Abstract
C27–C29 steranes are common constituents of crude oils and hydrocarbon source rocks and are usually dominated by either C27 or C29 compounds. In the current study, however, steranes dominated by C28 components were detected in eight lacustrine source rock samples in the first member of the Shahejie Formation in the Qikou Sag, Bohai Bay Basin of Eastern China. Ratios of C28/(C27 + C28 + C29) ααα 20R steranes in four samples can exceeded 50%. Furthermore, C27 triaromatic steroids, presumably originating from C28 steranes, were also observed at high levels, relative to C26 and C28 homologues. Stable carbon isotope compositions of a C28 ααα 20R sterane and C27 20R triaromatic steroid were found to be similar, with δ13C at −37‰ and −36‰ respectively, much lower than values for pristane (by ∼7‰) and total organic carbon (by ∼10‰). The abnormally low δ13C values of the steroids suggest that C28 ααα 20R sterane was biosynthesized by algae blooming in the water column during phases of overturn and mixing, such as in autumn, during which recycled CO2 and nutrients welling up from hypolimnion to the epilimnion are assimilated, contributing to a high algal productivity and low δ13C in algal organic matter.
Published Version
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