Gilbertown Field, established in 1944, is the oldest oil field in Alabama and produces oil from fractured chalk of the Cretaceous Selma Group and from glauconitic sandstone of the underlying Eutaw Formation. The field is situated along the southern margin of the Gilbertown graben, which is part of the peripheral fault trend of the Gulf Coast basin, and has produced more than 14 million barrels of low-gravity oil with virtually no associated gas. Production has declined to marginal levels, yet nearly all of Gilbertown Field is still in primary recovery because of high water saturation in the oil column. To identify opportunities to revitalize Gilbertown Field, we are conducting a thorough investigation of reservoir architecture that emphasizes structural geometry, fault seals, reservoir heterogeneity, and completion history.

The Selma Group is productive along normal faults in the western part of Gilbertown Field. Fault-seal diagrams and completion records establish that fracture permeability in the Selma Group is developed principally in the hanging-wall blocks of the faults where clay shale in the hanging wall is juxtaposed with chalk in the footwall. Along the eastern productive fault, wells are completed immediately below the shale seal. By contrast, wells are completed as much as 250 feet below the seal along the central part of the western fault, suggesting that a large quantity of oil remains untapped. Moreover, detailed structure mapping shows that a large segment of this fault has yet to be drilled. Therefore, infill drilling and deviation of existing boreholes may be viable options to improve recovery from the Selma Group. Another approach for improving recovery in the Selma Group is to drill horizontal wells through areas with high potential for untapped oil. Drilling horizontal wells at low angles to fault strike would maximize the amount of reservoir contacted by the wellbore.

Eutaw sandstone is productive in fault-bound footwall uplifts that are effectively sealed by Selma chalk. The Eutaw Formation in Gilbertown Field comprises seven glauconitic sandstone units that extend across the field and were deposited in transgressive shoreface and shelf environments. Glauconite content tends to increase upward in the Eutaw Formation, and in most of the field, oil is produced from the upper two sandstone units. In the eastern part of the field, which is structurally highest, the upper sandstone units are behind casing, and oil is produced only from older sandstone units. The reason the older sandstone units were preferred for completion is because they contain less glauconite and pseudomatrix and thus have a more open pore system than the younger units. If existing wells in the structurally highest part of the field are recompleted, an additional 60 feet of potentially productive section is accessible.

Careful reevaluation of reservoir architecture and completion practices in Gilbertown Field has helped identify solutions for redevelopment that can be implemented using existing technologies that are readily available to most operators. We suggest that thorough evaluation of other aging fields will also identify basic solutions that can significantly prolong reservoir life, thus reducing the risk of premature abandonment and slowing the decline of domestic oil production.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah