Organic Acids from Petroleum Source Rocks

Summary

The vastness and dynamics of sedimentary basins make it difficult to assess organic acid generation from petroleum source rocks on the basis of the limited subsurface data currently available. An alternative approach involves simulating the natural process in laboratory pyrolysis experiments that maintain a liquid-water phase, utilize whole rock, avoid extreme temperatures, and minimize reactor-wall effects. Appropriately conducted laboratory pyrolysis experiments show that saturated acyclic monocarboxylic acids are the dominant organic acids generated, that C₅-C₅ monocarboxylic acids dominate the aqueous phase assemblages, and that acetic acid (C₂) is typically the dominant aqueous organic acid. Low activation energies derived from laboratory pyrolysis experiments indicate that these organic acids are retained in sedimentary organic matter by weak noncovalent bonds. Thus, the organic acid potential of a source rock is largely dependent on the amount of C²-C⁵ monocarboxylic acids assimilated by noncovalent bonds into sedimentary organic matter during its early development into kerogen. Significant quantities of these acids may be released from petroleum source rocks by diffusion during early diagenesis. However, the sluggishness of this diffusion process is only likely to cause local enhancement of porosity within a source rock and in rocks immediately adjacent to it. Release of the remaining organic acids is most likely to occur during the expulsion of petroleum from a source rock. Dissolved organic acids in the expelled petroleum will redistribute themselves within the associated formation waters as the migrating or entrapped petroleum cools, degasses, or encounters lower salinity waters. As a result, enhanced porosity may occur within carrier beds during secondary petroleum migration or within reservoirs during or after petroleum entrapment.