Practical Limitations of the Interpretation of Deepwater Gulf of Mexico Subsalt Seismic Data

Louis Liro, Qingbo Liao, Brigida Fontecha, Wenying Cai, and Marcelo Benabentos
RepsolYPF, The Woodlands, TX

Exploration in the deepwater Gulf of Mexico is hampered by shallow allochthonous salt. The high seismic velocity of salt, contrasting with relatively slow velocities of adjacent clastics, results in difficult seismic imaging. Inadequate seismic image in certain settings is commonly rationalized as “poor seismic”; we review several common salt allochthon configurations where imaging is possible only under specific acquisition and processing workflows not typically found in “spec” data. We then consider their impact on geologic interpretation and risk evaluation: (1) Salt allochthon shape variations, creating irregular and often insufficient recovery of seismic signal. Resulting diminishment of seismic image prevents adequate definition of the salt body, as well as subsalt structure, particularly 4-way closures. (2) Seismic processing algorithm difficulties of near-salt imaging result in poor definition of vertical and near-vertical salt feeder stock and weld systems. This effect is particularly detrimental to the definition of 3-way traps against or near vertical salt. (3) Inadequate and irregular recovery of seismic trace stack, coupled with typically low gas-saturation in encountered oils, leads to overall inability to use amplitude-versus-offset (AVO) and other direct hydrocarbon indicator (DHI) methods subsalt as an effective risk determiner. (4) Imperfect preservation of amplitudes prevents adequate stratigraphic (i.e., reservoir) interpretation subsalt. Each of these issues contributes to overall interpretational inadequacies, allowing only basic structural interpretation subsalt. The paradox of this exploration situation is that while the most sophisticated seismic tools and software are being utilized, only the most basic structural interpretation is possible.