Hydrous pyrolysis of immature Monterey Phosphatic or Siliceous rock at progressively higher temperatures causes systematic changes in biomarker thermal maturity parameters of the generated hydrocarbons. Biomarker ratios based on proposed carbon-carbon cracking or aromatization reactions increase during hydrous pyrolysis along similar pathways for both Siliceous and Phosphatic members. An increase in these biomarker ratios is also observed for oils of increasing thermal maturity from the offshore Santa Maria Basin, although the rates of changes for each parameter differ between the hydrous pyrolysis and natural samples. Changes in some cracking parameters during maturation appear to result from differential thermal stability of the compounds rather than conversion of precursors to products. The behavior of isomerization-based biomarker ratios in these experiments is more complex than ratios based on carbon-carbon cracking or aromatization reactions. During heating, kerogen-bound precursors generate steranes and hopanes showing lower levels of thermal maturity based on isomerization ratios than those extracted from the unheated rock. Asymmetric centers in the kerogen-bound steroids and hopanoids appear to be protected from isomerization compared to those of free steranes or hopanes in the bitumen. The Phosphatic and Siliceous rocks can show different sterane or hopane isomerization ratios when heated under the same time/temperature conditions. Further, these isomerization ratios unexpectedly decrease at high hydrous pyrolysis temperatures (> 330°C) for the Phosphatic, but not for the Siliceous samples. This could be caused by the combined effects of isomerization and differential destruction of epimers, apparently mediated by rock mineralogy. Differences between the biomarker compositions of bitumens and expelled oils in these experiments mimic those caused by natural primary migration. Heavier, more polar compounds are preferentially retained in the bitumen. For example, bitumens are enriched in tri- over monoaromatic steroids, hopanes over tricyclic terpanes, and regular steranes over diasteranes compared to expelled oils. No significant fractionation of stereoisomers was observed, such as 22S vs 22R C32 17α(H),21β(H)-homohopanes or 20S vs 20R and ββ vs αα C29 5α(H)-steranes. © 1990.
