We have used a multipoint geostatistics algorithm called Snesim to evaluate its applicability to ... more We have used a multipoint geostatistics algorithm called Snesim to evaluate its applicability to reservoir modeling. To test the algorithm, we used a data from a deepwater reservoir analog from outcrops in the Tanqua Karoo Basin, South Africa. Our implementation demonstrated the ability of the algorithm to efficiently and faithfully reproduce the texture of geological facies while honoring a large number (127) hard data locations plus rotation and scaling fields and soft probability fields. We have used hand-drawn stationary training images with 3-5 facies to model the proximal to distal facies relationships seen in outcrop. The results show the ability to honor hard data, soft constraints, and complex geological relationships that vary over the reservoir grid.
Submarine slope channel-fills form complicated stratigraphy and lithofacies distributions through... more Submarine slope channel-fills form complicated stratigraphy and lithofacies distributions through repeated phases of erosion and deposition. This provides a challenge to accurate 3D modelling, particularly in representing lithofacies transitions within sand-poor areas. In this paper, traditional (sedimentary logs, palaeocurrent measurements, architectural panels) and non-conventional technologies (Light Detection and Ranging; Ground Penetrating Radar) were integrated to quantitatively describe lithofacies distributions and sedimentary architectures from two large-scale outcrops, one base of slope, high sandstone content system (Unit B) and one from a mid-slope, more mixed lithology system (Unit C), in the Laingsburg Formation, Karoo Basin, South Africa.
The stratigraphic distribution of two groups of soft-sediment deformation structures (SSDS)detach... more The stratigraphic distribution of two groups of soft-sediment deformation structures (SSDS)detached (dominated by lateral movement forming slides and slump deposits, and debrites) and in situ (dominated by vertical movement forming flame structures, pseudonodules, and ball-and-pillow structures)are compared in two Permian successions in the Tanqua and Laingsburg depocentres of the Karoo Basin. SSDS occur in a series of well-exposed 40 to 55 m thick sedimentary cycles. Cycle thicknesses decrease upwards and the proportion of deformed strata is around 30% in both successions. Each flooding surface-bounded cycle is characterized by a lower unit dominated by thin-bedded turbidites, overlain by an intermediate section dominated by soft-sediment deformed facies, above which are undeformed thick-bedded mouthbar and shoreface sandstones. The upper part of each cycle fines upwards into thin-bedded turbidites, which are overlain by the cycle-bounding flooding surface. The cycles, and the distribution of SSDS, are the response to delta outbuilding and retreat over a clinoform rollover and onto a clinoform slope. Repeatable trends in the distribution of SSDS during the evolution of clinoforms in both depocentres indicate a level of stratigraphic organisation and predictability in the otherwise complicated record of processes around the clinoform rollover.
The 380 m thick fine-grained Vischkuil Formation comprises laterally extensive hemipelagic mudsto... more The 380 m thick fine-grained Vischkuil Formation comprises laterally extensive hemipelagic mudstones, separated by packages of graded sandstone and siltstone turbidites, and volcanic ash beds, and is an argillaceous precursor to a 1 km thick sand-prone basin floor fan to shelf succession. The Vischkuil Formation provides an insight into the process by which regional sand supply is initiated and for testing sequence stratigraphic principles in a basin plain setting. Regionally mapped 1-2 m thick hemipelagic mudstone units are interpreted as condensed drapes that represent the starved basin plain equivalents of transgressive systems tracts and maximum flooding surface on the coeval shelf (now removed during later uplift). The section above each mudstone drape comprises siltstone turbidites interpreted as highstand systems tract deposits and a surface of regional extent, marked by an abrupt grain size shift to fine sandstone. These surfaces are interpreted as sequence boundaries, related to abrupt increases in flow volume and delivery of sand grade material to the basin-plain. The interpreted lowstand systems tract comprises sandstone-dominated turbidites and is overlain by another hemipelagic mudstone drape. The upper Vischkuil Formation is marked by three 20-45 m thick debrites, with intraformational sandstone clasts up to 20 cm in diameter that can be mapped over 3000 km 2 . In each case, debrite emplacement resulted in widespread deformation of the immediately underlying 3-10 m of silty turbidites. A sequence boundary is interpreted at the base of each deformation/debrite package. Six depositional sequences are recognised and the interfered energy shift across each successive sequence boundary and LSTs include a larger volume of sandstone increases up section. The lower two sequences thin to the NW and show NW-directed palaeocurrents. The four overlying sequences show a polarity switch in palaeocurrent directions and thinning, to the E and SE. Sequence 6 is overlain sharply by the 300 m thick sandstone dominated Fan A of the Laingsburg Formation. The LST debrites may indicate gradual development of major routing conduits that subsequently fed Fan A. The polarity shift from westward flowing turbidity currents to an eastward prograding deepwater to shelf system represents establishment of a long term feeder system from the west. Sand supply to the Karoo basin floor was established in an incremental, stepwise manner. Given the early post-glacial setting in an icehouse climate, glacio-eustatic sea-level changes are considered to have been the main control on sequence development.
… of subaqueous fold belts as a control on the timing and distribution of deepwater sedimentation: an example from the southwest Karoo Basin, South Africa
External Controls on …, 2009
ABSTRACT: The sediment source area for foreland-basin fills is typically inferred to be the adjac... more ABSTRACT: The sediment source area for foreland-basin fills is typically inferred to be the adjacent fold-and-thrust belt. In the case of the Karoo Basin, South Africa, however, the Cape Fold Belt (CFB) was not the source during the early phase of basin filling. Instead, ...
Sea floor and shallow seismic data sets of terminal submarine fan lobes can provide excellent pla... more Sea floor and shallow seismic data sets of terminal submarine fan lobes can provide excellent planform timeslices of distributive deep-water systems but commonly only limited information on cross-sectional architecture. Extensive outcrops in the Tanqua depocentre, south-west Karoo Basin, provide these three-dimensional constraints on lithofacies distributions, stacking patterns, depositional geometries and the stratigraphic evolution of submarine lobe deposits at a scale comparable with modern lobe systems. Detailed study (bed-scale) of a single-lobe complex (Fan 3) over a 15 km by 8 km area has helped to define a four-fold hierarchy of depositional elements from bed through to lobe element, lobe and lobe complex. The Fan 3 lobe complex comprises six distinct fine-grained sandstone packages, interpreted as lobes, which display compensational stacking patterns on a 5 km scale. Between successive lobes are thin-bedded, very fine-grained sandstones and siltstones that do not change lithofacies over several kilometres and therefore are identified as a different architectural element. Each lobe is built by many lobe elements, which also display compensational stacking patterns over a kilometre scale. Thickness variations of lobe elements can be extremely abrupt without erosion, particularly in distal areas where isopach maps reveal a finger-like distal fringe to lobes. Lobe deposits, therefore, are not simple radial sheet-dominated systems as commonly envisaged.
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Papers by David Hodgson