Petrophysical Analysis of Some Wilcox Wells

Abstract

Abstract Relationships are shown between some of the fundamental rock properties of the heterogeneous Eocene Wilcox formation along the central and eastern portion of the Texas Gulf Coast trend. Mention is made that quantitative evaluation, using the electrical logs and established correlations, often are difficult due to:variations in the shale content and cementation of the sand,variations in water salinities,thin bed effects, andinvasion effects. However, it is pointed out that this does not preclude the use of electrical logs but rather that extra care must be exercised in their interpretation. Several representative wells along a portion of the trend are reviewed ]to illustrate some of the general characteristics of the formation and the individual utility of some of the logging devices. Particular emphasis is placed on the value of drill cuttings and sidewall cores, as well as capillary pressure data which indicate pore size distribution and which, in turn, are related to porosity, permeability and water saturation. It was concluded that conventional coring usually is not essential for evaluation of exploratory wells along this portion of the Wilcox trend inasmuch as proper logs, in conjunction with cuttings and a few sidewall cores, generally provide adequate data for evaluation. A recommended evaluation program for down-dip Wilcox exploratory wells penetrating either the upper or both the upper and lower section is presented. It is stressed that the suggested program is necessarily general and therefore flexible and should be considered only as a guidein selecting evaluation procedures. Introduction Evaluation of the Eocene Wilcox along the Texas Gulf Coast trend varies from relatively simple cases, as exemplified by a typical well in the upper Wilcox at Mercy, Tex., to the more difficult ones such as found in the lower Wilcox at Sheridan, Tex. In either instance, quantitative evaluation of potential pay sections requires information concerning the porosity, permeability, and formational content (oil, gas or water), or more fundamentally, as Archie has shown, such evaluation should reveal the distribution of each phase (oil, gas or water) within the pores of the rock, The evaluation should not be limited only to permeable rocks containing hydrocarbons, but should include also the impermeable layers as well as the permeable layers containing water.