In-Situ, Micron-Scale Analyses of Carbonate δ13C by SIMS: A Method for Identifying Carbonate Cements That Form in Response to CO₂ Injection at the Illinois Basin Decatur Project Site

Abstract

A desirable consequence of supercritical CO₂ injection into saline reservoir sandstones at prospective sequestration sites is the precipitation of carbonate mineral cements. At the Illinois Basin Decatur Project (IBDP) demonstration site, it is estimated that at least 10-20% of the introduced CO₂ may be consumed, and thus permanently stored, via the precipitation of Fe-carbonates. The purpose of this work was a characterization of the mineralogy and isotopic fingerprints (δ13C and δ18O) of the carbonate cement generations that were present in the Mount Simon Sandstone reservoir prior to injection. Volumetrically significant abundances of carbonate cement occur locally throughout the unit, but are predominantly concentrated near the boundary with the overlying Eau Claire seal rock. Mineralogically, these cements belong to the dolomite-ankerite and the siderite to Mg-siderite solid solution series of the carbonate mineral family. As a first step towards establishing this pre-injection baseline, we developed standards and procedures employing a combination of petrographic and micro-analytical techniques (SIMS, EPMA, BSE- and CL-SEM). Recent advances in carbonate mineral analysis by secondary ion mass spectrometry (SIMS) are a particularly exciting development for future monitoring studies at sequestration sites that may seek to: 1) verify whether carbonate cements are indeed forming in response to CO₂ injection, 2) to quantify their abundance and spatial distribution and/or 3) to determine, by isotopic fingerprinting, if fracture filling cements in either the reservoir or seal rock are related to CO₂ plume migration. Here the SIMS technique can contribute uniquely as it allows for micrometer-scale measurements of δ18O and δ13C to be made in-situ from either thin sections or 1″ diameter core plugs; measured isotope values can thus be directly correlated to observable textures. An important aspect of analytical methods-advancement is the continued development of reference materials to correct for SIMS-specific sample matrix effects (currently focused on the magnesite-siderite solid solution series). We draw upon published data concerning ambient reservoir conditions at the IBDP site and the known C-isotope composition of the CO₂ that is being injected (δ13C ≈ -10‰ VPDB) to predict the probable C-isotope composition of the different possible carbonate cement types that may form within the reservoir in response to CO₂ sequestration.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016