New Mississippian Play Concepts and Fairways in the Montana Overthrust Belt

Richard Brandley
Rock Research Inc, Calgary, AB

The Mississippian Castle Reef Formation (Madison Group) in the Sawtooths and Big Belts represents deposition in Skeletal Shoal & Bank Complex and Shallow Restricted-Marine Mud depositional systems. Rocks are variably overprinted by the Sun River Dolomite (SRD) dolomitization front. The front has a diagenetic lower contact and erosional upper contact. Dolomitization decreases in intensity downward. Dolomitizing fluids were probably dense, magnesium-rich brines from evaporite pans. Earthy, microcrystalline, intercrystalline, and pinpoint porosity (7% average effective porosity, 5 md average permeability) formed during the Early Paleozoic SRD event. Moldic-intergranular porosity developed through post Mississippian-pre Jurassic meteoric diagenesis, and is currently overprinting outcrop exposures. Reservoir development occurs in all facies, but quality and distribution are strongly facies controlled. Basement-tectonic elements control both facies and reservoir distribution by defining edges and divisions of Sun River Dolomite Platform (SRDP). The western edge is defined by a crustal-scale ramp. The southern edge is defined by the basement-structure-controlled north edge of Central Montana Trough. Scapegoat Bannatyne Trend divides SRDP into 2 major blocks with very different facies, diagenesis, plays, and reservoir potential. Main fairways include Teton Block, Horse Mountain Block, Wolf Creek Promontory, and SRDP proper. Plays include dolomitized ooid, crinoid, pellet, and oncoid shoals, meteorically altered carbonates, and grainy fill in potential submarine canyons. Specific fairways contain only certain play types. Multiple generations of pyrobitumen and dead oil suggest irregular heating such that oil, gas, or CO2 could be encountered. These new plays and fairways create additional exploration opportunities in the Montana Fold and Thrust Belt.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005