Abstract: Transfer-Reaction Models of Diagenetic Processes in Volcanogenic Sandstones

James R. Wood, Ronald C. Surdam

The diagenesis of volcanogenic sandstones is characterized by alteration of glass and feldspar (plagioclase) to carbonate rocks, clays, and zeolites. These alteration products are distributed throughout the sandstone body in well-defined zones and commonly are associated with carbonate concretions and carbonate cements. A quantitative model describing these processes can be constructed by combining the equations describing chemical reaction with those governing mass transfer. This approach leads to a set of differential equations with associated boundary conditions which can be considered to be constrained by the mass-action equations. In this framework, the identity and spatial distribution of the reaction products can be shown to originate as a consequence of countercur ent mass flows in which different aqueous species either diffuse or are transported in opposite directions owing to boundary conditions and/or the distribution of internal sources and sinks. In the case of opposing diffusion currents containing components which are capable of reacting to form a solid precipitate, a well-defined series of precipitation zones will form. The positions of these zones in the body are a function of the boundary conditions and the strength of the sources and sinks.

The diagenetic evolution of an assemblage consisting of andesitic glass plus plagioclase can be calculated, given the initial mass distribution and specifying the boundary conditions. The calculation shows that it is particularly important to know whether or not the system was open or closed with respect to fluid flow and/or gas exchange. The Peclet number, which is the ratio of the fluid velocity to a characteristic diffusion distance, is perhaps the key parameter, as it not only affects the spatial distribution of the alteration products, but also affects whether the system will conserve or increase its porosity by flushing or will be sealed as a result of chemical reaction.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado