THE GENESIS OF PERMIAN AND LOWER TRIASSIC RESERVOIR SANDSTONES DURING PHASES OF SOUTHERN COOPER BASIN DEVELOPMENT

Abstract

The genetic differentiation of Permian Lower Triassic quartz sandstones and other rock-types has elucidated more fully our understanding of sedimentary processes during the course of time in the southern Cooper Basin. Core and ditch samples analyses in conjunction with log signatures provide a strong stratigraphic tool for constructing various types of maps which help in the exploration and development of hydrocarbons in this basin. Various percentage maps using gross genetic members have delineated areas of stream sediment transport, deltaic centres and shoreline-offshore sediments. Although channel-lag to point-bar sandstone reservoirs still offer gas and liquid potential, stratigraphic delta front-shoreline sandstones exhibiting excellent reservoir characteristics are also considered to be worthwhile targets in the basin.

Various depositional models are usually more widespread than sites of local anticlines which appear to have undergone varying rates of sinking, relative to one another and also to adjacent low areas during intervals of Permian-Lower Triassic sediment accumulation. Although unconformities are probably widespread in marginal areas of the basin, deposition is considered to have been essentially continuous in parts of the Nappamerri 'synclinoria- like' low - the main depositional and structural axis of the basin. Facies analysis tends to support the belief that anticlinal complexes have never shown abrupt relief during the sediment accumulation of the Gidgealpa Group even though in some areas crestal unconformities are common. It is suggested that local 'structural growth' is genetically related to the differential downwarping of the basin. Episodes of greater mobility of the basin rims are indicated by both unconformities and facies relationships. It is apparent, however, that facies patterns indicate only local entry of sediment transport into the basin particularly in its southern portions. During some phases of sediment accumulation climate may have blurred the larger-scale earth movements. It is suggested by this local study that similar genetic models and mapping techniques could be applied to other Permian basins in the region.