STRATIGRAPHY AND PETROLEUM PROSPECTS OF ANCIENT GEOSYNCLINES

Exploration seismology deals mainly with identifying anticlines, faults, salt domes, and stratigraphic units that are potential petroleum traps. The Paleozoic Cordilleran geosyncline, for instance, includes several contrasting physiographic provinces – deepest in the west and shallowing to the east. There seem to have been a cyclically active hinge zone sundering the miogeosyncline from the shelf zone on its eastern rim. In Wyoming, Montana, and Colorado, facies and lithostratigraphic units (formations and members) of all Upper Paleozoic systems have produced petroleum, and their chiefly miogeosynclinal equivalents in Idaho and eastern Nevada appear to contain both potential reservoir and source bed horizons. Eugeosynclinal sediments in western Nevada, and south central Idaho or similar strata in Utah are poor as source and reservoir beds. Thrusting in Nevada was also periodic throughout Upper Paleozoic and Mesozoic – thrusts punctuated eastward, and carried eugeosynclinal rocks over miogeosynclinal beds. The best potential traps occur between belts of strong deformation in gentle folds.