CONTRACTOR: LAWRENCE LIVERMORE NATIONAL LABORATORY

University of California

Livermore, California 94550

CONTRACT: W-6405-ENG-48

CATEGORY: Geophysics and Earth Dynamics

PERSON IN CHARGE: F. J. Ryerson

A. Effects of Heterogeneity on the Fracture of Rock (S.C. Blair [510-422-6467; Fax 510-423-1057; E-mail blair5@llnl.gov] collaborative research with L.R. Myer [Lawrence Berkeley Laboratory; 510-486-6456])

Objective: The objective of this research is to understand how microscale (or grain scale) heterogeneity affects macroscopic mechanical behavior of rocks, to study the process of progressive fracture of rock in compression, and to evaluate the role of crack interaction in rock deformation and fracture. Moreover, this work also aims to investigate the relationship of static to dynamic moduli (in collaboration with LBL) and to characterize the role of heterogeneity at a variety of scales in fracture and the scaling properties of rocks, including fluid flow properties.

Project Description: This project is concerned with simulation of rock deformation and fracture at grain and larger scales. During the initial phase of the project a 2-dimensional field-theory model for rock fracture was developed and then used to determine how heterogeneity in different microscale parameters affects behavior in the simulated compression tests, and how macroscopic stress strain behavior is related to the formation of cracks. More recently, we have extended our work to include simulation of rock deformation in 3-dimensions. This work uses high resolution 3-dimensional tomographs of the rock microstructure as input, and is focused on studying the fundamental physics of coupled rock deformation and fluid flow. This includes evaluation of the effects of local heterogeneity on the permeability field, and on the propagation and attenuation of seismic waves, as well as extending our work on crack nucleation and propagation to 3-dimensions.

Results: Results from the 2-D simulations show that the model exhibits an implicit size effect (inverse power law) that closely matches laboratory and field data for the dependence of sample strength on sample size, and that local heterogeneity in geometry and strength may lead to crack interactions along macroscopic surfaces that are similar to those caused by shear localization and that are often observed in real experiments. The model was also used to relate macroscopic stress strain behavior in compression to the formation of microcracks and to strain energy due to cracks.

Two patterns of cracking were found, including spatially uncorrelated, non-interacting cracks that have little effect on macroscopic deformation, and spatially correlated interacting cracks associated with extension or linking of cracks and fractures.  These cause strain softening and large changes in crack strain energy. 

During the past year we developed and tested a highly efficient finite-element model, custom designed to use high resolution X-ray tomographs of rock microstructure as input. These tomographs can resolve features as small as a few microns in size, but to fully resolve all the details of the tomographic image requires 107 by 108 elements, or in extreme cases 109 elements. This is far beyond the capacity of most finite-element codes, even using parallel computers. Fortunately, as all the elements in the tomograph are cubic, the stiffness matrix for a typical element can be calculated in advance which results in a much simplified code. Furthermore, we derived a new stiffness matrix from a lattice-spring model, as opposed to the usual strain-energy methods; it is a sparse matrix and requires only one tenth as many arithmetic operations to compute the nodal forces as a conventional finite element. This streamlined model can simulate systems as large as 107 elements on a single processor.   We have also completed preliminary development and testing of a skeletonization algorithm to study flow through complex pore spaces. A simplified flow structure is derived from the actual 3-dimensional pore structure using a skeletonization algorithm which preserves the connectivity of the original sample. The skeletonization is achieved by a uniform erosion of the surfaces of the structure with a constraint which prevents any erosions from breaking connections. Thus, the topology and connectivity are preserved during the skeletonizing process. The structure produced by this algorithm is topologically equivalent to original pore network. Moreover, since the algorithm keeps track of where the eroded pore space goes, it is possible to attach an effective size to each connection, which can be used to account for the volumetric flow of the eroded pore space that contributes to a particular connection.

B. The Role of Carbon and Temperature in Determining Electrical Conductivity of Basins, Crust, and Mantle (A. G. Duba [510-422-7306; Fax 510-423-1057; E-mail alduba@llnl.gov] collaborative research with T. J. Shankland, [505-667-4907; Fax 505-667-8487; E-mail shankland@lanl.gov], and E. A. Mathez [American Museum of Natural History; 212-769-5379; Fax 212-769-5339; E-mail mathez@amnh.org])

Objectives: The intent of this work is to comprehend the electrical conduction mechanisms in carbon-bearing rocks and in mantle minerals for the purpose of relating electrical conductivity (s) measured in the field to formation conditions and existing state of crustal rocks and to temperatures in the mantle.

Project Description: Electrical conductivity depends strongly on temperature T and on the presence of other phases such carbon, fluids, or ore minerals at the lower temperatures of the crust and basins. Thus, one research approach is to measure s of mantle minerals as functions of temperature, orientation, oxygen fugacity ƒO₂, and iron content. These data supply the best models for "electrogeotherms" yet available. Another approach is to document textures of carbon in crustal rocks from basins and metamorphic zones and relate them to rock conductivity. In this case texture of carbon distribution is mapped with electron microscopy in the same samples used for conductivity measurement. The approach here is to see how a trace element, hydrogen, can alter mineral conductivity.

Results: At depths of 80-200 km in Earth's mantle there is a high conductivity layer (HCL) that has electrical conductivity of order 10^-2 - 10^-1 S/m, which is 1 to 2 orders of magnitude greater than that measured in olivine at upper mantle temperatures. Understanding the physical cause of this layer has long been a perplexing geophysical problem. Explanations for elevated conductivity have included partial melts, aqueous fluids, elemental carbon in the rock matrix, and intracrystalline hydrogen H⁺. To investigate the latter hypothesis we measured conductivity of a mantle-derived mineral, San Carlos olivine, along its [100] crystallographic axis; this is the direction of highest H diffusivity. The sample was buffered both by CO₂/CO and by CO₂/H₂ atmospheres to give similar temperature-oxygen activity (T-ƒO₂) paths in the center of the olivine stability field. Conductivities were measured in two regimes: at temperatures of 900-1300°C and fixed CO₂/H₂; and at a constant 1200°C for a range of H₂ activities. Maximum values of ƒH₂ were 0.22 atm. Switching the buffering to a hydrogen-enriched atmosphere produced an increase in conductivity in all observations with no change in activation energy of conduction. For instance, switching ƒH₂ buffering to 0.03 atm (predicted H/Si of ~1 ppm) produced a 3.5% increase in conductivity . For increasing values of ƒH₂ the conductivity jump progressively increased. Although small, the effect matches predictions of enhanced conductivity due to hydrogen in the olivine crystal lattice. [For an ionic conductor diffusivity is related to carrier mobility so that conductivity can be calculated from H diffusivity and solubility.] While questions still remain, this result is in accord with dissolved H as the principal physical explanation for upper mantle HCLs. The results apply to our understanding of mantle dynamics, plate tectonics, and the distribution of heat sources beneath the crust.

C. Reactive Solute Transport and Processes of Dissolution and Deposition in Single Fractures in Rock (W. B. Durham [510-422-7046; Fax 510-423-1057; E-mail durham1@llnl.gov], B. P. Bonner, W. L. Bourcier, and A. Tompson)

Objective: The objective of this research is to measure local rates of dissolution and precipitation on the walls of individual fractures and correlate differences in reaction rates to changes local fracture aperture. Much use will be made of high resolution physical topography measurement and numerical simulation of reactive flow.

Project Description: The experiments and simulations will be done on rock samples containing a single laboratory-made or natural fracture. Detailed imaging of the fracture aperture before and after alteration will be coordinated with measurements of fracture deformation, permeability, dispersivity, and effluent composition, all as functions of pressure, temperature, temperature gradient, time, rock composition, fluid velocity, and fluid composition. For the most part we will work with simple but relevant systems in order to maximize our understanding and impact: samples will be monomineralic rocks with low porosity and low bulk permeability (such as quartzites and marbles), under fully saturated, single-phase flow conditions. We will attempt measurements in undersaturated, dual-porosity, and more chemically complex settings as success dictates.

Results: This project has only recently been initiated. All the experimental methodology has now been established and one experiment has been carried out. The first accomplishment was obtaining a large supply of Carrara marble to provide a large source of uniform sample material. The first cylindrical sample (50-mm diameter) was prepared from the Carrara, and a fresh axial tensile fracture created. The aperture topography was digitized at high spatial resolution (50 mm), and the sample is currently being subjected to chemically undersaturated fluid flow that should cause dissolution of material from the fracture faces. Aperture topography will be remeasured after a precalculated amount of material is removed into solution. The aperture topography will then be remeasured to provide a proof of concept of the experiments: if point to point variations are found in the amount of material removed from the fracture, simulations and testing of more samples under a wider range of conditions will proceed.

D. Water Distribution in Partially Saturated Porous Materials (J.J. Roberts [510-422-7108; Fax 510-423-1057; E-mail roberts17@llnl.gov] and W. Lin [510-422-7162])

Objective: To determine the distribution of water in partially saturated porous materials by measuring the complex electrical properties and to investigate the relationships between electrical transport and other transport properties in materials with well-characterized microstructures.

Project Description: The purpose of this project is to measure the unsaturated electrical properties of fused glass bead samples ranging in porosity from 1 to 43 percent. Measurements include dielectric constant and electrical resistivity as functions of saturation. The complex impedance from 10^-3 to 10⁶ Hz is measured because impedance spectra provide information regarding the number and arrangement of conduction mechanisms and the distribution of the liquid phase. The fluids used to saturate the samples have a range of ionic composition, and hence, electrical conductivity. This permits the comparison of impedance spectra of samples at similar saturations to better understand the relationship between fluid distribution and the corresponding conduction mechanisms. These measurements are of particular importance because field electrical measurements in unsaturated regions (including electrical resistance tomography, electromagnetic depth sounding, and induced polarization) depend on reliable laboratory measurements for accurate interpretation. The degree of difficulty of remediation problems in the vadose zone may depend on reliable information regarding the interconnectedness and distribution of the fluid phase.

The results will be analyzed in terms of mechanism of conduction and compared to existing measurements and models. These comparisons include permeability, cation exchange capacity, and ultrasonic velocity. All of these transport properties depend to some degree on microstructural properties as well as saturation level.

Detailed microstructural characterization of the material will be performed and relationships between the microstructure and interconnectedness of fluid as a function of saturation will be explored.

Results: All work preparatory to the laboratory measurements has been completed. This includes selection, machining, and characterization of samples, microstructural analysis using mercury intrusion and gas adsorption, and fabrication of sample holders. Complex impedance measurements between 10^-4 and 10⁶ Hz have been completed for eight samples ranging in porosity from 5 to 30% under the following conditions: distilled water, 25°C, wetting; distilled water, 35°C, drying; NaCl solution (70 mS/cm), 35°C, wetting. Measurements are currently being made for the drying cycle at 35°C with the NaCl saturating fluid. Thus far, the frequency-dependent resistivity indicates distinct conduction mechanisms that are observed over discreet frequency ranges. The specific nature of these mechanisms is under investigation. The number of conducting mechanisms changes with saturation; measurements performed with a third saturating fluid (more conductive) will help determine the nature of specific electrical responses at specific saturations. The comparison between laboratory-determined microstructural properties, measured electrical properties, and theoretical transport and microstructural determinations (specifically the two-point correlation functions) has begun. Permeabilities determined by image analysis and by the Katz and Thompson model are in agreement. Pore diameters determined via mercury intrusion and by image analysis compare favorably (within ~10%) while specific surface areas appear to differ by about a factor of two. An abstract summarizing these results was submitted to AGU for presentation at the Fall 97 meeting.

CATEGORY: Geochemistry

A. Thermodynamic and Transport Properties of Aqueous Geochemical Systems (Joseph A. Rard [510-422-6872; Fax 510-422-0208; E-mail rard1@llnl.gov] and Donald G. Miller [510-422-8074; Fax 510-422-6363; E-mail dmiller@llnl.gov])

Objectives: The objectives are to (1) measure precise and accurate osmotic/activity coefficients, solubilities, densities, and mutual (Fick's law) diffusion coefficients for aqueous brine salts and their mixtures and osmotic/activity coefficients for acidic sulfate mixtures; (2) develop reliable methods to estimate such properties for multicomponent solutions from binary solution properties; and (3) calculate generalized transport coefficients.

Project Description: The general techniques of classical thermodynamics and of linear irreversible thermodynamics are used to understand and model equilibrium and transport processes in brines and other aqueous electrolyte mixtures relevant to energy programs. Properties being measured are osmotic/activity coefficients and solubilities by the isopiestic method, densities by pycnometry and vibrating densimetry, and diffusion coefficients by Rayleigh and Gouy interferometry. 

One goal is to measure highly-accurate data for systems involving geochemical brines, radioactive waste isolation, and chemical pollutants. A second goal is to develop estimation methods for accurate predictions of these properties for aqueous electrolyte mixtures of arbitrary complexity, using the accurate new data as test systems. Transport data are being analyzed as Onsager transport coefficients and osmotic/activity coefficients are being analyzed using extended Pitzer's equations.

Results: Diffusion experiments were performed previously for four different solute ratios of mixtures of NaCl and Na₂SO₄ at 25ºC and at a total molarity of 0.5 mol/dm³, along with the limiting binary solutions NaCl and Na₂SO₄, using  Rayleigh and Gouy interferometry. These experiments have been extended to 1.0 mol/dm³ at 25ºC, using the newly automated Gosting diffusiometer with computer-controlled data collection in real time from a photodiode array. These diffusion experiments were performed in collaboration with Professor John G. Albright at Texas Christian University.    Isopiestic vapor-pressure experiments were completed previously for aqueous H₂SO₄ + MgSO₄ mixtures at 25EC in the acid-rich region of this system, at molality fractions z of H₂SO₄ of z 6/7, 5/7, and 4/7.

Additional experiments have now been performed for H₂SO₄ + MgSO₄ mixtures with z»3/7, 2/7, and 1/7. A total of 269 data points have been measured. The highest molalities for all three z values extend well into the supersaturated molality region. Lower molality experiments are complete in the acid-rich region but still are needed below 0.9 mol/kg for z.3/7, 2/7, and 1/7. The highest total molalities (sum of the molalities of H₂SO₄ and MgSO₄) are m_T = 12.050 mol/kg at z.6/7, m_T = 11.011 mol/kg at z.5/7, m_T = 7.2060 mol/kg at z.4/7 of m_T = 4.8474 mol/kg at z»3/7, m_T = 4.2843 mol/kg at z.2/7, and m_T = 3.9621 mol/kg at z»1/7. Once the experiments have been completed for the MgSO₄-rich region, the results will be modeled with extended versions of Pitzer's equations, and will yield reliable parameters for the interaction of Mg²⁺ with HSO₄^- . Surprisingly, the osmotic coefficients are predicted fairly well by Scatchard's neutral electrolyte binary-solution mixing approximation. This was definitely not expected because of the extensive ionic association present in these solutions. During this period four journal articles were published and another one completed. These papers report: 1) our diffusion results for aqueous NaCl + Na₂SO₄ mixtures at 0.5 mol/dm³; 2) diffusion results for aqueous NaCl + KCl mixtures at 1.0 mol/dm³; 3) diffusion results for aqueous HCl to high concentrations; and 4) the isopiestic results for the H₂SO₄ + MgSO₄ mixtures in the acid-rich region. An extensive review article was written on the osmotic and activity coefficients of aqueous CaCl₂ solutions to supersaturated molalities at 25ºC, and this article is now in press. These activity data for CaCl₂ solutions were represented with extended versions of Pitzer's equations with inclusion of association to form CaCl⁺ ion pairs. A short communication was written for the journal Applied Radiation and Isotopes (now in press) to correct some misconceptions in the literature about tracer diffusion. A talk was presented at the 52nd Annual Calorimetry Conference describing the isopiestic results for H₂SO₄ + MgSO₄ mixtures.

B. Uranium, Thorium, Lead and Oxygen Diffusion in Rock-forming Minerals: Implications for Reactive Transport (F.J. Ryerson [510-422-6170; Fax 510-422-1002; E-mail Ryerson@s91.es.llnl.gov] and K. D. McKeegan [University of California, Los Angeles; 310-825-3580; Fax 310-825-2779; E-mail kdm@argon.ess.ucla.edu])

Objectives: The spatial distributions of isotopes of elements such as uranium, thorium, lead, and oxygen, observed on a microscale, can be utilized to constrain thermal histories of crustal rocks and the extent of their interactions with fluids. There are two requirements necessary for application of this idea to real geologic systems: first, microanalytical techniques must be employed to quantitatively measure the isotopic or elemental heterogeneities, and second, fundamental diffusion data must be experimentally determined in order to know the rates at which equilibrium between fluid (either melts or aqueous fluids) and host rocks can be approached as a function of various external conditions.

Project Description: The diffusion coefficients for uranium, thorium, lead, and oxygen will be determined in a number of different minerals under a variety of external conditions. The experimental diffusion runs are performed at the Lawrence Livermore National Laboratory and the analytical phase of the work is done at UCLA using the Cameca ims 1270 ion microprobe. Techniques for the in situ measurement of either experimentally induced or naturally occurring variations of isotopic and elemental concentrations are being developed for the UCLA ion microprobe.

Results: Silicon and oxygen diffusion have been measured in a grossular, spessartine and pyrope almandine garnets at 800EC, 1 GPa and oxygen diffusion measured at 800ºC, 1 atm. 1 GPa runs employ an overgrowth technique in which a single crystal of garnet is annealed in an H₂¹⁸O fluid containing powdered ³⁰SiO₂; 1 atm runs employ a ¹⁸O-enriched gas reservoir. The garnet overgrowth formed in the 1 GPa runs (typically 300 nm thick) decreases the tendency for surface dissolution under hydrothermal conditions, and our experiments yield well-behaved diffusion profiles in many, but not all runs. We are currently conducting experiments at 100 MPa, using CO₂-H₂O fluids, in order to better assess the role of water fugacity on the oxygen diffusion in both garnet and clinopyroxene. Surface stability remains a major problem in these experiments.   We have also begun a series of experiments to determine the isotopic fractionation of oxygen between zircon and aqueous fluid by equilibration of a zircon alkoxide precusor and water at 1 GPa and temperatures between 600ºC and 1000ºC. The method has produces euhedral zircon grains with diameters of ~100 µm at temperatures as low at 600ºC. The solid run products will be analyzed using the laser fluorination technique.

C. An Experimental Investigation of Mechanisms Controlling Glass Dissolution (Susan A. Carroll [510 423-5694; Fax 510-422-0208; E-mail carroll6@llnl.gov] and William L. Bourcier [510-423-3745; Fax 510-422-0208; E-mail billb@llnl.gov])

Objectives: The objective of this project is to identify the underlying molecular mechanisms responsible for dissolution of glass and to utilize this understanding in the development of quantitative models for predicting glass dissolution rates in nature.

Project Description: This project uses a combination of conventional glass dissolution experiments, potentiometric surface titrations and NMR characterization of glass and its solution interface to determine the mechanisms controlling glass dissolution. The dissolution behaviors of three simple glass compositions (SiO₂, Na₂O·4SiO₂, and NaAlSi₃O₈) will be used to develop a mechanistic model based on three principal observations: (1) the pH dependence of dissolution rates, (2) the saturation effect and (3) the effect of absorbed alkali cations on dissolution rates. The results will be relevant to a number of problems including the stability of radioactive waste glasses, weathering of volcanic glasses, and obsidian hydration age dating, among others.

Results: We completed the experimental measurements of albite glass (NaAlSi₃O₈) dissolution rates at 25 and 70ºC in pH 2, 4, 6, 8, 10, and 12 solutions and in similar buffer solutions at pH 4 and 10 doped with NaCl and CsCl. The measured rates as a function of pH showed a v shape similar to crystalline albite, except that albite glass dissolves about 10 times faster than crystalline albite. A similar ratio of rates was observed for silica glass versus quartz. In the leachate solutions with added Cs and Na chlorides, we found trends similar to those observed for silica glass. Sodium enhanced the dissolution rate significantly. Cesium had little effect. The effects were larger in the pH 10 solutions than in the pH 4 solutions. This is consistent with the fact that in general the extent of surface sorption of Na⁺ on oxide surfaces is greater at higher pHs.

NMR work on the glasses reacted in NaCl and CsCl shows that adsorbed Na⁺ has a similar line shape and chemical shift as free Na⁺ dissolved in solution. However, adsorbed Cs⁺ has a negative chemical shift of about 5 ppm indicating a change in coordination, probably to lower coordination. We interpret this to mean that sorbed Na⁺ apparently has a similar hydration shell structure as free Na⁺ , but that Cs⁺ changes its hydration layer when it adsorbs onto the glass surface, perhaps forming an inner sphere complex. Apparently the inner sphere complex of Cs⁺ is not as effective in catalyzing increased surface dissolution rates as is the outer sphere complexed Na⁺ .

D. Mineral Dissolution and Precipitation Kinetics: A Combined Atomic-Scale and Macro-Scale Investigation (Kevin G. Knauss [510-422-1372; Fax 510-422-0209; E-mail knauss@s19.es.llnl.gov] and Carrick M. Eggleston [307-766-6769; Fax 307-766-6679; E-mail carrick @uwyo.edu])

Objectives: Our objectives are to build and test an atomic force microscope (AFM) capable of operation at up to 150ºC and 6 atm pressure, to apply this AFM to direct, in-situ, and real-time observation of step dynamics during dissolution and growth of oxide and silicate minerals at elevated temperature and pressure, and to use rate and stoichiometric data from parallel macroscopic dissolution and growth experiments to interpret mineral rates using a combined microscopic Burton-Cabrera-Frank and macroscopic surface-complexation model.

Project Description: This project combines atomic-scale and macro-scale approaches to the study of mineral-fluid interaction in order to significantly improve our understanding of, and ability to predict the course of, mineral dissolution and precipitation processes. We are building a high temperature-pressure flow-through AFM that will allow atomic-scale kinetic experiments under geologically relevant conditions for important oxide and aluminosilicate minerals. Identical conditions will be investigated using macroscopic wet-chemical rateexperiments, including conditions both near and far from equilibrium. We will measure rates of dissolution and precipitation, determine activation energies, measure rates of step motion across surfaces (including anisotropy), and investigate step-step interactions that affect rate.  We will then be able to address many still-open questions concerning the exact forms for rate laws near and far from equilibrium, the microscopic interpretation of these rate laws in terms of dissolution and precipitation mechanisms operating under various conditions, and the question of what exactly the "active area of interaction" and "active sites" are on mineral surfaces.

Results: After extensive evaluation of the major commercially-available AFM designs, one laser optical system and image analysis hardware/software package was selected. This decision allowed final designs to be made for the high temperature-pressure flow-through AFM, which had to be compatible with the commercial optical head and control electronics and which must withstand modest gas pressures. A prototype microscope was constructed and tested. This design uses a chemically-inert membrane (Kalrez) to isolate the fluid in the flow cell (containing the cantilever/tip and sample) from the microscope base (containing the piezo tube scanner, stepping motor, etc.). The same gas pressure that is used to initially pressurize the in-flowing solution is used to pressure the gas side of the membrane. Pulse-less flow is achieved using a precise digital back pressure regulator, which creates a small leak across a precision valve. The new AFM was successfully calibrated using a micron-scale grid, and a calcite surface was imaged, both dry and wet. After leak testing the cell, the design was modified to incorporate an additional o-ring seal, so that the membrane seal remains intact upon changing samples. This should allow calibration to remain intact between samples. Testing under pressure and at elevated temperature follow.

E. Collaborative Research: Studies for Surface Exposure Dating in Geomorphology (R.C. Finkel [510-422-2044; Fax 510-422-0208; E-mail rfinkel@llnl.gov], and M. Caffee [510-423-7896; Fax 510-422-1002; E-mail caffee1@llnl.gov])

Objective: The objective of this research is an experimental and theoretical program to fully develop the systematics of in situ produced cosmogenic nuclides in terrestrial surface samples and their application to the dating of surface features and processes. This work includes determination of precise production rates and production depth profiles, studies of altitude and latitude effects, intercalibration with other methods, isolation of in situ produced nuclides from other lithologies an development of in situ produced ¹⁴C. This research is a collaborative endeavor between LLNL (Caffee, Finkel, AMS), UC Berkeley (Dietrich, geomorphology; Nishiizumi, geochemistry) and LANL (Reedy, cosmogenic nuclide modeling; Poths, noble gas mass spectrometry).

Project Description: In the past year this project, in its LLNL manifestation, has attacked two components of the overall project objectives: In situ ¹⁴C and spallogenic ³⁶Cl. ¹⁴C has a half-life which is significantly shorter than the other commonly measured in situ cosmogenic nuclides. This makes it ideal for determining recent erosion rates and for burial dating of recent formations. Blank problems and difficulties in quantitatively extracting ¹⁴C from rocks have limited the applicability of this nuclide. Work is nearly completed on constructing and calibrating an extraction line for determining ¹⁴C in quartz samples. Many geologic problems, e.g. studies involving basalts, require dating of formations which do not contain quartz. ³⁶Cl is an alternative nuclide to use in these cases. The use of ³⁶Cl is made more complex by the existence of two modes of production: spallation from K and Ca and thermal neutron capture form Cl. The thermal neutron capture production has a very different depth profile and much greater dependence on rock composition and moisture than does spallation. We have begun chemical studies to develop methods for determining pure spallation ³⁶Cl.

Results: Results from the in situ ¹⁴C extraction: We have constructed a gas line for extracting ¹⁴C from quartz. The high melting temperature of quartz has proven an obstacle, which we have now surmounted by using a high-temperature tube furnace rather than a radio-frequency generator. Blank tests performed on samples spiked with dead carbon have shown that we can now melt 10 g of quartz and extract carbon with a blank of approximately 5 x 10⁶ atoms of ¹⁴C. The next phase of this work is the separation of the meteoric ¹⁴C from the in-situ ¹⁴C. Initial tests performed to date suggest the meteoric ¹⁴C is released at lower temperatures without the use of an oxidizing atmosphere. The in-situ ¹⁴C is released quantitatively at higher temperatures under oxidizing conditions. The work in progress now will determine the optimum temperature and oxygen fugacity for the separation of the two C components.

Results from spallation ³⁶Cl: There are several approaches to this problem. In the past year we have investigated the possibility of coupling ³⁶Cl determinations to our ¹⁰Be and ²⁶Al work in quartz by removing thermal-neutron capture ³⁶Cl using sequential leaching before determining the spallation component. We are in the process of carrying out sequential leaching experiments on several granitic samples. ³⁶Cl has been extracted from these leaches and measured by AMS. We are currently measuring the concentration of target elements and of chlorine in the samples so that we can assess the effectiveness of this approach. We plan to continue work on this leaching technique as well as undertaking investigations of techniques to measure spallation ³⁶Cl in other lithologies in the upcoming year.

CATEGORY: Energy Resource Recognition, Evaluation and Utilization

A. Velocity Analysis, Parameter Estimation, and Constraints on Lithology for Transversely Isotropic Sediments (P.A. Berge [510-423-4829; Fax 510-423-1057; E-mail berge@s44.es.llnl.gov], J. G. Berryman [510-423-2905; Fax 510-422-1002; E-mail berryman@s123.es.llnl.gov], I. Tsvankin [Colorado School of Mines; 303-273-3060; Fax 303-273-3478; E-mail ilya@dix.mines.edu], K. Larner [Colorado School of Mines; 303-273-3428; Fax 303-273-3478; E-mail klarner@dix.mines.edu], and F. Muir [Stanford University; 415-723-9390; Fax 415-723-1188; E-mail francis@pangea.stanford.edu])

Objectives: Our major objective is to obtain constraints on lithology using the anisotropy parameters recovered from seismic data, in order to improve analysis of seismic reflection data collected in areas where the geology is complicated by anisotropy and heterogeneity.

Project Description: The influence of anisotropy leads to significant distortions in seismic reflection data processing and errors in interpretation that can result in drilling in the wrong place or to the wrong depth, and errors in data analysis that may turn a play into a non-play or vice versa. Theoretical constraints on the elastic stiffnesses in a transversely isotropic medium and algorithms newly-developed at the Colorado School of Mines for processing seismic reflection data exhibiting transverse isotropy are being combined with rock physics analysis to determine how constraints on anisotropy translate into constraints on lithology and improved interpretation of seismic reflection data. Expected benefits of this project are improved processing and interpretation of seismic reflection data in the oil exploration industry and increased understanding of the connections between seismic properties and other physical properties of rocks and sediments exhibiting anisotropy.

Results: Investigators from the Colorado School of Mines have developed new algorithms and codes for processing 3-D seismic reflection data in transversely isotropic media and orthorhombic media, as well as 2-D codes for P-wave nonhyperbolic reflection moveout. Some of these codes have been applied to an ARCO dataset to estimate fracture orientation and some of the anisotropy parameters. These results have been published in Geophysics and in The Leading Edge and were presented at the 1996 annual meeting of the Society of Exploration Geophysicists (SEG) and at the European Association of Geoscientists and Engineers (EAGE) 59th Annual Meeting and Exhibition. These results have also been submitted to Geophysical Prospecting, to the Geophysical Journal International, and for presentation at the 1997 annual meeting of the SEG. Results from research at Stanford include the development of a set of mathematical tools for constructing the elastic properties of anisotropic rocks and for deconstructing elastic moduli into rock properties. These results were presented at the 1996 SEG-EAGE Summer Research Workshop on Wave Propagation in Rocks. LLNL researchers have investigated the range of values and algebraic sign of two key anisotropy parameters in layered transversely isotropic media and how these values are related to fluid content. These results were presented at the Third International Conference on Theoretical and Computational Acoustics Rock Acoustics and Reservoir Geophysics Symposium and have also been submitted for presentation at the 1997 annual meeting of the SEG. LLNL researchers collaborating with Schlumberger researchers have also developed models to describe stress-induced anisotropy in granular media. These results have been published in the ASME Journal of Applied Mechanics and Mechanics of Materials, and related work has been submitted to the Journal of Geophysical Research.

B. Compositional Kinetic Modeling of Oil and Gas Formation (Alan K. Burnham [510-443-8779; 510-423-7914; E-mail burnham1@llnl.gov])

Objectives: This work develops and tests models of petroleum generation, migration, and thermal stability. These models are used to reduce petroleum exploration risk and costs through integrated basin analysis, which combines many aspects of geology, geophysics, geochemistry, and hydrology to determine where and when oil is generated, migrates, and accumulates.

Project Description: Oil and gas generation kinetics and oil destruction kinetics have measured by a variety of techniques, including isothermal hydrous pyrolysis, temperature-programmed pyrolysis using various detectors, and sealed capillary tube pyrolysis, and kinetic models are developed to predict oil and gas generation and composition and their expulsion from the source rock. The final remaining experiments are directed towards understanding the kinetics of oil cracking to better understand the floor for oil survival and formation mechanisms of natural gas. Results: During the past year, experimental work was completed on using ¹³C isotopically labeled compounds to measure the decomposition reactions of hexadecane, dodecene, dodecylbenzene, and dodecylcyclohexane in hexadecane and crude oil matrices. The activation energy for decomposition of hexadecane in hexadecane is greater than in crude oil matrices, causing the net effect of the oil matrix to be slightly inhibiting at temperatures above 330ºC and slightly acceleratory at temperatures below 330ºC. Earlier work had suggested a difference in decomposition products in hexadecane and in crude oil matrices, and the studies using dodecene showed that the primary fate of alkene intermediates is different in the two cases: primarily saturation in the crude oils and primarily alkylation in the hexadecane. This work is in press in Geochim. Cosmoshim. Acta. The data on dodecylbenzene and dodecylcyclohexane is partially analyzed. The rate of dodecylcyclohexane decomposition is 2-3 times faster than that of hexadecane, and the rate of dodecylbenzene decomposition is about 4 times faster than that of dodecylcyclohexane. Scatter in the data made it difficult to discern effects of the matrix on decomposition rates of less than a facto of two, but the decomposition of dodecylcyclohexane appeared to be slightly faster in the hexadecane matrix than in the North Sea oil.