Geochemical characteristics and origins of natural gases in the eastern Cote d’Ivoire Basin, West Africa

• Published in: Acta oceanologica Sinica Vol. 43; no. 8; pp. 26 - 36
• Main Authors: Li, Li, Li, Quan, Cheng, Tao, Yang, Songling, Rao, Yong, Liu, Xinyu, Ding, Wenjing
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.08.2024; Springer Nature B.V; CNOOC International Ltd.,Beijing 100028,China; School of Natural Sciences,Macquarie University,Sydney,NSW,2109,Australia%CNOOC International Ltd.,Beijing 100028,China%School of Natural Sciences,Macquarie University,Sydney,NSW,2109,Australia; CNOOC Research Institute,Beijing 100028,China
• Subjects: Carbon; Carbon isotopes; Chemical composition; Climatology; Coal derived gases; Earth and Environmental Science; Earth Sciences; Ecology; Engineering Fluid Dynamics; Environmental Chemistry; Gases; Geochemistry; Hydrocarbons; Isotope composition; Isotopes; Kerogen; Marine & Freshwater Sciences; Natural gas; Natural gas reserves; Oceanography; Oil; Organic carbon; Origins; Petroleum; Reflectance; Rock; Rocks; Sapropels; Sediments; Total organic carbon; West Africa; Tano Basin; stable carbon isotopes; marine source rocks potential; molecular composition; Upper Albian
• DOI: 10.1007/s13131-024-2335-6
• ISSN: 0253-505X

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 (S 1 + S 2 ) 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 δ 13 CH 4 < δ 13 C 2 H 6 < δ 13 C 3 H 8 ( δ 13 C 1 < δ 13 C 2 < δ 13 C 3 ), the relationship between C 2 /C 3 molar ratio and δ 13 C 2 - δ 13 C 3 , and the plot of δ 13 C 1 versus C 1 /(C 2 +C 3 ) 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 δ 13 C 1 versus δ 13 C 2 and the established relationship between δ 13 C 1 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.