THE INFLUENCE OF STRUCTURAL GROWTH AND OTHER FACTORS ON THE CONFIGURATION OF FLUVIATILE SANDSTONES, PERMIAN COOPER BASIN

Abstract

The term structural growth is applied to subtle earth movements contemporaneous with sediment accumulation. To assess this factor, one must consider the roles of lateral sandstone accretion, river avulsion and sediment dispersal in the context of basin setting, climate and geological time.

Interpretation of stratigraphic relationships and isopach and structural maps of areas comprising structural highs in the subsurface of the Permian Cooper Basin is based on the correlation of thin chronostratigraphic units in the Patchawarra Formation (Tirrawarra Field) and the Toolachee Formation (Big Lake-Moomba fields). Unit C of the Toolachee Formation in the Moomba area comprises several fining upwards sequences that appear cyclic and sheet-like. This facies relationship is similar to those in the immediate downflank areas of Big Lake. However, upflank in crestal areas, fluviatile sequences are more typical of multistorey sandstones. These crestal sandstones are the result of structural growth contemporaneous with deposition from a fluviatile meandering system during a significant interval of geological time. Meandering river systems are considered to have crossed the Big Lake-Moomba area locally from south-west to north-east. Isopach and percentage sandstone maps indicate that streams preferred an avenue of transport towards the Nappamerri Trough.

Stratigraphic analysis of Patchawarra Formation Unit 2B in the Tirrawarra Field suggests a slightly greater rate of local subsidence there than in Big Lake and Moomba fields, resulting in a decreasing sand body interconnection. Although occasional fining upward sequences correlate across the field area, multistorey sandstone development occurs in an arcuate belt on its western side. This is the result of local structural growth and faulting contemporaneous with fluviatile deposition. Potential petroleum reservoirs of fluviatile origin occur in numerous basins worldwide. In basins exhibiting low rates of differential subsidence, potential reservoirs and seals occur as sheet-like bodies in structural traps near the margins of major depocentres. Knowledge of local and basinal differential subsidence and fluviatile architecture provides confidence for field exploitation and exploration for new plays.