The left column shows four scenarios, A to D. Seismic is shown as a backdrop, and the main geological layers are shown as light-colored regions, annotated with their resistivity value. The right column shows the constrained inversion results obtained by inverting synthetic MT data. The synthetic data were obtained by forward-computing the scenarios on the left and then fixing the resistivity in the region marked by “xx ?m [fixed].” The resistivity in the region between the thin red lines was left free to invert for.

Magnetotellurics (MT) makes use of electric and magnetic fields that occur naturally in the earth. By measuring these fields at surface, suitable processing and inversion techniques yield a subsurface resistivity image. This can be useful to discriminate various geological scenarios and improve identification of rock types in cases where seismic and other technologies fail to do so.

Shell conducted a feasibility study for an exploration prospect that turned out to be a seldom-occurring “textbook example” in the application of MT. The usual complicating factors appeared to be absent, and the piece of the de-risking information that MT can add was of high business value. The application of MT in the study’s particular geological setting was unique, as was the focus on the integration of various technologies (seismic, gravity/magnetics, MT) rather than relying on a single technology. As a result, Shell intends to increase screening of prospects for applicability of MT technology to see where it can add value, with a continued focus on how MT can help exploration efforts in environmentally sensitive areas.

MT is a technique used by many oil companies to map subsurface resistivity. The data that it yields is complementary to existing datasets such as seismic and gravity/magnetics and therefore can be of significant importance to complete the interpretation. Although MT is one of the lesser used means of gathering reservoir data, there are cases where it can be the technical solution of choice. The purpose of this article is not only to revive interest in this method, but also to demonstrate that real-life cases that are free from complicating factors do exist. Shell’s study shows where MT can, indeed, maximally enhance our understanding of the subsurface.

Launching the feasibility study

The prospect, which shall remain undisclosed for proprietary reasons, is located in a remote, environmentally sensitive area. It had been studied for quite some time with various techniques: 2-D seismic, gravity and magnetics. The interpretation of these data indicated that a very large structure existed at a depth of 13,124 and 14,765 ft (4,000 to 4,500 m), but the presence of potential reservoir lithologies remained unclear.

It could have been a Palaeozoic reefal carbonate reservoir flanked by basinal carbonates. Alternatively, the sediments on one side might not have been basinal carbonates but non-reservoir Permo-Triassic siliciclastic sediments and no significant carbonate body existed (scenarios are outlined below).

The high technical risk of the prospect combined with the very high drilling costs were a challenge to the commercial viability of the project. Relatively low-cost technologies such as MT offered the potential to help de-risk the prospect, thereby adding significant business value. In this context, the Shell study team requested a feasibility study to see whether the prospect at hand was suited for application of magnetotellurics.

While we knew that MT could add the missing piece of information, it was not clear in advance how successful MT would be in this particular case. Would there be complicating factors? Would the resolution and accuracy of the method be sufficient? Shell decided that a feasibility study could answer these questions and provide the confidence needed to commission a real survey.

Identifying the scenarios

A 2-D seismic line through the prospect was selected at high angles to the geological strike to ensure a 2-D geometry. Four scenarios were identified as different interpretations of this seismic line (Figure 1). How the Palaeozoic section is interpreted constitutes the differences between them:

A. Fault-dominated scenario, consisting of low-resistive Permo-Triassic clastic sediments on one side of a major vertical fault and high resistive basinal carbonates on the other side.

B. Reef carbonates scenario, consisting of medium- to high-resistivity reef carbonates at the prospect location, flanked by high-resistivity basinal carbonates.

C. Reef carbonates scenario, consisting of medium- to high-resistivity reef carbonates at the prospect location, flanked by low-resistivity basinal carbonates plus an additional salt

layer to the right of the prospect.

D. Reef carbonates scenario, consisting of medium- to high-resistivity reef carbonates at the prospect location flanked by low-resistivity basinal carbonates.

The various lithologies and their resistivities are listed in Table 1. Resistivity values were taken from logs of nearby wells or from analogs.

Scenario A would represent a non-reservoir facies, whereas the other scenarios are likely to contain good-quality hydrocarbon reservoirs.

MT inversion results

The Shell team took the following approach to see whether an MT survey would be able to distinguish the scenarios. First, for all four scenarios, a 2-D resistivity model was made and the expected station response was forward-calculated. These synthetic data were then used as “real” measured data to invert for subsurface resistivity. In this inversion process, use was made of the fact that we already know the shape of the main seismic horizons. Also, when the lithology was already known, the proper resistivity value was assigned

and kept fixed in the model. Only in those regions in the model with no information on resistivity available was the inversion routine allowed to adjust the resistivity until a good fit with the “observed” data was obtained. These “free regions” are the regions between the thin red lines in the right column of Figure 1. More specifically, for the results shown in Figure 1, that free region was subdivided into three or four compartments with no further internal structure. This approach is what is called a “constrained inversion.”

As one can see from Figure 1, the constrained inversion results almost perfectly recover the original models for all four scenarios. Not only the lateral positions of resistivity boundaries are correct, but also the values of the resistivities are recovered quite

accurately. Additional computations were done to assess the robustness of these results. For instance, a salt layer was included. Its presence and its exact resistivity value were found not to disturb the results. In addition, “unconstrained inversions” were performed on the same synthetic data, meaning that a fully flexible resistivity distribution was allowed to exist (no compartmentalization) and the results of those calculations were consistent with the results shown in Figure 1, albeit of coarser resolution.

Note that in a case where no reef carbonates exist, but with a stronger basement high, the

results would be indistinguishable (in the case of identical high resistivity). Confidence in the seismic interpretation ruled out this possibility and thus contributed to the value that MT was able to add here.

Assessing the benefits

This case study is situated in a region where costs for drilling and seismic acquisition are extremely high. Existing seismic is of sufficient quality to map the main horizons and overburden but insufficient to map the internal structure and/or identify lithology very well. The wider region is known to be very prolific, and large nearby oil fields do exist.

The technical feasibility and very low environmental impact of MT surveys (it is a passive technique that uses natural sources and lightweight, wireless equipment) made MT an ideal candidate for further de-risking of this particular prospect. A simple MT survey consisting of a few 2-D lines can be performed at a fraction of the cost of drilling or acquiring additional seismic, has a very small environmental footprint, and allows one to map the subsurface resistivity, which is a good indicator of lithology.