Investigation of hydrocarbon biodegradation from a downhole profile in Bohai Bay Basin: Implications for the origin of 25-norhopanes

► Inverse response of 22S/(22S + 22R) ratio between extended hopane and its 25-norhopane as severe biodegradation occured. ► Abundance of 25-norhopanes showed a minor increase as hopanes decreased significantly. ► Alternative degradation pathways for hopanes might occur in a reservoir, besides C-25 demethylation. ► 25-Norhopanes were formed prior to complete elimination of acyclic and bicyclic alkanes. ► Oil biodegradation in the subsurface was dominantly by anaerobic bacteria.