Seismic Exploration for Fractured Lower Dakota Alluvial Gas Sands, San Juan Basin, New Mexico

Reeves, James J. and W. Hoxie Smith
GeoSpectrum, Inc, Midland, TX

Reservoir fractures are predicted using multiple azimuth seismic lineament mapping in the Lower Dakota reservoir section. A seismic lineament is defined as a linear feature seen in a time slice or horizon slice through the seismic volume. For lineament mapping, each lineament must be recognizable in more than one seismic attribute volume. Seismic attributes investigated include: coherency, amplitude, frequency, phase, and acoustic impedance. We interpret that areas having high seismic lineament density with multi-directional lineaments are associated with high fracture density in the reservoir.

Lead areas defined by regions of “swarming” multi-directional lineaments are further screened by additional geologic attributes. These attributes include reservoir isopach thickness, indicating thicker reservoir section; seismic horizon slices, imaging potentially productive reservoir stratigraphy; and a collocated cokriged clay volume map for the reservoir zone computed from near trace seismic amplitude (an AVO attribute) and a comprehensive petrophysical analysis of the well data to determine discrete values of clay volume at each well. This map indicates where good/clean reservoir rock is located. We interpret that clean/low clay reservoir rock is brittle and likely to be highly fractured when seismic lineaments are present.

A gas sensitive AVO seismic attribute, near trace stacked phase minus far trace stacked phase, phase gradient, is used to further define drill locations having potential high gas saturation. The importance of this attribute cannot be understated, as reservoir fractures enhance reservoir permeability and volume, they may also penetrate water saturated zones in the Dakota and/or Morrison intervals and be responsible for the reservoir being water saturated and ruined.

Seismic interval velocity anisotropy is used to investigate reservoir potential in tight sands of the Upper Dakota up hole from the main reservoir target. We interpret that large interval velocity anisotropy is associated with fracture related anisotropy in these tight sands.