Abstract: Porosity Development in a Polyphase Paleokarst: Madison Group (Mississippian), Northern Montana

John C. Hopkins, Peter E. Putnam

Low-permeability carbonates of the Mississippian Madison Group of northern Montana, and their equivalents in western Canada, contain potentially large reserves of oil and gas. However, fracturing and complex permeability variations within subcrops limit predictions of reserve location. Outcrops of the Madison in the Little Belt Mountains of north-central Montana illustrate both the diagenetic history and reservoir geometry associated with polyphase karsted Mississippian carbonates.

Three phases of solution and brecciation are recognized in the Madison. Phase 1 is regional brecciation associated with penecontemporaneous interstratal solution and collapse of intertidal-supratidal parasequences. Brecciation is enhanced by dissolution in dolomitic paleosol horizons, but is essentially an in situ process as bedding is preserved within phase 1 breccia deposits. Phase 2 is recorded locally in karst solution pipes and paleocave deposits that formed during a period of regional karstification during late Mississippian exposure. Phase 3 is represented by dissolution of cements along fractures which have cut older breccias. Fracturing is associated with Cretaceous and later uplift of the Little Belt and Rocky Mountains, and continues to the present day.

Widespread potential reservoir rocks also occur in leached dolostones and limestones associated with underlying paleophreatic solution zones. In contrast, fractured and leached breccias would provide local reservoirs.

Another important aspect of paleokarst and paleocave deposits is collapse of the overlying section into caverns. Cavern collapse distorts or truncates regional stratigraphy both within the Madison and overlying formations. The presence of such discontinuities must be predicted for successful subsurface horizontal drilling developments.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994