Local Structural Configurations Research Articles

Structural Setting and Validation of Direct Hydrocarbon Indicators for Amauligak Oil Field, Canadian Beaufort Sea (1)

The recent discovery of a giant oil field in the southeastern Beaufort-Mackenzie basin has brought this frontier area closer to oil production despite severe environmental conditions. The Amauligak field is a fault-bounded growth structure developed in the Kugmallit Trough, within deltaic deposits of the Beaufort Sea Shelf. Shelf construction occurred during the Late Cretaceous-Tertiary by repeated progradation of the Mackenzie River delta in response to rift-induced opening of the Canada basin and extension of the Kugmallit Trough. The Amauligak field contains oil and gas in multiple sandstone reservoirs of the Oligocene Kugmallit sequence. The upper sandstones are truncated by an unconformity and sealed by the overlying shales of the Miocene Mackenzie Bay sequence. Based on two-dimensional seismic coverage, the field was initially described as structurally simple. Interactive interpretation on Landmark and SIDIS workstations of a three-dimensional seismic program revealed the local structural complications, spatial configuration, and detailed structural elements of the field. Direct hydrocarbon indicators (DHIs), including amplitude anomaly, phase change, flat spot, and low-frequency zone, associated with a large gas cap were investigated using full amplitude-range and attribute-extraction methods. Interpretation of seismic data and correlation with well results suggest that a combination of structural, stratigraphic, and hydrodynamic factors are responsible for the appearance and distribution of Amauligak DHIs. On the amplitude displays, a flui contact is seismically mappable over the field, clearly separating the gas cap from the wet reservoir.

Measurement of local structural configurations associated with reversible photostructural changes in arsenic trisulfide films.

Extended x-ray-absorption fine-structure measurements have been made on three reversible and reproducible cycles of thermally annealed and light-soaked amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ films. Associated with the light-soaked material are (1) a very small increase in the population of wrong bonds in the first shell, (2) an enlarged As---S---As bond angle with an expansion of As---As distance in the second shell, (3) a larger spread in the distribution of As---S---As bond angles, and (4) an absence of any change in the third As---S shell. From these data, we present the first quantitative correlation between observed local atomic structural changes and measured macroscopic properties that are associated with photodarkening. Our data demonstrate that the photoinduced structural changes mainly involve bonding alterations at S atoms as well as a change in the dihedral angle relationship between adjacent ${\mathrm{AsS}}_{3}$ pyramids joined at S atoms.

Polymorphism of Nickel-Phosphorus Metallic Glasses

It is shown that nickel-phosphorus metallic glass alloys not only exhibit two distinct local structural configurations for a given composition, but also that the configuration can be selected by choosing the appropriate processing parameters. It is also shown that the structure exhibiting the greater Knight shift exhibits a discontinuous transformation to the structure with the lower Knight shift. Measurements have been extended to amorphous alloys containing up to 42 at.% phosphorus.

Internal friction peaks in metallic glasses

Low-temperature, thermally-activated internal friction peaks have been observed in three binary metallic glasses containing only metallic components: Cu 50 Zr 50, Co 35 Y 65 and Co 35 Dy 65. The magnitudes of the activation energies and the characteristic relaxation times obtained for these materials as well as the sharpness of the peaks point to process involving either atomic jumps of a specific nature or similar localized reordering. Possible local structural configurations with built-in, non-uniform stress distributions that could lead to such relaxation effects are discussed.

Densities of valence states of amorphous and crystallineAs2S3,As2Se3, andAs2Te3: X-ray photoemission and theory

The density of valence states (DOVS) of amorphous and crystalline ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ and ${\mathrm{As}}_{2}$${\mathrm{Te}}_{3}$ and of amorphous ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ have been investigated with monochromatized x-ray photoemission spectroscopy (XPS). All three compounds exhibit similar XPS spectra consisting of a pronounced peak near the Fermi energy associated with the nonbonding $p$ orbital of the chalcogen atom followed by a shoulder 4 eV wide, ascribed to the bonding $p$ orbitals, and lower $s$ bands situated about 13 eV below the top of the valence band. These XPS spectra are well described by DOVS synthesized from those of the constituent elements. The similarity in the DOVS of the crystalline and amorphous forms of ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ is presumably attributable to the similarity of the local structural configurations of the two forms. In the case of ${\mathrm{As}}_{2}$${\mathrm{Te}}_{3}$, where the crystalline and amorphous structures are quite different, the very similar XPS spectra for the two forms are argued to be a consequence of the chemical composition and the maintenance of chemical order in the amorphous state. A band-structure calculation for a model of an isolated arsenic chalcogenide layer is performed using only important nearest-neighbor interactions. The few parameters in the calculation are derived from XPS spectra for the constituent elements and the resulting band structure is in good agreement with the gross features of the XPS DOVS for the arsenic chalcogenides. From this semiempirical band structure, the $s\ensuremath{-}p$ hybridization is estimated to be about 5%. This $s\ensuremath{-}p$ interaction, however, is sufficient to shift the $s$ and $p$ energy bands by 1 eV.