川中北部地区茅口组白云岩成因分析——来自岩石学、原位地球化学及年代学证据

Analysis of dolomite genesis in the Maokou Formation, northern slope area of centralSichuan, China:Petrologic, in situ geochemical, and chronological evidence

近期川中北部地区茅口组白云岩储层勘探获重大突破。为探讨白云岩储层成因,通过岩心观察、薄片镜下观察、阴极发光、激光原位主微量元素分析和U⁃Pb同位素定年等技术手段,对川中北斜坡地区茅口组白云岩储层进行了系统的地球化学特征研究,建立了研究区茅口组白云岩的成岩序列,并厘定了各类型白云岩的成岩流体特征,最终建立了研究区茅口组白云石化模式。主要认识有:①研究区主要发育生屑白云岩、晶粒白云岩和鞍状白云岩3种类型白云岩,根据岩石学特征和地球化学特征建立的成岩序列为第一世代生屑白云岩(D1⁃1)→第二世代它形细晶白云岩(D2⁃1)→第三世代非典型鞍状白云岩(SD1)→第四世代典型鞍状白云岩(SD2)→第五世代砂糖状白云岩(D2⁃2)。其中残余生屑白云岩先后受到第二世代它形细晶白云岩和第三世代非典型鞍状白云岩的影响所形成。②白云岩的地球化学特征均显示出明显的Ce负异常和Y正异常特征,表明各类型白云岩受到海源流体及其衍生流体不同程度的影响。同时各类型白云岩甚至生屑灰岩的部分样品存在δEu值正异常,表明存在后期构造—热液作用。③在同生期—中深埋藏期先后发育5期白云石化作用,在同生期—准同生期先后发生第一期白云石化作用和第二期白云石化作用,形成生屑白云岩和它形细晶白云岩等白云岩基质;在准同生期—浅埋藏期受构造作用的影响发生2期构造—热液白云石化作用,形成鞍状白云岩,并交代部分基质白云石;进入中—深埋藏期的第五期白云石化作用交代生屑白云岩和它形细晶白云岩形成大量砂糖状白云岩。④第一期白云石化作用和第二期白云石化作用由于发生时间很早,有利于原生孔隙的保存,第三期白云岩化作用和第四期白云石化作用属于典型的构造—热液白云石化作用,对白云岩储层的孔隙建造和破坏作用并重,而第五期埋藏期白云石化作用是在封闭体系中发生的,是研究区白云岩储层形成的最重要的成岩作用。

Recently,significant breakthroughs have been made in the exploration of dolomite reservoirs in the Maokou Formation in the northern slope area of central Sichuan.In order to explore the genesis of dolomite res⁃ervoirs,the author used techniques such as core observation,thin section microscopy,cathodoluminescence,la⁃ser in-situ analysis of major and trace elements,and U⁃Pb isotope dating to systematically study the geochemi⁃cal characteristics of the Maokou Formation dolomite reservoirs in the northern slope area of central Sichuan,and established the diagenetic sequence of the Maokou Formation dolomite in the study area.And the diagenetic fluid characteristics of various types of dolomite were determined,ultimately establishing a dolomitization model for the Maokou Formation in the study area.The main understanding includes:(1)Three types of dolomite are mainly developed in the study area,namely,biotite dolomite,crystalline dolomite and saddle dolomite,and the diagenetic sequence based on petrological and geochemical characteristics is as follows:the first generation of D1-1→the second generation of D2-1→the third generation of SD1→the fourth generation of SD2→the fifth generation of D2-2.The D1-2 was successively formed by the influence of the second-generation D2-1 and the third-generation SD1.(2)The geochemical characteristics of the dolomites show obvious Ce-negative and Y-positive anomalies,indicating that each type of dolomite has been affected by the sea-derived fluids and their derivatives to different degrees.At the same time,the positiveδEu anomalies in some samples of each type of dolomite and even clastic tuffs indicate the existence of late tectonic-hydrothermal effects.(3)Five phases of do⁃lomitization were developed successively in the syn-and medium-deep burial periods,and the first and second phases occurred successively in the syn-and quasi-syncline periods,resulting in the formation of the dolomite matrix,such as clastic dolomite and its fine-crystal dolomite,etc.;the tectonic effects were influenced by the tectonic effect in the quasi-and shallow burial periods;and the dolomites in the quasi-and shallow burial periods were influenced by the tectonic effect.In the quasi-symbiotic period,two phases of tectonic-hydrothermal dolo⁃mitization occurred under the influence of tectonics,forming saddle-shaped dolomite and accounting for part of the matrix dolomite;the fifth phase of dolomitization in the middle-deep burial period accounted for the forma⁃tion of a large number of trachyte dolomite and other fine-crystalline dolomite.(4)Phase I dolomitization and Phase II dolomitization are beneficial to the preservation of primary pore space because of their early occurrence.Phase III dolomitization and Phase IV dolomitization are typical tectonic-hydrothermal dolomitization,which have both constructive and destructive effects on the pore space of dolomite reservoirs,and Phase V dolomitiza⁃tion of the buried period occurs in a closed system,which is the most important diagenesis for the formation of the dolomite reservoirs in the study area.