Coherency mapping has become a widely used tool applied in both frontier exploration and exploitation areas. Several examples will be illustrated where 3-D seismic coherency and spectral decomposition have been applied to diverse carbonate depositional environments. These seismic attributes are shown to be particularly useful in identifying faults and fractures, and interpreting boundaries of different types of reefal bodies.

Coherency is particularly useful for locating faults, including basement faults and reefal fault blocks that could influence facies development and subsequent porosity enhancement. Location of more subtle features such as fracture patterns and possible higher permeability zones can also be important when exploring and producing from microporous carbonate reservoirs. In these cases, coherency maps can be used to help direct the wellbore to intersect fractures during horizontal drilling for increased production. Reef margins, including boundaries of pinnacle reefs, low relief, isolated reef buidups, and shelf margin buildups can also be interpreted from coherency images and verified by well data where possible. Accurate definition of reef margins determines trap size and economic reservoir limits that can be critical risk factors in making a play viable.

Spectral decomposition used in conjunction with coherency, helps confirm the interpretation of faults and reefal boundaries and can enhance the appearance of stratigraphic or diagenetic complexities in different frequency ranges. In contrast to seismic inversion, spectral decomposition is a quick, easy-to-use tool that allows us to detect lateral changes in reflectivity, including below the 1/4 wavelength seismic resolution limit. These lateral changes in spectral components provide provoking subjective interpretations. However, these interpretations are difficult to tie and calibrate to existing well and production data to define a productive horizon (i.e., zones of high porosity and permeability). Decomposing the data into multiple spectral components gives the interpreter additional attributes for testing geologic interpretations. This is particularly important in frontier areas where little well control is available.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah