B24 BERKINE BASIN OILS, ALGERIA GEOCHEMICAL CORRELATION BY GC AND SPECTROSCOPIC FTIR AND SUVF

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1 1 B24 BERKINE BASIN OILS, ALGERIA GEOCHEMICAL CORRELATION BY GC AND SPECTROSCOPIC FTIR AND SUVF A. PERMANYER 1, J.NAVARRO 2, O. ABBAS 3, C. RÉBUFA 3 and J. KISTER 3 1 Dt. Geoquímica, Petrologia i Prospecció Geològica, Universitat de Barcelona,. Barcelona, Spain 2 Cepsa E.P., S.A., c/ Ribera del Loira, Madrid, Spain 3 Université d Aix-Marseille III, CNRS UMR 6171, Marseille, France Abstract FTIR and SUVF helped to define complementary parameters to show differences between oils of the Berkine Basin in Algeria. These differences are probably linked to different oil source rocks, oil evolution and reservoir compartmentalization. Key words: Oil, Berkine Basin, Algeria, Reservoir Geochemistry, Source rock, Fourier Transform Infra Red Spectroscopy, Synchronous Ultra violet Fluorescence, Gas Chromatography. Introduction Earlier studies have demonstrated that the application of spectroscopic techniques such as Fourier Transformed Infra Red (FTIR) and Synchronous Ultra Violet Fluorescence (SUVF) integrated with gas chromatography (GC) may better assess reservoir compartmentalization (Permanyer et al., 2002a, b). Recent studies indicate that FTIR and SUVF could also successfully discriminate between oils of different origins and establish correlations with source rocks (Permanyer et al., 2004a, b). This study shows the results of the application of such techniques in the geochemical assessment of oils from the Berkine Basin in eastern Algeria. To this end, seven oil samples were analyzed. The oils share a number of common features, particularly related to high maturity indicated by high API gravity (44-45ºAPI) and biological marker depletion which prevent their use for correlation. Oils were obtained from two reservoir units, the TAGI (Trias Argilo-Gréseux Inférieur) and Carboniferous sandstones in two distinct fields, Rhourde El Krouf (RKF) and Ourhoud (ORD), located approximately 15 km. apart and separated by a structural low (Fig.1). The TAGI reservoir consists of stacked fluvial sandstones resting unconformably over Carboniferous rocks. Regional data suggests TAGI oils are sourced either from Frasnian and/or Silurian black shales. The Carboniferous reservoir can be described as a multi-layer system of shallow marine sandstone beds interbedded with siltyshale intervals containing thin organic-rich shales and coal beds. An intra-carboniferous source is postulated as the most likely source for the Carboniferous oils (Navarro et al, 2004).

2 2 Figure 1.- Schematic cross section through the Ourhoud (ORD) and Rhourde El Krouf (RKF) fields illustrating the oil samples location. Oils were obtained from two reservoir units: TAGI (Trias Argilo- Gréseux Inférieur) and Carboniferous (CARB) sandstones. Methodology High-resolution gas chromatography was carried out on whole oils. Chromatograms were compared and 22 minor inter-paraffin peak ratios from nc7 to nc13 alkanes were selected for correlation. The peak height ratios were plotted on star diagrams to establish similarities or dissimilarities according to the classic GC fingerprint method for reservoir continuity studies (Kaufman et al., 1990; Hwang et al., 1994; Hwang & Baskin, 1994). The samples were also analysed by FTIR and SUVF. The FTIR preparation and data acquisition were performed five times for each sample. Assignments of the main IR bands and indexes are determined according to previous studies (Guiliano et al., 1990; Pieri et al., 1996). The indexes were used to determine and compare the chemical composition of each sample. A deconvolution technique was applied to increase spectral resolution of overlapping infrared bands (Doumenq et al., 1991) Fluorescence intensity is related to the quantity of aromatic compounds present in the sample. The analysis of UV fluorescence spectra of standard polycyclic aromatic hydrocarbons, which are present in crude oils, allows us to define three main regions A1, A2 and A3. Each of these spectral regions is characterized by the number of condensed aromatic rings, yielding qualitative information on the nature of the aromatic species present in oils (spectral region from 280 to 580 nm) (Kister et al., 1996). The fluorescence index (A2/A1 ratio) represents the ratio of the aromatic compounds with 3 or 4 rings with respect to the compounds with 2 rings. The A3 represents aromatic condensation with 5 or more aromatic rings. GC fingerprints Results and Discussion Three distinct oil groups are identified when peak ratios are plotted in a star diagram (Fig. 2). The two Carboniferous oils from the RKF field compose the first group. A second group is made up of the four TAGI oils from the RFK field. A third group consists of one single oil sample from the TAGI reservoir in the ORD field.

3 CARB.-RKF CARB.-RKF 14 Figure 2.- Combined star diagrams of 22 selected picks ratios determined by GC whole oil. The diagram clearly indicates three oil groups:, and CARB-RKF. FTIR and SUVF data Various FTIR and SUVF parameters can be used for oil correlation purposes. In this study we use Aromaticity, Condensation, Substitution 1 and Substitution 2 parameters from FTIR and A3, A2 and A1 parameters from SUVF. Different cross-plots with these parameters were used for oil grouping. The A2/A1 Fluorescence versus Substitution 1 FTIR index (Fig. 3, A) diagram allows to differentiate two oil groups depending on the reservoir: TAGI and Carboniferous. Comparison of A2/A1 Fluorescence and Substitution 2 FTIR indexes indicates three oil families (Fig. 3, B): TAGI- RKF, and CARB-RKF oil. Relationships between Aromaticity, Substitution 1, Substitution 2, Condensation FTIR indexes and A3/A1 Fluorescence index (Fig. 3 C, D, E and F) allow identifying three oil groups. However, these relationships also highlight two subgroups within the TAGI oils from the RKF field. The two sub-groups were not distinguished by GC fingerprints, where they appear as a unique family of oils (Fig. 2). 10

4 4 A2 / A1 Fluorescence Index A3 / A1 Fluorescence Index A3 / A1 Fluorescence Index 0,60 0,55 0,50 0,45 0,40 0,35 A 0, ,054 0,044 0,034 0,024 0,014 C Substitution 1 Index 0,004 0,052 0,042 0,032 0,022 0,012 E 0,002 0,50 0,60 0,70 0,80 0,90 1,00 1,10 1,20 1,30 1,40 1,50 Condensation FTIR Index Figure 3.- Cross plots of FTIR and SUVF parameters. The cross-plots B, C, D, E and F clearly allow identifying three oil groups:, and CARB-RKF. Two oil sub-groups can be differentiated within the oil group in the cross-plots C, D, E and F. Conclusions The results provided by FTIR and SUVF analyses helped to refine the oil characterization by GC fingerprinting. Both techniques clearly show three distinctly different oil groups:, and CARB-RKF. However, FTIR and SUVF allowed identifying two oil subgroups within the oils, which was not evidenced by GC fingerprints. The differences observed in the TAGI oils from the RKF field suggest reservoir compartmentalization. This would require further geochemistry analysis and integration with reservoir engineering and production data for confirmation. A2 / A1 Fluorescence Index Condensation FTIR Index Substitution 2 FTIR Index 0,60 0,55 0,50 0,45 0,40 0,35 B 0, ,50 1,40 1,30 1,20 1,10 1,00 0, Substitution 2 FTIR Index D 0,80 F 22 Differences between TAGI oils from the Ourhoud and RKF fields were emphasized by both GC and spectroscopic techniques. These differences could be related to oils derived from different source rocks (Frasnian, Silurian or a mixture of both) or due to different post-emplacement histories. The Carboniferous oils form an oil group which probably was sourced by an intra-carboniferous source rock (Navarro et al, 2004), a third Paleozoic source rock present in the Berkine Basin (Miles & Sanger, 2003). This work demonstrates that FTIR and SUVF techniques can be very helpful for differentiating oils, even in high mature oils and/or with poor biomarker content.

5 5 Acknowledgements Authors are grateful to the Sonatrach-Cepsa Association for providing samples and geological data, and for authorizing publication of the results. This work was financed by the DURSI of the Catalonian Government (ACI2002/2; 2001SGR00075 "Grup de Geologia Sedimentària") and Project BTE , from the Spanish Ministry of Science and Technology. References Doumenq, P., Guiliano M., Mille G., Kister J., Approche méthodologique directe et continue du processus d oxydation des bitumes par spectroscopie infrarouge à Transformée de Fourier. Analytica Chimica Acta, 242, Guiliano M., Doumenq P., Mille G., Davin E., Kister J., 1990, Les Systèmes Chimiques Complexes: approche IRTF. Science Technique Technologie, 15: Hwang, R.J., Ahmed, A.S., Moldowan, J.M Oil composition variation and reservoir continuity: Unity Field, Sudan. Org.Geochem. 21: Hwang, R. J. & Baskin D. K. (1994): Reservoir connectivity and oil homogeneity in a large-scale reservoir. Middle East Petroleum Geoscience Geo94, 2: Kaufman R.L., Ahmed A.S., Elsinger R. L Gas Chromatography as a development and production tool for fingerprinting oils from individual reservoirs: applications in the Gulf of Mexico. GCS-SEPM GCS-SEPM Foundation 9th Annual Research Conf. Procc., (D.Shumaker and B.F.Perkins, eds), Austin, 1990; p. Kister J., Pieri N., Alvarez R., Diez M. A., Pis J.J Effects of Preheating and Oxidation on Two Bituminous Coals Assessed by Synchronous UV Fluorescence and FTIR Spectroscopy. Energy & Fuels, 10, Miles, J.A. & Sanger, B.R New geochemical insights on Palaeozoic Source Rocks, Berkine Basin, North Africa. Abstract AAPG Inter. Conf., September 21-24, 2003, Barcelona,Spain. Navarro, J., Batrina, T., Ferroukhi, I., Rossi, C., Ruiz, J.L. and Malagon. J., 2004, A new insight to the hydrocarbon potencial of the RKF field (Berkine Basin, Algeria), Sgp5, Cinquième Séminaire de Géologie Pétrolière, Janvier 2004, Boumerdes, Algérie. Permanyer, A., Douifi, L., Lahcini, A., Lamontagne, J., Kister, K., 2002a. FTIR and SUVF spectroscopy applied to reservoir compartmentalization: a comparative study with gas chromatography fingerprints results. Fuel, 81(7), Permanyer, A., Douifi, L., Alberdi, M., Rébufa, C., Kister, J. 2002b. Reservoir compartmentalization and oil maturity determined by FTIR. 8 th Latin-American Congress on Organic Geochemistry, p Permanyer, A., Douifi, Dupuy, N., Lahcini, A., Kister, J. 2004a. FTIR and SUVF spectroscopy as an alternative method in reservoir studies. Application to Western Mediterranean oils. Fuel, XX:1-10. Permanyer, A., Azevedo, D.A., Rébufa, C., Kister, J., Gonçalves, F.T.T. 2004b. Application of Spectroscopic Techniques to Geochemical Characterization and Correlation of Brazilian Oils. 9 th Latin-American Congress on Organic Geochemistry. Extended abstract, 4p. December 6-9, 2004, Mérida, Yucatán, México. Pieri N., Planche J.P., Kister J Caractérisation structurale des bitumes routiers par IRTF et Fluorescence UV en mode excitation émission synchrones. Analysis, 24: