Marine and Petroleum Geology 20 (2003) 631–648

www.elsevier.com/locate/marpetgeo

Lateral accretion packages (LAPs): an important reservoir element

in deep water sinuous channels
Vitor Abreu*, Morgan Sullivan1, Carlos Pirmez3, David Mohrig2
ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, TX 77252-2189, USA
Received 1 September 2002; received in revised form 12 August 2003; accepted 12 August 2003

Abstract
The Lower Miocene Green Channel Complex from the Dalia M9 Upper Field, Block 17, offshore Angola is an excellent example of a
deepwater sinuous channel. This sinuous Channel Complex is located in the upper portion of a Confined Channel System, which is
approximately 150 m deep, 2 km wide, and tens of kilometers long. The Green Channel Complex itself is approximately 40 m deep and 2 km
wide and was formed by the lateral migration and local avulsion of a single channel that was approximately 300 m wide and 40 m deep.
An important characteristic of the Green Channel Complex is the presence of shingled seismic reflections at channel margins. These
shingled reflections tend to be parallel to the channel, dipping toward the channel in most cases and sometimes dipping down flow. The
shingled reflections form well-defined packages always in the inner side of the channel bends. They are interpreted to be associated to
continuous lateral migration during channel evolution, resulting in the deposition of accretion packages in the inner side of the channel and
erosion at the outer side of the channel. These accretion packages are named in this paper Lateral Accretion Packages (LAPs). Typically,
lateral migration of individual sinuous channels produces laterally amalgamated Channel Complexes that have varying degrees of internal
amalgamation depending on the nature of the channel-fill.
Integration of high-resolution 3D seismic and well data from Block 17, offshore Angola with outcrop analogs of interpreted sinuous
channels has even further improved the understanding of these types of deepwater channels. Dominating the fill of many sinuous channels
observed in outcrops are inclined sandbodies that dip toward the channel axis, perpendicular to the paleoflow direction. These inclined
sandbodies are interpreted to be analogous to the LAPs described on seismic data. Lithologic composition of LAPs described from outcrop
and core data is variable. They tend, however, to be dominated by a mixture of coarse- and fine-grained sandstones at the base and finer
grained, less amalgamated beds towards the top. Importantly, LAPs can form sizable reservoir elements in the subsurface, with an individual
LAP reaching a thickness of 45 m over as much as 0.75 km2 in the Green Channel Complex.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Deepwater; Reservoir; Channels; Sinuous; Lateral accretion; Migration

1. Introduction offshore West Africa are large, erosionally confined deep-

water Channel Complexes, and a spectrum of architectural
Given the high cost of exploration and development of styles and channel-fill types are observed. Although many of
deepwater reservoirs in offshore West Africa, accurate pre- these high quality deepwater reservoirs have been thought to
drill prediction of reservoir characteristics and post-drill be dominated by low-sinuosity channels, recent advances in
assessment of reservoir heterogeneity away from well seismic data quality and resolution indicates that high-
penetrations is essential. The dominant reservoir types in sinuosity channels are also an important reservoir element.
In fact, many of the reservoir prone intervals associated with
* Corresponding author. Tel.: þ 1-713-431-2114; fax: þ1-713-431-4114. the large, high quality Channel Complexes in offshore West
1
Present address: Department of Geosciences, California State Africa contain highly sinuous channel elements. Deepwater
University, Chico, CA 95929-0205, USA. channels with various degrees of sinuosity were observed in
2
Present address: Department of Earth Atmospheric and Planetary modern deepwater fans (Garrison, Kenyon, & Bouma,
Sciences, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge MA 02139, USA.
1982) but their relevance as reservoirs was just more
3
Present address: Shell E&P Technology Company, P.O. Box 481 recently being assessed (Kolla, Bourges, Urruty, & Safa,
Houston, TX 77001-0481, USA. 2001; Stephens, Monson, & Reilly, 1996).
0264-8172/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpetgeo.2003.08.003
632 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

One of the best subsurface examples of a sinuous,

erosionally confined deepwater Channel Complex is the
Green Channel Complex (Fig. 1) at the Dalia M9 Upper
Field (Fig. 2), which is located in Block 17, offshore
Angola. Dalia Field is comprised of two Lower Miocene
confined channel systems: the M9 Lower and M9 Upper.
The Green Channel Complex is the youngest complex
within the Dalia M9 Upper Channel System. High-
resolution (approximately 65 Hz at the reservoir level) 3D
seismic data (Fig. 3) allowed the recognition and detailed
mapping of internal channel architectures that were not Fig. 2. Seismic profile from the high-resolution 3D seismic volume from the
clearly imaged in conventional resolution seismic data Dalia M9 Upper Channel System (boundary in red) and the interpreted
stratigraphy of the Confined Channel System. The Channel Complexes
(35 Hz). Block 17, offshore Angola contains a number of
within the Channel System are confined at the base and more weakly
large confined channel systems similar to those at the Dalia confined at the top. The Dalia M9 Upper Channel System can be further
Field. Common characteristics of these Confined Channel subdivided into three Channel Complex Sets. This study focuses on the
Systems are: Green Channel Complex, which is part of the youngest Channel Complex
Set, represented in this figure by green lines at its top and base. For location,
see Fig. 5.
† They are large features, up to 200 m thick, 1 – 5 km wide,
and tens of km long. † Based on core data, sandstone reservoirs in these
† They are characterized by complex, inter-cutting, sand- Channel Complexes are dominated by high-concen-
rich Channel Complexes. tration turbidites and traction deposits.
† They present a temporal and spatial evolution of
channel styles expressed by a vertical trend
of decreasing vertical and lateral amalgamation of
Channel Complexes and increasing channel sinuosity
towards the top of individual confined channel
systems.

The term channel is used here to refer to a long-term

conduit for sediment to be transported down slope as defined
by Mutti (1977) and not to infer the degree of erosion or
confinement. Confined channels in particular are elongated,
negative relief features produced by erosive down cutting of
turbidity currents. The term erosionally confined channel is
here used to define deepwater channels in which confine-
ment is mostly caused by erosion of older deposits and only
partially by coeval overbank deposits and levee building.
Erosionally confined channels often have a degradational
profile in cross-sectional view, being distinct from aggrada-
tional, leveed channels, whose confinement is mostly
caused by levee/overbanking processes. It is necessary,
however, to further define an individual channel or more
appropriately a channel-fill as the product of a single cycle
of channel cutting, filling, and avulsion or abandonment
(Sprague et al., 2002) so as not to confuse channels with
Channel Complexes or even lower order architectural
elements (Fig. 4). Two or more channel-fills of similar
architectural style are termed a Channel Complex and two
or more Channel Complexes each bounded at its base by a
basinward shift in facies and at its top by a surface of
abandonment is a Channel Complex Set. Genetically
related, stacked Channel Complex Sets form a Channel
Fig. 1. Horizon slice 26 ms below the top of the Green Channel
Complex system. In this paper, we propose that a Channel
Complex showing a moderate to high-sinuosity deepwater channel. For Complex can also include architectural styles resulting from
location, see Fig. 5. the migration of a single, contiguous channel through time.
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 633

Fig. 3. High-resolution 3D seismic data available in the Block 17 (offshore Angola) allowed the imaging of new reservoir elements in slope channels. Figure
shows conventional resolution (35 Hz) and high-resolution (65 Hz) seismic examples of (1) a cross-section view of the Dalia M9 Upper field (a and a0 ), (2) a
cross-section view of the Green Channel Complex (b and b0 ), and (3) an amplitude map at the base of the Green Channel complex (c and c0 ). Amplitude
extraction showed in c and c0 are from the green horizon at the base of the channel showed in b and b0 . Red squares in a and a0 and red lines in c and c0 indicate
the position of cross-sections shown in b and b0 . Shingled reflectors at channel margins are visible in the higher-resolution data in the cross-section (b0 ), showing
a characteristic ‘scroll bar’ pattern in map view (c0 ). For location, see Fig. 5.

The description and definition of these architectural styles is is the nature of the surface at its base, which is interpreted to
the main objective of this paper. have formed by migration and avulsion of a single channel.
Thus, an approximately 300-m wide channel formed a
Channel Complex approximately seven times wider (close
2. Dalia field subsurface data and the Green to 2000 m) than the width of the active channel at any one
Channel Complex time (Figs. 1 and 5). Aerially, the youngest channel occupies
approximately a third of the total area of the Channel
The development of the Dalia field requires an accurate
understanding of reservoir distribution and connectivity in
order to be economically successful. The upper portion of
the M9 Upper Confined Channel System at the Dalia field
contains the informally named Green Channel Complex
(Fig. 5) and connectivity in this portion of the reservoir is
a large uncertainty. To assist with characterization of
reservoir connectivity of high-sinuosity deepwater channels
such as the Green Channel Complex, deepwater outcrop
analog data and forward seismic models were integrated
with seismic and well data from the Dalia field. Well data
from the Girassol field, Block 17, offshore Angola, is also
presented.
The Dalia M9 Upper Channel System is moderately
sinuous in plan view, approximately 2 km wide and 150 m
deep (Figs. 2, 4 and 5). It can be further divided into three
Channel Complex Sets (Fig. 4). The Green Channel
Fig. 4. Comparison between the stratigraphic hierarchy of confined
Complex occurs near the top of the youngest Channel channels proposed by Sprague et al. (2002) and the stratigraphic hierarchy
Complex Set within the Dalia M9 Upper Channel System. for the Dalia M9 Upper Channel System. Stratigraphic model of the Dalia
An important characteristic of the Green Channel Complex M9 Upper channel is from Vincent Cornaglia.
634 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

Fig. 6. Isochron map of the youngest channel within the Green Channel
Complex, which is on average 40 m thick (1 ms ¼ 1 m at this depth). The
map shows the location of the 14 bends analyzed in this study. For location,
Fig. 5. Map view of the Dalia M9 Upper field (in brown), showing the
see Fig. 5.
Green Channel Complex (in yellow), the last position of a single, sinuous
channel (in gray) and cut-off meanders (in orange). Red arrow indicates
flow direction. Location of other figures presented in this paper are channel in the Green Channel Complex is approximately
indicated in this figure (red squares). Interpretation by Vincent Cornaglia 40 m thick and 300 m wide with an aspect ratio (width
(ExxonMobil Development Company). versus thickness) of 7.5:1. Isochron and seismic amplitude
Complex (Fig. 5). In the southwestern portion of the mapped extraction maps of this Channel Complex show moderate-
area, the channel presents three well-defined abandoned to high-sinuosity patterns (sinuosity of 2.7 in the studied
meander loops (channel bends 9, 12, and 14 in Fig. 6). The section of the channel).
abandoned meander loops are in a slightly higher position Detailed mapping of 14 bends of the Green Channel
than the last phase of the channel (i.e. terraced), indicating Complex in the study area (Fig. 6) was performed to define
that the flows passing through the channel were erosive (by- the internal geometry of the shingled reflections (spatial
passing) during most of the channel evolution. In the overall configuration of the channel remnants) and the relationship
evolution of the Dalia M9 Upper Channel System, the Green of these shingled seismic reflections to the last phase of
Channel Complex is interpreted to reflect the later phases of channel development (Fig. 7(a) – (c)). Attribute extractions
the evolution during waning deposition. were made at the base and top of the packages of shingled
reflections (peak and trough), as well as detailed mapping of
internal reflections (line by line) in few selected packages.
2.1. Green Channel Complex: detailed mapping Shingled seismic reflections were individually mapped to
and definition of LAPs achieve a better understanding of their spatial distribution
and vertical stacking (Fig. 8). Most of the shingled
Amplitude extractions and horizon slices in the Green reflections are well preserved, although a few are partially
Channel Complex display a moderate- to high-sinuosity eroded.
map pattern (Figs. 1 and 5). Bright amplitudes on the inside The geometry of the shingled reflections shows well-
bends are associated with shingled seismic reflections defined patterns:
observed on vertical seismic profiles (Fig. 3). In general,
the shingled reflections are parallel to the inside bend of the † Seismic reflections are parallel to the channel margin
last position of the channel-filling. along which they formed (Figs. 7 and 8).
A segment of the Green Channel Complex (approxi- † Reflections dip towards the channel along which they
mately 10 km in length) was mapped in the central portion formed (Figs. 7 and 8).
of the Dalia field where the M9 Upper confined channel † Often there is a downdip component in the lateral
system is penetrated by several wells (Fig. 6). The youngest migration of the channel (red square in Fig. 9(b)).
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 635

Fig. 7. (a)–(c) Bend 2 in the Green Channel complex (see Fig. 5 for location). Shingled reflections at the Bend 2 show the amplitude extraction (a) at the base of
the package curvilinear lineaments subparallel to the channel margin, representing the downlap of each shingled reflection within the LAP, which is about
40 ms thick (b). Amplitude extraction is acquired at the pink horizon (c). (c) shows a cross-sectional view of the channel. (d) and (e) Bend 13 in the Green
Channel Complex (see Fig. 5 for location). Vertical profile (d) across the bend (blue line in e) shows bi-directional downlap of shingled reflections. This
package of shingled reflections is approximately 750 m long and 500 m wide (e), with an average thickness of about 30 m. The dip of the accretion surfaces
varies from 11 to 268. Red line in (e) correspond to the vertical profile showed in Fig. 3b0 . (f) and (g) Bend 12 in the Green Channel Complex (see Fig. 5 for
location). Internal unconformity at the Bend 14 related to a change in the direction of the lateral migration of the channel.

† Reflections show bi-directional downlap perpendi- The Green Channel Complex not only displays lateral
cular to the direction of channel migration (Fig. 7(d) migration (swing) but also shows downdip migration
and (e)). (sweep, Fig. 9(b), red square). In Bend 8, which is in a
relatively straight portion of the channel, the internal
It is interpreted that each inclined seismic reflection geometry of the LAP indicates a strong downdip migration
marks the position of the inner bank of the channel during of the channel in this area during most of the Channel
migration. Shingled reflection packages would represent the Complex evolution. Additional common features in many of
record of the lateral and downdip migration of the channel, the LAPs are internal truncation surfaces (Fig. 7(f) and (g)).
with each reflection corresponding to accretionary beds These truncation surfaces are probably related to an abrupt
deposited in the inner side of channel bends. Shingled
reflections related to moderate to high-sinuosity, erosionally
confined channels associated with lateral and downdip
migration of the channel, and forming discrete bodies are
named in this paper a Lateral Accretion Package (hereafter
called LAP).
LAPs represent a gradual and/or constant lateral change
in the position of channel. Abrupt changes in channel
position are also observed in the Green Channel Complex
and are here interpreted to be associated with channel
avulsion (Fig. 9). In contrast to LAPs, channel avulsion is
marked on seismic by a cut and fill pattern, characterized by
subhorizontal reflections between inclined reflections in
cross-sectional view and an absence of the characteristic
‘scroll bar’ pattern in map view (Fig. 9(a), blue square,
Fig. 9(b), blue square, and Fig. 9(d)). Fig. 9 shows a horizon
slice at 30 ms below the top of the Green Channel Complex,
Fig. 8. Accretion surfaces within a LAP at the lee side of the Bend 3 in the
demonstrating that both accretion (gradual channel Green Channel complex. Note the accretion surfaces are parallel to the
migration) and avulsion (punctuated channel migration) margin of the channel, dipping towards the channel. Red arrows indicate
can occur within the same channel. paleoflow direction.
636 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

Fig. 9. Seismic profile (a) from channel bend 5 (Green Channel Complex) shows two distinct styles of channel migration: lateral migration (red square) and
avulsion (blue square). Horizon slice (b) corresponds to the dashed red line in (a) and is at 30 ms below top of the Green Channel Complex, encompassing
channel beds 5–7 (see Fig. 5 for location). Lateral channel migration (c and e) is characterized by shingled reflections in seismic profiles (red square in a and d)
and ‘scroll bars’ geometry in map view (red square in b). Channel avulsion (d and f) is characterized by ‘cut and fill’ pattern in seismic profiles (blue square in a
and d) and map view (blue square in b). Yellow square in (b) corresponds to a set of shingled reflections showing strong downdip migration of the channel.

shift in the direction of channel migration. Fig. 7(f) and (g) the shingled beds inside the LAPs. In general, the
shows one example of an internal unconformity within the lineaments are parallel to the channel, probably marking
LAP present in Bend 13. In this case, there is a trend of the position of the channel margin during the different
lateral channel migration toward the west followed by phases of the channel evolution.
another set of shingled reflections that migrate toward the The reconstruction of the evolution of the Green Channel
north (updip migration). There is a bounding surface Complex was completed using the position of the differ-
between the two sets, marked by truncation of bedsets to ent accretion surfaces within each LAP as a guide for
the south and onlapping of the bedsets to the north.
Dimensions of an individual LAP in the Green Channel
Complex are strongly linked to the dimensions of the last
phase of the channel. The length of an individual LAP can be
up to five times the channel width and its width tends to be
twice as wide as the channel. Furthermore, the accretion
packages commonly are thinner than the depth of the channel,
due to the continuous erosion at the base of the channel while
the LAPs were being deposited, indicating a continuous
increase in the depth of channel over time. The average
thickness of the Green Channel Complex is approximately
40 ms, corresponding to about 40 m. The deepest portions of
the channel are about 50 m and are located in the updip
portion of channel bends and in cut-off meanders.

2.2. Channel complex evolution

Seismic attribute extractions from the horizons at the

bases and tops of the LAPs (Fig. 7(c)) were used to help
define the internal spatial distribution of the shingled Fig. 10. LAPs were observed and mapped in each bend of the studied
reflections at every studied channel bend. Fig. 10 channel. Amplitude maps from the bases of these packages (a) define the
direction of accretion through time and have been used to reconstruct the
summarizes the interpretation for the position of the time evolution of lateral migration and (b) shows the interpreted LAPs in
shingled reflections at each channel bend. The lineaments the Green Channel Complex. Lineaments inside LAPs represent the
in every channel bend represent the downlap position of position of each shingled reflection inside the package.
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 637

the position of the channel margin through time (Fig. 10). Therefore, for each time interval during channel evolution,
Therefore, the reconstruction of the Green Channel Com- LAPs were interpreted as active, not active, or not present.
plex presented in this paper is an interpretation based on the Thus, the channel reconstruction presented here is based on
assumption that every shingled reflection represents the observed stratigraphic relationships within individual LAPs
position of the channel margin at a given time during and between different LAPs and the last phase of the
channel evolution. It is also assumed that in a given LAP, channel, rather than based on horizon slices or other seismic
shingled reflections distant from the last position of the attributes.
channel are older than the ones closer to the channel. Laterally amalgamated channels are present in the
Detailed analyses of each LAP in the Green Channel central portion of the mapped area with seismic evidence
Complex indicated that the packages are of varying age, as for cut and fill, indicating periods of abrupt shifts in the
suggested by erosional patterns at the edges of the packages channel migration (local channel avulsion). Internal uncon-
and by the presence of LAPs in cut-off meander loops. formities within the LAPs were also used to define abrupt

Fig. 11. Time lapses showing the evolution of the Green Channel Complex.
638 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

shifts of the channel. These shifts were often correlatable to 1). Channel cut-off becomes more prevalent in the later
abrupt changes in the channel configuration, such as after a stages of channel development (stages 2 – 0; Fig. 11). These
channel avulsion or the formation of a cut-off meander. changes are reflected in channel sinuosity, which increases
The evolution of the system indicates that the width of over the life of the channel from 1.3 to 2.7.
the Channel Complex increased through time, due to the
migration and avulsion of a single channel (Fig. 11). The 2.3. Lithologic calibration from wells and stratigraphic
width of the channel itself appears to have remained model
approximately constant based on the geometry of cut-off
loops. The increase in width of the Channel Complex is also Lithologic composition of the LAPs defined in seismic can
associated with the increase in sinuosity of the late stage be observed in cores from the Dalia and Girassol fields. Fig.
channel. The evolution of the Green Channel Complex is 12 shows the position of a well from the Girassol Field, which
exemplified and represented by six stages (Fig. 11). The cored the outer portion of a LAP similar to the ones present in
available data does not allow for confidently placing the Dalia Field. Two seismic profiles illustrate the position of
absolute time boundaries for these stages. the well with respect to the LAP. Well-logs show a fining
Meander geometries undergo an orderly progression upward pattern, with a marked decrease in sand content
from stages 5 to 0 whereby the initial, relatively straight towards the top of the package (Fig. 12). Well-logs also show
channel in stage 5 evolves into a highly sinuous channel that the impedance changes associated with each accretion
(Fig. 11). The geometric development is evident in surface in a LAP are in most cases related to a boundary
measurements of average meander wavelength, amplitude, between lower impedance sands (high-concentration turbi-
radius, and sinuosity (Fig. 11). Incremental accretion of dites) and higher impedance shales (low-concentration
beds on the inside bends and erosion of outside bends leads turbidites). Therefore, the LAP sampled in the Girassol well
to an increase in overall meander amplitude. This widening is predominantly composed of low-concentration turbidites
of the meander belt occurs simultaneously with a tightening (suspension), with coarser grains restricted to the base of the
of the meander loops, shown by a systematic shortening of package (mixed suspension and traction). Apparently, this
meander wavelength and radius. Tightening of the meander grain size variation occurs in every bed (from coarse at the
loops is also related to the ‘recurving’ of the channel and base to very fine at the top). Sandstones form an amalgamated
ultimately to the onset of channel cut-off and abandonment bed at the base of the LAPs in the Girassol example.
(the first such occurrence is observed between stages 2 and Therefore, lower portions of the LAPs have potentially better

Fig. 12. Lithologic composition of the LAPs in the Girassol field, Block 17 (offshore Angola).
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 639

Formation: Chapin, Davies, Gibson, & Pettingill, 1994;

Elliot, 2000; Martinsen, Lien, & Walker, 2000; Sullivan
et al., 2000; Jackfork Group: Coleman, 2000; Cook, Bouma,
Chapin, & Zhu, 1994; Sprague, 1985).

3.1. Studied outcrops

The Ross Formation is part of the Upper Carboniferous

Clare Basin in western Ireland and represents one-third
order lowstand systems tract deposited in an intracratonic
basin (Sullivan et al., 2000). The formation is interpreted to
represent an aggradational/progradational unit, formed by
sheet sands at the base and Channel Complexes at the top of
the formation, which is overlain by slope mudstones and
Fig. 13. Lithofacies distribution model for LAPs based on integration of
muddy debrites of the Gull Island Fm. (Martinsen et al.,
seismic, well-log, core and outcrop data.
2000). Shingled, inclined sandbodies at channel margins
connectivity, which deteriorates towards the top of the are observed in several outcrops in the upper portion of
package. In general, LAPs with this lithofacies composition the Ross Formation suggesting a trend of increasing
(suspension-dominated deposits) and stacking (semi-amal- occurrence of more sinuous channels towards the top of
gamated beds) are expected to have low vertical permeability the formation.
and variable horizontal permeability. Carboniferous outcrops of the Jackfork Group in the
Fig. 13 summarizes observations from high-resolution Ouachita Mountains, Arkansas (USA) represents a
3D seismic profiles and cores in a stratigraphic model for 10,000 m thick interval of deepwater sediments (Coleman,
LAPs. This model of lithofacies distribution in the LAPs 2000; Sprague, 1985) deposited during foreland basin
predicts the occurrence of an amalgamated bed at the base subsidence immediately prior to the Late Carboniferous
of the unit, consisting of coarse to pebbly sandstone thrust faulting and deformation of the Ouachita Orogeny.
deposits. This model is supported by the interpretation of Interpreted sinuous channels are present in several
the Girassol channel, where the greater amount of wells and localities, often stratigraphically located at the later phases
cores penetrating or close to LAPs allows a better under- of evolution of Channel Complex sets. In fact, the vertical
standing of facies distribution in these units than in the more evolution of Channel Complex Sets in the Jackfork Group
sparsely sampled Dalia channel. and in the Ross Formation (Ireland) is very similar. In both
cases, there seems to be an increase in the occurrence of
channels dominated by shingled, inclined sandbodies
3. Potential deepwater outcrop analogs interpreted to reflect high channel sinuosity towards the
top of individual Channel Complex Sets, with stratigraphi-
Most of the outcrop exposures of interpreted deepwater cally older channels dominated by simple channel-fills.
sinuous channels are not 3D and many of the subsurface The Tabernas Basin is located in SE Spain in the Almeria
channels in offshore Angola were not penetrated by wells. province and is one of the intermontane basins that developed
Therefore, integration of outcrop analogs with subsurface during the Miocene collision between Iberia and North
data can provide a better understanding of the geometry, Africa (Kleverlaan, 1989). The small and elongate basin
stacking, and lithology distribution of these reservoir (10 km wide and 25 km long) contains a wide spectrum of
elements. As a result of this integrated approach, a more deepwater facies preserved in close vertical and lateral
comprehensive depositional model for erosionally confined, juxtaposition (Kleverlaan, 1989). Three types of contrasting
deepwater sinuous channels is proposed here, aiming to deepwater architectural styles were defined by Kleverlaan
better explain and predict reservoir distribution and (1989) in the basin: (1) sand-rich, amalgamated Channel
performance in reservoirs dominated by LAPs. Complexes, (2) sand-rich sheet complexes, and (3) a
Outcrop analogs of interpreted sinuous, erosionally confined, mixed conglomerate, sandstone and shale filled
confined deepwater channels were studied in the Ross Channel Complex (Solitary Channel, Fig. 14). In the
Formation (Ireland), Jackfork Group (Arkansas, USA) and Tabernas Basin, the focus of the work was on the exhumed
Tabernas Basin (Spain). Several authors have already Solitary Channel, which is youngest of the Slope Channel
studied these outcrops, discussing the sinuosity of the Systems. It is at least 200 m wide and 40 m thick, and has a
channels and related accretion deposits (Tabernas Basin: heterolithic fill. It can be mapped almost continuously for
Cronin, 1995; Haughton, 2000; Kleverlaan, 1989; Picker- 7 km in the field. The channel architecture and lithofacies are
ing, Hodgson, Platzman, Clark, & Stephens, 2001; Ross very similar to many of the slope channels in West Africa.
640 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

Fig. 14. Southern end of the outcrop belt of the Solitary Channel. The red
line represents the master surface of the Slope Channel System. The dashed
light-blue line marks the top of the Slope Channel System. The dark blue
dashed lines mark major changes in vertical stacking patterns (top of
Channel Complex Sets).

3.2. Forward seismic modeling (solitary channel)

Due to the non-uniqueness inherent in seismic facies

analysis, translation of seismic facies into depositional
environments requires careful selection of an appropriate
analog, be it either subsurface or outcrop based (Sullivan
et al., 2000). Forward seismic models have been
constructed for the Solitary Channel outcrop (Fig. 14)
in order to calibrate this outcrop analog to the Dalia
subsurface data as this outcrop is interpreted to be the
most similar in lithology and amalgamation. The forward
seismic models were built using velocities and densities
similar to those at the Dalia field and were generated
using vertical incidence ray tracing and a zero-phase Fig. 16. (a) –(c) 15, 35, and 65 Hz models for the Solitary Channel, Spain.
Ricker wavelet.
The geologic model used for the forward seismic the high-resolution 3D survey of Block 17 covering the
modeling is from Haughton (2000), based on a schematic Girassol and Dalia fields in offshore Angola. The seismic
representation of the channel-fill architecture of the Solitary model of 35 Hz is similar in frequency to good quality 3D
Channel in the Tabernas Basin (Fig. 15). Three models seismic surveys.
varying in seismic frequency (15, 35, and 65 Hz) were In the high-resolution forward seismic model (65 Hz,
generated from the same geologic model (Fig. 16). The Fig. 16(c)), the shingled geometry of the LAPs is observed if
lower frequency models are similar in resolution to 2D the packages are thicker than 10 m. In the Green Channel
seismic profiles commonly used in regional exploration. Complex, well-imaged LAPs are on average 25 –40 m
The high-resolution 65 Hz model is similar in frequency to thick. The largest LAPs in the studied outcrops are
approximately 10 m thick in the Solitary Channel. The
seismic resolution on 35 Hz frequency data is approxi-
mately 14 m. This indicates that packages similar to the
ones in the Tabernas Basin would only be resolved on high-
frequency seismic data. Moreover, the well-developed
LAPs in the Green Channel Complex were not detected
on the 35 Hz seismic data, although they are up to three
times thicker than the largest outcrop examples. Therefore,
seismic resolution of most 3D volumes (approximately
35 Hz) is typically not sufficient to clearly resolve LAPs
based on seismic geometries alone.

4. Implication for reservoir connectivity

Fig. 15. Stratigraphic model (modified from Haughton, 2000) and seismic
forward model of the Solitary Channel, Spain (Fig. 23(a) and (a0 )),
compared with the stratigraphic model and seismic profile from the Dalia The map and cross-section views of the LAPs (Figs. 3
M9 Upper Channel System (Fig. 23(b) and (b0 )). and 4) imply poor connectivity both within and between
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 641

Fig. 17. Measured section at the uppermost (youngest) LAP in the Solitary Channel (Tabernas Basin, Spain) illustrating the lithofacies that dominate this
package. Red square in the upper photo and the red line in the lower photo show the location of the measured section.

individual accretion sets. However, connectivity may and composition of accretionary packages in outcrop and
improve depending on lithology type, stacking within a seismic examples may indicate an internal variation of
LAP, and sand distribution in the channel-fill. stacking inside the unit. This range in bed stacking in LAPs
could also be due to the differences in magnitude, energy,
4.1. Lithofacies association variability of discharge and sediment load composition of the
turbidity currents, indicating that similar geometries could be
LAPs associated to interpreted sinuous channels in associated with a range of lithofacies.
outcrops range in lithology from (1) traction-dominated
conglomerates and coarse-sandstones (R and S beds, sensu
Lowe, 1979), to (2) mixed traction –suspension pebbly 4.2. Bedset geometry and stacking
sandstones (R and/or S beds), shale rip-up clast conglom-
erates and high-concentration sandy turbidites, and to (3) The degree of amalgamation of laterally migrating
suspension-dominated high- and low-concentration sandy sinuous channels and associated LAPs is also quite variable.
and muddy turbidites. It is controlled by the spatial distribution of massive
Traction-dominated LAPs are well exposed in the Solitary sandstones (Ta/S3 beds) to muddy turbidites or mud-clast
Channel (Tabernas Basin, Spain), composed mostly of R and conglomerates and ranges from amalgamated to semi-
S sands (conglomerates and coarse-sandstones). LAPs amalgamated and to non-amalgamated LAPs (Fig. 18).
outcropping in the Ross Formation are composed of mud-
clast conglomerates interbedded with massive sandstone
beds (Clarke, 1998) and are a good example of mixed
traction – suspension deposits. The late stage channel
exposed at the top of the Solitary Channel (Tabernas
Basin) is composed of coarse-grained, high-concentration
turbidites at the base and low-concentration turbidites
interbedded with thin-bedded sandstones towards the top,
representing a suspension-dominated LAP, reflecting an
upward degradation in reservoir quality (Fig. 17). In fact, the
LAP at the late stage of the Solitary Channel is interpreted to
be very similar in composition and vertical organization to
the LAPs present at Girassol and Dalia fields.
Shingled sandstones at the channel margins observed in
outcrops typically thin and fine updip and downdip, forming
a downlap surface at the base of the package. Other examples
show an amalgamated bed at the base of the package.
Moreover, outcrop examples often show complex bed
relationships with common downlapping and onlapping Fig. 18. Degrees of amalgamation in Channel Complexes formed by
beds within a single package. The apparent range in geometry migrating sinuous confined channels and lithofacies association.
642 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

Reservoir quality in LAPs tends to degrade from amalga-

mated to non-amalgamated deposits due to a decrease in the
sand content and connectivity within individual sand beds
(Fig. 18).

4.2.1. Amalgamated LAPs

Amalgamated LAPs are relatively uncommon in the
studied outcrops. The best example of an amalgamated LAP
was observed in the Ross Formation (Rinevilla Point) and it
is composed of amalgamated (approximately 1 m), thick Ta
beds accreting at the margin of a sand-rich channel that is Fig. 20. Northern margin of the Solitary Channel showing Channel
about 10 m thick (Fig. 19). This interpreted sinuous channel Complex Sets 1 – 3. Dashed blue lines indicate Channel Complex
boundaries and dashed red lines indicate Channel Complex Set boundaries.
(Elliot, 2000; Martinsen et al., 2000; Sullivan et al., 2000) is
Channel Complex Set 2 is composed of three Channel Complexes. The
part of a laterally and vertically amalgamated Channel youngest Channel Complex (light blue) shows inclined sandbodies dipping
Complex with extremely high net/gross (ratio of sandstone perpendicular to the paleoflow direction.
to shale). Single channels stratigraphically below this LAP,
which are also part of the same Channel Complex Set, composed of traction beds (conglomerates and coarse-
present compensational stacking with gradual channel-axis grained sandstones) and high-concentration turbidites at
to -margin transition. In these channels, coarsening upwards the base, and high- to low-concentration turbidites
packages at channel margins correlate laterally to thick and towards the top. The final channel is mud-filled. There
amalgamated Ta beds (massive sandstones) in the channel is a decrease in the amalgamation of the beds towards
axis. These channels probably present lower sinuosity than the top of the package. A lithology change in a single
the younger (stratigraphically higher) channel with accre- bed is observed at the base of the LAP, passing from
tion beds at the channel margin. traction (conglomerates and coarse-grained sandstones) to
suspension (fine- to medium-grained sandstones).
4.2.2. Semi-amalgamated LAPs A different style of semi-amalgamated LAP outcrops in
Semi-amalgamated LAPs are a commonly observed the Jackfork Group (Arkansas) is observed at Baumgardner
in Channel Complexes associated with high-sinuosity Quarry (Fig. 21). This LAP is suspension-dominated and is
channels. In general, traction-dominated deposits are associated with a small-scale sinuous channel approxi-
common in amalgamated to semi-amalgamated LAPs, and mately 0.5 m thick, composed of semi-amalgamated
are often interbedded with high-concentration turbidites. massive sandstones interbedded with low-concentration
Semi-amalgamated LAPs frequently show a vertical trend turbidites. The massive sandstones thicken downdip form-
of amalgamated, thick bedded, coarse-grained traction ing an amalgamated layer at the base of the channel. The
deposits and/or high-concentration turbidites at the base to massive sandstones change in lithofacies updip to planar
non-amalgamated, thin-bedded, high- to low-concentration (Tb)- and ripple-laminated sandstones (Tc).
turbidites towards the top of the package.
A thick, semi-amalgamated, mixed traction –suspen- 4.2.3. Non-amalgamated LAPs
sion LAP occurs in the Channel Complex Set 2 (Figs. 14 Two different types of non-amalgamated LAPs are
and 20) of the Solitary Channel (Tabernas Basin, Spain). defined in outcrops based on lithofacies association:
The accretion beds dip to the west – northwest, and the mixed traction – suspension and suspension-dominated.
accretion package is at least 10 m thick. This LAP is

Fig. 19. Amalgamated, suspension-dominated LAP at the Rinevella Point,

Ross Formation (Ireland). Dashed red lines indicate the position of inclined Fig. 21. Semi-amalgamated, suspension-dominated LAP in the Jackfork
surfaces in the massive sandstones (Ta beds). Group (Baumgardmner Quarry, Arkansas).
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 643

Fig. 22. Lateral accretion at the margin of a mud-filled channel in the upper portion of the Ross Formation (Rehy Cliffs).

Non-amalgamated, mixed traction – suspension LAPs are deep. The LAPs probably represent about two-thirds of the
characterized by interbedded high-concentration turbidites Channel Complex width. Individual sand bodies have a
deposited by suspension (massive sandstones) and mud (rip- flattened lens shape that tapers both in downdip and updip
up)-clast conglomerates deposited by traction as bed load. directions. Vertical and lateral continuity in these LAPs
Examples of mixed traction – suspension, non-amalgamated would be low due to weak internal amalgamation of sandy
LAPs are present in the Ross Formation and in the Jackfork Ta beds (Figs. 22 and 23).
Group (Figs. 22 and 23). These outcrop examples of LAPs One important difference between the examples from
are very similar in internal geometry, composition, and the Ross Formation and the Jackfork Group is related to
dimensions. These LAPs are composed of non-amalga- the channel-fill. Fig. 23 shows a LAP from the Big Rock
mated, shingled, inclined, lensoid shaped sandbodies Quarry (Jackfork Group) associated with a late stage sand-
interbedded with shale rip-up clast conglomerates. Net to rich channel and Fig. 22 shows a LAP from the Rehy
gross in the packages is between 45 and 65%. These Cliff outcrop (Ross Formation) associated with a late
inclined bedforms dip up to 168 and are oriented stage mud-filled channel. The fill of the channel in the Big
perpendicular to paleoflow. They are interpreted to be the Rock Quarry is composed of thick and amalgamated
accretionary deposits (LAP) of a meandering deepwater massive sandstone onlapping against the LAP. Therefore,
channel system. The Channel Complexes comprising the turbiditic flows associated with LAPs are not related to
LAPs are approximately 200 –300 m wide and up to 10 m those that fill channels with sediment. Channel-plugging

Fig. 23. Semi-amalgamated, mixed traction–suspension LAP in the Big Rock Quarry (Jackfork Group, Arkansas).
644 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

deposits may be mud or sand-rich, depending on the 4.3. Reservoir connectivity between LAPs
character of the flows that ultimately fill the channel. In
fact, LAPs probably are related to periods of sediment As discussed in the previous chapter, connectivity is an
bypass, also indicated by the presence of erosion at the important reservoir issue in LAPs. Map patterns derived
top of sandstone beds inside the LAPs. from 3D seismic data imply poor connectivity between
Suspension-dominated, non-amalgamated LAPs are LAPs deposited in sinuous channels. In fact, connectivity
characterized by interbedded high-concentration sandy between LAPs will be greatly influenced by the nature of the
and low-concentration muddy turbidites, with a sharp fill of the last phase of the channel. In the case of mud-filled
decrease of bed thickness and sand content towards the channels, the connectivity between LAPs can be enhanced if
top of individual packages. In the Tabernas Basin, the the channel-fill is characterized by sandy barforms at its
youngest Channel Complex Set (Channel Complex Set 4, base.
Fig. 14) in the Solitary Channel presents a possible A low net to gross channel-fill generally characterizes the
example of non-amalgamated, suspension-dominated LAP last phase of the channel in the Green Channel Complex.
(Figs. 17 and 24). This package is also associated with a However, amplitude extractions at the base of the last phase
mud-filled channel. The LAP has a partially preserved, of the channel-fill indicate amplitude anomalies located at
maximum thickness of 10 m and is composed of high- specific positions in the channel. Comparison between the
concentration turbidites at the base and interbedded high- isochron map of the last phase channel and the amplitude
and low-concentration turbidites at the top of the package. map of the trough at the base of the channel indicates that
Individual Ta beds have a flattened lens shape that tapers most of the high amplitude anomalies are located at the
both in the downdip and updip directions. Fining, deepest portions of the channel (Fig. 25). This suggests that
thinning, and downlapping of the sandy turbidite beds coarser-grained sediments carried by turbidity flows were
define the base of the LAP, with no evidence for bed deposited in local scours at the base of the channel,
amalgamation at the base. Similar to other examples of developing sandy barforms. The amplitude anomaly showed
LAPs, the Ta beds show evidence of erosion at their tops, in Fig. 25 is located at the axis of the channel, updip from
indicating that the interbedded low-concentration turbi- Bend 14, in a wider portion of the channel.
dites are more likely related to tails of by-pass flows,
rather than a waning phase or abandonment of the
channel. Because the outcrop shows only partial preser- 5. Depositional model for LAPs
vation of the inclined beds in part of a channel-fill,
Channel Complex Set 4 within the Solitary Channel could The inferred depositional model of these shingled
be interpreted in alternative ways. The inclined beds seismic reflections is of deposition (accretion) in the inner
could, for instance represent tectonically rotated, onlap- side of the bend and erosion at the outer side of the bend
ping channel-fill. during lateral and downdip migration of the channel. This is

Fig. 24. LAP in the Channel Complex Set 4, located at the southwestern end of the outcrop belt (left side of photo 1). The LAP is approximately 10 m thick and
is composed by high-concentration turbidites and traction beds (S sands) at the base and low-concentration turbidites towards the top.
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 645

Fig. 25. (a) shows an amplitude map at the base of the Green Channel complex and isochron map for the last phase channel (red contours). Distinct amplitude
anomalies interpreted as barforms can be observed at the base channel (b), generally coinciding with the thick portions of the channel-fill. Black contour in (c)
indicates the outline of two amplitude anomalies located updip in the Bend 12. These amplitude anomalies are interpreted as barforms deposited in local scours
at the base of the channel.

similar to what is observed for point bars in sinuous fluvial slightly. Secondary flow circulation would further act to
channels. Peakall et al. (2000) proposed that the evolution of transport sediment from the outer bank to the inner bank,
deepwater and fluvial channels is significantly different. similar to what is observed in fluvial meander bends (Kassen
They suggested that common features in fluvial channels & Imran, 2003).
like downstream migration, point bars, and cut-off meanders There is an apparent discrepancy between the nature of
are rare or absent in deepwater channels, based on turbiditic flows (relatively short-lived and not necessarily
observations from aggradational deepwater channel-levee
systems (Mississippi Fan, Kastens & Shor, 1985; Amazon
Fan, Pirmez & Flood, 1995). In addition, the continuous
changes in fluvial channel geometries suggest that they do
not attain equilibrium geometry as observed for aggrada-
tional deepwater sinuous systems (Peakall et al., 2000). The
sinuous and erosionally confined Green Channel Complex,
however, displays strong lateral and downdip migration of
channels, accretion deposits at the inner portion of every
channel bend (LAPs) and frequent cut-off meanders (about
20% of the meanders in the studied area). The apparent
similarity of these deepwater deposits (LAPs) to fluvial
point-bar deposits suggests that the controls on their
formation may be similar. Therefore, the depositional
model proposed for these deepwater LAPs is that the
accretion surfaces would be formed by relatively continuous
and gradual lateral sweep of channel bends by systematic
erosion of the outer banks and deposition along inner banks
(Fig. 26). This is similar to the classic fluvial point-bar
model (Galloway & Hobday, 1983). Suspension deposition
of sandstones and mudstones may be preserved in a
primarily bypass dominated phase of the channel evolution
due to sedimentation in low-velocity, separation zones.
Deposition in recirculation zones (eddies) adjusts channel
morphology by smoothing channel irregularities. Further-
more, erosive currents may cause collapse of outer bank of Fig. 26. (a) Fluvial point-bar model (modified from Galloway & Hobday,
bend, producing a local widening of the channel. Collapsed 1983). (b) The cross-section view of a LAP in a deepwater, sinuous,
bank material serves as ‘fuel’ for subsequent currents. erosionally confined channel resembles the geometry of a fluvial point-bar.
(c) Depositional model proposed for the LAPs. The accretion surfaces
Selective deposition on inner bank of bend would be a would be formed by relatively continuous lateral sweep of channel bends by
consequence of local spatial deceleration of currents systematic erosion of the outer banks and deposition along inner banks (the
associated with flow expansion where the channel widens classic point-bar model).
646 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

cyclic) and the steady and continuous flows that seem to be † Handles multiple grain sizes, and
necessary to generate accretion units in sinuous channels. † Can be run with arbitrary channel shapes.
Elliot (2000), in describing accretion units related to
deepwater channels in the Ross Formation (Ireland), The numerical model of Das (2002) includes erosion and
implied that during periods of lowered sea level the deposition by traction and suspension. It uses the velocity
sedimentation rate was so high that sediment could have field computation to determine the locus of maximum
been supplied to the deepwater fan system by quasi- centerline velocity along the channel. The difference
continuous hyperpycnal currents from the delta mouth. between the geometric centerline and the locus of maximum
According to Elliot (2000), the lateral accretion deposits velocity represents the excess velocity or velocity anomaly.
seen in many of the submarine-channel outcrops in the Ross The rate of migration is assumed to be proportional to the
Formation could be better explained by this type of steady excess velocity and the migration direction is assumed to be
sediment supply process rather than by reconciling these normal to the locus of maximum velocity. This approach has
observations to ignitive event processes. Although hyper- been used successfully to predict lateral migration of fluvial
pycnal flows have characteristics that apparently could and deepwater systems (Das, 2002; Imran, Parker, &
better explain the occurrence of accretion deposits in Pirmez, 1999). The model calculates the velocity field and
deepwater sinuous channels, the relative importance of channel migration rate for specified channel geometry and
hyperpycnal flows in deepwater systems is not well steady flow conditions. The amount of lateral migration
understood. On the other hand, paleohydraulic reconstruc- depends on a pre-specified time step and bank erodibility
tion of turbidity currents in Amazon Channel suggest that factor. Map patterns of sinuous channels provide a good
individual flows, igniting at the canyon head, may have constraint on flow velocity because meander migration
lasted for days within the sinuous channel, perhaps geometry is sensitive to the locus of maximum flow
explaining the continuous nature needed to generate and velocity. Stream-wise velocity excess determines the sites
maintain such sinuous channels (Pirmez and Imran, this of maximum bank erosion and the channel-migration
volume). direction.
There is a good match between the interpreted channel
evolution and the numerical modeling of the Green Channel
5.1. Numerical model Complex (Fig. 27). Channel path and LAP interpretation
versus simulated depositional pattern in stages 5 and 4 of the
A numerical model (Das, 2002) was used to test the Green Channel Complex evolution are very similar.
evolution model proposed for the Green Channel Complex. Location of associated deposits are predicted just down-
This numerical model includes the following capabilities: stream of active LAPs in most bends as determined in Stage
5 to 4 match. A similar good match was obtained for
† Erosion and deposition of suspended sediment and evolution of Stage 4 to 3. This analysis predicts that flow
bedload transport on the channel bed, velocity increased over time from Stage 5 to 4.

Fig. 27. Numerical modeling of the stages 5 and 4 of the Green Channel Complex evolution, showing an increase in sinuosity during channel evolution. Red
colors indicate preferential sites for deposition and blue colors indicate preferential sites for erosion. Stream wise velocity excess determines the sites of
maximum bank erosion and the channel-migration direction.
 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648 647

6. Occurrence of LAPs being described ever more commonly in subsurface and

outcrop datasets. The high-resolution 3D seismic data from
Despite all the variations in internal stacking and Block 17, offshore Angola and elsewhere, however, shows
lithofacies association in LAPs, there seems to be a that these channels and associated LAPs are a common
predictable stratigraphic pattern associated to the occur- feature in the late phase of evolution of Slope Channel
rence of sinuous, erosionally confined channels in Confined Systems. Extensive volumes of hydrocarbons are being
Channel Systems. The Solitary Channel (Tabernas Basin) is assessed in Channel Complexes dominated by LAPs. Thus,
interpreted in this work as a Confined Channel System and it LAPs are an important reservoir element in Confined
can be further subdivided into four Channel Complex Sets Channel Systems that needs to be properly evaluated and
(Fig. 14). Each Channel Complex Set shows a trend of included in reservoir analyses of deepwater channels. In the
decreasing amalgamation and aspect ratio of individual Green Channel Complex (Dalia field), more than one-third
channels upward, which is similar to trends observed in of the Channel Complex area is occupied by LAPs, and
slope Channel Complex Sets in West Africa. In terms of probably most of the reservoir sands in this interval are also
lithofacies distribution, the coarsest grain sizes are observed associated with the lateral accretion units. Therefore, LAPs
at the base of the Channel Complex Sets (primarily are a significant reservoir element in sinuous channels that
conglomerates), with a general trend of decreasing grain had not been described in detail until now.
size towards the top. Typically, lateral migration of individual sinuous,
In the Big Rock Quarry (Jackfork Group), the main confined channels has produced laterally amalgamated
outcrop face is more than 250 m long and about 60 m high, Channel Complexes that have varying degrees of internal
representing a Channel Complex Set showing a decrease in amalgamation depending on the nature of the channel-fill.
lateral and vertical amalgamation of single channels Amplitude extractions in sinuous, erosionally confined
towards the top of the outcrop. There is also a trend of channels can be used to determine the position and
increasing sinuosity of the channels towards the top of the dimensions of each package. In addition, dimensions of
Channel Complex Set, as evidenced by the increasing LAPs are strongly related to the dimensions of the last stage
presence of LAPs within channel-fills. In fact, interpreted channel. Therefore, if the channel width is known, the
sinuous, erosionally confined channels are present in several length and width of the LAPs can be predicted. Further-
localities in the Jackfork Group (Arkansas), often strati- more, the accretion packages tend to be thinner than the
graphically located at the tops of Channel Complex Sets. depth of the channel, due to the continuous erosion at the
Moreover, the vertical evolution of Channel Complex Sets base of the channel while the LAPs are being deposited.
in the Jackfork Group and in the Ross Formation (Ireland) is Map patterns in the Green Channel Complex suggest
very similar. In both cases, there seems to be an increase in low-connectivity between individual LAPs, which can be
the occurrence of channels dominated by shingled, inclined enhanced if the late stage channel is sand-filled. Although
sandbodies interpreted to reflect high channel sinuosity there are several examples where LAPs are related to mud-
towards the top of individual Channel Complex Sets, with filled channels, this may not always be the case. In fact,
stratigraphically older channels dominated by simple, other confined channel systems in Block 17 (offshore
aggradational channel-fills. Angola) show a sand-rich channel associated with a well-
The studied seismic example of confined channel system, developed LAP. In outcrops, LAPs are also associated with
the Dalia M9 Upper field in offshore Angola can be divided sand-rich (Big Rock Quarry, Arkansas) or mud-rich (Rehy
in three Channel Complex Sets. Each Channel Complex Set Cliffs, Ireland) late stage channel-fills. The occurrence of
displays a vertical evolution from confined, laterally sandbars at the base of the channel can also enhance
amalgamated Channel Complexes with high-concentration connectivity between accretion units (Fig. 25). Cross-
turbidites at the base, to more highly sinuous, laterally section geometries also suggest better connectivity at the
migrating, lower aspect ratio channels dominated by high- base, deteriorating towards the top of individual LAPs.
and low-concentration turbidites at the top. Internal erosional surfaces within individual LAPs can also
Therefore, LAPs tend to occur at the top of Channel improve connectivity.
Complex Sets in Confined Channel Systems, marking a
decrease in aspect ratio and increase in sinuosity and
thickness of single channels through the evolution of a Acknowledgements
Channel Complex Set. This may be due to changes in
gradient and sediment supply as the system evolves. The authors thank ExxonMobil Exploration Company,
TotalFinaElf, BP, Statoil, Norsk Hydro and Sonangol for
permission to publish seismic data from Block 17, offshore
7. Conclusions Angola. This work greatly benefited from the review offered
by Mark Deptuck, to whom we are very grateful. We would
Sinuous, erosionally confined channels have been an also like to thank Bret Dixon, Christine Rossen, Kirt
under recognized deepwater channel type in the past, but are Campion, Ciaran O’Byrne and Ven Kolla for reviewing
648 V. Abreu et al. / Marine and Petroleum Geology 20 (2003) 631–648

the manuscript. Thanks to Vincent Cornaglia for providing Kastens, K. A., & Shor, A. N. (1985). Depositional processes of a
the stratigraphic framework of the Dalia M9 Upper Channel meandering channel on Mississippi Fan. AAPG Bulletin, 69,
190 –202.
System. This work also benefited from technical discussions
Kleverlaan, K. (1989). Three distinctive feeder-lobe systems within one
with the siliciclastic reservoir group of the ExxonMobil time-slice of the Tortonian Tabernas fan, SE Spain. Sedimentology,
Upstream Research Company and with the Block 17 team of 36(1), 25 –45.
the ExxonMobil Development Company. Finally, thanks to Kolla, V., Bourges, Ph., Urruty, J. M., & Safa, P. (2001). Evolution of deep-
the ExxonMobil Upstream Research Company for per- water tertiary sinuous channels offshore Angola (West Africa) and
mission to publish this work and for supporting this research. implications for reservoir architecture. AAPG Bulletin, 85, 1371– 1405.
Lowe, D. R. (1979). Sediment gravity flows: Their classification and
problems of application to natural flows and deposits. SEPM Special
Publication 27, pp. 75 –82.
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