How do CO₂-content, reservoir heterogeneity and fluid composition affect CO₂-induced diagenesis?

Abstract

The Pretty Hill Formation in the Otway Basin provides an exceptional example of a natural analogue for CO2 storage and has been the focus of this study. Results demonstrate differences in CO2 -water-rock reactions related to reservoir/ baffle heterogeneities and fluid composition, and clearly illustrate sequential changes in the degree of CO2- related diagenesis associated with present-day CO2-content. Samples from low-C02 sites « 1 % CO2 ) display little mineralogical alteration; kaolin minerals are rare with chlorite and illite as the dominant clay minerals. Samples from an intermediate-C02 site (c. 50% CO2 ) show significant CO2-related reactions have occurred and that kaolin minerals are the dominant clay. HyloggerTM data highlight variations in clay mineralogy associated with changes in lithology, facies and bedding, and also show a change from dominantly kaolin clay minerals above the free-water-Ieg to chlorite and illite below the free-water-Ieg. This illustrates differences in CO2-induced diagenesis between the gas-leg and water- leg. Samples from a high-C02 site (c. 98% CO2 ) show extreme diagenetic alteration; in the cored interval virtually all feldspar, chlorite, and illite/mica have undergone reaction, resulting in diagenetic quartz arenites with locally well developed secondary porosity. The very high CO2 concentrations at this site are comparable to those we would inject into a storage site. Results from this study provide a comparison of traditional thin section analyses, HyloggerTM data, QEMSCAN® and XRD data, while carbonate isotope analyses are used to support/challenge the model. These techniques help to better understand the effects of CO2 related to CO2 content, reservoir heterogeneity and fluid composition.