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Depositional Control on 
Hydrocarbon
Accumulations in Deepwater
Nigeria*
By
Xijin (CJ) Liu1
Search and Discovery Article #40226 (2006)
Posted December 12, 2006
*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006
1West Africa Exploration, ConocoPhillips, Houston, TX 77079 ([email protected])
Introduction
Seafloor geomorphology controls deepwater sand deposition that, combined with
structural configurations, controls hydrocarbon accumulation. Structural control
of hydrocarbon accumulations is very well known, but depositional control seems
to be very important in some deepwater exploration areas. This is demonstrated
by a case study on Nigeria deepwater discoveries.
Many discoveries have been made in anticlinal structures in the Nigeria deepwater. The anticlines are located from the shale-diapir province, through the inner thrust belt and translational province, to the outer toe-thrust belt. One of the most interesting observations is that most of the discoveries were made on the downslope (basinward) side of the anticlines.
Two possible explanations, among others, become obvious. One is that sand
deposition preferentially occurs on the downslope side of the structures at the
time of deposition synchronous with structural growth (generated by shale
diapirism or thrusting). This is the result of relatively greater accommodation
space and/or hydraulic jump of turbidity current flow across seafloor
topographic highs. The second one is that hydrocarbon migration and charge
preferentially occur, along a regional depositional slope, in an updip
(landward) direction. This extended abstract is aimed to expand the discussion
of the two explanations.
Examples of hydrocarbon discoveries in the deepwater Nigeria on the downslope
side of anticlinal structures (shale-cored diapirs or thrust-generated
anticlines) include Bonga, Bonga Southwest, Akpo, Uge, N’Golo, and Obo North. A
common characteristic of all the structures is that reservoir sand deposition
was synchronous with structural growth. The structures occurred as seafloor
topographic highs during the time of sand deposition. Turbidite elements of the
reservoir sands consist primarily of channel fill and associated frontal splays
(e.g., lobate sheet sands).
There are other
discoveries, such as Agbami, in the deepwater Nigeria, which do not exhibit this
pattern. A detailed examination of those discoveries reveals that the reservoir
sands were deposited prior to structural growth, and therefore the structures
did not affect sand distribution. Moreover, turbidite elements of the reservoir
are mainly basinal-plain terminal sheet-sand lobes.
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Depositional Control on Sand DistributionDeepwater 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.
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