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Surdam, Ronald C.1, Zun Sheng Jiao1, Yuri Ganshin1 
(1) Innovative Discovery Technologies, Laramie, WY

ABSTRACT: Rock-Fluid Systems Characteristics of Rocky Mountain Laramide Basins: An Example from the Wind River Basin, Wyoming

An anomalous velocity model was constructed for the Wind River Basin (WRB) based on ~2000 mi of 2-D seismic data and 175 sonic logs, for a total of 132,000 velocity/depth profiles. Ten cross sections were constructed through the model coincident with known gas fields. In each cross section, an intense, anomalously slow velocity domain coincided with the gas-productive rock/fluid interval. The anomalous velocity model: 
1. Easily isolates gas-charged rock/fluid systems characterized by anomalously slow velocities and water-rich rock/fluid systems characterized by normal velocities; and 
2. Delineates the regional velocity inversion surface, which is characterized by steepening of the Ro/depth gradient, a significant change in connate water composition, and acceleration of the reaction rate of smectite-to-illite diagenesis in mixed-layer clays. 
Gas chimneys are observed as topographic highs on the regional velocity inversion surface. Beneath the surface are significant fluid-flow compartments, which have a gas-charge in the fluid phase and are isolated from meteoric water recharge. Water-rich domains may occur within regional gas-charged compartments, but are not being recharged from the meteoric water system (i.e., trapped water). 
The WRB is divided into at least two regionally prominent fluid-flow compartments separated by the velocity inversion surface: a water-dominated upper compartment likely under strong meteoric water drive and a gas-charged, anomalously pressured lower compartment. Judging from cross sections, numerous gas-charged subcompartments occur within the regional compartment. Their geometries and boundaries are controlled by faults and low-permeability rocks. Commercial gas production results when a reservoir interval characterized by enhanced porosity/permeability intersects one of these gas-charged subcompartments.


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