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The early Eocene Green River Formation represents one of the largest petroleum reserves in the world. Although this oil shale has figured prominently in the literature for the past century, and has enormous potential to meet global energy requirements, no detailed geochemical characterization of these deposits exists in the context of the rhythmic sedimentary cycles that characterize the formation. There are two competing (though not mutually exclusive) hypotheses regarding causes of this cyclicity: one argues the predominant cause is climate fluctuations responding to orbital changes; the second suggests a tectonic origin. These competing hypotheses make different predictions about the nature of the oil-shale geochemistry and are thus important in understanding the origin and preservation of petroleum sources within the formation and in other lacustrine systems that are geologically less well-characterized. My project investigated the ecosystem context of the lacustrine cyclicity using sedimentary facies, oil-shale yield, bulk nitrogen and carbon isotopic signatures of organic matter, and compound-specific carbon isotopic composition of plant wax n-alkanes sampled at millennial resolution through a series of cores and outcrops in a transect across the Uinta and Piceance Creek Basins, spanning ~500 m stratigraphic thickness and 3.2 million years of time.

My results demonstrate co-variation in sedimentary facies, oil-shale yield, TOC, C/N ratios, and carbon isotopic composition of bulk organic matter in the expected orbital periodicities, all consistent with a climatic control of lake depth and ecosystem function, although significant lateral and vertical changes in accumulation make the relative spectral power in the precessional (~20 ky) range rather weak. In contrast, there are differences in the mode of response with oil-shale yield, TOC, and sedimentary facies showing more power in the precessional range, whereas C/N ratios track a 100 ky orbital cycle. C/N ratios tend to be high through multiple precessional cycles dominated by organic matter derived from microbes especially cyanobacteria and archaeobacteria, suggesting persistent anoxia (oxygen-poor waters) and a large resulting pool of hypolimnetic ammonium. This pool was eliminated during low lake levels and the C/N values then fluctuate through several precession-controlled steps. The carbon isotopic record, although showing a signal coherent with that of other proxies, is modulated by both differential preservation of organic matter with different characteristic values as well as changing efficacy of the biological pump. Compound-specific carbon isotopic analyses of n-alkanes suggests mixing of carbon from different sources of isotopic composition, and presents a dramatically different pattern from that seen in published studies of the saline facies of the formation.

The orbital cyclicity of the oil-shale yield records promise considerable predictability in exploration, suggesting intervals that while at the basin margin might be relatively low-grade, could improve dramatically towards the center of the basin.