Sedimentological and Stratigraphic Characteristics of Fluvial Sandstones Based on Outcrop Spectral-Gamma-Ray Data and Borehole Images, Williams Fork Formation, Piceance Basin, Colorado

Keeton, Gabriela I.

Graduate Thesis Or Dissertation

Sedimentological and Stratigraphic Characteristics of Fluvial Sandstones Based on Outcrop Spectral-Gamma-Ray Data and Borehole Images, Williams Fork Formation, Piceance Basin, Colorado Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/2n49t201v

Abstract

For subsurface characterization and mapping of fluvial reservoirs, and to evaluate the stratigraphic variability of fluvial deposits, it is useful and common to first interpret either lithofacies or architectural elements in non-cored wells based on well-log motifs and/or borehole images. To explore the uncertainty in well-log motif interpretation of lithofacies and fluvial architectural elements, measured sections (N=4; total length=1485 ft [452.6 m]) of exceptionally well-exposed outcrops from Coal and Plateau Creek canyons and detailed core descriptions (596 ft [181.7 m]) from the MWX #2 well in Rulison Field were used to assess how lithofacies and architectural elements relate to gamma-ray-log response (total and spectral values) in terms of log motif and thorium (Th) and potassium (K) abundance. For fluvial sandstones of the Upper Cretaceous Williams Fork Formation in the Piceance Basin, Th-K cross plots, Th/K values, and significant gamma-ray log-motif variability indicate that no practical correlation exists between outcrop- and core-derived spectral-gamma-ray-log signatures and lithofacies or architectural elements. Fluvial architectural-element log motifs also vary stratigraphically and commonly do not relate to changes in grain size. Th/K values are generally the highest for crevasse splays and are sequentially lower for single-story, multistory, and amalgamated channel bodies, respectively. Weathering, compositional changes, and lateral compositional variability all affect the spectral-gamma-ray motif. In contrast, a distinct set of criteria based on borehole images was established to aid in the interpretation of lithofacies and fluvial architectural elements. Four electrofacies that represent the most common lithofacies observed in core are identified in the Williams Fork Formation through comparison of paired, calibrated borehole images and core (N=5 cores, total length=441 ft [134.4 m]) from the same wells. Borehole-image analysis of dip type, dip pattern, color scheme, and electrofacies was used to establish a set of characteristics to aid in the interpretation of crevasse splays, tidally influenced channels, and single-story, multistory, and amalgamated channel bodies. For example, tidally influenced channels are characterized by dip types consisting of shale brakes and lamination, large dip scatter, alternating brown and yellow colors, and electrofacies representing bioturbated mudrock and sandstone. Results from this study support the interpretation that the lower and middle Williams Fork Formation were deposited by a low energy, meandering-fluvial system and a higher energy, low-sinuosity, braided-fluvial system, respectively. Unlike previous interpretations, the upper Williams Fork Formation does not represent purely low-sinuosity braided-fluvial deposits. The upper Williams Fork Formation can be divided into two intervals, Flaco and Ges intervals, based on changes in fluvial architecture. The Flaco interval consists of very-fine grained crevasse splays and single-story channel bodies and was deposited by a low energy, high-sinuosity meandering-fluvial system, while the overlying coarser-grained, amalgamated Ges interval represents deposits of a higher energy, low- to intermediate-sinuosity, braided-fluvial system. These changes in fluvial style are interpreted to be related to changes in accommodation space which affect the equilibrium profile and energy of the system.

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