ABSTRACT
The spatial distribution of burial diagenetic cements in sandstones of the Frio Formation is related to permeability and fluid flow. The relationship between sandstone diagenesis and fluid flow implies that the modification of the rocks is largely the outcome of reactions between the detrital and authigenic minerals and the formation water in the pores. During burial, formation water preferentially flowed through the most porous and permeable sandstones. Diagenetic alteration is more extensive in permeable sandstones because the minerals in those rocks were exposed to significantly more pore volumes of reactive water, each of which accomplished a small amount of diagenetic work. Consequently, porous and permeable units were also the preferred sites of diagenetic mineral precipitation. The heterogeneous distribution of authigenic minerals in sandstones is a result of changing preferential flow paths, due to diagenetic modification, during burial. Sandstones that are not the most porous or permeable at the time of deposition commonly preserve the best reservoir quality at depth because they are less modified by diagenesis.
Petrographic analyses indicate that the SiO2 needed for the quartz overgrowths in the sandstones had an allochthonous source. δ18O values for diagenetic quartz imply that the mineral precipitated from rapidly ascending, hot formation water. Quartz overgrowths are most abundant in distal-shelf facies sandstones, where the isolated nature of the sandstones in a predominately shaly section maximized preferential fluid flow. Mass balance calculations show that thousands of pore volumes of formation water are required to supply the SiO2 needed for the quartz overgrowths in the sandstones. The amount of water required to produce the diagenetic modification of the sandstones is much larger than the amount of water present in the entire Frio Formation, implying that reuse of formation water is an important and necessary process during diagenesis.
