Fracture-fillinng cements in the palaeozoic Warburton Basin, South Australia

In the Cambro-Ordovician Warburton Basin, South Australia, reservoir rock porosity and permeability are enhanced by open fractures and by partially-open or incompletely mineral-filled fractures. However in many cases, the fractures are completely filled by cements of several types. A knowledge of the nature and origin of the fracture-filling cements can help to identify areas where cement precipitation is relatively less intense, and where oil and gas production may be improved. In the present study, fluid inclusion microthermometry, and stable isotope and electron microprobe analyses were used to indicate the nature and timing of fracture-filling cements. The study shows that fracture-filling cements in the studied Warburton Basin wells include quartz, siderite, ankerite, kaolin and pyrite. Based upon precipitation temperatures, stable isotope compositions, elemental compositions and quartz cement textures, a hydrothermal source for the silica and carbonate cements is envisaged. The hydrothermal activity may be related to granite intrusions during the Carboniferous. Fluid inclusion microthermometry suggest that Warburton Basin rocks in some areas have experienced temperatures of more than 260°C and cannot therefore be a source of hydrocarbons. The observed low permeabilities occur because most fractures were completely or partially filled by cements precipitated during hydrothermal activity soon after granite intrusion. There is a greater likelihood of finding reservoir rocks with open fractures in areas far from the granite intrusions.