Abstract
Sesquiterpanes are ubiquitous components of crude oils and ancient sediments. Liquid saturated hydrocarbons from simulated pyrolysis experiments on immature organic-rich mudstone collected from the Lower Cretaceous Hesigewula Sag were analyzed by gas chromatography–mass spectrometry (GC–MS). C14 bicyclic sesquiterpanes, namely, 8β(H)-drimane, 8β(H)-homodrimane, and 8α(H)-homodrimane were detected and identified on basis of their diagnostic fragment ions (m/z 123, 179, 193, and 207), and previously published mass spectra data, and these bicyclic sesquiterpanes presented relatively regular characteristics in their thermal evolution. The ratios 8β(H)-drimane/8β(H)-homodrimane, 8β(H)-homodrimane/8α(H)-homodrimane, and 8β(H)-drimane/8α(H)-homodrimane all show a clear upward trend with increasing temperature below the temperature turning point. Thus, all these ratios can be used as evolution indexes of source rocks in the immature–low-maturity stage. However, the last two ratios may be more suitable than the first ratio as valid parameters for measuring the extent of thermal evolution of organic matter in the immature–low-maturity stage because their change amplitude with increasing temperature is more obvious.
Highlights
Bicyclic sesquiterpanes, a type of saturated hydrocarbon, are widely distributed in crude oil and sediments
Bendoraitis (1973) proposed that the high concentrations of bicyclic alkanes in crude oils from South Texas may result from the degradation of more complex precursors, in particular by the thermal degradation of higher plant triterpenes, whereas Alexander et al (1984) proposed that bacteriohopanoids were extensively degraded in petroleum source rocks during diagenesis, resulting in functionalized bicyclic compounds of the drimane type
The aim of this study was to identify bicyclic sesquiterpanes and analyze their evolution and organic geochemical significance by conducting simulated pyrolysis experiments on an immature organic-rich Lower Cretaceous mudstone collected from the Hesigewula Sag, a relatively new exploration area
Summary
A type of saturated hydrocarbon, are widely distributed in crude oil and sediments. Bendoraitis (1973) proposed that the high concentrations of bicyclic alkanes in crude oils from South Texas may result from the degradation of more complex precursors, in particular by the thermal degradation of higher plant triterpenes, whereas Alexander et al (1984) proposed that bacteriohopanoids were extensively degraded in petroleum source rocks during diagenesis, resulting in functionalized bicyclic compounds of the drimane type. These compounds could undergo rearrangement involving carbonium ion intermediates or be reduced directly to hydrocarbon. These compounds could undergo rearrangement involving carbonium ion intermediates or be reduced directly to hydrocarbon. Noble et al (1987) concluded that bicyclic alkanes were released from the insoluble organic matrix (kerogen) and that drimanes and Petroleum Science (2019) 16:502–512 homodrimanes occurred in sedimentary organic matter in both the free and the bound states
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