Heat-flow and Source Rock Maturity Modelling in the Distal Domain of the West African Margin

Abstract

Organic-rich intervals (ORIs) are extensively documented in the distal domain of passive continental margins, often associated with oceanic anoxic events. However, it is the general perception of the industry that potential source rocks overlying old oceanic crust, or ocean-continent transitional basement, lack the necessary thermal stress and/or sediment burial to mature and generate hydrocarbons. For this study we selected eight DSDP/ODP sites at key locations along the West African Margin, between NW Morocco and South Africa, to model the maturity of the regionally relevant ORIs. The models test a variety of plausible tectonic/thermal model scenarios, based on the available constraints on the structure of the underlying lithosphere, kinematic models and heat flow data. The work presented here is part of the “Eastern Atlantic (West African Margin) Oceanic Crust Project”, where we are also reviewing the evidence for the occurrence of ORIs on African plate oceanic/transitional crust based on a re-evaluation of biostratigraphic and geochemistry data. The 1-D modelling experiments predict that the majority of the DSDP/ODP drilled ORIs along the distal West Africa Margin are immature. The exceptions are arguably the Aptian, oil- and gas-prone source rock horizons (likely also Barremian) offshore Namibia and South Africa where, in agreement with the available maturity data, early- to mid-oil maturity may occur in less than 1500 m sediment burial, as a result of pre- and syn-break up voluminous magmatism and/or sub-lithosphere mantle dynamics. To the north of the Walvis Ridge the modelling results show that for a normal oceanic lithosphere setup, i.e. cooling towards steady-state following break-up, the maturity of the Jurassic-Cretaceous ORIs takes place for burial depths greater than 3000 m. Alternative thermal model scenarios, which include variations in the nature/thickness of the crust and lithosphere, magmatism and hot-spot activity, basin exhumation and by-pass of mid-ocean ridge segments across transform faults, suggest, however, that in many areas a sediment burial of approximately 2000 m may suffice for the maturation of regionally important ORIs to occur. The models also highlight the potential risk of early source rock maturity and hydrocarbon generation, possibly before trapping/sealing conditions have developed. Global sediment thickness compilations (e.g. CRUST-1; Laske et al., 2013) show vast areas with > 2000 m sediment cover in oceanic and ocean-continent transition domains off West Africa, which may thus encompass active petroleum systems. In the follow-up to the work discussed here we will focus in areas where we have better constraints on the sediment coverage, from new seismic and potential field data, and use 2-D and 3-D modelling techniques to help de-risk these large-scale petroleum systems.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018