The West African Margin

Defining the timing and magnitude of
paleo-thermal events controlling the
maturation history and distribution of reservoir sands

The following provides a brief summary of the original detailed proposal distributed to interested parties in July 2000, prior to the commencement of Geotrack's West African study. With sufficient support, the study commenced in October 2000, and the final of three volumes was completed in September 2001. The success of the study is a testament to the unique and valuable information available from AFTA on this margin.

The final study differs somewhat from the original proposal. Notably, more than 250 AFTA analyses were performed, significantly greater than originally proposed. Two successful field excursions were undertaken exclusively for this study. The study is heavily data intensive allowing a firm platform from which new concepts and new models for the tectonics, uplift, maturation and sediment-supply histories can be evaluated and developed. Furthermore, the study contains an organic petrography report, with valuable discussion about widespread problems in this region when using conventional techniques to evaluate maturity. The study fundamentally relies on Thermal History Reconstruction (THR) which is an approach developed by Geotrack International to better understand basin development and sediment supply.

The following summary of the original proposal outlines the main objectives of the study. For the original detailed proposal, or more detail on the final results and scope of the current study, we suggest you contact us for this information.

Defining the timing and magnitude of
paleo-thermal events controlling the
maturation history and distribution of reservoir sands

Executive Summary

Despite significant exploration success in the last five years along the West African margin, the margin remains under-explored and basic questions about the petroleum systems remain unanswered. Key questions relate to the unroofing (or uplift) history and the corresponding thermal history of the margin, in relation to its effect on hydrocarbon maturation history, and sources and distribution of reservoirs. This study is designed to answer those key questions on a broad regional basis by providing a comprehensive thermal history framework and exhumation history for the margin from Gabon to Namibia. Samples from wells in the offshore region, and from both wells and outcrops onshore will be included in this vast regional study. Please refer to the attached map and “Work program and sampling details” for more information.

Generalised location map showing the wells and outcrop locations considered for inclusion in this study. The well and outcrop localities are discussed in more detail in the full proposal in the “Work program and sample details” section in the proposal. The technical basis of the proposed work is squarely focussed on understanding the thermal history of the margin on a regional basis, and using these firm constraints to better understand not only thermally driven processes such as maturation and generation, but also the source and distribution of key reservoir horizons. The study necessarily requires a broad regional approach to appropriately utilise the thermal history techniques and approach, and to construct a well-constrained framework from which future exploration targets can be assessed. The regional aspect of this study, which spans 3500 km of the West African margin, is critical to its value. Basins and plays extend across national boundaries; thus a study restricted to a single country will necessarily fail to observe key relationships.

Key to the project is the use of AFTA^® (Apatite Fission Track Analysis) combined with VR (vitrinite reflectance) data. AFTA allows direct measurement of the timing and magnitude of thermal episodes responsible for maturity development and hydrocarbon generation, while VR data also provide control on the maximum paleotemperature. These direct measurements of timing and magnitude of past thermal events not only provide vital control on the maturation and generation history but additionally provide quantitative constraints on the amount of section removed during exhumation. Knowing these variables through direct measurement necessarily reduces exploration risk. The study will also provide critical data for sequence stratigraphic analysis of the Upper Cretaceous and Tertiary section of the margin. A better understanding of the configuration of the basin throughout its history will allow an improved assessment of likely reservoirs and migration pathways.

This project is designed to impact hydrocarbon exploration in two ways:1) To identify the magnitude and timing of Late Cretaceous and Tertiary exhumation of the West African margin, and using this new information to understand Tertiary sand distribution, including deepwater reservoirs and seals, and 2) To constrain the amount of uplift and erosion, and using this new information directly assess maturation levels and timing of hydrocarbon generation in relation to structural development in Lower and Upper Cretaceous source rocks. The regional aspect of this study, which spans 3500 km of the West African margin, is critical to its value. Basins and plays extend across national boundaries; thus a study restricted to a single country will necessarily fail to observe key relationships. Large-scale controls on the distribution of hydrocarbons can only be identified in comprehensive projects such as this.

The work program is extensive and includes thermal history reconstruction in at least 20 wells and over 70 separate outcrop locations in Angola, Congo Republic, D.R. Congo (Zaire), Gabon and Namibia. In total, more than 130 AFTA samples and 150 VR new samples will be collected and analysed for this study. Extensive fieldwork and sample collection will be carried out as part of this study. Valuable support from government bodies and industry is now available to immediately launch a field party. Time is presently available to invite companies to make suggestions of specific localities or wells to include in the study.

Field work commenced in November 2000 and continues to the present. Preliminary results will be available in the first quarter of 2001. For immediate subscribers the cost of the study is $US85,000 and the final report will be completed in July 2001. Details on the investigators, their experience in the region and contact details can be provided on request in a detailed proposal for the study.