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Campen Consultants, Inc., Billings, MT

Abstract: Basic Log Analysis In Coalbed Methane Exploration

Coal beds that are interbedded in sandstone and shales produce readily identifiable signatures on most commonly used open hole logs run in oil wells. With a modern log suite the different attributes of the coals make coal identification relatively accurate. Old logs, however, cause many uncertainties in this process. To avoid mistakes in the identification of coal seams, it is necessary to understand the reasons for the tool response as well as tool signatures.

Pure coals have densities that range from .7 to 1.8, making coal identification with a density log relatively certain. The porosity index from the neutron log is very high due to the high hydrogen and carbon content of coal. Delta T in a sonic log varies inversely with compaction, thus providing permeability indications. The gamma ray is used for correlation, and to judge the ash content of the coal. Old resistivity logs must be used with caution since the high resistance typical of coal does not enable a complete identification of a coal seam. A dense formation or freshwater sand can look similar to coal but will generally not have a discernable SP build-up.

Coal identification from oil and gas logs is a challenge that can be mastered by understanding the distinctive features of individual coal seams and the response of the logging tools to those particular characteristics.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana