De-risking offshore exploration and license round evaluation – an example from offshore Nigeria
Acquiring, processing and interpreting exploration data in offshore areas is time-consuming and expensive. When evaluating license round acreage, finding ways of quickly and cost-effectively evaluating large areas to enable you to focus your more detailed exploration on the best licenses on offer is absolutely vital.
In this blog we focus on the identification of potential hydrocarbon reservoirs offshore Nigeria. We describe the results of a collaboration between BRADE Consulting, an innovative energy consultancy based in Lagos, and Getech, that sought to examine if gravity data – commonly used in natural resource exploration – could be used to map intra-sedimentary sand channels.
The project leveraged Getech’s industry leading Multi-Sat gravity data (see Figure 1) to map sand channels. Identifying the location and extent of such potentially highly porous reservoir units can form a key stage in an early exploration workflow and help enormously in focusing future exploration effort and investment.

Figure 1.
Gravity Provides Understanding
Gravity data has long been recognised as an extremely valuable tool for mapping sub-surface geological structure and composition over large areas.
Our Multi-Sat data is now of such high resolution and accuracy that it provides previously unachievable levels of confidence in mapping intra-sedimentary features.
If a target feature has a measurable density contrast with respect to adjacent units, it is possible to model the anticipated gravity response and correlate this to anomalies observed in the gravity data.
In our project, we used a workflow that was most applicable to offshore Nigeria. However, the principle holds for onshore areas too – assuming good quality terrestrial data is available and a target reservoir unit with a density contrast is present.
Site Identification
BRADE Consulting staff identified several target sand channel reservoirs, using seismic and well data.
These sand channels were demonstrated to have a subtle but clear negative density contrast as shown in the well density log in Figure 2 below:

Figure 2.
Typically, these sand-stacked channels had a cumulative thickness of over 200 metres and were observed to have density contrasts of approximately 0.15 to 0.2 g/cc with respect to neighbouring sediments.
We then incorporated the locations and extents of these observed channels, together with the appropriate density values, into 2D models to assess the gravity response.
As shown in Figure 3, we observed a subtle but coherent and consistent negative gravity response of the sand channels, highlighted in the 20km high pass residual of the gravity.

Figure 3.
The striking correlation between these subtle gravity lows observed in the 20km high pass residual allowed us to use the similar high frequency filtered maps over the whole project area to infer the location and extents of these sand channels, away from the primary constraint data (Figure 4).

Figure 4.
Mapping of the local structures and depth-to-basement analysis enabled us to distinguish the gravity lows most likely to be associated with these sand channels versus those associated with greater sediment thickness.
This interpretation was reinforced by a 3D inversion of the Multi-Sat gravity for sedimentary density anomaly which showed a very good correlation of low density with the interpreted channels, as well as highlighting other local sub-basins and depo-centres (Figure 5).

Figure 5.
Make Informed Decisions
This project demonstrated how subtle gravity anomalies associated with target channel sand reservoirs offshore Nigeria can be identified and mapped with good quality, self-consistent gravity data.
A workflow using gravity data can significantly aid and accelerate identification of these targets in a multitude of domains where the location of suitably porous reservoirs is typically a critical factor – including hydrocarbon exploration, carbon storage and natural hydrogen storage.
The results help cost-effectively and efficiently focus subsequent stages of an exploration campaign where mapping potential reservoirs is an essential element – targeting project work more rapidly and helping avoid wasted effort.
Such an approach is particularly suited to the analysis of a large area as part of a licensing round evaluation where quick but informed decisions are needed as to where to focus more detailed exploration efforts.
Matthew Stewart and Simon Campbell (Getech), Ese Avanoma (BRADE Consulting)
BRADE: http://www.bradeafrica.com/