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uIntroduction
uFigure
captions
uDepositional
control
u Migration /charge
uComment
uConclusions
uReferences
uAcknowledgment
uIntroduction
uFigure
captions
uDepositional
control
uMigration/charge
uComment
uConclusions
uReferences
uAcknowledgment
uIntroduction
uFigure
captions
uDepositional
control
uMigration/charge
uComment
uConclusions
uReferences
uAcknowledgment
uIntroduction
uFigure
captions
uDepositional
control
uMigration/charge
uComment
uConclusions
uReferences
uAcknowledgment
uIntroduction
uFigure
captions
uDepositional
control
uMigration/charge
uComment
uConclusions
uReferences
uAcknowledgment
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Figure Captions
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Figure 1. A simple depsitional model for
sand deposition as ponded lobes on the downslope side of
anticlinal structure. |
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Figure 2. A present-day Nigeria slope
channel and its geomorphological parameters. The channel thalweg
depth is in red, thalweg depth in blue and the channel gradient
in light green. The presumed slope depositional equilibrium
profile is shown in dotted pink. The channel extends across
three seafloor topographic highs (i.e., High 1, High 2 and High
3) associated with either shale-cored diapirs or
thrust-generated anticlinal structures. Note the thalweg
elevation for the channel segment directly across these highs is
above the equilibrium profile, suggesting potential for further
down cutting and headward erosion. The deepest part of the
channel thalweg, as shown by the channel thalweg depth curve
(blue), occurs at approximately the intersection point between
the thalweg elevation and the equilibrium profile curves.
Channel depth gradually decreases downdip from the intersection
point as a result of deposition within the channel thalweg. Sand
deposition across the channel should be expected to occur
primarily on the downslope sides of the highs because of (1) a
relatively greater accommodation spaces, and (2) hydraulic jumps
within turbidity current flows emanating from the highs.
(Modified from Pirmez, et al., 2000.) |
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Figure 3. The impact of structural
growth rate and sedimentation rate on sand thickness and the
number of sands on the downslope side mini-basin. Assuming other
factors being equal, thicker but fewer sands occur where
structural growth rate is significantly greater than
sedimentation rate (A), and thinner and more sands will be
deposited if structural growth rate is close to or less than
sedimentation rate (B). |
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Figure 4. Schematic diagram illustrating
the relative sizes of the fetch areas from source-rock kitchens
on both the updip and the downdip sides of an anticlinal
structure. In a slope setting, the fetch area on the downslope
side of a given anticlinal structure is generally much greater
than that on the updip side of the same structure. |
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Deepwater gravity sedimentation in general and sand deposition in
particular is greatly influenced by seafloor topography with sediment
being largely focused into topographic lows. On a continental slope,
sedimentation of turbidite and associated gravity flow deposits are
controlled by local base level and accommodation space which, in turn,
is controlled by the slope equilibrium profile (Pirmez, 2000; Prather,
2003). Erosion/incision occurs in areas above the profile, and
deposition occurs in areas below the profile.
Growth of shale-core diapirs, due to either deep-seated duplexing/thrusting
as a result of downslope gravity gliding along detachment surfaces or
differential sediment loading, generate subtle seafloor topographic
highs, thus leading the depositional surface to deviate from the
equilibrium profile and creating local areas of erosion and
accommodation space for deposition. Pre-existing or syn-growth channels
along the continental slope across these seafloor highs have to adjust
themselves. Channel thalwegs will be subject to either down-cutting
(deepening) and possible headward erosion across these highs (if thalweg
elevation is above the equilibrium profile) or aggradation (shoaling) on
the downslope flank of the highs and the adjacent structural low area
(where thalweg is beneath the equilibrium profile).
Turbidity current flows emanating down from a seafloor topographic high
is forced to undergo a hydraulic jump from a Froude-supercritical flow
regime to a highly Froude-subcritical regime. This results in a deep,
placid, slow-moving turbidity current farther downstream (Parker, 2003),
leading to deposition of sands that cannot be carried farther
downstream. Skaloud and Cassidy (1998) used the hydraulic jump theory to
explain the sand deposition on the downslope flanks of both Bonga and
N’golo field. This process can explain sand distribution in some of the
fields mentioned previously. The depositional model is illustrated in
Figure 1.
It is possible that sands can be carried by a single turbidity current
across multiple structural highs and deposit them in the form of
terminal lobe if the channel thalweg is more or less close to the
equilibrium profile and flow thickness is less than the channel depth.
During periods of active structural growth, such equilibrium condition
will be disrupted and sand deposition should occur, although
non-uniformly, across different channel segments across multiple
structures. A modern example is shown in Figure
2.
Sands are expected to be predominantly deposited on the downslope sides
of structural highs due to a greater accommodation space (below the
regional depositional equilibrium profile). Hydraulic jump of turbidity
current flows emanating down from these structural highs is the dominant
hydrodynamic process for sand deposition. The upslope flank of the
structural high as manifested by the channel thalweg profile is
generally above the equilibrium profile. Consequently, little sand will
be deposited there and on the top of the highs, assuming turbidity
current flow thickness is less than the channel depth. Even if sands are
deposited across the top and upslope flanks of the highs during periods
when structural growth rate is far less than sedimentation rate across
the entire channel stretch, they may not be well preserved because
subsequent structural growth and turbidity current flows may lead to
erosion. For thick sand deposition within a particular channel segment
downslope of a structural high, it requires a greater accommodation
space and higher preservation potential, other factors being equal. This
can occur when structural growth rate is significantly greater than the
background sedimentation rate, such that a high topographic relief of
the seafloor is created. This could create a ponded mini-basin upslope
of the structural high, where sands could be deposited (Prather, 2003).
Once the mini-basin is filled to such a level that the thalweg elevation
plus a presumed turbidity current flow thickness exceeds the elevation
of the regional equilibrium profile, sand will start to be funneled down
the structural high into the downslope part of the channel and be
potentially deposited in the next basinward mini-basin. This will be the
time for relatively thick sand deposition, until the downslope
mini-basin is filled. Therefore, the timing of thick sand deposition in
a slope setting is either synchronous or soon after individual active
structural growth episodes.
The overall succession of the sand package generally exhibits a
thinning-upward trend (Figure 3). If
sedimentation rate keeps up with structural growth and no significant
accommodation space is available along the channel stretch, only thin
but possibly more sands will be deposited due to a less degree of
amalgamation. It is also possible that more unconfined lobate-shaped
sandbodies will be deposited due to relatively shallow thalweg depth and
ease of overbank flow stripping. It is clear from the above discussion
that the downslope flanks of shale-cored structural highs are loci for
sand deposition as long as structural growth continues and sedimentation
rate is not high enough to smooth out its seafloor topographic relief.
The thickness and number of sands are dependent upon, other factors
being equal, the relative rate between structural growth and
sedimentation. High rate of structural growth creates more opportunities
for thick sand deposition on the downslope flank of the structure, but a
greater degree of erosion at the top of the structures.
In addition to the depositional control over hydrocarbon accumulation
pattern, it is also important to note that hydrocarbon migration and
charge also play a critical role. Two most important factors are worthy
of consideration. First, hydrocarbon migration from source-rock kitchen
generally occurs in an updip direction. Secondly, the fetch area from
source-rock kitchen is generally much greater for its updip shale-cored
anticlinal structures than that for its downdip structures (Figure
4). Therefore, charge volume for a given structure is relatively
limited from the updip side source rock kitchen and the bulk of
hydrocarbon charge is from the source-rock kitchen on the downdip side
of the structure. Regardless of sand distribution differences, as long
as sands are not uniformly distributed across a given structure,
hydrocarbon migration direction and charge volume alone should lead to
more accumulations on the downslope side.
The finding of the hydrocarbon accumulation pattern in the deepwater
Nigeria can potentially be applied to other deepwater exploration
regions such as the Gulf of Mexico and offshore Angola, where the target
stratigraphic intervals were deposited synchronously with or shortly
after structural growth. However, it does not apply to the stratigraphic
intervals or areas where sand deposition was not affected by structural
growth (e.g., a basin-floor fan setting in a basinal plain without any
growing seafloor structures).
1.Several hydrocarbon discoveries in deepwater Nigeria suggest that
hydrocarbon accumulations preferentially occur on the downslope flank of
shale-cored structural highs.
2.The hydrocarbon accumulation pattern is primarily controlled by
preferential sand deposition on the downslope sides of the structures.
3.Hydrocarbon migration direction and charge volume may also play an
important role for the observed hydrocarbon accumulation pattern.
4.The finding of hydrocarbon accumulation pattern can be applied to
other deepwater basins where structural growth is synchronous with sand
deposition.
Chapin, M., P. Swinburn, R. Van der Weiden, D. Skaloud,
S. Adesanya, D. Stevens, C. Varley, and J. Wilkie, 2002, Integrated
seismic and subsurface characterization of Bonga Field, offshore
Nigeria: The Leading Edge, v. 21, p. 1125-1131.
Pirmez, Carlos, R.T. Beaubouef, and S.J. Friedmann, 2000,
Equilibrium Profile and Baselevel in Submarine Channels: Examples from
Late Pleistocene systems and Implications for the Architecture of
Deepwater Reservoirs, abstract, in GCSSEPM 20th Annual Bob F.
Perkins Research Conference, 2000, Houston, TX.
Prather, B.E., 2003, Controls on reservoir distribution,
architecture and stratigraphic trapping in slope settings: Marine and
Petroleum Geology, v. 20, p. 529-545.
Skaloud, Dieter K., and Phillip Cassidy, 1998,
Exploration of the Bonga and Ngolo Features in Deepwater Nigeria (abs.):
AAPG Bulletin, v. 82, p.1883-1984.
Toniolo and Parker, 2003, Depositional turbidity currents
in diapiric minibasins on the continental slope: Theory, experiments,
and numerical simulation, extended abstract for presentation at the AAPG
Annual Meeting, Salt Lake City, Utah, May 11-14, 2003.
The author
is very grateful for instructive discussion with and input from the
Deepwater Nigeria Exploration Team of ConocoPhillips. I thank Dave McGee
and Frank Snyder who provided very good technical editing. The author is
also thankful for ConocoPhillips management and coventures for approval
to publish this material.
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