Abstract

In the Rocky Mountain Laramide Basins (RMLB), over 80% of gas production is from anomalously pressured rock-fluid volumes that extend from an upper regional pressure surface boundary (i.e., the boundary between normal and anomalous pressure regimes) to 2000 feet below this boundary. In order to evaluate anomalously pressured gas assets in the RMLB and elsewhere, two exploration tasks are essential. First, it is vital to determine the position, and if possible, evaluate the three-dimensional pressure surface boundary between normal and anomalous pressure regimes (the surface is expressed as an inversion in both sonic and seismic velocities). Secondly, for successful exploration prospects it is essential to detect and delineate porosity and permeability sweet spots in the target reservoir interval within the anomalously pressured volume below the regional pressure surface boundary.

The most efficient means of accomplishing these two essential tasks is to construct a detailed sonic and seismic velocity study of the area of interest. The Siberia Ridge study described in this article provides a neat illustration of the type of velocity analysis that is necessary for evaluation of anomalously pressured gas assets in the RMLB.

The velocity study outlined in this article enables explorationists (1) to accurately determine the position of the regional pressure surface boundary at any location in the Siberia Ridge study area and (2) to evaluate, in detail, the gas distribution below the regional pressure surface boundary.