Figure 8. Plot of percentage C29αββ 20(S+R)/C27-29αββ 20(S+R) steranes against sterane/hopane ratios [C27-29ααα(20S+20R) + C27-29αββ(20S+20R) steranes]/[C29-30αβ + C31-33αβ(22S+22R) hopanes], showing marine contributions to the Niger Delta oil samples. Five Niger Delta oil samples biodegraded to level 6 or above, as described in Peters and Moldowan (1991), were excluded from the plot.