Geochemical characterization of an organic-rich carbonate from the Grosmont Formation, Alberta, Canada

Abstract

The Grosmont Formation is a massive, organic-rich carbonate deposit that subdivides into upper (UG) and lower (LG) units. Geochemical analyses of Grosmont carbonate rocks collected from wells 10-12-93-24W4 and 11-33-94-22W4 allowed us to differentiate the depositional and diagenetic conditions for these two units. The UG unit subdivides into three subunits (UG1–UG3) and consists of fine- to coarse-crystalline and subhedral dolomite. The LG unit consists of dolomitic limestone and calcareous dolostone with medium- to coarse-crystalline, euhedral to subhedral dolomite and calcite. These units also include small amounts of quartz, illite, pyrite, K-feldspar and other trace minerals. Petrographic and geochemical analyses indicate that the LG unit formed under influence of clastic sediment input. Isotopic ratios for UG dolomites suggest precipitation within a near surface environment under evaporitic/ hypersaline conditions. By contrast, the LG dolomite may have formed in a diagenetic environment at salinities lower than those inferred for UG samples, according to observed isotopic ratios. Both UG and LG carbonates exhibit 87Sr/86Sr ratios (0.7082 to 0.7086) and δ18O values (−4.03 to −6.09‰ PDB) that are consistent with formation from Devonian seawater at temperatures ranging from 44 °C to 57 °C.