Fig. 1. Variations of Pritane-to-nC17 alkane (Pr/nC17) (a) and Phytane-to-nC18 (Ph/nC18) (b) with burial depth of crude oils in the Termit Basin.
Systematic geochemical analysis of 39 crude oils from the Termit Basin, Niger, all belonging to the same oil family, reveals distinct differences in bulk properties and molecular compositions, which were undoubtedly caused by differences in the extent of biodegradation. Various degrees of microbial alteration were identified in crude oils, which were classified as non-, slightly, moderately, or heavily degraded oils, according to their physical properties, bulk chemical compositions, and certain molecular biomarker characteristics. Examination of the possible origins and geological backgrounds of the biodegraded oils shows that reservoirs with deeper burial depths (greater than 1800 m), thicker caprocks (>100 m), relatively higher stratigraphic temperatures (>80 ◦C), and which have been subject to the influence of silent faults had not suffered significant biodegradation. The burial depth of reservoirs is therefore considered to be the main factor controlling the biodegradation process, and may offset other factors if the depth is sufficient. The Paleogene reservoirs in the Dinga Step-Fault Zone (DSFZ) that meet all of these conditions are therefore dominated by normal oils. Degraded oils of varying degrees are widely distributed in other tectonic units where at least one of these factors is absent. This study may have practical application for further petroleum exploration, prediction of petroleum properties, and the reduction of production risk.