--> --> Disconformities and condensed sections of the Niobrara Formation and the effect on source and reservoir potential, DJ Basin, Colorado

AAPG Rocky Mountain Section Meeting

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Disconformities and condensed sections of the Niobrara Formation and the effect on source and reservoir potential, DJ Basin, Colorado

Abstract

The Niobrara Formation in the DJ Basin of Colorado and Wyoming shows significant variability in interval thickness, geometry, producibility and facies stacking. Most stratigraphic framework of the Niobrara utilize the four well-known 3rd-order chalk/marl cyclostratigraphic packages. These basinwide, well-correlated units are comprised of a hierarchy of 4th- and 5th-order, genetically-related facies successions separated by disconformities. Numerous studies have been published on the variability in producibility between the four Niobrara ‘benches’, however clear relationships between Niobrara productivity and intra-bench facies variation have yet to be established. We evaluate Niobrara productivity within the context of the regional stratigraphic framework, with a closer look at stratigraphic stacking and facies variability within one of the most productive Niobrara intervals. Historically the Niobrara is divided into four 3rd-order chalk and marl cyclostratigraphic packages that occur across the entire DJ basin—the Niobrara A, B C and D/Ft Hays Limestone. We integrated facies information from over fifty recent Niobrara cores within the regional stratigraphic framework based on a 10,000+ well logs. Depositional facies occur as cycles with individual (cm scale) marl to chalk couplets stacking into sequences of increasing chalk or marl upwards (meter scale). Facies successions at multiple scales range from laminated, burrowed marls to medium-bedded, microbioturbated pelletal, coccolithic chalks. The meter-scale, 4- and 5th-order packages are bound by either erosional surfaces with thin lag deposits or condensed sections with multiple ash beds. The surfaces show truncation of beds below and downlap above. This can result in the erosion or non-deposition of partial to entire sequences, both marl and chalk, and result from local variations in subsidence rate and sea floor currents. Conversely, locally expanded sequences are the result of increased relative subsidence rates and can lead to the preservation of some of the best reservoir rocks in the Niobrara. Depositional environment and resultant facies is the dominant control on Niobrara source and reservoir rock properties. Knowing the vertical and lateral variability of facies and the bounding surfaces of a depositional sequence is critical to understanding the distribution of rock properties and evaluating the resource potential of a sequence.