Abstract: The Shahejie Formation, located at the western and southwestern margins of the Qingdong Sag, contains extensive reservoirs of heavy oil. At present, there is a lack of in-depth research into and clear understanding of the formation mechanism of this oil. This knowledge gap has affected the evaluation of resources within the study area, as well as oil and gas exploration processes. A comprehensive analysis of the formation of heavy oil in the area was made based on the physical properties of the crude oil and composition characteristics of the biomarkers. The results indicate that the heavy oil contained native immature oil and distributions of heavy oil produced by secondary effects, such as biodegradation. The former was mainly distributed within the Qingdong Well 5 area, located in the southwestern section of the sag; the latter, biodegraded oil, was found at the western slope of the sag and Qingdong-Qingnan transition zone. Degradation of the crude oil was at levels 2-8, and generally had the following characteristics:heavier above and to the south, and lighter below and to the north. The formation and distribution of heavy oil in the study area was mainly affected by factors including the early hydrocarbon generation of organic matter, paleotopography, and tectonic setting.

Key words: immature oil, heavy oil, biodegradation, formation mechanism, Qingdong Sag