A multidisciplinary study of diagenesis and pore fluid evolution in frontier basins; an example from Canterbury and Great South basins, New Zealand

Cretaceous sandstones of the Canterbury/Great South basins of New Zealand have locally undergone advanced diagenetic alteration, and this is likely to have had a significant impact on sand quality. However, data and samples are very scarce and little is known about the rock properties of these potential reservoirs. In this study we assess the degree of diagenesis by using a multidisciplinary approach to determine the timing and causes of diagenetic reactions, and potential distribution of authigenic phases. Results show that sandstone diagenesis is related to differences in the original texture and composition of the sediment, the well location (flank-crest and proximity to source rock kitchen), seal integrity, and the thermal and pore fluid histories. Detrital feldspar was the main reactant, with paragenetic observations and available microthermometry data indicating that most feldspar reactions occurred at a relatively late stage in the burial history at temperatures in excess of 100 °C. Based on 1D basin models, organic CO2 is unlikely to be the prime source of acidity for diagenesis. Instead, most feldspar reactions are interpreted to be related to magmatic CO2 flux, which is consistent with reported thermal anomalies, gas compositions within fluid inclusions, and stable isotope data from CO2 sampled in the reservoirs. The diagenesis described here has implications in terms of locally enhancing reservoir properties through the development of secondary porosity, and reducing reservoir quality by precipitation of pervasive kaolinite and, in some cases, significant illitic clay (including illitised kaolinite). Alteration of fine-grained lithologies has locally resulted in poor seal quality, which may also be related to the migration of magmatic CO2. An understanding of the distribution and timing of igneous intrusions and associated fluid migration is therefore considered fundamental for predicting reservoir and seal quality in these basins. •An integrated study of petrography and basin modelling in a frontier basin.•Authigenic kaolinite primarily formed from feldspar during burial diagenesis.•The degree of diagenetic alteration is related to the burial and pore fluid history.•Organic CO2 is unlikely to be the main source of acidity for diagenesis.•Most feldspar reactions are interpreted to be related to magmatic CO2 flux.