Seismic Technologies for Independents:

Exploring for Unconventional Resources

Bob A. Hardage. Senior Research Scientist, Bureau of Economic Geology, Austin, TX 78758  [email protected]

Independents who explore and exploit unconventional resources need 1) a strategy to evaluate the fracture systems that dominate many unconventional resource plays, and 2) access to seismic contractors who specialize in acquiring small 3D seismic surveys. Shear (S) waves react to fractures in a more robust manner than do compressional (P) waves, thus S-wave seismic technology should be used to evaluate unconventional resources whenever possible. Nine-component (9C) seismic data provide all possible S-wave modes and are the optimal data-acquisition option to use across unconventional prospects. However, the lowest cost type of S-wave data that can be acquired is the P-to-SV converted shear mode that can be extracted from three-component (3C) seismic data. Consequently, the P-SV mode will be a more attractive S-wave option for many cost-conscious independents. For this reason, this paper discusses the use of P-P and P-SV data acquired in 3D3C data surveys for fracture evaluation.

Strategy for Evaluating Fractures

From a geological perspective, there are three fracture attributes that need to be known in order to conduct a successful exploration/exploitation program for unconventional resources: 1) fracture orientation, 2) fracture intensity, and 3) type of fracture-filling fluid. All three of these fracture properties can be determined with P-P and P-SV data. For example, a common-azimuth analysis of P-SV data is an excellent way to determine the azimuth of maximum P-SV reflectivity from a fractured interval, which should be the orientation direction of the principal fracture population within that interval. 

As a second example, fracture anisotropy should create S-wave splitting and cause the P-SV mode to segregate into fast-S and slow-S modes. Fracture intensity should then be a maximum at those seismic-image coordinates where the ratio of fast-S to slow-S reflection amplitudes from a fracture target exhibit local maxima, and where the ratio of slow-S to fast-S time thicknesses across a fracture interval is a local maximum. 

Third, amplitude-versus-angle (AVA) analysis of both P-P and P-SV data can identify whether the fracture-filling fluid is gas or brine. When P-P and P-SV AVA analyses are done across an area of brine-filled fractures and then repeated across an area of gas-filled fractures, there is no variation in P-SV AVA behavior, but there is a large difference in P-P AVA response, with gas-driven P-P AVA amplitudes being much larger than brine-driven P-P AVA amplitudes.

Thus by a proper combination of P-P and P-SV attributes, the critical information needed to understand the fracture systems found in most unconventional resource plays should become available to influence exploration and exploitation business strategies.

Small-Survey Seismic Contractors

All U.S.-based onshore seismic contractors offer good services, and independents should contact any of these service companies to discuss their seismic-data needs. However, many independents desire that there be some number of seismic data-acquisition contractors who are more focused on acquiring the small seismic programs that many independents use in their operations. A new technology called cableless seismic data acquisition may allow this new paradigm of small-survey contractors to emerge. Cableless data acquisition is based on the same technology as cellular telephones. The attraction of cableless data acquisition is that seismic-spread cables are eliminated by connecting a small box that acquires the data directly to each geophone string. The data acquired by this cellular box are either relayed to a central data-storage unit via a system of data-antennae links or are downloaded manually at appropriate time intervals. By eliminating spread cables, a smaller capital investment is required to start a new seismic data-acquisition company, and less operating cost is involved in transporting, deploying, and maintaining a seismic system. These two factors may cause a new class of seismic contractor to arise; a class of companies that is tailored to serve the independent operator community by concentrating on small-survey services.