CARBON DIOXIDE AND CARBONATE CEMENTS IN THE OTWAY BASIN: IMPLICATIONS FOR GEOLOGICAL STORAGE OF CARBON DIOXIDE

Abstract

Understanding CO2 source and carbonate cements in natural gas accumulations is important for predicting the behaviour of anthropogenic CO2 in a reservoir system. The Otway Basin offers an excellent opportunity to examine late CO2-derived cements as an analogue for mineralogical storage of CO2. Understanding Otway Basin diagenesis and carbonate cement distribution is also of great significance to petroleum production in the region.

Elemental and textural examination of Otway Basin cements has identified five carbonates in reservoir rock from CO2-rich gas accumulations. These carbonates show an overall increase in Fe2+ and Mg2+ relative to the calcites in CO2-free reservoir rock, indicating cation derivation from CO2 interaction with labile minerals.

δ13C isotopes of 2.18‰ to -6.7‰ PDB from late carbonate cements in reservoirs containing CO2, confirm an inorganic CO2 origin. 3He/4He gas isotopic ratios of R/Ra > 1 indicate a predominantly mantle input for the CO2-rich accumulations.

Degassing of magma associated with Pleistocene to Recent volcanics is suggested as the dominant, CO2 source for the existing CO2 accumulations. CO2 influx from the magmatic source was rapid, and is the most analogous scenario to injection of anthropogenic CO2. Natural influx of CO2 and the opportunity for mineralisation of CO2 is variable, with CO2 dissolving some original carbonate and precipitation dependant on pH, ρCO2, and available cations. Positive mineralogical CO2 storage occurs in the Pretty Hill Formation, due to a higher content of labile lithic minerals, with ~36 kg/m3 of CO2 (~48 kg/m3 carbonate) stored in the Ladbroke Grove Field from the current CO2 phase. The Waarre Sandstone has negative mineralogical storage of CO2, with less carbonate than similar reservoir rock without CO2, and therefore more CO2 being released from dissolution of early carbonates.