ABSTRACT

Two major depositional facies characterize producing Lower Tuscaloosa Formation (Upper Cretaceous) sandstone reservoirs in the McComb-Little Creek Field areas; a lower fluvial sequence ("Denkman Sand", Little Creek) topped by near shore marine deposits ("McComb Sand"). Petrographic studies reveal that the sandstones of both fields were deposited as very fine to medium-grained quartz arenites and quartz litharenites. Petrographic and X-ray diffraction analysis of the clay mineral content of the sandstones indicate that most of the clays are authigenic and comprise a suite which includes chlorite, kaolinite, mixed-layer illite/smectite and illite. Mixed-layer illite/smectite appears to be considerably more abundant in the McComb reservoir sandstones than at Little Creek. The reservoir sandstones in the McComb and Little Creek Fields are highlighted by good secondary porosity (average 19%, range 32 to 27%) developed through dissolution of rock fragments and carbonate cements.

The combined petrographic and X-ray diffraction results suggest a diagenetic model which begins with mechanical compaction of the sediments and the precipitation of quartz overgrowths. Carbonate cements replaced quartz overgrowths and filled interstices between framework grains during the first stages of mesodiagenesis. During a more mature diagenetic stage, perhaps corresponding to hydrocarbon migration and emplacement, decarboxylation of contained organic matter in interbedded and adjacent fine-grained units led to wide-spread decarbonization of the reservoir sandstones and the creation of hybrid, oversized, moldic and intergranular pores with good pore-throat interconnection. Complete and partial alteration of rock fragments and feldspars was accompanied by neoformation of chlorite (as grain coatings), kaolinite, and illite.

The formation of vermicular kaolinite occurred after the main phase of secondary porosity generation and hydrocarbon emplacement. Late stage diagenetic events also include the precipitation of euhedral quartz crystals in some pore spaces and over chlorite-coated grains.