Organic Geochemistry and Sedimentology of Middle Proterozoic Nonesuch Formation—Hydrocarbon Source Rock Assessment of a Lacustrine Rift Deposit

Abstract The middle Proterozoic Nonesuch Formation is part of a transgressive-regressive sequence that fills the Keweenawan trough in northern Michigan (Upper Peninsula) and northern Wisconsin. The Nonesuch is conformable with the underlying Copper Harbor Conglomerate (alluvial) and overlying Freda Sandstone (fluvial). Based on integration of outcrop and core data, three genetic facies assemblages have been recognized. A marginal-lacustrine assemblage, characterized by interbedded sandstone, siltstone, mudstone, and sandstone/shale couplets, represents deposition on a sandflat/mudflat complex. A lacustrine assemblage is characterized by massive to well-laminated, dark shaly siltstone, carbonate laminites, shale, siltstone, and mudstone. These sediments were deposited in a progressively shallowing perennial lake that periodically may have been thermally stratified. A gradual transition from a lacustrine environment to a fluvial environment is represented by red, horizontally laminated and rippled, finegrained sandstone and siltstone of the fluvial-lacustrine assemblage. Interactions among subsidence rates, sedimentation rates, lake-level fluctuations, and possible climatic changes have resulted in variable vertical facies sequences. Total organic carbon analyses show a strong correlation between organic richness and the shale facies (lacustrine assemblage) and parts of the marginal-lacustrine assemblage. Quantitative assessment of organic-carbon levels for the shale facies reveals the presence of organic-prone lithologies (average >0.50% TOC) comprising at least 50% of five of the eight core sections considered. Organic pétrographie and geochemical analyses of selected samples, including incident white light and reflected blue-light fluorescent microscopy, pyrolysis-flame ionization detection, Rock-Eval pyrolysis, and pyrolysis-gas chromatography- mass spectrometry, indicate that most kerogens may be classified as type I and/or type II. A type III designation for some specimens is suggested based on pyrolysis results. Distinct pétrographie and geochemical characteristics among these samples, viewed in terms of geographic and stratigraphie distribution, may be interpreted as the result of differential preservation of similar source organic materials rather than differential incorporation of source materials or varying thermal maturation histories within the basin. Consideration of all pétrographie and geochemical data suggests that limited intervals of the Nonesuch Formation qualify as moderate to good hydrocarbon source rocks that have experienced a mild thermal history (i.e., “oil window” thermal regime). Successful hydrocarbon exploration efforts in the Mid- Continent rift system will depend on how well one can correlate the presence of better source rocks with identification of suitable reservoir rocks and trapping mechanisms. Given the antiquity of these rocks, however, details about timing of hydrocarbon migration and accumulation and preservation of reservoirs also must be addressed.