渤海海域石臼坨地区古近系沙河街组湖相生屑白云岩成因*

渤海海域石臼坨地区石臼坨凸起东倾末端南北两侧的陡坡带在古近系沙河街组发育多套生屑白云岩,其中南侧生屑白云岩主要发育在沙河街组二段,北侧生屑白云岩集中在沙河街组一段。为了明确该区生屑白云岩的成因,通过古生物、岩石薄片和地球化学等资料对该区生屑白云岩的岩石学、地球化学特征进行较为系统的研究。按照岩石组分特征,研究区生屑白云岩可划分为亮晶生屑白云岩和泥晶生屑白云岩2种类型。生屑的主要组分腹足类及介形类典型属种分析显示,南北两侧生屑白云岩均形成于浅水湖泊环境。地球化学Pr/Ph(姥鲛烷/植烷)参数反映,南侧生屑白云岩形成于氧化环境,北侧以还原性环境为主。碳氧同位素数据反映南侧生屑白云岩形成于低温高盐的封闭湖泊环境,以准同生期的回流渗透成因为主。北侧生屑白云岩的碳氧同位素相对分散,显示为受不同流体介质作用的结果,可以划分出3种成因类型: 准同生期的回流渗透成因、埋藏期的铁白云石化交代成因和晚期局部受热液作用影响,因此,北侧生屑白云岩是多期次多成因综合作用的结果。该研究成果对开展这类中深层优质储集层预测具有重要意义。

Origin of lacustrine bioclastic dolostone in the Paleogene Shahejie Formation: A case study in Shijiutuo area,Bohai Sea

There are several sets of bioclastic dolostone in the Paleogene Shahejie Formation on the north and south of the eastern margin of Shijiutuo uplift,Shijiutuo area, Bohai Sea. The bioclastic dolostone on the south of the eastern margin of Shijiutuo uplift is mainly developed in the Member 2 of Shahejie Formation,while that on the north is mainly developed in the Member 1 of Shahejie Formation. Based on paleontology,thin sections,geochemical data,the petrological and geochemical characteristic are analyzed for the genetic mechanism of the widely developed bioclastic dolostone in this area. According to the rock composition of the bioclastic dolostone,they can be classified into sparry bioclastic dolostone and micrite bioclastic dolostone. Gastropods and ostracodes in the bioclastic dolostone indicate that the bioclastic dolostone on the north and south of Shijiutuo uplift were formed in shallow lake. Pr/Ph values indicate that the bioclastic dolostone in the south formed in an oxidizing environment,while in the north formed in a reducing environment. The carbon and oxygen isotopic values show that the bioclastic dolostone in the south formed in a closed lake with low-temperature and high-salinity environment,and was the result of seepage-reflux dolomitization during the penecontemporaneous period. The compostion of carbon and oxygen isotopes fluctuate greatly in the north. This indicates the bioclastic dolostone was affected by different fluid media in the north. The process of dolomitization in the north can be divided into three stages: seepage-reflux dolomitization during penecontemporaneous period,the isomorphous replacement during burial diagenesis period and hydrothermal action during the late period. It was the result of multi-stage and multi-genesis in the north. This study is of great significance for similar high-quality reservoir prediction.