Papers by Zoltan Sylvester
Mathematical Geosciences, 2015
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ABSTRACT The modern seafloor and shallow subsurface of the western Niger Delta slope exhibits sin... more ABSTRACT The modern seafloor and shallow subsurface of the western Niger Delta slope exhibits sinuous submarine channels with multiple phases of evolution. Investigations of these channels using three-dimensional (3D) high-resolution seismic-reflection data, and piston coring offer a rare opportunity to examine a lithologically calibrated submarine channel system. At least three phases of channel development and evolution are related to changes in sediment supply coincident with updip avulsions. The first phase is predominantly incisional and creates a large valley within which the subsequent phases evolve. The second phase creates a wide, low sinuosity channel within the valley. This channel displays fluvial point-bar like accretion and expansion of the meander bends. The third and most recent phase is initiated when drainage from a nearby channel is captured by the valley system. This addition of sediment supply causes a narrowing and downcutting of the channel. This narrowing occurs via deposition along the inner bends of the channel and results in very complex inner levee geometries and architectures, which have been intricately mapped on the 3D seismic data. Piston coring of these inner levees reveal heterogeneous facies, while facies in the channel thalweg consist of coarser grained sands that are often amalgamated. The modern seafloor and grain size distributions from the piston cores were used as inputs for a three-dimensional numerical model (TCSolver). Particularly important was the inputted vertical grain size distributions obtained from the piston coring. Results from simulations of turbidity currents in this lithologically calibrated model indicate a normal sense of helical flow in the channel bends, supporting the observations from the 3D seismic data.
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Sedimentological observations, planktonic microfossil data, terrigenous index proxy data, benthic... more Sedimentological observations, planktonic microfossil data, terrigenous index proxy data, benthic morphogroup analyses, pressure analyses and geochemical fingerprinting were integrated in order to assess the nature of mud-stones in the Cardamom Field in the Auger salt withdrawal minibasin (Gulf of Mexico). Both ponded and slope accommodation have occurred through the salt basin's history resulting in complex stratigraphic architecture of stacked submarine lobes and channels. Continuous core (164 ft) from the Messinian reservoir unit enabled this study to assess the depositional nature of the mud-rich intervals. Four mudstone facies (Mudstones 1–4) and respective depositional settings were interpreted. A modified benthic foraminifera microfacies model, in conjunction with variations recorded in planktonic flora and fauna abundances and reworking, serves as the key reference for the paleoecological interpretations. Of particular interest is the potential of a 41 ft thick (below seismic resolution) intra-reservoir mudstone (Mudstone 3) to act as a baffle/barrier to fluid flow between the lower U Sand and the upper U Sand. Mudstone 3 shows a hemipelagic character containing a diverse benthic community, high abundances of autochthonous planktonic flora and fauna, reduced terrigenous input and extensive bioturbation. Downhole formation pressure data and fluid fingerprinting from the reservoir pay sands (upper and lower U Sands) show significant differences in the calculated pressure gradients and fluid composition. This suggests that most likely the two reservoir pay sands are not in vertical communication. Mudstone 3 paleoenvironmental analyses suggest the lateral extension of a possible baffle/barrier to vertical fluid flow. These results show that benthic morphogroup analyses in mudstones can be a robust method to assess reservoir compartmentalization and consequently, impact field development planning and reservoir modeling.
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Changes in sediment supply and caliber during the last , 130 ka have resulted in a complex archit... more Changes in sediment supply and caliber during the last , 130 ka have resulted in a complex architectural evolution of the Y channel system on the western Niger Delta slope. This evolution consists of four phases, each with documented or inferred changes in sediment supply. Phase 1 flows created wide (1,000 m), low-sinuosity (1.1) channel forms with lateral migration and little to no aggradation. During Phase 2, the Y channel system began to aggrade, creating more narrow (300 m) and sinuous (1.4) channel forms with many meander cutoffs. This system was abandoned at , 130 ka, perhaps related to rapid relative sea-level rise during Marine Isotope Stage (MIS) 5. Phase 3 flows were mud-rich and deposited sediment on the outer bends of the channel form, resulting in the narrowing (to 250 m), straightening (to a sinuosity of 1.22), and aggradation of the Y channel system. Renewed influx of sand into the Y channel system occurred with Phase 4 at , 50 ka, during MIS 3 sea-level fall. The onset of Phase 4 is marked by the initiation of the Y9 tributary channel, which re-established sand deposition in the Y channel system. Flows entering the Y channel from the Y9 channel were underfit, resulting in inner levee deposition that is most prevalent on outer banks, acting to further straighten (1.21) and narrow (to 200 m wide) the Y channel. The inner levees accumulated quickly as the flows sought equilibrium, with deposition rates. 200 cm/ky. Marked by the presence of the last sand bed, abandonment occurred at , 19 ka in the Y channel and , 15 ka in the Y9 channel and is likely related to progressive abandonment due to shelf-edge delta avulsion and/or progressive sea level rise associated with Melt Water Pulse 1-A. The muddy, 5-meter-thick Holocene layer has thickness variations that mimic those seen in the sandy part of Phase 4, suggesting that dilute, muddy flows continue to affect the modern Y channel system. This unique dataset allows us to unequivocally link changes in submarine channel architecture to variations in sediment supply and caliber. Changes in the updip sediment routing system (i.e., the channel ''plumbing'') are shown to have profound implications for submarine channel architecture and reservoir connectivity.
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ABSTRACT
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Journal of Sedimentary Research, 2015
ABSTRACT Changes in sediment supply and caliber during the last ∼ 130 ka have resulted in a compl... more ABSTRACT Changes in sediment supply and caliber during the last ∼ 130 ka have resulted in a complex architectural evolution of the Y channel system on the western Niger Delta slope. This evolution consists of four phases, each with documented or inferred changes in sediment supply. Phase 1 flows created wide (1,000 m), low-sinuosity (1.1) channel forms with lateral migration and little to no aggradation. During Phase 2, the Y channel system began to aggrade, creating more narrow (300 m) and sinuous (1.4) channel forms with many meander cutoffs. This system was abandoned at ∼ 130 ka, perhaps related to rapid relative sea-level rise during Marine Isotope Stage (MIS) 5. Phase 3 flows were mud-rich and deposited sediment on the outer bends of the channel form, resulting in the narrowing (to 250 m), straightening (to a sinuosity of 1.22), and aggradation of the Y channel system. Renewed influx of sand into the Y channel system occurred with Phase 4 at ∼ 50 ka, during MIS 3 sea-level fall. The onset of Phase 4 is marked by the initiation of the Y′ tributary channel, which re-established sand deposition in the Y channel system. Flows entering the Y channel from the Y′ channel were underfit, resulting in inner levee deposition that is most prevalent on outer banks, acting to further straighten (1.21) and narrow (to 200 m wide) the Y channel. The inner levees accumulated quickly as the flows sought equilibrium, with deposition rates > 200 cm/ky. Marked by the presence of the last sand bed, abandonment occurred at ∼ 19 ka in the Y channel and ∼ 15 ka in the Y′ channel and is likely related to progressive abandonment due to shelf-edge delta avulsion and/or progressive sea level rise associated with Melt Water Pulse 1-A. The muddy, 5-meter-thick Holocene layer has thickness variations that mimic those seen in the sandy part of Phase 4, suggesting that dilute, muddy flows continue to affect the modern Y channel system. This unique dataset allows us to unequivocally link changes in submarine channel architecture to variations in sediment supply and caliber. Changes in the updip sediment routing system (i.e., the channel “plumbing”) are shown to have profound implications for submarine channel architecture and reservoir connectivity.
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Application of the Principles of Seismic Geomorphology to Continental-Slope and Base-of-Slope Systems: Case Studies from Seafloor and Near-Seafloor Analogues, 2012
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Application of the Principles of Seismic Geomorphology to Continental-Slope and Base-of-Slope Systems: Case Studies from Seafloor and Near-Seafloor Analogues, 2012
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The scale of heterogeneities (flow baffles and barriers) in hydrocarbon reservoirs is such that d... more The scale of heterogeneities (flow baffles and barriers) in hydrocarbon reservoirs is such that despite major advances in 3d imaging and detailed outcrop studies a wide range of uncertainty needs to be included in static model scenarios. Understanding of physical processes of erosion and deposition, and the resulting stratal architecture can be gained from detailed outcrop analogue studies, but these are largely limited to two-dimensional exposures. Depositional systems at the seafloor offer the ability to study reservoir analogues at very high-resolution, but sampling at the scale of outcrop sections is still limited. A complementary approach is to generate synthetic deposits in the laboratory, where one can directly link the flow processes and the resultant deposits. We have generated synthetic channel fills that are scalable to natural depositional systems. Such laboratory-scale reservoir models, coupled with numerical models of flow, erosion and deposition, provide the ability t...
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This submarine apron is an analog for the stratigraphic architecture of shallow ponded basins com... more This submarine apron is an analog for the stratigraphic architecture of shallow ponded basins common to stepped, above-grade slopes, where late-stage bypass valleys and channels did not form. Deposition of this apron began within shallow ponded accommodation. Sediment gravity flows entering the basin pass through a leveed channel that incises underlying slope muds. Flows spread, becoming depositional once reaching lower-gradient area within ponded accommodation. Incisions at the distal end of the basin suggest that gravity flows downcut the basin sill as they bypass the basin during filling of ponded accommodation. A channelized apron downlaps the ponded deposits, healing the stepped topographic profile formed after ponded accommodation fills. Collapsing flows exiting the entry-point channel create plunge-pool scours in the proximal part of the apron. Sediment gravity flows exiting the plunge-pool scour accelerate over the steeper face of the apron, eroding bypass channels as healin...
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Submarine channel-levee systems on continental slopes commonly show stratigraphic architectures w... more Submarine channel-levee systems on continental slopes commonly show stratigraphic architectures with multiple erosional surfaces and complicated relationships between channel and overbank deposits. This complexity is often interpreted in terms of numerous phases of erosion and deposition that take place on scales ranging from individual beds to entire valley fills. Unless related to autogenic avulsions, these cut-and-fill events imply allogenic control, as they require repeated and frequent changes either in flow parameters or in channel slope. However, growing evidence from 3D seismic and outcrop data suggests much of this complexity is due to autogenic channel migration processes. The influence of long-term channel migration on stratigraphic architecture is inadequately understood in fluvial and in deep-water channel systems; in addition, lateral migration of submarine channels is viewed as an anomaly. Apart from a larger width/wavelength ratio, the planform morphology of sinuous ...
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AAPG Bulletin, 2015
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Papers by Zoltan Sylvester