Propyl ergostanoic acids: Possible new indicator for oil biodegradation

Representative oils having similar maturity, derived from a saline lacustrine source rock and accumulated at different depths in stacked reservoirs in the Campos Basin, Brazil were selected for study. Geological conditions allowed the development of distinct extents of biodegradation that increase from the deeper to the shallower reservoirs as shown by increased sulfur content, decreased saturated hydrocarbons and lower API gravity. Gas chromatography (GC), GC-mass spectrometry (GC-MS) and GC-MS–MS show the presence of hopanoic acids with the C32 ββR biological configuration and a suite of 3-propylsteranoic acids with the carboxylic function in the C-17 side chain, suggesting that the compounds are proxies for biodegradation. Systematic variation in the relative abundance of these new biomarkers with the extent of biodegradation were also observed. To the best of our knowledge, this is the first report of the occurrence of 3-propylergostanoic compounds in geological samples.