Distribution and properties of biomarkers in severely biodegraded crude oil of Gudao reservoir, China

Abstract

Biodegraded crude oils in China are distributed widely and make up a certain portion of heavy-oil resources. An unresolved complex mixture (UCM) of hydrocarbons was isolated from three biodegraded crude oils in the Gudao reservoir of the Shengli Oilfield, China. The biomarker composition of biodegraded oils was determined using GC–MS technique. The results show that n-alkanes were completely depleted, isoprenoids and diasteranes were almost absent. The hydrocarbons expect for tricyclic terpane, steranes, hopane, were preferentially depleted in saturated fraction. Meanwhile, the 25-norhopane series occurs, indicating that the crude oils experienced severe degradation equivalent to levels 5–6. Based on biomarker proxies, the depositional environment of the Gudao Oils can be divided into two phases. In the first (GD2-19-55 and GD8-27N12), deposition occurred completely in seawater-dominated oxic to suboxic conditions, with primary input of terrestrial higher plants with an abundant influx of aquatic organisms. The second phase (GD2-7-41, GD-KD-162, GD-KD-18-168 and GD-KD-45) consisted of mainly seawater anoxic to suboxic/oxic conditions along with a small freshwater influence, with diluted organic matte derived from phytoplankton and a lesser influence from terrestrial higher plants. In addition, the vitrinite reflectance (Ro) of ca. 0.73–0.89% and methylphenanthrene index (MPI-1) of ca. 0.55–0.82 indicate the organic matter to be mature.