Introduction to CSEM

Basics – why CSEM?

CSEM finds its place in the exploration workflow as a risk-reducing tool. It offers resistivity information, which often can be related directly to the presence or absence of hydrocarbons in a reservoir. This information is complementary to other sources of information, contributes to lowering the exploration risk and therefore adds value for the client.
In the search for hydrocarbons, seismic is by far the most common exploration method. It relies on the transmission and reflection of acoustic waves into the subsurface, and results in a detailed structural image of the various geological layers based on rock density. On the other hand, electromagnetic fields propagate by diffusion, and are sensitive to the electrical properties of the subsurface. By measuring the rock resistivity, CSEM can be used to discriminate between different fluids in a reservoir, and may therefore indicate the presence of oil and gas. The main principles of the marine CSEM methods in use today are the use of (1) an electric source which excites the subsurface with an electromagnetic field, and (2) multiple receivers which record the subsurface response to the electromagnetic excitation. The sources in use are either stationary and vertical like PetroMarker’s transmitter system, or moving and horizontal as those used by other companies. Similarly, the receivers can be stationary (like PetroMarker) or moving, and they can record the horizontal and/or the vertical components of the electromagnetic field.

Basics – why PetroMarker?

PetroMarker is the only company offering EM services based on vertical transmitter and receiver technology, which is known to have higher sensitivity to thin resistors like a hydrocarbon reservoir compared to methods based on horizontal transmitters and receivers.
PetroMarker’s EM technology differs notably from other marine CSEM technologies based on horizontal transmitter and receiver dipoles, which commonly work in the frequency domain. Working at short offsets (in the near zone) and in time domain, with alternate pulsing and receiving sequences, PetroMarker’s EM technology avoids disturbances like the air wave and the direct signal. As a result, the data processing is much easier and faster than with similar technologies. The main features of PetroMarker’s EM technology are:

Accuracy – Due to the short offset setup (typically 1000 – 1500 m from transmitter to receiver), the subsurface volume “illuminated” by the method is mostly confined to the region between the transmitter and the receiver
Sensitivity – The vertical component of the EM field is more sensitive to thin resistors in the subsurface than the horizontal components. To compensate for the relatively weak signal strength of the vertical component, receivers with ultra-low noise floor have been developed
Depth penetration – Depending on the geological conditions (complexity and resistivity variations) and transmitter output, PetroMarker may penetrate to depths around 5000 m below seabed
Vertical dependent – The receiver antenna must be as vertical as possible to avoid contamination from the horizontal field components. PetroMarker has developed a new range of receivers which allows the antenna to be aligned vertically to within a high degree of accuracy.