Sedimentary Characteristics of the Lower Cambrian Yuertusi Formation and the Organic Matter Enrichment Model in the Tarim Basin
The Tarim Basin is the largest oil-bearing basin located in Northwestern China.The Lower Cambrian Yuertusi Formation comprises mudstone and carbonate deposits rich in organic matter,making it one of the most important source rocks in the basin.We conducted a comprehensive analysis of outcrops,including logging da-ta,seismic data,cores,and cuttings,to reveal the lithofacies,sedimentary facies,and paleogeographic distri-bution characteristics of the Yuertusi Formation and to understand the distribution and constraints on organic-rich deposits.The Yuertusi Formation can be divided into three sequences based on the boundaries of the ex-posed surfaces,and it generally formed in a mixed ramp shelf environment on a passive continental margin.Ten lithofacies,seven facies associations(Fa),and four major sedimentary facies have been identified in the Yuer-tusi Formation.The sedimentary palaeogeography map shows the distribution of sedimentary facies belts from southwest to northeast,including mixed tidal flat,shallow inner shelf,deep outer shelf,and deep basin.The organic-enriched deposits mainly developed in the deep outer shelf zone and are characterized by black mud shales with unusually high TOC values and extremely high trace element abundances.The enrichment of organ-ic matter is related to high paleo-productivity and favorable preservation conditions.Hydrothermal activities introduced many nutrient elements,and upwelling currents promoted the mixing and diffusion of these nutrients with normal seawater,leading to the formation of eutrophic surface water.Additionally,the anoxic conditions in the deep outer shelf are conducive to the preservation and enrichment of organic matter.
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