Regional Paleotopographic Trends and Production, Muddy Sandstone (Lower Cretaceous), Montana, Wyoming, and Colorado

J. C. Dolson, D. S. Muller, M. J. Evetts, J. A. Stein

The Muddy formation (Lower Cretaceous) of the Rocky Mountains has produced nearly 2 billion bbl of oil-equivalent hydrocarbons. Production is primarily from unconformity-related traps.

An Albian epeiric uplift and/or sea level drop exposed older Muddy and Skull Creek strata across most of the foreland. Conformable strata are preserved in a lowstand foreland basin in northwestern Wyoming and west-central Montana.

At least seven paleodrainage basins developed. Isopachs, combined with bentonite and faunal zone correlations, document a regional drainage divide across southern Wyoming that separated southeast-from northwest-flowing drainages. Older Muddy delta-front sandstones eroded from drainage divides provided local sand sources for younger valley fills. Through-going drainages had eastern and southern provenances of Paleozoic strata.

Reservoir data from over 100 field studies show little or no correlation between depth and porosity or permeability. Secondary porosity is common at all depths. Valley fill and channel traps have produced over 350 million BOE; onlap cycles another 250 million BOE; and buried hill traps over 500 million BOE. Best per-field reserves are from marine sandstones. Regional production patterns reflect proximity to mature source beds or favorable migration routes within individual paleodrainages.

Future exploration success will require deeper drilling and a better understanding of the role of unconformities and diagenesis in controlling hydrocarbon accumulations.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988