Porosity Development at Unconformities: Ancient Examples, Modern Analogous, Numerical Models and Prediction

Melvyn R. Giles, Sarah L. Indrelid, Joachim E. Amthor, Gerard Beynon

Porosity enhancement and destruction beneath unconformities is common worldwide. Qualitatively, the factors controlling dissolution and precipitation during exposure (e.g., climate, reactivity of pore-fluid type, mineralogy, duration of exposure, etc.) are well established. Because all these factors are interlinked, quantitative prediction of the occurrence and distribution of specific products associated with unconformities remains difficult, and existing descriptive and thermodynamic diagenetic models are not adequate to predict porosity associated with subaerial exposure.

Aquifer systems provide a natural laboratory which can be used to aid our understanding of diagenetic processes and reactions occurring at unconformities. Hydrochemical investigations show that dissolution fronts formed below a subaerial unconformity extend hundreds of metres to kilometres down-dip in the direction of groundwater movement. In fact, there are a number of different reaction fronts passing through the rocks. Depending on the mineralogy of the aquifer, these fronts include a total carbonate dissolution front, an incongruent dolomite dissolution front, a gypsum dissolution front and a K-feldspar dissolution front. The reaction fronts are spatially related but not coincident, rather there is a distinct pattern of zoning which reflects the approach toward equilibrium of the ore fluid with the rock. The rate at which the reaction fronts move, and hence the time scale for significant porosity modification, will depend on the fluid-flow rate, on the competition between different reactions, and on the (low-temperature) reaction kinetics of the diagenetic processes. Hence, to model dissolution and cementation at unconformities requires a kinetic model which can link a number of reactions.

Using continuum mechanics it is possible i) to model kinetically controlled diagenetic reactions in which fluid, heat and mass transport are coupled through feedback loops, and ii) to link the factors critical to porosity modification. Rates of porosity modification obtained in this way can be related to the duration of exposure and/or to the rate of propagation of the unconformity surface, making predictions of porosity development at unconformities more reliable.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995