Controlled-source electromagnetics (CSEM) is an emerging technology for marine hydrocarbon exploration that has seen a variable degree of acceptance across the industry despite empirical results showing that the success rate for exploration wells drilled on prospects with a significant CSEM anomaly is as high as 70%, twice that of exploration wells drilled on prospects with no or only weak CSEM. Part of the challenge for this technology has been a lack of understanding on how to integrate it with more traditional geological and geophysical workflows.

A case study from the Norwegian Continental Shelf (NCS) illustrates the potential of CSEM technology’s ability to identify new prospectivity in an explored basin.

Rocksource ASA is a young Norwegian E&P company that focuses heavily on the use of CSEM technology and its integration into the exploration workflow . In the APA 2009 licensing round on the NCS, Rocksource was awarded 60% of license PL 559, where feasibility studies have shown that the prospectivity from several different reservoir intervals should be feasible for de-risking with CSEM data.

This map shows the location of PL 559 (blue area) in relation to nearby fields and wells. The acreage covered by PL 559 previously was part of PL 128. (Images courtesy of Rock-source ASA)

The new play concept

The Upper Triassic Grey and Red Bed formations underlying the Jurassic B?t Group previously were not considered interesting targets for exploration, although most of the discovery wells have proven reservoirs and some oil shows in this stratigraphic interval

In recent studies, new prospectivity has been defined where these stratigraphic units play an important role, not only due to the sandy reservoir intervals, but also as a base sealing unit for prospective hydrocarbon traps. The presence of these Upper Triassic sands also could be the key to hydrocarbon migration in areas where traditional Jurassic carrier beds do not reach.

Proprietary CSEM and seismic data have been used as part of an integrated approach to test and de-risk the identified prospectivity.

Re-investigating an old prospect

In 2003, the PL 128 license acquired data that indicated a CSEM response related to the Linerle prospect. Well 6608/11-4 was drilled in 2004 and proved an oil leg in the Tilje formation. This was argued as a clear indication that CSEM technology was able to de-risk prospectivity in the area. The CSEM line also showed an elevated response to the northeast of Linerle corresponding to the Valkyrie prospect. Encouraged by these results, a further CSEM survey consisting of several lines was acquired in this area to test the Valkyrie prospect.

This image illustrates the CSEM (seabed logging) survey acquired over the Valkyrie prospect and the elevated response over this prospect. To the left is a plot showing normalized data (top) and the inversion results (bottom) along the Lo5n line (marked with a yellow line in the map). Well 6608/11-5 was drilled after the Statoil publication, but its location has been marked with a red line in the cross sections by the authors. In the map, both the Linerle and the Valkyrie wells are marked as blue-filled circles (all lines and well locations have been added in this study).

Well 6608/11-5 drilled the Valkyrie in 2006. Only very small amounts of hydrocarbons were discovered, and it was classified as dry by the operator. The well contains insufficient hydrocarbons to explain the observed CSEM anomaly, and the Valkyrie case was taken as evidence that the CSEM tool was not applicable or at least challenging to apply in this area. Several of the lines showed an elevated response over the prospect, but clear variations were observed along the CSEM lines. A model was made where a highly anisotropic layer was introduced to explain the results.

In the current study, the published data were compared with seismic and well data from the area. This comparison suggests that the Valkyrie well was drilled southwest of the main CSEM anomaly. This observation applies for both the basic “normalized” data and for the inversion results. Since the published information is limited and the raw data were unavailable, it was too early to conclude whether the observed response represents a major hydrocarbon-filled target. These initial observations, however, were sufficient to encourage further study. Special studies were carried out for both the Linerle discovery and the Valkyrie prospect to assess whether the observed CSEM anomaly over Valkyrie could be explained by electrical anisotropy or whether the well simply missed the CSEM anomaly. These studies show that it is highly unlikely that anisotropy can cause the observed anomaly. However, a synthetic model with a resistive hydrocarbon-filled target located just northeast of the well – at the target level in the older Triassic Grey Beds – gave a good match to the real data.

At the Linerle prospect, a hydrocarbon-filled target is needed to match the observed response in the real data, although the expected response at the well location likely is below detection level (the CSEM line is slightly off the location of the well and covers areas that might have a different hydrocarbon column thickness compared to the well location). The study also showed that a match between synthetic models and real data can be achieved without including anisotropy at all, whereas significant electrical anisotropy is more likely to create a significant mismatch.

The inversion results (2005, upper left – well location and rotated normalized plot have been added in this study) are consistent with the revised mapping and new CSEM inversion results from the Phoenix and Amazon prospects (lower left). The electromagnetic results from different CSEM surveys are consistent with geological mapping of the area (right figure).

The results from these studies led to a process that eventually resulted in Rocksource being awarded PL 559 together with VNG Norge and Ska-gen44. Subsequently, the license holders acquired a new CSEM line that tested both the extension of the Linerle discovery to the east (named the Phoenix prospect) and a new prospect (Amazon) to the north and east of the Valkyrie prospect. The new CSEM data clearly show anomalies that match the lateral extent of the Phoenix and Amazon prospects. The observed anomalies across the Phoenix and Amazon prospects are consistent with the modeling of a significant hydrocarbon-filled target. The license owners have decided to drill a well to test the Phoenix prospect and obtain information about the Amazon stratigraphy in 2011. The results from this well will be invaluable for understanding the observed responses in the CSEM data.

Based on the significant scientific interest related to CSEM data, contractor EMGS has established a consortium called EDDA. The purpose of the EDDA project is to evaluate and accelerate the use of 3-D CSEM data in E&P. As a part of the consortium, a large 3-D CSEM dataset recently was acquired over this region. The good-quality data should provide insight into the geological complexity of the area. Together with a well on the Phoenix/Amazon prospect, the data are expected to lead to an increased understanding of the potential of the CSEM technology.