Internal architecture of an ephemeral fluvial shadow bar from Ground Penetrating Radar Umbum Creek, Lake Eyre, South Australia
Gardiner, Nathan Paul
Honours Degree, 2006
University of Adelaide
Modern dryland fluvial environments can be used as analogues for the interpretation of many currently producing petroleum fields around the world, such as those found in the Caspian Sea, Chad Basin, North Sea and Algeria. The present day Lake Eyre Basin provides an ideal modern analogue for the investigation of dryland, fluvio-aeolian-lacustrine sedimentary deposits.
Located in an arid zone, the Umbum Creek flows into the western side of Lake Eyre in Central Australia. In its lower reaches the Umbum Creek occupies a relatively straight incised channel belt except for two incised meanders immediately downstream from a basement fault. The meanders are overprinted by a series of sandy chute channels with complex shadow bars, which vary in size from 5x5 to 50x50 metres. The shadow bars form in response to a low velocity zone created in the lee of an obstacle, dominantly vegetation.
A high resolution 30x30 metre Ground Penetrating Radar (GPR) survey was acquired over one of these shadow bars within the Umbum Creek using a Mala X3M array with a 250 MHz antenna. The resultant dataset was processed and interpreted using the commercial software package Reflex-W. The compositional architectures of the shadow bar were identified and characterised into seven GPR facies elements. These facies consisted of two linear sub-facies, a convex smiley facies, a stacked oblique facies, a stacked/prograding facies and a mounded complex facies.
Two trenches were excavated parallel to the GPR lines to allow for a ground truthing comparison of the GPR facies with the internal architectures present in outcrop. The internal facies architecture of the investigated shadow bar is characterized by multiple erosional and reactivation surfaces, discontinuous 0.5 to 5 centimetre thick mud drapes and fining upward cross-bedding sandy gravel packages.
A total of seven architectural elements were recognised in outcrop: extensive basal surfaces (BS), scour hollows (SH) and (SHc), broad based mounds (MO), lateral accretion elements (LA) and (LAs) and downstream accretions (DA). Correlations were made between the GPR facies elements and these seven architectural elements and lithofacies associations assigned for each. Following on from the characterisation of GPR facies elements in terms of their outcrop equivalents, the 3D extent of several elements were mapped. By using the lithofacies associations and assigning a hierarchical order ranking to each architecture, it was then possible to map the distribution of sediments and architectures in the shadow bar using the GPR dataset.
The characterisation of architectural elements of a shadow bar in terms of surface hierarchies, outcrop architectures and lithofacies associations describes in detail the composition of singular elements within the Umbum Creek fluvial system. Using such a detailed characterisation it is then possible to upscale the information to characterise the entire convex bar, of which the shadow bar forms one major component. Such information of a convex bar can be later incorporated into detailed reservoir models.