ABSTRACT

Geopressured shale samples from wells located offshore Louisiana, Gulf of Mexico were examined by scanning electron microscopy (SEM), and compared with well-log 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. 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.