Organic/Inorganic Water-Rock Reactions and Sandstone Diagenesis

Donald B. MacGowan, Ronald C. Surdam

Carboxylic acid anions are important in clastic diagenesis; not only do they buffer solution Eh and pH, but they also dominate formation-fluid alkalinity from about 80°-130°C and therefore exert a major control on carbonate mineral stability. Additionally, they enhance the solubility of aluminosilicate mineral framework grains by complexation and transportation of Al and Si from the site of dissolution. Carboxylic acid anions are released to the diagenetic environment by the action of methanogenic and sulfate-reducing bacteria; thermocatalytically during kerogen maturation by biogenic or chemical alteration of crude oil (i.e., oil-water interaction); by mineral oxidation; or by oxidation of hydrocarbons during sulfate reduction. Hydrocarbon reduction of sulfate o H₂S proceeds by oxidation of hydrocarbons to carboxylic acids or CO₂. This reaction is catalyzed by a number of geocatalysts and has been documented to occur abiotically at temperatures as low as 100°C. This organic/inorganic interaction releases highly reactive H₂S, carboxylic acids, and CO₂ species to the diagenetic environment and has been documented to cause both carbonate and aluminosilicate dissolution in the subsurface. Sources of SO₄^2- in subsurface fluids include evaporite minerals and connate waters.

A holistic diagenetic model incorporating organic/inorganic reactions is coupled with a time-temperature model, burial history reconstructions, and petrographic observations from the Louisiana Gulf Coast to explain the observed diagenetic sequence. Reaction kinetics determined from static thermochemical and hydrous pyrolysis experiments for the organic/inorganic reactions are used to evaluate the timing of diagenetic events.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.