In situ δ13C and δ18O microanalysis by SIMS: A method for characterizing the carbonate components of natural and engineered CO2-reservoirs

Abstract

This work addresses the potential utility of in situ carbon and oxygen isotope microanalysis (δ13C and δ18O) by secondary ion mass spectrometry (SIMS) in carbon sequestration research. A desirable long-term consequence of CO2-injection into underground rock formations at prospective sequestration sites (such as deep saline sandstone aquifers capped by impermeable strata) is the precipitation of carbonate mineral cements, the isotopic fingerprinting of which is a central theme here. More specifically, we focus on the unique advantage of the SIMS technique, which lies in the capability of analyzing very small sample volumes that are otherwise inaccessible to sampling techniques in conventional isotope ratio mass spectrometry (IRMS). For example, single carbonate crystallites as small as 3–10μm across can be readily analyzed by SIMS with sub per-mil (‰) accuracy and precision. Importantly, the ability to perform micrometer-scale measurements in situ from either thin sections or 1-in. (25mm) diameter polished core plugs preserves the petrographic context of measured carbonate δ18O and δ13C values.We provide a preliminary characterization of the pre-injection mineralogy and isotopic fingerprints of carbonate cements in the Mount Simon Sandstone reservoir and the overlying silty-shaly caprock (the Eau Claire Formation) at the Illinois Basin Decatur Project, a demonstration and research site for exploring the feasibility of long-term CO2 storage in a deep saline aquifer. By drawing upon published data on ambient reservoir conditions and the isotopic composition of the injected CO2, we make simple predictions regarding possible δ13C values of calcite, dolomite-ankerite, and siderite cements that may form in response to long-term CO2 storage.