"Good Log Characteristics" for Mesaverde Reservoirs of the Piceance Basin, Colorado: Evaluating Movable Water in Tight, Shaly Sandstones

Paul E. Devine
Exploration, Williams Production RMT, Denver, CO

Accurate analysis of wireline log data through thick intervals of shaly-sandstone reservoirs is a critical aspect of geologic interpretation in tight-sand gas plays. The current science reflects a system designed to calculate water saturation with equations that account for local relationships in well calibrated areas of development. For exploration, it is useful to have a reconnaissance technique based on simple concepts and rapid calculations to corroborate or refute conclusions derived from the development standards.

This presentation describes an experimental log-evaluation method based on data from the Mesaverde Group in the basin-centered gas play of the Piceance Basin which includes >2000’ of shale and shaly-sandstone reservoirs deposited in alluvial environments. The analysis relies on two derived parameters: apparent water resistivity (RWA) and cross-over or approach of density and neutron porosities (DNXO). For both of these quantities, the addition of gas to the system tends to increase the response whereas increase of clay will cause a decrease. Because responses are measured in relation to a well-specific baseline for each parameter, no normalization of curves is required.

Quantification of good log characteristics delineates zones with the highest responses for RWA and DNXO for a given shaliness as defined by the gamma-ray curve. Inverse modeling of various shaly-sandstone rock types shows that this method can successfully discriminate the effects of gas and clay and therefore identify movable water. Results from this technique have been used to optimize completion intervals based on risk for high water-cut from the sandstone matrix or, in special cases, natural fractures.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas