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

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. [ABSTRACT FROM AUTHOR]