The Paleocene–Eocene Wilcox Group in the northern Gulf of Mexico Basin records the history of very large and long-lived submarine fans that formed during global greenhouse conditions (i.e., no continental ice sheets). These sand-rich, submarine-fan deposits are up to 8000 ft (2400 m) thick and cover an area of at least 66,000 mi2 (170,000 km2). They were deposited over 14.5 m.y. and are important oil and gas reservoirs. Our mapping of sediment across different depositional environments suggests that up to half of the sand routed to the basin was deposited in these submarine fans. The high volume of sand routed to deep water was a function of high, but relatively stable, sea level; large, integrated fluvial systems that drained the mountains of the Laramide Orogeny; narrow continental shelves; and long-lived, shelf-penetrating submarine canyons that provided an efficient connection between deltas and submarine fans. Due to the vast areal extent and prominent thickness of these submarine-fan reservoirs, this play has had a high exploration success ratio (59%), despite being below the floor for direct detection of hydrocarbons from seismic data. Although the volume of sediment routed to deep water is massive, these sandstones are very fine-grained. This fine grain size, combined with a high silt content, a feldspar and lithic-rich composition, and burial diagenesis, results in low permeability. It has taken advances in drilling and completion technology to open the door to commercializing these large, but complex, reservoirs.