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Salt Tectonics and its Controls on the Prospectivity of Miocene Sands, Offshore Angola

Borsato, Ron¹; McDonald, Kevin¹; Mathew, Frances¹; Morse, Steve¹; Joaquim, Lourenco M.²; Jose, Jose G.²; Pedrodeoliveira, Arante M.²
¹MultiClient Reservoir, Petroleum GeoServices, Weybridge, United Kingdom.
²Sonangol, Luanda, Angola.

Introduction
Petroleum Geo-Services in conjunction with Sonangol have merged over 60,000 km2 of 3D seismic data offshore Angola. The post-salt deep-water petroleum system contains abundant source and reservoir potential, the latter in the form of stacked channel complexes and turbidites. This presentation will show some results of RMS amplitude extraction windows within prospective areas, and relate examples of how Miocene channel structures have been influenced by structural salt evolution.

Geological Setting
Asymmetrical rift grabens formed along the South Atlantic margin of West Africa between the Upper Jurassic and Lower Cretaceous. During the Aptian, the Loeme salts were deposited during a period of marine restriction. Significant post-rift deformation has been attributed to salt movement, with salt structures influencing the overlying geology and subsequently the regional petroleum system. Salt diapirs have altered the trajectory of these sand fairways and salt walls have restricted depositional extent. Salt movement through time has also created trapping mechanisms for hydrocarbons within these reservoir sands.

Method
Salt tectonics have created seafloor folding and faulting which has resulted in significant temporal and special changes in channel geometry from the early Oligocene to late Miocene. The 3D seismic data indicates a variety of growth fault related basins, turtle structures and mini basins on salt canopies where sediments can pond and stack (Mayall et al., 2010). A variety of geological interpretation software will be used to find examples of abrupt increases in channel width and thickness in the vicinity of salt as discussed by Gee and Gawthorpe (2006). The movement of salt diapirs is also recorded in the lateral migration of the channels as channel avulsion causes facies to migrate around and away from salt domes. Accommodation space is often created in slope depressions as salt structures grow, resulting in sedimentary packages which may thicken towards fault related salt movement.

Conclusions
Angola is another example of where Miocene channels have been influenced by salt movement and diapirs during the Miocene. It is very important to understand the basin geometry on a regional scale. The use of time structure, isochron or amplitude extractions are not new, but using these tools in a more regional basin strategy will provide regional prospectivity. Understanding the regional geologic model will help to reduce risk.

 

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012