Distribution and Characteristics of the Homohopane Molecular Parameters in Paleogene System of the Dongying Sag

Key words: 
• homohopane /
• isomerization /
• biomarkers /
• lacustrine source rocks /
• Dongying sag

Abstract: The Paleogene system in the Dongying sag is a representative lacustrine basin in the eastern China. To investigate the distribution and characteristics of the homohopane maturation parameters, the main source rocks from Member 3 (deposited in fresh and brackish water) and Member 4 (deposited in salty water) in Shahejie Formation (burial depth from 1 300 m to 4 000 m) were investigated. The range 2 800~4 200 m coincides with the "oil window" from the early of oil generation to the late of oil generation, which can well represent the evolution of homohopane maturation parameters during the process of oil generation.
〓〓The results demonstrate that the main controlling factors for the distribution of C31, C32, C33, C34, C35 (17α) homohopane parameters are the isomerization and different decomposition rates as well as generation rates between different isomers. Depositional environment (such as highsalt environment) also impacts their distribution to some extent. For immature and lowmature samples, the related parameters 22S/(22S+22R) increase with burial depth from a disperse state to a equilibrium state, while for the matured samples the parameters increase slowly and then transit to a continuous thermal equilibrium state. In highsalt environment most homohopanes show reversal in different degree except C33 homohopane which remains unchanged in an equilibrium state, indicating that high content of saline minerals also has inhibited or retarded the homohopane maturity parameters. The thermal equilibrium of the parameters coincides with the threshold of oil generation and can be seen as a good indicator for oil generation. The distribution patterns of C31, C32, C33, C34, C35(17α) 22S/(22S+22R) are complex and diversified. For the matured source rock samples, the C35 homohopane 22S/ (22S+22 R) shows a strong change and is characterized by obvious "rise and" "fall". While for the immature samples, C3222S/(22S+22R) displays some remarkable high value, and most immature samples demonstrate characteristics of C31 C33 C35 and exhibits an advantage of evennumber carbon. The study shows that the thermal equilibrium values for C31, C32, C33 isomerization parameters are 0.6 consistently , while the equilibrium values for C34 and C35 homohopanes are comparatively high (the equilibrium of C3422S/(22S+22R) for Member 3 and Member 4 are 0.63, 0.62 respectively, and the equilibrium of C3522S/(22S+22R) for Member 4 reaches 0.65). The homohopane index C35/Σ(C31C35) is also impacted by thermal evolution. In the oil window the index firstly decreases as the burial depth increases, and in the 3.6km highsalt environment, the index shows opposite trend and relatively high values. The study indicates that its change is dominated by 22R isomers. The parameters C31/Σ(C31C35), C32/Σ(C31C35), C33/Σ(C31C35), C34/Σ(C31C35), C35/Σ(C31C35) show different characteristics in their distribution and evolution in the profile. 