Hydrocarbon prospectivity, salt tectonics offshore Angola

Multiple structures, plays present in deepwater

Michael Keaveny,
Western Geophysical

Generalized cross-section showing extentional and compressional tectonics offshore Angola. [25,881 bytes]
In the deep water, the top and base of salt appear to converge in a featheredge. [34,138 bytes]
Interpreted thrust planes creating an accretionary wedge of salt within possible sediment rafts. [38,786 bytes]
A series of the eastward dipping reflectors terminate against a low-frequency reflector. [23,307 bytes]

In recent years, speculative seismic surveys have been separated into two main categories: (1) 3-D surveys that have particular objectives and are in or adjacent to mature exploration areas, or (2) Huge frontier areas of under-explored acreage, particularly in deep water.

In many of these latter instances, multi-client seismic data provide the first high-resolution look with which the exploration community is able to start exploring a particular area. One such survey is Western Geophysical's multi-client dataset for offshore Angola.

Western Geophysical has a large database of multi-client seismic data for offshore West Africa. Working in a joint venture with the Angolan state oil company, Sonangol, Western has coverage in Angola from water depths of 200 meters to over 3,500 meters. This coverage provides an opportunity to examine complex structural relationships over traverses 200-300 km long. These include relationships between:

Deep structure
Salt movement and varying degrees of sedimentation
The role of volcanic activity in sediment distribution.

Figure 1 shows Western Geophysical's multi-client data coverage for offshore Angola. Grid density is typically 5 km by 5 km in shallow-water areas, and in deepwater blocks 31 km to 34 km. In other deepwater and ultra-deepwater areas, density of coverage decreases to a minimum of 40 km by 40 km.

Structural description

A pattern of tectonic change is manifest from north to south offshore Angola. Extensional tectonics occur eastward in the upper margins of the basin while compressional tectonics occur in the lower basin to the west (Figure 2).

Shoreward, salt tectonism is reflected in swells, stocks, walls, and piercement diapirs. Complete salt withdrawal and touchdowns (where postsalt sediments and presalt sediments lie horizontally adjacent to each other) are common. Also present are whole ranges of structural and stratigraphic features associated with dynamic salt movement. In these areas, sediment thickness between salt features often reaches two to three seconds signal travel time (TWT), with a similar amount of rift section below the base-of-salt reflector.

Oceanward, salt-induced structures become more complex due to a more pronounced compressional environment induced by a basinward sediment movement. This results in:

Small, detached bodies of salt near the ocean bottom
Highly faulted domed structures above incipient diapirs
Extensive salt canopies.

These canopies often evidence further detachment, generating salt tongue structures. The structures are often uniformly asymmetrical, trending oceanward. As salt migration becomes more extreme, overlying sediments thin, and the top surfaces of the canopies come close to the ocean floor.

Further into deep waters, the top and base salt appear to converge in a featheredge, until they finally meet and salt structure is no longer expressed seismically. A number of interesting phenomena are observed in this region.

Where the salt pinches out, there is a sharp break in the slope, with a completely different style of sediment either side of this break (Figure 3). The break in slope is analogous to the Sigsbee Escarpment in the Gulf of Mexico.

Oceanward of the break, there is no salt activity and sediments lie in flat uniform layers with little or no disruption and no tectonic structures. Here, good evidence exists for the presence of channels, bodies of reworked sediment, and erosive contacts, indicating clastic sedimentary systems.

Landward of the break, we see a familiar pattern of mobile salt overlain by sediments that have been disrupted by the salt activity as the salt thickens to the east. Generally, the upper surface of the salt has an undulating relief, often with bathymetrically expressed, tight to open symmetrical folds in the overlying sediments. At the seafloor, these undulations have amplitude of several tens of meters and wavelengths of 2-5 km.

In Figure 4, an undulating top surface of the salt can be seen, as can folds in the overlying sediments and water-bottom. Within the salt tongue, a series of dipping reflectors are clearly resolved. These dip landward and appear to sole out against a basal reflector.

Conversely, where they reach the top surface of the tongue, they often appear to pass through and interrupt the overlying sediments, possibly giving rise to the undulating top surface of the salt body and to the associated folding within the sedimentary cover. These have been interpreted by some to be thrust planes caused by compression, creating an accretionary wedge of salt within possible sediment rafts.

Below the salt tongue are yet more intriguing structural elements (Figure 5). Here, a series of the eastward dipping reflectors terminate against a low-frequency reflector. This reflector lies flat and correlates eastward with an unequivocal base-of-salt reflector.

Westwards, this low-frequency reflector bifurcates into a west-dipping event and an event that appears to be the foot of an escarpment, which rises rapidly westwards and then flattens. It then terminates in the west where a fault marks the edge of the salt basin (Sigsbee Scarp equivalent).

Deeper in the section, at over 8 seconds TWT, we find two families of crosscutting reflectors that respectively show west and east dip. Seismic coverage in this area is too sparse to be definitive, but this area is believed to mark the junction between the oceanic and the continental crust.

Hydrocarbon habitat

The implications for hydrocarbon habitats from the previous observations re significant. Abundant salt-induced structures accommodate many trap types, from rollovers adjacent to salt structures and against normal faults within the intervening sediments, to enormous drape closures above remnant salt bodies and active salt diapirs.

Stratigraphic plays may also exist within vast prograding slope deposits and turbidite channels and fans. Following recent major discoveries in the deepwater salt basin in the north of Angola there is mounting evidence that conditions exist for a world-class hydrocarbon basin.

The Western Geophysical program of speculative data projects is continuing in West Africa in general and very actively in Angola. Western is engaged in surveys that will explore more fully the oceanward extent of the hydrocarbon potential within the Lower Congo, Kwanza, and Namibe Basins, including an extensive new survey being shot offshore west of Blocks 31 to 34.

It is hoped that these data will extend knowledge of the basin extent, halokinesis, and oceanic crust relationships gained from the existing data. This, in turn, will help define new models for active petroleum systems and corroborate existing play fairway models.