Recurrent Faulting and Petroleum Accumulation, Cat Creek Anticline, Central Montana

NELSON, W. JOHN, Illinois State Geological Survey, Champaign, IL

The Cat Creek anticline, scene of central Montana's first significant oil discovery, is underlain by a south-dipping high-angle fault (Cat Creek fault) that has undergone several episodes of movement with opposite sense of displacement. Borehole data suggest that the Cat Creek fault originated as a normal fault during Proterozoic rifting concurrent with deposition of the Belt Supergroup. Reverse faulting took place in Late Cambrian time, and again near the end of the Devonian Period. The Devonian episode, coeval with the Antler orogeny, raised the southern block several hundred feet. The southern block remained high through Meramecian time, then began to subside. Post-Atokan, pre-Middle Jurassic normal faulting lowered the southern block as much as 1500 ft.

During the Laramide orogeny (latest Cretaceous-Eocene) the Cat Creek fault underwent as much as 4000 ft of reverse displacement and a comparable amount of left-lateral displacement. The Cat Creek anticline is a fault-propagation fold; en echelon domes and listric normal faults developed along its crest in response to wrenching.

Oil was generated mainly in organic-rich shales of the Heath Formation (upper Chesterian Series) and migrated upward along tectonic fractures into Pennsylvanian, Jurassic, and Cretaceous reservoir rocks in structural traps in en echelon domes. Production has been achieved only from those domes where structural closure was retained from Jurassic through Holocene time.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)