Log Response as Model for Production: Deep Wilcox Sandstones, Provident City Field, Lavaca County, Texas

James L. Collins

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 fines produced in the 13 wells penetrating this section. Structural and stratigraphic maps further delineated the reservoir and helped reconstruct the original trapping mechanism.

Log data, plotted on resistivity-porosity crossplots as a means of comparative analysis, defined a water saturation equation. Cutoffs of less than 50% salt water for commercially productive wells, 50-65% salt water for marginal wells, and more than 65% salt water for wet wells are applied using Rw = 0.1, a = 0.7, m = 1.90, and n = 2 in the standard Archie equation.

Whole core taken from the 50-65% salt water interval in one producing well shows most of the reservoir porosity to be secondary, with a wide variety of clays 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 salt water less than 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 definition implies gas migrated into secondary pore spaces that were created just prior to hydrocarbon formation and inhibited precipitation of clays in the main reservoir while off-structure pore throats contain water, gas, and mobile fine material.

A recovery factor of 1,720 mcf/ac-foot was calculated for reservoirs with salt water less than 50%.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.