Central Montana has been tectonically active at least since pre-Devonian time. This activity has been in seven stages: (1) pre-Cambrian to post-Devonian uplift and erosion, (2) early Mississippian Big Snowy downwarp, (3) Heath uplift, (4) pre-Jurassic epeirogeny, (5) late Laramide orogeny, (6) late Eocene or post-Eocene igneous intrusions, (7) pre-Pleistocene epeirogeny. The late Laramide orogeny and late Eocene or post-Eocene igneous intrusions account for the asymmetrical (Big Snowy Mountains) tectonic features and the laccolithic domes (Judith Mountains) visible to-day. Accumulation of oil in central Montana is found in both structural (Cat Creek field) and a combination of stratigraphic and structural traps (Northwest Sumatra field). Geologically the oldest producing ormation is the Mississippian Kibbey sand (Ragged Point field) and the youngest is the Lower Cretaceous First Cat Creek sand (Cat Creek field).

The conclusion can be made that central Montana tectonics were not restricted to Laramide time. Oil has been found in lenticular or isolated sandstones but very little has been found in the widespread sandstones of the Jurassic and Lower Cretaceous. One may speculate that oil which initially accumulated in these sands migrated from the central Montana area or its margins prior to the late Laramide orogeny. The subsequent cropping out of the Jurassic and Lower Cretaceous strata during late Laramide deformation and erosion caused an increase in formation hydrostatic pressures making it more difficult for the oil to migrate back into central Montana and into the present structures.

End_of_Article - Last_Page 788------------