MATURITY ASSESSMENT AND CHARACTERIZATION OF BIG-HORN BASIN PALEOZOIC 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 dif ferent oil types and subtypes, (2) assess the thermal maturity of the oils and (3) determine occurrence and extent of oil alteration. Oils f rom all productive formations throughout the basin were analysed by ga s chromatography (GC) and combined GC-mass spectrometry (GC-MS) to obt ain molecular parameters. Bulk parameters, such as API gravity, per ce nt sulphur, the vanadium/nickel ratio and stable carbon isotope ratio were also included. Previous studies had shown that Big Horn Basin Pal aeozoic oils are generally similar in composition and originated prima rily in organic-rich phosphatic shales in the Permian Phosphoria Forma tion. The source rocks now lie mostly outside the basin to the west. B ulk parameter measurements and GC analyses showed a general similarity among oils analysed in this study a nd tended to confirm that they or iginated in the sa me or similar sources. The GC-MS analyses indicate the presence of five oil subtypes. Most oils constitute two of the sub types; it is suggested that one originated in the Meade Peak Member an d the other in the Retort Shale Member of the Phosphoria Formation. Th e other three subtypes represent single-field occurrences and may be d erived, at least in part, from local sources. The most useful maturity indicator for Big Horn Basin Palaeozoic oils was found to be extent o f thermal cracking of triaromatic steroid wide chains. It indicated co nsiderable variation in oil maturity, not directly related to present reservoir depths. The GC analyses showed varying degrees of biodegrada tion and water washing among oils from shallower reservoirs. Biodegrad ation was not necessarily accompanied by water washing, and vice versa . Aromatic compound and sulphur compound distributions indicate signif icant 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.