Geologic Framework of Norphlet and Pre-Norphlet Strata of the Onshore and Offshore Eastern Gulf of Mexico Area

TEW, BERRY H., ROBERT M. MINK, STEVEN D. MANN, BENNETT L. BEARDEN, and ERNEST A. MANCINI, Geological Survey of Alabama, Tuscaloosa, AL

Hydrocarbon accumulations have been discovered in the Jurassic Norphlet Formation in the onshore and offshore eastern Gulf of Mexico area. An understanding of the regional geologic framework of the Norphlet and pre-Norphlet stratigraphic succession in the study area is crucial to successful exploration for and development of Norphlet hydrocarbon reservoirs in the eastern Gulf region. These strata include Jurassic sedimentary rocks and pre-Jurassic sedimentary and crystalline rocks. Of these strata, only the Norphlet Formation has produced economic quantities of hydrocarbons from reservoirs which include eolian dune, interdune, wadi, and marine sandstones. However, the pre-Norphlet geology and paleotopography of the area controlled, to a large degree, Norphlet depositional patterns and subsequent structural development. Especially important were the locations of pre-Jurassic paleohighs and the distribution and thickness patterns of the underlying Jurassic Louann Salt. Norphlet hydrocarbon traps are generally associated with structures that have resulted from halokinesis of the Louann. Traps include salt anticlines, faulted salt anticlines, and extensional faults associated with salt movement. In addition to these salt-related structural traps, traps resulting from presalt basement paleotopography may exist in the study area. These traps might include Norphlet structural highs associated with preexisting basement highs and Norphlet stratigraphic terminations along the flanks of basement highs.

Where Norphlet reservoir facies are present, the key factor controlling hydrocarbon accumulation potential is structural setting. In the Mississippi-Alabama-Florida (MAFLA) region that is underlain by the Norphlet Formation, four structural settings have been identified. Types A and B structural settings are characterized by Norphlet sediments overlying basement or thin salt; no Norphlet fields have been established in these settings. Types C and D structural settings are characterized by structures related to movement of thick Louann Salt; all Norphlet fields established in the MAFLA region occur in these settings. Four geographic regions have been defined in the study area for the purpose of characterizing Norphlet petroleum potential. Region 1 has excellent potential for future Nor hlet discoveries, region 2 has low to moderate potential, and regions 3 and 4 have very little to no potential.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)