CSEM: more than just a high-grading tool

A RELATIVELY new geophysical technique is offering offshore explorers the chance to significantly de-risk deepwater prospects before taking the plunge with a drill rig.
CSEM: more than just a high-grading tool CSEM: more than just a high-grading tool CSEM: more than just a high-grading tool CSEM: more than just a high-grading tool CSEM: more than just a high-grading tool

CSEM, or controlled source electromagnetic imaging, uses electromagnetic remote sensing technology to detect the presence and extent of hydrocarbon accumulations beneath the seabed.

Usually considered a complement to seismic, the technology offers the promise of fewer dry wells, improved ranking of development prospects, and more accurate or reduced appraisal drilling. It is even being suggested that companies may sometimes be able to go straight from 2D seismic to CSEM without the need for costly 3D seismic.

As BPC Ltd exploration manager Mike Middleton explains, CSEM adds an “extra dimension” of information to data modelling.

“Ideally what happens is you can see the oil or gas zone identified by its electrical characteristics,” he said.

“You need a preliminary understanding of the geology to understand how to use it. So it’s effectively a high-grading technique.”

Middleton’s company, in conjunction with US-based Offshore Hydrocarbon Mapping (OHM), is about to take-over operatorship of Westralian Gas & Power’s recently awarded Vlaming Sub-basin permits off the coast of Perth, where the joint venture is currently finalising the details of a CSEM survey due to begin early next year.

Westralian won the two offshore petroleum exploration permits, WO5-21 and WO5-22, covering 4530 square kilometres between Rottnest Island and Mandurah in August.

At that time, Westralian executive chairman Alan Burns said the company and the Western Australian Government believed the area was promising but 1970s and 80s exploration had shown it was “a hard nut to crack”.

Previously, the area’s complex structural geology, combined with limited seismic resolution, resulted in failure to accurately predict the distribution, thickness and quality of reservoirs and seals.

But Westralian believes CSEM will be the key to successfully exploring the Vlaming.

Another junior counting down to the start of its own CSEM survey is AIM-listed Falklands Oil and Gas (FOGL).

OHM has once again been contracted to undertake the survey, which will be undertaken in FOGL’s acreage in the South and East Falklands Basins.

FOGL CEO Tim Bushell has said that CSEM imaging works best under certain conditions, including deepwater (less than 500m), simple sand and shale geological sequences, and large traps, all of which are prevalent in the region.

“The use of CSEM is an appropriate exploration tool given the large number of prospects and leads that we have identified,” he said.

“We believe that CSEM can help to significantly reduce risk, improve the chance of success and allow us to focus on the best prospects for drilling in 2008.”

FOGL’s strategy differs to Westralian’s approach, in that the company plans to begin by acquiring CSEM data over many of the largest prospects before using the results to set priorities for infill 2D seismic to determine the best sites for exploration wells.

At last report, both the CSEM and 2D seismic surveys were due to begin before the end of the year, with results expected to be announced in early 2007.

The petroleum industry was first introduced to CSEM technology about six years ago, when Norway’s Statoil undertook a full-scale trial survey over an area with proven hydrocarbon reserves offshore West Africa.

Before this, CSEM has been used successfully for at least a quarter of a century to study ocean basins. But the November 2000 survey was the first time the technology had been applied in the hunt for hydrocarbons. Results from this pioneering effort confirmed that the received signals were sensitive to buried resistive layers.

CSEM exploits the significant difference between resistive hydrocarbon-saturated reservoirs and the more conductive surrounding layers saturated with saline water.

A CSEM survey involves deploying a horizontal electric dipole source that transmits a low frequency electromagnetic signal to a series of seabed receivers that measure the electromagnetic field at the seafloor.

As the source is towed over the receivers, the variation and phase of the received signal indicates the resistivity of the subsurface structure down to depths of several kilometres.

Using electromagnetic techniques to determine the nature of the subsurface is certainly not new to the oil and gas industry – wireline logging is now a routine part of exploration and reservoir development.

But CSEM offers a non-invasive solution, albeit at a lower vertical resolution, which under the right conditions can distinguish between water-filled and petroleum-filled drilling targets.

Statoil might have been the first to prove its usefulness to the oil and gas sector, but ExxonMobil is also believed to have benefited from its application.

A 2004 Wall Street Journal article credited CSEM with contributing to the oil giant’s remarkable run of 13 out of 13 discovery wells, offshore Angola.

Since this time, enthusiasm for CSEM as a way of enhancing seismic data and confirming the presence of hydrocarbons in offshore structures has been growing. This has become especially important as industry moves into deeper and deeper waters where a single well can cost in excess of $50 million.