South Gabon Shallow Water Pre-Salt Play Creaming Curve Revival

Abstract

Successful exploration for oil in the South Gabon pre-salt play has focussed on the Early Cretaceous Gamba sandstone play. The shallow water discovery creaming curve in this play had begun to plateau due to the limitations of inadequate imaging of available seismic. However, recent developments in 3D seismic acquisition and processing have revealed the intra syn-rift play, never before deliberatly targetted;. A review of legacy data and historical operations records showed the velocity heterogeniity challenge in the post-salt Albian “Madiela” carbonate. Integration of a survey design study, efficient operations management, the use of long offset streamers and application of source and reciever de-ghosting with an interpretation-driven pre-stack depth migration, have resulted in an unprecedented image in the prospective pre- and post-salt sections. Fluvial-lacustrine deltaic sand sequences (the Dentale and Kissenda Formations) and two main anoxic lacustrine source rocks (Kissenda and Melania Formations) were depositied during the syn-rift. Upon break-up, the first marine transgression eroded the upper part of this section, creating a low relief monocline upon which the transgressive shoreface Gamba sandstone was deposited. As the Madiela platform carbonate developed, it loaded the Ezanga salt, quickly creating a heterogeneous post-salt fabric leading to complexities in imaging the pre-salt syn-rift sequence.These have been addressed using modern multiple elimination strategies and careful velocity building control on pre-stack depth migration. For the first time the syn-rift is imaged, revealing both Dentale targets and pre-Dentale structures with no expression at Gamba level. The lower syn-rift play has seen recent exploration success offshore Congo. To create a new creaming curve of discovery in South Gabon, requires new technology, new plays and/or fresh access to acreage. In South Gabon we now have 3D seismic revealing new intra- syn-rift tilted fault block structures, heterogeneous Gamba distribution, undrilled Madiela structures with karstification textures and platform margin geometries, as well as undrilled post-salt traps and an understanding of reservoir disctribution using advanced seismic attribute analysis. All this in open acreage and coincident with news of a significant liquid discovery in the pre-salt adjacent to the 3D area. It’s time to re-appraise this exciting oil prone basin.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018