Abstract:

Obvious variations of oil properties were observed from the oils of the Bamianhe region, Bohai Bay Basin, eastern China Heavy oils are mainly distributed in high position tectonically whereas normal oils are in low position with relatively short distance from the depo-center of source rock. Detailedly geochemical investigations of 31 oil samples show that the heavy oils of Bamianhe oilfield are characterized by low concentration of saturated hydrocarbons with low ratios of saturated/aromatic hydrocarbons, and low content of hydrocarbon fractionation with low molecular weight i.e. alkyl alkanes, naphthalene and phenanthrene, suggesting light to middle degree of secondary alteration.Oils from adjacent Chaoqiao oilfield are featured by occurrence of 25norhopane series besides the similar characteristics of biodegradation in the Bamianhe oilfield. It is suggested that secondary alteration is crucial for the formation of the heavy oils in the Bamianhe area. Additionally, the immature oils in the Shaziling oilfield with similar source rocks to the Bamianhe display chacteristics of normal oils rather than that of the heavy oils, which infers that early hydrocarbon degradation does not result in indigenous heavy oils, which further proves secondary alteration is the main factor responsible for the heavy oils in the Bamianhe oilfield.Oil pools with shallow burial depth are liable to be destroyed under relatively bad reserve conditions.

Key words: heavy oil, origin of oil and gas, biodegradation, secondary alteration