Elemental Chemical Zonation of Black Shales Revisited--Application to Petroleum Source Rocks

All ‘black' shales are not chemically similar. Quinby-Hunt and Wilde, (1991, 1994, 1996) placed black shales into four zones on an elemental and thermodynamic basis using a cluster approach on the 287 syngenetic sample data base of the Marine Sciences Group (Quinby-Hunt et al., 1989). These zones are redox based going from oxic to highly anoxic even though lithologically these rocks are considered ‘black' shales. The ‘black' color seems to be related to the Mn-Fe content and solubility, with oxic black shales having stable Mn oxides that color the rock. In anoxic black shales with lower Mn concentrations, Fe minerals color the rock. We have compared our zones with (1) other published analyses of aperiodic black shale events from the Archean, Early Proterozoic, Late Proterzoic, Ordovician, Silurian and Jurassic using Drever's (1988) average shale values as a baseline. Certain trace metal concentrations are higher in the more anoxic zones. These signatures might be used to identify potential cryptic petrochemical source beds even when the initial high carbon values may be lost via metamorphism or migration. This assumes that trace metal signatures in the potential petroleum source rocks are from marine organic matter (Martin and Knauer, 1973, Yamamoto et al., 1985), which can be distinguished from that of detrital sediments from non-organic rich rocks. Carbon (Wilde, Lyons, and Quinby-Hunt, 2004) and Sulfur (Wilde, Quinby-Hunt and Lyons, 2005) initial values might be recovered using trace metal proxy equations for such depleted rocks.