Approximately 1 million bbl of oil and 2 bcf (5.7 × 10⁷m³) of gas have been produced from the Vivian field since its discovery in June 1981. Ultimate recoverable reserves after water flooding are estimated to be 5.1 million bbl of oil and 8.3 bcf (2.4 × 10⁸m³) of gas. The field is a stratigraphic trap resulting from porous, permeable grainstones pinching out into muddy limestones and shale. It is located on the northwestern flank of the Caddo-Pine Island structure in Caddo Parish, Louisiana, and has 56 productive wells, 52 of which produce from one or more reservoirs in the Pettet "B" interval. (The Pettet "B" is a term used by the operator of the field to denote the second of four limestone units encountered when dril ing through the Lower Cretaceous Sligo Formation in the vicinity of Vivian field.) The reservoir rock is primarily skeletal-rich grainstone (rarely oolitic). The occurrence of these high-energy deposits at this location indicates that the broad Sligo platform was divided by an oolite-shoal complex into a platform lagoon on the landward side of the grainstones and an outer platform on the basinward side. A similar situation was described for the Sligo of south Texas.

The Pettet "B" interval contains eight microfacies: oolitic grainstone, skeletal grainstone, skeletal-oolitic grainstone, skeletal packstone, oolitic packstone, skeletal-oolitic packstone, skeletal wackestone, and shale. Three depositional environments (high, moderate, and low energy) are represented by these microfacies.

Reservoir grade permeability of 0.5 md or greater is restricted to the grainstone microfacies in the Pettet "B." Normally, this occurs in the skeletal and skeletal-oolitic grainstone microfacies where permeabilities can be as high as 80 md. A portion of the lower B2, however, does have lower limit reservoir permeability in a medium-grained oolitic grainstone. Porosities associated with reservoir-grade permeabilities range from 9 to 21%.

The Pettet "B" represents an oolite-shoal complex which built up on the broad Sligo platform. An apron of skeletal grains, primarily aragonite mollusc shells, accumulated in a narrow belt on the seaward flank of the oolite-shoal complex. Porosity and permeability are highest in this skeletal grainstone, which has both primary and skeletal-moldic porosity.

End_of_Article - Last_Page 1211------------