Reverse Weathering, the Carbonate-Feldspar System, and Porosity Evolution during Burial of Sandstones

MILLIKEN, K. L., and L. S. LAND, University of Texas, Austin, TX

Acid generated by reverse weathering in mudrocks drives linked reactions involving carbonates and feldspars that in turn have a profound impact on the evolution of porosity during burial diagenesis of sandstones. In the Oligocene Frio Formation along the Texas Gulf Coast, petrographic evidence at various scales in both sandstones and mudrocks, regional compositional variations in sandstones and shales, and modern pore fluid compositions are all consistent with this idea. Acid released during illitization of smectite dissolves marine skeletal and detrital carbonate in shales. Shales export excess H+, dissolved CA++, and CO(2) to the sandstones. Resultant interaction of sandstones with these shale-derived fluids is primarily controlled by the feldspar content of the sandstones. If the s ndstones contain reactive feldspar (i.e., detrital igneous and metamorphic feldspars), acid from the shales is effectively buffered, thus allowing precipitation of calcite as cements and grain replacements in the sandstones and maintenance of fluids with low pCO(2). Once the supply of reactive detrital feldspar is exhausted through dissolution and albitization, shale-derived H+ remobilizes carbonate in sandstones, generating secondary porosity and fluids with high pCO(2).

Acid generated through thermal maturation of organic matter plays only a trivial role in this system of reactions because the quantity of acid required for dissolution of carbonate and feldspar far exceeds the amount of kerogen in the system. The balance between the local acid generation capacity of shales, the local buffering capacity of feldspars in sandstones, and the availability of externally derived acids and ions is the primary control on the sequence of reactions occurring during burial metamorphism of sandstones.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)