ABSTRACT

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 205 thin sections from 11 wells in the Vocation oil field area. The field is located in the eastern portion of the Manila Sub-basin and along the southern part of the western flank of the Conecuh Ridge. In this area, marine sediments of the Smackover Formation define an upward shallowing cycle, including microbial reef complex, shallow subtidal, shallow lagoon, shoal complex, and tidal flat-sabkha environments deposited in an evaporate-carbonate setting. This sequence was deposited over an irregular topography produced by block faulting and tilting related to Early Mesozoic rifting events. As a result, the Smackover Formation rests unconformably on Paleozoic basement paleo-highs and overlies siliciclastic sediments of the Norphlet Formation in a down-dip direction.

Petrographic descriptions indicate that the Smackover Formation was affected by extensive dolomitization and different episodes of carbonate 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 pores especially significant in the oolitic grainstone facies, (4) deep burial non-fabric selective dissolution that yielded vuggy and channel pores, characteristic of the reef facies, and (5) fracturing. The variability in the depositional textures and the superposition of various types of diagenetic events experienced by these rocks produce significant 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 potential reservoirs in the shoal grainstone and the microbial reef lithofacies, which are the main exploration targets.