The effects of depositional environment and formation water chemistry on the diagenesis of Frio Formation (Oligocene) sandstones and shales, Aransas, Nueces, and San Patricio Counties, Texas
Shorezone and shelf facies sandstones of the Frio Formation from five growth fault blocks in the Corpus Christi, Texas, area were studied by petrographic, isotopic, and XRD techniques in order to determine the factors influencing their diagenesis. The distribution of quartz overgrowths, calcite, kaolinite and secondary porosity in the sandstones suggests that permeability and fluid flow largely govern the diagenetic modification of the rocks. The most important control on the original permeability of a sandstone is clay content and is related to depositional environment. Mixed-layer I/S is the most common clay mineral in both Frio sandstones and shales. During burial diagenesis, the amounts of I/S and chlorite increase and the amounts of kaolinite and discrete illite decrease in both rock types. Mass balance calculations indicate the Frio shales act as open systems during diagenesis and require significant import of potassium from a yet unidentified source. The calculations further show that Frio shales can supply all the SiO₂ needed for quartz overgrowths in the associated sandstones. The quantitative relationships between the clay minerals in Frio shales do not support a smectite-cannibalization mechanism for the smectite to illite reaction. [delta] ¹⁸O values of calcite cement in sandstones become progressively depleted with depth, implying that the calcite recrystallizes throughout the burial history of the rock. The ⁸⁷Sr/⁸⁶Sr value and trace element chemistry of diagenetic calcite is largely a function of the chemistry of the formation water causing recrystallization. The presence of chemically distinct calcites and formation waters from adjacent growth fault blocks indicates that these blocks have been hydrologically separate from each other since at least the time of calcite precipitation (~26 m.y.b.p.). There is no evidence that sandstone diagenesis is different in fault blocks characterized by organic acid-rich formation waters. Most of the diagenetic modification of Frio sandstones occurs between 6,000 ft (1,829 m) and 9,000 ft (2,743 m). The [delta] ¹⁸O of quartz overgrowths indicates that they precipitated from hot, rapidly ascending formation water. The water was probably part of a flow (convection?) system that developed in the transition zone from hydrostatic to nearly lithostatic fluid pressure. The [delta] ¹⁸O values of calcite and kaolinite/dickite support this model for the diagenesis of Frio Formation sandstones.