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AAPG Bulletin

Abstract


Volume: 77 (1993)

Issue: 9. (September)

First Page: 1509

Last Page: 1518

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

Author(s): Ronald C. Surdam, Zun S. Jiao, Donald B. MacGowan (2)

Abstract:

Hydrocarbon invasion into a sandstone containing mineral oxidants and carbonate 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 are reactions of iron oxides (± sulfate) with hydrocarbons. The iron oxides (± 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 carbonate and sulfate cements, whereas the white zones within them are more porous.

These redox reactions are of three types:

[EQUATION]

or

[EQUATION]

or

[EQUATION]

The produced organic acids are available to dissolve carbonate cements via the reaction [Equation]. 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 could 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. These redox reactions may explain why hydrocarbon accumulations appear to have created porosity in some cases.

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