Abstract: SINOPEC Shengli Oilfield has reached the period when crude oil is produced with extremely high water content
(approximately 99%). However, the oil saturations of cores are mainly between 60%-80%. This means that the water saturations of
cores are relatively low, there is a high water content in the production fluid during water driving, and the produced liquid by electric
heating is almost pure oil. To explain these seemingly contradictory phenomena, we present a new explanation in this paper. It is the
gradual biodegradation that made the crude oil more viscous. The oil field underwent rapid biodegradation due to water flooding after
long-term exploitation. Under experimental conditions, biodegradation of 6th level or above only took about 8 weeks. Due to the gradual
densification of oil, a super-high water content of surface produced fluid appeared at the early period in the Shengli oilfield. However,
the underground oil saturation is still high, and the oil is mainly composed of heavy oil and bitumen, which is difficult to be recovered.
As a result, the residual oil was enriched underground. Although water flooding has some influences on the recovery factor in the past,
but at present, it has preservative effects on the enrichment of residual oil instead.

Key words: oilsaturationofcore, waterdriving, oildensification, bitumen, enrichmentofresidualoil, preservation