Maturity assessment and characterization of Big Horn Basin Palaeozoic oils

Fifty oils from the Big Horn Basin plus three from the adjoining Wind River Basin were analysed to (1) look for the possible presence of different oil types and subtypes, (2) assess the thermal maturity of the oils and (3) determine occurrence and extent of oil alteration. Oils from all productive formations throughout the basin were analysed by gas chromatography (GC) and combined GC-mass spectrometry (GC-MS) to obtain molecular parameters. Bulk parameters, such as API gravity, per cent sulphur, the vanadium/nickel ratio and stable carbon isotope ratio were also included. Previous studies had shown that Big Horn Basin Palaeozoic oils are generally similar in composition and originated primarily in organic-rich phosphatic shales in the Permian Phosphoria Formation. The source rocks now lie mostly outside the basin to the west. Bulk parameter measurements and GC analyses showed a general similarity among oils analysed in this study a nd tended to confirm that they originated in the sa me or similar sources. The GC-MS analyses indicate the presence of five oil subtypes. Most oils constitute two of the subtypes; it is suggested that one originated in the Meade Peak Member and the other in the Retort Shale Member of the Phosphoria Formation. The other three subtypes represent single-field occurrences and may be derived, at least in part, from local sources. The most useful maturity indicator for Big Horn Basin Palaeozoic oils was found to be extent of thermal cracking of triaromatic steroid wide chains. It indicated considerable variation in oil maturity, not directly related to present reservoir depths. The GC analyses showed varying degrees of biodegradation and water washing among oils from shallower reservoirs. Biodegradation was not necessarily accompanied by water washing, and vice versa. Aromatic compound and sulphur compound distributions indicate significant alteration, not believed to be due to biodegradation, but rather to reservoir lithology. The presence of appreciable anhydrite in the reservoir rock appears to be an important factor.