AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Assessment of Porosity and Diagenesis in the Lower Cretaceous Aptian-Albian Sligo Formation, South Texas
(1) Department of Geosciences, Mississippi State University, Mississippi, MS.
Approximately 120 ft (37 m) of core from the Mobil McElroy-1 well (Lower Cretaceous Aptian-Albian, Sligo Formation) was studied to evaluate diagenetic processes resulting in porosity evolution and occlusion. After detailed core description, samples were taken at approximately 5 ft (1.5 m) intervals for thin sections and SEM. Thin sections were stained with alizarin red-S and potassium ferricyanide solution and petrographically analyzed. Thin section mapping and point counting were used to establish the diagenetic sequence and evolution of porosity. Organic components and cement microstructure were investigated using scanning electron microscopy. Post-depositional porosity-impacting structures in the Mc-Elroy-1 well (including stylolites, fractures and microfractures) occluded porosity with up to three generations of cement. Medium (1 mm - 3 mm) to large (> 3 mm) equant calcite and non-ferroan dolomite cement encasing compacted grains was interpreted as late relative to compaction and jointly responsible for more than 10% of the primary porosity lost. Isopachous rims of cement were followed by dissolution of aragonitic allochems and precipitation of fine (≤ 1 mm) equant calcite. Evidence of contemporaneous neomorphism is also present. Emplacement of hydrocarbon inclusions within medium-sized (1 mm - 3 mm) equant cement is interpreted to have preceded the precipitation of large (> 3 mm) calcite crystals, some of which are poikilotopic. These were followed by baroque dolomite. Late-stage cementation is observed to be the main porosity modification event below 14,950 ft (4,557 m) core depth. Hydrocarbon inclusions trapped within fracture and microfracture cements are thought to suggest that fractures served as hydrocarbon escape pathways in the McElroy-1 well prior to fracture cementation. Primary porosity loss is greatest below 16,950 ft (5,166 m) core depth where pressure solution was observed to be most intense.