Effects of depositional environment on rearranged hopanes in lacustrine and coal measure rocks

Abstract

On the basis of GC-MS analysis, a suite of 97 lacustrine and coal measure rocks from the Songliao Basin, Ordos Basin and Tarim Basin were found to contain abundant rearranged hopanes. Except for peak ratios, the absolute concentrations of rearranged hopanes were determined based on addition of an internal standard (5α-androstane) normalized to bitumen “A”, which in lacustrine source rocks are far more than those in coal measure rocks. To examine the effects of depositional environment on relative abundance and absolute concentration of rearranged hopanes, the clay content, redox condition and water salinity of sedimentary rocks from two sample sets were analyzed. The results reveal that the acidic clay acted as clay-mediated acidic catalysis in the formation and degradation of rearranged hopanes, but there is no positive correlation between clay content and rearranged hopanes in lacustrine source rocks. In contrast, the absolute concentrations of rearranged hopanes in coal measure rocks, show a general increase with clay content. Some molecular parameters, such as pristane/phytane (Pr/Ph), C35 homohopane index and gammacerane index, were found to change as a function of depositional environment. With increasing Pr/Ph, C35 homohopane index and gammacerane index, the ratios of C30 diahopane/C30 hopane and C29Ts/C29 hopane display an initial increase and subsequent decrease trend. The absolute concentrations of C30 diahopane and C29Ts in lacustrine source rocks also increase at first, followed by a decrease at the largest value. Whereas the absolute concentrations of rearranged hopanes in coal measure rocks show an apparent positive correlation with some depositional environmental parameters. On the whole, abundant rearranged hopanes in lacustrine source rocks appear to be distributed in suboxic-weak reduction and fresh-brackish condition, while abundant rearranged hopanes in coal measure rocks appear to be present in oxic and fresh condition.