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The Application of Land “Active” EM in Oman

Ismaily, Ali *1; Brandenburg, Aly 1; Farran, Herve 1; Mahrooqi, Said 1
(1) Exploration, PDO, Muscat, Oman.

Controlled Source Electromagnetic can be a useful tool to de-risk the fluid fill of exploration prospects, and has been used with some success in the deep water environment. It relies on detecting resistivity anomalies in the subsurface, related to hydrocarbons. The success of the method is dependant upon the shallow and deep subsurface geology and can be negatively impacted by the presence of high resistivity rocks at near surface or in the deep subsurface, like for instance high resistive limestones or salt.

Onshore CSEM acquisition and processing has been challenging to date. BGP has recently developed the TFEM method. Time Frequency Electromagnetic (He 2003) uses both time domain resistivity data together with frequency domain IP (Induced Polarization) data to deliver a subsurface resistivity and chargeability inversion results.

This method has been tested in Oman, where an added complication is formed by the surface and near surface resistive rocks which as result coursed high contact resistance between ground and the electrodes. In the desert the dry near surface rocks and sands result in a very high contact resistance, which prevents sufficient current to penetration to the target. To mitigate this problem, shallow source holes needed to be drilled. These were filled with salt water and clay to lower the contact resistance and achieve better Signal to Noise ratios. Shallow TDEM was used to image the near surface resistivity and allow planning of the source hole drilling campaign.

The target for the survey was selected following a 1D resistivity screening exercise on a range of working hydrocarbon plays within Oman. This identified that the Sub Nahr Umr Haima/Al Khlata reservoirs of the South Oman Eastern Flank would provide a suitable candidate for resistivity imaging. Subsequent 2D and 3D modelling confirmed that signal to noise resolution would allow hydrocarbon detection. The resistivity contrast between hydrocarbon filled reservoirs compared to water filled reservoir or seals is a factor of 2-30, and the column is thought to be sufficiently thick (up to 70 m)given the depth of top reservoir at 1 km to allow detection.

The location of the survey was carefully selected to cover both a know oil field and a know water filled structure, and several un-drilled prospects.

The existing regional Petrel model will be used to achieve optimal inversion of the acquired data.

NB: update conclusions once we have full results

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain