High-Resolution Logging: The Key to Accurate Formation Evaluation

D. C. McCall, D. F. Allen, J. S. Culbertson

Homogeneous geological zones, even 1 ft thick, are rarely seen in nature. The wireline logs used to evaluate reservoirs typically have bed resolutions ranging from 2 to 10 ft and cannot detect sudden changes in porosity, lithology, or saturation. Now, however, log analysts can accurately evaluate formations that are less than 1 ft thick through new wireline measurements and enhancements of older measurements.

Whether viewed as outcrops, road cuts, cores, cuttings, or thin sections, most geological zones are very heterogeneous. Very few homogeneous zones can actually be observed, even over a 1 ft interval. Despite what is seen in nature, log analysts tend to refer to zones as 20-ft thick with 22% porosity, implying that the zone is homogeneous. Even thick Gulf Coast sands typically have thin shale laminations in them. In fact, if zones were truly homogeneous, the Dipmeter service would not work. Even a thick shale is normally a heterogeneous laminated interval.

Recently, new measurements and enhancements of older measurements make it possible to accurately evaluate many thin heterogeneous zones. While many consider the problems of evaluating thin zones to primarily apply to laminated sands, the problems of thin-bed resolution may actually be most severe in carbonate reservoirs where porosity may change dramatically in just a few inches. In all cases, porous, productive zones will be overlooked and hydrocarbon reserve calculations will be incorrect if the log data cannot resolve these changes in porosity and resistivity.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.