Flow-Substrate Interaction at the Fringes of Deep-Marine Lobes: Skoorsteenberg Fm., Tanqua Karoo

Abstract

Observations and interpretations of sedimentary facies in deep-marine lobe deposits have been strongly influenced by models developed from relatively small, coarse-grained foreland basins. Whilst these models are valid for similar systems, today's ultra deep-water subsurface exploration targets are typically associated with sedimentary systems that are up to orders of magnitude larger, with a much narrower grain-size range. In this contribution, the spatial and stratigraphic distribution of the various facies associated with a fine-grained, deep-marine lobe complex (Fan 3, Skoorsteenberg Fm., Tanqua Karoo) are presented and characterized to improve understanding sediment transport processes in such environments. The stratigraphy of Fan 3 is exceptionally well-exposed and well-constrained, making it an ideal place to observe this variability. The dataset includes helicopter-based photomosaics, measured sections and thin sections from oriented samples. QEMSCAN® (Quantitative Evaluation of Minerals by SCANning electron microscopy) analysis, including mineralogical and textural analysis of different bed types, was undertaken to support outcrop observations. Grain-size distributions, including quantification of clay content, can be established from these data and, in conjunction with the outcrop data demonstrate a progressive enrichment of clay and fine grained particles towards the distal and marginal parts Fan 3 lobes. It is demonstrated that predictable spatial and stratigraphic facies distributions can be recognised. Her, this distribution is attributed to an increase in near bed flow concentration due to flow deceleration and collapse in response to flow expansion and entrainment of clay and silt from the substrate; this is recorded in the deposits, from axis to off-axis positions, by increased clay content, decreased erosional capability of flows and progressively stronger internal deposit heterogeneity. The model differs from previous models as the flow transformation is thought to be highly localized, occurring due to autocyclic flow evolution in medial to distal lobe localities; the model and quantification of flow transformation distance has important implications for estimating the spatial and stratigraphic distributions of heterogeneities/reservoir quality for such beds in deep-marine lobe deposits, which in many areas form important hydrocarbon reservoirs, and for interpreting the significance of these deposits in core and outcrop datasets.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015