Geochemical characteristics and correlation of continuous charge mixing and biodegradation of heavy oil in southeastern Dongying Sag, Bohai Bay basin, China

Conventional geochemical diagnostic parameters in the saturated hydrocarbon fraction are affected by biodegradation. Based on the intensity of n-alkanes, hopanes and 25-NHs, many samples from Caoqiao appear to have experienced both biodegradation and continuous charge. A wide range of geochemical parameter values from different components point to multiple oil charge mixing. The relationship between the absolute concentrations of n-alkanes to hopanes and hopanes to 25-NHs of the samples can qualitatively differentiate whether oil charge mixing occurs in reservoirs or not. The cross plot of n-alkanes and hopanes dynamically describes the processes of continuous charge mixing and biodegradation. The positive correlation between the concentrations of hopanes and 25-NHs may suggest that oil charge mixing and biodegradation proceed continuously, while the negative correlation may indicate charge stops due to some geological factors.