Wireline shear and AVO modeling: Application to AVO investigations of the Tertiary, U.K. Central North Sea

Abstract

Numerous examples exist in the literature of the successes and failures of the application of amplitude variation with offset (AVO) “gas effect” theory in hydrocarbon exploration. The acquisition of wireline and VSP shear data is considered critical information for accurate calibration, quantification, modeling, and successful application of AVO analysis techniques in a particular geologic environment. Wireline compressional, wireline dipole shear, well vertical seismic profiling (VSP), and surface seismic data are used to investigate the AVO behavior of the major lithologic boundaries and hydrocarbon bearing zones. Within the study area, lithologic boundaries produce significant AVO anomalies that may be easily confused with Class III AVO gas effect. In addition, the principal economic target, oil bearing sands, does not appear to produce AVO gas effect anomalies; although a Class III AVO gas effect response is observed when as little as 2 m of free gas is present at the top of the reservoir. Confirmation of an (Class I, II, or III) AVO gas effect in these Tertiary sands can be significant for detecting and delineating reservoirs; however, excessive emphasis on the importance of AVO gas effect, or high amplitudes on relative amplitude seismic sections, will result in an underestimate of (oil) hydrocarbon potential within the region.