The Upper Cretaceous Paaratte Formation, Otway Basin, Australia, is a deltaic reservoir-seal succession and currently the target for field-scale CO2 injection research experiments of the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) (now CO2CRC Ltd.) Otway Project. A relatively homogeneous ∼7-m-thick injection interval was identified for stage 2 CO2 injection experiments by analysis of wire-line and core log data. Core from above, below, and through the injection zone was sampled to quantify the effects of CO2 storage on physical and chemical heterogeneities to determine storage efficiency.

The injection interval consists of two distinct yet related facies associations of a marine tract of an upper-deltaic, depositional environment. They exhibit broadly similar bulk petrophysical reservoir properties, though their mineralogy and fine-scale sedimentary fabrics differ. A dolomitic horizon above the reservoir constitutes an intraformational barrier.

Early diagenetic alterations (ca. 80 Ma) occurred within distributary delta channel and mouth-bar sandstones. These alterations include formation of early authigenic clays that inhibited quartz overgrowth development while simultaneously preserving intergranular porosity.

Primary dolomitic cement was precipitated from anoxic organic fluids passing through proximal, mouth-bar sandstones, along with processes such as quartz dissolution and minor kaolin precipitation until maximum burial (ca. 40 Ma). Mineralogical markers constrained the position (proximal-to-distal and lateral) within the deltaic depositional tract, and low-resolution sedimentation rates derived from palynological dating provided information to estimate the spatial scale of delta facies. Clarification of the lithological heterogeneity within the injection zone has helped to determine CO2 storage efficiency by establishing an in-depth assessment of sequence stratigraphic and depositional facies architecture.

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