Bates, Terry G.¹, James W. Hankla², Thomas E. Voytovich³, Gary Hall⁴
(1) Baker Atlas, Houston, TX
(2) Baker Atlas, Tyler, TX
(3) Apache Corporation, Tulsa, OK
(4) Devon Energy, Houston, TX

ABSTRACT: Identifying Low Resistivity Pay in the Cotton Valley Sand Using a Pulse Neutron Tool

The Cotton Valley Sand trend on the Western Shelf of the East Texas Basin consists predominately of tight gas sands with porosities seldom exceeding 10%. Scattered throughout this trend are stratigraphic traps in porous sands that exhibit a low resistivity. These sands typically have porosities ranging from 13-22% and resistivities of less than 2 ohms. Using conventional openhole log analysis, the calculated water saturations are high, yet many of these sands are gas productive, with little or no water cut.
The low resistivities are due to clay coating, generally either chlorite or illite, on the grains. To compensate for the conductive clays, it was suggested that another device such as a pulsed neutron tool be used as a measure of the formation salinity for deriving valid water saturations, independent of electrical properties.
To date, thirty-five wells with porous, low resistivity clay coated sands have been logged utilizing pulse neutron tools. Nineteen of these wells were tested, yielding 12 successful gas completions. In both the successful and unsuccessful wells, the combination pulsed neutron/openhole neutron overlays, sigma cut-offs, and pulse neutron derived water saturations have been significantly more reliable in predicting the presence or absence of producible gas than other conventional openhole log-derived solutions. This technique provides another method of increasing reserves by identifying often overlooked and bypassed low resistivity clay coated pay sands.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.