摘要
西沙群岛新近纪以来广泛发育生物礁白云岩,在西永2井白云岩层段发现大量的铁白云石,关于该区的铁白云石研究较少。笔者主要通过X射线粉晶衍射数据,结合前人研究资料,对西永2井407~570 m白云岩-铁白云岩进行了分析。研究发现西沙群岛西永2井中新统铁白云岩主要集中在上中新统黄流组一段,其黄流组二段主要发育含铁白云岩。白云岩孔隙度分布与其铁白云石分布有较好的对应关系,孔隙度高的层位铁白云石分布更密集。从中新统白云石和铁白云石的晶胞参数可以得知,该区铁白云石和白云石的晶胞参数都较大,呈现出富钙铁的特征。西永2井白云岩-铁白云岩有序度受Fe^(2+)的影响较大,Fe^(2+)的加入使其有序度降低,因此有序度低的现象不能简单的认为是结晶速度快的准同生作用造成。西沙群岛晚中新世有大量火山活动的现象,岩浆热液提供了铁白云石形成所需要的Fe^(2+)和热量来源。在成岩后期,孔隙性较好的黄流组一段受火山活动影响大,其热源及富Fe^(2+)离子促使白云石向铁白云石转变,形成了大量的铁白云石;而孔隙性较差的黄流二组仅仅形成大量的含铁白云岩。
Post-Neogene reef dolostone occurs widely in the Xisha Islands and a great amount of ankerite has been found in the well of Xiyong 2. However, the research on ankerite is rare in literatures. In this paper, the ankerite-dolomites in the interval of 407~ 570 m of the well Xiyong 2 are analyzed with X-ray powder diffraction upon the basis of previous research data. It is found that the Miocene ankerite mainly occurs in the first Member of the Upper Miocene Huangliu Formation of the well Xiyong 2, while the ferroan dolomite dominates the second member. The porosity of dolomite shows a positive relationship with the amount of ankerite. The higher the porosity of dolomite rock, the higher the ankerite content. The crystal cell parameters of the Mesozoic dolomite and ankerite in this region are large, suggesting that they are rich in calcium and iron. The order degree of the dolomite-ankerite in well Xiyong 2 is greatly affected by Fe^(2+) content and the increase in Fe^(2+) will reduce its order degree. Therefore, it is not appropriate to suppose that the dolomite in the well Xiyong 2 of Xisha is simply formed by penecontemporaneous dolomitization. There was a great deal of volcanic activity in the Xisha Islands during Late Miocene, and the magmatic hydrothermal fluid may provide enough Fe^(2+) and heat for the formation of ankerite. In the late diagenetic period, the first member of Huangliu Formation with good porosity was greatly affected by volcanic activities. The heat and rich Fe^(2+) input promoted the transformation of dolomite to ankerite,and a large number of ankerite was then formed. However, in the second Member of Huangliu Formation with poor porosity there formed only ferroan dolomites.
作者
沈江远
闫琢玉
付和平
马骁
张海洋
赵强
许红
SHENG Jiangyuan;YAN Zhuoyu;FU Heping;MA Xiao;ZHANG Haiyang;ZHAO Qiang;XU Hong(Yangtze University,Wuhan 430100,China;First Institute of Oceanography,Ministry of Natural Resources,Qingdao 266061,China;CNOOC EnerTech-Drilling and Production Co.,Zhanjiang 524057,China;Qingdao Institute of Marine Geology,China Geological Survey,Qingdao 266071,China;China University of Petroleum(East China),Qingdao 266580,China;Baoding Center Seismic Station,Hebei Earthquake Agency,Baoding 071000,China)
出处
《海洋地质前沿》
CSCD
北大核心
2021年第6期39-48,共10页
Marine Geology Frontiers
基金
国家自然科学基金(41872114)
国家科技基础资源调查专项课题(2017FY201407)。
关键词
西永2井
铁白云岩
X射线衍射
有序度
成因分析
well Xiyong 2
ankerite
X-ray diffraction
degree of order
genesis analysis
