ABSTRACT

The first commercial production of hydrocarbons from the Jurassic Haynesville Formation in southwestern Alabama was from the Frisco City field. The field currently produces 57.8° API gravity oil on 160-acre well spacing from a depth of about 12,000 ft (3,660 m). Perforations are in the "Frisco City sand" interval, in the lower part of the Haynesville Formation, which has an average porosity of 21 percent and an average permeability of 45 md. Currently, the field has two producing wells with cumulative production of over 180,858 barrels of oil and 275,747 Mcf of gas.

The trapping mechanism in the Frisco City field is a combination structural-stratigraphic trap. The structural component consists of a basement paleohigh, and the stratigraphic trap is produced by the termination of porous sandstone against the basement paleohigh.

The lower part of the Haynesville Formation in this area is composed of, in ascending order, the Buckner Anhydrite Member, the "Frisco City sand," and interbedded shale and anhydrite. Sediment of the "Frisco City sand" interval was probably deposited in a shallow marine, braid delta-front setting and has a blanket-like morphology. The sandstone is moderately to well-sorted, very fine to fine-grained, subarkose to plagioclase arkose, and contains angular pebbles of predominantly chlorite and mica schists. The sandstone is interbedded with thinly bedded, burrowed, sandy mudstone which produces, along with a variable distribution of carbonate and anhydrite cement, considerable reservoir heterogeneity. Porosity is predominantly intergranular and textural analysis revealed a significant amount of framework grain dissolution and decementation.