The Geologic History of Submarine Fans in the Deepwater Gulf of Mexico: Mesozoic to Modern

摘要

Submarine fans are among the most important reservoirs in the prolific Gulf of Mexico Basin and are known from industry drilling and seismic data to be contained within strata extending from the Mesozoic to modern. However, little has been published on how these systems evolve over this time frame, responding to extra-basinal factors like tectonics, climate, sediment discharge, etc. A comparison of Mesozoic age submarine fans with those of the most important Cenozoic fans in the Gulf reveals important new insights into the basin formation and evolution. We interpret the newly formed basin, just after termination of Callovian salt deposition, to be rather shallow but not entirely terrestrial. The initial middle-Jurassic low-dipping ramp from land to sea transitioned into a shelf-slope-basinal profile, with sea floor spreading ranging in age from Kimmeridgian to Berriasian. This in turn provided impetus for development of the earliest poorly organized submarine aprons in the late Jurassic and early Cretaceous. These eventually evolved into submarine fans but their sizes were constrained by rather short river lengths until reorganization of North American drainage systems in the mid-Cretaceous. Mapping major sand-prone fairways reveal that most of the Mesozoic slope and basin systems were fed by eastern North American source terrains. Siliciclastic delivery into the basin was also hampered by reef blocking, as large rimmed shelf reef systems of Barremanian to Albian age restricted sediment bypass to a few discrete basin entry points. The dramatic drainage reorganization in the Cenomanian ushered in the first of the large and extensive submarine fan systems, known from exploration wildcats to have extended nearly 500 km from the coeval shelf edge. Deep crustal tectonic events likely played a role in both drainage re-routing but also allowed siliciclastic systems to surmount the rimmed shelf reef systems and develop shelf-edge/slope-basin pathways over broad fairways. The Cenomanian fan systems look more like Cenozoic fan successors, with long run-outs, variable sediment gravity flow products, and tendency to respond to the para-autochthonous salt seascape and developing primary basins. The alteration of the seascape caused by the Chicxulub impact event subsequently altered deepwater transport pathways in some areas. But this event did set the stage for Paleogene and later fans that responded to continental scale drainage networks developed with the Laramide orogeny. Subsequent evolution of these fans was modulated by salt tectonics, climate-related sediment discharge variations, volcanism, and eventually the Neogene ice sheets. The Value of Information: Wilcox Play Mapping, 1998-2015 Our knowledge of submarine fan systems in the Gulf of Mexico has advanced with migration of exploration from onshore to shelf to deepwater over the last 90 years. A historical examination of the Paleogene Wilcox Play maps constructed by the Gulf Basin Depositional Synthesis (GBDS) research project at the University of Texas reveals how our expectations of size and extent of this prolific hydrocarbon reservoir play have been episodically reset with evolving regional depositional models and new drilling results. The analysis also documents the value of information in reducing risk and uncertainty for high-cost drilling targets like the deepwater Wilcox. Prior to 1998, the Wilcox was largely viewed as an economically challenged onshore deep gas play, with discoveries peaking in the early 1980s (Snedden et al., 2002; Zarra, 2007). Few wells were drilled offshore for the Wilcox and in the deep water, as strata older than Oligocene were only penetrated in salt carapaces and rafts at shallow depths (Fig. 1, top). The presence of large Paleogene fluvial and deltaic systems, documented in outcrop and onshore subsurface wells, suggested the presence of a Lower Wilcox sandy apron system in offshore areas (Fig. 1, bottom).