ABSTRACT

Depth-related mineralogic changes in Frio and Vicksburg shales from Ann-Mag Field, Brooks County, Texas, were determined in order to assess the nature and extent of burial diagenesis and geochemical alterations and their relationship to development of high fluid pressures. Mixed-layer illite-smectite and illite dominate the mineralogy of the shales. Burial diagenesis has resulted in the transformation of randomly interstratified illite/smectite to a regularly interstratified illite/smectite and to illite over a depth range of 6000 to 10,400 ft (1828 to 3170 m). The amount of expandable smectite layers in the fine clay fraction decreases from 70% in the shallowest sample to 22% in the deepest sample, whereas illite increases from 27% to 61%. Alteration of feldspar in the fine silt fraction probably provided the potassium required in this reaction.

An abrupt increase of smectite to illite alteration occurs at a depth of approximately 8900 ft (2713 m), corresponding to a temperature of 224°F (107°C). Fluid pressures estimated from conductivity data and measured pressures show that at this depth a significant increase in fluid pressure takes place. Associated with this is also a decrease in shale density. These relationships suggest that high fluid pressures in this field are caused by clay transformation reactions which expel interlayer water into the pore spaces. Upward movement of pore fluids in thick shales is also suggested by extractable cation determinations.