A comparative study on the geochemical characteristics of expelled and retained oil from hydrocarbon generation simulation of Australian Tasmanian oil shale Ⅱ: molecular geochemical characteristics

Abstract

The Australian Tasmanian oil shale is a special set of source rocks in which a single species of planktonic algae (Tasmanite) is significantly enriched and has relatively lower degree of maturity with equivalent vitrinite reflectance of 0.5%. It can be considered as a good material for artificial maturation experiment. In order to compare the molecular geochemical characteristics of expelled oil and retained oil at different simulation temperatures, a hydrocarbon generation and expulsion simulation experiment was carried out. Results show that: (1) The extracts of original rock sample and the retained oil indicate a reducing environment, while a mixed source region of both reduction and oxidation is indicated by the expelled oil. (2) The maturity-related biomarker parameters (e.g C29 steranes 20S/(20S+20R), C29 hopanes ββ/(αα+ββ) and Ts/(Ts+Tm)) indicate that the retained and expelled oil are mature at 350 ℃ of the experiment. These biomarker parameters in retained oil increase with temperature before the simulation temperature lower than 350 ℃; however, the correlation is poor for expelled oil. When temperature is higher than 350 ℃, the parameters of retained or expelled oil have irrelevant correlation with the simulated temperature. (3) The distribution of C27, C28 and C29 steranes in retained and expelled oil shows variations with maturation degree, and changes fromreverse "L" type, to slightly asymmetric "V" type, and finally to "L" type at 400 ℃. At the same temperature, retained oil and expelled oil are comparable. Therefore, at the same maturity stage, the sterane distribution characteristics is effective for oil source correlation; while at different maturity stages, the comparability of expelled oil and retained oil may vary greatly. It is revealed by this study the differences of molecules between expelled oil and retained oil, as well as the influence of simulated temperature on the parameters. The influence of maturity to the biomarker parameters has to be considered when studies of depositional environment, maturity and oil-source correlation are carried out, especially after the peak of oil generation.

Keywords

• thermal simulation experiment
• retained oil
• expelled oil
• molecular geochemical characteristics
• maturity-related biomarker
• oil-source correlation
• tasmanian oil shale
• australia