Microcracks and matrix deformation in stressed gas-bearing sandstones
The microstructure of laboratory and nuclear explosively deformed low-permeability sandstones is studied by SEM examination of ion-thinned polished thick sections. Samples prepared in this manner can be oriented relative to coordinate axes, enabling semi-quantitative descriptions of pore structure and matrix deformation. The undeformed rock contains an intricate intergranular network of authigenic chlorite and kaolinite causing the average pore size to be 2--3 ..mu..m. Chlorite platelets are typically oriented in random directions. Uniaxial-strain testing causes elongation and bending of chlorite platelets. Preferential orientation of platelets perpendicular to the direction of applied stress is associated with increased strains. Grain fracturing rarely extends further than a grain diameter. The effect of uniaxial-stress testing depends on the confining pressure of the test. Fracturing occurs primarily at grain boundaries within the immediate fault zone in samples tested at less than 50 MPa confining pressure. Away from the fault, matrix and pore structure are essentially unaltered. At higher confining pressures, grain fracturing is common and occurs further from the fault, although rarely extending beyond a grain diameter. Compaction and preferential alignment of chlorites is common. At 600-MPa confining pressure, the pore size associated with chlorite-filled areas is less than a micron. Nuclear explosively deformed samples contain long macroscopic fractures oriented perpendicular to the shock-front direction. In the vicinity of these fractures, chlorite deformation is similar to that observed in the laboratory-tested samples. Away from these areas, the microstructure is unaltered.
- Research Organization:
- California Univ., Livermore (USA). Lawrence Livermore Lab.; California Univ., Berkeley (USA). Dept. of Geology and Geophysics
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 6775540
- Report Number(s):
- UCRL-80328; CONF-770484-1
- Country of Publication:
- United States
- Language:
- English
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030200* -- Natural Gas-- Reserves
Geology
& Exploration
ALUMINIUM COMPOUNDS
ALUMINIUM SILICATES
BENCH-SCALE EXPERIMENTS
CRYSTAL STRUCTURE
DEFORMATION
EXPLOSIONS
GEOLOGIC DEPOSITS
GRAIN SIZE
KAOLINITE
MICROSTRUCTURE
MINERALS
NATURAL GAS DEPOSITS
NUCLEAR EXPLOSIONS
OXYGEN COMPOUNDS
POROSITY
RESERVOIR ROCK
RESOURCES
ROCKS
SANDSTONES
SEDIMENTARY ROCKS
SILICATES
SILICON COMPOUNDS
SIZE
STRAINS