Thomas Dewers

A coupled reaction/transport/mechanical model for intergranular pressure solution, stylolites, and differential compaction and cementation in clean sandstones

Abstract “Pressure solution” at grain-grain contacts and free-face dissolution and growth kinetics coupled to solute transport and texture-dependent effective stress are shown to lead to pervasive intergranular compaction and the development of stylolites in sandstones. Reaction/transport modeling studies presented here demonstrate that stylolite formation may arise from an instability of a chemically compacting rock, leading to the enhancement of spatial heterogeneities in rock texture. Textural and geochemical criteria distinguishing between conditions favoring stylolites vs. pervasive intergranular pressure solution for both the water film diffusion (WFD) and free-face pressure solution (FFPS) mechanisms (so denoted in Tada et al., 1987) are outlined. WFD and FFPS each show a characteristic grain size dependence which aids in the recognition of the kinetic mechanism responsible for pressure solution. Results from our simulations concerning the effect of grain size on the stable compaction of sandstones compare favorably to observations and data in Houseknecht (1988). They suggest that FFPS is operative in clean sandstones, while WFD is operative in clay-rich sandstones. Spatially discrete domains of heightened cementation and intergranular pressure solution-induced compaction are shown to develop from certain types of initial spatial variations in grain textural parameters. These features, predicted by our computer simulation, show differential cementation/compaction trends similar to examples observed in Paleozoic sandstones. The greatest contrasts in cementation, compaction, and porosity attend the FFPS mechanism.

Force of crystallization during the growth of siliceous concretions

The force of crystallization is of renewed interest as a diagenetic replacement mechanism. We present a quantitative reaction-transport model for mineral replacement driven by the pressure exerted by crystal growth, based on continuum equations accounting for conservation of mass and momentum. A condition of volume-for-volume replacement during growth and dissolution is shown to arise naturally from the interaction of mechanical and reactive deformation. When applied to the postnucleation growth of quartz in a calcite plus amorphous silica host, a centimetre-scale concretion-like feature is shown from our simulations to arise within roughly 17 ka.

Mechano-chemical coupling in stressed rocks

Abstract The free energy of a grain in a rock at depth varies as a function of the texture of its surroundings, either due to fluctuations in stress, strain energy, or interfacial factors. Herein we discuss methods Used to estimate the contribution by texture to grain free energy in deforming rocks, as well as implications for textural evolution when included in kinetic reaction-transport models. A key feature is the introduction of a formalism coupling rock flow and mineral reaction by means of a Navier-Stokes equation. The resulting set of equations describes mechano-chemical interactions under a wide range of conditions and may pose constraints on more descriptive models. Under certain circumstances, this free energy may lead to the autonomous enhancement of spatial ihhomogeneities in texture when coupled to reaction and transport in an intergranular fluid. We hold that such phenomena as metamorphic layering, spaced (solution) cleavage, geodes, and certain types of concretions are examples of mechano-chemical selforganization. The dynamics leading to formation of such texture may thus be understood in analogy with other examples of geochemical self-organization and may be quantified accordingly.

Influences of clay minerals on sandstone cementation and pressure solution

The authors investigate hypotheses concerning observed associations between clay minerals and sandstone pressure solution and cementation. By use of a quantitative reaction-transport-deformation model that the authors set forth earlier, simulations of diagenetic interactions in clay-bearing sandstones in time and space show that different combinations of kinetic mechanisms acting as quartz grain-free faces and contacts produce characteristic spatial patterns in clay content, quartz-overgrowth abundance, porosity, and amount of pressure solution-related compaction. Comparison of simulation results to existing data from sandstones provides constraints on the operating diagenetic processes. The authors conclude that thin clay coatings on sand grains sufficiently retard development of quartz overgrowth by slowing precipitation kinetics, intracontact clay minerals promote water-film diffusion pressure solution, and free-face pressure solution operates within clay-free sand-grain boundaries.

The role of geochemical self-organization in the migration and trapping of hydrocarbons

Abstract Many sedimentary basins of the world contain abnormally pressured “compartments” surrounded by impermeable seals, which may serve as hydrocarbon traps. Observations of the many types of seals that may exist point to an origin due to self-organization. Self-organization is the development of repetitive or other low symmetry patterns from an initially unpatterned system in the absence of an externally imposed template, and involves feedback generated by the interaction of transport, chemical reactions, and grain growth and dissolution. Herein we focus on the self-organizing mechanisms of reaction driven advection as they pertain to the formation of diagenetic traps.

Differentiated structures arising from mechano-chemical feedback in stressed rocks

Abstract We review some of our mathematical modeling efforts toward describing the interaction between a deforming rock and its contained fluid. Specifically, we advocate the concept of self-organization as a means of understanding the evolution of patterned structures observed in rocks ranging from diagenetic to metamorphic conditions. These arise from feedbacks resulting from the coupling of mechanical forces with chemical reactions and transport, due to the dependence of mineral free energy on surrounding rock texture. The same formalism in different limiting or initial conditions captures much of the behavior observed with such deformation-related structures as stylolites, intergranular pressure solution, spaced cleavage, metamorphic layering, and mineralized segregations arising from the force exerted by crystal growth. The equations predict the likelihood of any tendency toward differentiation, and show how chemical factors impose strong constraints on the deformational response of a rock.

Cellular and oscillatory self-induced methane migration

Abstract Methane genesis from kerogen maturation can induce kilometer-scale flow in a porous medium due to the fluid mass density dependence on methane concentration. The decrease in density with increasing methane content in solution may destabilize a water column, and initiate convective overturns. Conditions favoring these buoyancy-driven flows include a narrow thermal-depth window for methanogenesis and a large initial organic matter content. The self-organizing aspects of methanogenesis-induced flow are explored via the numerical simulation of reaction-transport equations. Cellular convection is demonstrated, and under certain conditions, the flow velocities are found to oscillate in time. For systems with initially random kerogen distribution the convection cells organize into one of a discrete set of well ordered convection cell patterns despite the initial disorder. Simulations initialized with heterogeneous porosity and permeability show that varying sizes and patterns of convection flows may develop which reflect the interplay between the inherent tendency toward self-organization and the nonuniformities in hydraulic properties.

Chapter 7 Formation of Stylolites, Marl/Limestone Alternations, And Dissolution (Clay) Seams by Unstable Chemical Compaction Of Argillaceous Carbonates

Publisher Summary This chapter focuses on the reaction-transport modeling to describe the evolution of clay seams, stylolites, and marl/limestone alternations during chemical compaction in terms of a few coupled diagenetic processes. The model discussed in the chapter accounts for diffusional mass transport, texture-dependent effective stress distribution, different reaction rates and driving forces for reaction at grain contacts and at grain “free” faces, and rate inhibition due both to solute inhibitors and clay coatings. In particular, it is shown that the reaction rate at grain-grain contacts relative to that at free grain faces is one deciding factor between behaviors in porous rocks. As the rates vary with pore fluid chemistry, temperature, clay content, and amount of overgrowth cement at grain contacts, the response of a porous rock to pressure solution may vary widely with different conditions. In addition, an example of clay seam formation in a low-porosity calcite-clay rock is examined using a model developed in an earlier study. This model incorporates results on the elastic matrix-inclusion problem into reaction-transport formalism. The effect of enhanced diffusion at phyllosilicate-calcite grain boundaries on the length scale and timing of clay seam formation is explored in this limit of low porosity.

Modeling Diagenetic Bedding, Stylolites, Concretions, and Other Mechanochemical Structures

A quantitative reaction-transport-mechanical model of pressure solution is used to explain a variety of differentiated features in carbonate rocks. The development of stylolites, bands of compaction alternating with bands of augmented cementation, and carbonate bands in sandstones are shown to be the consequence of an unstable dynamic that takes place during compaction. This dynamic leads to the intensification of textural contrasts during burial diagenesis. The quantitative model allows for the prediction of the range of existence and properties of these phenomena with respect to constraints. These constraints include carbonate grain size, clay content, burial and thermal history, and fluid chemistry.