ABSTRACT: Redox Reactions Involving Hydrocarbons and Mineral Oxidants: A Mechanism for Significant Porosity Enhancement in Sandstones

SURDAM, RONALD C., JOHN JIAO, and DONALD B. MACGOWAN, University of Wyoming, Laramie, WY

Hydrocarbon invasion into a sandstone containing mineral oxidants and carbonate and/or sulfate intergranular cements may result in redox reactions and significantly enhanced porosity.

For years geologists have noted that when hydrocarbons invade red sandstones significant bleaching (i.e., iron reduction) takes place. The reactions responsible for the color distribution in the red (oxidized) and white (reduced) zones involve the reaction of iron oxides (+/- sulfate) and hydrocarbons. The iron oxides (+/- sulfate) provide the oxygen to oxidize the hydrocarbons (reductant) to oxygenated organic compounds; the Fe2O3 (oxidant) is reduced by hydrocarbons to pyrite (+/- chlorite). Typically, the red sandstones are tight because of carbonate and/or sulfate cements, and the white zones are porous.

It is suggested that the responsible redox reactions are of the following types:

1. C9H20 + 0.5 Fe2O3 + 2 SO + 4.25 CO2 + 3.25 H2O generating 6.625 CH3 COOH + FeS2,
2. C9H20 + 0.25 Fe2O3 + CaSO4 + 1.125 H2O + 3.125 CO2 generating 4.0625 CH3 COOH + 0.5 FeS2 + Ca++ + 2 CH3 COO-, and
3. C9H20 + 0.5 Fe2O3 + 0.5 Al4Si4O10(OH)8 + 4.75 CO2 + 6.75 H2O + Mg[-2] generating 6.875 CH3COOH + 0.5 Fe2Mg2Al4Si2O10(OH)8 + H4SiO4 + 2H-.

The produced organic acids are available to dissolve carbonate cements via the reaction CH3COOH + CaCO3 CH3COO- + Ca+2 + HCO3-. If the reaction product is CO2 and CH4, instead of CH3COOH, the implications to porosity enhancement are the same. Volumetric calculations demonstrate that if a hematite stained sandstone (1.5% Fe2O3) is invaded by a fluid containing a 50/50 mixture of water and hydrocarbons, and redox reactions result, enough organic acid and consequent carbonate dissolution can occur to generate 8-14% additional porosity. More subtle redox reactions involving hydrocarbons and mineral oxidants have the potential to significantly enhance porosity in any sandstone type. These redox reactions may explain why in some cases, hydrocarbon accumulations appear to have created porosi y.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)