THE ROLE OF CLAY COATINGS ON QUARTZ CEMENTATION IN THE ST. PETER SANDSTONE, MI BASIN, USA

Newly acquired petrological data was collected from the St. Peter Sandstone in the Brown Snowplow 1-5 well (Alpena Co.), Michigan Basin. The studied interval, positioned at 7230.5-7231.5 ft below ground level, is characterized by alternating decimeter-scale intervals of red and white sandstone. Petrographic analysis of the red and white sandstones suggests that they are comprised of fine to medium, well-sorted, rounded detrital quartz grains lithified by quartz cement. Unlike the white sandstone, the quartz grains in the red sandstone are coated by thin reddish-brown material. Scanning electron microscope (SEM) images show that the coating has a platy, clay-like texture. SEM-energy dispersive spectroscopy (SEM-EDS) analyses of this material show elevated concentrations of Ca, Mg, Al, K, and Si relative to the detrital and authigenic quartz. These observations are consistent with bulk rock x-ray fluorescence (XRF) data, which show that the red sandstone intervals exhibit higher concentrations of Fe and common clay-forming elements, such as Al, Ca, and K, compared to the white sandstone intervals. Powder x-ray diffraction (XRD) data show that in addition to the predominant quartz peaks, the red sandstone also exhibits some small diffraction peaks in the low 2-theta range typical of clay minerals. Importantly, the white intervals typically have more cement and less pore space than the red intervals, an observation consistent with other studies that have suggested that the presence of clay coatings can inhibit the growth of quartz cement in deeply buried sandstones, which has implications for retaining primary porosity and preserving reservoir quality.