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Three-dimensional facies and process-regime variability in shelf-edge deltas: implications for shelf-margin progradation and deepwater sediment delivery
Laugier, Fabien J.
Laugier, Fabien J.
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2014
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2014
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2015-11-01
Abstract
Prograding shelf-slope systems are the main constituents of continental margins. In particular, shelf-edge deltas are the primary mechanism by which shelf margins are constructed and play a critical role in the delivery of sediment to the shelf edge, as well as facilitating sediment dispersal to the slope and basin floor. The overall architecture of shelf-edge deltas and their association to the shelf margin and deepwater deposits is well-understood from seismic data; however, much remains to be understood about the mechanisms by which sediment is brought to and partitioned along the shelf edge, and downslope. The shelf edge is a zone of significant gradient change separating a relatively flat-lying shelf from a steeper slope region, and has a physiographic expression that can be observed in 2-D and 3-D seismic data and in seismic-scale, dip-parallel, continuous outcrops; however, the physiographic shelf edge needs to be defined in well log datasets or non-continuous outcrops. Defining this zone of gradient change is essential because it segregates areas dominated by shelf currents and waves from areas where sedimentation is governed by gravity-driven processes. Moreover, the location of the shelf edge must be known in order to understand the linkage between shelf-edge deltas and their coeval slope and basin floor deposits. In the Tanqua depocenter of the Karoo Basin (South Africa), the Permian Kookfontein Formation is exposed at seismic scale in 3-D outcrops on a series of mountainsides. The Kookfontein Formation consists of stacked shelf-slope clinothems that occur as gently basinward-dipping, generally upward-coarsening packages that can be walked out across 21 km in depositional-dip and depositional-strike orientations. The quality and aerial extent of the exposure permits investigation at multiple stratigraphic scales and allows detailed analyses of the 3-D linkage between sedimentation from the shelf to the slope. This study (Chapters 2-3) focuses on three clinothems that prograded to the northeast. First, two independent methods were used to identify the shelf-edge position: the shelf-edge zone was determined by facies distribution, as well as clinothem thickness changes and gradient breaks. In particular, results from this dissertation show that a transition from predominantly shelf-current and wave deposits to gravitational deposits, together with the increase in degree and style of soft-sediment deformation, correlates with the position of gradient increase and clinothem thickening, and thus defines the shelf-edge position. Recognition of the shelf edge permits 3-D documentation of the distribution of facies along the shelf edge and from shelf to slope. Two main varieties of shelf and shelf-edge depositional environments are recognized along the Kookfontein shelf margin and indicate spatial variation in process-regime dominance along the shelf edge. In places, the shelf and shelf-edge deposition occurred from river-dominated deltas with gravitational delta-front deposits. Basinward of such shelf-edge zones, the slope is sand-rich, channelized (with channels widening downslope), and rich in slump and collapse features. In other places, the shelf and shelf-edge deposits indicate considerable tidal reworking. This tidal influence was recognized by the occurrence of facies with bidirectional paleocurrent signatures, thickening-thinning trends that approach a neap-spring tidal cyclicity, and deposits that indicate repeated and systematic fluctuations in current velocities. Tidal deposits occur as thin, localized depositional units, as well as laterally-extensive zones along the shelf edge. Thin, localized packages of tidal deposits associated with mouth bars and distributary channels indicate that tidal currents were locally amplified in topographically-restricted areas. Laterally-extensive zones of significant tidal influence indicates large-scale topographically-restricted areas, possibly between delta lobes, or that decreased fluvial drive allowed tidal currents to have greater effects. Basinward of tide-influenced shelf-edge zones, the slope is largely devoid of sand, shelf-edge-delta deposits are thinner, and only one small channel was observed. This analysis suggests that process-regime variability along the shelf edge exercises significant control on the character of shelf-edge progradation, slope channelization, and sediment delivery to the deepwater areas. Though process regime variability is locally observed along the shelf edge, the shelf-edge deltas of the Kookfontein Formation are fluvial-dominated. Data from Clinothem 6 is compared to high-resolution outcrops datasets from other fluvial-dominated shelf-edge delta-to-slope systems (Chapter 4). These outcrops include the Eocene Battfjellet Formation in the Central Basin of Spitsbergen; the Eocene Sobrarbe Formation in the Ainsa Basin, Spain; and the Fox-Hills-Lewis Formations in the Washakie Basin, Wyoming, U.S.A. Results from this chapter highlight greater variability in the processes of delta progradation, shelf-margin accretion, and sediment delivery than previously recognized in fluvial-dominated shelf-edge deltas. In particular, this study indicates that within fluvial-dominated shelf-edge deltas, processes of shelf-margin accretion and deepwater sediment delivery can be controlled by sediment failure, hyperpycnal flows, episodic bypass from distributary channel avulsions, or a combination thereof. This investigation also includes a case study of deriving simple geologic concepts from outcrops, and integrating them with quantitative outcrop data into novel rule-based reservoir models to address the impact of fine-scale stratigraphy on flow performance in deepwater lobe systems (Chapter 5). Data was collected from well-exposed outcrops of the Permian Skoorsteenberg Formation in the Tanqua Karoo Basin. Simple geologic concepts of lobe hierarchy, within-bed lithofacies distribution, and bed amalgamation were derived from outcrop analysis and form the foundation of the models. Two end member models were constructed: 1) a complex of deepwater lobe elements that were only subdivided into lobe sandstone and overbank mudstone; and 2) a complex of deepwater lobe elements constructed from individual sedimentation units with distinct trends in lithofacies and associated rock properties. Simplified flow performance simulated within each model highlights the significant impact that heterogeneity related to individual bed-scale trends within the lobe elements has on reservoir performance at the complex scale.
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