EVOLUTION AND OCCLUSION OF POROSITY IN THE LOWER CRETACEOUS APTIAN-ALBIAN SLIGO FORMATION, SOUTH TEXAS

Approximately 120 ft of core was studied to document the pre-, syn-, and post-depositional processes that resulted in the evolution and occlusion of porosity in the dry Mobil McElroy-1 well (Lower Cretaceous Aptian-Albian, Sligo Formation). The core was described and samples were taken at every 5 ft (1.5 m) interval for thin section and SEM. Thin sections were stained with alizarin red-S and potassium ferricyanide solution to distinguish between different carbonate cements, and were petrographically analyzed under standard and cathode luminescence microscopes. Stable isotope analyses were used to characterize pore-occluding carbonate cements and suggest that the main pore-modifying diagenetic environment of the McElroy-1 well was meteoric. The δ¹⁸O (‰ V-PDB) isotope values between -1.9 and -2.4 of late cements are less definitive but suggest marine influence. Trace and minor element compositions of strontium, magnesium, sodium, manganese and iron within the sampled cements were used to corroborate interpreted sources and diagenetic environments of cement-precipitating fluids.

Fracture and other pore-filling carbonate cements occluded porosity with up to three generations of cements. Cement generations were inferred from stable isotope signatures and cathode luminescence intensities of sampled cements. Early through late medium (1 mm – 3mm) 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. Baroque dolomites replacing stylolite-associated late diagenesis calcite negatively impacted porosity, especially at depth where pressure solution was seen to be most intense.

The varying petrographic characteristics including luminescence intensities of the carbonate cements are thought to be related to the transformation of calcite to dolomite. Horizontal and non-horizontal stylolites contributed to post-depositional porosity loss and loss of reservoir rock thickness. This was observed to be greatest below 16,963 ft (5,170 m) . Thin section porosity analyses showed that generally porosity loss was greatest at depths below 16,952 ft (5,167 m) in the McElroy-1 well.