Geochemical characteristics of the lower part of Shayi Member in Lixian Slope, Raoyang Sag, Bohai Bay Basin, Northern China: Implications for organic matters origin, thermal maturity and depositional environment

• Published in: Geochemistry international 2017-12, Vol.55 (12), p.1140-1153
• Main Authors: He, Fugui, Gao, Xianzhi, Zhao, Xianzheng, Yang, Dexiang, Lu, XueJun, Liu, Jingwang, Dong, Xiongying, Wang, Huilai, Wu, Dongsheng
• Format: Article
• Language: English
• Subjects: Algae; Alkanes; Anoxia; Bacteria; Barium; Earth and Environmental Science; Earth Sciences; Geochemistry; Kerogen; Macerals; Maturity; Microscopy; Mudstone; Organic matter; Petrology; Saline water; Sapropels; Sedimentary rocks; Shale; Strontium
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702917140026

The lower part of Shayi Member (Es1x Sub-Member) composed mainly of dark mudstones and shales is the dominant source rocks for the Lixian Slope. Based on organic petrology, organic and inorganic geochemistry analyses of several mudstone and shale samples selected from Es1x Sub-Member, this research provides an overview on type, origin and thermal maturity of organic matters, as well as depositional environment of Es1x Sub-Member. Kerogen microscopy observation shows that the macerals are dominated by sapropelinite with a significant mixture of vitrinite and inertinite, indicating that aquatic algal-bacterial organic matter inputs are dominate with a significant contribution of terrigenous organic matter inputs. This statement is supported by n -alkane patterns distribution characteristics, high ( n -C 21 + n -C 22 )/( n -C 28 + n -C 29 ) values (average = 1.77), the plot of high Ph/n -C 18 values (average = 4.15) versus low Pr/n -C 17 values (average = 1.13), and high proportion of C 27 sterane and C 29 sterane (average = 37.7 and 42.0%, respectively). In addition, the rather low Pr/Ph values (average = 0.38), high gammacerane index values (average = 0.30), high V/Ni and V/(V + Ni) values (average = 11.84 and 0.89, respectively), high Sr/Ba and Sr/Cu values (average = 8.54 and 108, respectively), indicative of a saline water condition and a anoxic depositional environment. The low C29 sterane ααα 20S/(20S + 20R), C 29 sterane αββ/(αββ + ααα), C 31 homohopane 22S/(22S + 22R), C 32 homohopane 22S/(22S + 22R), Ts /( Ts + Tm ) values and relatively high moretane/hopane values show that the level of thermal maturity of organic matters in Es1x Sub-Member are low.