--> Axis to Fringe Changes in Sedimentology and Architecture of Submarine Lobe Complexes: Results From an Integrated Core and Outcrop Dataset, Karoo Basin, South Africa

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Axis to Fringe Changes in Sedimentology and Architecture of Submarine Lobe Complexes: Results From an Integrated Core and Outcrop Dataset, Karoo Basin, South Africa

Abstract

Documenting the range of sedimentary facies associations in the different sub-environments of submarine lobe deposits is important in the accurate prediction of reservoir quality and connectivity in subsurface datasets. Lobe axis (thick bedded structureless to planar laminated sandstone) and off-axis (structured medium bedded sandstone) facies associations are somewhat predictable but lobe fringe facies associations can vary between and within different deep-water systems, such that the length-scales of facies transitions from lobe axes to fringes are poorly understood. This means that reservoir cut-off in fringe settings is difficult to predict. The integration of extensive outcrop data and newly acquired core data from behind-outcrops research wells of Fan 4, Skoorstenberg Formation, permit the range of facies associations and rate of facies transitions in lobes and lobe complexes to be quantified within a well constrained palaeogeographic context. Outcrop and core sections were logged bed-by-bed, integrated with palaeocurrent measurements and correlated over a 1050 km2 study area with good 3D data distribution. Fan 4 is interpreted as a lowstand systems tract lobe complex set, comprises two lobe complexes separated by ~70 cm of hemipelagic claystone and stacked in a compensational pattern. Statistical analysis was performed to establish unbiased facies trends over the fan sub-environments. Lobe fringe associations can be i) thick bedded structureless to planar laminated sandstones that show pinching and swelling and are associated with underlying debrites, ii) current ripple laminated sandstones and siltstones, or iii) prone to argillaceous and clast-rich linked debrites depending on confinement and position in the lobe complex. Commonly, frontal fringes are more concentrated in linked debrites and transition from thick-bedded sandstones over length-scales of 1–2 km, whereas lateral fringes tend to be current ripple-laminated sandstones and siltstones that transition from thick-bedded lobe axis sandstones over several kilometres. A stratigraphic trend is noted, with fringes of earlier lobes being more prone to clast-rich linked debrites. This trend is interpreted to reflect the development of slope degradation and entrainment of slope mud into flows during the initiation of a lowstand systems tract. Constraining geometries, stacking patterns, facies trends and distributions in lobe complexes is critical to reservoir evaluation and prediction.