Multi-Scale Controls on Organic Carbon Enrichment in the Upper Cretaceous Niobrara Formation, Western Interior Basin, USA

Rob Locklair¹ and Brad Sageman^2
1Chevron Energy Technology Company, Houston, TX, USA
²Northwestern University, Evanston, IL, USA

The Cretaceous Western Interior basin of North America preserves a thick section of mud-rich deposits in more distal environments and age-equivalent, shoreline deposits in more proximal settings. There are a number of stratigraphic intervals within the thick, mud-rich section that contain appreciable quantities of organic matter. A number of approaches have been taken with regard to evaluation of the depositional controls of organic-carbon enrichment and distribution of organic-rich facies in these, and other, successions. Some of these studies have focused on source rock quality and/or potential for unconventional resources. This presentation concentrates on Late Cretaceous (Turonian-Campanian) deposition of the Niobrara Formation and multi-scale controls on the distribution of intraformational, organic-rich facies.

The Niobrara Formation exhibits deci-meter to deca-meter scale compositional oscillations of carbonate and/or organic content (Figure 1). The organic fraction is dominantly type II marine kerogen. Compositional oscillations span the entire formation, although the character of lithologic rhythms varies between the Lower Fort Hays Limestone and Upper Smoky Hill Chalk Members. Cyclostratigraphic analyses have been conducted on a variety of Niobrara time series collected from the subsurface. The high frequency oscillations are interpreted to reflect the influence of orbital-scale climate change. Geochemical and sedimentologic observations, in conjunction with a relative orbital timescale and bulk accumulation rate history, are used to differentiate between possible variations in siliciclastic, carbonate, and organic flux as a mechanism for generating observed lithologic cycles. Lower frequency oscillations are interpreted in the context of global carbon isotope stratigraphy, eustasy, and regional stratigraphy. Niobrara deposition spans the time associated with the Coniacian-Santonian Oceanic Anoxic Event III and corresponding 0.5 per mil positive carbon isotope excursion. From a regional perspective, a well establish micro- and macro-fossil biozonation is used to correlate formal and informal members of the Niobrara Fm. to age equivalents in various sub-basins, including shoreline deposits. In this way, various outcrop and subsurface successions can be correlated in time to identify important stratigraphic relations such as: 1) time-transgressive facies, 2) comparison of offshore geochemistry to nearshore stratigraphic architecture, and 3) a regional picture of relative sea-level history that includes observations of paleobathymetric indicators.

Recognition and quantification of cycle hierarchies in the Niobrara Fm. indicate that multiple depositional controls appear to influence source rock, reservoir quality, and mechanical stratigraphy. There is evidence that the primary drivers of sedimentation patterns operate across a spectrum of geographic (local to global) and temporal (~20 kyr to ~myr) scales. Integration of techniques for delineating multi-scale controls on deposition facilitates regional predictions of stratigraphy and rock composition that bear on prospectivity for unconventionals.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013