Approximately 37 m (120 ft) of core was studied with the objective of evaluating and documenting the development and loss of porosity in the dry Mobil McElroy-1 well (Lower Cretaceous Aptian – Albian Sligo Formation). Core slabs were described and thin section samples, taken every 1.5 m (5 ft), were stained and analyzed under standard petrographic, cathode luminescence, confocal and scanning electron microscopes. The main conclusion is that average porosity significantly reduced with depth. Carbon and oxygen isotope values obtained for 20 samples show that the main pore-occluding diagenetic environment was meteoric with most samples having relatively low delta18O (-3.1%o to -6.7%o V- PDB) values. Early through late stage medium (1 mm – 3 mm) to large (> 3 mm) calcite and nonerroan dolomite jointly contributed to more than 10% of primary porosity loss. This study significantly contributes to the understanding of the Sligo Formation and promotes development of natural gas resources.