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For over 30 years literature describing relations between well log velocities and densities have been only sporadic, uncoordinated and often excluded one or more key parameters such as lithology or geological age. Model relationships are presumed to hold and tested only qualitatively with little to no effort directed toward refining the models or quantitative reconciliation.

In 1986 the concept of the sand/shale acoustic impedance cross-over on Zone II was introduced. Where Zone II exists, it separates strongly contrasting low acoustic impedance sands within shales from the more consolidated but still strongly contrasting high acoustic impedance sands within shales at greater depth. The inconsistent and highly muted reflections characterizing Zone II, and corresponding small fluid signatures make seismic definition especially difficult. Also, familiar tools such as seismogram synthesis and AVO studies give unreliable results in this environment. We also now recognize the relatively widespread occurrence of Zone II.

Important potential for hydrocarbons has been established for Zone II—albeit mostly by accident. It remains largely unrecognized and unexplored except for obvious simple structures. Conscious and detailed seismic procedures in conjunction with appropriately organized cataloging of subsurface parameters offer the most effective methods for defining Zone II to date. Applying such techniques constitutes the Challenge, but the rewards could be great indeed.

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