Abstract of paper presented at American Association of Petroleum Geologists annual convention, April, 1991, Dallas, Texas.
AAPG Bulletin, v. 75, p. 687.

Paleomagnetic core-orientation for characterizing reservoir anisotropy: Case histories from fractured reservoirs in Abu Dhabi and Venezuela

D. R. Van Alstine and J. E. Butterworth, Applied Paleomagnetics, Inc.
E. J. M. Willemse and W. J. E. Van de Graaff, Koninklijke/Shell Exploratie En Produktie Laboratorium

Over the past decade, we have tested and refined the paleomagnetic core-orientation technique we first applied on the U.S. Department of Energy's Multiwell Experiment in the early 1980's. Since then, we have paleomagnetically oriented over 10,000 feet of core from over 100 wells in all lithologies from hydrocarbon provinces around the world.

Paleomagnetic orientation accuracy within 5° is routinely achievable, as demonstrated in over 100 tests against dipmeter, seismic, borehole televiewer, and mechanical core-orientation (conventional multishot and MWD) techniques. These tests reveal that mechanical core-orientations commonly suffer from torques acting on downhole equipment and on the core. In contrast, the paleomagnetic core-orientation technique is not prone to downhole mechanical problems, uses no rigtime, is cost-effective, and can selectively orient individual core pieces, including ASR and DSCA samples.

Accurate paleomagnetic orientations can be obtained even from cores drilled decades ago. In Abu Dhabi, paleomagnetically-oriented 10-year-old slabbed cores from Habshan and Dukhan carbonates yield fracture patterns consistent with the geologic structure in the Asab and Sahil fields. In Venezuela, paleomagnetic orientation of fresh and decade-old archived cores of Cogollo limestones and La Luna black shales yield fracture and bedding directions consistent with dipmeter data and outcrop observations around Lake Maracaibo.

Paleomagnetic core-orientation permits access to the vast resource of reservoir anisotropy data in previously unoriented cores in storage throughout the world. Paleomagnetic core-orientation is a versatile technique with proven accuracy not only in vertical wells, but also in slant holes and horizontal wells.