Applications of the Carbon/Oxygen Log in Johnson (Glorieta) Field
Authors: Barton Wayne , Flynn Jack M. 
Published: December 2007

Summary
Use of the carbon/oxygen log in Johnson (Glorieta) field in Ector County, TX, has led to successful completions in zones previously believed to be wet. This paper reviews Glorieta development in the area and discusses the logging system. Possible applications of the log in similar carbonate reservoirs are suggested.

Introduction
In June 1978, the carbon/oxygen (C/O) log began aiding formation evaluation in the Glorieta reservoir of Johnson field, a moderately porous carbonate trend in central Ector County, 8 miles northwest of Odessa, TX. Production there is from four separate reservoirs - the Grayburg-San Andres, the Holt, the Glorieta, and the Upper Pennsylvanian. Until recently, Glorieta development was limited by lack of oil shows in drilling samples, by poor drillstem test performance, and by high water saturations calculated from openhole logging packages.But results of the C/O log substantially contradict openhole log interpretations and have led to successful completion in zones previously believed to be wet. Based on these data, 11 wells have been either drilled or recompleted to the Glorieta.The paper will review the history of Glorieta development in the Johnson area prior to use of the C/O log and briefly will discuss the operation of the logging system. It will describe the logging suite now in use, showing a comparison of openhole and C/O log interpretations for the same wells. The C/O log has seen only limited use in the carbonate formations of the Permian basin, but possible suitability of the log in other west Texas applications is suggested.Information from this project will help other operators to evaluate the suitability of the logging system in areas where openhole logs are unavailable or unreliable.Principal conclusions drawn from the project are as follows. 1. Openhole logging suites used in the area did not yield reliable Sw data in the Glorieta.2. The C/O tool has applications in carbonate reservoirs such as the Glorieta.3. Porosity limitations on the use of the C/O tool have not been determined. Qualitative accuracy has been obtained in this project through a porosity range of approximately 10 to 20%

Field History
Production in the Johnson field area (Fig. 1) comes from four zones: the Grayburg-San Andres, Holt, Glorieta, and Upper Pennsylvanian (Fig. 2). Development of the three deeper zones began with completion of the Johnson Deep Unit (JDU) No. 1 in April 1973. Openhole log analysis indicated high water saturations in most of the potentially productive intervals in the Glorieta, and the well was completed dually in the Holt and Upper Pennsylvanian formations.Three subsequent wells were drilled as Holt or Holt/Upper Pennsylvanian producers. When the productive limits of the Upper Pennsylvanian were reached, a Glorieta completion was attempted in the JDU No. 5. The attempt was successful, and nine wells on the east side of the field eventually were completed as Glorieta producers.Because of poor shows in drilling samples, lack of encouragement from the openhole logs, and an unsuccessful drillstem test in the JDU No. 2, no additional development was attempted on the west side until 1977. In Nov. 1977 the Sun Johnson "D" No. 6, located on the western edge of the field, was recompleted successfully. Production was from an upper Glorieta zone.
JPT
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