渤海湾盆地东营凹陷沙四上亚段—沙三下亚段岩相特征及沉积环境——以樊页1井为例

通过岩心、显微薄片的观察,依据有机质含量、无机矿物组成、沉积构造等特征,将渤海湾盆地东营凹陷沙河街组四段上亚段(沙四上亚段)-沙三下亚段樊页1井的细粒沉积岩划分出11种岩相类型。利用常量、微量元素含量及特定元素比值的垂向变化,从古气候、古物源、古水深、古盐度和古氧化还原性5个方面对沉积环境的垂向变化特征进行分析。研究层段发育的细粒沉积岩形成于半深湖-深湖、半咸水-咸水、相对湿润、还原-强还原的古环境条件,且环境的垂向演化具有明显的阶段性,经历了水体加深、盐度降低、气候由干旱转为湿润、强还原-弱还原-还原、陆源供给由多减少再增多的演化过程。5个沉积环境要素共同影响着岩相类型的垂向分布与组合特征。随水体加深,岩相组合中碳酸盐岩所占比重逐渐降低。不考虑重力流事件,岩石组分中石英、长石含量与水体深度、离岸距离、陆源碎屑供给量密切相关,从而影响粉砂岩及粉砂质细粒沉积岩的发育程度。高盐、强还原的水介质环境有利于有机质的保存,多形成富有机质岩相。古气候的变化使其他古环境要素也相应发生改变,进而影响岩相类型的分布与组合特征。

Lithofacies characteristics and sedimentary environment of Es4U and Es3L: A case study of Well FY1 in Dongying sag,Bohai Bay Basin

Characteristics summarized through core and microscopic thin section observations, including the organic matter content, inorganic mineral composition and sedimentary structure, were used to categorize the lithofacies types of the fine-grained sedimentary rocks in the upper section of the fourth member of Shahejie Formation(Es^4U) and the lower section of the third member of Shahejie Formation(Es^3L) in Dongying sag, Bohai Bay Basin.As a result, 11 lithofacies types were identified.The vertical variation of the sedimentary environment was analyzed in terms of paleoclimate, palaeosource, paleo-water-depth, paleosalinity and paleo-oxidation-reduction, based on the content of major and trace elements as well as vertical variation of specific element ratios.The fine-grained sedimentary rocks formed in semi-deep to deep brackish-to-saline lacustrine environments with relatively humid climate and reduction-to-strong reduction.In addition, the evolution of sedimentary environment is obviously periodic, featuring increase in water depth, decrease in salinity, climate change from arid to humid, change from strong reduction to weak reduction then to reduction, and terrigenous sediment supply change from large to small then to large quantities.All the five elements of sedimentary environment collectively influence the vertical distribution and combination of lithofacies types.With the water depth increasing, the proportion of carbonates to lithofacies assemblages will decrease.Without considering the gravity flows, a combination of water depth, offshore distance and terrigenous debris supply will affect the contents of quartz and feldspar, which will in turn influence the development of siltstone and silty fine-grained sedimentary rocks.The setting featuring high salinity and strong reduction facilitates the preservation of organic matters, prone to produce organic-rich lithofacies types.Variation of paleoclimate makes other paleoenvironmental elements change correspondingly, then affecting the distribution and combination of lithofacies types.