In 2014 Petroleum Geo-Services (PGS) began a large, integrated 2-D geosciences project with the aim of increasing the geological understanding of the Angolan and Namibian offshore margins (red lines, Figure 1). The project employed an integrated approach using regional 2-D interpretation, gravity and magnetics, and quantitative seismic interpretation to achieve this aim.

Seismic acquisition took place offshore Angola in 2011 and offshore Namibia in 2013. Both datasets were acquired using PGS’ GeoStreamer dual-sensor towed streamer acquisition platform, and the 2013 survey also used PGS’ proprietary GeoSource technology. The true broadband nature of this type of acquisition system allows a superior seismic image, providing broader frequencies across the amplitude spectrum making it ideally suited for reservoir characterization/quantitative interpretation and geological interpretation.

While shooting the seismic data, PGS also acquired gravity and magnetics data along the same lines. As such, the PGS Access project combines regional 2-D geological interpretation, quantitative interpretation, and gravity and magnetics. It also ties to important pre- and post-salt wells and discoveries.

FIGURE 1. PGS Access West Africa project lines are shown in red. Lines in magenta are the additional 2-D dataset that runs across Angola and Namibia. (Source: PGS)

Geology

Both basins were formed during the Late Jurassic to Early Cretaceous due to the rift and subsequent drift and separation of the South American and African plates. The major stage of rifting occurred from the early Jurassic to early Cretaceous (Neocomian) from south to north. The basins that formed during this rifting phase are stratigraphically divided up into pre-rift, syn-rift, sag-phase and post-rift successions.

The main reservoir targets offshore Angola are in syn-rift sandstones, sag-phase lacustrine carbonates (the focus for exploration over the past five years) and post-rift shallow marine carbonates and channel and turbidite systems. In Namibia, targets are typically in post-rift channel sandstones and shelf carbonates but also are chasing the pre-rift basin potential. However, in terms of exploratory wells drilled, Namibia remains far more immature than Angola and, as a consequence, less is understood about its petroleum system.

FIGURE 2. This example shows an interpreted PGS Access West Africa zoomed profile over the Cameia Field. The image to the bottom right shows the whole interpreted PGS Access West Africa profile line. (Source: PGS)

Regional interpretation

The first stage of the project was to complete a large-scale 2-D seismic interpretation. Total line coverage used covers almost 8,000 km (4,970 miles), so having the best regional interpretation to carry forward into quantitative interpretation is crucial for gaining accurate insight. The lines cover a variety of basins, from the Kwanza Basin in the north of Angola to the Orange Basin in the south of Namibia, each with both large-scale and subtle differences in structure and stratigraphy (Figure 2).

The 2-D grid chosen for the dataset deliberately covers a number of presalt discoveries, including Baleia/Orca, Cameia, Azul and Denden. By interpreting the seismic data across these discoveries, a better regional geological understanding has been gained and a number of viable presalt leads have been developed through the Kwanza, Benguela and Namibe basins.

FIGURE 3. The density model is overlain on the seismic, with the synthetic gravity response (red dashed line) matching the recorded data (black line, recorded Bouguer gravity response). This interpretation shows a clear syn-rift basin (fuschia dashed box) on the east of the seismic line with salt and a thick post-rift sedimentary sequence overlying. This syn-rift basin is a highly prospective area in the Namibe Basin. (Source: PGS)

Gravity and magnetics

The gravity and magnetics interpretation project was completed in conjunction with the regional 2-D interpretation. A two-stage approach was taken. The first stage entailed making a regional qualitative interpretation using gridded gravity and magnetics data, drawing from both PGS’ acquired 2-D and public domain data sources. The second stage entailed 2-D gravity modeling, whereby an iterative process was used to modify 2-D density models until the synthetic gravity response matched the high-resolution gravity data recorded by PGS during acquisition (Figure 3).

Densities used in the 2-D modeling were constrained using a combination of density logs derived from well data and literature available in the public domain. Horizons were taken from the 2-D interpretation to help constrain the initial stages of the model build and were modified until a good fit was made between observed and modeled data. This process meant that various competing seismic interpretations could be tested.

The gravity and magnetics interpretation gave insight into the nature of the deeper structure, the basement and syn-rift graben structure and the volcanics across Angola and Namibia, which otherwise in just 2-D seismic interpretation may have been inconclusive.

FIGURE 4. This figure shows a) the interpreted Baleia-1 logs, b) the well-to-seismic tie at the Baleia-1 location based on full-stack seismic data, and c) the interpreted induced polarization (Ip) inverted from Baleia-1 logs and Ip-relative from seismic data. (Source: PGS)

Rock physics and seismic inversion

The Baleia-1 well (offshore Angola) was drilled in 1996, encountering a 91.4-m (300-ft) presalt oil column in a dolomitic reservoir. In-place hydrocarbon volumes are estimated to exceed 1 Bbbl of oil. To better understand the presalt dolomitic limestone reservoir, compressional- and shear-wave velocities, density and gamma ray logs from the Baleia-1 well were used for rock physics analysis.

Two carbonate intervals were discriminated. Carbonate 1 is an upper interval composed primarily of dolomitic limestone, and the lower interval is predominantly argillaceous limestone, referred to as Carbonate 2 (Figure 4a). The acoustic impedance volume (Figure 4c) shows a clear distinction between Carbonate 1 and 2, caused by a sharp decrease in velocity at the interface between the two intervals.

A very good well-to-seismic tie at Baleia-1 is observed both on the seismic (Figure 4b) and on the relative inversion version—acoustic impedance in this case (Figure 4c)—leading to a greater confidence and understanding of the presalt reservoir and its distribution.

Quantitative interpretation work performed on the PGS Access West Africa data in the Kwanza Basin is indicating a similar relative acoustic impedance response to that seen in the Santos Basin. This may indicate that reservoir-quality carbonates and dolomites deposited within the sag phase have similar elastic properties on both sides of the mid-Atlantic conjugate margin system.

By using an integrated reservoir approach, geoscientists at PGS have been able to gain insight into both the regional- and reservoir-scale geology in the West African offshore margins. The PGS Access West Africa project demonstrates the value of this integrated approach for extracting maximum information from a multitude of geophysical measurements. The additional insight gained from this product will lead to better decision-making as to where to best invest time and resources along the Angolan and Namibian margin and ultimately increase the likelihood of exploration success.

Acknowledgment

The author would like to thank Sonangol, NAMCOR and PGS for permission to publish this work.

References available.

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