A correct understanding of the quality distribution and origin of super heavy crudes is critical to their exploration assessment and development planning.A quality assessment model for characterizing heavy crude samples from the Neogene Minghuazhen and Guantao Formations in the steep slope zone of LX structure in Liaoxi sag of Bohai Sea was established based on a method combining rock pyrolysis with seismic multi-attribute analysis.The quality distribution and origin of the samples was also studied through analyses of paleo fluid activities and physical properties of relevant reservoirs.The result shows that

vertically speaking

the quality of samples from the top is worse than that from the bottom of the Minghuazhen Formation while the Guantao Formation is the opposite with samples from the top better than that from the bottom.Horizontally

the quality of samples from areas near structural highs is less favorable while those near faults and reservoir boundaries yield better-quality samples.As a whole

sample quality shows a zonal distribution

which is considered to be closely linked to the "two-stage reservoiring and densification" of the crude.It is suggested that the crude of the Dongying Formation experienced a reservoiring and densification process in the early stage

and later the reservoirs that had experienced the first densification in the Dongying Fm were destroyed

leading to the re-reservoiring of the crude in the Minghuazhen and Guantao formations.It is also suggested that crude quality is poorer in intervals with better physical properties as a result of more extensive densification.The second stage of reservoiring is proposed to be accompanied by charges of newly-generated better-quality crude along faults and reservoir boundaries that helped improving the quality of existing crude in these areas and resulting in the current spatial distribution of crude quality.