The oil and gas exploration industry currently spends more than US $5 billion a year on gathering 3-D seismic data. Seismic can identify the geological structures that may contain hydrocarbons but under many circumstances does not reveal the presence of hydrocarbons themselves. And with frontier exploration moving into deeper waters where the costs of drilling an exploration well run to tens of million dollars, the industry needs new, non-invasive, lower-cost exploration techniques.
A new sounding technique called Controlled Source Electromagnetic Imaging (CSEMI) can detect offshore hydrocarbon reserves. The technique comes into its own in frontier exploration, where it can be used to test high-risk prospects such as stratigraphic plays, which often have little or no seismic expression at all.
The technique involves transmitting a low-frequency controlled electromagnetic (EM) signal (typically in the range of 0.01Hz to 10Hz) in close proximity to the seafloor and recording the resulting EM field response with an array of stationary receivers deployed on the ocean bottom. The resulting data is processed, with changes in the EM field used to provide information on the resistivity structure of the subsurface. Hydrocarbon-bearing formations are typically between 10 times and 100 times more resistive than the surrounding strata, and this contrast allows detection of the presence or absence of hydrocarbon reservoirs and their distribution.
CSEMI is now regarded as a key value-adding exploration tool and is applied by supermajors and exploration and production (E&P) minnows alike in their exploration efforts. CSEMI gives a clear benefit in the move to deeper water frontier exploration as it provides enhanced understanding of the geologic setting. In addition, recent developments in the technique now mean that it can be carried out in water depths as shallow as 165 ft (50 m) to look at structures down to several thousand feet.
CSEMI sounding can also be applied as a complementary method in environments where seismic data struggle. There are numerous regions in the world where the presence of shallow-high velocity layers makes the imaging of deeper structure using conventional seismic reflection techniques a difficult task. In some continental shelf areas, for example, potentially oil-bearing sedimentary structures are obscured by layers of basalt, carbonate or salt. High-velocity layers such as these limit the penetration of seismic waves and can cause reverberations which mask reflections from deeper sedimentary structures, leading to ambiguities in interpretation. In this instance it may be possible to use CSEMI surveying to detect and constrain the base of the salt or basalt layer - useful information in itself.
For example, Figure 1 shows the results of a feasibility study examining the detection of a reservoir trapped against the flank of a salt dome, a situation encountered in, for example, the Gulf of Mexico or West Africa. Despite the presence of the massive resistor (the salt in this case, with a resistivity of 200 ohms) the reservoir can be delineated using CSEMI data.
Key in the success of the technique is the interpretation of the retrieved data. Ohm has developed software packages and technical skills which turn the EM data gathered in the field into decision-ready images of the geo-electric structure of the Earth.
While global demand for energy continues to rise unabated, with E&P moving into frontier areas to acknowledge that demand, the need remains to reduce both the cost and environmental impact of exploration and extraction. CSEMI is a non-invasive technology able to minimize the risk of drilling unsuccessful exploration wells. It can also provide a cost-effective way of proving many more prospects within a portfolio, preventing potentially economic reserves of hydrocarbons from being missed.

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