Abstract

The sequence of authigenic minerals in a Pennsylvanian deltaic sandstone from the Strawn Series, north-central Texas, is: a) growth of chlorite rims around quartz grains, b) cementation by an average of 11% syntaxial quartz overgrowths, c) cementation by calcite, d) dissolution of calcite cement, feldspar grains and rock fragments, and finally e) cementation by Fe-calcite, ankerite and kaolinite. Isotopic analyses of the authigenic phases and formation water lead to the following conclusions. Chlorite formed during shallow burial soon after deposition when amorphous aluminosilicates and iron oxides-hydroxides reacted with pore waters to form the iron-rich, lb polytype. Quartz overgrowths have a delta O 18 of approximately +24 per thousand and formed under conditions of near maximum burial (2.1 km) as compaction of adjacent shales and simultaneous degradation of smectite released silica-bearing solutions which escaped through the porous sandstone aquifers. Calcite cement formed when the rate of burial slowed sharply in the middle Mesozoic and simultaneously ended the smectite reaction and the fluid migration required for quartz cementation. The CaCO 3 was probably derived from slow stylolitization of underlying carbonates. Dissolution may have been caused by the late maturation and migration of hydrocarbons at the relatively cool burial temperatures of these rocks. Today the off field is being cemented at a depth of approximately 1.4 km by Fe-calcite and ankerite, and the H (super +) thus released is forming kaolinite at the expense of detrital feldspar. The formation fluid today appears to be in near ionic equilibrium with kaolinite, albite, K-spar, calcite, and ankerite, and in near isotopic equilibrium with kaolinite, calcite, and ankerite.

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