Abstract: Geologic History--Key to Accumulation at Cedar Creek

James H. Clement

More than 240 million bbl of oil has been produced from Mississippian, Silurian, and Ordovician carbonate reservoirs in the crestal part of the Cedar Creek anticline of eastern Montana and North and South Dakota. Ultimate primary and supplemental phases of production probably will exceed 315 million bbl of oil and 60 Bcf of solution gas; more than 94 percent of this is expected from the Silurian and Ordovician reservoirs. This pronounced surface fold, more than 120 mi (193 km) long and 2,000 sq mi (5,180 km) in area, developed through a complex geologic history of recurrent tectonic movements along a northwest-southeast-striking fault zone on the southwest flank of the Williston basin.

Subsurface data from nearly 1,000 exploratory and development wells document four major periods of tectonism from early Paleozoic through middle Tertiary.

1. Post-Silurian-pre-Middle Devonian: uplift and probable fault movement accompanied initial north and east tilting of the main Cedar Creek block.

2. Late Devonian-pre-Mississippian: fault movement and uplift created a broad northwest-southeast-striking anticlinal feature and significant structural closures.

3. Late Mississippian (Chesterian) through Triassic: repeated subsidence occurred along the crestal block with relative down-to-the-east normal fault movement along part of the ancestral master and subsidiary faults.

4. Post-Paleocene: major uplift of the Cedar Creek block accompanied asymmetric drape folding, minor fault adjustment, and significant increase of northwest plunge along the crestal part.

Although favorable traps of significant size were present in the ancestral Cedar Creek region during Paleozoic and Mesozoic periods, geochemical and LOM studies indicate oil migration from organic-rich Ordovician and Mississippian source-rock sequences in the basin eastward did not begin until the middle Late Cretaceous. Present structural closures only partly appear to control accumulation; known limits of productivity generally align more closely with mappable paleostructural closures. Other geologic parameters of equal or greater importance than structure in controlling entrapment and productivity are: (1) abrupt lateral and vertical changes in primary stratigraphic and diagenetic facies with consequent variations in porosity and permeability in the multiple dolomite and limestone eservoirs; (2) geomorphic conditions at major unconformities; (3) fault displacements; and (4) hydrodynamic conditions.

Early recognition of such hydrocarbon generation, accumulation, and paleostructural relations can contribute significantly to successful exploration.

AAPG Search and Discovery Article #90973©1976-1977 AAPG Distinguished Lectures