Polycyclic alkanes in a biodegraded oil from the Kelamayi oilfield, northwestern China

Abstract

A comparison is made of the distributions of certain classes of saturated hydrocarbon biomarkers in an unaltered oil and a severely biodegraded oil from the Kelamayi oilfield in the Zhungeer Basin of northwestern China. Both oils are thought to have the same source rock. The n-alkanes, acyclic isoprenoids, carotenoid-derived alkanes and regular C 27C 29 steranes which are present in the nondegraded oil were not detected in the biodegraded sample. Additionally, the biodegraded oil shows an increase in the abundance oil, and the presence of 17α(H)-25-norhopanes, 17ß(H)-25-norhopanes, C-10 demethylated tricyclic terpanes and a C-10 demethylated C 23 tetracyclic terpane which were not detected in the nondegraded oil. C 26 and C 25 regular steranes appear to show a much greater resistance to microbial attack than the C 27C 29 regular steranes, as indicated by an isomer distribution for these low molecular weight steranes that is expected for nondegraded samples in the biodegraded oil. Unknown tricyclic compounds showing a base fragment ion of m/ z 219 and a similar carbon number abundance to the steranes in the nonbiodegraded sample are present in relatively high concentrations in the biodegraded oil. These compounds are also present in very low abundance in some nondegraded oils from the Kelamayi Basin, indicating that their much higher concentration in the microbialy altered oil is possibly due to their resistance to biodegradation. However, at present their derivation from the microbial alteration of steranes cannot be ruled out. Several series of bicyclic compounds are also present in higher concentrations in the biodegraded oil.