Abstract The slope morphologies of the Magdalena deepwater fan exhibit a series of channel-levee complexes (CLCs), recording the evolution of the Magdalena delta. Detailed morphological analysis of the seafloor expression of the channels and their lateral relationship allows the reconstruction of the history of Pleistocene fan development. The Magdalena deepwater fan was deposited on the northern offshore Colombia accretionary wedge (Caribbean Sea), initiated during the late Miocene. Fan evolution is closely related to the Magdalena River delta migration and reflects control by tectonic processes occurring from Pliocene to present. Major delta shifts toward the southwest (Canal del Dique) and northeast (Cienaga de Santa Marta region) create a submarine fan that migrated with the river, becoming younger toward the southwest. The main fan was abandoned during the Holocene, focusing deposition on the Barranquilla region to the northeast with modern active sedimentation. The depositional processes in the active fan area are mainly dominated by turbidity currents, alternating with slumps/debris flows that generated large mass transport deposits (MTDs). Eight river delta phases were identified, linked to the onshore geology and their corresponded submarine fan expression, which is characterized by the presence of CLCs and MTDs. Seven CLCs were studied using multi-beam bathymetry and seismic profiles. The CLCs showed a big variation of sinuosity and gradient throughout the slope. The higher sinuosity values were encountered at areas of high gradients, suggesting that the channels attempt to reestablish its equilibrium profile by increasing sinuosity as a response of changes in the slope. Highly sinuous channels in the western fan suggest that sinuosity changes are controlled by changes on the slope associated with the deformation of the fold-and-thrust belt along the margin. In addition, channel’s forced migration, avulsions, convex-up profiles, and the presence of knickpoints (KPs) suggest ongoing deformation during western CLC deposition. Conversely, the northeastern section of the fan exhibits channel thalweg profiles with lower sinuosity values at deeper depths. Convex-up thalweg profiles in this area may represent disequilibrium profiles or post-abandonment deformation. Older CLCs are highly affected by degradational processes after the abandonment of the systems, increasing channel width and modifying levee walls. These processes should be considered when evaluating dimensions of buried deposits and reservoir quality prediction. A sequence of KPs in the western fan seems to connect sediment flows from the shelf break downslope through a series of steps in the slope, culminating with lobate unconfined deposits. Upstream KP migration in slope steps as a response to deformation may represent a key process to explain the initiation of deepwater channel systems on the Magdalena Fan, as well as channel systems deposited on other tectonically active basins. This study provides new understanding of the processes involved in the Magdalena deepwater fan and implications for channel systems characterization in areas with active deformation during deposition.

Abstract The present-day continental slope offshore Brunei Darussalam (NW Borneo) displays several networks of submarine channels possessing planform attributes similar to those observed in better-studied river systems. We use shallow 3D seismic data to study one tributary network in detail. This network is located directly downslope from the shelf-edge Champion Delta and encompasses an area approximately 8 km by 24 km in the strike and dip directions. The channels in this network initiate 1–2 km down dip of the shelf edge and are not directly linked to a terrestrial river system. Mapping of shallow seismic horizons reveals that the tributary channel network is an aggradational feature constructed on top of a relatively smooth slide plane associated with a large mass-failure event. This observation highlights differences between network construction in submarine settings compared to terrestrial settings where tributary networks are net erosional features. The smooth slide plane provides us with the simplest possible initial condition for studying the deposit architecture of an aggradational submarine-channel network. An isopach map between the seafloor and the slide plane is used to unravel sedimentation trends, particularly relative rates of levee and overbank sedimentation as a function of channel relief, lateral distance from the nearest channel centerline, and distance from the shelf edge. We observe an anti-correlation between channel relief and deposit thickness, which suggests that the degree to which currents are confined within channels exerts a first-order control on local deposition rates. We also find that over 80% of the deposit volume associated with the aggradational network is within levees. Observations suggest that this channel network was constructed from turbidity currents that initiated at the shelf edge as sheet flows prior to transitioning down slope into weakly confined flows through the construction of aggradational channels. Thicknesses of channel-forming turbidity currents are estimated using the distance between channel heads and the ratio of channel to overbank deposit thickness. These two methods yield estimates for flow thicknesses that are between 1.1 and 3 times the mean relief of channels in the network.

Abstract The Brazos–Trinity depositional system consists of four linked late Pleistocene intraslope basins (I–IV) located on the upper slope, offshore Texas, U.S.A. Conceptual understanding of the fill history in these basin include “fill and spill” models where basins fill sequentially in the seaward direction, to models which invoke coeval basin filling with the coarse fraction retained preferentially in the up-dip basins and synchronous early bypass of the fine fraction to down-dip basins. Integration of recent coring results with nearly complete 3D seismic coverage has improved age dating and reconstruction of infill history. Initially sediment gravity flows bypassed the upper basins, as they had not yet formed, depositing a basal sandy unit in Basin IV. Higher net/gross sands in Basin II deposited from mixed flows, with mud suspended high within the flows exiting Basin II through a tributary-like flow-gathering zone near the basin exit point. The muddy parts of these flows preferentially accumulated in the lower part of Basin IV, which was a three-dimensionally closed basin with deep ponded accommodation. In contrast, the upper fill in Basin IV comprises a submarine apron that is sourced by a continuous channel system that extends directly from a shelf-margin delta located in Basin I. Within this apron, the observed seaward tapering is controlled by lower-efficiency sandy sediment gravity flows of relatively small volume with respect to basin size. These observations allow us to distinguish perched aprons from ponded aprons, with direct implications for reservoir continuity. We further recognize that low-relief ponded aprons have lower ratios of sand net/gross than either high-relief ponded aprons or perched aprons.

Abstract A series of four intraslope basins linked by submarine channels in the northwestern Gulf of Mexico form part of a source-to-sink depositional system that starts in the headwaters of the Brazos and Trinity Rivers and terminates in a ponded intraslope basin offshore Texas—the Brazos–Trinity depositional system. The system is well imaged with 3D seismic data, and two of the basins have been drilled, with three Integrated Ocean Drilling Program wells and two geotechnical wells. Using an integrated approach, we have combined seismic-litho-bio-tephro-stable-isotope-radioisotope stratigraphic methods, using both new and published results, to generate a millennial-scale-resolution chronostratigraphy for this system. Basins I through IV are infilled with about 62 km 3 of sand-rich sediments (∼ 1.6 ×10 11 metric tons) transported by sediment gravity flows since the last interglacial (Oxygen Isotope Stage 5e). The bulk of the sediments, about 49 km 3 , were deposited within a short time period within Oxygen Isotope Stage 2, starting at 24.3 ka at the latest and ending at ∼ 15.3 ka, before meltwater pulse 1A. Sediment accumulated in the slope basins at rates which varied over time between 1.4 and 55 million tons per year. Except for a short time interval when the Brazos River was diverted to the shelf edge at the head of Basin 1, sediment flux to deepwater was on average less than the present-day sediment discharge of the Trinity–Brazos–Sabine Rivers combined. In the period 24-15 ka the sediment sinks comprising the slope basins and shelf-margin delta can be balanced against the fluvial sources if their discharges are somewhat lower than present day, and if the contribution from incised-valley erosion was relatively small. The history of sedimentation on the slope basins is modulated by sea-level changes, but it is strongly influenced by basin topography and by the dynamics of delta development on the shelf. During peak high stands of sea level the slope area receives only pelagic sediments; during low sea-level stands, the sedimentation in each basin results from a complex combination between fluvial input at the head of the first basin, and the rate of subsidence/sedimentation causing basin topography. The ages of sediments in separate basins show that sedimentation occurs at the same time in multiple basins with trapping of sand in updip basins, while mud is preferentially deposited in downdip basins.

Abstract This submarine apron is an analog for the stratigraphic architecture of shallow ponded basins common to stepped, above-grade slopes, where late-stage bypass valleys and channels did not form. Deposition of this apron began within shallow ponded accommodation. Sediment gravity flows entering the basin pass through a leveed channel that incises underlying slope muds. Flows spread, becoming depositional once reaching lower-gradient area within ponded accommodation. Incisions at the distal end of the basin suggest that gravity flows downcut the basin sill as they bypass the basin during filling of ponded accommodation. A channelized apron downlaps the ponded deposits, healing the stepped topographic profile formed after ponded accommodation fills. Collapsing flows exiting the entry-point channel create plunge-pool scours in the proximal part of the apron. Sediment gravity flows exiting the plunge-pool scour accelerate over the steeper face of the apron, eroding bypass channels as healing progresses. Avulsion takes place as the height of the lower apron unit builds, forcing flows to bypass and erode the southwestern flank of the lower apron. Avulsion leads to deposition of an upper apron unit. Throughout deposition of the aprons, flows leave the basin through a gather zone at the exit point of the basin, forming a tributary scour pattern. Acceleration of these flows as they top the basin sill forms a deeply incised submarine valley. Erosion of the sill progresses by headward-migrating knickpoints that truncate apron deposits.

Abstract The Pleistocene Fuji–Einstein system in the northeastern Gulf of Mexico consists of a shelf-edge delta that is directly linked to and coeval with two submarine channel–levee systems, Fuji and Einstein. There is a continuous transition between the channel fills and the delta clinoforms, and the seismic reflections of the prodelta are continuous with the levee deposits. Five smaller delta lobes within the Fuji–Einstein delta formed through autocyclic lobe switching that was superimposed on a single falling-to-rising sea-level cycle. The corresponding stratigraphic complexity is difficult to interpret in single downdip seismic sections, especially where elongated mudbelts are attached to some of the delta lobes. The two slope channel systems, Fuji and Einstein, deeply incise the shelf-edge delta. However, late-stage delta progradation was coeval with slope-channel development, and, as a result, there is no easily mappable, single erosional surface separating channel deposits from deltaic sediments. During early delta-lobe development, a gully field forms on the upper slope, directly downdip from the delta lobe. As the delta progrades, one of the larger gullies in the middle of the field captures most of the denser flows and gradually evolves into a sinuous channel. The larger delta-related slope channels source 2–4 km-wide submarine aprons where they encounter areas with lower gradients. If the slope gully or channel remains active for a long enough time, its corresponding submarine apron smooths out the slope and becomes incised by the later bypassing flows. The well-preserved and mappable 3D shelf-edge architecture provides a rare opportunity to understand relationships between deltaic and slope depositional systems.

Abstract Field and simulation studies indicate that channel architecture and the presence of channel-base drapes (CBDs) can have a significant impact on oil recovery and represent key uncertainties in the understanding of a turbidite channel reservoir. Accordingly, understanding the frequency and distribution of CBDs provides valuable insights into reservoir performance. Core and dipmeter data contain information that can be used to recognize channel-base disconformities and associated CBDs. By comparing the observed number of channel-base disconformities to the observed number of disconformities overlain by mudstone, a statistical assessment of their frequency and distribution can be made. In a spatial sense, the fraction observed in the wells represents the average percentage of the channel elements within the reservoir that are overlain by a drape.

Abstract Accurate modeling of flow path connectivity is critical to reservoir flow performance prediction. Flow path connectivity is controlled by the complex shape, extent, and spatial relationships between pay intervals, their intersection with wells, and the existence of flow barriers between wells. This reservoir heterogeneity can be captured in a flow simulation model as facies patterns among cells and as effective properties within cells (porosity and permeability). However, fine-scale, irregularly-shaped flow barriers between cells can not be accurately represented with pixel-based modeling techniques. To preserve these important fine-scale geological features at the flow simulation block scale, an additional modeling variable is introduced as the edge of a model cell. This cell edge is a continuous or categorical value associated with the cell face and is defined in conjunction with the cell centered property which is often reserved for facies types and/or petrophysical properties. An edge model is created that captures the facies and edge properties as a vector of information at each cell location. For the flow simulation model, the edge properties are easily translated into transmissibil-ity multipliers. Using the example of 3D shale-drapes attached to channel-sand bodies in a deep-water depositional setting, a methodology is presented in which these shale drapes are accurately up-scaled and history matched to production data while maintaining the geological concept that describes the drape geometry. The perturbation parameter in history matching is the continuity of the shales as an edge property. More generally, this coupled modeling of cell-center and cell-edge allows for more flexible reservoir modeling, opening up the potential for modeling and history matching complex geological features effectively at the scale that they are relevant, without additional computational cost of flow simulation.

Abstract A 3,000 km 2 3D seismic data set on the slope (600-1500 m water depth) offshore Trinidad provides detailed images of the seafloor and shallow subsurface. This data aids in the interpretation of the stratigraphic architecture and depositional/erosional processes in this frontier deep water basin. Three main elements comprise the deposits imaged: channel complexes, mass-transport complexes, and mud volcanoes. Channel complex range from nearly straight to highly meandering and from single trunk to distributary and braided/anastomosed patterns. Two main channel systems having seafloor expression are interpreted as the most recent conduits for Orinoco River sediment to the braided fan at the toe of the Barbados accretionary prism. Seismic images help define the genetic evolution of these channels. Sediment pathways are affected by the presence of large mud volcanoes on the present day slope. These mud volcanoes have an average height of 100 m and an average radius of 2km. Failure of the continental slope is revealed by several thick, widespread mass-transport complexes (MTCs) in the study area. Seismic images illustrate variations in size, transport direction/distance, and emplacement mechanisms for these MTCs. Syndepositional thrusting and linear basal scours as opposed to internally chaotic deposits are interpreted to indicate slump-dominated versus debris-flow dominated MTCs, respectively. The main depositional elements are organized commonly in sequences beginning with basal MTCs, followed by channel complexes capped by a pelagic abandonment interval. This vertical organization is similar to that observed in other intraslope basin and basin-floor fans and is interpreted to reflect base-level control on the stratal architecture.