CSEM Efficiency - Evaluation of Recent Drilling Results

Hesthammer, Jonny ¹; Sperrevik, Susanne ²; Stefatos, Aristofanis ¹
(1)Rocksource Geotech, Bergen, Norway. (2) Rocksource, Bergen, Norway.

In 2011, Rocksource ASA planned for 5 exploration wells to be drilled on prospects that show a clear CSEM anomaly. These wells are located offshore West Africa (1), in the Barents Sea (2), in the North Sea (1) and in the Norwegian Sea (1). The outcome of these wells allows testing of the effectiveness of a CSEM-driven de-risking process in a portfolio setting and also allows comparison with empirical results published by Hesthammer et al. (2010).

The CSEM technology for marine hydrocarbon exploration was introduced around year 2000, and Hesthammer et al. (2010) evaluated the results of 86 wells drilled on prospects and fields with available CSEM data. Fifty of the wells are exploration wells drilled after the acquisition of CSEM data in the period 2004-2009. When considering wells drilled on prospects with a significant CSEM anomaly (normalized anomalous response, NAR, above 15% for fundamental frequency), 21 out of 30 exploration wells are discoveries, providing a technical discovery rate of 70%. For exploration wells drilled on prospects without a significant CSEM anomaly, 7 out of 20 wells are discoveries, providing a technical discovery rate of 35%.

Another publication by Fanavoll et al. (2010) builds on the results from Hesthammer et al. (2010) but views the data in terms of commercial success rates. This study demonstrates that of 8 technical discoveries with weak or no CSEM anomaly, the commercial success rate is only between 5-14% (1 declared commercial, 5 declared non-commercial, 2 with unknown commerciality). The commerciality of the discoveries associated with a significant CSEM anomaly is not known due mainly to the need for further work (including appraisal drilling) prior to a declaration of commerciality. However, several of these discoveries are announced as significant. This indicates a clear link between the observed EM response and the size of hydrocarbon-filled prospects. From an exploration point of view, this is important as the technology provides means for the oil companies to identify likely commercial volumes of hydrocarbons prior to drilling.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.