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 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.