Deep-water channel-levee system: their geomorphic evolution, stratigraphic architecture, and interaction with salt diapirs in the base of slope to basin floor environment

Abstract

This thesis combines an interpretation of 3D seismic and bore hole data (drill cuttings and cores) to analyse and describe the geomorphic evolution, and the stratigraphic architecture of deep-water depositional systems in (i) channel dominated Offshore Taranaki Basin, and (ii) a channel-lobe dominated Central North Sea Basin. In the Taranaki basin, (Offshore New Zealand), two seismic units (SU1-2) were mapped, identifying three depositional elements: (channel and intra channel-fills, levees/overbank, and basin floor mudstones). Within SU1, several north-trending, channel-levee complexes, characterised by increasing sinuosity (SI 1.15 – 2.25), and having average width and depth of 346 m and 34 m, respectively, was identified. These channel-levee complexes were deposited in a basin-floor environment. In SU2, NW-trending channel-levee complexes, deposited in the base of slope were observed, however a decrease in sinuosity (1.01 – 1.40) and an increase in the width and depth (108 m – 104 m respectively) of the channels was noticed. Well data show that the channel fill facies are variable, comprising sandstone, muddy-sandstone, siltstone, and mudstone, with sandstone content decreasing upward. The levee/overbank facies are overall finer grained, and characterised by muddy, very fine-to-fine-grained sandstone interbedded with siltstone and mudstone. The slope/basin floor are mudstone dominated. Key differences between SU1 and SU2 channel complexes are: (i) palaeoflow direction was northwards during SU1 times and northwestwards during SU2 times; (ii) SU1 channel complex-fills are sandier (20-45% sand content), compared to those in SU2 (10-18% sand content); and (iii) SU1 channel complexes are relatively unconfined and characterised by lateral migration, whereas SU2 channel complexes are more commonly confined within valley systems. In the Central North Sea Basin, the upper Palaeocene deep-water sandstone bodies are ca. 40-150 m thick and characterised by a high sandstone (i.e., 30 - 58%) content. Using core data, three facies associations; turbidite sandstone, heterolithics, and basin floor mudstones were identified. The turbidite sandstone represent the blocky or bell shape GR pattern, and in core are fine to very fine grained and are either structureless or are characterised by dewatering pipes, dish structures, mudstone clast, and oxidized quartz veins. The heterolithics facies represents the intervals characterised by high and low GR serrated GR well log patterns, and in core comprise interbedded units that display remobilisation features. The mudstone represents the intervals with low serrated GR patterns, and in core either appears massive or laminated. The seismic characteristics of the interval of interest shows; (i) subparallel to parallel, high amplitude reflection (HAR) packages interpreted as sandstones, (ii) subparallel, interbedded HAR and low amplitude reflection packages interpreted as heterolithics and (iii) low amplitude reflection packages interpreted as basin floor mudstones. The top and base reflection characteristics of the interval of interest shows continuous, parallel to subparallel, HAR packages that are only truncated by salt diapirs. Internally, the seismic characteristics of the interval of interest show chaotic characteristics. Vclay analysis shows that these packages are characterised by high sand to clay contents. Salt diapir was shown to control the pre-existing sea floor bathymetry during deposition. Analysis of cores facies and attribute map show (i) post depositional structures resulting from salt movement (ii) high amplitude reflection packages distributed around the diapirs flanks suggesting lobes deflection and diversion and (iii) Thickness variability suggesting local depocentres created by moving salt diapirs. This study contributes significantly to the understanding of the stratigraphic evolution of basin floor fan system and provide insights into reservoir facies distribution in salt influenced basins.