Abstract

A 3D seismic exploration method for fractured gas reservoirs is developed in a study conducted for the U. S. Department of Energy. A comprehensive petrophysical analysis was done on the Lower Dakota sandstone and integrated to a high resolution 3D seismic volume in a gas Unit in Rio Arriba County, New Mexico.

The interpretation methodology is based on four principal seismic attributes. Seismic lineament analysis is used to map lineaments through the Lower Dakota zone using horizon slices and time slices. We interpret that in a probabilistic sense where lineaments swarm and cluster together is where reservoir fractures are most likely to be found. Leads identified using lineament density are further screened using rock typing to identify reservoir that is more likely to fracture. A collocated cokriged clay volume map using near trace seismic amplitude (an AVO attribute) is used to identify reservoir having low clay that is interpreted to be more brittle and more prone to fracturing. Fractured reservoir and good reservoir rock do not necessarily make a drillable prospect, as reservoir fractures may provide a plumbing system to both water and gas. For prospect development a gas sensitive phase gradient AVO attribute is used to further screen the leads to insure that gas is present in the reservoir. Finally, in the Upper Dakota, fractured reservoir potential up hole is interpreted using a seismic interval velocity anisotropy attribute.

The resulting interpretation is further validated by the unified set of seismic attributes. For example, rock typing is supported both by the unique directional distributions of lineaments in each rock type and clay volume. Clay volume is supported both by near trace seismic amplitude and phase gradient AVO seismic attributes.

The first well was drilled and completed using the interpretation methodology in January 2004 and produced 4000 MCFGPD out of the Lower Dakota, a very good well in this region of the San Juan Basin. Two other good wells have also been recently drilled. Results indicate a success ratio of nearly 100 percent using the exploration method. The technology is ready for commercialization and industry use in exploration for tight gas fractured reservoirs.