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Cretaceous Niobrara Carbonates — Facies Successions, Architecture and Stratigraphy — Hydrodynamic and Biotic Controls, DJ Basin, Colorado

McDonough, Katie Joe *1; Deacon, Marshall W.2; Brinton, Lise 3; Friedman, Scott 2; Geesaman, Richard 4
(1) KJM Consulting, Pine, CO.
(2) Noble Energy, Denver, CO.
(3) LithoLogic, Inc., Denver, CO.
(4) Indian Peaks Exploration, Gold Hill, CO.

The Cretaceous Niobrara Formation has produced oil and gas from fractured chalks and ‘shales’ (marls) for over 50 years. Recent advances in and successes with horizontal drilling and completion technologies have led to burgeoning petroleum industry interest in understanding regional variability of Niobrara rock properties.

Sedimentologic data from Niobrara cores and thin sections identified macro- and microfacies and their corresponding rock properties. Facies delineation used physical sedimentary structures, macrofauna, ichnofauna and chalk/marl proportion and vertical trend. Facies successions were then used to delineate cycles. Cycle stacking patterns in sequence stratigraphic context reveal that reservoir and source rock facies occur in discrete positions within the stratigraphic architecture.

Facies cycles occur in patterns of upward-varying carbonate/marl proportions (“chalking” or “marling”). Upward “marling” indicates fresh water influx and detrital clay dilution (“regression”), driving ocean chemistry change and concomitant decrease in chalk-producing organisms. Physical sedimentary processes governed deposition in Niobrara regressive cycles. Upward “chalking” represents less detrital dilution, optimum conditions for chalk-producing organisms (“transgression”), and exhibits dominantly biotic and chemical controls on sedimentologic distribution. Aggradational cycles show little upward variation in chalk/marl proportion, occur commonly at cycle turnarounds, and are associated with unique facies types. At chalking-to-marling turnarounds, reservoir facies occur interbedded with thin organic-rich zones having a crinkled, microbial mat-like aspect. In contrast, highest TOC facies occur at marling-to-chalking turnarounds.

Basin-wide changes in stratigraphic architecture are dependent on relative subsidence rates within structurally controlled sub-basins. Areas of relatively higher subsidence are separated from basinal highs that influenced depositional patterns, composition, and carbonate dilution. Stratigraphic variation can be seen in basin-wide multi-well gamma-ray cross sections, which mimic seismic profiles. Production across the Wattenberg Field is strongly controlled by stratigraphic variability and depositional facies. Areas of increased thickness and dilution correspond to areas of lower resistivity and lower hydrocarbon production. Understanding and predicting these trends are critical to the future development of the Niobrara.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California