ONSHORE OTWAY BASIN CARBON DIOXIDE ACCUMULATIONS: CO2-INDUCED DIAGENESIS IN NATURAL ANALOGUES FOR UNDERGROUND STORAGE OF GREENHOUSE GAS

Abstract

The study of natural carbon dioxide (CO2) accumulations, such as those found in the onshore Otway Basin, is necessary for the validation of underground long-term storage technology as an option for decreasing greenhouse gas emissions.

The investigation of natural CO2 occurrences is being investigated as part of the Geological Disposal of Carbon Dioxide (GEODISC) research program. This study identifies the effects of CO2 on reservoir rock’s mineralogy through time as well as its porosity and permeability. The Otway Basin CO2 accumulations display variations in reservoir type, CO2 concentration and time of injection. A range of typical reservoirs types for the CO2 accumulations occurs in the Otway Basin, including feldspathic litharenites, subfeldsarenite and quartz arenite. CO2 concentrations in the Otway Basin vary greatly in the accumulations studied, ranging from 10 mol% within the Port Campbell Field to 99 mol % in the Caroline Field. The source of the CO2 is degassing of the deep-seated magmas of the Newer Volcanics, with CO2 influx occurring between ~2 million to as recently as 5,000 years ago. This study investigated three areas of the Otway Basin;

Penola Trough—Ladbroke Grove, Katnook (non-CO2)

Port Campbell Embayment—Boggy Creek, Langley, Port Campbell; and

Gambier Sub-Basin—Caroline

Due to their close proximity and similar geological history prior to CO2 influx, the Ladbroke Grove-Katnook gas accumulations are particularly useful for examining differences between a CO2-rich (Ladbroke Grove) and a CO2-absent field (Katnook) and for developing a post- CO2 diagenetic history. Variation in grain size and CO2 concentration affects the degree of reaction of CO2 with the reservoir rock. Petrology and formation water chemistry of these fields indicate that CO2 has modified the rock properties. In all CO2-rich reservoirs examined (>10 mol % CO2), dissolution and alteration of lithic and felsic framework grains has occurred (e.g. albite dissolution). Clays and cements throughout most of the Otway Basin CO2 accumulations are modified to minerals more stable in the changed gas compositions (e.g. chlorite to kaolin). The change in mineralogy after the recent CO2 influx shows that the Pretty Hill Formation with high amounts of reactive minerals and smaller grain size is an effective reservoir unit for mineral storage of CO2. Longterm storage in the Waarre Sandstone quartz-rich reservoirs also displays the effectiveness of CO2 storage in pore space.

This study of natural accumulations of CO2 has demonstrated that geological storage of CO2 is a viable option. Understanding of the mineral reactions involved with CO2 in reservoir rock is vital for selection of storage sites and modelling the behaviour of CO2 in the subsurface.