Facies Architecture of a Dryland River Convex Bar, Umbum Creek, Lake Eyre, Central Australia

Carmen B. Krapf¹, Tobias Payenberg², Mark Reilly³, and Simon Lang^4
1Australian School of Petroleum, Adelaide, SA, Australia
²Chevron Energy Technology Company Pty Ltd, Perth, WA, Australia
³Whistler Research, Brisbane, QLD, Australia
⁴Woodside Energy Ltd, Perth, WA, Australia

Dryland rivers are complex depositional systems which are increasingly recognised as important petroleum reservoirs. Modern analogues that quantify the facies architecture are essential data sets to aid in the development of reservoir models for comparable ancient subsurface deposits.

The Umbum Creek is a sandy ephemeral river which drains into the western side of Lake Eyre, central Australia, and displays a complex morphology typical of arid-zone rivers. In its lower reaches it occupies a relatively straight incised channel belt. Within the incised channel belt, two prominent convex bars, occurring about 5km upstream of the playa edge, have formed in response to a locally steeper fluvial gradient, where the fluvial profile is trying to equilibrate by meandering. Those 1x2 km large and approx. 5 m thick sedimentary features are overall characterised by a variety of fine- to coarse-grained facies, that are caused by fluctuations during flood events indicative for varying flow regime in dryland settings. The convex bars are composed of a combination of lateral and downstream accreting bars, simple unit bars, chute channels with associated splays, vegetated shadow bars and deep scour pools. A chute channel, which is only activated during large-flood events, represents the lateral boundary of each bar towards the channel belt margin.

Understanding the spatial occurrence and distribution of such convex bars within a dryland fluvial system in combination with a sound understanding of their facies architecture can provide new exploration strategies for reservoirs in similar setting.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas