贵州地区下寒武统牛蹄塘组烃源岩热水沉积成因鉴别与强度评价

Genesis and intensity of hydrothermal sedimentation in hydrocarbon source rocks in the Lower Cambrian Niutitang Formation,Guizhou area

为了准确地判别烃源岩是否受热水沉积作用影响并对其影响强度进行评价,以贵州地区下寒武统牛蹄塘组烃源岩为主要研究对象,通过对其微量元素进行分析后发现,V/Sc-V/Cr图能够用于判别研究区烃源岩是否为热水沉积成因并反映其氧化还原环境的变化,还可利用斜率Sc/Cr比值来评价热水沉积作用的影响强度。当Sc/Cr≥0.144时,以正常沉积作用为主,很少受到热水沉积作用的影响,V/Cr比值和V/Sc比值之间呈线性相关关系,能够反映氧化还原环境的变化;当Sc/Cr≤0.120时,以热水沉积作用为主。受热水沉积物中元素含量异常影响,V/Cr比值受到干扰,无法反映真实的氧化还原环境变化,但V/Sc比值仍可用于指示氧化还原环境。当0.120≤Sc/Cr≤0.144时,受两种沉积作用共同影响,此时V/Cr比值对氧化还原变化的指示作用受到影响,但仍能反映氧化还原环境的变化趋势,V/Sc比值则仍能用于反映氧化还原环境的变化。V/Sc-V/Cr图能够准确地判别烃源岩受热水沉积作用影响并对其影响强度进行评价。

The study case is the source rocks of the Lower Cambrian Niutitang Formation in Guizhou area. Their trace element content is analyzed to accurately identify the hydrothermal sedimentation effect on source rocks and evaluate the intensity of the effect. The results show that（ 1） the diagram V/Sc-V/Cr can be used to judge whether hydrothermal sedimentation effect is a contributing factor for the composition of source rocks in the study area,which in turn,can reflect the redox environment change;（ 2） the slope Sc/Cr ratio is applicable for evaluating the intensity of hydrothermal sedimentation： when the Sc/Cr ratio is more than or equal to 0. 144,normal sedimentation dominates and hydrothermal sedimentation effect is weak,and there is a linear correlation between V/Sc and V/Cr ratios,reflecting the redox environment change; when the Sc/Cr ratio is less than or equal to 0. 120,hydrothermal sedimentation dominates,and the V/Cr ratio,which is distorted by the element content anomaly in the hydrothermal sediments,can not indicate the actual change of the redox environment,but remain useful to indicate the environment change; however,when the Sc/Cr ratio varies from0. 120 to 0. 144（ with both ends included）,both sedimentary processes function,and the value of V/Cr ratio is influenced by the hydrothermal sediments,but can still indicate the trend of redox environment change,and thus can be applied nor-mally. The conclusion can be drawn that the diagram V/Sc-V/Cr can accurately identify the influence and indicate the intensity of the hydrothermal sedimentation on source rocks.