Geochemical characteristics of trace elements in the Es₄^x Formation of the Bonan Sag and their implications for sedimentary environment

The Bonan Sag is an important oil and gas-rich region of Bohai Bay basin. In order to clarify the depositional environment and hydrocarbon potential of the hydrocarbon source rocks in the lower subsection of Es₄^x in the Bonan Sag, and the hydrocarbon potential of the hydrocarbon source rocks of the Es₄^x, we analyzed the typical mudstone samples of the Es₄^x in the Bonan Sag by trace element testing and organic geochemical characteristics. Through selecting the elements and element combinations sensitive to the depositional environment, such as δCe_n, Ce_anom, Ni/Co, V/Ni, V/(V+Ni) , V/Cr, B/Ga etc., in combination with the organic geochemical characteristics, this paper clarified the climate, redox environment and salinity during the sedimentary period of the Es₄^x in the Bonan Sag. The applicability of these parameters in the paleoenvironment study for the working area and the factors affecting the development of source rocks with hydrocarbon generation potential were discussed. The results show that the La_n/Yb_n values range from 1.62 to 3.00; B/Ga range from 1.43 to 3.73; the blackish mudstones have Pr/Ph＜1, δCe_n＞1, and Ce_anom＞0, and the reddish mudstones samples have δCe_n values ranging from 0.94 to 0.96, and Ce_anom＜0. It indicates a warm and humid climate in some areas during the depositional period of the Es₄^x in the Bonan Sag, a slow sediment deposition rate, and a freshwater-slightly saline environment of the whole area, while that part of the area was in an anoxic environment, except for the oxygen-rich environment. The results of comparative analysis show that using δCe_n and Ce_anom is reliable to discriminate redox environments, and the relative magnitudes of mudstone Ni/Co, V/Ni, and V/（V+Ni） can infer the anoxicity of environment during their deposition, and the B/Ga ratio can distinguish freshwater-brackish water and brackish water environment. In addition, the mudstone in the study area of the Es₄^x has a high content of sapropelinite, the kerogen is type II-III; TOC, S₁+S₂, and chloroform bitumen “A” contents of some mudstones indicate that they have reached the level of general hydrocarbon source rocks. Compared with the non-hydrocarbon source rock in the study area, they have smaller B/Ga and larger ETR index and gammacerane index. This suggests that the development of hydrocarbon-generating potential source rocks in anoxic environments may depend on the salinity of the sedimentary water.