Geochemical characteristics of sedimentary environment on He 8 Member of Shihezi Formation in eastern Ordos Basin
Guo Yanqin1, Yu Fang2, Li Yang1, Liang Dayong1, Li Wenhou3, Zhao Jingzhou1, Ma Yanping1
1. School of Earth Sciences and Engineering Xi′an Shiyou University，Key Laboratory of Shaanxi Province for Oil and Gas Accumulation Geology，Xi′an Shiyou University， Xi′an 710065；
2. Petroleum Exploration and Development Research Institute of PetroChina Changqing Oilfield Company， Xi′an 710018；
3. Department of Geology， Northwest University，Xi′an 710069
This study comprehensively analyzed about the salinity of ancient sedimentary environment， redox state and paleoclimate of He 8 Member of Shihezi Formation in east part of Ordos Basin. Various methods were used in the study such as the mass fraction of Sr、 Sr/Ba、 Rb、 Rb/K、 K+Na， element method of K/Na、 B/Ga、 B， Ce anomaly（Ceanom）value，trace element and macroelement， etc. The results show that the paleosalinity of study area is between 2.76‰ to 14.32‰ with an average of 8.62‰， which reflects fresh water to brackish water sediments； The Ceanom is mainly concentrated in the -0.05 to 0.1. The enrichment of Ce as a whole indicates a weak reductive water environment； Sr/Cu ratio ranges from 1.66 to 9.41 with an average of 4.60， which indicates humid climate. The paleosalinity of upper He 8 is higher than lower He 8 Member on vertical， and its climate is slightly arid than lower He 8 Member. The paleosalinity of well 79 and well 28 of Mizhi in the southern study area and Tongganglanggou—Shihanshuili on the northern study area is higher than 10‰ in a plane， which indicates brackish—saline water deposits. Correspondingly， the ancient climate of the two blocks is relatively more droughtier than other blocks. This achieves high conformity with the distribution characteristics of depositional system. The analysis suggests that the lake shoreline during He 8 period in study area is located in well 79 and well 28 of Mizhi area. Therefore the paleosalinity is higher on the affects by the lake and the arid climate. And braided river sedimentary was developed in Tongganglanggou—Shihanshuili on the northern part. The paleosalinity increased under the influence of arid climate and evaporation environmental. The brackish—salt water depositional environment of the study area is conducive to the development of chlorite cementation， and also to the development of reservoir. The conclusion not only plays a complementary role in the research of sedimentary environment on He 8 Member in Ordos Basin， but also has an important meaning for gas exploration in He 8 Member.
Guo Yanqin,Yu Fang,Li Yang et al. Geochemical characteristics of sedimentary environment on He 8 Member of Shihezi Formation in eastern Ordos Basin[J]. Chinese Journal of Geology, 2016, 51(3): 872-890.