David R. Pyles

Multiscale stratigraphic analysis of a structurally confined submarine fan: Carboniferous Ross Sandstone, Ireland

An integrated, multiscale analysis of the Ross Sandstone documents important evolutionary patterns of a structurally confined submarine fan. Through time, the following are evident: (1) depocenters of high-frequency stratigraphic cycles stack vertically, (2) the depositional area increased, (3) regional paleocurrent patterns define a fanlike pattern, (4) the Ross Sandstone correlates in the landward direction to a bypass surface, (5) the Ross Sandstone inherited the basin physiography from the preceding Visean carbonates, and (6) the Ross Sandstone is ponded in the Carboniferous Shannon Basin. These observations demonstrate that the submarine fan filled an actively subsiding, structurally confined basin in an aggradational pattern. A high-resolution geologic map and a cross section of the Ross Sandstone at Loop Head Peninsula depict the location and stratigraphic architecture of every exposure of the Ross Sandstone. These data reveal that (1) lobes are the dominant architectural element (by area) at all stratigraphic positions in the Ross Sandstone; (2) although channels are the most commonly studied features in the Ross Sandstone, they occupy 7% or less of the total cross sectional area of the formation; (3) channels increase upward at the expense of lobes; (4) slumps increase upward; and (5) percent sandstone decreases upward mostly because of an increase in slumps. These trends define an increasing architectural diversity upward within the Ross Sandstone. Observations in the Ross Sandstone coupled with regional patterns for younger formations in the shallowing-upward, basin-fill succession are used to introduce a unifying model for the stratigraphic evolution of the Namurian strata in the basin. The model proposes that each formation in the shallowing-upward succession reflects its own depositional system related to an evolving landscape, and each was sourced from a different direction. This study quantifies vertical trends in stratigraphic features and analyzes them in a statistical manner to test the significance of external controls on local stacking patterns. The statistical analysis reveals that temporal changes in regional conditions, including sediment supply, source area, and basin shape, are statistically and empirically related to local changes in stratigraphy, including architectural element associations and percent sandstone. These relationships suggest that fans with a larger area have more architectural diversity and heterogeneity than their smaller counterparts. Regardless of fan area, lobes are the dominant architectural element. Also, source area is related to the type of architectural element present and percent sandstone. Lobes and their genetically related channels were sourced from the south, whereas slumps and heterolithic strata are mostly sourced from the west. Physical similarities, including climate, eustasy, size and shape of the basin, thickness of stratigraphic cycles, regional stacking patterns, subsidence, percent sandstone, and architectural elements between the Ross Sandstone and the ponded phase of the northern Gulf of Mexico minibasin-fill successions, imply that the Ross Sandstone is an excellent outcrop analog for these successions.

Quantification of static connectivity between deep-water channels and stratigraphically adjacent architectural elements using outcrop analogs

Paleozoic and Mesozoic outcrop and core samples (REMINA-Dekese and REMINA-Samba wells) covering various stratigraphic intervals from the central Congo Basin were analyzed for total organic carbon (Corg), total inorganic carbon (Cinorg), and total sulfur content. Rock-Eval analysis and vitrinite reflectance (Ro) measurements were performed on the basis of the Corg content. Fifteen samples were chosen for molecular organic geochemistry. Nonaromatic hydrocarbons (HCs) were analyzed by gas chromatography (GC) flame ionization detection and GC mass spectrometry. Samples of the Alolo shales from the Aruwimi Group (Lindi Supergroup, late Neoproterozoic to early Paleozoic) are in general very poor inCorg (most samples <0.5%) and contain a high amount of degraded organic matter (OM). All samples of this group revealed a type III to IV kerogen and cannot be considered as a potential source rock. Permian Carboniferous sediments from the Lukuga Group (REMINA-Dekese well and outcrop samples) contain moderate contents of organic carbon (<2%). The Tmax values (heating temperature at which top peak of S2 occurs) indicate early mature OM, partly also a higher level of maturity because of Ro (0.6 0.7%) and production index values (S1/S1 + S2 < 0.2). All samples contain hydrogen-poor type III to IV kerogen with low HC generation potential, only having a very minor gas generation potential. The kinds of OM, as well as the biological markers, indicate a terrestrial-dominated depositional environment.This article uses data from well-exposed outcrops and published information to document static connectivity in deep-water channelized systems. Two measures of static reservoir connectivity on outcrop analogs are proposed: margin connectivity and sand-on-sand connectivity. Margin connectivity (Cm) is the length between two stratigraphically adjacent elements not obstructed by a barrier normalized by the total length of the interface. Sand-on-sand connectivity (Cs) is the length of sand-on-sand contacts between two stratigraphically adjacent elements normalized by the total length of the interface. The Cm and Cs are analyzed with regard to four categories: (1) association of architectural elements, (2) stacking pattern of channel elements, (3) setting on the slope-to-basin profile, and (4) net sand content. Results are as follows. First, connectivity varies by association of architectural elements. Channel-lobe contacts have higher Cm and Cs than channel-channel and channel-levee contacts. Second, connectivity varies by stacking pattern of channel elements. Predominantly vertically stacked channel elements have higher Cm and Cs than predominantly laterally stacked channel elements. Also, disorganized nonsequentially stacked channel elements have higher Cm than organized systematically stacked channel elements. Third, connectivity varies by setting on the slope-to-basin profile. Channel elements in confined settings have higher Cm than both weakly confined and unconfined-distributive settings. Fourth, connectivity varies by net sand content. Channel elements with a high net sand content have higher Cm than those with a low net sand content. Therefore, knowledge of a reservoirs placement in these categories can be used to aid in the prediction of static connectivity.

Flow processes and sedimentation associated with erosion and filling of sinuous submarine channels

This article uses measurements from a three-dimensional exposure of a sinuous submarine channel and its fill to document, for the first time, how flow properties and sedimentation differ between erosional and filling stages. Two units, recording this sequential evolution, are documented. Unit 1 records gravity flows that deepened and laterally migrated the channel, resulting in the deposition of point bars, with coarsening-upward profiles, on the inner part of channel bends. These gravity flows were mud-rich with a wide grain-size distribution. Flow heights exceeded the depth of the channel resulting in the deposition of levees. Strong secondary flow is evident with a helical pattern reversed to their subaerial counterparts. Strata in point bars and levees are inclined and deposited primarily by tractive processes. Unit 2 records gravity flows that filled the channel. These gravity flows were sand-rich with a relatively narrow grain-size distribution. Flow heights scaled to the depth of the channel, and they contain no evidence for secondary flow. Associated strata are horizontal and deposited primarily from suspension.

Quantitative outcrop characterization of an analog to weakly confined submarine channel systems: Morillo 1 member, Ainsa Basin, Spain

Weakly confined channel systems are common in low-relief minibasins on continental margins and are important hydrocarbon reservoirs. They are characterized by channels that diverge in the proximal part of the basin and converge because of topographic confinement in the distal part of the basin. The Morillo 1 member, in the Ainsa Basin, Spain, is an excellent outcrop analog of a weakly confined submarine channel system. Data from the Morillo 1 member are used to quantitatively document how reservoir characteristics vary laterally and longitudinally in weakly confined submarine channel reservoirs. The key axis-to-margin patterns are the proportions of channel elements, channel complexes, channel-complex sets, reservoir facies, and net sand content; static connectivity decreases laterally from the axis to the margins of the system. The key longitudinal patterns in the updip area are channel elements that have levees, are spatially dispersive, and have a radially divergent map pattern. In the downdip area, channel elements are spatially focused and have uniform orientations, and the proportion of channel elements does not change along the longitudinal profile. However, the size of channel elements, percentage of reservoir facies, and connectivity of channel elements are higher in the downdip area. Patterns identified herein are significant because they cannot be resolved using subsurface or sea-floor data. Results of this study can therefore be used to reduce uncertainty in the interpretation of subsurface data, provide input to constrain rule-based forward stratigraphic models, and provide input to constrain reservoir models.

A hierarchical approach for evaluating fluvial systems: Architectural analysis and sequential evolution of the high net-sand content, middle Wasatch Formation, Uinta Basin, Utah

Well-exposed three-dimensional fluvial outcrops of the high net-sand content middle Wasatch Formation in Three Canyon, Uinta Basin, Utah, were used to create and develop a new methodology for describing the architecture of fluvial systems. The methodology builds on the works of Campbell, Jackson, Allen, and Miall, and addresses sedimentary processes, scale, and temporal context for reservoir and non-reservoir bodies. The methodology developed herein is a three-level hierarchical framework that classifies meso- and macroscale architecture of fluvial systems. The three-level hierarchy contains, from smallest to largest: stories, elements, and archetypes. Eight story types provide the foundational building blocks of this framework and account for sedimentation in both channel-belt and floodplain-belt elements, including (1) downstream accreting; (2) laterally accreting; (3) erosionally-based fine-grained fill; (4) fine-grained fill associated with laterally accreting; (5) levee; (6) splay; (7) crevasse or overbank channels; and (8) floodplain fines. Two types of elements are recognized: (1) channel belt and (2) floodplain belt. An archetype consists of a channel-belt element and its genetically related floodplain-belt elements. Two distinct upward-stacking patterns differentiate braided and meandering archetypes. In deconstructing the evolution of archetypes, three distinct associations between channel-belt elements and their adjacent splays are documented: (1) unassociated splays; (2) associated coeval splays; and (3) associated non-coeval splays.Width and thickness for stories, channel-belt elements, and archetypes are documented providing dimensional constraints for analog high-net-sand-content fluvial systems. Additionally, this methodology provides object-based models with shape-defined reservoir and nonreservoir geobodies that realistically compare to fluvial systems.

Shelf-edge trajectories and stratal stacking patterns: Their sequence-stratigraphic significance and relation to styles of deep-water sedimentation and amount of deep-water sandstone

Using a seismic database from the Qiongdongnan Basin in the South China Sea, this study demonstrates that shelf-edge trajectories and stratal stacking patterns are reliable, but understated, predictors of deep-water sedimentation styles and volumes of deep-water sand deposits, assisting greatly in locating sand-rich environments and in developing a more predictive and dynamic stratigraphy. Three main types of shelf-edge trajectories and their associated stratal stacking patterns were recognized: (1) flat to slightly falling trajectories with negative trajectory angles () (−2° to 0°) and negative shelf-edge aggradation to progradation ratios () (−0.04 to 0) and associated progradational and downstepping stacking patterns with low clinoform relief () (150–550 m [492–1804 ft]) and negative differential sedimentation on the shelf and basin () (−0.6 to 0); (2) slightly rising trajectories with moderate (0°–2°) and medium (0–0.04), and associated progradational and aggradational stacking patterns with intermediate (250–400 m [820–1312 ft]) and intermediate (0–0.6); and (3) steeply rising trajectories with high (2°–6°) and high (0.04–0.10) and associated dominantly aggradational stacking patterns with high (350–650 m [1148–2132 ft]) and high (1–2). Each trajectory regime represents a specific stratal stacking patterns, providing new tools to define a model-independent methodology for sequence stratigraphy. Flat to slightly falling shelf-edge trajectories and progradational and downstepping stacking patterns are empirically related to large-scale, sand-rich gravity flows and associated bigger and thicker sand-rich submarine fan systems. Slightly rising shelf-edge trajectories and progradational and aggradational stacking patterns are associated with mixed sand/mud gravity flows and moderate-scale slope-sand deposits. Steeply rising shelf-edge trajectories and dominantly aggradational stacking patterns are fronted by large-scale mass-wasting processes and associated areally extensive mass-transport systems. Therefore, given a constant sediment supply, then , , , and are all proportional to intensity of mass-wasting processes and to amounts of mass-transport deposits, and are inversely proportional to the intensity of sand-rich gravity flows and to amounts of deep-water sandstone. These relationships can be employed to relate quantitative characteristics of shelf-edge trajectories and stratal stacking patterns to deep-water sedimentation styles.

The use of spectral recomposition in tailored forward seismic modeling of outcrop analogs

Spectral recomposition is an improved methodology for generating forward seismic models of outcrop analogs that make use of the full range of frequencies found within real-world seismic volumes. It overcomes the issue common to conventional forward seismic models in which single-peak frequencies are modeled, with the resultant models not capturing the range of frequency content that is found in the real world image. This variable frequency content of real-world seismic volumes has long been known to contain more detailed information within different spectral bands, which forms the basis of the spectral decomposition process, in which different spectral frequencies can correlate to different stratigraphic thicknesses, for example. In this article, we introduce and illustrate the use of a new method termed spectral recomposition. Spectral recomposition is a poststack seismic method operating in the frequency domain that recomposes forward seismic models generated at separate peak frequencies to derive a final image, which has a frequency spectrum similar to that of a targeted real-world image. Using this method, forward seismic models can be tailored to specific real-world images and be used as a more appropriate comparison between the known outcrop geometries and those that are imaged in the subsurface.

Lateral juxtapositions of channel and lobe elements in distributive submarine fans: Three-dimensional outcrop study of the Ross Sandstone and geometric model

Distributive submarine fans contain channel-lobe elements that compensationally stack to build a radially dispersive map pattern. The middle parts of some submarine fans contain juxtapositions of channel elements and lobe elements due to longitudinal and lateral shifts in their channel-lobe transition zones. This article uses an exceptionally well-exposed three-dimensional outcrop of the Ross Sandstone at Bridges of Ross (Ireland) to document the stratigraphic and plan-view manifestation of lateral juxtapositions of channel elements and lobe elements in submarine fans. Observations made herein compare favorably to those in seafloor studies of Navy Submarine Fan (offshore southern California, USA) by William Normark and others, indicating that these systems can be used as paired outcrop-seafloor analogs for distributive fans in which the channel-lobe transition zones are located in longitudinally variable positions. In addition, data from Bridges of Ross and Navy Submarine Fan are integrated to constrain a geometric model that predicts the fractional length of a fan that contains lateral juxtapositions of channel elements and lobe elements. Lateral juxtapositions of channel elements and lobe elements are important because they enhance vertical and lateral connectivity within subsurface reservoirs.