Reducing exploration risk the CSEM way
  from: Asian Oil & Gas
  by: Dave Pratt
  Friday, October 27, 2006

With exploration becoming increasingly challenging and drilling costs soaring as wells become more and more complex, the oil and gas industry is turning to the latest innovative techniques in a bid to reduce the risks of drilling non-commercial wells and to stretch exploration budgets to test many more prospects than ever before. OHM chief executive Dave Pratt discusses the provenance and potential of one such technique - controlled source electro-magnetic imaging (CSEM),

Until recently the only way to identify whether hydrocarbons were present was to drill. But with as many as two thirds of exploration wells generally failing to result in commercial discoveries, over the last few years oil and gas companies have begun using controlled source electromagnetic imaging (CSEM) as a key component in their exploration tool kits to complement their seismic data, gain a more complete picture of the sub-surface, delineate reservoirs and reduce the risk of drilling costly dry holes.

CSEM has been used for over 20 years by researchers to examine the properties of fluid in the earth, initially over active hydrothermal and volcanic systems on mid-ocean ridges. Uptake in its use by oil and gas exploration and production companies has grown steadily over the past few years, with CSEM survey provider Offshore Hydrocarbon Mapping (OHM) seeing ever-increasing levels of enquiries from supermajors to junior exploration and production companies; this year the company's order book for surveys has reached higher levels than ever before.

In prior years, the primary users of CSEM were the supermajor and major oil companies. We are now conducting surveys for mid-sized and junior exploration companies whose managements and shareholders recognise the value of reducing exploration risk - and therefore cost - using CSEM.

What is CSEM?
The technique uses electro-magnetic signals to map the variations in the resistivity of the geological formations below the seabed. The resistivity in hydrocarbon bearing layers is typically between 10 and 100 times greater than the surrounding strata. CSEM uses this contrast in the resistivity between water and hydrocarbons to detect and delineate structures that are likely to contain oil and gas.

The survey method involves towing a high-powered electromagnetic source through the water, close to the seabed. The source transmits a low-frequency signal which is detected by electromagnetic receivers placed on the seabed. Each receiver detects and records the electromagnetic fields at the ocean floor at offsets of up to 10-15km. The variation of the received signal as the source is towed allows the team of scientists to determine the resistivity of the underlying geological formations.

Formations that are water saturated or with low hydrocarbon saturations generally show a small CSEM response while commercially exploitable hydrocarbon deposits, in general, have a higher resistivity and a correspondingly stronger CSEM response.

The data interpreation team then uses complex algorithms, including forward modelling and inversion techniques to turn the electro-magnetic data into 2D and 3D images of the geo-electric structure of the subsurface. The technique can map complex reservoir structures in subsurface structures anywhere from a few metres to several kilometres deep.

Survey design
The success of the technique relies on careful planning of the survey and skilful interpretation of the electro-magnetic data, especially as some geological conditions can prove challenging for the technique. Some subsurface formations can display the same high resistivity as hydrocarbons and in these cases the CSEM team will look at the extent of the resistive anomaly and the surrounding geology to identify whether the data relates to hydrocarbon-saturated sands or another highly-resistive formation such as carbonates.

Pre-survey modelling is a vital part of the process as it determines, to a large extent, just how valuable the work to be undertaken will be. As the electromagnetic source provides a three dimensional signal, the accurate placement of the receivers is vital.

Dr Lucy MacGregor, chief scientific officer with Offshore Hydrocarbon Mapping explains: 'This is not a one-size fits all process. Each survey must be carefully planned, taking into account the survey objectives, to ensure that the optimum dataset is collected. Post-survey, we take great care in the analysis and interpretation to arrive at a geo-electric section or volume, providing intuitive images that the client can usefully use in their exploration process.'

The technique not only offers exploration companies the opportunity to distinguish between water-filled and hydrocarbon bearing structures but can also map out their location, indicating both presence and distribution. It can also provide extremely valuable information about the level of gas saturation, overcoming a key pitfall of using seismic alone.

Shallow water challenges
Until recently, CSEM surveys had been limited to exploration for oil and gas reserves in tracts of deep water of 1000m or more because of technical challenges posed by 'airwave' interference swamping the electromagnetic response from below the seabed in shallower water. However, research at OHM funded by a consortium including Total, BG, Shell, Eni and Chevron with the UK's Industry Technology Facilitator has largely removed this limitation. The research culminated in a successful trial on the Nuggets-1 reservoir in the UK North Sea in summer 2006 in just 116m of water, demonstrating the effectiveness of the technique in shallower water depths than previously thought possible.

Overcoming the technical challenges of operating the technique in shallow water was a key breakthrough in the technology, and saw an increase in the use of the CSEM by oil and gas exploration companies.

Commercial surveys have now been completed in 70m of water and surveys are planned soon in even shallower water depths.

Asian focus
Offshore Hydrocarbon Mapping has been active in Southeast Asia, with the technique having been used in complex stratigraphic trap plays in the region. The company has worked with a Malaysian partner, Petra Resources on a contract from Newfield Sarawak Malaysia to acquire and process CSEM data over a number of prospects in the deepwater block 2C offshore Sarawak, with OHM undertaking the survey planning, data acquisition and processing and interpretation.

Asia continues to be a focus for OHM with the company actively focussing on the significant opportunities the region continues to offer for exploration and production with increased resources from late 2006, including the opening of a new regional office based in Singapore.

The company is also to undertake CSEM surveys offshore Western Australia as part of a total geophysical multi-client study to be offered in partnership with other oil services companies. The CSEM survey will be over the WA 06-9 licence, which has been opened for competitive bidding through a work programme bidding system with a deadline for applications of May next year. The multi-client data study will integrate Offshore Hydrocarbon Mapping's CSEM data with seismic well log data to comprehensively de-risk the exploration acreage, which is close to the Scarborough and Jansz gas fields. OHM is working with Global Geo Services, a specialist in Asia Pacific exploration consulting, particularly for seismic survey design and process management, Australian Seismic Brokers and Rock Solid Images, a private company based in Houston, Texas, which uses the latest techniques for understanding reservoir characterisation.

Elsewhere, the company has operated in the deepwater clastic basins off West Africa, in the more complex geology off Brazil and in technically challenging imaging conditions in the Gulf of Mexico. The company has also undertaken two surveys in the North Falklands Basin in the South Atlantic for Rockhopper Exploration.

The surveys were conducted in 110- 140m of water over an untested play concept in a basin with no current production and a reputation for very high drilling costs.

Surveys off northwest Africa have examined traps bounded by diapiric salt, and the recent shallow survey off the Falkland Islands for Rockhopper Exploration has successful identified and created images of resistive bodies in settings with complex basement topography. Multi-client surveys in West Africa and off Norway, which are selling well, have de-risked investments committed by oil and gas exploration companies in bid rounds.

Falkland Oil & Gas, which has rights to vast tracts of deepwater exploration acreage offshore the Falkland Islands, recently asked Offshore Hydrocarbon Mapping to conduct an extensive CSEM survey in the South and East Falkland basins. The programme will start late in 2006 with the intention of acquiring CSEM data over many of the larger prospects identified by Falkland Oil & Gas from seismic interpretation. The oil and gas exploration company intends to use the data to help determine the best sites for exploration wells.

With exploration becoming ever more costly and the need to de-risk prospects higher than ever, the uptake of CSEM can only continue to surge ahead. With the oil and gas exploration industry from the supermajors to the smallest start-up exploration and production companies adopting the technique as a key part of their strategies, the days of simply identifying seismic events and then committing to risky and costly appraisal drilling are clearly over. CSEM is becoming an increasingly vital exploration tool. AOG

Click here to register to receive your own copy of Asian Oil & Gas each month.