GEOCHEMISTRY AND ORIGIN OF UPPER CRETACEOUS OILS FROM THE TERMIT BASIN, NIGER

Bang Liu^1*, Guangya Zhang¹, Fengjun Mao¹, Jiguo Liu¹ and Mingsheng Lü²

¹ Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China.

² China National Oil and Gas Exploration and Development Corporation, PetroChina, Beijing 100034, China.

^* corresponding author: liubang@petrochina.com.cn

Crude oil samples (n = 16) from Upper Cretaceous reservoir rocks together with cuttings samples of Upper Cretaceous and Paleogene mudstone source rocks (n = 12) from wells in the Termit Basin were characterized by a variety of biomarker parameters using GC and GC-MS techniques. Organic geochemical analyses of source rock samples from the Upper Cretaceous Yogou Formation demonstrate poor to excellent hydrocarbon generation potential; the samples are characterized by Type II kerogen grading to mixed Types II-III and III kerogen. The oil samples have pristane/phytane (Pr/Ph) ratios ranging from 0.73 to 1.27, low C₂₂/C₂₁ and high C₂₄/C₂₃ tricyclic terpane ratios, and values of the gammacerane index (gammacerane/C₃₀hopane) of 0.29-0.49, suggesting derivation from carbonate-poor source rocks deposited under suboxic to anoxic and moderate to high salinity conditions. Relatively high C₂₉ sterane concentrations with C₂₉/C₂₇ sterane ratios ranging from 2.18-3.93 and low values of the regular steranes/17α(H)-hopanes ratio suggest that the oils were mainly derived from kerogen dominated by terrigenous higher plant material. Both aromatic maturity parameters (MPI-1, MPI-2 and R_c) and C₂₉ sterane parameters (20S/(20S+20R) and ββ/(αα + ββ)) suggest that the oils are early-mature to mature. Oil-to-oil correlations suggest that the Upper Cretaceous oils belongs to the same genetic family. Parameters including the Pr/Ph ratio, gammacerane index and C₂₆/C₂₅ tricyclic terpanes, and similar positions on a sterane ternary plot, suggest that the Upper Cretaceous oils originated from Upper Cretaceous source rocks rather than from Paleogene source rocks. The Yogou Formation can therefore be considered as an effective source rock.

Key words: Termit Basin, Niger, Upper Cretaceous, oil, biomarkers, oil-source correlation, source rocks.

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