Diagenetic History of the Upper Jurassic Smackover Formation and Its Effects on Reservoir Properties: Manila Sub-basin, Eastern Gulf Coastal Plain
LLINAS, JUAN CARLOS
Department of Geological Sciences, The University of Alabama, Tuscaloosa, AL
Diagenetic processes play an important role in the development of reservoir properties of carbonates, not only due to the high susceptibility to diagenesis of these rocks, but also because these processes are initiated immediately after sediment deposition, and evolve through time, modifying the original rock fabric and therefore the reservoir quality. This is the case for the Upper Jurassic Smackover Formation, which is one of the major oil producing carbonate reservoirs in the Gulf of Mexico. The diagenetic history of the Smackover Formation in the Manila Sub-basin is described and illustrated in this study. The results of the study are based on core descriptions and petrographic analyses of approximately 200 thin sections from 11 wells in the Vocation oil field.
In this area, the Smackover Formation was affected by extensive dolomitization, and different episodes of carbonate recrystallization, cementation, compaction, and dissolution that occurred in the eogenetic and mesogenetic environments. The most important diagenetic events for the enhancement of the reservoir properties include: (1) extensive dolomitization that not only generated new porosity, but also enhanced permeability, (2) early calcite cementation that preserved primary porosity from later compaction, (3) selective dissolution of aragonite allochems that produced moldic porosity especially significant in the oolitic grainstone facies, (4) deep non-fabric selective dissolution that yielded vuggy and channel porosity, characteristic of the reef facies. The variability in the depositional textures and the superposition of various types of diagenetic events experienced by these rocks produce sharp changes in porosity and permeability as indicated by core analyses and thin section observations. Despite the numerous and complex diagenetic modifications, the original depositional fabric controls the distribution of the main reservoirs in the shoal grainstone and the microbial reef lithofacies, which are the main exploration targets.