MIXING OF CRUDE OILS AND DEPOSITION OF SOLID HYDROCARBONS IN
THE HITCH SANDSTONE RESERVOIR
,
Dongwon Kim,
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.