川东北阆中地区二叠系茅口组白云岩化成因

川东北阆中地区茅二段有效储集岩类主要为白云岩,对其成因研究较少,还未有统一认识。结合区域地质背景,通过岩石薄片、阴极发光、碳氧同位素、锶同位素、包裹体等定量分析,研究了阆中地区茅二段白云岩岩石学特征及地球化学特征,探讨了其成因及意义。研究认为:研究区白云岩以细—中晶白云岩为主,晶面较平直,以自形—半自形晶为主,成岩环境温度不高;研究区细—中晶白云岩阴极发光以暗色光为主,具δ^13C低正值、δ^18O高负值、^87Sr/^86Sr略高于同时期海水的特征,表明其形成于浅埋藏环境,受高温流体作用影响不明显;白云岩化流体来自于浅埋藏期海水,以粒屑滩颗粒灰岩为载体,形成层状、似层状白云岩。浅埋藏期白云岩化作用形成大规模储集空间,是碳酸盐岩储集层最重要的成岩方式之一。

Genesis of Dolomitization of Permian Maokou Formation in Langzhong Area,Northeastern Sichuan Basin

As an important oil and gas reservoir in Langzhong area of northeastern Sichuan basin, the second member of Permian Maokou formation is primarily composed of dolomite. There is no general understanding on dolomite genesis due to limited research literatures. Based on thin section authentication, cathodoluminescence analysis, quantitative analysis on inclusions and isotopes of carbon, oxygen and strontium, and combined with regional geological setting, the paper studies the petrologic and geochemical features of the dolomite in the second member of Maokou formation in Langzhong area, and discusses the genesis and significance of the dolomite. The results show that the dolomite in the study area is dominated by fine-medium crystalline, automorphic-semi-automorphic dolomite with relatively flat surface, which indicates the temperature of diagenetic environment was not high. The fine-medium crystalline dolomite in the study area is characterized by dark cathodoluminescence, low positive value of δ ¹³C and high negative value of δ ¹⁸O, ⁸⁷Sr/ ⁸⁶Sr slightly higher than that of the seawater of the same period, indicating a shallow burial and unobvious influences of high-temperature fluid. The fluid in the dolomitized rocks came from the seawater of the shallow-burial period, took grain-limestone as a carrier and then laminar and layered dolomite formed. Large scale reservoir spaces formed due to the dolomitization in the shallow-burial period, which is one of the most important diagenetic modes of carbonate reservoirs.