CSEM Anomolies Indicate Hydrocarbon Potential in the North Sea
The alignment of the sources in vertical Controlled Source Electro Magnetic (CSEM) technology, combined with source signals transmitted as pulses in time domain, result in a deeper imaging as well as the ability to record near-offset data, giving rise to an increased lateral resolution of subsurface resistivity compared to other CSEM technologies.
Vertical CSEM uses a vertical electric dipole source that is adjustable to the water depth of the actual survey area and a grid of receivers with vertical electric dipole antennas that is dropped in pre-planned positions on the sea floor. The dipole source has two transmitter electrodes, one placed and positioned accurately on the seabed, while the upper electrode hangs below the survey vessel, positioned above the lower electrode by DP2. It is important that the vertical alignment is strictly maintained to obtain the best possible data quality. During pulsing the vessel is stationary and the dynamic positioning of the vessel is used to ensure verticality. The electric current transmitted between the two electrodes is 5,000A and the source signal is pulsed to acquire data in time domain. A typical source signal could be 9s on at 5,000A, followed by 9s off for a listening period. The pulse sequence is repeated for a pre-planned number of stacks to achieve the required signal to noise ratio.
The receivers are equipped with an active verticality correction for the vertical electric dipole sensors (each receiver has four parallel Ez dipoles). They are dropped to the sea floor by gravity and positioned accurately by acoustic signals and then the verticality is adjusted for a potentially tilted seabed. After data acquisition the receivers are released by an acoustic signal and retrieved by the survey vessel. Horizontal sensors complement the recording suite.
One of many important features of the vertical CSEM technology is its application in zones close to existing infrastructure, or so-called ‘near zone’. Vertical time domain CSEM is a near-offset technology, with the strongest response being on the receivers closest to the transmitter. A typical offset range could be from 500m to 5,000m. In the near offset, the subsurface layers impacting the pulse signal are narrowly concentrated between the transmitter and its nearby receivers. Most of the vertical energy is therefore used to penetrate the layers of interest and is not passing through geology at far offsets before reaching the target of interest. Another important factor in vertical CSEM is the stationary pulsing. More energy can be transmitted by increasing the pulsing time on each location to optimize signal to noise ratio. The stationary approach also gives more freedom when operating close to existing installations.
The North Sea Gjøa Area Survey
PetroMarker was established in 2005 to develop this marine CSEM acquisition method using 2D surveys, but improved technology and an increased number of receivers mean that it is now possible to acquire 3D data by this method. In 2018 the company acquired 700 km2 of vertical 3D CSEM data around the Gjøa field in the Norwegian part the North Sea.
The survey also covered the Cara discovery, now renamed Duva, located north-east of Gjøa, as well as interesting exploration opportunities to the east and south of the field, as seen on the survey layout map on the following page. The field reached a record high production of 43.8 MMboe in 2016 and this coupled with the Cara discovery in 2016 have increased interest in further exploration potential around the Gjøa area.
The CSEM receivers were laid out in a grid pattern with 1.7 km distance between each receiver location. The source positions were asymmetric between receivers, with a distance of 700m and 1,000m to the closest receivers, getting a richer dataset with more offsets for the inversion. Pulsing strategy was set to 9s transmitting time and 9s listening time, repeated over a period of 67 minutes at each pulsing location. PetroMarker’s technology allows for tailormade grids without generating operational issues around infrastructure.
Analyzing CSEM Anomolies
There are a couple of dry wells in the survey area that show an absence of any elevated resistivity in the respective drilling locations, in line with the unsuccessful outcome of these wells. Offset from them, however, three prominent CSEM anomalies stand out from regional variations (Anomaly 1, 2 and 3 in the figures below). The anomalies indicate the presence of additional exploration possibilities in the area, with potential for significant remaining volumes. This view is supported by an analysis of the main characteristics of the three anomalies, including an evaluation of the most likely resistive alternative models.
Of the three observed anomalies, Anomaly 2 exhibits the characteristics that are most commonly observed as resulting from hydrocarbon effects in rotated fault blocks. A clear up-dip termination of the anomaly against the fault is present, with a limited extent in the down-dip direction which may indicate the presence of a fluid contact. In addition, the limited down-dip extent also reduces the likelihood that a resistive lithology effect has caused the anomaly, as a sharp lateral lithology boundary with very different resistivities would need to be invoked.
Anomaly 3 shows a similar apparent up-dip termination against a fault; however, no clearly defined rotated fault block geometry is present. At the same time, a distinct geo-body is present approximately 150m below the anomaly, which needs to be considered within the depth uncertainty inherent in the CSEM method. The highest resistivities can be observed in the up-dip location where this body pinches out towards the fault. This lack of correlation between body thickness and resistivity makes a lithology model very unlikely if the implied correlation of seismic feature and CSEM feature is correct.
Anomaly 1 represents a more challenging case as it appears to cross stratigraphic boundaries. However, this does not only represent a challenge for a hydrocarbon scenario, but also for any alternative models to be evaluated. While the juxtaposition of two resistors could represent one scenario, only two intervals with large resistivity contrasts are proven by wells in the area. These comprise the Lower Cretaceous shales and the Heather shales, both of which were penetrated in the nearby well 35/12-1, but no marked CSEM response is observed at the well location.
The Lower Cretaceous shales are the most unlikely alternative model due to the depth difference, primarily because of a thinning away from the well location and the limited overall resistivity contrast. The Heather Formation shales could represent a different model for a juxtaposition, provided that a resistivity increase is postulated. This is primarily due to the more rapid variations in thickness at this level, but it still requires the resistivity placed in the syncline where the Heather Formation shales in this location would be either absent or very thin. Solving this puzzle will probably be the key to properly qualifying and quantifying the anti-model risk and for the more prominent anomalies with more distinct fluid characteristics.
Further Reading on CSEM in Oil and Gas Exploration
A selection of recommended GEO ExPro articles relating to the use of CSEM in the exploration and production of oil and gas resources.
A Strong Start to CSEM Services in 2019
EMGS has to date acquired 950 surveys in 36 countries and territories, from Spitsbergen to New Zealand, all calling on their specialist CSEM services.
This article appeared in Vol. 16, No. 3 - 2019
A New Approach to Hydrocarbon Exploration
Aslak Myklebostad; PetroMarker
PetroMarker’s unique 3D vertical EM technology acquires robust data for new development opportunities in already-explored areas such as the Norwegian Sea.
This article appeared in Vol. 15, No. 3 - 2018
An Electrical Rock Physics Framework For CSEM Interpretation
Daniel Baltar, Neville Barker (EMGS), Matthew Carr and Christopher Schmidt (QI Petrophysics)
A low-resolution rock physics framework relating CSEM-derived Rn measurements to reservoir properties can provide precise property estimates even in high-uncertainty settings.
This article appeared in Vol. 13, No. 1 - 2016
Improving Reservoir Characterisation
Lucy MacGregor, RSI Inc.
Reservoir characterisation can be improved through the integrated analysis of CSEM, seismic and well log data.
This article appeared in Vol. 12, No. 1 - 2015