ABSTRACT: Diagenesis of Oligocene Frio Formation Sandstones from South Texas: The Role of Calcite Precipitation and Dissolution in Porosity Evolution

DIGGS, T. N., Shell Western Exploration and Production, Inc., Houston, TX

Petrographic observations, integrated with stable isotope and trace element compositions of the carbonate fraction of sandstones in the Oligocene Frio Formation (Kenedy and Kleberg Counties, south Texas), provide evidence for three episodes of calcite precipitation and at least two episodes of calcite and Fe-calcite dissolution, regenerating porosity at depth. Fe-poor Calcite I was an early phase of limited lateral extent; stable isotopes and other data suggest emplacement at temperatures less than or equal to 35 degrees C. Fe-poor Calcite II was the first volumetrically significant authigenic phase to precipitate. Average Oxygen 18 (-6.5o/oo PDB) and Strontium 87/Strontium 86 (0.7072) of Calcite II and considerations of probable pore water Oxygen 18 suggest precipitation at ~75 degre s C in waters slightly less radiogenic than Frio seawater (0.7079). Based on paragenetic relations, present geothermal gradients, and Oxygen 18 of Calcite II and Calcite III, the onset of Calcite II dissolution is interpreted to have occurred at depths >1.8 km between 80 degrees and 100 degrees C. Fe-rich Calcite III was the second volumetrically important subsurface phase to appear, precipitating from waters of more variable O and Sr isotopic compositions and/or over a greater temperature range than Calcite II. Oxygen 18 of Calcite III ranges from -6.5o/oo to nearly -9o/oo PDB, and Strontium 87/Strontium 86 ratios vary from 0.7076 to 0.7087. Using a representative Oxygen 18 of -7.5o/oo PDB for Calcite III, and assuming formation waters had been buffered to about +6o/oo SMOW by the sm ctite to illite transformation, Calcite III precipitation is constrained to ~100 degrees C. Calcite III dissolution therefore must have occurred at >100 degrees C (>2.5 km), but no precise temperature limits have been placed on this event by isotopic geothermometry.

The radiogenic nature of Calcite III relative to Calcite II, and the presence of two Calcite-III-dominated sandstones with Sr ratios more radiogenic than Frio seawater suggest precipitation after: (1) emplacement of allochthonous pore fluids, possibly from the Mesozoic section; or (2) reaction of small volumes of older cratonic silicates. Present-day formation waters in the fields under consideration are similar in composition to waters in the updip Mesozoic section, suggesting the waters may be, at least in part, allochthonous. Association of "Mesozoic-type" waters with calcite dissolution and porosity enhancement in the Tertiary clastic section has been observed but is poorly understood.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)