Permeability Heterogeneities Within Modern Ephemeral Fluvial Deposits: Examples from Cooper Creek, Southwest Queensland, Australia

Alan T. McInally, Gary J. Nichols

Sand deposits along dry river beds of Cooper Creek, Southwest Queensland, Australia, have been investigated for use in reservoir analogue studies. Cooper Creek is a large scale, internally drained, ephemeral, anastomosing fluvial system situated in Australia's semi-arid interior. Flow within the river is controlled by monsoonal incursions into the Cooper's catchment, with large floods occuring on a 10-20 year time scale. Sand deposition occurs predominantly in low width to depth ratio channels lying within a mud dominated floodplain. The erratic and transitory flow regime of the river produces permeability heterogeneities within the sand deposits of the river, these are:

1. Individual cross-strata show crude coarse/fine grainsize alternations at a centimetre scale between individual avalanche laminae.

2. Reactivation surfaces are common and are marked by drapes of fine sand and mud.

3. Remnant bar surfaces are also marked by drapes of fine sand and mud which become indurated during subaerial exposure. The induration prevents reworking and later "cleaning" of the sediments.

4. Deposition of mud from suspension and infiltration from standing water, following flood events, can produce 2-3 cm mud deposits across the channel which thicken towards the thalweg.

5. Sand deposits occur in discrete, laterally stable channels within a mud dominated floodplain.

The ephemeral flow regime on Cooper Creek therefore leads to permeability compartmentalisation within the sand deposits at a relatively small scale, and may explain poor hydrocarbon production rates from ephemeral fluvial reservoirs.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995