Salinity of deep formation water in the Canning Basin, Western Australia

Salinity of deep formation water in the Canning Basin, estimated from well log data or measured on water samples from oil-production wells and drill stem tests, ranges from almost fresh water (<1000 mg/L TDS) to brines which contain >300,000 mg/L TDS. Salinity is generally below 10,000 mg/L and rarely above 100,000 mg/L, partly because most wells have been drilled near the margins of the Canning Basin or its sub-basins and/or they intersect only the top few km of sedimentary sections. Most very high salinity water (> 100,000 mg/L) was found in the south of the Canning Basin, particularly in the Willara and Kidson Sub-Basins, which were the sites of sabkhas (e.g. the Mellinjerie Limestone) and extensive deposition of evaporites (e.g. the Carribuddy Formation) in the Ordovician and Silurian. Salinity depends strongly on depth and sometimes increases from the top to the base of a stratigraphic unit or group of units before abruptly decreasing at the top of the underlying unit and then rising again. This type of pattern is most evident where the stratigraphic units are relatively thick, and the profiles are best preserved in low to moderate permeability sediments located in present-day low recharge areas of the basin. The pattern is probably formed by stacking of a series of palaeo-salinity profiles, produced during a marine-continental or other depositional cycle in which the relatively high-salinity marine or non-marine water in the upper parts of the aquifer was partly replaced by low-salinity terrigenous groundwater. Average salinity of individual stratigraphic units within the basin increases linearly with increasing depth of burial, which suggests that the more saline deeper water interacts with the nearer-surface meteoric systems, probably via molecular diffusion of salt. Salinity is abnormally high in those parts of the Grant Group where re-solution of evaporites from the underlying Carribuddy Formation can occur, and in the low-permeability Mellinjerie Limestone where original highly saline pore water is retained and/or there is dissolution of associated evaporites. Higher than expected salinity also occurs in the Devonian Reef Complex and the Nita Formation, where water may contain remnants of palaeo-brines which entered the sediments before their permeability was reduced by cementation and compaction.