GRANTEE: ARIZONA STATE UNIVERSITY

Departments of Geology and Chemistry/Biochemistry

Box 871404

Tempe, Arizona 85287-1404

GRANT: DE-FG03-95ER14533

PERSONS IN CHARGE: Peter R. Buseck (602-965-3945; E-mail: pbuseck@asu.edu) and Huifang Xu (fangxu@asu.edu)

Objectives: To gain an improved understanding of the microstructures and reaction mechanisms for the following diagenetic reactions: (1) berthierine-to-chamosite and polytype transformation in chamosite, (2) smectite illitization and mechanism for the formation of periodically interstratified illite/smectite, and (3) textural and structural evolution of the organic matter vitrinite.

Project Description: We are studying clay and detrital minerals as well organic matter in sequences of Upper Cretaceous and Lower Tertiary clastic rocks from the southern Rocky Mountains using a range of analytical and structural techniques. We are especially interested in: (1) the berthierine-to-chamosite reaction, (2) smectite illitization, and (3) organic diagenesis.

This research will lead to an improved understanding of the states of I/S, C/B, organic matter, and detrital minerals in diagenetic environments. The results will provide important information for determinations of reaction mechanisms and establishment of kinetic models that permit one to predict the extent to which the formation of certain clay minerals occurs at certain depths, temperatures, and times. Such fundamental data will help with evaluations of basinal diagenetic patterns in hydrocarbon exploration.

Results: We have successfully used a slow-scan CCD camera to record high-resolution images of mixed-layer chlorite-pyrophyllite minerals, thereby minimizing the limiting effects of damage by the high-energy electron beams used for high-resolution imaging. The stacking sequence of chlorite/pyrophyllite intergrowths is commonly nonperiodic along the c-axis. Three 1:1 polytypes (1-layer, 2-layer, and 1M_d) have been identified from selected-area electron diffraction patterns and on-dimensional HRTEM images. Two-dimensional HRTEM images show that neighboring chlorite and pyrophyllite layers are coherent in the mixed-layer crystals.

Electron microprobe analysis shows that interstratified crystals are heterogenous in composition and vary from area to area within a nominally single crystal. The mole fractions of chlorite range from 0.46 to 0.23. We suggest that the chlorite/pyrophyllite crystals with both periodic mixed layers formed by nonequilibrium crystallization from Al-rich solutions.

We also obtained atomic-force microscope results for the transformation from anhedral smectite to euhedral smectite-rich I/S mixed-layers. We are using the microtopographic differences between them to determine the transformation mechanisms of dissolution-precipitation.

In an attempt to obtain more information about the bonding crystallinity of organic matter in geological environments, we have used electron energy-loss spectrometry (EELS) to study vitrinite from high-and low-volatile bituminous coals, from Pittsburgh, PA and Pocahontas, VA, respectively. The EELS measurements can be used to avoid artifacts from the preferred orientation of particles during reflectivity measurements. Preliminary results suggest that we can characterize the cluster crystallinity and C bonding features of these samples by studying the p * and s* carbon peaks of the EELS spectra.

GRANTEE: ARIZONA STATE UNIVERSITY

Center for Solid State Science

Tempe, Arizona 85287-1704

GRANT: DE-FG03-94ER14414

TITLE: Chemical Dynamics of Hydrocarbon Reservoirs Investigated by Secondary Ion Mass Spectrometry

PERSON IN CHARGE: Richard L. Hervig (602-965-3107; Fax 602-965-9004; E-mail richard.hervig@asu.edu) and Lynda Williams (602-965-5081; Fax 602-965-8102; E-mail lynda.williams@asu.edu)

Objectives: Microanalyses of oxygen and boron isotopes in authigenic silicates are being obtained to determine their variation in hydrocarbon-producing sedimentary basins. These analyses can be used to constrain mass transport processes occurring during diagenesis and hydrocarbon migration.

Project Description: Current research examines the isotopic heterogeneity of authigenic silicates (quartz, feldspar, clay minerals) in the Gulf Coast Sedimentary Basin in order to interpret the chemical environment of precipitation during burial diagenesis. In situ analyses of O-isotopes help determine the timing of reservoir cementation and the volumes and chemistry of the paleofluids responsible. B-isotopes are being investigated as an aid to interpreting the concurrent changes in O-isotopes, and identifying the fluid sources. An understanding of the sources of mineral components is required to interpret mass transfer processes and develop better models to explain fluid flow and hydrocarbon migration.

Results: This year research focused on 1) testing the O-isotope analytical technique on various clay mineral standards in order to examine variations in instrumental calibration, and 2) developing a better understanding of the B-isotope systematics related to clay minerals undergoing diagenesis.

Three clay mineral standards were analyzed repeatedly for O isotopes revealing identical calibrations (within error) for smectite (SWy-1) and illite (IMt-1). In-situ analyses of mixed-layer I/S in thin section should be readily interpreted. Kaolinite calibration (KGa-1) differs from I/S by ~10 per mil.

Boron isotope microanalyses have been made on minerals characterized by bulk techniques and on a Cretaceous bentonite (Pierre shale) heated by a cross-cutting basaltic dike which drove illitization from 20%-100% (200- 450EC). The B content of the <2mm size fraction (300ppm - 100 ppm) generally decreased with increasing temperature. The d¹¹B of the fixed-B was initially -5 per mil, but decreased linearly (to -19 per mil) as illitization progressed from 64% to 96% (R1 ordered). Under this thermal regime, ¹¹B is released from the clay fraction during illitization rather than ¹⁰B being incorporated in the illite. It was observed that the B-content of the bentonite decreased with increasing organic matter maturity (decreasing H/C and O/C). This indicates that clays may reflect the boron chemistry of organic matter.