Model for Isopaching Jurassic-Age Norphlet Formation in Mobile Bay, Alabama Area

Lorraine Fischer Torres

Deep gas was discovered in the Norphlet Sandstone of Mobile Bay, Alabama in 1979. Sixteen wells, of which Exxon Company, U.S.A. has had an interest in eight, have tested gas from depths greater than 20,000 ft and at an average rate of 19 million ft³ of gas per day.

The dominant structural features in Mobile Bay are large east-west-trending salt-supported anticlines associated with salt pull-apart listric normal faulting. Throws on these faults measure up to 1,000 ft. Individual structures have dimensions as large as 15 mi in an east-west strike direction and 8 mi in a north-south dip direction.

The Jurassic age (Callovian) Norphlet of Mobile Bay is characterized by eolian dune sand deposits up to 700 ft thick. An important factor affecting future development drilling is the accurate prediction of reservoir thickness. This presentation shows that an integrated study of seismic and well data has facilitated the development of a geological model for isopaching the Norphlet Formation. The isopach exhibits a strong north-northwest-south-southeast orientation of parallel thicks and thins. These trends are believed to be the result of original eolian deposition of complex linear dunes in the Norphlet Sandstone. The major east-west structural grain of faults and anticlines overprints this preserved depositional trend.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.