Insight into new exploration targets may be gained by extrapolating from known trap types and extending them into undrilled areas.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990