MODERN FLUVIAL-LACUSTRINE RESERVOIR ANALOGUES FROM LAKE EYRE, CENTRAL AUSTRALIA

Simon C. Lang¹, Tobias H.D. Payenberg¹, Mark R.W. Reilly¹, and Jochen Kassan^1,2
1 Australian School of Petroleum (ASP) and Australian Petroleum Cooperative Research Centre (APCRC), University of Adelaide, Adelaide, SA 5005, Australia
¹ Whistler Research Pty Ltd, Whistler Court, Greenbank, QLD 4124, Australia

Modern ephemeral fluvial-lacustrine depositional environments offer useful analogues for ancient reservoirs deposited in comparable successions (eg. the Triassic of the North Sea and the Berkine Basin Algeria, and the Pliocene of the south Caspian Sea). The ephemeral inland rivers of the intracratonic Lake Eyre basin, Central Australia provide a field laboratory of natural analogues that provide data on the variable scales, geometries and potential connectivity of sandy fluvial-lacustrine delta and terminal splay facies. This study focuses on the variation in depositional processes, fluvial style and sandstone composition that result from several major ephemeral rivers entering Lake Eyre, and the implications for improving our understanding of ancient comparable reservoirs.

The significant rivers that enter Lake Eyre include the Cooper, Diamantina, Warburton-Kalaweerina, Macumba, Neales, Umbum and Frome systems, and these drain over a million square kilometres of generally low relief subtropical dryland savannah and arid to hyper-arid desert environments. The rivers are normally characterised by variable seasonal discharge into a large arid playa lake. Cyclonic monsoonal rainfall in the upper catchments particularly of the Cooper and Diamantina systems result in rare major lake fillings (e.g. 1974), but normally smaller flood events are typically out-of-phase with the lake level, resulting in the construction of terminal splays that only rarely function as deltas building into standing lake water. Sedimentation around the lake fluctuates from deposition through a spectrum of high or low river flow in high or low lake levels, punctuated by either short periods of wave erosion around the lake shoreline or more prolonged periods of aeolian deflation.

The western rivers (Neales and Umbum systems) have been the main focus of sedimentological study because they have built accessible fluvial-dominated terminal splay complexes incised into a much larger Pleistocene low-gradient geomorphic fan including several highstand lacustrine and falling-stage fluvial deposits. The active rivers lie approximately 50 km apart at the lake shoreline, but satellite imagery indicate a bifurcating network of older fluvial channel belts all pinned structurally at the fan apex lying approximately 50km from the lake shoreline.

Both the Neales and the Umbum flow through incised, relatively straight channel belts with a range of low to high sinuosity channel planform geometries. Both rivers comprise a coarse-grained sandy meandering fluvial channel belt (W/T ratios of 250:1), but the differing sediment type carried by the rivers results in contrasting styles of terminal splays. The Neales contains a mixed load of gravel, sand and mud derived from a large provenance area, and it typically bifurcates into numerous straight distributary channels typically 1– 5m deep, with smaller branches formed by active sandy crevasse splay channels (W/T ratios range from 40:1 to 80:1). In contrast the Umbum has a smaller but steeper provenance area, with only one major coarse-grained fluvial channel (<5 m deep) feeding a coarse-grained amalgamated frontal splay complex with only a few significant, shallow distributary channels (W/T ratios of 50:1 to over 100:1). Both systems have splay lobes (W/T ratios of 300:1 to 1000:1 and typically <1m thick), and these dominate the splay-plain and comprise medium to fine-grained sand, with parallel lamination, convex-upward parallel stratification, and climbing ripples. The distal frontal splays comprises an extensive apron of thin (<0.5m), fine-grained sandy mouthbars cleaned-up by aeolian processes (W/T ratios >1000), prograding across dark brown or bluish transgressive lacustrine prodelta clay.

GPR profiling in the Umbum system across the relict interdistributary plain (characterised by a surficial gibber-plain lag), shows several downlapping reflectors, erosional surfaces and distributary channels >2m deep and 10m wide, overlying an extensive, amalgamated sandsheet with interbedded shale-prone intervals. These indicate that important potential barriers to fluid flow may occur in an otherwise high net to gross succession.

Despite the overall low accommodation setting, the more variable net to gross of the Neales system compared to the high net to gross of the Umbum system highlights the importance of differing provenances yielding muddy versus sandy/gravelly sediments during flood episodes. The lateral switching of these systems though avulsion near the fan apex will result in an interdigitation of these differing styles of sedimentation, provided that aeolian processes do not rework completely the surface. These reservoir analogues may be useful for developing reservoir models in both high and low net to gross ancient ephemeral fluvial systems.