Bar-scale alluvial geometry : controlling parameters, identification in the rock record and implications for reservoir modelling

• Main Author: Holzweber, Barbara Isabella
• Published: University of Aberdeen 2012
• Subjects: 551
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.571685

Observations from modern day rivers are linked to the rock record and the implications for reservoir modelling of fluvial systems are investigated. In order to investigate the geomorphology of modern day fluvial systems, scale invariance and bar dimensions were evaluated. To provide a link between modern analogues and ancient deposits, an outcrop of the Kimmeridgian Salt Wash Member (Morrison Formation) in south-central Utah was described in detail, focusing on lithofacies distributions and dimensions of fluvial deposits in the rock record. The outcrop illustrates the difficulties in distinguishing between different planform geometries in the rock record. Collected data include detailed outcrop descriptions, sedimentary logs, GPS (Global Positioning System) measurements and a LiDAR (Light Detection And Ranging) survey. The interpretation of the different fluvial facies types was imported into the modelling software Paradigm GOCAD®. The resulting threedimensional digital outcrop model of the sedimentary facies was used as a training image to generate geologically realistic heterogeneities within three-dimensional models of fluvial systems, focusing on the small scale intra-channel and intra-bar heterogeneities. This study shows that bar dimensions are scale invariant and independent of planform geometry, climate, tectonic setting and gradient. Each river has the potential to create a range of planform geometries as well as facies assemblies. And without exposure in planview, it is not possible to confidently define a rock record fluvial deposit as braided or meandering. Irrespective of channel planfom, bars are the key architectural element that is primarily preserved in the rock record. Stacked bar complexes are bounded by accretion surfaces that are likely to represent the main barriers to fluid flow within fluvial channel deposits. Consequently, to increase the accuracy of fluvial reservoir models it is important to populate any channel element with accurate bar dimensions and associated bounding surfaces.