Alabama Ferry Field: Study of a Lower Cretaceous Stratigraphic Trap

Nick Pollard

Since discovery in September 1983, nearly 200 wells have extended Alabama Ferry field to a surface area of approximately 70 mi². Located primarily in southeastern Leon County, east Texas, Alabama Ferry is oil productive from stratigraphically trapped carbonate reservoirs in the Lower Cretaceous upper Glen Rose "D" interval.

Deposition within the field was the end result of a very large tidal-dominated system most likely initiated at a break along the upper Glen Rose shelf margin. Progradation of the tidal system lagoonward during "D" time resulted in the deposition of five separate, vertically stacked, shoaling-upward flood tidal delta sequences (subintervals).

Environmental interpretations were based on four subinterval characteristics delineated from core descriptions and detailed mapping. Lateral subinterval migration, morphology, a common southeastern channel system, and facies zonation all provide evidence that deposition resulted from prograding flood tidal deltas.

The northwest-southeast-trending tidal delta complexes have been divided into five environments based on facies zonations and Holocene examples: (1) mud-rich lagoonal, (2) oolitic-dominated outer fringe, (3) skeletal-dominated inner fringe, (4) oolitic-dominated channel margin, and (5) intraclast-dominated channel environments.

Porosity distribution in the field area is primarily facies related. Oolitic-dominated marginal and mud-rich lagoonal facies have low porosities, intraclast-dominated channel facies have low to moderate porosities, and skeletal-dominated inner fringe facies have moderate to high porosity values. Preservation of primary porosity is evident in all facies. Porosity values in the oolitic (fine-grained) vs. intraclast (coarse-grained) facies are related to differences in grain sizes. Coarse grain sizes, irregular shapes, and secondary dissolution of aragonitic pelecypods are responsible for high porosities in the skeletal facies.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.