川东南地区中二叠统茅口组三段白云岩成因机理

Genetic mechanism of dolomite in third member of Maokou Formation in Middle Permian, southeastern Sichuan Basin

杨浩 潘磊 全力 李让彬 王广伟 高俊杰 周玲 徐芮

YANG Hao PAN Lei QUAN Li LI Rangbin WANG Guangwei GAO Junjie ZHOU Ling XU Rui

1. 中国石油大学(华东) 地球科学与技术学院, 山东 青岛 266580; 2. 中国石化 勘探分公司, 成都 610041

1. School of Geosciences, China University of Petroleum(East China), Qingdao, Shandong 266580, China; 2. SINOPEC Exploration Company, Chengdu, Sichuan 610041, China

白云岩是四川盆地海相层系最为重要的天然气储层类型之一，也是深层碳酸盐岩油气勘探的重点，白云岩成因机理的研究是预测深层优质白云岩储层分布的基础。通过川东南地区茅口组三段白云岩露头、岩心、薄片等资料，结合碳氧同位素、锶同位素、稀土元素等研究了白云岩的成因机理。川东南地区茅三段白云岩可分为层状粉晶白云岩、层状细—中晶白云岩以及鞍形白云石胶结物。层状细—中晶白云岩是最主要的白云岩类型，未受热液影响的该类型白云岩δ18OVPDB值为-7.04‰～-6.08‰，与泥晶灰岩相似，87Sr/86Sr比值为0.707 550～0.707 740，处于同期海水范围值内，主要成因为准同生—埋藏期封闭局限海水渗透回流白云岩化；受到热液影响的该类型白云岩δ18OVPDB值为-7.33‰～-7.11‰，与泥晶灰岩相比略微偏负，87Sr/86Sr比值为0.707 950～0.708 171，高于同期海水。层状粉晶白云岩未明显受到热液影响改造，其成因与上述未受热液影响的层状细—中晶白云岩一致。而溶洞、裂缝中的鞍形白云石胶结物则是热液流体直接沉淀或重结晶所形成，具有较高的87Sr/86Sr比值(均值0.707 970)、明显偏负的δ18OVPDB值(均值-7.32‰)以及Eu的正异常。从层状粉晶白云岩、层状细—中晶白云岩到鞍形白云石胶结物，呈现出一种受控于封闭局限海水到热液流体的梯度性变化，研究区内的白云岩是由多期次流体、多次成岩作用的结果。

Dolomite formation is a crucial aspect of natural gas reservoirs in the marine strata of the Sichuan Basin, particularly in the exploration of deep carbonate reserved oil and gas resources. Understanding the genetic mechanism of dolomite is essential for predicting the distribution of favorable dolomite reservoirs in deep formations. Taking the third member of the Maokou Formation in the southeastern Sichuan Basin as an example, present work systematically investigates the genetic mechanism of dolomite by analyzing geochemical characteristics such as carbon and oxygen isotopes, strontium isotopes and rare earth elements. The analysis is based on data from outcrops, core samples and thin sections. The results reveal that the dolomites of the third member of the Maokou Formation in the southeastern Sichuan Basin can be categorized into laminated fine-crystalline dolomite, laminated fine to medium-crystalline dolomite, and saddle dolomite cement. Among these, the laminated fine to medium-crystalline dolomite is the most common type. The δ18O values of this dolomite type (-7.04‰ to -6.08‰) appeared unaffected by hydrothermal fluids, and are similar to those of mud-crystal limestone. Additionally, the 87Sr/86Sr ratio (0.707 550 to 0.707 740) locate within the range of contemporaneous seawater, indicating a diagenesis process of quasi-syngenetic to burial-induced restricted marine water infiltration dolomitization. The δ18O values of hydrothermally influenced dolomite range from -7.33‰ to -7.11‰, showing slightly negative compared to those of mud-crystal limestone. These dolomites also showed relatively higher 87Sr/86Sr ratios (0.707 950 to 0.708 171) than those of contemporaneous seawater. The laminated fine-crystalline dolomite, which was not affected by hydrothermal fluids, shares a common genetic mechanism with the previously mentioned laminated fine to medium-crystalline dolomite. Saddle dolomite cement found in caves and fractures is either directly precipitated or recrystallized from hydrothermal fluids. This type of dolomite is characterized by higher 87Sr/86Sr ratios (with a mean value of 0.707 970), significantly negative δ18O values (with a mean value of -7.32‰), and positive Eu anomalies. A gradient change from laminated fine-crystalline dolomite to laminated fine to medium-crystalline dolomite and finally to saddle dolomite cement is observed, controlled by a transition from marine water to hydrothermal fluids. The dolomite in the study area is proposed to be the product of multi-stage fluid-induced diagenesis.

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https://doi.org/10.11781/sysydz202402311