| Shale samples from wells located off-shore Louisiana, Gulf of Mexico have been examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray powder diffraction (XRD) to study the relationship between clay mineralogical and fabric changes during burial diagenesis, and the effects of these changes on petrophysical properties. For electron microscopic observations a new sample preparation method was used which allows elucidation of the details of clay fabric.; Neutron-density logs from the wells studied show an abrupt density decrease and porosity increase at an approximate sub-bottom depth of 2200 m, which marks the beginning of the geopressured zone. Conductivity and sonic logs also show features typical of a geopressured zone at the same depth. The active smectite-to-illite transition zone obtained from CEC, total K, and XRD data also corresponds to the top of the geopressured zone. Systematic changes in both clay mineralogy and clay fabric are the primary causes for the well log responses. Above 2200-m depth the shales are smectite rich, generally lack particle orientation, and contain appreciable pores and crystal dislocations, features indicative of modest local permeability. From 2200-m depth downward, with increasing burial, the shales become more illite rich, better laminated, and less porous. The fabric changes and concomitant growth of illite crystals render a significant decrease in permeability and a rise in the fluid pressure gradient.; To determine the maximum sealing depth, shale compaction curve as a function of burial depth was used. 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. |
