Comparison of magnetotelluric data with drilling results in Papua New Guinea

Abstract

Since 1988, more than 800 magnetotelluric (MT) stations have been acquired in Papua New Guinea. Most of the stations have been acquired in the highlands where conventional exploration (seismic) is difficult owing to the outcropping limestone, intense karstification and severe topography. The seismic data that are acquired are expensive and, in many cases, poor to no-record. In an effort to provide useful geophysical data in the exploration effort, several companies have acquired MT data. The survey size has ranged from nine to over one hundred MT stations. Most of the data have been acquired along profiles, with station spacing of between 200 m and 1500 m. With the release of MT and drilling data by the PNG government, a comparison of MT data with well data can now be shown. MT data have now been acquired at more than 15 well locations, which were drilled either before or after the MT data surveys. These comparisons confirm the usefulness and application of the MT technique in tough exploration regions such as PNG. Although MT does not have the accuracy of seismic, it can be very useful in predicting pre-drill depths and lithologies, at best to within 5% of depth. Perhaps MT is more useful in determining gross structure at depth, such as mapping repeats of thrusted section and structural style, which combined with geologic modelling can help to determine the best well location or, in some cases, lower the prospectivity of a prospect. When recorded properly as closely-spaced stations along dip profiles, and interpreted using 2-D algorithms integrated with geology, MT can be very successful in predicting pre-drill depths and helping to determine well locations.