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Plays Prospectivity and Exploration Direction of Pre-Salt Section in South Gabon Basin, West Africa

Abstract

The South Gabon Basin is a typical salt basin located in the middle portion of the West African passive continental margin, which has been found to be prospective in Pre-salt section especially after the discoveries of Diaman & Leopard in deep-water area. Two depressions with an uplift tectonic framework (Western Outer Depression, Central Uplift and Eastern Inner Depression) have been identified based on joint analysis of gravity, magnetic and seismic anomalies, with thicker depositions and deeper burial depth in the west while less sediments of shallower burial depth towards the east. Two sets of high-quality mature lacustrine source rock (Neocomian Kissenda in Early rift & Barremian Melania in Late rift) accompanied with multiple sets of reservoir-cap assemblage as well as a series of structural traps in rifting stage of Early Cretaceous were attributed to the tectonic-sedimentary evolution, and three plays (lower, middle, upper) can be divided in the Pre-salt sequence. An integrated analysis of each play unveiled that the Pre-salt oil and gas shows the distribution regularity of “inner oil and outer gas in plane, upper more and lower less in vertical”,which mainly controlled by the source rock maturity, structural traps and reservoirs development. Based on the analysis of geological differences and hydrocarbon accumulation controlling factors, the three tectonic units have different exploration directions: (i)the Outer Depression mainly focused on the upper play for finding the large structural gas reservoirs(Anticline & Faulted-anticline of Aptian Gamba & Upper Barremian Dentale sandstones), (ii)the Central Uplift mainly focused on the upper play for seeking the structural oil reservoirs of Dentale sandstones in the fault terrace belt (Anticline & Faulted-anticline),and(iii) the Inner Depression mainly focused on the middle-lower play for searching the structural oil reservoirs of high-quality reservoir (Faulted-block of Lower Neocomian Basal sandstone & Lower Barremian Lucina Sandstone). What is worth mentioning is that the poor seismic imaging restricts the Pre-salt exploration, so it is a crucial issue to obtain high-quality seismic data.