Texture, 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