Li Li, Quan Li, Tao Cheng, Songling Yang, Yong Rao, Xinyu Liu, Wenjing Ding. Geochemical characteristics and origins of natural gases in the Eastern Cote d’Ivoire Basin, West Africa[J]. Acta Oceanologica Sinica.
Citation:
Li Li, Quan Li, Tao Cheng, Songling Yang, Yong Rao, Xinyu Liu, Wenjing Ding. Geochemical characteristics and origins of natural gases in the Eastern Cote d’Ivoire Basin, West Africa[J]. Acta Oceanologica Sinica.
Li Li, Quan Li, Tao Cheng, Songling Yang, Yong Rao, Xinyu Liu, Wenjing Ding. Geochemical characteristics and origins of natural gases in the Eastern Cote d’Ivoire Basin, West Africa[J]. Acta Oceanologica Sinica.
Citation:
Li Li, Quan Li, Tao Cheng, Songling Yang, Yong Rao, Xinyu Liu, Wenjing Ding. Geochemical characteristics and origins of natural gases in the Eastern Cote d’Ivoire Basin, West Africa[J]. Acta Oceanologica Sinica.
Macquarie University, Sydney, NSW, 2109, Australia
3.
CNOOC Research Institute, Beijing 100028, China
Funds:
The Major Science and Technology Project of CNOOC under contract No. KJGG2022-0902 and the National Natural Science Foundation of China under contracts Nos 42202184 and 42272177.
The gas sources in the Eastern Cote d’Ivoire Basin (Tano Basin) are seldom reported and remain controversial due to multiple sets of potential source rocks and poorly documented geochemical characteristics of natural gases. The marine source rock potential from the Upper Albian to Turonian as well as the molecular composition and the stable carbon isotope composition of natural gases in the Eastern Cote d’Ivoire Basin were studied in detail to investigate the origins of natural gases. The total organic carbon (TOC), hydrogen index (HI), and generation potential (S1 + S2) of source rocks indicate that both sapropelic source rocks and humic source rocks developed during the Late Albian, whereas sapropelic source rocks developed during the Cenomanian and the Turonian. The normal order of δ13CH4< δ13C2H6< δ13C3H8 (δ13C1< δ13C2< δ13C3), the relationship between C2/C3 molar ratio and δ13C2 - δ13C3, and the plot of δ13C1 versus C1/(C2+C3) collectively show that the natural gases are thermogenic due to the primary cracking of kerogen, including the typical oil-associated gases from Well D-1, the mixed oil-associated gases and coal-derived gases from Well G-1 and Well L-1. Based on the plot of δ13C1 versus δ13C2 and the established relationship between δ13C1 and equivalent vitrinite reflectance (Ro), we proposed that the natural gases are in a mature stage (Ro generally varies from 1.0% to 1.3%). Combined with results of basin modelling and oil-to-source correlation, the transitional to marine source rocks during the Late Albian were thought to have made a great contribution to the natural gases. Our study will make a better understanding on petroleum system in the Eastern Cote d’Ivoire Basin.
Figure 1. (a) Simplified structural map of the Cote d’Ivoire Basin and distributions of the three sampled wells in the basin. (b) Stratigraphic table of the major Cretaceous strata in the Cote d’Ivoire Basin (modified from Morrison et al., 2000).
Figure 2. (a) Plot of hydrogen index (HI) versus maximum hydrocarbon generation (Tmax) showing kerogen types (refer to Espitalié et al., 1984). (b) Plot of total organic carbon (TOC) versus generated hydrocarbons (S1 + S2) showing source rock quality. HI = S2/total organic carbon × 100.
Figure 3. The stable carbon isotopes distribution of gaseous alkanes (methane CH4, ethane C2H6, and propane C3H8) in the Eastern Cote d’Ivoire Basin.
Figure 4. The origin of natural gas interpretative diagram based on δ13C-CH4, δ13C-C2H6, and δ13C-C3H8 data in the Eastern Cote d’Ivoire Basin (according to Dai et al., 2014).
Figure 5. The plot of ethane/propane (C2/C3) molar ratio versus δ13C-C2H6−δ13C-C3H8 (according to Prinzhofer and Battani, 2003) showing the natural gases of the Eastern Cote d’Ivoire Basin are dominated by primary cracking gases.
Figure 6. Genetic diagram of δ13C-CH4 versus C1/(C2+C3) showing the genetic type of the natural gases in the Eastern Cote d’Ivoire Basin. The boundary lines are from Milkov and Etiope (2018).
Figure 7. The plot of δ13C-CH4 and C1/(C2+C3) distinguishing the genetic types of the natural gases in the Eastern Cote d’Ivoire Basin. (A. biogenic gas; B. biogenic gas and sub-biogenic gas; C. sub-biogenic gas; D. oil-type gas; E: oil-type cracked gas; F: oil-type cracked gas and coal-derived gas; G condensate oil-associated gas and coal-derived gas; H: coal-derived gas; I: inorganic gas; J: inorganic gas and coal-derived gas (plot after Dai, 1992).
Figure 8. The types and thermal maturity of natural gas classification diagram based on δ13C-CH4 and δ13C-C2H6 data in the Eastern Cote d’Ivoire Basin the boundary lines are based on (Dai, 1992; Dai et al., 2005; and Li et al., 2022).
Figure 9. (a) Down-hole variation of source rock quality (total organic carbon (TOC), generation potential (S1 + S2), and hydrogen index (HI)) from Turonian to Upper Albain in the Eastern Cote d’Ivoire Basin. (b) Present temperature and maturity fields of the Eastern Cote d’Ivoire Basin (modified from Rüpke et al., 2010).
Figure 10. Mass chromatograms (m/z 191 and m/z 217) of the Upper Albian source rock and crude oil from Well D-1 (data from preliminary report of Geochemical Solutions International, Inc.).
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