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ABSTRACT: Maturation and Migration of Petroleum in the Lake Superior Portion of the Proterozoic Midcontinent Rift

Jeffrey L. Mauk, Philip A. Meyers

The White Pine mine in Ontonagon County, Michigan, contains both active oil seeps and bitumen representing former petroleum migration. Bitumen has two main occurrences: (1) inclusions within veins along faults and joints; and (2) associated with native copper as cement in lithic arenites. Ultraviolet microscopy reveals aromatic hydrocarbons occur as: (3) halos above bitumen droplets enclosed in vein calcite, and (4) inclusion trains along unroofing-related microfractures that cut both detrital grains and cement. Recent work at the mine established a relative chronology of events between approximately 1.1 and 1.0 Ga: (A) sedimentation and concomitant growth faulting; (B) diagenesis and main-stage copper mineralization; and (C) thrust faulting, folding, second-stage copper ineralization, and uplift. Since then, the area has been relatively stable at shallow depths of less than 1 km. Phase 1, 2, and 4 hydrocarbons are spatially and temporally related to event C, suggesting that the first arrival of petroleum at White Pine was coincident with Grenvillian compression, which acted to expel fluids updip from the 6 to 10 km deep basin axis to the north. Thus, temperatures were sufficiently high in the axial portion of the basin to generate petroleum during the roughly 100 m.y. between Keweenawan deposition of the Nonesuch Formation and initial thrusting. Active oil seeps from organic-rich shale of the Nonesuch Formation at White Pine require that this unit is currently in the oil window, and that the oil window has moved approximately 80 km from basin axis to it present margin in the past billion years.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990