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Evolution and Occlusion of Porosity in the Lower Cretaceous Aptian-Albian Sligo Formation Shelf Edge Reef, South Texas

Eyitayo Aina and Brenda Kirkland
[email protected]

Approximately 120 ft (37 m) of core (14,940 – 15,015 ft and 16,950 – 17,005 ft) from the dry Mobil McElroy-1 well (Lower Cretaceous Aptian-Albian, Sligo Formation) was studied to evaluate porosity and document the diagenetic processes that lead to porosity evolution and occlusion in the Sligo Formation. Detailed core description facilitated the careful selection of samples for thin sections and SEM from approximately every 5 ft of core available. Alizarin red-S and potassium ferricyanide solution was used to stain thin sections that were petrographically analyzed under standard and cathode luminescence microscopes. XRD was used to confirm calcite and dolomite carbonate cements. Stable isotope analyses facilitated the characterization of pore-occluding carbonate cements and suggest that the main pore-modifying diagenetic environment of the McElroy-1 well was meteoric. The ?18O (‰ V-PDB) isotope values between -1.9 and -2.4 of a few pore-occluding cements are less definitive but suggest marine influence. Thin section porosity analyses showed decrease in average porosity in the McElroy-1 well with depth. Porosity loss was greatest below 16,952 ft (5,167 m) where pressure solution appeared to be most intense. Stylolite-associated, late diagenetic calcite, in places completely filling unloading fractures and tension gashes negatively impacted porosity at depth. SEM analyses revealed fine quartz and calcite occluding porosity in micropores and microfractures throughout diagenesis. Early through late, medium (1mm – 3 mm) to large (> 3 mm) calcite and non-ferroan dolomite were observed to jointly contribute to more than 10% of primary porosity loss in the well.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013