The hysteresis of asphaltene-trapped saturated hydrocarbons during thermal evolution

Abstract

Asphaltene-trapped (adsorbed or occluded) biomarkers are considered a valid source of information for crude oil with severe secondary alterations. However, thermal stress might change asphaltene-trapped hydrocarbons to some extent. Therefore, it is indispensable to investigate the thermal evolution behavior of asphaltene-trapped biomarkers. In this study, low-maturity bitumen from the Kuangshanliang area of the Sichuan Basin (China) was selected as a sample. The thermal evolution behavior of free and asphaltene-trapped saturated biomarkers was investigated by thermal simulation experiments. The results revealed, in addition to normal biomarkers, also a series of even carbon number n-alk-(1)-enes in asphaltene-occluded hydrocarbons. All of them were hardly influenced by secondary alterations. Moreover, due to the restriction of macromolecular structure, the thermal evolution of asphaltene-trapped biomarkers lags behind that of free biomarkers. Most biomarker parameters of occluded hydrocarbons changed little with increasing maturity. In particular, the distributions of occluded terpanes or steranes retained the characteristics of early diagenesis of organic matter, even if they suffered from strong thermal alteration.