PALAEO-HYDROLOGY OF THE VULCAN SUB-BASIN: IMPLICATIONS FOR TRAP INTEGRITY

Abstract

A major fluid-flow event has been recognised in the Vulcan Sub-basin from an analysis of palaeo-formation waters trapped as fluid inclusions. These pore waters, with maximum salinities in excess of 200,000 ppm, record the migration of high-salinity brines through sandstones in the Mesozoic and Tertiary sequences. Although the source of these fluids cannot be demonstrated conclusively, Palaeozoic evaporites, occurring as both salt diapirs and bedded salt at depth, represent the only recognised source of such fluids in this area. Significantly, the distribution of high-salinity fluids cannot be reconciled through the dissolution of intruded salt diapirs alone; fault controlled injection of brine from bedded salt at depths of up to 10 km is considered to be the main source for brine migration. The absence of these hyper- saline fluids In samples taken from intact hydrocarbon columns indicates the flow of brine occurred subsequent to hydrocarbon charge. Further, the presence of high salinities in samples from recognised residual oil zones suggests that breaching of these traps facilitated the ingress of high-salinity brines, thus making the brines a marker for fault reactivation. Renewed salt diapirism and fault reactivation during Mio-Pliocene collision of the Australian and Eurasian plates is seen as the trigger for brine flow. The geographically dispersed location of wells containing high-salinity fluids argues for an areally extensive bedded salt layer that may play a very significant role in controlling the style and distribution of fault reactivation. Thermal anomalies associated with the migration of hot brines are recognised in some wells and while substantial on a localised scale, they may not greatly influence the overall thermal state of the basin. Nonetheless examples of convective heating do appear to characterise traps located close to potential up-flow zones, while convective cooling may affect traps that are distal from injection points, with downward-flow of brine acting as a heat sink. However, perturbation of the current thermal and salinity conditions is generally not apparent, probably reflecting the waning of this fluid-flow event Importantly, conclusions about trap integrity derived from these palaeo-salinity data complement previously published predictive seal integrity models allowing such models to be applied with greater confidence.