The Pretty Hill Formation as a natural analogue for CO 2 storage; An investigation of mineralogical and isotopic changes associated with sandstones exposed to low, intermediate and high CO 2 concentrations over geological time This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.A C C E P T E D M A N U S C R I P T
ACCEPTED MANUSCRIPT
ABSTRACTThe Pretty Hill Formation of the Otway Basin (Australia) has been studied as a natural analogue for geological storage of anthropogenic CO 2 in order to examine the effects that CO 2 concentration and reservoir heterogeneity have on CO 2 -related reactions. New petrographic data are presented, which validate the use of Hylogger TM as a tool to investigate high-resolution vertical changes in reservoir mineralogy. The integrated data set confirms earlier interpretations, showing that chlorite has been altered to kaolinite and siderite/ankerite in reservoir facies exposed to moderate and high CO 2 concentrations, while chlorite remains the dominant clay mineral in all parts of the formation where CO 2 content is low.Differences have been observed in the degree of CO 2 -related reaction relative to CO 2 concentration and reservoir heterogeneity. Where CO 2 content is very high (c. 98 mol%) and associated with high water saturations, both chlorite and detrital feldspars have undergone complete reaction in the reservoir facies, resulting in quartzose sandstones with a kaolinite matrix, and with siderite as the dominant carbonate precipitate. Conversely, where CO 2 content is moderate (c. 29-57 mol%) and within the gas leg of the reservoir, chlorite has undergone significant reaction, but much of the original feldspar is preserved, suggesting relatively minor reaction. Carbonate cements from the moderate CO 2 gas-leg comprise calcite, siderite and ankerite, occurring as cemented zones
A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTassociated with rock heterogeneities and the present-day gas-water contact. Heterogeneities within the gas-leg are likely to have associated pore fluid contacts, whereby relatively high water saturations will be present in the fine-grained baffles and seals. The most advanced feldspar reaction occurs locally at the contact between baffles and reservoir rock, while reactions have been significantly impeded in the finer grained units due to their low permeabilities. These studies illustrate the importance of understanding both the reservoir composition and vertical heterogeneity of potential storage systems. Fluid-mineral reactions are likely to be advanced within stacked reservoir facies and impeded within siltstone layers, while the distributi...
