Geochemistry of Seep and Produced Oil Samples From Southern Cameroon

Abstract

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.

Neocomian – Barremian Lacustrine Shales in the Mayo Oulo‐lere Basin, North Cameroon: Depositional Environment and Petroleum Potential

“…Similar Early Cretaceous depositional environments have been interpreted in the Benue Trough (Babangida et al, 2015;Sarki Yandoka et al, 2017); in the Doba, Bongor, Termit and Muglad Basins (locations in Fig. 1; Dou et al, 2018aDou et al, , 2020Liu et al, 2015); in the Kribi/Campo, Douala and Rio-Del-Rey Basins of southern coastal Cameroon (Ntamak-Nida et al, 2008;Agyingi et al, 2019); in the North Gabon margin (N'Komi Basin: Mbina et al, 2009); and in the northern part of the Sergipe -Alagoas Basin along the conjugate margin of Brazil (Mello and Katz, 2000). The results therefore suggest that organic matter accumulated during the Neocomian -Barremian in the Mayo Oulo-Léré Basin in a lacustrine environment with variable depositional conditions.…”

“…The main oil and gas production in Cameroon is from the coastal and offshore Douala/Kribi-Campo and Rio del Rey Basins which are relatively well known (e.g. Nguene et al, 1992;Pauken, 1992;Loule et al, 1997;Brownfield and Charpentier, 2006;Ntamak-Nida et al, 2008;Mfayakouo et al, 2014;Agyingi et al, 2019). By contrast and with the exception of the Logone Birni and Mamfe Basins, onshore basins in Cameroon have been poorly studied.…”

Inorganic Geochemistry of Crude Oils of Northern Eurasia after ICP-MS Data as Clear Evidence for Their Deep Origin

“…The study of inorganic geochemistry of crude oils by the ICP-MS method should be continued, especially bearing in mind the difficulties arisen, associated with very low contents and (in some cases) insufficiently good reproducibility of single analyses. Similar ICP-Ms studies were carried out by many scientific groups with different purposes-some of them aimed to improve the methodology [36][37][38][39][40][41] and others tried to assess the geochemical specialization of crudes [42][43][44][45]. It is always important to consider the analytical error; in our case this error is ±8-10% and every point in our accompanying figures is in reality a field that also increases the error probability.…”

Geochemical Analyses of Seep Oils in the Southern Gafsa Basin, Sw Tunisia: Reassessment of Regional Hydrocarbon Potential

“…The MS was run in selective ion monitoring (SIM) mode to determine the following ions: alkanes (m/z 85), terpanes (m/z 191), and steranes (m/z 217, m/z 218 and m/z 259) (e.g. Agyingi et al, 2019). Analyses took place in the ETAP laboratories.…”