Abstract:

The sedimentary environment of the Pinghu Formation in the BW area of the Xihu sag has long been controversial. In this study,19 mudstone samples were collected and analyzed for trace elements and rare earth elements(REEs)using two methods: conventional whole-rock total digestion and selective acetic acid extraction. Based on the geochemical data obtained,multiple proxies were integrated to systematically analyze the redox conditions,paleoclimatic characteristics,and paleosalinity evolution during the depositional period of the Pinghu Formation. The results show that: (1)Element concentrations obtained via acetic acid extraction are significantly lower than those from whole-rock digestion,but both methods reveal consistent elemental distribution patterns. Moreover,the acetic acid extraction method is proven to be more effective than conventional whole-rock digestion when applying specific elemental proxies to identify sedimentary environments,especially for distinguishing terrestrial and marine facies.(2)The redox conditions during the depositional period of the Pinghu Formation were predominantly suboxic to oxic,indicating a relatively oxidizing environment;paleoclimate and paleosalinity evolved concurrently,characterized by an initial warm and humid freshwater setting,which shifted to an arid and saline environment by the end of the early stage. This was followed by a transition to a warm and humid brackish setting in the middle stage,a return to arid and saline conditions by the mid-stage,and finally a return to a warm and humid freshwater environment in the late stage. In light of the sedimentary context of the study area,the synchronous variations in paleoclimate and paleosalinity are interpreted as recording four successive changes,collectively indicating at least two major marine transgressions during the depositional period of the Pinghu Formation. The above results have significant scientific guiding significance and application value for deepening oil and gas exploration in the Xihu sag of the East China Sea Shelf Basin.

Key words: sedimentary environment, palaeoclimate, redox state, paleosalinity, elemental characteristics, Pinghu Formation, Xihu sag

摘要：

东海陆架盆地西湖凹陷BW地区始新统平湖组为该地区油气的主要储集层以及烃源岩,但对其沉积环境认识尚存在争议。本研究选取19块泥岩样品,采用传统的全样总量分析法和醋酸选择性提取法分别提取其微量及稀土元素,并基于多项地球化学指标,系统分析了平湖组沉积时期的氧化还原状态、古气候特征及古盐度演化。结果表明: (1)2种方法揭示的元素分布特征具有一致性,但醋酸选择性提取法所得元素质量分数显著低于全样提取法,在利用特定元素指标判别沉积环境(特别是区分陆相与海相)时,醋酸选择性提取法相较于传统的全样总量分析法更具优势。(2)平湖组沉积时期,研究区氧化程度整体为贫氧—富氧的偏氧化环境。 该时期古气候与古盐度协同演化,同步记录了4次连续变化,表现为沉积初期为温暖湿润的淡水环境,早期末为干旱咸水环境,中期转变为温暖湿润的半咸水环境,中期末则再次转变为干旱的咸水环境,晚期又变回温暖湿润的淡水环境,指示平湖组沉积时期研究区至少存在2次较大规模的海侵作用。上述结果对深化东海陆架盆地西湖凹陷油气勘探具有重要的科学指导意义和应用价值。

关键词: 沉积环境, 古气候, 氧化还原条件, 古盐度, 元素特征, 平湖组, 西湖凹陷

CLC Number: 

• P594