Texas Pacific Oil Co. has used the composite three porosity tool lithology movable oil plot on eight wells in the Wasson Northeast Clearfork field in Yoakum County, Tex. Single porosity movable oil plots have been used on porosity movable oil plots have been used on five wells. Results of the completions have been good. The Clearfork formation is a Permian-age carbonate reservoir composed of Permian-age carbonate reservoir composed of dolomite and anhydrite and containing lesser amounts of gypsum, silica and limestone. Sonic, density and neutron logs all respond differently to porosity as the lithology varies. A mathematical solution may be used to define the fractions of major lithologic components and calculate the correct porosity. A comparison of single porosity movable oil plots has indicated that the sidewall neutron plots has indicated that the sidewall neutron log presents the most accurate porosity log when only one porosity tool is to be used. The laterolog and microlaterolog are used to determine the movable oil relationship.
Development of the Middle Clearfork formation in the northeast portion of the Wasson field began in Feb., 1967, with the completion of Sunray DX Ruth Bennett Well No. 9. Since then, 32 wells have been drilled and completed as oil producers. All wells have been drilled through the Middle Clearfork section at an average depth of 6,600 to 6,800 ft. After logging, casing is set to bottom and cemented. The limited entry perforating technique has been employed for most of the wells. Initial stimulation is accomplished by acidizing, usually with a high strength acid. If the desired oil producing rates are not obtained, large volume sand-water fracture treatments are used.
The logging program on early wells consisted of a gamma ray log for correlation and a sonic log for porosity readings. On occasion, a laterolog was used to determine water saturation. Results of these completions varied widely.
Realizing that unknown variations in lithology of the Clearfork formation would give calculated porosity values which are different from the actual rock porosity, the first three wells were completed using a composite three porosity lithology movable oil plot. Only the porosity lithology movable oil plot. Only the porous sections that contained oil and porous sections that contained oil and indicated permeability were perforated and stimulated. Better completions resulted from this logging program.
On the third well, movable oil plots were also computed using each of the single porosity logs—sonic, density and sidewall neutron—to determine if comparable results could be obtained. Although none of the single porosity logs indicated the same porosity values as the three porosity log, it was found that perforation depths could be chosen with reasonable perforation depths could be chosen with reasonable accuracy from the epithermal sidewall neutron log. The next four wells were logged using a single porosity epithermal sidewall neutron logarithmic movable oil plot. The purpose of this report is to discuss each type of movable oil plot with its application to the Middle Clearfork formation of the Wasson field.
The composite three porosity lithology movable oil plot is calculated by means of a computer program from data obtained from three porosity logs, such as the sonic, density, and porosity logs, such as the sonic, density, and epithermal sidewall neutron logs. The waterfilled porosity and movable oil relationship is computed from deep and shallow investigation resistivity logs such as the laterolog and microlaterolog. A typical presentation is shown in Fig. 1.