Jurassic formations in the Queensland portion of the Surat Basin were used as a case study, representing prospective low‑salinity, siliciclastic geological CO2 storage reservoir systems. Geochemical investigations showed that the principle reaction pathways in low-salinity aquifers are the same as in high-salinity aquifers.
However, since more acid is formed in low-salinity water and the acid buffer capacity is low in formation water of the Surat Basin, Queensland, the formation water becomes relatively acidic, leading to a typical pH of 4. The prospective reservoir in the Surat Basin is the Precipice Sandstone, a very homogenous rock unit, largely dominated by quartz. As this mineral is hardly reactive under CO2 storage conditions, the geochemical reactivity of this unit overall is very low. Consequently, the long-term CO2 trapping capacity in the form of carbonate mineral precipitation is very low as well.
Detailed mineral analysis of units above the Precipice Sandstone revealed the Boxvale Sandstone Member may be suitable for above-reservoir monitoring purposes.
Relatively high porosity, permeability and a thickness of several meters are characteristic for the Boxvale Sandstone Member, making it a good secondary containment formation with the Evergreen Formation sealing strata above. This rock unit is also distinct in its mineral composition as it contains a large proportion of feldspar, a mineral known to dissolve relatively quickly in CO2– enriched water. This would lead to rapid changes in the water composition and could serve as an indication of CO2 leakage from the primary storage reservoir (Precipice Sandstone).
In addition, the compilation of stress field data led to a much higher data density in the Surat Basin than anything previously published and thereby reduced the uncertainty in predicting the rock mechanical response to CO2 injection and storage. Preliminary rock mechanical considerations suggest faults with strikes that are approximately at 30 degrees to the maximum horizontal stress direction will be at greatest risk of reactivating due to the fact they have the highest shear to normal stress on the fault plane.