GEOCHEMISTRY OF SEEP AND PRODUCED OIL SAMPLES FROM SOUTHERN CAMEROON

C. M. Agyingi^a, P. A. Abanda^b, K. F. Fozao^a, O. A. Njoh^a, A. F. Epanty^a N. C. Tangko^a, F. N. Kwangkam^a, and E.Eseme^c,*

^a Petroleum Research Group, Department of Geology, University of Buea, PO Box 63, Buea, Cameroon.

^b Houston Community College, Houston, TX 77002, USA.

^c Activation Laboratories Ltd, Ancaster, ON, L9G 4V5, Canada.

^* Corresponding author, email: sm_mmnl@yahoo.com

Crude oil samples from surface seeps in the Douala Basin (southern Cameroon) and from producing fields in the nearby Rio del Rey and Kribi-Campo sub-basins were analysed for bulk and molecular geochemical parameters by inductively coupled plasma – mass spectrometry (ICP-MS), gas chromatography – mass spectrometry (GC-MS) and isotope ratio mass spectrometry (IRMS). The aims of the study were to assess the composition of the oils, to evaluate the relationship between the seep oils and the oils from producing fields, and to highlight the significance of the data for oil exploration in the region. Chromatograms of the saturate fractions of the oils exhibit biodegradation ranging from very light (PM1 on the scale of Peters and Moldowan, 1993) in oil from the offshore Lokele field in the Rio del Rey sub-basin, to severe (PM 6+) for seep oils from the Douala Basin. A plot of Pr/n-C₁₇ (1.3– 5.0) versus Ph/n-C₁₈ (0.8–2.6) for the samples further supports mild biodegradation in some samples (Lokele, Kole, Ebome), and demonstrates that the oils from the Lokele and Kole fields (Rio Del Rey sub-basin) and from Ebome field (Kribi-Campo sub-basin) originated from mixed organic matter with a dominant marine contribution. The Pr/Ph ratio (1.8–2.3) for the Lokele, Kole and Ebome oil samples, and the V/(V+Ni) ratios (< 0.5) for the seep oils (Douala Basin) and the oils from the Lokele, Kole and Ebome fields, indicate derivation from source rocks deposited in oxic – dysoxic environments. The CPI (1.0–1.1) demonstrates that the Lokele and Ebome oils originated from mature source rocks, with the ratios of C₃₁ 22S/(S+R) (0.57 to 0.63) and C₃₀-βαH/C₃₀-αβH (0.18–0.23) for the Lokele, Kole and Moudi samples indicating early oil window maturity. Both V/(V+Ni) ratios (0.06–0.22) and δ¹³C (-26.96 to -24.89 ‰) were used for correlation of the oils, with the seep oils from the Douala Basin showing the closest relationship to the oil from the Lokele field. The presence of mature Type II / III source rocks in different basins in southern Cameroon suggests significant potential for oil exploration in the region.

Key words: Douala Basin, Cameroon, source rocks, oil seep, petroleum geochemistry, biodegradation.

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