Architectural Hierarchy and Anatomy of an Exhumed Submarine Slope Channel Complex

Figueiredo, Jorge P.¹; Hodgson, David M.²; Flint, Stephen S.²
(1)Exploration, Petrobras, Rio de Janeiro, Brazil. (2) School of Environmental Sciences, University of Liverpool, Liverpool, United Kingdom.

A well exposed upper slope channel complex set in Unit F, Fort Brown Formation (Permian) SW Karoo Basin, South Africa, allows the identification of a hierarchy of erosion surfaces overlain by characteristic architectural elements and lithofacies associations. Outcrop constraints and palaeocurrent measurements indicate that the channel complex set is 1.1km wide, >60m thick, oriented WNW-ESE (flow to ESE) and is partially confined by adjacent external levee deposits. One channel complex was studied in detail, where two scales of erosion surface are identified. The larger is a composite surface >300m in width and 35m in depth. Three smaller-scale surfaces, <200m wide and shallower than 25m, are nested within the larger cut. Sedimentary facies in the channel-fills show an overall fining- and thinning-upward distribution. At the deepest part of the main composite erosion surface, the channel fill is composed of contorted fine-grained deposits interpreted as slumps. These are overlain by mudstone clast conglomerates, interpreted as lag deposits formed during a period of sediment bypass. The conglomerates are overlain by fine-grained structureless and amalgamated sandstones, which pass laterally into channel margin thin-bedded siltstones. The contact between beds and bedsets in this oldest channel-fill is usually erosive. The magnitude of erosion decreases upward in the successive channel fills. Each erosion surface and related depositional succession represents the cut-bypass-and-fill of a single channel. Four single channels were identified that form a laterally offset stacking pattern. This set of channels is interpreted as a channel complex that formed during a period of waxing-to-waning energy, possibly related to a high frequency allocyclic control.

The scales of the single channel-fills and the channel complex revealed in this study shows that channel complexes can easily be imaged in 3D seismic data; nevertheless it is not always true for single channels. Hence, it is likely that most of the features interpreted in 3D seismic data as single channels are in fact channel complexes, which has a serious consequence for the analysis of the internal heterogeneities in these reservoir types. High resolution outcrop based study such this presented here complement seismic interpretation and resolve the internal anatomy of the elements that cannot be imaged even in the high quality 3D seismic data.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.