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Tectonic Controls on the Lower Eocene Upper Wilcox Deepwater Fan System Offshore Northern Mexico: Implications for Reservoir Presence and Quality Prediction

Abstract

The deepwater Lower Eocene Wilcox Formation in the western Gulf of Mexico has long been identified as a key reservoir interval in US waters. Its discovery in 2001 by the Baha 2 well revealed a new fairway more than 350km downdip from the long produced paralic deposits of the onshore Texas shelf. Since then about a dozen wells have targeted the Upper Wilcox in the deepwater Perdido area. Commercial success has been mixed and defining the fairway has proven difficult because of challenging sub-salt imaging. In 2012, Petróleos Mexicanos (PEMEX) started exploratory drilling in the southern extension of the Perdido foldbelt, confirming the presence of Wilcox-aged deepwater reservoirs in Mexico. As industry focus grows in this region, defining the Upper Wilcox fairway has taken on renewed importance. Recent insights reveal an extension of the Texas-sourced Wilcox deepwater trend at least 150km further to the south into Mexico, illustrating the regional scale of the Wilcox deepwater fan. Highly efficient depositional systems of this scale are typical of fine-grained dominated fans, confirmed by observations in both the US and Mexican Perdido wells. However, tectonic processes exert an equally important control on sedimentation, with plate subduction to the west, age-controlled thermal subsidence of oceanic crust and a regional salt allochton combining to effectively funnel Lower Eocene deepwater sands along a southwesterly oriented trough along the western Gulf of Mexico. This mirrors the present day Rio Grande fan system which shows a similar basinal refocus of sedimentation to the south, crucial for preventing the fan from running out to the present day abyssal plain and instead constraining deposition closer to the margin in more manageable water depths. Integration of log, 3D seismic and core data demonstrate that the Wilcox fan is the product of a series of individual, sinuous channel systems up to 100 kilometres in length, 1000 meters wide and 100 meters deep. Many exhibit well developed levees, with associated overbank and splay deposits. These observations also assist in characterizing the reservoir quality of the fairway, both in terms of a) mineralogical composition by reconstructing sediment pathways back to their provenance terranes, and b) sedimentary texture, largely controlled by grain-size characteristics and depositional processes on the basinal fan, as evidenced by debrite deposition (stand alone and linked) in the reservoir section.