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Geophysical Interpretation of Jurassic Structures and Norphlet Deposition, Alabama State and Federal Waters

Nat S. Turner, Gordon W. Fielder

Since the 1979 discovery of deep (20,000 ft) gas in Alabama's Mobile Bay, the Jurassic Norphlet Formation has become the target of intense exploration activity. Reserve estimates range from 12 to 21 tcf of gas.

Reserves of this magnitude could make the developing Norphlet trend into one of the top five gas producing areas in the United States.

The Norphlet Formation is a predominately eolian sandstone sequence offshore Alabama. Seismic response at the Norphlet level is one of high amplitude and lateral continuity. Interpretations of a seismic grid revealed an extensive network of 500 to 2,000-ft faults cutting the Norphlet Formation. These faults and related closures appear to be caused by structural movement in the subsalt Triassic and/or Paleozoic sections. Salt movement does not appear to be the primary cause of the faulting and structural configuration at the Norphlet level. This pattern of faulting illustrates a different structural style in the Mesozoic section of the Gulf Coast and provides an alternate mechanism to salt tectonics for Jurassic exploration. Jurassic subsidence related to this faulting may have influen ed the accumulation of the eolian Norphlet. Limited exploratory drilling suggests that the Norphlet section is thicker in offstructure tests relative to wells on the structural crests, thereby suggesting the influence of topographic relief upon eolian deposition. Recognition of the patterns and mechanisms of faulting, with respect to related subsidence, should help define the areal extent of the eolian Norphlet sandstone.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.