Sinuous Deep-water Channels Research Articles

Using sea-floor morphometrics to constrain stratigraphic models of sinuous submarine channel systems

Constructing geologically accurate reservoir models of deep-water strata is challenging due to the reliance on incomplete or limited resolution datasets. Connecting areas of high-certainty across areas where data is sparse or non-existent (e.g., between wellbores) is difficult and requires numerous interpretations and assumptions. In this study, morphometric data from the Lucia Chica Channel System, offshore California, provides high-resolution 3-D information that is used to constrain correlation and characterization of ancient submarine channel fill deposits.A statistical relationship between cross-sectional asymmetry and planform morphology in sinuous submarine channels is determined from bathymetric data. Submarine channel cross-sectional asymmetry was quantified by calculating the ratio between the distance from the inner bend margin to the thalweg by the entire channel width. This metric was calculated for 243 cross-sections, from 27 channel bends ranging in sinuosity from 1.0 to 3.0. Three distinct channel geometries are classified based on their thalweg position; normal asymmetrical, symmetrical and inverse asymmetrical. Straight channel segments (sinuosities 1.0–1.05) exhibit the most symmetrical cross-sectional morphologies. Low (sinuosities 1.05–1.2) and high sinuosity (sinuosities >1.2) channel bends exhibit maximum cross-sectional asymmetries at bend apexes, with symmetrical cross-sectional morphologies at inflection points. As expected, asymmetry values systematically increase from straight to high sinuosity channel segments; however, the most significant increase occurs at the threshold from straight to low sinuosity channel segments, which suggests that even minor deviation from straight channels promotes development of asymmetrical cross-sectional morphologies. Defined relationships are utilized to inform correlation of channelform surfaces between two well-exposed outcrops of a sinuous deep-water channel system that are ∼1 km apart (Cretaceous Tres Pasos Formation, southern Chile). The developed methodology can be applied in the subsurface to model realistic channelform sedimentary bodies guided by channel planform interpretations from limited-resolution 3-D seismic data, augmented by well data.

Flow dynamics and sedimentation of lateral accretion packages in sinuous deep-water channels: A 3D seismic case study from the northwestern South China Sea margin

The current study uses 3D seismic data to document architectural styles and flow dynamics of lateral accretion packages (LAPs) associated with sinuous deep-water channels, contributing to a better understanding of flow processes and sedimentation associated with LAPs. The documented LAPs underwent three main stages of architectural evolution, including the early incision stages characterized by intense downcutting, active migration stages characterized by active migration and avulsion of the individual channels, and late abandonment stages characterized by the termination of sediment gravity-flows and LAP growth. These three stages of LAP growth repeated through time, yielding a fining-upward pattern from sandy channel-fill turbidites, into sand-mud couplets, all capped by muddy turbidites. A river-reversed helical flow circulation was created by an imbalance, through the flow depth, of inwardly directed pressure gradient forces near the bed and outwardly directed centrifugal forces near the surface. It consists of low-velocity cores near the outer banks and low-velocity cores along the inner banks. Such river-reversed helical flow pattern is evidenced by volumetrically extensive LAPs and toplap and downlap terminations along the gentle banks and by aerially restricted, seismically unresolvable levees and truncation terminations near the steep banks. This river-reversed helical flow circulation favors asymmetric intra-channel deposition characterized by inner bank deposition versus outer bank erosion, and which, in turn, forced individual channels to consistently migrate towards outer banks, resulting in significant asymmetric cross-channel profiles with aerially extensive LAPs along inner banks.

Reservoir quality variations within a sinuous deep water channel system in the Niger Delta Basin, offshore West Africa

Submarine fan channel reservoirs exhibit strong heterogeneity not only on channel architecture but also on the qualities of reservoirs, which can exert direct control over the fluid flow and remaining oil distribution during oilfield development. However, the distribution of the variations in the reservoir quality within submarine fan channel systems is poorly understood. This paper, which takes the submarine fan channel reservoirs of the X oilfield in the Niger Delta Basin as an example, aims to study the variations in reservoir qualities and the controlling mechanisms within a sinuous deep-water channel system. The studies are based on the integration of core analysis, well-logging interpretation, seismic inversion and interpretation constrained by channel architecture.The results show that variations in the reservoir quality within a sinuous deep-water channel system are primarily controlled by rock texture, lithofacies (association) and channel architecture under the circumstance of weak diagenesis. (1) The grain sorting and clay content mainly control the reservoir's porosity and permeability, respectively, while the relationship between grain size and the reservoir's properties is complicated because of the strong heterogeneity in the rock textures for different lithofacies. Cross-laminated fine sandstones have the highest porosity because of their good sorting, while massive medium to coarse sandstones have the highest permeability because of their low clay content. (2) Channel fillings are divided into four types (Type I – Type IV), which bear different reservoir qualities according to the lithofacies associations. Within a complete channel system, the channel fill types evolve progressively upward as the dominant gravity flow types evolve, which leads to the vertical differential distribution of porosity and permeability. (3) The relatively high porosity-permeability zones in the plane of a channel belt exhibit two patterns: multiple lenticular strips and single lenticular strips, which are controlled by the channel's architecture.

Sedimentary Architecture in Meanders of a Submarine Channel: Detailed Study of the Present Congo Turbidite Channel (Zaiango Project)

Abstract Sinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a “reservoir” scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel migration, also provides new data for interpretation of flow dynamics in submarine meandering channels.

Stratal attributes and evolution of asymmetric inner- and outer-bend levee deposits associated with an ancient deep-water channel-levee complex within the Isaac Formation, southern Canada

Isaac Channel 3 is a rare outcrop example of a perpendicular cut through a sinuous deep-water channel, and also where levee deposits formed on opposite sides of the channel are well exposed. Strata flanking the outer- and inner-bend margin of the channel show important differences in lithofacies, architecture and association with channel-fill strata. Proximal outer-bend levee deposits are sand-rich (N:G up to 0.68) and comprise medium- to thick-bedded, T a–d turbidites interstratified with thinly-bedded, T cd turbidites. The thicker-bedded deposits show lateral variation in grain size and thickness over hundreds of meters whereas thin-bedded strata thin and fine negligibly over similar distances. The distal outer-bend levee (up to 700 m laterally away from the channel) consists predominantly of thin-bedded turbidites interstratified with up to 5 m thick coarse-grained splay deposits. In contrast to the outer-bend, the inner-bend levee deposits are significantly more mud-rich (N:G as low as 0.15) and consist mostly of thin-bedded, T cd turbidites with less common thicker-bedded, T a–d turbidites. Lateral thinning and fining trends associated with these less common thicker-bedded deposits occur more rapidly than their outer-bend counterparts. Erosion associated with lateral migration of the channel axis produced a sharp contact along the outer-bend channel margin causing coarse-grained channel-fill deposits to be in erosional contact with levee deposits. This suggests that the crest of the outer-bend levee was elevated above the channel floor and produced a channel margin upon which channel-fill strata onlapped. Positive topography is interpreted to have developed by overspilling processes that deposited abundant sand on the outer-bend levee while the majority of the flow continued through the channel bend and bypassed to areas further downslope. In contrast, some thick-bedded, amalgamated channel-fill deposits in the axial channel area grade laterally over 140 m into thinly-bedded turbidites on the inner-bend levee. The lack of channel-fill on lap relationships implies that topography along the inner bend was sufficiently subtle that at least some flows were able to expand laterally and over the overbank area without becoming separated from the main throughgoing channel flow. Stratal relationships observed in Isaac Channel Complex 3 suggests three main episodes of channel-levee growth that were each initiated by a period of increased levee relief followed by channel filling and distal levee deposition. This consistent depositional history points to the regular variations, in both time and space, of sediment transport and deposition in a deep-marine sinuous channel-levee system.

Sinuous deep-water channels: Genesis, geometry and architecture

Sinuous deep-water channels display a wide range of geometries and internal architectures. Most modern examples have been documented from large passive-margin fans, supplied by major rivers carrying huge volumes of dominantly fine-grained sediments, e.g. Amazon, Mississippi, Zaire, Bengal, Indus, Rhône and Nile Fans. However, similar examples have also been documented from tectonically active margins, e.g. Magdalena Fan. In most cases, modern sinuous channels comprise the core element of laterally extensive channel–levee systems that often aggrade significantly above a low-gradient (0.1–0.5°) fan surface; individual channels may extend downslope for 100s of kilometres. Typically, channels are subject to frequent avulsions, with only one channel active at any given time. Present highstand conditions have ensured that activity in many modern sinuous channels is much reduced due to the disconnect between fluvial feeder system and canyon head, and some have even been heavily modified or destroyed by major mass-transport deposits. Exceptions include Zaire Fan, where recent activity has provided useful insights into flow processes. The majority of detailed studies relating to sinuous channels in the subsurface originate from offshore west Africa, where channels typically occur within incisional (confined) slope–channel complexes and often represent the latter stages of channel complex fill. Both dominantly aggradational and laterally migrating styles are recognised, while modern seafloor channels in this region display a similar incisional character, e.g. Cap Timiris Canyon. Morphologic expression of sinuosity is harder to recognise at outcrop, but there are an increasing number of documented examples of lateral accretion deposits, representing point-bar growth, that are currently thought to be diagnostic of sinuous channel forms. Sinuous channel lateral migration, and point-bar growth, appears to be driven by sustained flow of fluvial-sourced (probably hyperpycnal) low-density turbidity currents, although there does appear to be variation in energy conditions, with some outcrop examples showing switches in erosive and depositional phases of activity. Previous studies have frequently focussed on the obvious gross planform morphologic similarities between fluvial and deep-water sinuous channels, e.g. nature of sinuosity, presence of point bars and cut-off loops. However, we suggest here that the differences between submarine and river channels are of greater significance, in terms of geometry, flow processes, migration style and deposit character. Sinuous deep-water channels typically form initially by a moderate amount of incision followed by rapid initial bend growth (associated with bypass of sediment). Channels that show significant aggradation then reach a point where there is a near cessation of planform movement (ossification), and growth is dominated by vertical aggradation. A new process model is proposed for this developmental sequence that synthesises observations and experiments that were previously paradoxical.

Slope and basin-floor reservoirs from the Miocene and Pliocene of the Veracruz Basin, southeastern Mexico

A regional study of the Veracruz Basin provided an excellent view of long-term deepwater sedimentation patterns from an evolving foreland-type basin. The regional seismic and well-log data set allows for an accurate reconstruction of slope and basin-floor depositional patterns, lithologic compositions, and paleogradients from a continuous succession of bathyal strata that span the Miocene to the lower Pliocene. Variations in Miocene and Pliocene deepwater reservoirs can be linked to prevailing slope characteristics. The Miocene basin had a high-gradient, tectonically generated slope, and the Pliocene basin had a low-gradient constructional slope. The Miocene basin owes its steep margin to the tectonic stacking of early Tertiary, Laramide-age thrust sheets. The Miocene margin shed a mixture of coarse elastic sediments (sands, gravels, and cobbles) and fines (silts and clays) that were transported into the deep basin via turbidity currents and debris flows. Channelized deposits dominate the Miocene slope, and reservoirs occur in long-lasting basement-confined canyons and shorter-lived shallower erosional gulleys. Thick and areally-extensive basin-floor fans exist outboard of the strongly channelized Miocene slope. Fan distribution is strongly controlled by synsedimentary contractional anticlines and synclines. In contrast, the latest Miocene to early Pliocene basin development was dominated by a strongly prograding wedge of shelf and slope deposits that was induced by volcanogenic uplift and increased sediment supply. During this phase, turbidite reservoirs are limited to narrow and sinuous deepwater channels that reside at the toe of the constructional clinoforms and areally limited, thinner basinal fans.

Evolution of deep-water Tertiary sinuous channels offshore Angola (west Africa) and implications for reservoir architecture

Simple to complex, highly sinuous deep-water channels are common in the subsurface Tertiary off Angola, west Africa, and have been important exploration targets there in recent years. We discuss in this article three examples of these sinuous channels that are generally characterized by high-amplitude seismic reflection events, resulting from reservoir-prone lithologies. The planform characteristics of these channels--sinuosities and sinuous loop wavelengths--are similar to those of the fluvial channels; however, the planform combined with sectional seismic characteristics of the deep-water channels suggest that high sinuosities generally evolved through repeated channel aggradation and lateral migration. In detail, the influence of lateral migration vs. vertical aggradation in the evolution of the deep-water channel sinuosities varies from one channel to another and even along the length of a single channel or across a single sinuous loop. The lateral migrations may be continuous or discrete, separated in distance, and resolvable in seismic. Along some parts of the channels, very high initial sinuosities, not significantly affected by lateral migration, suggest that the initial sea-floor topography and gradients were a major factor in their development. In such cases, the channels essentially aggraded vertically. Thus, the modes of deep-water channel migration and sinuosity evolution are complex and different from those of fluvial channels that exhibit largely lateral (without much aggradational component) shifts within a meander loop. These differences are the result of unique hydrodynamic characteristics of the currents in these systems. The gross reservoir shapes in both the fluvial and deep-water channels are obviously dictated by their high sinuosities. The degree of lateral migration vs. vertical aggradation, however, determines the details of the lateral extent vs. vertical stacking of reservoir lithologies and their connectivities in deep-water sinuous channels. A fluvial, (Begin page 1374) point-bar-type continuity may be present in a part of a deep-water sinuous channel loop but is not expected to extend all across it.

Abstract: Deep-Water Clastic Depositional Geometry in Small, Sandy, Topographically Constrained Systems: Analogues for Basins without Sinuous Deep-Water Channels

Abstract: Deep-Water Clastic Depositional Geometry in Small, Sandy, Topographically Constrained Systems: Analogues for Basins without Sinuous Deep-Water Channels