地质科技通报 (Jan 2023)
Geochemical characteristics and environmental implications of source rocks of the Dongying Formation in southwest subsag of Bozhong Sag
Abstract
Source rocks of the Dongying Formation are widely distributed in the Bozhong Sag. It is of great significance to clarify the characteristics and paleoenvironment of the source rocks to promote conventional and unconventional oil and gas exploration in the Bozhong Sag. In this study, organic and inorganic geochemical experiments were carried out on the source rock samples collected from the Dongying Formation in the Bozhong Sag, with aims to analyze the quality and paleoenvironment and compare it with the Shahejie Formation. The results show that: ①the characteristics of source rocks in different layers of the Dongying Formation are significantly different from those in Es1-2.The abundance of organic matter in the lower and third members of the Dongying Formation (Ed2L and Ed3) is high and has reached the mature-high maturity stage. The organic matter of the Dongying Formation is mainly type Ⅱ. ②According to the Pr/nC17-Ph/nC18 charts, the source of organic matter in the Dongying Formation is mixed type, which is obviously different from the low algae of Es1-2.③The paleoenvironment during the formation of source rocks was analyzed by combining the contents or ratios of 16 elements, such as Sr/Cu, Mn/Fe, Pr/Ph and the gammacerane index. Different from the strong reduction-salinized lacustrine basin in the arid climate of Es1-2, the Dongying Formation was formed in a warm and humid climate, accompanied by strong terrigenous input, and controlled in a weakly oxidized freshwater environment. Longitudinally, the paleoclimate changes little from Ed3 to Ed1, and the water salinity is basically the same, but the water depth becomes shallower and the water oxidation conditions show a trend of increasing gradually.④Using Co and La contents to quantitatively restore the paleo-water depth during the deposition of the Dongying Formation, and suggests that Ed3 (50.4 m)>Ed2 (35.7 m)≈Es1-2(33.2 m)≈Ed1(31.7 m). The calculated results are consistent with the previous qualitative results of paleo-water depth.
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