Depositional and Stratigraphic Complexities of the Niobrara Formation and the Relationship to Producibility, DJ Basin, Colorado

Abstract

A number of geologic variables assert some element of control on the producibility of the Niobrara Formation. Maturity, source, porosity, wettability, thickness, lithology, fracturing, depositional facies and stratigraphy are all critical to the Niobrara petroleum system. Understanding the distribution, variability, and role they play is critical to unraveling the economic viability across the basin. Depositional facies and stratigraphic framework are two of the most important as they form the basis and context for evaluating these controls. Over 50 cores have been taken in the Niobrara in the DJ and over 10,000 well logs tied to the cores allow the depositional and stratigraphic framework to be delineated.

The Niobrara is stratigraphically divided into chalk and marl sequences at multiple scales that occur across the entire basin. Depositional facies record this cyclicity with individual (cm scale) marl to chalk couplets stacking into chalkening upwards and marling upwards sequences (meter scale) as a result of climatic and sea level fluctuations. Marl beds are deposited during periods of increased terrigenous input with higher preservation of organic material. Chalk beds form during times of decreased terrigenous input and increased oceanic circulation with an associated increase in bioturbation resulting in higher porosity and permeability. Several basin-wide surfaces separate the Niobrara into genetically related sequences creating a unique depositional and stratigraphic framework for each zone. The surfaces are associated with regional chalk or marl beds and occur both at the base and within the C, B, and A intervals. They can be overlain by a thin lag deposit and show truncation of beds below and down lap above the surface. This can result in the erosion or non-deposition of entire intervals, both marl and chalk. Changes in sea floor currents and local sub-basin subsidence rates may explain their genesis. Conversely, locally expanded sequences are the result of increased relative subsidence rates wth a corresponding change in rock properties.

Depositional facies is the dominant control on Niobrara source and reservoir rock properties and thus its producibility. The vertical and lateral variability of the facies and recognition of the sequence and bounding surfaces is thus critical in understanding the distribution of reservoir attributes.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018