New Insights into Petroleum Systems and Plays inAngola-Congo-Gabon from PSDM Sub-Salt Imaging by Steve Henry - Innovative Exploration Services, Al Danforth - Consultant, C. Willacy & S. VenketramanA paper presented at the HGS International Explorationists Dinner on February 21, 2005SUMMARY:In this paper we present the preliminary results from a new regional survey of over 13,000 km of 2D pre-stack depth migrated (PSDM) seismic data that has been acquired by GX Technology over the West African salt basins of Angola, Congo and Gabon in 2004 (CongoSpan). This data is providing the first consistent acquisition and processing from the shallow water (50 m), past the limit of the salt, and into the ultra-deep water (4,000+ m) (Fig 1). The new profiles provide multiple transects crossing the continental margin, from rifted continental crust, across the crustal transition zone and on to the interpreted volcanic terrain associated with break-up of the South Atlantic.The advantage of these depth images is that they provide improved reflector geometry. These improved images have been used in identifying a consistent set of tectono-stratigraphic units, and relating post-salt extensional and compressional structures to underlying basement structure. These structural and stratigraphic insights are generating new ideas on the petroleum systems and will be useful for developing new plays. SEISMIC ACQUISITION – PROCESSING:The seismic acquisition parameters follow the concept of a “Span”, with long record length, a long cable, and a large source. The goals of a Span are to provide long (250-2500 km) regional profiles that are positioned geologically to improve imaging beneath the salt, cross post-salt, pre-salt, and basement structural elements and to follow sediment distribution pathways. The positioning of these 2D lines used the known orientation of the major pre and post-salt features to minimize out of the plane effects. The dip lines are spaced on an average of about 50 km, with strike lines selected to pass through sediment depocenters and avoid the poor signal areas associated with salt canopies. In map view, the lines are gently curved to achieve these geologic goals and to tie key wells.Seismic processing utilized propriety routines for converting the wavelet to zero phase, attenuating multiples and noise, and various migration algorithms designed to preserve both steep dips and the range of frequencies needed for stratigraphic interpretation. Building the velocity model is extremely important and strongly contributes to the final quality of the PSDM data. The process is iterative and begins with picking sediment velocities, then applying a “salt flood” based on interpreted horizons for top and then base salt, and finally applying sediment velocities beneath the salt. Interaction between interpreters and seismic processors has provided a very useful quality control check on the velocity model, ensuring that the final model satisfied both the seismic imaging requirements and geologic constraints.INTERPRETATIONThe primary advantage from this PSDM data is the ability to minimize the structural distortions due to lateral velocity variations and improve imaging of reflectors beneath the salt. This data is revealing the syn-rift basement structure, sequence boundaries within the pre-salt, and enabling more realistic estimates of the post tectonic and even the original depositional thickness of salt. The quality of this data is excellent, with faults and sequence boundaries well resolved in both the post and pre-salt. Interpretation of this data is providing support for some published West African salt basin models and generating new ideas that will undoubtedly drive future exploration in the next decade.This PSDM data is supporting and helping to further refine a tectono-stratigraphic model (Fig. 2) that was developed over the past ten years using nonexclusive 2D seismic acquired primarily in Angola. The main elements of this model are: 1) a westward migration of the rifting axis, 2) thermal subsidence in the abandoned rifts with the development of a sag basin prior to the continental separation, and 3) the development of a massive subaerial volcanic terrain in the successful rift axis at the time of break-up. Major tectono-stratigraphic (TS) units are associated with each of these elements, and their imaging on the PSDM data is much improved over the existing time migrated seismic data.Tectono-stratigraphic units group sediments into packages with characteristics associated with the tectonic setting in which they were deposited. The main TS sequences are: 1) Pre-rift, 2) Syn-rift, 3) Sag, and 4) Drift. Thermal energy uplifts the crust prior to (Late Pre-rift) and during Syn-rift, and the loss of this thermal energy results in subsidence, the tectonic setting for both Sag and Drift. Sag is common for intra-cratonic rifts that fail to develop a spreading ridge. In the rifting of the South Atlantic, where extension occurred over a very broad (250-300km) zone, Sag developed as the axis of rifting migrated to the location of the future spreading ridge. This westward migration is supported by maps of the locations of the Early Sag, Late Sag, and even the overlying salt depocenters, that are progressively shifted to the west (Fig. 2).Using these TS units, an interpreter can explain how one area can be actively involved in uplift and basement involved rifting (Syn-rift), while during the same geologic time an adjacent area overlying an abandoned rift is subsiding (Sag) with little or no basement-involved faulting. Initially the Sag basin is above sea level and fills with fluvial - lacustrine sediments, as subsidence continues the Sag basin may reach and even drop below sea level. The Sag basin (Fig. 1), as mapped in West Africa, is a large (200 x 1200 km) interior basin that in the Late Sag was periodically flooded with marine waters spilling over the Walvis Ridge. Restricted circulation could have produce additional rich Late Sag source rocks and periodic marine flooding provides a mechanism for the subsequent thick deposits of salt.SEISMIC RECOGNITION CRITERIA Illustrated in Figure 2 is the reflector geometry that has been used to recognize the major TS units. These units are the: 1) Pre-rift, 2) Syn-rift, 3) Sag, and 4) Drift. Pre-rift sediments are sub-parallel to basement and their reflectors are not easily identified on seismic data. It is likely that pre-rift sediments exist in this area, but they would be preserved only in the deeper parts of the Syn-rift basins. The sediments of the Syn-rift are easily identifiable from reflectors that show g