Abstract: The Role of Hydrocarbons in Sandstone Porosity Enhancement

Mamdouh A. Shebl, Ronald C. Surdam

Experiments were designed to evaluate the potential for, and the extent of, oil-rock reactions in hydrocarbon reservoirs. Ten to 20 cc of oxidized ("red") Tensleep and Casper sandstones were heated with 400 cc of deionized water and 200 cc of liquid petroleum for 3 to 14 days at 100° to 350°C. These experiments demonstrate that there is significant potential for redox reactions between oxidized mineral phases and crude oil. Moreover, these redox reactions can significantly alter the porosity/permeability characteristics of a elastic hydrocarbon reservoir.

The oxidized red sandstones utilized in these experiments contain significant carbonate, anhydrite, and clay intergranular cements, and the porosity typically ranges from 6 to 14%. After the experiments the sandstones are light gray or white and are characterized by an increase of 15 to 20% over the original porosity. Petrographic observation suggests that the increase in porosity is due primarily to carbonate dissolution.

It is suggested that the iron oxides (+/- sulphate) react with hydrocarbon, with the iron oxides oxidizing the hydrocarbons to oxygenated organic compounds as the iron oxides are reduced. Oxygenated organic compounds, including organic acids and CO₂, are produced in the experimental runs and are responsible for destabilizing the carbonate cements and enhancing the porosity. It is concluded that redox reactions involving crude oil and the mineral matrix of a reservoir rock can be significant, and can result in significantly enhanced porosity.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994