渤海湾盆地歧口凹陷沙河街组烃源岩沉积环境及形成机理

The paleo-sedimentary environment and formation mechanism of the source rocks in Shahejie Formation, Qikou Sag, Bohai Bay Basin

烃源岩分布与其形成环境密切相关,对油气分布具有重要影响。对烃源岩的评价及其形成环境的研究可为渤海湾盆地黄骅坳陷歧口凹陷下一步的勘探指明方向。从烃源岩发育特征和演化过程入手,深入研究了歧口凹陷沙河街组烃源岩的地球化学特征、分布规律及形成机理。结果表明:沙河街组烃源岩有机质丰度达到了中等—最好的级别,有机碳含量整体呈现从沙三段(Es_(3))到沙一下亚段(Es_(1)^(x))到沙一上亚段(Es_(1)^(s))先增大后减小的特征;有机质类型为Ⅰ型—Ⅲ型,且沙一上亚段、沙一中亚段以Ⅰ型—Ⅱ_(1)型有机质为主,沙一下亚段、沙二段(Es_(2))、沙三段Ⅰ型—Ⅲ型有机质均有分布,反映沙河街组沉积期有机质来源为湖泊原生有机质和陆源有机质混源。沙河街组烃源岩在平面上的分布具有分布范围广、沉积厚度大的特征,优质烃源岩在斜坡区更为发育。古气候研究结果表明,在沙河街组沉积期,该地区风化强度中等,整体以温暖湿润的古气候条件为主,有利于生物的发育。歧口凹陷沙河街组沉积期水体古盐度较大,属于半咸水环境。古盐度与古气候之间没有明显的相关关系,水体咸化的原因可能与海水入侵有关。古盐度和有机质含量的关系表明:在低盐度阶段,有机质含量随古盐度的升高而增大;在高盐度阶段,有机质含量随古盐度的升高而减小。根据微量元素比值和C_(org)∶P摩尔比值,歧口凹陷沙河街组沉积期发育氧化—亚氧化的沉积环境,Pr/Ph值显示在Es_(1)^(x)间歇性发育强还原环境,提高了有机质的保存条件。通过对沙河街组沉积期湖泊古生产力的恢复,沙河街组沉积期歧口凹陷整体上为中—富营养湖泊,板桥地区古生产力较低,为贫—中营养湖泊。歧口凹陷整体相对较高的古生产力水平及较高的陆源有机质输入量为该地区有效烃源岩的形成提供了物质基础。在纵向上,沙河街组沉积期古生产力水平具有Es_(1)>Es_(3)>Es_(2)的特征。古生产力条件是影响歧口凹陷沙河街组烃源岩形成的主要因素。由于斜坡区陆源有机质和水生有机质的双重贡献,优质烃源岩更为发育。

The distribution of source rocks is closely related to their sedimentary environment and has an important influence on the distribution of oil and gas.The evaluation of the source rocks and the study of their sedimentary environment can point out the direction for the next exploration in the Qikou Sag.In this paper,starting from the development and evolution of source rocks,the geochemical characteristics,distribution law and formation mechanism of source rocks in the Shahejie Formation in Qikou Sag are studied in depth.The geochemical analysis shows that the abundance of organic matter has reached the medium-best level and the contents of organic matter has the characteristics of increasing firstly from Es_(3)to Es_(1)^(x)and then decreasing from Es_(1)^(x)to Es_(1)^(s).The types of organic matter are typeⅠto typeⅢ,and the organic matter are dominated by typeⅠand typeⅡ_(1)in Es_(1)^(s)and Es_(1)^(z),while by typeⅠto typeⅢin Es_(1)^(x),Es_(2),Es_(3),reflecting that the source of organic matter in the sedimentary period of the Shahejie Formation was lacustrine primary organic matter mixed with terrigenous organic matter.The distribution of source rocks in the Shahejie Formation on the plane has the characteristics of wide area and large thickness,and high-quality source rocks are more developed in the slope area.The paleoclimate during the sedimentary period of the Shahejie Formation was affected by moderate weathering intensity,and the overall warm and humid paleoclimate conditions during this period were conducive to the development of organisms.The high paleosalinity of the water belongs to the brackish water.There is no obvious correlation between paleosalinity and paleoclimate,and the high paleosalinity of water may be influenced by seawater intrusion.The relationship between paleosalinity and the content of organic matter shows that in the low paleosalinity stage,the abundance of organic matter increases with the increase of paleosalinity,and in the high paleosalinity stage,the abundance of organic matter decreases when the paleosalinity increases.According to the ratio of trace elements and C_(org)∶P molar ratio,an oxidative-suboxidative depositional environment developed during the deposition of the Shahejie Formation in Qikou Sag,and the Pr/Ph ratio show that a strong reducing environment developed intermittently in Es_(1)^(x),which is conducive to the preservation of organic matter.The evaluation of the paleoproductivity at the period of the Shahejie Formation deposited shows that the lake had medium-high nutrient content and primary productivity was medium-high,while the Banqiao Sag had low-medium nutrient content and had low-medium primary productivity.The relatively high paleoproductivity and the high input of terrigenous organic matter in the Qikou Sag,with the highest value in Es_(1)period and the lowest value in Es_(2)period,provide a rich material basis for the formation of source rocks.The paleoproductivity is the main factor affecting the formation of source rocks in the Shahejie Formation in Qikou Sag.Due to the dual contribution of terrigenous organic matter and aquatic organic matter in the slope area,high-quality source rocks are more developed here.