Chemical Reaction and Solute Transport Rates as Constraints on Diagenetic Models

Geoffrey Thyne, Knut Bjorlykke, and Wendy Harrison

We present a conceptual model for clastic diagenesis that applies basic principles of chemical equilibria and solute transport. For shallow burial depths, fluid velocities are on the order of 10 meters/year and temperatures less than 60°C. Under these conditions mineral dissolution and precipitation rates are much slower than solute transport rates and models that using advective transport and kinetically-controlled mineral reactions are appropriate. During deeper burial, porewater velocities are less than 0.1 cm/year and diffusion is the dominant mode of transport. If mineral reaction rates exceed transport rates, the porewater can reach metastable equilibrium with the local mineral assemblage. The pH is dominated by silicate mineral reactions and dissolved carbonat controlled by carbonate mineral equilibrium. As a result porewater chemistry in adjacent sedimentary formations that have different mineral assemblages is not the same. Differences in porewater chemistry form diffusion gradients that produce mass transfer between adjacent formations. Diffusion produces mixing between the porewaters which may cause precipitation of mineral phases that act as a sink for dissolved ions. Diffusive mass transport can continue until the source of dissolved ions is exhausted or mineral precipitation occludes porosity and inhibits solute transport. In the case of diffusive mass transfer, the water is the medium of transport and mass transport occurs in "closed" systems whose dimensions are defined by diffusive transport This eliminates the water volume problem hich has been a source of concern in many diagenetic models.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California