Differentiating between carbonate formed via different mechanisms, and determining controls on the extent of authigenic carbonate formation, could lead to options for engineered accelerated mineralisation in reservoirs.
This work recognises authigenic carbonates as a natural analogue of mineralisation trapping. It seeks to understand control mechanisms for their formation in low salinity, siliciclastic aquifers of the Great Artesian Basin (GAB).
More than 250 well completion reports were selected, from among tens of thousands of publically available petroleum, coal seam gas and stratigraphic drilling records, on the basis of spatial and geological coverage, the detail of included information, and type and availability of associated samples. The well completion reports were assessed in detail for the presence of significant carbonate mineralisation, and samples of carbonate cemented sandstone, as well as carbonate fracture mineralisation, were taken from some 50 localities. All Mesozoic units within the chosen wells were subject to sampling, whether the strong carbonate cement was sporadic or extensive. The samples included both chipped and cored intervals.
Key parameters derived from petrological and geochemical analyses of the carbonates were fed into a model for carbonate authigenesis within the GAB Laboratory experiments were undertaken to explore processes of enhanced carbonate mineral trapping of CO2 in Precipice and Hutton sandstone core.
The study concluded:
- The oxygen isotope values and fluid inclusion data for GAB cement and vein carbonates indicate that deeper, hotter fluids mixed with shallower and fresher fluids during, or just prior to, carbonate precipitation in a number of cases.
- Most of the modelled fluid carbon isotope compositions are indicative of remobilised marine carbonate or mantle/ magmatic CO2, whereas a subset of carbonate samples had very negative modelled carbon isotopes consistent with an organic carbon source.
- Fault-assisted fluid migration apparently played a major role in the carbonate authigenesis, and a subset of samples was associated with hydrocarbon migration.
- Elevated fluid inclusion temperatures ≥120°C, in samples from wells located within the Moonie-Goondiwindi and Leichardt-Burunga fault corridor in the eastern Surat, are anomalously high relative to what is known about the regional thermal history.
- Gaseous hydrocarbons found in fluid inclusions in the Eromanga Basin samples are sourced from the underlying Cooper Basin.
- The availability of cations for precipitation of dissolved CO2 as carbonate minerals can be a rate limiting step in the process of CO2 mineral trapping. Engineering injection, to take advantage of CO2 migration paths (e.g. injecting below baffle units and down-dip from a structural closure), is one way to maximise carbonic acid dissolution of minerals encountered by the CO2-water mixing front.
- Co-injecting CO2 dissolved in brine or other waste water would increase the available cations for carbonate precipitation as would co-injecting a small quantity of SO2 to form dilute sulphurous or sulphuric acid at specific depths, e.g. below baffle units.