REDOX REACTIONS INVOLVING HYDROCARBONS AND MINERAL OXIDANTS - A MECHANISM FOR SIGNIFICANT POROSITY ENHANCEMENT IN SANDSTONES

Hydrocarbon invasion into a sandstone containing mineral oxidants and carbonate or sulfate intergranular cements may result in redox reactio ns and significantly enhanced porosity. For years, geologists have not ed 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 ar e reactions of iron oxides (+/- sulfate) with hydrocarbons. The iron o xides (+/- sulfate) oxidize the hydrocarbons (reductant) to oxygenated organic compounds; the Fe2O3 (oxidant) is reduced by hydrocarbons to pyrite (+/-chlorite). Commonly, the red sandstones are tight due to ca rbonate and sulfate cements, whereas the white zones within them are m ore porous. These redox reactions are of three types: C9H2O + 0.5 Fe2O 3 + 2S0 + 4.25 CO2 + 3.25 H2O --> 6.625 CH3COOH + FeS2 or C9H2O + 0.25 Fe2O3 + CaSO4 + 1.125 H2O + 3.125 CO2 --> 4.0625 CH3COOH + 0.5 FeS2 Ca++ + 2 CH3COO-or C9H2O + 0.5 Fe2O3 + 0.5 Al4Si4O10 (OH)8 + 4.75 CO2 + 6.75 H2O + Mg2+ --> 6.875 CH3COOH + 0.5 Fe2Mg2Al4Si2O10(OH)8 + H4Si O4 + 2H+. The produced organic acids are available to dissolve carbona te cements via the reaction CH3COOH + CaCO3 --> CH3COOH- + Ca+2 + HCO3 -. Volumetric calculations demonstrate that if a hematite-stained sand stone (1.5% Fe2O3) is invaded by a fluid containing a 50/50 mixture of water and hydrocarbons, and redox reactions result, enough organic ac id and consequent carbonate dissolution could occur to generate 8-14% additional porosity. More subtle redox reactions involving hydrocarbon s and mineral oxidants have the potential to significantly enhance por osity in any sandstone. These redox reactions may explain why hydrocar bon accumulations appear to have created porosity in some cases.