The Importance of a Complete Suite of Logs in an Old Reservoir
Authors: Hall Jon E. 
Published: December 2007

Summary
A field study conducted with the new well data from a recent infill drilling program resulted in a major change in the log-evaluation technique for the Ownby (Upper Clearfork) field of the Permian basin. An on-location review of the new log and core data resulted in the running of a gamma ray spectral log to confirm our on-site analysis. Through the use of the complete suite of logs run on the new wells in the field in conjunction with the spectral log, we determined that the use of a gamma ray shale cutoff is not necessary for net pay determination. The results of the new log-analysis technique for the 23 old producing wells in this field indicated an average increase of 34 ft net pay per well, an average increase of 55%.

Introduction
The main objective of this paper is to show by example the importance of a complete suite of logs in new wells in an old reservoir. During a recent infill drilling program in the Ownby (Upper Clearfork) Unit in the Permian basin, a complete suite of logs was run on all 10 new wells. With these data and the core data from the three new wells that were cored, a revised log-evaluation technique was implemented. Thus, this paper discusses one field example for which a complete suite of logs was necessary to determine what log cutoffs were necessary for accurate log evaluation.

Importance of a Complete Suite of Logs in an Old Reservoir
Whether or not a complete suite of logs should be run on a new well in an old field depends solely on the information available from the old wells. In some fields, correlation tools are the only tools that need to be run. In others, a complete suite of logs should be run; if done, this produces a wealth of information.
A complete suite of logs consists of two or more porosity logs, a resistivity log, and a correlation log. Logs run in a recent 10-well infill drilling project in the Ownby (Upper Clearfork) Unit includeddensity, neutron, and acoustic logs as porosity devices,a resistivity log to aid in saturation determination, anda gamma ray device with each tool for ease in correlation.
The multiple porosity devices allowed for crossplot identification of lithology, secondary porosity, and gas detection. Through a thorough evaluation of the log data, new pay intervals were found that could increase production and, ultimately, the field's present worth.
Without a complete suite of logs in an old reservoir, pay intervals can easily be overlooked. Although running a complete suite of logs does not guarantee that further pay intervals will be found, the data enhance the pay evaluation. The following example is just one case where a complete pay evaluation through the use of a complete suite of logs could increase recoveries from an old reservoir.

Example

Reservoir Description and Field History
The Ownby (Upper Clearfork) field is located in eastern Yoakum County, TX, and is part of a combination stratigraphic and structural trap. The trap runs along the northeast-southwest-trending anticline on the south-eastern side of the North Basin platform. The Upper Clearfork formation is a Permian-Age dolomite with anhydrite nodules and small shale laminations throughout.
JPT
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