ABSTRACT: Origin and Exploration Significance of Lensing in the Norphlet Formation, Offshore Mafla

Robert Hoar, C. A. Taylor, K. R. Story, E. Weise, C. L. Sharp, W. Von Herneberg

In the northeastern Gulf of Mexico, seismic and borehole data indicate abrupt thickness changes where the Late Jurassic Norphlet Formation overlies the Louann Salt. The thickness ranges from below seismic resolution to over 300 m in less than 2 km, and the variations form linear trends that can be more than 60 km long. More than 100 m of structural relief is associated with the thicks. Although some of this is topographic, much is due to salt movement or density effects during lower Hainesville deposition.

In the Destin dome area, the lenses are oriented east-west, perpendicular to a southerly paleowind direction, suggesting a transverse dune origin. In the Mobile Bay area, the lenses are oriented north-south, subparallel to a prevailing northerly paleowind direction, suggesting linear dune deposition. However, in both areas, the dune, interdune, sandsheet composition of the lenses suggests a more complex origin. The lenses may have formed by early differential loading of the salt by small dunes (relative to the size of the lenses) and that, once in place, differential loading perpetuated sand deposition in these areas.

The Norphlet sand lenses have important exploration significance because they are a reliable indicator of eolian sand deposition on salt. Furthermore, Norphlet Formation thins, as interpreted on seismic, may indicate areas of tight or absent sand. Such barriers would provide stratigraphic traps, as well as regional migration problems for otherwise good structural traps.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990