关键词: 沙三段, 层序划分, 地球化学特征, 古环境, 东濮凹陷, 沙三段, 层序划分, 地球化学特征, 古环境

Abstract: Sedimentary environments serve as a critical constraint on hydrocarbon accumulation, while the geochemical characteristics of sediments provide an effective proxy for tracing paleoenvironmental evolution. To investigate the sedimentary characteristics of the Sha 3 Member of the Paleogene in the Weicheng-Guyunji area of the Dongpu depression, Bohai Bay basin, this paper conducts a systematic analysis of the target interval utilizing sequence stratigraphy, geochemistry, and geophysical well logging. Based on well logging curves, the Sha 3 Member in Well Y3 of the Dongpu depression is divided into three long-term sequences, which are designated as sequence Ⅰ, sequence Ⅱ, and sequence Ⅲ from bottom to top. Based on the geochemical characteristics of major, trace, and rare earth elements, this study evaluates paleosalinity, paleoclimate, paleo-redox conditions, and tectonic settings within these three sequences, and elucidates the geochemical characteristics and paleoenvironmental evolution of the Sha 3 Member in the Weicheng-Guyunji area of the Dongpu depression. The results indicate that the chondrite-normalized REE patterns of the three sequences in Well Y3 are generally consistent with those of post-Archean Australian shale (PAAS). These patterns exhibit a “V” shape and an overall right-inclined trend with a gentle variation. This indicates a relative enrichment of light rare earth elements and a relative depletion of heavy rare earth elements. The Sr/Ba ratios of the Sha 3 Member samples in the study area range from 0.50 to 16.33, with an average of 2.65, indicating that the depositional water body during the Sha 3 Member period was characterized by a saline to brackish environment. The Sr/Cu ratios range from 5.88 to 653.31, with an average of 90.41, suggesting that the study area experienced an overall arid and hot climate during this period. Similarly, the La_N/Yb_N ratios range from 7.85 to 12.25, with an average of 10.14, which further corroborates the dry and hot paleoclimate during the deposition of the Sha 3 Member. The V/(V+Ni) ratios range from 0.45 to 0.79, with an average of 0.69, indicating that the study area was characterized by an anoxic environment with weak water column stratification. Furthermore, the δCe values range from 0.90 to 1.02, with an average of 0.94, indicating that the deep water body of the Sha 3 Member was predominantly in a reducing environment. Overall, the climate in the study area during the deposition of the Sha 3 Member was predominantly arid, with reducing conditions and a saline to brackish water environment. Discrimination diagrams of La/Th versus Hf and La/Yb versus ΣREE indicate that the source rocks were composed primarily of intermediate to acidic felsic rocks, as well as mixed felsic/mafic rocks.

Key words: Dongpu depression, Sha 3 Member, sequence division, geochemical characterization, paleoenvironment