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

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.