基于镁同位素的规模埋藏白云岩形成过程——以塔里木盆地蓬莱坝组为例  

Magnesium isotope-based forming process of large sized burial dolomite:A case study of the Penglaiba Formation in Tarim basin

作　　者：乔占峰[1,2] 沈安江 梁峰[1,2] 邵冠铭 张天付[1,2] 罗宪婴 孙晓伟 董建华[3] QIAO Zhanfeng;SHEN Anjiang;LIANG Feng;SHAO Guanming;ZHANG Tianfu;LUO Xianying;SUN Xiaowei;DONG Jianhua(PetroChina Hangzhou Research Institute of Geology,Hangzhou,Zhejiang 310023,China;Key Laboratory of Carbonate Reservoir,CNPC,Hangzhou,Zhejiang 310023,China;China University of Petroleum(Beijing),Beijing 102249,China)

机构地区：[1]中国石油天然气股份有限公司杭州地质研究院,浙江杭州310023 [2]中国石油天然气集团有限公司碳酸盐岩储层重点实验室,浙江杭州310023 [3]中国石油大学(北京),北京102249

出　　处：《地质学报》2023年第7期2293-2310,共18页Acta Geologica Sinica

基　　金：中国石油天然气股份有限公司基础性前瞻性科技项目(编号2021DJ0503)资助的成果。

摘　　要：地下富镁流体运移机制问题,制约着规模埋藏白云岩形成过程的认识,一直是白云岩成因中地质学家争论的焦点。以塔里木盆地蓬莱坝组埋藏白云岩为研究对象,在岩石学和碳氧同位素、锶同位素与锶元素含量等常规地球化学分析的基础上,通过镁同位素分析,认识了规模埋藏白云岩的形成过程。分析显示,蓬莱坝组不同类型白云岩的镁同位素分布虽有重叠,但是差异明显,藻纹层白云岩镁同位素为-2.34‰~-2.02‰;细中晶残余颗粒白云岩的镁同位素分布范围较广,为-2.24‰~-1.66‰,平均为-2.04‰;粗晶白云岩的镁同位素主要集中在-2.24‰~-1.89,平均为-2.05‰;蓬莱坝组灰岩镁同位素为-3.63‰和-2.82‰,较白云岩明显更偏负。镁同位素与氧同位素、锶同位素和锶元素含量在高频旋回中表现出规律性的旋回变化,对应三种白云岩形成过程:向上变轻-渗透回流叠加埋藏云化、向上变重-蒸发泵叠加埋藏云化和向上变重-埋藏云化模式。进一步,通过白云石晶体变化与锶元素含量变化明确了埋藏云化流体的侧向运移和白云石晶体响应规律。认识到厚层白云岩是由多期白云石化作用叠加而成,既有层内云化流体也受源外云化流体影响,受沉积相和构造埋藏演化史共同控制,海平面波动下大量叠置发育的准同生白云岩是规模埋藏白云岩形成的关键。特别是,基质孔发育的渗透回流型准同生白云岩,在高频层序格架下占比越高、越频繁,越有利于埋藏云化的顺层渗透扩散,进而形成规模埋藏白云岩。The formation of large-scale burial dolomite,especially the migration mechanism of magnesium-enriched fluid,has always been the focus of debate among geologists in the genesis of dolomite.Considering the burial dolomite of Penglaiba Formation in Tarim basin,the formation mechanism of massive burial dolomite was studied by integrating magnesium isotope and petrology,conventional geochemical analysis of carbon and oxygen isotopes,strontium isotopes and strontium content.The analysis shows that although the magnesium isotope distribution of different types of dolomites in the Penglaiba Formation overlaps,the difference is obvious.The microbial dolomites have the magnesium isotope value of 2.34‰~2.02‰while those of fine to medium crystalline dolomite range from 2.24‰to 1.66‰,with an average of 2.04‰.The magnesium isotopes of coarse crystalline dolomite are concentrated in 2.24‰~1.89‰range,with an average value of 2.21‰.In contrast,the magnesium isotope of limestone of the Penglaiba Formation ranges between 3.63‰to 2.82‰,which is obviously more negative than that of dolomite.The compositions of magnesium isotope,oxygen isotope,and strontium isotope and strontium contents show regular cyclic changes in high-frequency cycles,corresponding to three processes of dolomitization:upward lightening-reflux superimposed by burial dolomitization,upward weighting-Sabkha superimposed by burial dolomitization,and upward weighting-burial dolomitization.Furthermore,according to the lateral variation of dolomite crystal and strontium content within cycles,the response principle of dolomite crystal in the process of lateral migration of buried dolomitizing fluid is clarified.It is recognized that the massive dolomite is formed by the superposition of multi-stage dolomitization.There are both intralayer dolomitized fluids and exotic dolomitized fluids.It is jointly controlled by sedimentary facies and tectonic burial evolution history.A large number of stacking penecontemporaneous dolomites formed under sea level flu

关 键 词：规模白云岩 埋藏白云岩 镁同位素 塔里木盆地 蓬莱坝组 

分 类 号：P618.13[天文地球—矿床学]