WONHAM, JONATHAN P., Elf Exploration (UK), London, UK, STANISLAS JAYR,TSurf Corporation, Houston,TX, DOMINIQUE CLAUDE, Elf Exploration and Production, Pau, France, RIGOBERT MOUGAMBA, Lille University, France; and *ENZO INSALACO, Elf Exploration UK, London, UK

Abstract: 3D Sedimentary Evolution of Turbidite Channel Reservoirs, Gulf of Guinea, West Africa

Recent developments in deep-water drilling technology have opened new domains for hydrocarbon exploration along the Atlantic passive margins. In these settings Tertiary stratigraphic traps of turbidite facies are important hydrocarbon targets. Nevertheless passive margin turbidites remain poorly understood sedimentary systems, particularly regarding the controls on channel geometry and intra-channel facies distribution. This poor understanding stems from the fact that modern deep-water clastic systems remain understudied and that outcrop analogues of passive margin turbidites are rare. In order to improve our understanding of these systems this study has focused on the detailed analysis of subsurface data from an Atlantic margin settings. In such a "subsurface analogue" approach the technical challenge is in the effective integration and interpretation of different techniques and datasets.

The study data comes from the Baudroie Marine and Baliste fields of offshore Gabon where the reservoir is the early Miocene Mandorové Formation. The system comprises a series of deep-water clastic sands and shales deposited within the Baliste-Crécerelle canyon. The reservoir body has a ribbon-like geometry and is a low net-to-gross unit up to 450 m thick. Standard seismic stratigraphic study has defined the internal architecture of the reservoir interval.

New methodologies for integrating advanced 3D seismic interpretation tools and 3D modelling have been tested.The 3D modelling tool, gOcad, was used to integrate and help generate the results derived from the study of the well and seismic data. It has been possible to use detailed seismic attribute analysis to identify numerous sedimentary bodies which were modelled in 3D. The seismic attribute analyses includes generation of maximum and mean amplitude maps, and the use of semi-automated picks of high amplitude events in small 3D seismic volumes.

The depositional elements identified from the analysis include meandering turbidite channels and more extensive sand-sheets. These sedimentary bodies are then placed into their true 3D position within the canyon through a carefully mapped stratigraphic framework. Using this approach over 150 channel and sand-sheet bodies have been identified and picked from the dataset. This 3D picking of the bodies has allowed changes in channel morphology to be considered in the stratigraphic context of unconformity surface development and aggradational channel filling.

Parameters such as the sinuosity, width and meander radius of turbidite channels form essential components of stochastic reservoir modelling techniques which are tested on these reservoirs. Our results allow better quantification of hydrocarbon volumetrics and reservoir connectivity, thereby improving risk assessment in this type of field development. In addition, these studies help to address fundamental questions about the origin and evolution of meandering turbidite channels systems and can document important temporal channel dynamics at fine spatial and temporal resolution.

The technical achievements of this 3D geological modelling are in effective integration of.datasets, improved interpretation of sedimentary evolution, quantification of channel parameters, development of a coherent framework for property distribution, and in high quality visualization. Future developments in such studies should attempt to refine the methodology, resolve intra-channel architecture and develop techniques for facies population within both the intra- and inter-channel areas.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England