How broadband pre-stack depth reprocessing, with a post-stack survey merge for the migrated OBC and streamer volumes, has enabled the development of 50-year-old gas fields

Multiclient reprocessing of recovered legacy 3D seismic datasets has re-imaged historic gas fields, providing reliable attributes for reservoir modelling and the location of appraisal and production wells. Both OBC and streamer data was reprocessed, with workflows tailored to each dataset and a velocity model derived from processed well data.

Exploration on the shallow water shelf of the Lower Congo Basin during the 1970s and 1980s discovered numerous fields con­taining oil and gas within the post-salt succession. Although several of the oil fields were developed, with oil production from Block 2 set to reach 40,000 bopd, from 17 fields, the discovered gas fields have lain undeveloped.

The largest of the gas fields are Quiluma and Garoupa, discovered in 1970 and 1981, respectively, and Maboqueiro, discovered in 1995. Despite significant gas volumes in place (Quiluma and Maboqueiro are estimated at 2,551 BCF of gas), companies were historically un­able to commercialize these resources due to regulatory limita­tions as well as challenges related to resource density and market access. Consequentially, these gas fields have lain undeveloped for over 50 years.

Why are the gas fields, discovered over 50 years ago, being developed now?

With a desire to increase the country’s hydrocarbon production headline figure, ANPG (the state body in charge of hydrocarbon supervision, regulation and pro­motional activities) has, in recent years, introduced legislative decrees to incentivise gas produc­tion. The most notable of these, in relation to gas, is Presidential De­cree No. 7/18 of May 18, 2018, which established the framework for the rights related to natural gas. This decree, which governs its exploration, production, and sale, aims to attract investment and promote the development of the natural gas sector.

An outcome of this legislative change has been the formation of the New Gas Consortium (NGC), a collaboration between Azule Energy (ENI & BP JV), Sonangol Pesquisa e Produção, TotalEnergies and Cabinda Gulf Oil Company (Chevron), covering Angola’s Blocks 1, 2 and 3 (as well as a small deeper water area named Block 15/14). This $2.4 billion project aims to share knowledge and risks, reduce costs and pool resources in such a way as to develop and produce the non-as­sociated gas in a profitable way.

The first gas field to be developed within this consortium will be Quilu­ma, followed by Maboqueiro and Garoupa, with considerable potential to appraise and develop smaller, neighbouring gas fields in the future.

Seismic image limitations associated with returning to the historic gas fields

With the NGC formed, and large historic gas fields identified for ap­praisal and development, a notable challenge has been the availability and image quality of the vintage seismic data. Oil companies ac­quired some 19 3D seismic surveys over the blocks in the period from 1989 to 1998 (with a following phase of 3D surveys focused only on oil field development). Consequently, the discovered gas fields that are now to be developed by the NGC are imaged by 35-year-old 3D seismic data, tempering confidence in res­ervoir models and drilling decisions.

To overcome these data limitations, a multiclient data collection and broad­band depth reprocessing project was undertaken on the OBC and Towed Streamer seismic data. Locating the field data and navigation data for these surveys took several years, with visits to oil company and regulator archives, a close collaboration between oil companies and ANPG was required to retrieve and catalogue the data. With so many mergers and acquisitions between interested oil companies over the 35 years, this process involved a fascinating delve into oil companies’ historical data archives, together with the handling of some very dusty boxes of data tapes!

Broadband reprocessing to re-image the gas fields

With the original seismic data finally retrieved, all surveys were able to be reprocessed from field tapes using a full integrity VTI pre-stack time im­aging sequence and TTI anisotropic pre-stack depth imaging sequence.

The reprocessing of the two OBC surveys, acquired in 2008, used the hydrophone and geophone re­cordings in a P-Z summation centred workflow to suppress the receiver side multiples. This was followed by source-side deghosting and source-side multiple attenuation using SRME and Shallow Water SRME. The data were 5D COV regularised to ensure azimuthal information was preserved through the migration.

Streamer data reprocessing incorporated bespoke noise atten­uation workflows for each survey, deterministic source and receiver deghosting, plus extensive 3D SW-SRME, traditional 3D SRME and 3D muted SRME multiple modelling combined with adaptive subtraction techniques. Significant coverage gaps were recovered through 4D regularisation passes, and diverse azimuth imprints were suppressed by robust acquisition footprint removal.

The velocity model building sequence, which incorporated en­vironmentally corrected, processed and interpreted well data, includ­ed several passes of anisotropic tomography update. They utilised both RMO and First Break picks from all surveys in order to update the model covering the surveys. This was followed by multi-survey diving wave FWI that was run up to 12 Hz (3 dB down). Recorded shots from all surveys together with correspond­ing source signatures were used to simultaneously update the model. Kirchhoff pre-stack depth migration was used to image the data.

The applied velocity model build­ing workflow included D-FWI, resulting in a reliable, seamless model that accurately represents the subsurface, capturing intricate details such as low-velocity gas pockets, ultra-low velocity regions, and sharp unconformity edges. Integrating this improved model with advanced data reprocessing techniques yield­ed significant enhancements in the broadband seismic image, resulting in improved structural and fault defi­nition, better focusing, and overall clearer and more reliable imaging.

Future outlook

The development of the Quiluma and Maboqueiro fields, which constitutes the first phase of the NGC project, consists of two offshore wellhead platforms, the construction of a new onshore gas processing plant at Kivinca Nvemba and also an upgrade of the existing Angola LNG plant at Soyo. With the offshore platforms now in place, first gas is expected from Quiluma in early 2026, with production from Maboqueiro expect­ed later that year.

These projects are expected to increase Angola’s production rates by 350 mmscf of gas a day, directly increasing Angola’s LNG production and the availability of domestic gas for the country’s industrial development.

The broadband reprocessed 3D seismic data has guided the place­ment of the recent appraisal well, and will also support the location of any additional production wells. Furthermore, it will provide a robust AVO response to allow rock physics models to be extrapolated in the data and will also provide a baseline sur­vey for any future 4D monitor surveys.

Despite taking 55 years to come to fruition, it seems that the country’s first non-associated gas development will help fuel the country’s growth and development into the future.