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

摘要

The Sligo Formation (Lower Cretaceous Aptian-Albian) and equivalent formations are part of thick carbonate and terrigenous sediments that accumulated around the Gulf of Mexico in the Early Cretaceous (Bebout and Loucks, 1983). The Mobil McElroy-1 wildcat well drilled into the Lower Cretaceous Aptian-Albian Sligo Formation, onshore South Texas, was a dry hole. Figure 1 shows the location of the McElroy-1 well in Webb County, onshore South Texas. This study makes use of approximately 120 ft (37 m) of core from two sections of the dry Mobil McEIroy-1 wildcat well-upper core (14,940-15,015 ft) (4554-4577 m) and lower core (16,950-17,005 ft) (5166-5183 m)-to document the porosity of the Sligo Formation and the pre-, syn- and post-depositional processes that resulted in the evolution and occlusion ofthat porosity in the Sligo Formation. Detailed core description facilitated the careful selection of samples for thin sections and scanning electron microscope (SEM) from approximately every 5 ft (1.5 m) of core available. Alizarin red-S and potassium ferricyanide solution were used to stain thin sections that were petrographically analyzed under standard and cathode luminescence microscopes. X-ray diffraction (XRD) was used to confirm calcite and dolomite carbonate cements. Stable isotope analyses facilitated the characterization of pore-occluding carbonate cements.