Rex D. Cole

Analysis and modeling of intermediate-scale reservoir heterogeneity based on a fluvial point-bar outcrop analog, Williams Fork Formation, Piceance Basin, Colorado

This study presents results of outcrop characterization and modeling of lithologic heterogeneity within a well-exposed point bar of the Williams Fork Formation in Coal Canyon, Piceance Basin, Colorado. This deposit represents an intermediate-scale depositional element that developed from a single meandering channel within a low net-to-gross ratio fluvial system. Williams Fork outcrops are analogs to petroleum reservoirs in the Piceance Basin and elsewhere. Analysis and modeling of the point bar involved outcrop measurements and ground-based high-resolution light detection and ranging data; thus, the stratigraphic frameworks accurately represent the channel-fill architecture. Two- and three-dimensional (2-D and 3-D) outcrop models and streamline simulations compare scenarios based on different lithologies, shale drapes, observed grain-size trends, petrophysical properties, and modeling methods. For 2-D models, continuous and discontinuous shale drapes on lateral-accretion surfaces result in a 79% increase and 24% decrease in breakthrough time (BTT), respectively, compared to models without shale drapes. The discontinuous shale drapes in the 2-D and 3-D models cause a 30% and 107% decrease, respectively, in sweep efficiency because they focus fluid flow downward to the base of the point bar. For similar reasons, 2-D models based on grain size exhibit 67–267% shorter BTT and 44–57% lower sweep efficiency compared to other model scenarios. Unlike the 2-D models, the continuous shale drapes in the 3-D models cause the fluid front to spread out and contact more of the reservoir, resulting in 42–53% longer BTT and 41–52% higher sweep efficiency compared to the other models. These results provide additional insight into the significance of intermediate-scale heterogeneity of fluvial reservoirs.

New incision rates along the Colorado River system based on cosmogenic burial dating of terraces: Implications for regional controls on Quaternary incision

New cosmogenic burial and published dates of Colorado and Green river terraces are used to infer variable incision rates along the rivers in the past 10 Ma. A knickpoint at Lees Ferry separates the lower and upper Colorado River basins. We obtained an isochron cosmogenic burial date of 1.5 ± 0.13 Ma on a 190-m-high strath terrace near Bullfrog Basin, Utah (upstream of Lees Ferry). This age yields an average incision rate of 126 +12/–10 m/Ma above the knickpoint and is three times older than a cosmogenic surface age on the same terrace, suggesting that surface dates inferred by exposure dating may be minimum ages. Incision rates below Lees Ferry are faster, ∼170 m/Ma–230 m/Ma, suggesting upstream knickpoint migration over the past several million years. A terrace at Hite (above Lees Ferry) yields an isochron burial age of 0.29 ± 0.17 Ma, and a rate of ∼300–900 m/Ma, corroborating incision acceleration in Glen Canyon. Within the upper basin, isochron cosmogenic burial dates of 1.48 ± 0.12 Ma on a 60 m terrace near the Green River in Desolation Canyon, Utah, and 1.2 ± 0.3 Ma on a 120 m terrace upstream of Flaming Gorge, Wyoming, give incision rates of 41± 3 m/Ma and 100 +33/–20 m/Ma, respectively. In contrast, incision rates along the upper Colorado River are 150 m/Ma over 0.64 and 10 Ma time frames. Higher incision rates, gradient, and discharge along the upper Colorado River relative to the Green River are consistent with differential rock uplift of the Colorado Rockies relative to the Colorado Plateau.

Sandstone-body dimensions in a lower coastal-plain depositional setting: Lower Williams Fork Formation, Coal Canyon, Piceance Basin, Colorado

This study addresses the field-scale architecture and dimensions of fluvial deposits of the lower Williams Fork Formation through analysis of outcrops in Coal Canyon, Piceance Basin, Colorado. The lower Williams Fork Formation primarily consists of mud rock with numerous isolated, lenticular to channel-form sandstone bodies that were deposited by meandering river systems within a coastal-plain setting. Field descriptions, global positioning system traverses, and a combination of high-resolution aerial light detection and ranging data, digital orthophotography, and ground-based photomosaics were used to map and document the abundance, stratigraphic position, and dimensions of single-story and multistory channel bodies and crevasse splays. The mean thickness and apparent width of the 688 measured sandstone bodies are 12.1 ft (3.7 m) and 364.9 ft (111.2 m), respectively. Single-story sandstone bodies (N = 116) range in thickness from 3.9 to 29.9 ft (1.2 to 9.1 m) and from 44.1 to 1699.8 ft (13.4 to 518.1 m) in apparent width. Multistory sandstone bodies (N = 273) range in thickness from 5.0 to 47.1 ft (1.5 to 14.4 m) and from 53.2 to 2791.1 ft (16.2 to 850.7 m) in apparent width. Crevasse splays (N = 279) range in thickness from 0.5 to 15.0 ft (0.2 to 4.6 m) and from 40.1 to 843.3 ft (12.2 to 257.0 m) in apparent width. These data show that most sandstone bodies are smaller than the distance between wells at 10-ac spacing (660 ft [201 m]). Analyses of interwell sandstone-body connectivity suggest that even at 10-ac spacing, only half of the sandstone bodies are intersected and few are intersected by more than one well.

Fluvial architecture and connectivity of the Williams Fork Formation: use of outcrop analogues for stratigraphic characterization and reservoir modelling

Abstract This study addresses the stratigraphic architecture and connectivity of fluvial sandstones of the Williams Fork Formation through outcrop analysis, and static and dynamic modelling of equivalent reservoirs in the Piceance Basin, Colorado. The Williams Fork Formation is a succession of fluvial channel sandstones, crevasse splays, floodplain mudstones and paludal coals that were deposited by meandering- and braided-river systems within coastal- and alluvial-plain settings. Three-dimensional (3D) static and dynamic reservoir models that are constrained to both outcrop-derived and subsurface data show how static connectivity is sensitive to sandstone-body type and width, and varies with net to gross ratio. Connectivity analyses of 3D outcrop-based architectural-element models show how relatively wide sandstone bodies enhance connectivity. At Mamm Creek Field, connectivity of sandstones that are pay within the middle Williams Fork Formation is 12–18% higher than for the lower Williams Fork Formation. For highly constrained 3D object-based models of architectural elements, connectivity is only 4% higher when crevasse splays are included as reservoir-quality sandstones. Dynamic simulation results also suggest that the best history match is possible by considering only point bars and channel bars (reservoir-quality sandstones) as pay. Additional research is necessary to determine the impact of crevasse splays on reservoir connectivity.

Stratigraphic architecture of fluvial deposits from borehole images, spectral-gamma-ray response, and outcrop analogs, Piceance Basin, Colorado

Lithofacies, architectural-element abundance, and estimates of dune-bedform height and channel sinuosity from borehole images (BHIs) and well-exposed outcrops allow for an expanded interpretation of the fluvial stratigraphic architecture of the Upper Cretaceous Williams Fork Formation. Sedimentologic and stratigraphic data from outcrops and detailed core descriptions of the Williams Fork Formation, Piceance Basin, Colorado, were used to compare attributes of fluvial architectural elements to BHI characteristics and spectral-gamma-ray (SGR) log motifs. Results show a distinct set of criteria based on BHIs that aid in the interpretation of lithofacies and fluvial reservoir architecture. In contrast, a practical correlation does not exist between outcrop- and core-derived SGR log motifs or thorium and potassium abundances and fluvial lithofacies or architectural elements. Four electrofacies based on BHI characteristics (e.g., dip type, dip pattern, and color scheme) represent the most common fluvial lithofacies and are identified through comparison of paired, calibrated BHIs and core. Cross-bed-set thickness values from BHIs are used to calculate dune height as a proxy for flow energy. The lower and middle Williams Fork Formation represent low-energy meandering and higher energy braided systems, respectively, as evident by changes in channel sinuosity and architectural-element type. The upper Williams Fork Formation is divided into two intervals based on lithofacies, architectural elements, channel sinuosity, and net-to-gross ratio. The subdivision for the upper Williams Fork Formation represents a change from a lower energy, meandering fluvial system to a higher energy, lower sinuosity braided system as related to changes in accommodation through time.

Anomalous cold in the Pangaean tropics: COMMENT COMMENT

[Soreghan et al. (2008)][1] present the hypothesis that glaciation existed to near sea level in the tropical region of Pangaea in late Paleozoic time. Their most important evidence for this comes from Unaweep Canyon, a wind gap that crosses the Uncompahgre Plateau, (Colorado, United States). The

Fluvial architecture of the Burro Canyon Formation using UAV-based photogrammetry and outcrop-based modeling: implications for reservoir performance, Rattlesnake Canyon, southwestern Piceance Basin, Colorado

The stratigraphic variability of fluvial architectural elements and their internal lithological and petrophysical heterogeneity influence static connectivity and fluid flow. Analysis of the fluvial architecture and facies heterogeneity of the Lower Cretaceous Burro Canyon Formation provides insight regarding their impact on reservoir performance. The Burro Canyon Formation as exposed in Rattlesnake Canyon, Colorado, forms stacked amalgamated and semi-amalgamated channel complexes that consist of amalgamated and isolated fluvial-bar channel deposits and floodplain fines, and represents a perennial, braidedfluvial system. Detailed two(2-D) and three-dimensional (3-D) static and dynamic reservoir models are constrained using stratigraphic measured sections, outcrop gamma-ray measurements, and Unmanned Aerial Vehicle (UAV)-based photogrammetry. Resulting breakthrough time and sweep efficiency suggest subsurface reservoir performance is most effective perpendicular to paleoflow direction in amalgamated channels. Perpendicular to paleoflow, breakthrough time is 9% shorter than parallel to the paleoflow and sweep efficiency is, on average, 16% greater due to greater sandstone connectivity in this orientation. Variability of preserved channels and lateral pitchouts results in lower recovery efficiency. Facies heterogeneity can account for 50% variation in breakthrough time and slightly lower recovery efficiency (5%). Cemented conglomerates that form channel lags above basal scour surfaces can also create fluid-flow barriers that increase breakthrough time and decrease sweep efficiency (25%) and recovery efficiency (22%).

Mancos B"Interval of Upper Cretaceous Mancos Shale, Douglas Creek Arch, Northwest Colorado: A "Shelf-Sand"Complex: ABSTRACT"

The Mancos B interval of the Mancos Shale, a major gas producer on the Douglas Creek arch is a shelf-sand complex represented by up to 1345 ft (404 m) of thinly interstratified claystone, siltstone, and very fine to fine-grained sandstone deposited approximately 100 mi (161 km) offshore in the Western Interior seaway. In the subsurface, the Mancos B is subdivided into four major units, ranging in thickness from 175 to 480 ft 953 to 146 m). Each unit thins over the archs crest and, in the process, becomes more sand rich. In outcrops along the southern flank of the arch, the lower Mancos B forms a series of coarsening-upward sequences, 25 to 90 ft (8 to 27 m) thick. A typical sequence begins with silty claystone or bioturbated mudstone at the base, followed upward by bioturbated muddy sandstone, and topped with sandy dolomite (as concretions or thin beds). Sedimentary structures include horizontal lamination, wavy lamination, lenticular bedding, flaser bedding, and ripple lamination. Paleocurrent measurements from ripple foresets have a mean azimuth of 111/sup 0/. Burrows characteristic of the Cruziana ichnofacies are common to abundant in all lithofacies.

Primary and Secondary Sedimentary Structures in Fine-grained Lacustrine Rocks of Green River Formation (Eocene), Piceance Creek Basin, Colorado: ABSTRACT

A study of 285 polished slabs (59.0% oil shale, 23.0% carbonate, and 18.0% fine-grained clastic rock) collected from four measured sections along the southern and eastern edge of the Piceance basin reveals important sedimentologic information on the distribution of primary and secondary sedimentary structures. The slabs were studied under low-power binocular magnification, and individual stratification characteristics were noted. A total of 528 primary structures and 334 secondary structures were observed in the slabs. Eleven descriptive classes of primary structures are important: (1) even parallel stratification; (2) discontinuous even parallel stratification; (3) wavy parallel and nonparallel stratification; (4) discontinuous wavy parallel and nonparallel stratification; (5) discontinuous curved parallel stratification; (6) curved nonparallel stratification; (7) structureless; (8) mottled; (9) brecciated; (10) algal stratification; and (11) graded stratification. Of these classes, the oil shale is dominated by classes 1, 2, 3, and 4, and the carbonate and fine-grained clastic rocks by classes 6, 7, 8, and 10. Classes 5 and 9 are rarely represented. Apparently there is a correlation between the organic content and the stratification type of the oil shale. As oil shale increases in organic content, classes 2 and 4 become more abundant and classes 1 and 3 are less so. In the oil shale of the Parachute Creek Member of the two easternmost measured sections, class 1 decreases, whereas classes 2 and 4 increase upward through the sections. The older classes remain approximately the same. These vertical changes correlate with indications of desiccation in the depositional environment in the upper parts of the Parachute Creek Member. Six classes of secondary sedimentary structures are common: (1) loop structure; (2) fault displacement; (3) crystal-growth displacement; (4) bioturbation; (5) contortion; and (6) total disruption. Most of these classes are restricted to oil shale, and loop, fault and crystal-growth types are most abundant. The frequency End_Page 912------------------------------ of occurrence of the secondary structures, like the primary structures, varies with lithology. Crystal-growth disruption (sulfides and carbonate clots) in the oil shale increases with increasing organic content. In a vertical sequence of oil shale in the Parachute Creek Member, crystal-growth disruption of laminae increases upward through the section, and loop and fault structures decrease. Contortion of laminae is almost exclusive to the oil shale, and bioturbation is restricted to claystone and very limy claystone. End_of_Article - Last_Page 913------------

Stable Oxygen Isotopic Composition of Carbonate Rocks in Green River Formation, Eastern Utah and Western Colorado: ABSTRACT

Stable oxygen isotope ratios and calcite-dolomite ratios were determined for 40 samples of various lacustrine carbonate rocks from the Green River Formation (Eocene) of the eastern Uinta basin and the central Piceance Creek basin. Petrographic studies indicate there are five main carbonate rock types: (1) micrite and dolomicrite; (2) algal biolithite; (3) oolite and pisolite; (4) structureless microcrystalline carbonate aggregate (pellet); and (5) kerogen-rich dolomitic claystone (oil shale). The respective ^dgr O18PDB isotopic ranges in per mille for these rocks are: -3.27 to -15.85; -2.43 to -7.19; +2.73 to -4.54; +2.60 to -3.43; and +0.67 to -9.51. The percent dolomite in the carbonate fraction is from 0 to 100. These isotopic values, which are similar to values obtained by other workers for lacustrine carbonate of various ages, suggest that the oxygen isotopes in the carbonate material comprising the algal biolithite, oolite, pisolite, and oil shale were biologically fractionated to isotopically heavier values relative to the inorganically precipitated micrite. The similarity in isotopic values between the structureless microcrystalline carbonate (+2.60 to -3.43) and the oolite (+2.73 to -4.54) also suggests that the former may be a dolomitized and recrystalized form of the latter. No correlation between percent dolomite in the carbonate fraction and the oxygen isotopic composition was found for the oolite, pisolite, and algal biolithite rocks, suggesting that the dolomite in these samples f rmed by diagenetic replacement of primarily precipitated calcite. A positive correlation was found for the oil shale and some of the micrite, suggesting that the dolomite in these samples was primary. End_of_Article - Last_Page 956------------