AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Exploration in the Deep Water Gulf of Guinea off eastern São Tomé and Príncipe Islands, a High Thermal Regime Setting.

Abstract

Historically, the petroleum industry has shown little interest in exploring for hydrocarbons in the deep waters of the Gulf of Guinea, just east of São Tomé and Príncipe islands (Figure 1). The main geological reasons for this have centred around the uncertainty in the presence of source rock facies in the area, on the premise that this whole area lies upon Oceanic Crust and, therefore, under a cold thermal regime. This, coupled with the limited overburden that would make it difficult to mature any source rock, have represented the traditional challenges. A review of the different crustal models of the region, the regional gravity and the public surface heat flow data allow us to consider that the region actually resides on a continental-transitional crustal domain. The volcanic islands of São Tomé and Príncipe form part of the Cameroon Volcanic Line (CVL), which records non-linear age related volcanism and constitutes a regional upper mantle anomaly that seems to have played an important thermal role in the whole region since its emplacement, approximately 40-30 Ma (Burke, 2001; De Plaen et al. 2014). Exploration across the São Tomé and Príncipe eastern border in Equatorial Guinea and North Gabon has proven that the Upper Cretaceous contains good reservoir facies and several levels of source rock spanning from Aptian-Albian, through Cenomanian-Turonian and up to Santonian times (Dailly, 2000; Katz et al. 2000). Deposition of the Upper Cretaceous sequence was, therefore, not affected by the CVL and source rock facies and turbiditic sandstones may well have been deposited in that passive opening margin. Proof of the deposition of good source rock facies in that distal setting are the oil seeps present in São Tomé and Príncipe islands. Recent geochemical analysis of two of these oil seeps, one from São Tomé and the other from Príncipe island, suggest that they were sourced by two different marine source rocks of Upper Cretaceous age. Their source, degree of biodegradation and maturity has been assessed using biomarker, isotope and diamondoid analyses. The Petroleum Systems modelling predicts that Upper Cretaceous source rock facies should have reached good oil expulsion maturities over a wide area east of São Tomé and Príncipe islands.