Abstract
Analyses are presented of samples of Posidonia Shale (Toarcian), SW Germany, Allgau formation (Toarcian), Austria, and Kimmeridge Clay formation, S England, for TOC, quartz, pyrite, clay, diasterane ratio, beta alpha -diasterane ratio, 20S/(S + R) cholestane and beta beta /alpha + beta beta ) cholestane. The results show that the diasterane/sterane ratio is poorly correlated with the clay mineral content, but strongly correlates with the ratio of clay mineral to organic matter content. These results explain the anomalously high diasteranes in some carbonate-derived petroleums, and suggest a major role of clay minerals in the reduction of Delta (super 13(17)) -diasterenes. The last-named may be partly reduced to diasteranes and partly degraded during diagenesis in a ratio determined mainly by the availability of clay minerals.