西湖凹陷古近系平湖组泥质岩地球化学特征及其地质意义

Geochemistry of argillaceous rocks of the Eocene Pinghu Formation from Xihu depression in East China Sea basin and its geological significance

西湖凹陷是东海陆架盆地重要的富生烃凹陷之一,平湖组是主要烃源岩和油气富集层系。为了探讨平湖组沉积期构造背景、源区性质及古环境,本文对平湖组泥质岩地球化学特征进行了研究。地球化学分析结果表明,平湖组沉积期同时具有大陆岛弧和活动大陆边缘构造背景。平湖组泥质岩的物源区母岩主要由上地壳岩石演化而来,并通过对比周围隆起区母岩特征,认为西侧的海礁隆起为西湖凹陷西部斜坡区平湖组沉积期的主要物源区,母岩主要以上地壳的长英质火成岩为主,并受到少量来自幔源岩浆和上地壳古生界变质岩系的影响,母岩类型主要为花岗闪长岩和英安质凝灰岩,少量安山岩和变质岩。泥质岩CIA分析表明,平湖组泥质岩的母岩经历了中等—强烈的风化作用,结合Sr/Cu值分析,推断西湖凹陷在平湖组沉积期古气候较为温暖湿润。微量元素和古盐度分析结果以及对盆地古地貌特征分析表明,西湖凹陷平湖组沉积期自北向南沉积背景有明显的地区差异性,北部(A-1)沉积水体为淡水,主要发育河控三角洲沉积体系,而中部(A-2)和南部(B-1)表现为淡水和半咸水交替演化的特征,主要发育受潮汐影响三角洲沉积体系。平湖组泥质岩主要形成于含氧环境中。

The Xihu depression in the East China Sea basin contains significant oil and gas resources.The Pinghu Formation coal measures are the primary source rock and oil-gas reservoirs.In order to better understand its tectonic setting,provenance characteristics and paleoenvironment,we conduct detailed geochemistry study of the argillaceous rocks of the Eocene Pinghu Formation.La-Th-Sc and Th-Sc-Zr/10 diagrams indicate that the study area was mainly subjected to the continental island arc and active continental margin tectonic background.Argillaceous rocks were predominantly derived from upper crustal felsic igneous rocks based on the values Al;O;/TiO;,K;O/Al;O;,Cr/Zr,LREE/HREE,Eu,La;/Yb;,Gd;/Yb;and plots of Th/U-Th,and the fairly uniform REE patterns.By comparing the characteristics of the magmatic rocks in the uplift area,it is concluded that the parent rocks are mainly granitic amphibolite and ingenious tuff,with a few andesites and metamorphic rocks.The A-CN-K triangle showed that mudstones were affected by the K metasomatic processes,and in this regard,we corrected the chemical index of alteration(CIA).The Eocene Pinghu Formation mudstones have moderate to high K-corrected CIA(61.40~83.49),indicating moderate to intense chemical weathering conditions.By analyzing and comparing with the CIA of the Oligocene Huagang Formation,the chemical weathering history suggests that a warm and humid climate during the Eocene,whereas a cool and humid to semi-humid climate prevailed during the Oligocene.Using the corrected boron contents(Bc)and integrating the analysis of paleomorphological and paleontological features,it is concluded that there is regional variability in the depositional background of the Pinghu Formation.The northern part(A-1)shows a terrestrial depositional environment as a river-dominated deltaic depositional system.The central part(A-2)and southern part(B-1)were dominated by the marginal marine environments as the tidally influenced deltaic depositional system.Trace element U/Th ratios,V/Cr-Ni/Co and Mo-Ni/Co discrimination diagrams indicate that the mudstones of the Pinghu Formation were formed in an oxic environment.