Joel S. Watkins1,
Thomas T. Tieh1
(1) Texas A&M University, College Station, TX
Abstract: Preliminary evidence for fracture permeability in geopressured shales of the Texas Gulf Coast
Samples representative of Tertiary geopressured shales at four locations of the Texas Gulf Coast were examined utilizing Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD) methods to assess the nature of shale seal and the possible presence of fracture permeability. Well log data were used to determine the top of geopressured zone at each location.
At three of the four locations the shales show presence of micro-fractures. The fractures range in width from 1 to 10 µm, are generally bedding parallel, and contain authigenic minerals, most commonly pyrite. These authigenic minerals indicate that drained compaction occurred whereby fluids went through the fractures which were initiated by geopressure. Geopressure reduces the effective pressure resulting in an increase of fluid pressure and thus development of the micro-fractures. The geopressured shales with micro-fractures are illite-rich and contains little silt. At the fourth location the shale samples are also illite-rich, but contain abundant silt particles. These silty shale samples are devoid of any micro-fractures from SEM observations. Geopressure may not be built up high enough to develop the fractures in the silty shales because of their high porosity and permeability.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana