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 μm, nearly monomineralic mixed-layer illite/smectite (I/S)were isolated from nineteen whole-cores of shale of the Frio Formation from the Texas Gulf Coast. Between ∼7,000 ft (2133 m) and ∼15,000 ft (4572 m) burial depth I/S changes from random-interstratification (R = 0) with low %I to ordered-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 shift in the REE content of the mineral. There is no chemical or mineralogical evidence for a cessation of I/S reaction at ∼80% I as is commonly accepted; both chemical and mineralogical changes occur throughout burial. Continuous change in the oxygen isotopic composition of I/S implies that the conversion of smectite-layers to illite-layers is a dissolution and reprecipitation process.Mass balance calculations indicate that burial diagenesis of Frio Formation shales is an open-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 the associated Frio sandstones. Iron reduction in I/S is a significant source of the acid required for the diagenesis of both Frio shales and sandstones.