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Primary Depositional Controls on Reservoir Architecture, Facies, and Quality in Deepwater Wilcox Sands, Gulf of Mexico


The Wilcox deposition, Paleocene to early Eocene in age between 60-48.5 Ma, records one of the largest periods of sand deposition into the Gulf of Mexico basin. The Wilcox sediment drainage system incorporates most of the continental United States during the Paleocene. Massive amounts of sediment were transported through intervening fluvial-deltaic systems into the deepwater. Sand presence is ubiquitous in deepwater Wilcox wells. The key challenge is a high degree of variance in reservoir quality is observed in numerous deepwater Wilcox well penetrations. Reservoirs can range from 12-25% in porosity and 0.1 to 300 mD in permeability. Although temperature and compaction trends play a role in reservoir quality, primary depositional fabric is the main contributor. Two primary depositional controls on distribution of reservoir architecture, facies, and quality are: 1) Local changes in paleotopography and its impact on flow mechanics, and 2) basin position relative to sediment entry points in deepwater Gulf of Mexico sub-basins. Deepwater Wilcox reservoir prediction integrates well log, core, outcrop, and seismic data to distinguish reservoir heterogeneity at varying scales. Local gradient changes influence sediment gravity flow mechanics. Flows that exit topographic confinement into areas of decreased gradient transition into distributive flows. Higher density grains typically drop out of transport as flows exit zones of confinement where flow velocities begin to diminish and lower density grains such as silt/clay particles continue to travel further basinward. Identification of preferential sediment flow pathways from linked inboard basins is important in predicting vertical and lateral stacking patterns of reservoir elements. Deepwater reservoir architectural elements are classified into: primary channel/feeder, proximal lobe, medial lobe, and distal lobe elements. Reservoir facies both proximal and axial to the initial sediment input points along proximal basin margin settings are predicted to have better quality reservoir, higher net-to-gross, increased sand amalgamation, and maximum grain sizes. Reservoir facies both distal and off-axis to sediment input points are predicted to have poorer reservoir quality due to increased percentages of silts and clays and increase in thinly bedded sands.