ASSESSING THE UTILITY OF WELL CUTTINGS TO CHARACTERIZE WEATHERING PROFILES OF CRYSTALLINE BASEMENT

Microseismic events coincident with subsurface CO₂ sequestration sites occur in the Precambrian crystalline basement in central Illinois. Recent work to characterize the heterogeneity of the basement is limited by scarce well cores and lack of outcrop. One potential source of data is an abundance of well cuttings captured during drilling from basement penetrations; however, the process of drilling may effect bulk-rock geochemistry. This study evaluates the utility of geochemical analyses of those well cuttings. Four wells were sampled at two separate depths within 40 feet of the great unconformity. We use rock chips, commonly 1-4 mm, as a proxy for unaltered rock; however, since rock chips are relatively rare and time consuming to pick, we compare it to a sample of whole-sample cuttings. To prepare samples for analyses, four grams each of rock chips and whole-sample cuttings are weighed and processed at coeval well depths. Samples are analyzed using x-ray fluorescence (XRF) to compare major oxides.

Data collected shows that SiO₂, Al₂O₃, K₂O, Na₂O compare very well across all depths and wells, which indicates that sampling whole cuttings is a sufficient process of obtaining data for these major oxides. Results also show that CaO and Fe₂O₃ are the most variable. Magnesium, manganese, sodium, phosphorous, and titanium oxides are agreeable, but show variance in two wells. Preliminary conclusions suggest that using whole-sample cuttings is sufficient, rather than picking chips, for all oxides except calcium and iron. This study presumes that the variation in calcium and iron can be attributed to locally variable CaCO₃ content in the altered or weathered rock directly below the great unconformity. Given that the cuttings average five feet interval of well depth, this technique does not appear sufficient to accurately assess calcium and iron. Thus, studies that focus on alteration or weathering of the Precambrian basement using well cuttings should focus on weathering techniques that do not incorporate calcium and iron.