Once the exploration well is drilled, pre-drill/post-drill reconciliation begins. Often the structural dip interpreted from seismic conflicts with the structural dip interpreted from dipmeter or image logs. An independent measure of dip, “Rhythm of the Rocks”, can help to reconcile the conflict. The “Rhythm of the Rocks” is seen in the oscillatory response of well logging tools. One attribute of the oscillating log response is measured depth wavelength. If the wavelength over a particular succession of rocks is known from a wellbore penetrated normal to the rock layers, then an increase in wavelength for the same or similar succession of rocks penetrated by a non-normal wellbore can be explained by the angular relationship between the wellbore and rock layers. Measurement of normal and non-normal wavelengths can be accomplished graphically or digitally. A trigonometric relationship of normal and non-normal wavelengths determines the relative angle between the wellbore and a line normal to the rock layers. A plot of the relative angle centered at wellbore deviation and direction on a Stereonet yields true dip magnitude of the rock layers as a function of azimuth. If the wellbore is vertical, only the true dip magnitude is known; dip direction remains a mystery. In deviated wells, the true dip magnitude is known, but now an understanding into dip direction is achieved. Greater the deviation, greater is the understanding. “Rhythm of the Rocks” is used to determine true dip magnitude and help substantiate dipmeter results in an example from Garden Banks, Gulf of Mexico.

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