PARAGLACIAL AEOLIANITES: POTENTIAL NEW HYDROCARBON RESERVOIRS, GIDGEALPA GROUP, SOUTHERN COOPER BASIN

Abstract

Facies analysis of core from the Gidgealpa Group has led to the first recognition of sandstones of aeolian origin in the Cooper Basin. The aeolian suite was recognised in core from the Merrimelia Formation penetrated by wells within the Merrimelia field.

The Merrimelia 5 aeolianites occur in core between 8603 and 8659 ft drill depth. These were correlated to similar facies in core from Merrimelia 1 between 9649 and 9674 ft drill depth, where other lithotypes of the suite were identified.

Gamma ray-sonic log response over cored aeolianite intervals can be correlated to Merrimelia 13, where similar aeolian sediments are interpreted for an uncored interval of Merrimelia Formation.

The overall glacigenic nature of the Merrimelia Formation is well documented but this discovery records the first aeolian suite in the Gidgealpa Group and may also document the first subsurface example of cold-climate aeolianites.

The aeolianites are porous, poorly cemented, coarse and fine grained sandstones dominated by parallel lamination. Sharp, planar truncation surfaces divide the suite into units composed mainly of translatent wind ripple lamination with minor amounts of sandflow bedding. Primary depositional dip (post-compaction) varies between 0° and 25° suggesting preservation of dune lee-slope strata. Local deformation of avalanche foresets is also visible in the core.

The gamma ray-sonic logs of the thick (1190 ft (363 m)) Merrimelia Formation in Merrimelia 5 indicate thick (760 ft (232 m)) porous sandstones. These originate from distal aeolian and distal sandy braidplain environments. In core from Merrimelia 1, aeolianites are interbedded with proximal outwash fan conglomerates. Also gamma ray-sonic logs of the interval indicate thinning of the aeolianite sands. These two observations indicate a nearby depositional edge to the aeolianites.

A low interval transit time on sonic logs is characteristic of the porous aeolianite sand. This response produces an identifiable change in seismic reflections where the sands are developed, which allows mapping of the sand distribution from seismic data.

A 6 to 8-km wide band of aeolianite sand facies trending southeast — northwest has been mapped. This porous aeolianite sand facies is interpreted to pass laterally into outwash fan, braidplain and interdune deposits.

Potential hydrocarbon traps may occur at the top of the aeolianites, or within them beneath intraformational seals formed by muddy interdune facies. The recognition of an aeolian suite at Merrimelia indicates the potential for similar facies development elsewhere in the southern Cooper Basin. Locally these could form important reservoirs beneath the level of existing production.