Thunder Horse and Mensa are two of the largest fields of oil or gas, respectively, in the northern deep-water Gulf of Mexico. The fields are present in adjacent intraslope minibasins, located approximately 12 mi (19 km) apart in Mississippi Canyon. Both fields illustrate important complexities of deep-water sedimentation. Analysis is based on the integration of wire-line logs, biostratigraphy, and a 378-mi2 (979-km2), three-dimensional seismic data set.
Thunder Horse and Mensa reservoirs were deposited during the middle to late Miocene. Changes in paleobathymetry controlled the reservoir deposition, initially as salt withdrawal and later as turtle structures. From 125 to 24 Ma, the lithologies in both intraslope basins are interpreted as dominantly deep-water marls with interbedded shales. From 24 to 14.35 Ma, major input of deep-water siliciclastic sediments began. Sands were deposited in amalgamated sheets and amalgamated channel-fill units within the two major paleobathymetric lows; by contrast, shales were deposited across paleobathymetric highs. Between 14.35 and 13.05 Ma, the Thunder Horse turtle formed, creating a paleobathymetric high. Channelized sands were diverted around and deposited on the flanks of the structure. Meanwhile, to the north at Mensa, thick channel-fill sediments continued to be deposited. From 12.2 to 8.2 Ma, the lithologies throughout the entire area are dominantly overbank shales with thin channel-fill sands, suggesting that large volumes of sand bypassed the study area farther downslope to the south. Finally, at 9.0 Ma, Mensa's sheet-sand reservoir represents a different setting; sands were deposited near the crest of the Mensa turtle, which had subtle bathymetric expression.