FLANK PLAYS AND FAULTED BASEMENT: NEW DIRECTIONS FOR THE COOPER BASIN

Abstract

The Moomba and Big Lake Gas Field area has been actively explored for 25 years. However, recent drilling and field studies have identified new reservoir objectives for appraisal of established fields and for exploration in wildcat areas. Cooper Basin reserves have been increased and further additions are likely. Integration of drilling, production and pressure data for the Moomba and Big Lake Fields has resulted in the discovery of a structural-stratigraphic trap on the south-west flank of the Moomba Dome. Moomba-65 flowed gas at 9.8 MMCFD (0.27 Mm3/d) from deltaic sandstone of the Epsilon Formation (Early Permian). Similar plays are likely to be found on the flanks of other Cooper Basin fields and will become increasingly important as opportunities for conventional crestal tests of anticlines diminish.

Exploration to the south-west of the Moomba Field has established the first significant gas flows from rocks beneath the conventional reservoirs of the Cooper Basin. Lycosa-1 drilled a faulted anticline and achieved a maximum gas flow of 5.0 MMCFD (0.14 Mm3/d) from fractured metasiltstone of the Dullingari Group (Ordovician). Moo- lalla-1 drilled a low-side fault terrace and flowed gas at 9.6 MMCFD (0.27 Mm3/d) from 'protoquartzite' tentatively assigned to the Dullingari Group. Consequently, structures where 'basement' reservoirs are faulted against mature Patchawarra Formation source rocks are attractive exploration targets.

Petrological studies have identified 'glauconitic illite' in the Cooper Basin sequence suggesting hitherto unrecognised marine conditions. A reassessment of the source and reservoir potential of the region will be necessary if the presence of marine environments is substantiated by further studies.