Martin Cassidy
Department of Earth and Atmospheric Sciences, University of Houston, 312 Science and Research Bldg., Rm. 312, Houston, Texas 77204–5007

The Norphlet Desert and its Sand Dunes: In a Sub-Aerial Environment, but below Sea Level

Onshore Gulf of Mexico Exploration II (GRBCC, Assembly Area B)
Tuesday, September 22, 2015, 3:20 pm

In the late Triassic extension of Pangaea, the future Gulf of Mexico rifting began with a horst and graben terrain filled with clastic non­marine sediments. As extension and subsidence continued, the ocean entered depositing evaporates culminating with the formation of the Louann Salt. At that time the entrance from the Pacific Ocean closed, but the area of the present Gulf of Mexico continued to subside well below sea level. The arid environment continued and desert sand dunes were deposited in the Norphlet desert. Sand on the shores of the basin were swept into cross-bedded red sand dunes distributed throughout the basin by regional and adiabatic winds. When the barrier to the ocean finally broke, the sea flooded in. Water arose like the filling of a bathtub. The dunes were not eroded, but submerged by hundreds of feet of water. Deepwater, black limestones (e.g., the Smackover) were deposited directly over the dunes without transitions.

The sandstones are good reservoirs even at 20,000 feet below sea level, as seen in Mobil 76–1 well in Mobile Bay, Alabama. The 412 feet of fine to medium grained sandstone within the gas column had an average porosity of 11.1% and 7.7 md permeability. With a 11,240 psi formation pressure and a bottom hole temperature of 414°F, the well had an absolute open flow (AOF) of 37.3 million cubic ft of gas per day. Chlorite coatings of grains prevented quartz overgrowths therefore preserving porosity and permeability to great depths. Norphlet reservoirs should exist in local areas throughout the Gulf of Mexico, and be prospective of gas production wherever they are within drilling depth. Quality seismic data will help predict the location of thick deposits. The Norphlet Sandstone is the last prospective reservoir positioned stratigraphically above the original Louann Salt, where the salt layer used to be prior to mobilization including first-generation salt domes.