Chemical and Mechanical Disequilibrium in Sediments: Diagenesis, Compaction, and Mass-transfer

Anthony J. Park¹, David Budd², Geoffrey Thyne³, Kagan Tuncay⁴ and Peter Ortolev^4
1Sienna Geodynamics and Consulting, Inc., Bloomington, IN <[email protected]>
²University of Colorado, Boulder, CO
³Colorado School of Mines, Golden, CO
⁴Indiana University, Bloomington, IN

Sediments are in states of chemical and mechanical disequilibrium throughout their geologic histories. Chemical disequilibrium arises from the thermodynamic instability among minerals, thus driving reactions that direct the system toward more stable phase configurations. Mechanical disequilibrium arises from incomplete packing and textural arrangements, therefore causing sediment to compact and achieve a more stable mechanical state. 

Both diagenesis and compaction are strongly dependent on compositional and textural heterogeneity of sediments. Thus, sands, shales, and carbonates compact differently, and paragenesis of sands, shales, and carbonates also follow different paths. Further complicating the processes are mass-transfer mechanisms that redistribute matter on scales of centimeters to kilometers. Also, sedimentation and burial histories are critical factors that affect the rate of the processes, such that similar sediments subjected to different burial and thermal histories produce very different final sediment properties. Finally, all of the above processes are interdependent with each other. In other words, the chemical and physical processes that govern diagenesis, compaction, fluid flow, etc., are strongly nonlinear.

Results from WRIS.TEQ and CIRF.B simulations are presented to demonstrate each of the processes mentioned above and their combined aspects. CIRF.B is a process-oriented finite-element simulator that accounts for multi-mineral water-rock interaction, dynamic texture evolution, elasto-visco-plastic rheology, fluid flow, fracturing, and gas/oil generation. WRIS.TEQ is a configuration of CIRF.B specifically packaged to assess reservoir-scale diagenesis, compaction, fluid flow, and mass transfer.

Results demonstrate the extreme geologic cases, such as chemical instability driven early diagenesis of carbonate sediments, diffusion-controlled interactions between clastic sediment layers, compaction-driven overpressuring and fracturing in mixed carbonate-clastic rocks, as well as the cases where more than one processes are responsible for overall alteration of sediment properties.

Simulation results closely corroborate observed patterns of diagenetic and compaction products. As such, the simulation results provide unprecedented insight into the geologic mechanisms and scenarios responsible for the evolution of any particular observable pattern. Thus, the process-oriented program CIRF.B and WRIS.TEQ are viable prediction tools for assessing geologic evolution of sediments. In other words, since sediments evolve due to their inherent chemical and mechanical disequilibrium, the most effective means of modeling them is by implementing the fundamental principles that describe the states of disequilibrium.

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