AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Is a conjugate an analogue?

Abstract

The recent exploration history of the South Atlantic is one of conjugate chasing. The emergence of a play on one side of the Atlantic sees the industry trying to unravel its potential in the conjugate margin. This are the cases for the Early Cretaceous pre-salt carbonate and the Late Cretaceous turbidite plays of the South and Equatorial Atlantic, which have resulted in mixed technical and commercial successes. This study analyses 175 exploration wells drilled in the last 12 years on both sides of the Atlantic to highlight the geological factors controlling the asymmetric exploration performance and lessons that can be learnt when trying to replicate success across the rift. The discovery of the giant Lula field in the Santos Basin (Brazil) in 2006 and follow-on discoveries in the Santos and Campos basins triggered a wave of exploration in the Kwanza Basin (Angola) between 2012 and 2016, chasing the same Early Cretaceous pre-salt carbonate play. The lower Commercial Success Rate (CSR) of 27% for the play on the West African margin, compared to a CSR of 40% in Brazil, is explained by the higher gas content discovered in Angola. Although the plays on both sides of the Atlantic share similarities in the reservoir, seal and source rock stratigraphy, the difference in the thermal history of the opposite margins has had a direct impact on the commerciality of the play. After the discovery of the Jubilee field in the Tano Basin (Ghana) in 2007 the focus turned on the Late Cretaceous turbidite play with 84 exploration wells drilled on both sides of the Equatorial Transform Margin between 2007 and 2017. The Jubilee play concept triggered the discovery of the Liza oil discovery in 2015 in the Suriname-Guyana Basin, part of the Central Atlantic Margin. With a Technical Success Rate of 80% and a CSR of 48%, the Tano Basin is the only basin to have delivered commercial oil in the deepwater Equatorial Transform Margin so far. The topography of the Tano Basin dictated by its position relative to the fracture zone allows for connected turbidite sand systems to pond and create a hydrocarbon migration focal point. Well failure analysis reveals that reservoir quality and charge history are play elements that are not easily replicable. The dataset shows that a conjugate is rarely a true analogue. The number of components of a petroleum system that can be considered analogues on both sides of a conjugate margin diminishes as the rifting and drifting phases evolve in time and space. The early phases of rifting to thermal subsidence have the source rock deposition and early thermal history in common, coupled with reservoir deposition in the case of carbonates. As the drifting and uplift of the margins occur, local structural architecture plays a bigger part in the reservoir distribution, late thermal evolution and charge history of the opposite margins. Large scale plate tectonic reconstruction is necessary to identify play elements that are generated during the rift and thermal subsidence phases and that can be de-risked from a conjugate margin. Local scale basin evolution during the drift phase reveals play elements that are more unique to a basin. Major discoveries tempt explorers into extrapolating a play from one flank of a rift into its conjugate. It has brought asymmetrical success mirroring the asymmetry of the rift itself.