MIXING OF CRUDE OILS AND DEPOSITION OF SOLID HYDROCARBONS IN
THE HITCH SANDSTONE RESERVOIR,
A suite of crude oils and their respective core extracts from Upper Mississippian sandstone reservoirs in the Hitch and Etzold fields were analyzed by various geochemical techniques to study the geological and geochemical controls on the formation of solid hydrocarbons. The occurrence of solid hydrocarbons was unexpected. The solid hydrocarbons were deposited in the lower part of the Hitch reservoir and are significantly enriched in asphaltenes. The solid organic materials account for about 50 wt.% of the total hydrocarbons in the solid bitumen layers. The deposition of the solid hydrocarbons can be explained by mixing of oils of different geochemical compositions. The mixing model is supported by a simple laboratory mixing experiment using several different types of oils, indicating that compositional changes after oil-mixing were insignificant, but the amount of solid organic material precipitated increased up to 60 wt.% more than expected. The oil-mixing resulted in slight 13C depletion in saturate and aromatic fractions. The precipitated solid hydrocarbons consisted of 25-45 wt.% of paraffinic waxes and 55-75 wt.% of asphaltenes. There is no clear evidence to support other possible solid deposition mechanisms such as biodegradation or thermal alteration for the Hitch oils. The solid hydrocarbons in the Hitch field are believed, therefore, to be formed by mixing of oils from multiple source rocks filling the reservoir over an extended period of time. In addition, regional pressure and temperature drops caused by post-Laramide uplift may have contributed to a phase change in the reservoir fluid to precipitate solid hydrocarbons.