Experimental Quantification of “Good Log Characteristics” for Mesaverde Reservoirs of the Piceance Basin, Colorado: Toward a Reconnaissance Method for Evaluating Movable Water in Tight, Shaly Sandstones

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

Petrophysical analysis of wireline data through thick intervals of shaly-sandstone is a critical aspect of geologic interpretation in tight-sand gas plays. Established practices commonly apply water-saturation calculations adjusted for local relationships with variables such as water resistivity, Archie exponents, matrix density and shale properties calibrated to core in developed areas. In exploration, it is helpful to have a reconnaissance technique to use in uncalibrated areas as a check on conclusions computed using the development standards.

An experimental log-evaluation method is described based on data from the Mesaverde Formation, comprising >2000’ of alluvial shale and shaly-sandstone reservoirs, in the Piceance Basin, Colorado. The analysis uses two derived parameters: apparent water resistivity (RWA) and approach or cross-over of density and neutron porosities (DNXO). For both quantities, additional gas in the system increases the response whereas higher clay content decreases the response. Graphical modeling of various shaly-sandstone rock types shows that this method can successfully discriminate the effects of gas and clay without estimating the typical calculation variables previously listed.

Initially, the RWA and DNXO responses are calculated as a ratio to each curve’s baseline which is specific to a subject well, so extensive curve normalization is not required. Next, log characteristics are quantified for gas saturation based on the highest coordinated responses of both parameters for an equivalent shaliness, defined by the gamma-ray curve. Finally, this pseudo-gas saturation is correlated with porosity in a water-risk calculation. Results from this technique of targeting a high chance for gas with low water-risk have been used to optimize completion intervals based on properties of the sandstone matrix or, in special cases, natural fractures.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado