Burial Diagenesis and Geochemical Alterations and Their Relationship to High Fluid Pressures, Frio and Vicksburg Shales: ABSTRACT

Abstract

Depth-related mineralogic changes in Frio and Vicksburg shales from Ann-Mag field, Brooks County, Texas, were studied to assess the nature and extent of burial diagenesis and geochemical alterations, and to determine their relationship to development of high fluid pressures. Mixed-layer illite-smectite and illite dominate the mineralogy of the shales. Burial diagenesis has transformed randomly interstratified illite/smectite to regularly interstratified illite/smectite and to illite, over a depth range of 6000-10,400 ft (1828-3170 m). The quantity 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/sup 0/F (107/sup 0/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 increase is a decrease in shale density. These relationships suggest that high fluid pressures in this field are caused by clay transformation reactions that expel interlayer water into themore » pore spaces. Upward movement of pore fluids in thick shales is also suggested by extractable cation determinations.« less