Lang, Simon C.1, Tobias H.D. Payenberg2, Mark Reilly2, Jochen
(1) University of Adelaide, South Australia, Australia
(2) Australian School of Petroleum, Univeristy of Adelaide, Adelaide, Australia
(3) Whistler Research, Brisbane, Australia
ABSTRACT: Reservoir Analogues of Ephemeral Lacustrine Deltas and Terminal Splays - Examples from the Lake Eyre Basin, Central Australia
Ephemeral fluvial-lacustrine sandstone reservoirs (eg. North Sea or Algerian Triassic, South Caspian Pliocene) can benefit from studies of modern ephemeral depositional analogues. This study focuses on the variation in depositional processes, scale, planform geometries and potential connectivity of sandy facies around Lake Eyre, the terminus of a vast inland drainage system (> 1 Million km2) in arid central Australia. A spectrum of high or low river flow into high or low lake level, results in several fluvial channels feeding terminal splay complexes interdigitated with rare shallow lacustrine deltas. Wave reworking occurs during highstands, but aeolian deflation dominates the lowstands during prolonged aridity.
The ephemeral river channels are typically coarse-grained, <5m deep (W/T ratios of 250:1), and flow through incised, low-sinuosity channel belts, before entering the terminal splay or lacustrine delta complex . Mixed-load rivers typically bifurcate into numerous straight distributary channels with smaller digitate sandy crevasse splay channel branches (W/T ratios 40:1 to 80:1) feeding discrete fine-grained frontal splay/mouthbar lobes. The lobes are <1m thick, comprising fining-upward successions of medium to fine-grained sand, with parallel lamination, convex-upward parallel stratification, and climbing ripples (W/T ratios of 300:1 to 1000:1). In contrast, coarser-grained rivers construct an amalgamated frontal splay complex with only a few shallow, coarse-grained distributary channels (W/T ratios 50:1 to 100:1), and a distal frontal splay complex comprising a thin apron (<0.5m) of coarse to medium-grained sands (W/T ratios >1000). These reservoir analogues may be useful for developing reservoir models in low-accommodation, high or low net-to-gross ancient ephemeral fluvial systems.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.