Unlocking Prospectivity on the Kwanza Shelf

Angola

New 3D GeoStreamer acquisition combined with advanced processing techniques unveil key elements for a successful petroleum system in both the pre- and postsalt sections on the Kwanza Shelf

Location: Kwanza Shelf, Angola
Survey size: 8 304 sq. km
Survey year: 2019 and 2020
Survey type: 3D GeoStreamer
Streamer configuration: 12 streamers x 112.5 m x 8 025 m
Source: Triple

In partnership with Agência Nacional de Petróleo, Gás e Biocombustíveis (ANPG), PGS has acquired a new multisensor GeoStreamer survey over the Kwanza Shelf (orange polygon). This data provides enhanced subsurface illumination through broadband acquisition and the use of modern processing workflows tailored to the unique imaging challenges of shallow water and salt presence. This broadband 3D multiclient dataset will equip explorers with the tools to identify and unlock the pre- and post-salt plays in this underexplored area of the Angolan offshore. Upcoming license round areas are marked with a yellow outline.

Challenge

The historical absence of high-quality seismic data over the Kwanza Shelf has presented a significant challenge to recent hydrocarbon exploration efforts. With proven plays encountered in the syn-rift, sag phase, and post-rift sections, clearer imaging of presalt stratigraphy and structure is needed to unlock prospectivity. Complex geometries associated with the overlying Loeme Salt Formation require detailed velocity model building to create an accurate presalt image.

Solution

Data: Broadband 3D GeoStreamer
Processing: Deblending, full 3D demultiple and 4D regularization
Imaging: Kirchhoff, RTM and SWIM
Velocity model building: Reflection and refraction tomography, PGS FWI and RTM for salt model building
Q modeling: Variable Q model using tomographic and scanning method

A combination of reflection and refraction tomography has been used in conjunction with FWI to generate a detailed velocity model which gives enhanced imaging of complex salt bodies and presalt reflections. A variety of migration algorithms including SWIM, RTM and KPSDM have been utilized to produce multiple volumes which contribute to improved interpretation in a geologically complex region.

SWIM Delivers High Resolution Shallow Imaging

Final KPSDM full-stack versus the merged final SWIM & KPSDM full-stack. Merging the SWIM and the final KPSDM volume utilizes the best of both imaging algorithms. SWIM improves the near-surface image leading to greater resolution of the near-surface geology by resolving shallow channels and removing the acquisition footprint.

Salt and Presalt Better Imaged with RTM

Final KPSDM full-stack volume versus the final RTM stack (migrated to a maximum frequency of 40 Hz). The RTM gives greater continuity along the top and base salt reflectors (blue arrows). The presalt also shows an uplift in imaging, particularly where the salt structures above are more complex (yellow arrows).

Results

Tertiary sandstones deposited in turbidite channel systems offer excellent potential reservoirs (26-30% porosity and >400md permeabilities) and are analogous to those established in the prolific Lower Congo Basin. High-quality 3D data allows for detailed mapping of extensive stacked channel networks with additional reservoir potential also occurring in postsalt Albian carbonates. Quantitative Interpretation has identified AVO anomalies associated with hydrocarbon presence in these deposits. Hydrocarbons generated from source rocks in the presalt section have the potential to migrate into the post-rift via salt welds. Reservoirs can also be charged from postsalt sources through faults created by halokenisis, or by simple up-dip migration through carrier beds.

QI Reveals a Working Petroleum System

The salt, highlighted in pink, has welded out and post-rift mini basin touch-down has occurred onto the top of the sag phase presenting a potential migration route for hydrocarbons (orange arrow). Faulting, shown in black, can be observed to act as a conduit for hydrocarbons to migrate from the presalt, into the postsalt. P-Impedance anomalies can be seen in the post-rift suggesting that there is potential for these fluids to be trapped in high-quality Tertiary sandstones and postsalt Albian carbonate reservoirs.

CaseStudy-Kwanza-Shelf-ResAnimation