1. 1
Structural controls on turbidity current evolution and factors affecting1
reservoir quality2
BEN J. THOMAS1
*3
TIAGO ALVES
2
4
1
Department of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place,5
Cardiff.6
Abstract7
Many factors contribute to the quality of reservoirs found in deep water facies. The main8
control on being able to predict this is being able to interpret seismic data accurately;9
meaning that advancement in technology will increase our ability to predict deep water10
reservoir quality and continuity. Structural controls on turbidity flows include sinuosity of the11
channel, which is controlled by seafloor topography, slope angle and sedimentation rate.12
Stacking patterns within the channel is one of the most influential factors affecting reservoir13
quality; if the stacking results in massive sandstone units, reservoir potential will be high,14
however if sandstones are interbedded with muds or shale, reservoir potential is significantly15
lowered.16
Keywords: Reservoir, turbidity current, submarine channel, slope classification, sinuous17

2. 2
1. INTRODUCTION18
This paper combines the most important aspects of previous research carried out into19
structural and morphological controls on factors affecting the potential of high quality20
reservoir rocks being deposited and preserved in submarine channels and on submarine21
slopes, with the aim of identifying the most influential factors in order to ascertain whether it22
is currently possible to predict reservoir quality and quantity in deep water facies.23
Submarine canyons are becoming increasingly important and are the focus of current24
research, in order to grasp a better understanding of their origin, development and as this25
paper will concentrate on; their potential to create high quality hydrocarbon reservoir rocks.26
In order to analyse a channels reservoir potential, its structural properties and architectural27
development must be identified.28
In the last 10 years, the petroleum industry has become increasingly interested in29
submarine channels, as exploration of deep water facies intensifies (Mayall, 2006). Advances30
in technology have allowed for high resolution seismic surveys to be carried out across31
channels around the world. Previous work on submarine channels (e.g. West Africa and the32
Gulf of Mexico) has resulted in a comprehensive and detailed knowledge on channel33
morphology, classification and depositional processes within the channels.34
1.1 Origin of submarine canyons35
Shepard (1936) concluded that canyons off New England were not developed as a result36
of a single excavating event, because of the height of the canyon walls; instead they were37
subject to multiple emersion, erosion and filling events (Fig 1; Baztan et al., 2005; Shepard,38
1936). In the same year, Daly (1936) was first to put forward the theory that submarine39

3. 3
canyons were eroded by turbidity currents; an opinion that is now widely accepted (Baztan et40
al., 2005).41
Fig 1: Profile of a canyon example illustrating extremely high walls, suggesting that it42
was not formed by a single erosional event (Source: Shepard 1972)43
1.2 Oil resources in deep water44
It is currently estimated that there are between 1200 and 1300 hydrocarbon plays in45
deep water turbidite and slope systems around the world (Fig 2; Stow & Mayall, 2000). Many46
of the known fields are found in the North Sea, California, West Africa and the Gulf of47
Mexico. Fields first discovered were in convergent margins, but as technology and48
knowledge has advanced, plays have been located in divergent and more recently passive49
margins (Stow & Mayall, 2000).50
Fig 2: Map showing (highlighted in black) deep water hydrocarbon exploration across the51
globe (Source: Stow & Mayall, 2000).52
1.2.1 Massive sand deposits53
Turbiditity channels and submarine canyons are known to be very common features54
of continental slopes globally. However, as will be discussed in this paper, the term55
‘turbidite’ covers a vast range of depositional facies; resulting in the deposition of small56
(1cm) thick silt beds, to 100m thick boulder conglomerates. The evolution and structural57
components of every turbidity current is unique.58
Massive sandstone deposits are the key to high quality reservoir rocks in deep water59
facies. The deposition of these massive beds has been a subject of disagreement over the last60
several decades, which lead to Stow & Johansson (2000) classifying them in four main61
geometries: (1) chutes/flows, (2) channel ribbons, (3) lobes/lenses and (4) basin fill sheets62

4. 4
(Stow & Mayall, 2000), this classification is still broadly used today. The major controls on63
their formation include tectonic activity and clean sand supply (Johansson & Stow, 1998;64
Stow & Mayall, 2000).65
1.3 Submarine canyon evolution66
A study carried out by Baztan et al. (2005) into canyon evolution in the western Gulf of Lion67
resulted in three types of present day geometries being present in the canyon’s major valley:68
(1) infilled canyons, (2) incised canyons and (3) empty canyons.69
1.3.1 Infilled Canyons70
Infilled canyons comprise of total or partial sediment fill, with no axial incision;71
showing that these are channels that are currently in the process of being filled and have not,72
as of yet, been subject to erosion (Fig 3; Baztan et al. 2005).73
Fig 3: Seismic line in Herault Canyon head in the western Gulf of Lion, showing an infilled74
canyon (Source: Baztan et al., 2005)75
1.3.2 Incised Canyons76
This type of canyon shows that deposition of sediment is being exceeded by the rate77
of erosion from bypassing turbidity currents (Fig 4; Baztan et al. 2005).78
Fig 4: Seismic line in Herault Canyon head in the western Gulf of Lion, showing an incised79
canyon (Source: Baztan et al., 2005)80
1.3.3 Empty Canyons81
Empty canyons represent channels that have been subject to erosion, but have not82
since been filled by subsequent deposition (Baztan et al. 2005).83

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It is important to recognise these geometries in present day channels, as it inevitable84
that such fossil canyons will be recognisable in seismic lines when carrying out a survey to85
predict the quality of reservoir present of a given area.86
2. METHOD OF COLLECTING DATA
87
The data collected for this paper include research papers from this century (2000’s), in order88
to analyse and compare the most up to date data and knowledge. Works and theories89
formulated in the 1900’s have since been updated and modified in these more recent90
publications, as advances in technology and knowledge have allowed a more detailed and91
thorough analysis of seismic surveys and their interpretations. Papers referenced in this report92
have been carefully selected, in order to get the most relevant and accurate data in the93
selected field: Structural controls on turbidity current evolution and factors affecting reservoir94
quality. Data in the scientific papers used have been compiled by the collection of first hand95
data by the author(s), in conjunction with knowledge of previous studies, in order to come to96
an accurate and well researched conclusion.
97
3. ARCITECTURE OF SUBMARINE CHANNELS – A CASE STUDY FROM98
MAYALL ET AL., (2006).99
Much of the knowledge gained about the architecture of submarine channels and the100
importance of their structural elements in their ability to deposit high quality reservoirs has101
been made possible by the advance in 3D seismic technology in the last decade, however the102
process of deciphering seismic lines can still be a daunting task (Fig 5; Mayall et al., 2006).103
Fig 5: Seismic line of a large, erosional channel, with its subsequent interpretation of104
erosional surfaces (Source: Mayall et al., 2006).105

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Mayall et al. (2006) also state, every channel is unique and it is therefore difficult to produce106
an accurate general model for their formation and morphology, but there are several features107
that should be analysed in any turbidity channel; these are: (1) The nature of the sinuosity of108
the channel; (2) the facies of the channel; (3) Repeated cutting and filling sequences and (4)109
the stacking pattern within the channel (Mayall et al., 2006).110
3.1 Sinuosity111
Every modern day submarine turbidity flow seen on the seafloor today has varying112
levels of sinuosity (Fig 6). The reason for this varying sinuosity has been identified by Mayall113
et al. (2006) as: the architecture of the original erosional base; sea-floor topography; lateral114
stacking and lateral accretion. These factors have strong implications on the reservoir quality115
and distribution (Mayall et al., 2006).116
Fig 6: Plan view showing sinuosity of a large, 3rd
order erosional channel (Source: Mayall et117
al., 2006).118
3.1.1 Original erosional base119
The vast majority of channels studied have a sinuous erosional base (Fig 7). The120
reason for this is not fully understood, as in many cases there is no obvious link with121
underlying lithologies as to why the channel changes course.122
Fig 7: Sinuosity of an erosional base (arrowed), seen in plan view and seismic cross section123
(Source: Mayall et al., 2006).124
Mayall et al. (2006) concluded that it may be due to many flows by-passing each other as125
they travel down the slope; i.e. the first flow may come to a halt, which causes the following126
flow to have to change course slightly, as it reaches the build-up of material in order to pass127
and in doing so, reactivates a proportion of the previous turbidity material. It is also proposed128

7. 7
that this sinuosity results in the deposition of sands at the channel bends, due to the lowered129
velocity of the turbidity current. This however is difficult to identify on seismic cross sections130
(Mayall et al., 2006).131
3.1.2 Seafloor topography132
Seafloor topography inevitably has an effect on the sinuosity of turbidity currents and133
the submarine channels originating from them. As is true in areal mass movements, the134
materials will find the quickest route down the continental slope. This route is often the135
steepest. However, past studies have identified anomalies in the direction of material flow;136
namely the presence of faults on the seafloor. It has been observed that in the presences of137
faults associated with salt diapirism, turbidity currents will divert around the seafloor138
maximum high and will flow down-slope at the lateral tip-out of the topography, resulting in139
major diversions and increased channel sinuosity (Mayall et al., 2006).140
3.1.3 Lateral stacking141
Sinuosity is also affected by the lateral stacking of subsequent channels. This is due to142
filling of channels and later incisions being slightly off-set from the original erosional event143
(Fig 8; Mayall et al., 2006).144
Fig 8: Plan view and seismic cross section showing lateral stacking creating a sinuous145
channel (Source: Mayall et al., 2006)146
3.1.4 Lateral accretion147
Lateral accretion, unlike lateral stacking, where several erosional and filling events148
are incorporated, lateral accretion refers to lateral movements within a single, open channel149
(Fig 9). This process is not yet fully understood, but Mayall et al. (2006) suggest that this150

8. 8
process is due to the depositional facies, rather than a combination of factors, including151
seafloor topography.152
Fig 9: Plan view and seismic cross section, showing sinuosity created by lateral accretion153
(Source: Mayall et al., 2006).154
3.2 Facies of deposition155
Previous studies have resulted in a classification of four main turbidity channel types,156
based on the type of sediment transported (ranging from fine mud to boulders), and a range of157
gravity driven depositional processes (Fig 10).158
Fig 10: Simple model of facies in a channel fill (Mayall & Stewart, 2000; Mayall & O’Byrne,159
2002) and an interpreted seismic cross section of said fill (Source: Mayall et al., 2006).160
These divisions are reached as they are important indicators for identifying reservoir161
potential; they are often visible in seismic lines, even if they are of poor quality; they can162
identify risks in facies predictions and they often occur in vertical sequences on seismic lines163
(Mayall et al., 2006; Mayall & Stewart, 2000).164
• Slumps & debris flows,165
• Basal lags,166
• High N:G stacked channels,167
• Low N:G channel-levee.168
3.2.1 Slumps & debris flows169
Eschard et al. (2003) identified that slumps and debris flows mainly occur during the170
early stages of channel sequences; however they have been identified in the middle of the171
stratigraphy, this early predominance of slumps has been concurred by Mayall et al. (2006).172

9. 9
Slumps are seen in seismic cross section as a ‘mess’ of weak discordant reflectors. The latter173
have stated that these are sometimes difficult to observe from massive sands. Due to the174
muddy matrix and heavily compacted fine grains of slumps and debris flows, they do not175
produce effective reservoir rocks, but instead can form impermeable seals, which are just as176
important in the production of hydrocarbon plays as the reservoir itself (Mayall et al., 2006).177
3.2.2 Basal Lags178
Coarse sands and conglomerates are the most abundant form of basal lags, which are179
located at the base of the erosional channel, deposited as turbidity currents pass, carrying the180
majority of the sediment down slope. Basal lags range from 50cm to 5m in thickness. This181
variation in thickness is usually dependant on the topography of the erosional base, with more182
sediments being deposited if the erosional base temporarily levels out. Basal lags are a very183
good seismic indicator of the basement of the channel, particularly if the channel fill has not184
been subject to vast amounts of burial, as the pebbles of the conglomerates produce a strong185
seismic reflector (Mayall et al., 2006).186
The reservoir potential of this layer is dependent on the amount of compaction and the187
nature of sediment; coarse sands and conglomerates that have not been subject to much188
compaction potentially offer high quality reservoir rocks, whereas basal lags consisting of189
‘mudclast conglomerates’, which are mudclasts within a sandy matrix, originating from the190
erosion of the channel base and walls form a layer of low permeability (Mayall et al., 2006).191
Gardner & Borer (2000); Eschard et al., (2003) and Gardner et al., (2003) all identified192
deposits at the base of channels, now termed ‘shale drapes’, which are thought to represent193
deposition of mud and silt from the tail of turbidity flows. These drapes, like the mudclast194
conglomerates produce a layer of low permeability, which lowers the reservoir potential of195
the succession (Mayall et al., 2006). Coarse sand and conglomerate layers are easily196

10. 10
recognisable in seismic interpretation; mudclast conglomerates and shale drapes are not197
however, so it is therefore important to identify the nature of the basal lag in each channel198
analysed, as it has significant consequences on reservoir potential.199
3.2.3 Stacked high net to gross channels200
Mayall et al. (2006) identified this facies as the most important to reservoir potential, as it201
incorporates the stacking of channels, which are typically 1-10m thick and 100-500m wide.202
The separate channels can be identified on seismic surveys, by identifying the basal lags and203
erosional bases. The majority of channel fill consists of massive sand units, of which thick204
amalgamated beds dominate the axis of the channels whilst the margins are dominated by205
shale beds, deposited at times of lower net to gross facies (Fig 11; Campion et al., 2000;206
Gardner et al., 2003; Eschard et al., 2003; Beaubouf, 2004; Mayall et al., 2006).207
Fig 11: Seismic line, showing slumps and debris as part of channel fill (Source: Mayall et al.,208
2006).209
Stacking of smaller channels within a larger erosional confinement results in ‘axis210
dominated’ and ‘margin dominated’ areas that can easily be identified in seismic profiles.211
This variation results in different compaction rates across the channel, which has important212
consequences to reservoir potential; therefore identifying and quantifying the amount of213
compaction is imperative in estimating reservoir potential. This important analysis tool is214
compromised by seismic lines that show no variation in compaction rate, as it is therefore215
near impossible to estimate the extremity of compaction (Mayall et al., 2006).216
Being one of the most important factors controlling reservoir potential in large erosional217
channels, a lot of research has been done in this area. Net to gross values are typically 40-218
70% and are fairly continuous; only broken by shale drapes and shale deposits on the margins219

11. 11
of the channels. Identifying this facies in seismic analysis can be difficult, especially220
distinguishing it from a debris slump. If the net to gross is high there are very few internal221
reflections, making it hard to distinguish between channels. A weak top and base reflector222
may be visible, but this is very similar to the profile seen in debris flows and slumps. It is223
very important to identify these two facies correctly, as high N:G facies represent the224
potential for very high quality reservoir rock, but debris flows and slumps produce very poor225
reservoir potential (Mayall et al., 2006).226
3.2.4 Low net to gross channel levees227
Large erosional channels comprise of a highly sinuous leveed channel, which often228
extend laterally further than the main channel. These features can be seen as thin (>10m229
thick, 50-100m wide) that is represented poorly in the seismic line, or as prominent levee230
systems up to 500m wide and tens of metres thick. They are comprised of sands at the base231
and interbedded layers of sands and muds further up the deposit. It has been recognised by232
previous studies that this facies often occupies an important percentage of the hydrocarbon233
volume in the trap (Mayall et al., 2006). Patchy distribution of the sands and the thin nature234
of deposition can result in this horizon not being continuous. Low N:G erosional channel fills235
contain a basal lag and predominantly slumps and debris flows, with further mudstone236
deposited as the channel is abandoned (Mayall et al., 2006).237
3.3 Repeated cutting and filling238
3rd
order channels are seen to exist for long periods of time, resulting in re-incision239
and erosion of the channel fill, up to 5 times (Fig 12). This re-incision can result in minor or240
major erosion of previous channel fill. Mayall & O’Byrne (2002) identified three major241
implications for reservoir distribution and connectivity:242

12. 12
Fig 12: Seismic line showing repeated cutting and stacking in channel fill (erosional bases in243
yellow (Source: Mayall et al., 2006)).244
1) Extensive erosion can result in the scattering of remnants of previous channel fill245
throughout the channel. This can result in seismic data that is difficult to analyse, in246
terms of identifying each depositional facies; even with bore hole data, correlation can247
be near impossible (Mayall & O’Byrne, 2002; Mayall et al., 2006).248
2) Multiple erosional channels in succession can result in mudclast conglomerates or249
shale drapes forming impermeable barriers within the reservoir; therefore preventing250
the migration of hydrocarbons (Mayall & O’Byrne, 2002; Mayall et al., 2006).251
3) 4th
and 5th
order channel fill can vary greatly; if dominated by mudstones or silts, this252
entire re-incised channel can act as a barrier for hydrocarbon migration. If incised253
again, the scattering of this impermeable fill can compromise the permeability of the254
entire reservoir. This pitfall is exacerbated by the fact that muddy channel fill can be255
very difficult to identify in predominantly sand filled channels, due to the complex256
internal stratigraphy. This can result in greatly lowered reservoir potential than257
anticipated (Mayall & O’Byrne, 2002; Mayall et al., 2006).258
The repeated cutting and filling of channels can be hard to distinguish in outcrops, except259
those showing large exposures; they are however very common occurrences in most seismic260
surveys and are seen as a very important factor that must be addressed early in the evaluation261
of reservoir potential (Mayall et al., 2006).262
3.4 Stacking Patterns263
Stacking patterns vary from vertically stacked to laterally stacked channels. Most264
channels show traits of both, but are dominated by one or the other. The variation in these265
stacking patterns can be seen over short distances along channels, as a result of change in266

13. 13
seafloor topography or the amount of compaction and therefore subsidence within lower267
channel fills (Mayall et al., 2006). Dramatic changes in stacking patterns within a channel is268
very common; it would therefore be naïve and inaccurate to apply a general reservoir model269
for an entire channel based only on a small section of seismic analysis (Fig 13).270
Fig 13: Seismic data showing variation in stacking style in a single channel – seismic lines271
are 1km apart (Source: Mayall et al., 2006)272
Mayall & O’Byrne (2002) recognised that stacking patterns within the channels is273
also critical in identifying the location and orientation of development wells (Mayall et al.,274
2006). The positioning of both producer wells and injector wells need to be emplaced in such275
a manner that they mitigate the risk of producing low hydrocarbon volume, by passing276
through as many barriers previously described (e.g. shale drapes) and therefore connecting as277
many high quality reservoirs, such as massive sand units and conglomerates as possible.278
4. IMPORTANCE OF LATERAL ACCRETION, AFFECTING RESERVOIRS – A279
CASE STUDY FROM ABREU ET AL., (2003)280
Abreu et al., (2003) carried out research into the Green Channel Complex located281
offshore Angola. Most of the reservoir types in offshore West Africa are dominated by low-282
sinuosity channels; however this work carried out, coupled with advances in the quality of283
seismic data, identified the importance of high-sinuosity channels as reservoir elements.284
The Confined Channel Systems are up to 200m thick, 1-5km wide and tens of285
kilometres long. Core data has shown that the sandstone reservoirs consist of predominantly286
turbidites and traction deposits, which are characterised by complex, inter-cutting sand-rich287

14. 14
channel complexes. Towards the top of the individual channels sinuosity increases and288
vertical and lateral amalgamation decreases (Abreu et al., 2003).289
4.1 Dalia field and the Green Channel Complex290
The Dalia M9 Upper Channel System is of medium sinuosity when viewed from291
above, approximately 2km wide and 150m deep (Fig 14). There are three Channel Complex292
Sets (Two or more Channel Complexes, each bounded at its base by a basinward shift in293
facies at its top by a surface of abandonment (Abreu et al., 2003)), within this channel294
system. The Green Channel Complex (Two or more channel fills of similar style (Abreu et295
al., 2003)), occurs near the top of the youngest Channel Complex Set within the channel296
system. Abreu et al., (2003) concluded that the Green Channel complex was formed by the297
migration of a single channel, meaning a channel around 300m wide formed a Channel298
Complex approximately seven times wider than the width of the active channel at any one299
time. Arial view shows that the youngest channel only occupies about a third of the total300
Channel Complex.301
Fig 14: Plan view of the Green Channel Complex, showing its sinuosity (Source: Abreu et al.,302
2003)303
4.1.2 Lateral Accretion Packages304
LAPs form on the inside of bends in a channel, where sediment flow slows305
sufficiently to deposit coarser, sandy material (Fig 15). These usually run parallel to the306
margin of the channel and form vital sandstone nodules, which can host hydrocarbons (Abreu307
et al., 2003).308
Fig 15: Birds-eye view of accretion surfaces in a LAP; showing them parallel to the margin309
of the channel (Source: Abreu et al., 2003).310

15. 15
4.2 Reservoir connectivity311
Cross-section views of the LAPs suggest poor connectivity within and between312
accretion sets; connectivity is dependent on stacking within the LAP, sand distribution in the313
channel-fill and lithology type (Abreu et al., 2003).314
4.2.1 Geometry and Bed Stacking315
The distribution of massive sandstones, mudclast conglomerates and muddy turbidites316
controls the amount of amalgamation seen in laterally migrating channels and LAPs. This317
amalgamation ranges from non-amalgamated, through semi amalgamated, to amalgamated318
LAPs (Fig 16). Reservoir quality of said LAPs decreases from amalgamated to non-319
amalgamated deposits, because of increased muddy sediments and therefore decreased320
connectivity between sand beds (Abreu et al., 2003).321
Fig 16: Model showing different stages of amalgamation, formed by migrating sinuous322
channels (Source: Abreu et al., 2003).323
4.2.1.1 Non-amalgamated LAPs324
Non-amalgamated LAPs can either be suspension dominated or mixed traction-325
suspension dominated. Mixed LAPs consist of mainly massive sandstones, formed from326
multiple suspension deposited turbidites, with mudclast conglomerates deposited by traction,327
bed-load deposits (Abreu et al., 2003). LAPs typically represent about two thirds of highly328
sinuous Channel Complexes. The vertical and lateral presence of LAPs is low because of the329
lack of amalgamation within the sandy beds. Abreu et al. (2003) hypothesise that LAPs are330
related to periods of sediment by-pass, which is shown by erosion at the top of the sandstone331
beds in the LAPs.332

16. 16
Suspension-dominated LAPs consist of high quantities of sandy turbidites and low333
quantities of muddy turbidites, which decrease in thickness and sand content towards the top334
of the package (Abreu et al. 2003).335
4.2.1.2 Semi-amalgamated LAPs336
The majority of semi-amalgamated LAPs are traction dominated, comprising of337
interbedded high-concentration turbidites, regularly showing vertical amalgamation and338
coarse-grained, thick bedded traction deposits or high concentration turbidites at the base,339
getting less amalgamated and thinner beds, further up sequence (Abreu et al 2003).340
Suspension dominated deposition is also seen in semi-amalgamated LAPs, they generally341
comprise of massive sandstones interbedded with low-concentration turbidites. The sandstone342
thickens downdip, resulting in an amalgamated layer forming at the base of the channel343
(Abreu et al., 2003)344
4.2.1.3 Amalgamated LAPs345
Completely amalgamated LAPs are rare, but the sinuous channel studied at Rinevilla346
Point in Spain by Sullivan et al. (2000) showed thick sandstone beds accreting at the margin347
of a sand-rich channel, about 10m thick (Abreu et al., 2003; Sullivan et al., 2000). High348
rations of sandstone to shale were identified, with the sediment generally coarsening349
upwards.350
4.2.2 Reservoir Connectivity between LAPs351
The connection between LAPs and therefore reservoir potential is greatly dependent352
by the nature of the channel fill in the last phase of the channel. If sandy nodules are present353
at the base, the connectivity between mud-filled channels can be greatly enhanced (Abreu et354

17. 17
al., 2003). Studies suggest that coarse grained sediments were deposited in pits in the355
erosional base of the channel, resulting in the nodular effect of the sandstone deposits.356
4.3 Importance of LAPs of reservoir potential in highly sinuous channels357
LAPs, a common feature of late development of highly sinuous channel systems have358
become an important part of research in the last decade into reservoir potential in a channel359
type that was previously regarded to hold low reservoir potential. Large volumes of360
hydrocarbons have been identified in channels associated with LAPs, meaning that their361
structure and evolution need to be fully understood. Abreu et al. (2003) were the first to362
analyse these features in detail in offshore Angola. This research showed that over a third of363
the area in the Green Channel Complex comprises of LAPs, meaning that a high percentage364
of the reservoir is associated with lateral accretion. Their research has resulted in the possible365
prediction of LAP dimensions, just by knowing the width of said channel. Reservoir potential366
is affected greatly by the nature of channel fill and whether the sandy nodular bases of LAPs367
are able to be connected. An amalgamated, sand-rich channel fill produces the best potential368
for high reservoir quality, but it is important that the nature of the deposit and LAPs are369
identified early in the analysis of such channels.370
5. AFFECTS OF SLOPE SETTING ON RESERVOIR POTENTIAL – A CASE371
STUDY FROM BRADFORD E. PRATHER (2003)372
Prather (2003) classified slope grades into three catagories: (1) above-grade slopes, with373
well-developed ponded accommodation and large amounts of mid- to upper-slope healed-374
slope accommodation, (2) above-grade slopes with stepped profiles that lack well-developed375
ponded accommodation and (3) graded slopes that lack significant topography (Prather,376
2003). Richards, Bowman & Reading (1998) came up with three components of analysis that377
must be identified in order to predict reservoir potential: (1) basin screening, (2) fan378

18. 18
delineation and (3) fan characterization. This characterisation has resulted in the discovery379
that the majority of recent deepwater discoveries have been located in reservoirs from muddy380
above-grade slopes on passive margins (Prather, 2003). This highlights the need to carry out381
further studies into deep water facies of hydrocarbon reservoirs, as most submarine382
discoveries in the past have been in shallow waters, in sand rich reservoirs, associated with383
graded slopes (Prather, 2003).384
5.1 Accommodation385
Accommodation space is the amount of room for deposition in a certain facies;386
topography of depositional surfaces and the slope affect the accommodation space in387
submarine channels. Accommodation types have been split into three main areas: (1) Ponded,388
(2) healed-slope and basin floor positions, (3) slope (Prather, 2003).389
5.1.1 Ponded (Fig 17)390
Salt or shale is removed from a mobile substrate, in an intraslope basin, resulting in391
ponded accommodation space. Ponded accommodation can also be a result of structural392
activity within the slope, resulting in depressions in the slope, allowing deposition of393
sediment to in fill the cavities (Prather, 2003). ‘Fill and spill’ depositional processes make up394
the majority of depositional facies on slopes with ponded intraslope basins, which results in a395
shallow angled, stepped slope being formed (Prather, 2003). Ponded deposition represents396
isolated and confined sediment deposition over a long period of time; i.e. it is not the only397
deposition occurring within the intraslope at the time; periods of ‘spilling’ over the edge of398
the cavities that results in deposition on the slope is no longer termed ‘ponded accumulated,’399
despite being a consequence of this deposition (Prather, 2003; Booth, Dean, DuVernay &400
Styzen, 2003).401

19. 19
Fig 17: Cross section of a seafloor profile, illustrating the positions of the different slope402
types (Source: Prather, 2000; 2003).403
5.1.2 Slope accommodation (Fig 17)404
Sediments deposited on the continental slope is subject to several parameters,405
meaning that deposition is possible. Stable slope angle depends on pore pressure within muds406
deposited on the slope; high pore pressure decreases the shear strength of muds deposited, so407
a mass wasting event is therefore more likely to occur. Slope accommodation is defined by408
Prather (2003) as ‘the space between the highest stable graded-slope angle and the top of409
healed-slope accommodation or other lower-grade depositional profiles (Prather, 2000;410
Prather, 2003).411
5.1.3 Healed-slope accommodation (Fig 17)412
In intraslope facies, healed-slope accommodation is located above stepped-413
equilibrium profiles (Prather, 2000; Prather et al., 1998; Prather, 2003). Originally identified414
by Prather (1998) and Modified in 2000, healed slope accommodation was recognised in the415
Gulf of Mexico. Located above ponded accommodation and below the seafloor topography,416
healed slope accommodation occurs more frequently than ponded accommodation and417
consists of more sediment, due to the larger depositional surface area (Prather, 2003).418
Where sedimentation rate exceeds basin subsidence (above grade slopes) stepped419
slopes are frequently formed, lying on top of ponded accumulates. These deposits grade420
upwards from sands to finer muds and silts (Booth et al., 2003; Prather & Pirmez, 2003;421
Prather, 2003). These form submarine fans and channels when mass wasting event occur,422
which can be caused by a large influx of sediment or basinward tilting/deltaic progradation,423
resulting in the critical angle of said slope to be surpassed (Prather, 2003).424

20. 20
5.2 Slope types425
Slope types are categorised into three major categories: (1) above-grade slopes with426
well developed, enclosed intaslope basins; (2) above-grade slopes with changes in427
depostional gradient, resulting in terraced topography, and (3) graded slopes that lack428
significant topography (Prather, 2003).429
5.2.1 Graded Slopes430
The absence of ponded accommodation and healed-slope accommodation separates431
graded slopes from above-grade slope systems (Prather, 2003). Due to this lack of varied432
topography, sediment type and slope angle are key to preserving this depositional facies. This433
results in little to no deposition of sands in the mid-shelf area, due to gradient of the slope. In434
areas of low sediment influx, the slope angle is relatively shallow, whereas areas of high435
sediment deposition results in over pressured shales (Prather, 2003). This results in reservoir436
quality being poor in mid slope environments, but increasing downslope as the gradient levels437
out, allowing sands to be deposited. Leveed channels are a common facies for high quality438
reservoirs in this environment (Droz & Bellaiche, 1985; Galloway, 1998; Mutti & Ricci439
Lucchi, 1975, Prather, 2003). Flood et al. (1991) suggest that additional reservoir types may440
exist in muddy basin floor settings, but the lack of massive sandstone deposits make this441
improbable and highlight the fact that high concentration turbidites are poor at carrying sand442
past the toe of the slopes (Prather, 2003).443
5.2.1.1 Unconfined slopes444
Studies carried out in the Gulf of Mexico by Kendrick (2000) have shown that quality445
reservoirs can be identified in intraslope facies of graded slopes. The reservoirs proved to be446
very localised, as any sand deposited in this environment are susceptible to consequent447

21. 21
erosion by submarine mass wasting events. Sand deposition occurs in topographical448
depressions in submarine valleys or leveed channels (Prather, 2003). Reservoirs formed in449
this way are extremely thin and regularly consist of impermeable barriers, resulting in low450
hydrocarbon volumes.451
5.2.1.2 Toe of slope452
Toe of slope environments represent the highest quality of reservoir found on graded453
slopes, even on muddy slopes (Prather, 2003). Deposition is a result of gravity flows from454
deltaic settings, that bypass the mid-slope region, due to the high content of sand, so are455
therefore deposited at the toe of the slope, forming submarine fans (Prather, 2003). These456
deposits are found in areas of tectonic activity or high, rapid sediment influx.457
Large hydrocarbon volumes have been found in this environment offshore Brazil,458
where a break in slope on the basin floor results in massive sands being deposited, in laterally459
migrating turbidity currents (Prather, 2003). However, faulting and slumping can result in the460
pinch out seal of the reservoir being broken, as noted by Staccia & Prather, 1999; 2000.461
5.2.3 Above-grade slope462
Above grade slopes are characterised by the presence of large amounts of ponded463
accommodation and intraslope healed-slope accommodation, in conjunction with movement464
of salt or shale, resulting in structural highs and stepped profiles (Fig 18; Prather, 2003). Of465
all the slope types, reservoirs in above-grade slopes are the least understood. Most of the sand466
deposition occurs in the mid slope region, in ponded healed-slope accumulations. Sandstone467
deposition is also associated with incised slope canyons of a highly sinuous nature. More468
detailed research into above grade slopes is needed in order to predict the quality of reservoir469
produced (Prather, 2003).470

22. 22
Fig 18:Seismic section showing an above-grade slope, recording shallow ponded basins and471
submarine aprons (Source: Prather, 2003).472
5.2.4 Intraslope Basins473
Common in the Gulf of Mexico, intraslope basin reservoirs occur in two major474
environments; (1) ponded sheet sands and (2) slope wedges of mixed ponded and sheet sands.475
The most common hydrocarbon traps are of healed slope origin, in structurally high sections,476
directly below condensed units, which act as a seal (Booth et al., 2002; Prather, 2003). An477
ideal sequence within intraslope basins originate with channels and lobes at a break in slope,478
below the basin entry point, with sheet deposits down slope in ponded accommodation479
(Booth et al., 2003; Prather, 2003). Erosion and formation of a topographic ‘knick’ at spill480
over points of ponded accumlations begins and gravity flows spill downslope as the sill481
separating the up-slope basin from the down slope basin is topped (Pirmez, 2000; Beaubouef482
& Friedmann, 2000; Prather, 2003)483
5.2.5 Slope Canyons484
Recent discoveries of reservoirs in incised canyons in offshore West Africa have485
shown environments producing amalgamated sinuous, pod and ribbon shaped sand bodies, as486
previously discussed. Slope canyons can form on any of the slope types discussed in this487
section, but studies have shown that those on above-grade stepped slopes are far more488
common (Prather, 2003).489
An excellent example analysed by Prather (2003) shows deposition commencing with the490
incision of a previous channel, forming a broad canyon floor, then deposition of highly491
amalgamated, highly sinuous channel within the newly formed valley. Less tightly confined492
deposition follows, eroding most of the southern levee. There is little evidence of sand within493

23. 23
the channel, due to the lack of reflectivity on the bottom of the canyon. The seismic494
evaluation also shows a highly sinuous channel with sweeping plan geometries; suggesting a495
migrating channel (Prather, 2003). This example incorporates many of the structural controls496
discussed in this paper, but the difficulty of predicting reservoir potential is highlighted by497
Prather (2003) as many of the seismic units are hard to distinguish and therefore identify. In498
this example in particular, the edges of the channel are particularly complex, even with high499
frequency seismics. Static reservoir modelling is difficult, as connectivity of quality reservoir500
units needs to be clear and the position of producer and injector wells depends on the location501
of impermeable barriers, breaking up the reservoir (Prather, 2003).502
6. DISSCUSSION503
This paper has combined many of the most important factors affecting reservoir potential in504
deep water facies; however the author recognises that not all parameters have been discussed.505
Further research is required into these factors in order to produce a single document506
evaluating them all.507
6.1 Channel architecture508
The research carried out has highlighted the fact that submarine channels and slopes509
form very complicated reservoir rock facies, which requires significant knowledge of every510
element affecting reservoir quality, in order to ‘unravel the entire picture’ and come to an511
accurate conclusion when estimating reservoir quality and quantity; as every channel is seen512
to be unique and may contain different combinations, if not all of these factors. Certain513
features occur in many channels however, which can act as a stepping block to understanding514
unique channels. The following factors are recurring features within large (1-3km wide), 3rd
-515
order channels, as set out by Mayall et al. (2006), with which the author of this paper516
concurs:517

24. 24
(1) Sinuosity – every channel has a varied degree of sinuosity, which is affected by518
seafloor topography, lateral accretion, lateral stacking and erosion. Highly sinuous,519
confined channels have only just been discovered as potential sources of quality520
reservoirs. They were previously overlook, due to the fact that the presence of massive521
sand beds is lowered, due to the lateral migration of said channel. However, advances522
in 3D seismic technology and research carried out by Abreu et al. (2003) has resulted in523
the recognition of lateral accretion packages and their importance in the exploration of524
vast quantities of oil in offshore Angola. Over one third of the Green Channel Complex525
studied contains LAPs, which have the potential to connect porous sandstones in a526
continuous reservoir, depending on channel fill.527
(2) Facies – divided into four main categories, the facies that fill turbidite channels play a528
pinnacle in estimating reservoir potential. (i) The basal lag of each channel needs to be529
analysed and identified accurately, as a coarse conglomerate base offers a good530
reservoir continuation, but a mudclast conglomerate or shale drape will act as a barrier,531
preventing migration of hydrocarbons. (ii) Low net-gross sinuous channels often532
feature levees, which exceed the confined area of the channel, resulting in the533
possibility of hydrocarbons migrating out of the trap and dispersing, through faults or534
such like features. This channel type can also see deposition of mudstone layers in the535
channel, which will act as a barrier to oil migration. (iii) Slumps and debris flows from536
either channel collapse, or as deltaic sediment influx can cause the deposition of537
impermeable layers within the channel. (iv) High net-gross stacked channels form the538
best quality reservoirs, due to the potential to accumulate massive sandstone units539
(Mayall et al., 2006).540
(3) Repeated cutting &filling – re-incision of previous channels is common and can result541
in previous channel fill being scattered across the channel, not only making the seismic542

25. 25
data hard to interpret, but also potentially scattering muds and silts across sand deposits,543
therefore lowering the reservoir potential (Mayall et al., 2006).544
(4) Stacking patterns – stacking patterns can change dramatically in a short distance, not545
only meaning that detailed seismic surveys need to be carried out along the entire546
channel, but they also affect the placement of both production and injection wells547
(Mayall et al., 2006).548
6.2 Continental slope morphology549
Three types of slopes were researched in Prather’s work (2003): (1) above-grade with550
ponded basins, (2) stepped or terraced slopes and (3) graded slopes. Erosion controls the sand551
distribution on graded slopes that are absent of mobile components (e.g. salt or shale).552
Sediment that bypasses the steeper high and mid-slope areas proceeds to be deposited in553
aggrading turbidite sequences at the toe of the slope and in the basin. These turbidites and554
fans result in the basin shallowing, allowing sediment to prograde over the shelf created.555
Slow aggradation of these fans results in erosion and subsequent gravity flow processes,556
which reduces the reservoir potential through compaction (Prather, 2003).557
Above-graded slopes are formed from unconfined graded slopes and contain mobile558
substrata. This author concurs with Prather (2000; 2003) that episodes of high sediment559
deposition must be followed by low sediment flux, in order to form ponded basin deposits.560
Ponded deposits are vital in reservoir potential in over graded slopes, as without the hollows561
to ‘catch’ the coarser sediments, it would all bypass the intraslope section and be deposited at562
the toe or within the basin (Prather 2000; 2003).563
Prather (2003) concludes that the highest concentration of reservoirs in muddy slopes564
occur in ponded deposits mid slope and at the toe of the slope. It is important to emphasise565
that the majority of high reservoir potential will be deposited at the toe or within the basin,566

26. 26
due to the bypassing effect of the turbidity flows. Sand is preferentially deposited at the base567
of healed-slope accommodation, where it is associated with cyclic arrangements of sand-rich568
sheets overlain by channels (Prather, 2003).569
6.3 Limitations570
Research of a far greater magnitude is needed to cover this extremely broad and relatively571
unknown topic in order to come to a definite conclusion to whether reservoir potential in572
deep water facies can be accurately predicted. Constraints of time and journal accessibility573
has meant that this paper has not included every aspect of the topic has been covered in574
adequate detail.575
7. CONCLUSION576
This paper set out to identify whether it is currently possible to accurately predict and577
quantify reservoir potential in deep water facies. Important factors have been identified in a578
topic that is extremely broad and requires detailed knowledge in many areas involving deep579
water sedimentation and submarine channel architecture and evolution. Accurate predictions580
can be made, but they require very detailed analysis of seismic lines, taken at regular581
intervals across a submarine channel or slope, as they frequently change composition and582
structurally in short distances, resulting in a completely different interpretation when looking583
at the big picture. Previous work and work on going in areas such as the Gulf of Mexico and584
West Africa prove that these deep water systems have the potential to contain vast volumes585
of hydrocarbons.586
The most important features and skills required to come to an accurate conclusion of a587
channels reservoir potential are as follows:588

27. 27
(1) Seismic data must be collected in the highest possible resolution, as many of the589
processes seen in submarine flows result in extremely complex seismic profiles,590
which can be hard to decipher, even to the trained eye.591
(2) Important features that will be present in most, if not all channel deposits must be592
identified and quantified. These include: (1) the sinuosity of said channel, (2) the593
stacking patterns within the channel, including lateral accretion and the presence and594
distribution of LAPs, (3) identification and classification of the basal lags of each595
channel in the stacked stratigraphy, (4) identification of possible scattering of older596
sediments due to subsequent incision and erosion and other potential migration597
barriers, including mud/shale deposits within the channel; (5) Presence of levees and598
the potential that they could be acting as an ‘escape route’ for hydrocarbons in the599
reservoir.600
Future advances in technology and knowledge will increase the accuracy with which deep601
water reservoirs can be quantified, which will be at the forefront of oil exploration in602
years to come, as knowledge grows and shallow water reservoirs are exploited.603
Acknowledgements604
Ben Thomas acknowledges the support of Dr. Tiago Alves of the Department of Earth605
and Ocean Sciences at Cardiff University for the topic set. Thanks also goes to Mike606
Mayall of BP exploration for his permission to use his research and figures in the writing607
of this paper and to Bradford E. Prather of Shell International for the same, kind608
sentiment.609
610
611

28. 28
8. REFERENCES612
Abreu V., Sullivan M., Pirmez C., Mohrig D., (2003). Lateral accretion packages (LAPs): an613
important reservoir element in deep water sinuous channels. Marine and Petrol. Geol. 20,614
631–648.615
Baztan J., Berne S., Olivet J. L., Rabineau M., Aslanian D., Gaudin M., Rehault J. P., Canals616
M. (2005). Axial incision: The key to understand submarine canyon evolution (in the617
western Gulf of Lion). Marine and Petrol. Geol. 22, 805-826618
Beaubouef R. T. (2004). Deep-water leveed-channel complexes of the Cerro Torro619
Formation, Upper Cretaceous, southern Chile. AAPG. 88, 1471-1500.620
Beauboeuf R. T., & Friedmann S. J. (2000). High resolution seismic/sequence stratigraphic621
framework for the evolution of Pleistoceneintraslope basins, western Gulf of Mexico. In P.622
Weimer, R. M. Slatt, J.Coleman, N. C. Rosen, H. Nelson, A. H. Bouma, M. J. Styzen, &623
D. T. Lawrence (Eds.), Deep-water reservoirs of the world, GCS-SEPM.624
Booth J. R., Dean M. C., DuVernay III A. E., & Styzen M. J. (2003). Paleo-bathymetric625
controls on stratigraphic architecture and reservoir development of confined fans in the626
Auger Basin: central Gulf of Mexico slope. Marine and Petrol. Geol. 20.627
Booth J. R., Prather B. E., & Steffens G. S. (2002). Depositional models for ponded and628
healed-slope accommodation on above-grade slopes: implications for reservoir629
characterization. Proceedings from the 2002 AAPG Annual Meeting CD.630
Campion K. M., Sprague A. R., Mohrig D., Lovell R. W., Drzewiecki P. A., Sullivan M. D.,631
Ardill J. A., Jensen G. N., Sickafoose D. K. (2000). Outcrop expression of confined632
channel complexes - Global Deep-Water Reservoirs. Gulf Coast Section SEPM633
Foundation 20th Annual Bob F Perkins Research Conference, pp. 127–151.634
Daly R. A. (1936). Origin of the submarine canyons. A. Jour. Sci. 31, 401-420.635

29. 29
Droz L., & Bellaiche G. (1985). Rhone deep sea fan: morphostructure and growth pattern.636
AAPG Bulletin. 69, 460–479.637
Eschard R., Elbouy E., Deschamps R., Euzen T., Ayub A. (2003). Downstream evolution of638
turbidite channel complexes in the Pab Range outcrops (Maastrichtian, Pakistan). Marine639
and Petrol. Geol. 20, 691-710.640
Flood R. D., Manley P. L., Kowsmann R. O., Appi C. J., & Peirmez C. (1991). Seismic facies641
and Late Quaternary growth of Amazon submarine fan. New York: Springer, pp 415–433.642
Galloway W. E. (1998). Siliciclastic slope and base-of-slope depositional systems:643
components facies, stratigraphic architecture, and classification. AAPG Bulletin. 82, 589–644
595.645
Gardner M. H. and Borer J. M. (2000). Submarine channel architecture along a slope to basin646
profile, Brushy Canyon Formation, West Texas. AAPG Memoir 7. 68, 195-214.647
Gardner M. H., Borer J. M., Melik J. J., Mavilla N., Dechesne M., Wagerle R. D. (2003).648
Stratigraphic process-response model for submarine channels and related features from649
studies of Permian Brushy Canyon outcrops, West Texas. Marine and Petrol. Geol. 20,650
757-788.651
Johansson M. and Stow D. A. V. (1998). Deep-water massive sands, California: tectonic652
setting and sand supply controls. Geoscience 98, Abstracts. pp 12653
Kendrick J. W. (2000). Turbidite reservoir architecture in the northern Gulf of Mexico654
deepwater: insights from the development of Auger, Tahoe and Ram/Powell fields. In P.655
Weimer, R. M. Slatt, J. Coleman, N. C. Rosen, H. Nelson, A. H. Bouma, M. J. Styzen, &656
D. T. Lawrence (Eds.), Deep-water reservoirs of the world, GCS-SEPM foundation, 20th657
Annual Bob F. Perkins Research Conference pp. 450–468.658

30. 30
Mayall M. and O’Byrne C. (2002). Reservoir Prediction and Development Challenges in659
Turbidite Slope Channels. OTC Conference. 14029660
Mayall M. and Stewart I. (2000). The architecture of turbidite slope channels. Gulf Coast661
Section SEPM Foundation 20th
Annual Bob F. Perkins Research Conference, pp 578-586662
Mayall M., Jones E., and Casey M. (2006). Turbidite channel reservoirs-Key elements in663
facies prediction and effective development. Marine and Petrol. Geol. 23, 821-841.664
Mutti E., & Ricci Lucchi F. (1975). Turbidite facies and facies associations. Field Trip665
Guidebook A-11. 9th International Sedimentology Congress, Nice, France. 21–36, 12.32,666
12.4.3.667
Pirmez C., Beaubouef R. T., Friedmann S. J., Mohrig D. C. (2000). Equilibrium profile and668
base level in submarine channels: examples from Late Pleistocene systems and669
implications for architecture in deepwater reservoirs. In P. Weimer, R. M. Slatt, J.670
Coleman, N. C. Rosen, H. Nelson, A. H. Bouma, M. J. Styzen, & D. T. Lawrence (Eds.),671
Deep-water reservoirs of the world, GCS-SEPM672
Prather B. E. (2000). Calibration and visualization of depositional process models for above-673
grade slopes: a case study from the Gulf of Mexico. Marine and Petrol. Geol. 17, 619–674
638.675
Prather B. E. (2003). Controls on reservoir distribution, architecture and stratigraphic676
trapping in slope settings. Marine and Petrol. Geol. 20, 529-545.677
Prather B. E. and Pirmez C. (2003). Evolution of a shallow ponded basin, Niger delta slope.678
AAPG Conference Abstract CD.679
Prather B. E., Booth J. R., Steffens G. S., Craig P. A. (1998). Classification, lithologic680
calibration and stratigraphic succession of seismic facies from intraslope basins, deep681
water Gulf of Mexico, USA. AAPG Bulletin, 82, 701–728.682

31. 31
Richards M., Bowman M., & Reading H. (1998). Submarine-fan systems I: characterization683
and stratigraphic prediction. Marine and Petrol. Geol. 15, 689–717.684
Shepard F.P. (1936). The underlying causes of submarine canyons. Natl. Acad. Sci. Natl.685
Research Council. 22, 496-502686
Shepard F. P. (1972). Submarine canyons. Earth-Sci. Rev. 8, 1-12687
Sullivan M. D., Jensen G. N., Goulding F. J., Jennette D. C., Foreman J. L., & Stern D.688
(2000). Architectural analysis of deep-water outcrops: Implications for exploration and689
development of the Diana sub-basin, western Gulf of Mexico. Gulf Coast Section Society690
of Economic Paleontologists and Mineralogists Foundation 20th
annual research691
conference: Deep-water reservoirs of the world, pp. 1010–1032.692
Stow D. A. V. and Johansson M. (2000). Deep-water massive sands: nature, origin and693
hydrocarbon implications. Marine and Petrol. Geol. 17694
Stow D. A. V., Mayall M. (2000). Deep-water sedimentary systems: New models for the 21st
695
century. Marine and Petrol. Geol. 17, 125-135696
Straccia J. R., and Prather B. E. (1999). Stratigraphic traps in deep-water turbidite reservoirs697
at the base of depositional slope. Society of Petroleum Engineers, Offshore Europe698
Conference, SPE 56894.699
Straccia J. R., & Prather B. E. (2000). Stratigraphic traps in deepwater turbidite reservoirs at700
the base of depositional slope. Proceedings from the 2000 AAPG Annual Meeting.701