West Africa Source Rock and Oil Study

West Africa Exploration
West African exploration efforts have been one of the most dynamic offshore plays in the world. A total of 48 new discoveries have been made over the past 2 years has fueled this play. These discoveries have not only been in known petroleum provinces such as Nigeria, Gabon, Congo, and Angola but also in totally new locales and in deepwater such as Equatorial Guinea and Cote d'Ivorie. Of 55 deepwater wildcat wells, 11 new discoveries have commercial oil prospects of over 45 million barrels at a cost of $0.50 per barrel ¹.

Numerous geochemical studies have been completed in various regions and countries. This study is meant to complement and extend these studies and enhance the capability to evaluate prospects based on hydrocarbon charge. Hydrocarbon charge includes an evaluation of the timing of hydrocarbon generation and also whether gas or oil is likely to be found. Recent work at Humble has shown that bulk kinetic data grossly underestimates the timing of earliest hydrocarbon generation compared to compositional kinetic assessments (Figure 1) ( et al., 1999).

Obviously, if bulk kinetic data is overestimating the temperature regimes at which oil and gas are generated, Tertiary source rocks would be predicted to not generate oil and gas. On the other hand, compositional kinetic data shows that these Tertiary source rocks can generate oil given their maximum thermal exposure. This is consistent with recent correlations that have been made between Tertiary source rocks and oils in West Africa (unpublished data).

In fact the question remains, for the most part, are Tertiary source rocks effective in these basins? This question will be answered but the availability of recently drilled Tertiary source rocks in a couple basins.

Several key developments necessitate the need for additional work to understand the these petroleum systems:

• heretofore unavailable source rocks in Nigeria, Equatorial Guinea and elsewhere have been drilled and recovered
• techniques for assessing the oil and gas (compositional) yields and kinetics on rock and from oil samples have been developed by Humble
• techniques for screening larger numbers of oil samples using very high resolution gas chromatography allows assessment of oil types and generation expulsion temperatures

Objectives of Study
This study has direct exploration objectives. These are to high-grade prospects by providing a detailed assessment of oil types, i.e., source rock or organic facies variations, alteration effects, mixing, and other oil alteration effects, This will be accomplished by providing:

1. assessment of available source rock characteristics to assess whether they are effective source rocks
2. determination of thermal maturity profiles from well profiles
3. typing of oils and correlate oils and available source rock samples
4. assess the rates and yields of decomposition of source rocks into hydrocarbons:
1. bulk organic matter decomposition rates (oil/gas formation rates)
2. compositional (dry and wet gas, light and normal oil) decomposition rates
3. compositional yields of oil and gas at various levels of maturity
5. assess the yield of nonhydrocarbon gases at various levels of thermal maturity
6. assess the rate of source rock decomposition directly from oil samples both when proven effective source rocks at an immature stage are available and when they are not available using oil samples

Regional Scope of Study
This study will focus on identified source rocks and oils from the following countries:

• Angola
• Cabinda
• Congo
• Equatorial Guinea
• Gabon
• Ivory Coast
• Morocco
• Nigeria

Samples
Each participant must contribute rock and oil samples to the study. Rock samples may include complete well samples, e.g., cuttings, on which geochemical profiles can be generated, whether source rocks are present or not. In the absence of source rock material, maturity profiles can be generated for optimization of basin models. Oil samples are needed to characterize and confirm source rocks or to infer source rock characteristics if a positive correlation is not made (Figure 2, 3 and 4). Oil samples from West Africa are preferred, but other oils may be contributed in exchange.

In addition participants must be members of Humble's Worldwide Geochemical Library™ (WGL) from which rock and oil samples are being used in this study. Membership requires contribution of 15-30 source rock samples, complete wells (for maturity and conversion profiles), or oil samples. Sample contributions require accurate and complete descriptions for inclusion in the Library

Rock Samples
Various source rock samples as well as complete well profiles are to be included in the study. Source rock samples have both vertical and lateral variations so samples duplicating available samples are desired.

Source rock and rock sample availability is summarized

Oil Samples
Oil samples will be included both for typing and correlation purposes as well as for comparison to kerogen compositional kinetic yields and for oil cracking analysis
Oil samples are solicited and related production information is requested including gas-to-oil (GOR) data.

Oil availability and needs are summarized

Analytical and Interpretive Program
The primary focus of this study is providing data useful in high-grading exploration targets. New technology allows the determination of compositional kinetic and yield assessments of immature source rocks that have also become available from these regions. There are two phases to the study: (1) acquisition and interpretation of standard geochemical data for assessing source rocks, maturity, and oil types as well as oil-to-oil and oil-to-source rock correlations; and (2) kinetic analysis of immature petroleum source rocks and oils (either due to the unavailability of immature sources or for oil decomposition analysis). These two aspects of the study are described as:

Detailed petroleum systems analysis

1. Rock samples from the Humble WGL, submitted well samples, or data
1. Assessment of source potential
1. Screening analysis such as TOC, Rock-Eval, Pyrolysis/GC
2. Detailed analysis including extraction, GC fingerprinting, biomarker analysis
2. Assessment of thermal maturity and visual kerogen content
2. Oil samples from the Humble WGL, submitted samples, or data (if useful):
1. Oil typing using very high resolution light hydrocarbon and whole oil fingerprinting
2. Assessment of oil types by biomarker analysis
3. Prediction of oil generation temperatures from calibrated light hydrocarbon data
3. Assessment of the rates of kerogen decomposition into oil and gas
1. bulk rates of organic matter decomposition into hydrocarbons
2. compositional (oil and gas) generation rates and yields
4. Basin modeling of studied wells and selected seismic shot points

Availability
TOC, Rock-Eval, bulk kinetic data, compositional kinetics (selected samples), and oil fingerprints have been completed on samples in the study and are available for immediate purchase.

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