Palaeo-formation water evolution in the Latrobe aquifer, Gippsland Basin, south-eastern Australia continental shelf

摘要

Offshore fresh or brackish groundwater has been observed around the globe and represents an interesting but unusual freshwater reserve. Formation waters in sedimentary basins evolve at geological time through fluid-rock interactions and water movements in aquifers. However, the mechanism and timing of freshwater displacing and mixing with pre-existing formation water offshore under the seafloor has not been investigated in many cases. The growing need for developing freshwater resources in deeper parts of sedimentary basins that have not been economic or technically feasible in the past, may potentially lead to an increasing conflict with petroleum production or injection of carbon dioxide. For being able to assess and mitigate possible impacts of fluid production or injection on groundwater flow and quality, a better understanding of the natural history of the interaction between fresh meteoric water and deep basin formation water is necessary. A low-salinity wedge of meteoric origin with less than 5000ppm currently extends to about 20km offshore in the confined Latrobe aquifer in the Gippsland Basin (Australia). The Latrobe aquifer is a freshwater resource in the onshore, hosts major petroleum reservoirs and has been considered for carbon dioxide storage in the offshore parts of the basin. The objective of this study is to constrain the evolution of formation water in the Latrobe aquifer by investigating the water naturally trapped in fluid inclusions during burial. The measured palaeo-salinities from onshore and offshore rock samples have a minimum of about 12500ppm (NaCl equivalent) and a maximum of about 50000ppm. Most of the salinities are in the 32000-35000ppm range. There is no evidence for freshwater in fluid inclusions and the variation in palaeo-salinity across the basin is consistent with the palaeogeography of deposition of the sedimentary rocks. The current low-salinity water wedge must have started to form recently after most of the diagenetic processes that led to the trapping of water in fluid inclusions happened. The minimum homogenisation temperatures (T-h) recorded are consistent with current formation temperature. However, they are generally higher than present day suggesting that hotter temperatures were attained in the past. The T-h and salinity data together suggest that the fluid inclusions record the diagenetic modification of connate water to higher salinities over a time period that was accompanied by an increase in temperature, consistent with a westward palaeo-fluid flow from the deeper part of the basin through the aquifer. Subsequent pore-water evolution from palaeo- to current day conditions is consistent with an influx of fresher and cooler meteoric water into the Latrobe Group. The meteoric recharge originates from the area of the Baragwanath anticline in the onshore part of the basin where the Latrobe Group subcrops at high elevations.