ABSTRACT

Oaks field is a stratigraphically trapped Smackover field which produces from at least three separate reservoirs. Individual reservoirs are shoaling-upward carbonate grainstone cycles. The three reservoirs are offlapping and are separated from each other by a nonporous terrigenous-rich clastic facies. The oldest two reservoirs were deposited as well-defined barrier island complexes up to 4 1/2 mi long and less than one-half mile wide. The youngest reservoir is less well defined and was deposited primarily as coalescing bars which were only occasionally emergent. All carbonate facies were deposited as mud-free oolitic and rhodolitic grainstones. The majority of porosity occlusion was by early cementation in the meteoric phreatic, mixed phreatic, and vadose zones, paralleling depositional strike. These environments are confirmed by stable isotope analysis.

Ten of the 13 producing wells and all five dry holes were conventionally cored. Thin sections were made at 1-ft intervals from permeability plugs, corresponding with measured values of porosity and permeability. Detailed petrographic correlation of wells allowed the individual reservoirs to be subdivided into distinct mappable units based on a plot of the diameter of the largest coated carbonate grain ("clasticity index") in each thin section. Clasticity index provides a simple tool for rapid and precise well correlation within individual reservoirs. Such detailed correlations are not possible by conventional log methods in this field.

Prior to incorporating petrographic analysis into the development drilling program, the field consisted of seven producing wells and three dry holes. By combining clasticity plots and other petrographic information with porosity isopach values, the field size was almost doubled with the successful completion of the next six holes drilled.