Distribution and isomerization of C31–C35 homohopanes and C29 steranes in oligocene saline lacustrine sediments from Qaidam Basin, Northwest China

The 22S/(22S + 22R) ratios for C31–C35 homohopanes and the 20S/(20S + 20R) ratio for C29 steranes were measured by gas chromatography–mass spectrometry (GC–MS) on six Oligocene saline lacustrine rock samples from a borehole located in the western Qaidam Basin, northwest China. These ratios vary significantly and irregularly with burial depth from 2831 m to 3054 m around the threshold of the oil generation window. In addition, 22S/(22S + 22R) ratios also vary substantially among different homologs within the same sample, generally decreasing from C31 to C35 homohopanes. The distribution patterns of homohopanes for the six samples are unusual, such as, C31 < C32 > C33 > C34 < C35, or C31 > C32 > C33 < C34 < C35, or C31 < C32 > C33 < C34 < C35. All of these observations can be mainly ascribed to the release of bound hopanes and steranes from kerogen and other macromolecules. The 22S/(22S + 22R) ratios of C31, C32 and C33 homohopanes decreased, while that of C34 homohopanes increased for two samples, YHS1 and YHS6, after pyrolysis at low temperatures (180–270 °C). A major reversal of 20S/(20S + 20R) ratio for C29 steranes was observed at 240 °C for sample YHS1, but at 270 °C for sample YHS6. The distributions of homohopanes for these two samples after pyrolysis changed from their original patterns toward the normal pattern C31 > C32 > C33 > C34 > C35. These results reflect the influences of thermal degradation and direct isomerization of the free biomarkers and the release of bound biomarkers from kerogen and other macromolecules during pyrolysis.