Abstract

New technology is needed for effective stratigraphic trap exploration. Surface-measured resistivity (Electromagnetic Imaging) appears to be the solution. Commonly used exploration methods such as seismic, gravity, and magnetics have not always been successful in stratigraphic trap exploration. Geochemical methods are used for reconnaissance purposes and are inferred to depth. There is a large gap between the resolution capabilities of 2-D and 3-D seismic and potential fields methods such as gravity and magnetics. The resolution capability of the Electromagnetic Imaging (EMI) method falls between the two groups. The EMI method is a low-frequency, time-domain electromagnetic sounding technique utilizing a grounded-wire transmitter and an induction-loop receiver. It is the best of all the electrical methods because of its high degree of resolution, large depth of penetration, portability, and cost-effectiveness.

When hydrocarbons accumulate in reservoirs as a result of suitable trapping conditions, the reservoir rocks and a large volume of rock closely associated with and generally above the reservoirs undergo resistivity changes creating a large exploration target. Under these conditions, using a surface-based electrical method such as EMI to map stratigraphic traps becomes possible. Using the EMI method, we have found that stratigraphic traps, in almost all cases, manifest themselves as areas of deep-seated, high-resistivity anomalies which correlate closely with the hydrocarbon reservoirs.

The East Kinsler oil and gas field located in Morton County, Kansas produces from Morrow sandstones. An EMI line run across this field produced a robust resistivity anomaly delineating the field. Using this field as a model, a multi-line EMI survey was carried out in southwestern Kansas. A test hole drilled on one of the resulting EMI anomalies discovered gas in the lower Morrow sandstones and oil in the Mississippian St. Louis Formation.