渤海莱州湾凹陷中部古近系沙三段烃源岩地球化学特征及沉积环境

Geochemical characteristics and sedimentary environment of source rocks of the third member of Paleogene Shahejie Formation in central Laizhouwan Sag,Bohai Sea

通过对渤海海域莱州湾凹陷中部古近系沙三段烃源岩样品的热解、TOC、饱和烃GC/MS分析以及主、微量元素分析,研究了其烃源岩的地球化学特征及沉积环境。研究结果表明:(1)莱州湾凹陷中部古近系沙三段烃源岩有机质丰度差异较大,总体为好—优质烃源岩,有机质类型以Ⅰ—Ⅱ型为主,整体处于低熟—成熟阶段;(2)通过Pr/nC_(17)-Ph/nC_(18)图版、C_(24)四环萜烷/C_(26)三环萜烷(C_(24)TeT/C_(26)TT)、C_(27)/C_(29)规则甾烷、奥利烷指数等参数判断沙三段烃源岩有机质来源为低等水生生物和高等植物的混源输入;(3)综合CIA,Sr/Ba,Pr/Ph,V/(V+Ni),U/Th值和伽马蜡烷指数等分析了烃源岩形成时期的古环境,认为沙三段沉积时期气候温暖湿润,降水丰富,水体为淡水—微咸水,陆源输入较多,具有较高的初始生产力,沉积时期整体表现为贫氧的弱氧化弱还原环境,沙三下段—沙三上段沉积环境整体变化不大,古气候和古水体盐度基本保持不变,但陆源输入及水体氧化条件随沉积进程呈逐渐增加的趋势。

The geochemical characteristics and sedimentary environment of the third member of Paleogene Shahejie Formation(E_(2)s^(3))in central Laizhouwan Sag of Bohai Sea were studied by means of pyrolysis,TOC,GC/MS analysis of saturated hydrocarbons,and major and trace elements analysis of source rock samples.The results show that:(1)The source rocks of E_(2)s^(3) have different organic matter abundances,they are generally good to high-quality source rocks.The organic matters are mainly typeⅠ-Ⅱand in the low mature to mature stage as a whole.(2)According to Pr/nC_(17)-Ph/nC_(18)plates,C24tetracyclic terpanes/C_(26)tricyclic terpanes(C_(24)TeT/C_(26)TT),C_(27)/C_(29)regular sterane and olliane index,it can be concluded that the organic matters of E_(2)s^(3) source rocks are derived from mixed inputs of lower aquatic organisms and terrigenous higher plants.(3)CIA,Sr/Ba,Pr/Ph,V/(V+Ni),U/Th and gammacerane index were used to analyze the paleoenvironment of the source rocks.During the sedimentary period of E_(2)s^(3),the climate was warm and humid with abundant precipitation.The water environment was fresh-brackish water,accompanied by strong terrigenous input,with high initial productivity,and the sedimentary environment is weak oxidation and weak reduction environment with poor oxygen.The sedimentary environment of E_(2)s^(3) has little change,and the paleoclimate and water salinity are basically the same.However,terrigenous input and water oxidation conditions gradually increase with the deposition.