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42588-AC8
Origin and Character of Organic Carbon Rich Chalky Facies (Source Rocks) and the Niobrara Unconventional Gas Play
The objective of our PRF-supported project was to test for the presence of orbital forcing in the rhythmically bedded facies of the Niobrara Formation using advanced spectral analysis techniques, and if successfully identified, employ such signals to develop a more highly resolved time scale for the study interval (mainly Coniacian-Santonian) in the Western Interior of North America. With improved time control, hypotheses for the origin of the Niobrara depositional system are more effectively evaluated. In recent years the Niobrara Formation has proven to be an economic shale gas play in the Denver Basin, and interest in understanding its potential in other Rocky Mountain basins provided excellent motivation for achieving improved spatial and temporal constraints on the depositional system.
PRF funding supported one graduate student, Robert Locklair, and the Niobrara project formed the bulk of his dissertation. Locklair successfully defended his thesis in April 2007 and is currently finishing a post doctoral project at Johns Hopkins University before he joins Chevron Corporation in Houston. The major results of the Niobrara study are developed in four chapters of the Locklair dissertation. The first documents the application of spectral techniques to test for orbital forcing in the Niobrara Formation and the resulting development of a new orbital time scale for the Coniacian-Santonian stages. This work, which focuses on refinement of the Niobrara temporal framework, is currently in press in Earth and Planetary Science Letters (Locklair, R.E., Sageman, B.B., 2008, Cyclostratigraphy of the Upper Cretaceous Niobrara Formation, Santonian orbital timescale, Earth Planet. Sci. Lett., doi:10.1016/j.epsl.2008.03.021). The second chapter addresses the spatial context of the Niobrara depositional system and is titled “Upper Turonian-Lower Campanian Regional Stratigraphy, Depositional Environments, and Relative Sea-level Change in the Western Interior Basin, U.S.A.” In this very extensive manuscript the new orbital time scale is integrated with regional stratigraphic data to develop an improved interpretation of Niobrara depositional history. The third Niobrara chapter, titled “Cyclostratigraphy of the Upper Turonian-Middle Coniacian Fort Hays Limestone Member of the Niobrara Formation, Western Interior, United States” employs regional stratigraphic observations and spectral analysis techniques to resolve one of the long-standing puzzles of Western Interior stratigraphy – why is the Fort Hays limestone the most carbonate-rich stratigraphic interval in the entire stack of Western Interior Cretaceous strata? The final chapter on the Niobrara Formation employs the orbital time scale to calculate mass accumulation rates for carbonate and organic carbon in the study interval and employs these data to develop a new hypothesis that better accounts for the character of the putative Coniacian (–Santonian?) Ocean Anoxic Event III.
Development of the orbital time scale showed that the study interval has a duration of ~6.8 myr and the Coniacian and Santonian stages have durations of 3.40 and 2.39 myr, respectively. A major interpretive result of the PRF-funded study of the Niobrara Formation is the quantitative confirmation, through bulk accumulation rate calculations, that changes in siliciclastic dilution were the dominant mechanism modulating sedimentary cyclicity. This result applies to all Niobrara facies, including the Fort Hays, which had previously been interpreted to reflect orbitally forced changes in carbonate production. Regional stratigraphic observations also suggest two new models for Niobrara depositional history: 1) fine-grained siliciclastic sediment was bypassed south into Texas during Fort Hays time, possibly as a consequence of a forebulge structure; and 2) contrary to previous sea level interpretations, the Fort Hays most likely accumulated at shallow depths during transgression in a highly sediment-starved Western Interior sea. As such, the Fort Hays represents a new type of transgressive systems tract in which a combination of siliciclastic bypass and clear water conditions favorable for coccolith production lead to accumulation of a very pure limestone. Lastly, geochemical modeling based on analysis of mass accumulation rates of carbonate and organic carbon, calculated using the new orbital time scale, suggest that the muted d13C signal associated with OAE III may simply be the consequence of enhanced global carbonate production and burial during Coniacian-Santonian time. The intensity of the Conicacian-Santonian anoxic event may have been equal to, or greater than the Cenomanian-Turonian event in some areas, but the latter is characterized by a much more pronounced positive d13C excursion. Understanding and predicting both the organic richness of Coniacian – Santonian facies, which bears on their source characteristics, as well as their carbonate contents, which influences fracture properties and thus the quality of shale gas production, are important factors in the development of these hydrocarbon-rich resources.
