Print this pageTexture, Mineralogy, and Petrophysical 
Properties
of Geopressured Shales, Gulf of Mexico
J-W. Kim, R. R. Berg, J. Watkins, and T. T. Tieh
Department of Geology & Geophysics, Texas
A&M University, College Station, TX
An understanding of the origin and characteristics of overpressured shales is of critical importance to the safe production of overpressured gas or oil reservoirs. Geopressured shale samples from wells located offshore Louisiana, Gulf of Mexico were examined by scanning electron microscopy (SEM), x-ray diffraction (XRD) and welllog analysis to study the characteristics of fabrics, fractures, and changes in porosity and permeability caused by high fluid pressures.
Well logs from this study area including neutron-density, conductivity
and sonic logs show features typical of a geopressured zone at
a depth of 2200 m. The active smectite-to-illite transition zone indicated
by changes in CEC, and total K also corresponds to the top
of the geopressured zone. Because of the denser packing arrangement
of water molecules in the interlayer water, it will expand and abnormal
fluid pressure may result when the interlayer water is released
to a pore system during clay transformation. Geopressured shales
with increasing burial depth become more illitic, better oriented, and
less porous; they show a significant decrease in permeability and a
rise in the fluid pressure gradient. The shale compaction curve
as a
function of burial depth was used to determine the maximum sealing
depth for hydrocarbon. In this study shales effectively seal above
2400-m depth with the maximum sealing depth of 1260 m. Shales
undergo slow-drained compaction down to 2400 m while below
2400-m depth shales undergo drained compaction and fractures begin
to develop.
AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana