Wendy J. Harrison

Predictions of diagenetic reactions in the presence of organic acids

Abstract Stability constants have been estimated for cation complexes with anions of monofunctional and difunctional acids (combinations of Ca, Mg, Fe, Al, Sr, Mn, U, Th, Pb, Cu, Zn with formate, acetate, propionate, oxalate, malonate, succinate, and salicylate) between 0 and 200°C. Difunctional acid anions form much more stable complexes than monofunctional acid anions with aluminum; the importance of the aluminum-acetate complex is relatively minor in comparison to aluminum oxalate and malonate complexes. Divalent metal cations such as Mg, Ca, and Fe form more stable complexes with acetate than with difunctional acid anions. Aluminum-oxalate can dominate the species distribution of aluminum under acidic pH conditions, whereas the divalent cation-acetate and oxalate complexes rarely account for more than 60% of the total dissolved cation, and then only in more alkaline waters. Mineral thermodynamic affinities were calculated using the reaction path model EQ3/6 for waters having variable organic acid anion (OAA) contents under conditions representative of those found during normal burial diagenesis. The following scenarios are possible: 1. 1) K-feldspar and albite are stable, anorthite dissolves 2. 2) All feldpars are stable 3. 3) Carbonates can be very unstable to slightly unstable, but never increase in stability. Organic acid anions are ineffective at neutral to alkaline pH in modifying stabilities of aluminosilicate minerals whereas the anions are variably effective under a wide range of pH in modifying carbonate mineral stabilities. Reaction path calculations demonstrate that the sequence of mineral reactions occurring in an arkosic sandstone-fluid system is only slightly modified by the presence of OAA. A spectrum of possible sandstone alteration mineralogies can be obtained depending on the selected boundary conditions: EQ3/6 predictions include quartz overgrowth, calcite replacement of plagioclase, albitization of plagioclase, and the formation of porosity-occluding calcite cement, smectite, and illite, all of which are commonly documented in rocks. Under some circumstances, OAA-bearing waters are less effective at producing porosity in an arkosic sandstone than are OAA-free waters. In the scenarios modeled in this study the role of OAA in fluid-rock interactions is to contribute to the total alteration assemblage but not necessarily to dominate it, except under exceptional circumstances that might include, for example, hydrocarbon contaminant plumes in aquifers, wetland environments, and within hydrocarbon source-rocks.

Geochemical interactions resulting from carbon dioxide disposal on the seafloor

Abstract The storage of CO 2(liquid) on the seafloor has been proposed as a method of mitigating the accumulation of greenhouse gases in the Earths atmosphere. Storage is possible below 3000 m water depth because the density of CO 2(liquid) exceeds that of seawater and, thus, injected CO 2(liquid) will remain as a stable, density stratified layer on the seafloor. The geochemical consequences of the storage of CO 2(liquid) on the seafloor have been investigated using calculations of chemical equilibrium among complex aqueous solutions, gases, and minerals. At 3000 m water depth and 4°C, the stable phases are CO 2(hydrate) and a brine. The hydrate composition is CO 2 ·6.3H 2 O. The equilibrium composition of the brine is a 1.3 molal sodium-calcium-carbonate solution with pH ranging from 3.5 to 5.0. This acidified brine has a density of 1.04 g cm −3 and will displace normal seawater and react with underlying sediments. Seafloor sediment has an intrinsic capacity to neutralize the acid brine by dissolution of calcite and clay minerals and by incorporation of CO 2 into carbonates including magnesite and dawsonite. Large volumes of acidified brine, however, can deplete the sediments buffer capacity, resulting in growth of additional CO 2(hydrates) in the sediment. Volcanic sediments have the greatest buffer capacity whereas calcareous and siliceous oozes have the least capacity. The conditions that favor carbonate mineral stability and CO 2(hydrates) stability are, in general, mutually exclusive although the two phases may coexist under restricted conditions. The brine is likely to cause mortality in both plant and animal comunities: it is acidic, it does not resemble seawater in composition, and it will have reduced capacity to hold oxygen because of the high solute content. Lack of oxygen will, consequently, produce anoxic conditions, however, the reduction of CO 2 to CH 4 is slow and redox disequilibrium mixtures of CO 2 and CH 4 are likely. Seismic or volcanic activity may cause conversion of CO 2(liquid) to gas with potentially catastrophic release in a Lake Nyos-like event. The long term stability of the CO 2(hydrate) may be limited: once isolated from the CO 2(liquid) pool, either through burial or through depletion of the CO 2 pool, the hydrate will decopose, releasing CO 2 back into the sediment-water system.

Simulation of burial diagenesis in the Eocene Wilcox Group of the Gulf of Mexico basin

Abstract Diagenesis may be evaluated quantitatively by using petrographic observations and results of paleohydrologic reconstructions in combination with geochemical reaction path model calculations. The authors have applied a reaction path method by simulating diagenesis in the Eocene Wilcox sandstones in the Gulf of Mexico basin to evaluate the effects of variable Pco 2 , fluid composition, amount of rock reaction and burial history. The results show that increases in Pco 2 cause the amount of carbonate phases to increase, instead of creating secondary porosity, and closed system reactions with a chemically evolved pore fluid cause a reduction in the amount of carbonate phases, thereby preserving primary porosity. Diagenesis resulting from increased rock reaction per pore volume is characterized by a dominance of Fe-free mineral phases, and albite forms in simulations at temperatures above 100°C with neutral pH evolved fluids. The results approximate petrographic observations of previous workers on the Wilcox with only a few exceptions. Continued simulations using different fluid compositions and organic acid anions may increase the capability to reproduce observed paragenetic sequences.

Mineralogical responses of siliciclastic carbonate-cemented reservoirs to steamflood enhanced oil recovery

Abstract Rock–fluid interactions induced by steamflood enhanced oil recovery were investigated in laboratory simulations to determine the geochemical reactions and the effects of these reactions on reservoir permeability. Flow-through laboratory experiments using mixtures of quartz, kaolinite, and siderite were performed in a high temperature/high pressure permeameter at a confining pressure of 1200 psi and temperatures between 150–250°C. Fluid compositions used in the experiments simulated the vapor and residual liquid phases encountered in steamflood operations as well as an intermediate fluid composition. Effects of fluid pH, fluid salinity and flow rate were systematically investigated in the experiments. The most extensive fluid–rock interactions were observed in the vapor phase simulations and high temperature/high pH condition simulations. Smectite, chlorite, illite, mixed-layer clays, greenalite, analcime, and K-feldspar were all identified as products of rock fluid interaction in the experiments. Smectite was the dominant authigenic phase to reduce permeability in the experiments. The experiments showed that the formation of smectite in Fe-rich environments does not require a clay precursor. Smectite is likely the most damaging neoformed mineral to reservoir permeability under different hydrogeochemical conditions for several reasons including: (1) its relatively high surface area (including microporosity in the “honeycomb texture”), (2) its propensity to migrate and block pore throats during fluid flow in porous media because of its small particle size, pore-lining texture, and electrochemical surface properties, and (3) the wide range of stability of smectites in the physical and chemical conditions that exist in reservoirs undergoing steamflood EOR. The rapid precipitation of authigenic minerals in these experiments suggests that the period required for fluids and rock to reach equilibrium in diagenetic environments are extremely short when considering geologic time scales. The armoring of pre-existing minerals by grain-coating authigenic minerals appears to result in the attainment of local equilibrium conditions prior to when one would predict assuming a continuous supply of reactant minerals was present.

Rietveld refinement of Ba5(AsO4)3Cl from high-resolution synchrotron data

The apatite-type compound, pentastrontium tris[arsenate(V)] chloride, Sr5(AsO4)3Cl, has been synthesized by ion exchange at high temperature from a synthetic sample of mimetite [Pb5(AsO4)3Cl] with SrCO3 as a by-product. The results of the Rietveld refinement, based on high resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the same structure as other halogenoapatites with general formula A 5(YO4)3 X (A = divalent cation, Y = pentavalent cation, and X = F, Cl or Br) in the space group P63/m. The structure consists of isolated tetrahedral AsO4 3− anions (the As atom and two O atoms have m symmetry), separated by two crystallographically independent Sr2+ cations that are located on mirror planes and threefold rotation axes, respectively. One Sr atom is coordinated by nine O atoms and the other by six. The chloride anions (site symmetry ) are at the 2a sites and are located in the channels of the structure.

Basin‐Wide Diagenetic Patterns: Integrated Petrologic, Geochemical, and Hydrologic Considerations

The serious scholar of petrology is well aware of the discrepant level of understanding that exists between igneous and metamorphic petrogenesis and sediment diagenesis (also known as postdepositional evolution). While nucleation and crystallization theory, phase equilibria, and principles of heat and mass transport have provided guidance for scientists to understand the formation and subsequent evolution of igneous and metamorphic rocks, sedimentary petrologists seem to have been left behind in detailed, descriptive characterization, with little chemical and physical basis to unify the diagenesis of one rock unit with any other. Of course, one cannot lay blame entirely on the scientist: the study of sediment diagenesis is plagued by problems of incomplete chemical reaction, the need to characterize amorphous solids and poorly crystalline mineral phases, low temperature and pressure conditions that result in reaction kinetics dominating over thermodynamic equilibria, and a lot of fugitive water! Further complications arise because most of the samples available for extensive regional study of sediment diagenesis are those recovered during the exploration and exploitation of hydrocarbon and mineral resources. The processes that have controlled postdepositional chemical and physical evolution of such samples are undeniably nonrepresentative of those affecting the vast volumes of most sedimentary sequences.