Diagenesis Through Coupled Processes: Modeling Approach, Self-Organization, and Implications for Exploration

The coupling of diagenetic reaction-transport processes can lead to patterns of petroleum and mineral distributions that are not a trivial reflection of imposed basin features such as sedimentary beds, folds, and faults. In this article we review examples of diagenetically differentiated features of this type. They include oscillatory intracrystalline zoning, banded cements (observed to serve as diagenetic petroleum traps), reaction-front fingering (which can lead to local inhomogeneities of porosity and permeability), oscillatory ejection of fluids from overpressurized, sealed compartments, and patterns of buoyancy-driven convection cells that can arise from mineral or kerogen reactions or the geothermal gradient. Mathematical reaction-transport modeling provides a metho for determining conditions under which these phenomena may exist and for assessing their importance in petroleum genesis, migration, and trapping. Both qualitative and quantitative arguments are used to demonstrate the feedback mechanisms and orders of magnitude of the properties of these diagenetic structures. As illustrative examples, we focus on patterns of the migration of methane driven by its own buoyancy, flow self-focusing at reaction fronts in carbonate rocks, and autonomous oscillatory fluid release from overpressurized compartments. The development of banded pressure seals through a mechano-chemical feedback destabilizing the state of uniform compaction is studied in a companion chapter (Dewers and Ortoleva, 1990). With these examples, we illustrate how reaction-transport coupling can generate spatial patterns of petroleum and minerals that can play a central role in determining reservoir quality and the diagenetic in luences on petroleum distribution within the basin. Because experiments on geological length and time scales are often unfeasible, the development of quantitative models and the implementation of computer codes to simulate them are becoming a third branch of geological investigation that is distinct from classical, experimental, and theoretical geoscience. In the present context, then, this "computational geochemistry" allows us to identify and characterize these diagenetic phenomena born of the strong coupling between reaction, transport, and mechanical processes. End_Page 103------------------------