EARLY GENERATION OF 20S-5-ALPHA(H),14-ALPHA(H),17-ALPHA(H)-STERANES AND 5-ALPHA(H),14-BETA(H),17-BETA(H)-STERANES IN LOW-SALINITY LACUSTRINE SHALES

A quantitative sterane biomarker study was conducted on a series of pa ralic freshwater lacustrine shale samples ranging in maturity from imm ature to near oil window maturity taken from Section 3 of the Shahejie Formation (Es3) in the Liaohe Basin, N.E. China. Concentrations of 5 alpha(H),14 alpha(H),17 alpha(H)-20S and 5 alpha(H),14 beta(H),17 beta (H)-steranes remain nearly constant throughout the sample suite. Howev er, the decrease in the absolute concentrations of the 20R-5 alpha(H), 14 alpha(H),17 alpha(H)-C-29 steranes with increasing maturity results in an increase in the conventionally defined maturity parameters, 20S /(20S + 20R)-alpha alpha alpha and alpha beta beta/(alpha alpha alpha + alpha beta beta) sterane ratios. In addition, the data suggest that relatively early generation of 5 alpha(H),14 alpha(H),17 alpha(H)-20S and 5 alpha(H),14 beta(K),17 beta(H)-steranes has occurred in lacustri ne sediments with a vitrinite reflectance similar to 0.3% (R(o)). The data provide strong support for the major importance of relative therm al stability of epimers, but do not exclude the possibility of isomeri zation as a viable mechanism for production.