RESERVOIR POTENTIAL OF GLACIO-FLUVIAL SANDSTONES: MERRIMELIA FORMATION, COOPER BASIN, SOUTH AUSTRALIA

Abstract

Sedimentological analysis of cored sections within the Merrimelia Formation (basal Gidgealpa Group, Cooper Basin, S.A.) reveals a complex glacigenic environment, including glacio-lacustrine, deltaic, shorezone, fluvial, aeolian and other terrestrial facies. These facies are observed within terminoglacial and proglacial environments, and interfinger constantly throughout the Merrimelia Formation, exhibiting rapid environment change related to position of the ice sheet. Detailed sedimentological and petrological analysis, suggests that Tirrawarra sandstone-type facies belongs within the Merrimelia depositional realm. Provenance data indicates that the lithic component of the Tirrawarra Sandstone, is sourced from reworked Merrimelia coeval depositional facies. A high proportion of labile grains, which are commonly observed in the Merrimelia Formation, are only observed rarely in the Tirrawarra Sandstone (sensu stricto). It is proposed that the fluvial sandstones of the Merrimelia Formation are part of the same fluvial deposystem as the Tirrawarra Sandstone (sensu stricto) and the two sandstone sequences are the natural progression of coarsening sediment as more detritus was released from melting glaciers. It is suggested that there no longer remains any sedimentological, stratigraphical or petrographical reasons why the Tirrawarra Sandstone should not be included within the Merrimelia Formation. It is further proposed that all the braided glacio- fluvial sandstones within the Merrimelia Formation and Tirrawarra Sandstone (sensu stricto) be grouped together as 'Tirrawarra-type' facies within the Merrimelia glacigenic domain.

The concept that the glacio-fluvial sandstones within the Merrimelia Formation are of the same genetic origin as the Tirrawarra Sandstone (sensu stricto), must point to the economic potential of similar sandstones found throughout the Merrimelia Formation. The thickness, sediment style and lateral extent of the proglacial braidplains is controlled by the duration of glacial melting. The Merrimelia Formation reveals a sequence of major freezing and thawing episodes with the overlying Patchawarra Formation representing the cessation of glacial influence in the Cooper Basin. Therefore, the youngest 'Tirrawarra-type' sandstone facies found at the top of the Merrimelia Formation is likely to contain the least amount of labile grains, a highly rigid siliceous framework and will be well sorted. The risk of lower reservoir quality increases down section where, with time, freezing dominated over thawing and the sediments were less rinsed, and where the resultant sandstones are more likely to be thin, more poorly sorted and chemically immature with a high proportion of labile framework grains. Thus compositional variation, controlling the diagenetic overprint, together with sandstone facies and bedform style exert profound control on the reservoir quality in the Tirrawarra-Moorari Field area, on potential gas reservoirs in the Merrimelia Formation.