Exploration in West Africa has a rather low commercial success rate when it comes to frontier, deepwater wells, as evidenced by recent drilling campaigns. Of the nine West African frontier exploration wells drilled in 2017, only one was reported as a significant discovery. That corresponds to a disappointing success rate of 11%. The key challenge is the nature of the stratigraphic trap being chased here, where in particular seal is notoriously difficult to assess, and fluid type through quantitative interpretation using seismic data is ambiguous. This is well illustrated when viewing two of the wells drilled in 2017: one being the discovery Yakaar and the other the neighboring dry well Requin Tigre-1 (Figure 1). Both wells are tied to neighboring discoveries (Teranga and Tortue, respectively) and show large similarities in seismic definition as well as calibrated, positive amplitude- versus-offset responses. Yet, the outcome of the two wells is largely different. A similar result can be seen across the Atlantic on the South American Margin in the Guyana Basin, where the two prospects Liza and Skipjack had almost identical seismic expression and risking profile. However, the outcome of the drilling was very different— Liza came in as a discovery, while Skipjack was dry.

FIGURE 1. This map covers eight of the nine African frontier exploration wells drilled in 2017. Results were disappointing, with only one discovery (gas), five dry holes and two wells with shows. This strongly suggests that a seismic direct hydrocarbon indicator might say something about charge but does not properly distinguish between low- and high-saturation reservoirs. The two seismic lines show the discovery Yakaar-1 and the neighboring dry well Requin Tigre-1. Both wells are tied to nearby discoveries (Teranga and Tortue, respectively). (Source: EMGS)

Controlled-source electromagnetic (CSEM) technology is a marine remote geophysical method imaging the earth resistivity. The method is proven as a technology to identify oil and gas reservoirs and to separate low-saturation noncommercial reservoirs from commercial high-saturation reservoirs. Generic CSEM sensitivity studies based on public knowledge relevant for the nine West Africa wells from 2017 show that CSEM data should give significant contributions to the de-risking of these prospects. Application of CSEM for such a target would separate false positive seismic amplitude responses from true positives, in addition to reducing uncertainty in in-place hydrocarbon estimates. CSEM data acquisition in these basins in Africa should be considered to facilitate better decision-making of the next wells.

In November 2000 the first full-scale field test of the CSEM technology was carried out in Angola over the Girasol Field. The results of this survey were encouraging, bringing a new remote geophysical method in the toolbox to detect hydrocarbons offshore. Since then the development of equipment, acquisition, imaging and interpretation tools has been enormous. EMGS has acquired a significant amount of CSEM datasets in Africa, covering, among others, known discoveries as the Fortuna discovery in Equatorial Guinea and the Jubilee discovery in Ghana.

One of the Africa CSEM case stories is from the deeper parts of a delta system. Here, reservoir sands may not be deposited over structural closures such as delta toe thrusts. The distribution of channels and basin floor fans is normally controlled by other factors, such as transform faults. Traps will be of stratigraphic type, with high risk on seal. In an African CSEM campaign in 2004, several 2-D lines targeted mapped structural closures, but none of these structures could be correlated to resistive anomalies. However, along one of the lines, a strong resistive anomaly appeared toward the end. A closer look at the seismic data in the area revealed a channel/fan system that could be correlated to the resistive anomaly. Subsequently, an extension of the original line was acquired and the resistive anomaly from the extended line had a very good lateral match to the extension of the channel/fan system identified on the seismic. A well drilled some years later confirmed the presence of gas (Figure 2). Another well was drilled on the structural high where no resistive anomaly was observed. The well turned out to be dry as predicted by the CSEM results.

FIGURE 2. A strong resistive anomaly (red) is highlighting a channel system identified on seismic data located off a structural high. A well drilled some years later confirmed the presence of gas identified by the CSEM data. (Source: EMGS)

The exploration history shows that there are large uncertainties in the risking of prospects on the African shelf. It is a known fact that seismic is best suited for mapping trap and reservoir and not to determine the fluid content. It is often seen that the majority of failures are due to charge or seal, which often cannot be properly de-risked by the use of seismic data when it comes to fluid prediction. Many wells are now targeting stratigraphic fan type plays and these plays are typically characterized by high seal risk and large uncertainty on in-place volume. This risk and uncertainty are worth keeping in mind, especially in light of the recent dry Fatala-1 well in Guinea on the West African Atlantic margin, drilled on exactly such a play. What if Fatala was a “Skipjack”; could the next well be a “Liza”?

Could this be a similar case as in the Yakaar-1 and Requin Tigre-1 case, as illustrated in Figure 1?

CSEM data are sensitive to the fluid content in a reservoir separating seismic prospects with residual saturation from high hydrocarbon saturations. Therefore, CSEM data act as a complementary tool to seismic data in the prospect risking process. In-house sensitivity modeling based on public knowledge has shown that the Fatala target, as well as most of the other ones, is within the sensitivity range for the CSEM methodology (Figure 3). Therefore, wise use of CSEM can cost-efficiently improve the drilling sequence toward drilling high-volume/lower-risk prospects early on in the campaign.

FIGURE 3. A CSEM sensitivity study used a resistivity background model relevant for the Fatala-1 well. Scanning for depth versus areal size of target clearly illustrates that CSEM data will be sensitive to a target with size and reservoir depth similar to the Fatala prospect. The green area means high sensitivity to the target, while red means low to no target sensitivity. Being within the yellow transition zone will require more detailed work. (Source: EMGS)

References available. Contact Rhonda Duey at rduey@hartenergy.com for more information.

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