A Strategy to Reduce Risk in Searching for Gas Accumulations in Tight-Gas Sand
In Rock Mountain Laramide Basins, many commercial gas accumulations occurring in the lower Tertiary/Mesozoic strategraphic sections are characterized by the following critical attributes:
1) anomalously pressured (both over- and underpressured), but can appear to be normally pressured;
2) occur beneath a regional velocity inversion surface;
3) compartmentalized and gas-charged;
4) located at intersections of reservoirs facies and gas-charged domains;
5) productive intersections are typically enhanced by a combination of structural, stratigraphic, and diagenetic elements;
6) the reservoir facies commonly are considered tight-gas sands;
7) no apparent meteoric water connection.
In order to maximize risk reduction in searching for these
types of gas accumulations, the highest priority tasks are: 1) evaluating gas
distribution in the fluid system; and, 2) determining the distribution of
reservoir facies in the rock system. Our work in the
past 10+ years has documented that the distribution of gas-charged domains can
be effectively delineated by detailed sonic/seismic velocity evaluations. More
recent work suggests that a variety of seismic attributes can be used to
adequately define the distribution of reservoir facies.
Thus, at present, there are tools to determine the spatial intersections of
gas-charged domains and reservoir facies. This
approach has been tested in six known gas-producing fields in the