Organic Geochemistry and Sedimentology of Middle Proterozoic Nonesuch Formation—Hydrocarbon Source Rock Assessment of a Lacustrine Rift Deposit
Scott W. Imbus, Michael H. Engel, R. Douglas Elmore, 1990. "Organic Geochemistry and Sedimentology of Middle Proterozoic Nonesuch Formation—Hydrocarbon Source Rock Assessment of a Lacustrine Rift Deposit", Lacustrine Basin Exploration: Case Studies and Modern Analogs, Barry J. Katz
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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.
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Lacustrine Basin Exploration: Case Studies and Modern Analogs
Lacustrine environments are a major contributor of petroleum source rocks. Lacustrine source rock prediction is, however, influenced by numerous, complex variables governing lake sedimentation. Current predictive capability can be improved by attempting to map essential climatic variables to limit in space and time the area of lacustrine source rock exploration. Climatic characteristics that govern lake occurrence and the potential for stratification have been investigated with a General Circulation Model of the atmosphere for the present and for the mid-Cretaceous. In this analysis, the distribution of areas with a positive water balance first is used as an indicator of the distribution of areas conducive to lake formation. Second, the distribution of areas that experience large annual climatic variations is used as an indicator of the distribution of lakes that are less likely to be stratified and, hence, less likely to be sites of high organic-carbon preservation. Four factors used to define large climatic variations include (1) seasonal temperature cycle in excess of 40°C; (2) seasonal temperature extreme of less than 4C°; (3) average seasonal differences in precipitation minus evaporation balance in excess of 5 mm/ day; and (4) distribution of mid-latitude winter storms. Evidence is presented to support the capability of climate models that add insight into lacustrine source rock prediction by simulating geographic regions conducive to lake development and to stratification and organic-carbon preservation