In the search for oil on asymmetrical anticlinal structures it is of prime importance to know the position of the axial “plane” of the fold, for it is never safe, even where asymmetry is only moderate, to drill a test well across the axial “plane” and so penetrate the steep flank. Where such a thing has happened, the evidence in regard to the oil prospects of the area can generally be ruled out as inconclusive, and in that way much money and effort have been wasted.