FRACTURE FORMATION AND FLUID FLOW IN THE PALM VALLEY GAS FIELD, CENTRAL AUSTRALIA

Abstract

Palaeo-fluid flow in the fracture network in the Palm Valley gas field (Amadeus Basin, central Australia) was investigated using fluid inclusion, isotopic and petrographic methods. The Ordovician Pacoota and Stairway Sandstone reservoir rocks have exceedingly low matrix porosity and permeability and economic gas flow rates, therefore, depend on the fracture network.

Pre-fracture cementation of the matrix involved precipitation of pyrite, haematite, chlorite, illite and quartz. However, matrix cementation, as well as the fracture mineralisation, is now dominated by barite, ankerite and quartz. This indicates that subsequent to being fractured, connectivity between matrix porosity and fractures allowed invasion of the host sandstones by mineralising fluids from the fracture network. Fluid inclusion palaeo-temperature analyses indicate temperatures of 90–115°C prevailed at the time of formation of these minerals which was contemporaneous with maximum burial estimated to have occurred during the Alice Springs orogeny at ~340–240 Ma.

Aqueous fluids in the sandstones were derived from three sources. Connate waters comprise one source and were parental to pre-fracture diagenetic minerals. The reservoir was accessed by two other fluids via the fracture network. Basinal brines comprise one source, whilst low salinity waters of surface meteoric origin comprise the other. One component of the basinal brine had had prior contact with Precambrian Bitter Springs Formation evaporites whilst another had been in contact with rocks characterised by high barium contents and radiogenic strontium isotope ratios. The total vertical component of fluid flow appears to have been ~7–8 km.

Hydrocarbon migration was in part synchronous with fracture development and was accompanied by migration of basinal brines. Liquid hydrocarbons and wet gas migrated during cementation of the fractures. Temperatures continued to rise and dry gas was generated which displaced the wet gas now only observed in fluid inclusions in the mineral cements.