Channel-fill Elements Research Articles

Fluvial architecture and sequence stratigraphy of the Burro Canyon Formation, southwestern Piceance Basin, Colorado

The Lower Cretaceous Burro Canyon Formation in the southwestern Piceance Basin, Colorado, is composed of deposits that represent a braided fluvial system with high net to gross that transitions stratigraphically upward into a low net-to-gross, low-sinuosity, meandering fluvial system. The fluvial deposits are composed of multiple upward-fining, conglomeratic-to-sandstone successions forming bars and bar sets that exhibit inclined heterolithic strata that we have interpreted to have formed by oblique and downstream accretion. We used well-exposed outcrops, detailed measured sections, and unmanned aerial system-based imagery to describe the fluvial architecture of the Late Cretaceous formation using a hierarchical approach. We described the Burro Canyon Formation as comprising sandstone-rich amalgamated channel complexes (ACC) overlain by non- to semiamalgamated channel complexes. The lower interval of the formation is composed of ACC that contain channel-fill elements with cross-stratification and numerous truncated contacts. These stacked channel-fill elements exhibit an apparent width range of 137–1300 ft (40–420 m) and a thickness range of 5–60 ft (1.5–18 m). The upper interval of the Burro Canyon Formation comprises mudstone-prone intervals of the nonamalgamated channel complex with isolated channel-fill elements interbedded with floodplain mudstones that represent a period of relatively high base level. Associate channel fill elements range in apparent width from 200 to 1000 ft (60 to 300 m) and thickness from 20 to 30 ft (6 to 18 m). The characteristics and spatial distribution of architectural elements of the Burro Canyon Formation correspond to one depositional sequence. The erosional basal surface of the formation, as well as lateral changes in thickness and net to gross, suggest that the Burro Canyon Formation within this study area was deposited as an incised valley fill. Fluvial deposits of the Burro Canyon Formation serve as outcrop analogs for subsurface interpretations in similar reservoirs.

Seismic-driven geocellular modeling of fluvial meander-belt reservoirs using a rule-based method

A novel workflow is presented for building static models of fluvial reservoirs composed of large point-bar architectural elements, based on the application of a specialized forward stratigraphic model, named ‘Point-Bar Sedimentary Architecture Numerical Deduction’ (PB-SAND). The approach uses interpreted horizontal slices from 3D seismic datasets to reconstruct the planform evolution of meander loops, on which basis the geometry of point-bar deposits and associated accretion units can be simulated deterministically. The resulting meander-belt geometry is then populated with different types of facies, through a rule-based algorithm that generates facies architectures that reflect geologic understanding, enabling users to establish linkages between styles of meander evolution (e.g., meander growth via expansion, translation, rotation) and facies distributions. Input parameters define the proportions, geometries and distributions of types of point-bar deposits, as captured from subsurface data and/or from geologic analogs. Multiple stochastic realizations of facies architecture can be generated. To demonstrate the application of this modeling approach, the workflow has been applied to a meander-belt reservoir where large point-bar and channel-fill elements are imaged in seismic. A detailed example is used to illustrate workflows that can be used to build high-resolution sector models in pre-drill contexts, suitable for guiding development plans. An additional example is used to show how to achieve well match for densely drilled sectors, by means of a hybrid approach that combines the new algorithm with traditional geostatistical techniques. It is shown how the workflow allows consideration of point-bar growth styles, as inferred from seismic data, on distribution and geometry of heterogeneities, and how this facilitates the reproduction of geologic features that are important controls on the static connectivity of point-bar reservoirs (e.g., distribution and characteristics of bar-front mud drapes, and of mud-prone packages arising from progressive meander-bend tightening or from downstream fining of deposits beyond the apex of a meander bend). A comparison with traditional variogram-based methods is undertaken to compare metrics that describe intra-point-bar static connectivity and that represent proxies for the degree of compartmentalization of upper-bar sands by mud drapes.

Palaeoenvironment of braided fluvial systems in different tectonic realms of the Triassic Sherwood Sandstone Group, UK

Fluvial successions comprising the fills of sedimentary basins occur in a variety of tectonic realms related to extensional, compressional and strike-slip settings, as well as on slowly subsiding, passive basin margins. A major rifting phase affected NW Europe during the Triassic and resulted in the generation of numerous sedimentary basins. In the UK, much of the fill of these basins is represented by fluvial and aeolian successions of the Sherwood Sandstone Group. Additionally, regions that experienced slow rates of Mesozoic subsidence unrelated to Triassic rifting also acted as sites of accumulation of the Sherwood Sandstone Group, one well-exposed example being the eastern England Shelf. The fluvial depositional architecture of deposits of the Sherwood Sandstone Group of the eastern England Shelf (a shelf-edge basin) is compared with similar fluvial deposits of the St Bees Sandstone Formation, eastern Irish Sea Basin (a half-graben). The two studied successions represent the preserved deposits of braided fluvial systems that were influenced by common allogenic factors (climate, sediment source, delivery style); differences in preserved sedimentary style principally reflect their different tectonics settings. Analysis of lithofacies and architectural elements demonstrates that both studied successions are characterized by amalgamated channel-fill elements that are recorded predominantly by downstream-accreting sandy barforms. The different tectonic settings in which the two braided-fluvial systems accumulated exerted a dominant control on preserved sedimentary style and long-term preservation potential. On the eastern England Shelf, the vertical stacking of pebbly units and the general absence of fine-grained units reflect a slow rate of sediment accommodation generation (18–19.4m/Myr). In this shelf-edge basin, successive fluvial cycles repeatedly reworked the uppermost parts of earlier fluvial deposits such that only the lowermost channel lags tend to be preserved. By contrast, in the eastern Irish Sea Basin of west Cumbria, the rate of sediment accommodation generation was substantially greater (119m/Myr) such that space was available to preserve complete fluvial cycles, including silty drape units that cap the channelized deposits.

Anatomy and Evolution of a Slope Channel-Complex Set (Neoproterozoic Isaac Formation, Windermere Supergroup, Southern Canadian Cordillera): Implications for Reservoir Characterization

Abstract A detailed architectural analysis was conducted in Isaac Unit 5 of the Isaac Formation in the Castle Creek area (east-central B.C., Canada). Isaac Unit 5 developed within a turbidite-dominated slope system on the Neoproterozoic passive margin of western North America where sediment gravity flows and mass movements were common. Isaac Unit 5 crops out over a 3.5-km-long section oriented oblique to mean paleoflow direction and represents a long-lived pathway for transport and deposition that accumulated ~ 100 m of mostly sand in a deep-marine slope setting. It consists of three stacked, high net-to-gross channel-complex fills (8–30 m thick) that correspond to shorter-term flow conduits, which, in turn, are capped by mudstone-rich units. Fine-grained conglomerate and sandstone beds deposited from high-concentration sediment flows constitute most of channel-complex fills. In addition, muddy debrite and slump deposits occur, but although laterally extensive, are volumetrically minor. Laterally persistent, thin-bedded strata (4–20 m thick) composed mostly of Bouma Tc–e turbidites occur at the top of each channel complex and indicate episodes of local complex abandonment (interchannel complex deposition). Different kinds of channel-fill elements were identified within Isaac Unit 5, each characterized by a unique combination of facies assemblage, internal geometry, and bounding surfaces. Most commonly, channel fills consist of amalgamated, thick-bedded, normally graded sandstone (Bouma Ta and Tab divisions) associated with backfilling processes. Poorly stratified mudstone-clast breccia, associated with Ta beds and dune cross-stratified sandstones occur in channel fills that exhibit aggradational and laterally migrating stacking patterns. Inner-bend levee deposits are associated with this type of channel fill. Channel fills with inclined sandstone- and mudstone-rich strata, on the other hand, relate to non-aggradational, high-sinuosity channel conditions that developed during gradual abandonment of the pathways for coarse-sediment transport. In contrast, sudden deactivation of these pathways, most probably related to abrupt updip channel avulsion, led to accumulation of structureless sandstone passively filling the deactivated thalweg. In terms of hydrocarbon reservoir analogues, each of the five different channel-fill elements have unique reservoir attributes (connectivity and continuity), amalgamated and poorly stratified elements having the best (excellent to good) reservoir attributes. Due to high amalgamation at channel-fill scale, channel complexes would represent individual fluid-flow units where only laterally discontinuous permeability barriers (< 500 m long) are present. On the other hand, extensive thin-bedded elements and muddy debrites constitute kilometer-scale barrier-type facies that would effectively compartmentalize channel complexes within the channel-complex set. Strata of Isaac Unit 5 document the detailed stratigraphic complexity, evolution, and reservoir characterization that can be expected in turbidite-dominated slope channel systems developed on passive margins. Further, it is a potential analogue for similar systems developed in continent-margin basins that until now were known mostly from subsurface core and seismic data.

Constraints on paleolake levels, spillways and glacial lake history, north-central Ontario, Canada

The recognition of ice-marginal deltas constructed during the formation of the Nakina II moraine and a previously unrecognized spillway, in the vicinity of Longlac, northern Ontario, indicates that existing concepts of ancestral lake level history and drainage systems in the Lake Superior–Lake Nipigon region is inadequate. Based on isostatically corrected digital elevation maps, ice-marginal deltas of the Nakina II moraine probably formed at the level of glacial Lake Minong, most likely Minong III, and not glacial Lake Nakina as has been commonly suggested. In addition, the presence of a spillway near Longlac indicates that lake water drained southward through the Mullet Outlet–Pic River system immediately following ice-marginal retreat from the Nakina II moraine and not eastward as previously proposed. Architectural-element analysis of exposures within the spillway indicates hyperconcentrated outbursts of meltwater produced thick channel-fill elements during flood conditions with peak-velocities exceeding 3 m/s. Subsequent retreat of ice from the Pic River valley to the east, may have allowed waters of Lake Agassiz, Lake Barlow–Ojibway, or both, to drain into post-Minong lake levels in the Lake Superior basin. These findings place major constraints on previously proposed concepts of northeastern or eastern outlets of Lake Agassiz.

Origin, genetic interrelationships, and stratigraphy over the continuum of fluvial channel-form bounding surfaces: an illustration from middle Cretaceous strata, southeastern Colorado

Abstract A hierarchy of channel-form bounding surfaces generally partitions fluvial strata, each reflecting river scouring processes at a different scale. Low-order (e.g. channel scours, channel-belt boundaries, etc.) and high-order (i.e. valley-form sequence boundaries) surfaces are discussed extensively in the literature. The intermediate-scale boundaries, however, are largely overlooked. An understanding of the range of processes generating these channel-form surfaces is pivotal for interpreting fluvial architecture and understanding sequence-boundary genesis. This paper examines origin, interrelationships, and significance for the full range of channel-form bounding surfaces (channel-fill to sequence boundary) in middle Cretaceous strata of southeastern Colorado, and assesses the role of such surfaces in sequence and allostratigraphy. Middle Cretaceous strata of the Muddy Sandstone were photographed over the entire length (14 km) of the Huerfano Canyon. Lithofacies were then mapped in detail for all Muddy deposits, and bounding surfaces were identified by architectural-element analysis within Muddy fluvial strata. Two alloformations were established based on recognition of three continuous discontinuities. The Cucharas Canyon Alloformation is floored by one sequence-bounding unconformity that is smooth (surface SB3), and is capped by surface SB4. It is mostly a basal sandstone cliff composed of fluvial lithofacies assemblages, but is characterized commonly by paralic lithofacies assemblages in its upper part. The Huerfano Canyon Alloformation is floored by a locally incised sequence boundary SB4, and is capped by a smooth transgressive surface of erosion. It comprises cliff-forming fluvial lithofacies assemblages. Both alloformations reveal six orders of channel-form bounding surfaces. Each order is bundled by surfaces of the succeedingly higher orders; thus, the surfaces can be considered to fit in a hierarchical or ‘nested’ progression. The surfaces record, in order of ascending rank: nested-channel cuts, channel scours, channel-belt boundaries, nested-valley boundaries, valley-fill boundaries, and sequence-bounding unconformities. Though generated by discrete processes, each of these surfaces is formed as a composite of the lower-order surfaces. These surfaces are thus genetically interrelated into a bounding-surface continuum. This continuum is best constrained by allostratigraphic techniques, with sequence stratigraphy and architectural-element analysis treated as approaches under the auspice of allostratigraphy. Middle-Cretaceous strata reveal a close interrelationship between sequence-boundary morphology and architecture of overlying fluvial deposits. Valley-form sequence boundaries tend to record composite surfaces formed of multiple nested-valley scours. Smooth sequence boundaries tend to reflect composites of multilateral channel-fill elements or valley-fills. These observations promote a set of predictive relationships between sequence boundaries and overlying fluvial sandstone that are here categorized into four sequence architectural styles with valley-form boundaries (simple valley, complex valley, compound valley, and compound-complex valley) and four with smooth boundaries (channel sheet, stacked channel sheet, multivalley, and stacked multivalley). Valley and nested-valley incisional boundaries in middle Cretaceous alloformations most likely result from climatic forcing, and do not necessarily record regional surfaces. Such surfaces are abundant in modern deposits, and should be far more common in ancient strata than previously reported. Because of their potential impact on results, they need to be considered as a factor when estimating ratios of accommodation space versus sediment supply, evaluating regional permeability trends, and assessing up-dip fate of sequence-bounding unconformities.

Channel Belt Architecture and Geometry- A Function of Depositional Parasequence Set Stacking Pattern, Ferron Sandstone, East-Central Utah: ABSTRACT

SUMMARY Ferron Sandstone fluvial-deltaic parasequence sets exhibit a well-developed seaward-stepping, aggradational , and landward-stepping pattern. Detailed mapping has quantified the architecture and geometry of fluvial channel belts, as a function of parasequence set stacking patterns. Channel belts within the seaward-stepping parasequence sets, formed in river-dominat ed deltas, when the available sediment supply exceeded the available accommodation space. They have vertically stacked internal channel-fill elements. Thicknesses from 10-15 feet and 4045 feet and widths from 300-700 feet and 1200-1400 feet are most common. Width/thickness aspect ratios range from 7.8-52.9 and average about 29.5. Channel belts bifurcate as they approach the paleo-shoreline and become narrower, averaging 638 feet in width, with lower aspect ratios, averaging about 16.6. Channel belts within the aggradational parasequence sets, formed in more stormdominated deltas, when the sediment supply was balanced with rate of development of accommodation space. They have laterally stacked channel-fill elements. Thicknesses from 25-35 feet and 55-60 feet and widths from 1250-1900 feet are most common. Aspect ratios range from 31.9-97.4 and average about 57.2. Channel belts closer to the paleoshoreline have aspect ratios averaging about 12.0. Channel belts within back-stepping parasequence sets, formed in wave-dominated deltas, when the available sediment supply was less than the available accommodation space. They range in thickness from 9-27 feet and 749-2652 feet in width, with vertically stacked elements. Aspect ratios of 6590 and 185-195 are most common. Construction of geologic models of fluvial-delta ic channel belt reservoir systems must honor changes in architecture and geometry as a function of parasequence set stacking patterns and utilize realistic architectural objects.

Complex Fluvial Response to Low Gradients at Maximum Regression: A Genetic Link Between Smooth Sequence-Boundary Morphology and Architecture of Overlying Sheet Sandstone

ABSTRACT The Mesa Rica Sandstone is an extensive fluvial sandstone sheet that is separated from underlying marine strata throughout northeastern New Mexico by a regionally smooth and laterally continuous sequence-bounding unconformity. The Mesa Rica Sandstone consists mostly of channel-fill elements, with lesser proportions of lateral-accretion elements, and almost everywhere is of the same thickness as the largest channel fills. This broad sandstone sheet reflects deposition on a coastal plain by frequent avulsion and minor lateral migration of straight and low-sinuosity streams under nearly stable base-level conditions, which are here attributed to rapid progradation of the adjacent Mesa Rica deltaic shoreline relative to change in relative sea level during maximum Kiowa-Skull Creek regressi n. Rapid deltaic shoreline progradation under such conditions caused seaward extension and gradient reduction in the adjacent Mesa Rica coastal plain. Overextension and resultant loss of power in Mesa Rica rivers crossing this coastal plain forced both stream straightening and storage of nontransportable coarser sediment in coastal-plain channels. Because base level was stable, Mesa Rica channels could aggrade only minimally, and were forced to spread excess sediments mostly laterally by avulsion. Frequent avulsion and minor lateral migration of Mesa Rica channels under these conditions caused both regional scouring of a smooth sequence-bounding unconformity and deposition of a sheet sandstone with minimal channel stacking. The Mesa Rica Sandstone and the sequence boundary that underlies it represent a distinctive genetic association of single-story fluvial sheet sandstone and smooth sequence-boundary morphology that is probably not unique to the Lower Cretaceous of northeastern New Mexico. Such unincised sequence boundaries are also implied in conceptual models and flume experiments of previous authors.