Microanalysis of carbonate cement delta O-18 in a CO2-storage system seal: Insights into the diagenetic history of the Eau Claire Formation (Upper Cambrian), Illinois Basin

摘要

Oxygen isotope (delta O-18) zonation in carbonate mineral cements is often employed as a proxy record (typically with millimeter-scale resolution) of changing temperature regimes during different stages of sediment diagenesis. Recent advances in secondary ion mass spectrometry allow for highly precise and accurate determinations of cement delta O-18 values to be made in situ on a micrometer scale, thus significantly increasing the spatial resolution available to studies of diagenesis in sandstone-shale and carbonate systems. Chemo-isotopically zoned dolomite-ankerite cements within shaly sandstone beds of the predominantly silty-shaly Eau Claire Formation (Cambrian, Illinois Basin) were investigated, revealing the following: with increasing depth of burial (from <0.5 to similar to 2 km [<1500 to 6500 ft]), cement delta O-18 values decrease from a high of approximately 24 parts per thousand down to approximately 14 parts per thousand (on the Vienna standard mean ocean water [VSMOW] scale, equivalent to -6.5 parts per thousand to -16.5 parts per thousand on the Vienna Peedee belemnite [VPDB] scale). The observed cross-basin trend is largely consistent with cements having formed in response to progressive sediment burial and heating. Within the context of independent burial and thermal history models for the Illinois Basin, cementation began soon after deposition and continued intermittently into the mid-Permian. However, temperatures in excess of burial model predictions are inferred at the time of latest ankerite cement precipitation, which we propose overlapped in time with conductive heating of the Eau Claire Formation (a closed system) from under- and overlying sandstone aquifers that channeled the flow of hot, Mississippi Valley type mineralizing brines during the mid-Permian (ca. 270 Ma).