Acta Petrolei Sinica ›› 2017, Vol. 38 ›› Issue (4): 386-398.DOI: 10.7623/syxb201704003

• Petroleum Exploration • Previous Articles     Next Articles

Characteristics and genetic mechanism of the dolomite in the Middle Permian Maokou Formation, central Sichuan area

Liu Jianqiang1,2, Zheng Haofu1,2, Liu Bo2, Liu Hongguang1,2, Shi Kaibo1,2, Guo Rongtao1,2, Zhang Xuefeng2   

  1. 1. School of Earth and Space Sciences, Peking University, Beijing 100871, China;
    2. Institute of Oil & Gas, Peking University, Beijing 100871, China
  • Received:2016-09-14 Revised:2017-01-22 Online:2017-04-25 Published:2017-05-04

川中地区中二叠统茅口组白云岩特征及成因机理

刘建强1,2, 郑浩夫1,2, 刘波2, 刘红光1,2, 石开波1,2, 郭荣涛1,2, 张学丰2   

  1. 1. 北京大学地球与空间科学学院 北京 100871;
    2. 北京大学石油与天然气研究中心 北京 100871
  • 通讯作者: 刘建强,男,1986年10月生,2009年获西南石油大学学士学位,现为北京大学地球与空间科学学院博士研究生,主要从事碳酸盐岩沉积-成岩作用、油气储层地质学研究。Email:liujq86@163.com
  • 作者简介:刘建强,男,1986年10月生,2009年获西南石油大学学士学位,现为北京大学地球与空间科学学院博士研究生,主要从事碳酸盐岩沉积-成岩作用、油气储层地质学研究。Email:liujq86@163.com
  • 基金资助:

    国家自然科学基金项目(No.41672123,No.41272137)资助。

Abstract:

The dolomites in Maokou Formation, central Sichuan area are mainly developed in the mid-upper Member 2 and the lower Member 3 of Maokou Formation, which can be divided into fine-medium calcareous dolomite, fine-medium crystalline dolomite and brecciaous dolomite according to petrological characteristics, and the latter two are dominant. As viewed from the contrast in geochemical characteristics between micritic limestone and other types of dolomite samples, dolomites have lower Fe content, higher Mn content and lower Sr content than other limestone at the same layer. The micritic limestone and different types of dolomite present similar rare earth element (REE) distribution patterns with depletion of light REE , weak positive anomaly of La and negative anomaly of Ce, and dolomite sample also has weak positive anomaly of Eu. Fine-medium dolomite and brecciaous dolomite have lower δ18O values than primitive limestone, and obviously higher 87Sr/86Sr ratios than primitive micritic limestone. The similar REE distribution patterns between limestone and different types of dolomite indicate that some geochemical characters of primitive limestone are preserved during the dolomitization process in Maokou Formation, while the difference between the weak positive anomaly of Eu in dolomite and the limestone isotopes may be caused by the participation of high-temperature geo-hydrotherm during the dolomitization process. The dolomitization of Maokou Formation is dominated by multiple factors as follows: (1) the stratigraphic residual seawater and hydrotherm associated with magmatism are the major sources of Mg in dolomitization;(2) the thermal effect of Emeishan large igneous province provides abnormally high paleo-geothermal context for dolomitization;(3) the fault systems offer rapid migration channels for abnormal geothermal convection and dolomitization fluid;(4) the limestone with grain beach facies possesses favorable porosity and permeability, and thus have a higher possibility of dolomitization than micritic limestone. The abnormal geotherm associated with Emeishan Large igneous province in and abundant Mg content are the two key factors for dolomitization in Maokou Formation, and different types of dolomites were formed in different phases of dolomization process.

Key words: genetic mechanism, geochemical characteristics, dolomite, Maokou Formation, central Sichuan area

摘要:

川中地区茅口组白云岩主要位于茅口组二段中、上部和茅口组三段下部,根据岩石学特征可分为含灰质细-中晶白云岩、细-中晶白云岩、角砾白云岩,且以后两者为主。对比泥晶灰岩与不同类型白云岩样品地球化学特征后发现,白云岩相对于同层灰岩具有较低的Fe含量、较高的Mn含量和较低的Sr含量;泥晶灰岩与不同类型白云岩具有相似的稀土配分特征,均表现为LREE亏损、轻微正La异常和负Ce异常,且白云岩样品具有微弱的Eu正异常;细-中晶白云岩与角砾白云岩δ18O值均明显低于原始灰岩,而87Sr/86Sr比值则明显高于原始泥晶灰岩。不同类型白云岩与灰岩相似的稀土配分特征表明茅口组白云岩形成过程中保留了原始灰岩的部分地球化学特征,而白云岩微弱Eu正异常及其与灰岩同位素差异性可能是由于白云岩化过程中高温地质热流体的参与而引发的。茅口组白云岩化作用受多种因素控制:地层残余海水和岩浆活动伴生的热液流体为白云岩化作用中Mg的主要来源;峨眉山大火成岩省热效应为白云岩化提供异常高的古地温背景;断裂系统为异常地温对流和白云岩化流体提供了快速运移通道;颗粒滩相灰岩较好的孔、渗性使之比泥晶灰岩更易发生白云岩化。峨眉山大火成岩省伴生的异常地温和充足Mg供给是茅口组发生白云岩化的关键,不同类型白云岩形成于白云岩化过程的不同阶段。

关键词: 成因机理, 地球化学特征, 白云岩, 茅口组, 川中地区

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