Monoaromatic Steroid Hydrocarbons Research Articles

Steroid hydrocarbons and the thermal history of sediments

Assessment of the extents to which certain organic chemical reactions have occurred in sedimentary rocks with increasing burial depth and the associated temperature rise, can distinguish differences in the extent of their thermal maturation1–10. Heating experiments with sediments have suggested that these different reaction types have different kinetic constants11. Therefore, a measurement of the extent to which one reaction type has occurred might be expected to correspond to different values of a measurement based on a different reaction type, depending, for example, on the average sedimentary heating rate (°C Myr−1)12 of the sample. We use here two reaction types to investigate this hypothesis: (1) configurational isomerization in steroid alkanes, represented by the conversion of the biologically inherited configuration, 5α(H), 14α(H), 17α(H), 20R, in a C29 sterane to an approximately equal mixture of itself and the corresponding 20S configuration formed in the sediment6,13; and (2) apparent aromatization of two C29 monoaromatic steroid hydrocarbons (Fig. 2) assumed to be isomeric at C-5 (refs 14–21), based on retention time comparisons with synthesized C-27 analogues, to a presumed product, the C28 triaromatic steroid hydrocarbon9,14. Comparison of the extents to which these two reactions have occurred in suites of sediment samples has allowed three basins [Pannonian Basin (Pliocene), Mahakam Delta (Miocene), Paris Basin (Toarcian)] with different thermal histories to be distinguished. Extension of the hypothesis to two other sedimentary sequences suggests a higher average heating rate for the Oligocene of the Zhanhua Depression (north-east China) than for the Cretaceous of the Wyoming Overthrust Belt.

Molecular parameters of maturation in the Toarcian shales, Paris Basin, France—III. Changes in aromatic steroid hydrocarbons

Three series of monoaromatic steroid hydrocarbons ( base peak = m/ z 239, 253 and 267) and four series of triaromatic steroid hydrocarbons ( base peak = m/ z 217, 231, 245 and 259) have been recognised by computerised gas chromatography-mass spectrometry in a variety of samples of Lower Toarcian shales from the Paris Basin. An increase in the extent of aromatisation of the monoaromatic steroid hydrocarbons in the m/ z 253 series (measured by the abundance of monoaromatic components relative to triaromatic components in the m/ z 231 series) is observed with increasing maturity. Variations between reported maximum burial depth and extent of aromatisation are explicable partly in terms of new maximum burial depth information for some of the shales. The extent of carbon-carbon bond cracking in the side chains of the aromatic steroid hydrocarbons has only begun to be significant in certain of the deepest samples.