ABSTRACT

Producibility of overpressured middle and lower Wilcox sandstone reservoirs along the central Texas Gulf Coast is notoriously difficult to determine from standard logging suites. Typically however, this is the only means available. The middle Wilcox "S" sandstone of Provident City field, examined in terms of porosity, resistivity and shaliness explained highly variable rates of gas, water and formation fines produced in the thirteen wells penetrating this section. Structure and stratigraphic maps further delineated the reservoir and helped reconstruct the original trapping mechanism.

Log data, plotted on resistivity-porosity crossplots (Pickett, 1966) as a means of comparative analysis, defined a water saturation equation. Cutoffs of < 50% S_w for commercially productive wells, 50-65% S_w for marginal wells and > 65% S_w for wet wells were applied using R_w=0.1, a=0.7, m=1.90 and n=2 in the standard Archie equation.

Whole core taken from the 50-65% S_w interval in one producing well shows most of the reservoir porosity to be secondary, with a wide variety of precipitated materials suspended in the pore throats. A palinspastic structure map, constructed by omitting late faulting, revealed a simple downthrown rollover anticline. Net feet of section with S_w < 50%, contoured for each of three "S" sandstone lobes, defined commercially productive limits of the reservoir and confirmed the timing of hydrocarbon migration into this structure. This implies gas migrated into secondary pore spaces, which were created just prior to hydrocarbon formation, and inhibited late precipitation in the main reservoir; while off-structure pore throats contain water, gas and mobile fines.

Using the derived S_w equation, reconstructed structure maps and production figures, a recovery factor of 1720 mcf/acre-foot was calculated for reservoirs with S_w < 50%.