BURIAL DIAGENESIS OF ILLITE SMECTITE IN SHALES AND THE ORIGINS OF AUTHIGENIC QUARTZ AND SECONDARY POROSITY IN SANDSTONES/

Several gram-size samples of <0.1 mu m, nearly monomineralic mixed-lay er illite/smectite (I/S) were isolated from nineteen whole-cores of sh ale of the Frio Formation from the Texas Gulf Coast. Between similar t o 7,000 ft (2133 m) and similar to 15,000 ft (4572 m) burial depth I/S changes from random-interstratification (R = 0) with low %I to ordere d-interstratification (R = 3) with high %I. The accompanying chemical changes are loss of Si, Mg, and Fe+3, gain of K, Al, and Fe+2, and a s hift in the KEE content of the mineral. There is no chemical or minera logical evidence for a cessation of I/S reaction at similar to 80% I a s is commonly accepted; both chemical and mineralogical changes occur throughout burial. Continuous change in the oxygen isotopic compositio n of I/S implies that the conversion of smectite-layers to illite-laye rs is a dissolution and reprecipitation process. Mass balance calculat ions indicate that burial diagenesis of Frio Formation shales is an op en-system process that requires addition of K2O and Al2O3 and results in loss of SiO2. The amount of SiO2 made available by shale diagenesis is sufficient to be the source of the quartz-overgrowth cements in th e associated Frio sandstones. Iron reduction in I/S is a significant s ource of the acid required for the diagenesis of both Frio shales and sandstones. Copyright (C) 1997 Elsevier Science Ltd.