Sedimentary environment and geochemical characteristics of Shahejie Formation in Huimin Sag, Bohai bay basin, China

The Shahejie Formation in Huimin Sag is an important crude oil/natural gas reservoir stratum. The results show that (1) in the study area, the Sr/Ba value of Shahejie Formation ES4, ES3, and ES2 in Huimin Sag is 0.55–0.58, showing that these were freshwater-brackish water environment with a continental deposition; the average Sr/Ba value of the first member of Shahejie Formation (ES1) is 1.88, showing that it was a salt water environment. (2) B element method of paleo-salinity analysis shows that in ancient times, the salinity of ES1 was the highest; ES2 and ES4 had basically the same salinity that were lower than that of ES1; the third formation of Shahejie Formation(ES3) had the lowest salinity. (3) Analysis of values of Fe/Mn, Mg/Ca, and paleoclimate index C shows that the Shahejie Formation had generally a semi-dry climate. (4) Based on the analysis of the paleoenvironment, combined with the description of the cores, a sedimentary evolution model is built: ES4 was a debris flow fan-shaped river delta sediment in a shallow lake environment; ES3 was a delta-deep water fan-shaped system in a semi-deep-deep lake environment; ES2 was a delta system in semi-shallow lake environment; ES1 is the semi-shallow lake environment, the main sedimentary facies is the shore shallow lake facies.