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The behavior of REEs indicates the dolomite petrogenesis and a unique dolomitization model of the Middle Permian Qixia formation in Northwest Sichuan Basin, China
Carbonates and Evaporites ( IF 1.4 ) Pub Date : 2022-04-15 , DOI: 10.1007/s13146-022-00765-6
Shengyang Xie , Xingzhi Wang , Bo Li , Long Wen , Liang Xu , Yao Du , Lin Xie , Yong Li , Yaping Wang , Yawen Li

The study on dolomite characteristics and its forming mechanism is of great significance for dolomitization theory and dolomite reservoir exploration. In recent years, the exploration of dolomite reservoirs in the Sichuan Basin has reached a bottleneck state. This study takes the Qixia Formation, Middle Permian in Northwest Sichuan basin as an example, the characteristics, distribution, diagenetic fluid, diagenetic environment of dolomite, and the dolomite formation mechanism were analyzed by means of observation and sampling of outcrops and cores, electronic scanning microscopy (SEM), and measurements of rare-earth elements (REEs), carbon isotopes, oxygen isotopes, and strontium isotopes. The results show that the dolomite exists in two forms, P-dol (dolomite in the dolomitic part of the mottled dolomitic limestone), and C-dol (crystalline dolomite). Core photos show that C-dol can be divided into two types: dolomite-A (dol-A) and dolomite-B (dol-B). The samples are low ∑REEs + Y, 13C (1.32‰–4.32‰), negative δ18O (− 5.87‰ to − 1.99‰) and δCe, and high in δEu and 87Sr/86Sr ratio (0.70724–0.70923). The partition pattern of REEs in the samples shows seawater characteristics of LREE loss, La enrichment, Ce weak loss, and Gd enrichment. These characteristics indicate that the diagenetic fluid of dolomite in the study area was a hybrid of seawater and freshwater and the diagenetic environment was at relatively high temperature. Moreover, after extending the “Mg cage effect” theory proposed by Folk, it is found that Mg2+ removed from sediments that were flushed by fresh water during the syndiagenetic stage facilitate the formation of dolomites. In accordance with the geological setting, a unique dolomitization model of the Middle Permian Qixia formation in the northwestern Sichuan Basin was built. The stratum in this model can be divided into four zones from top to bottom: freshwater vadose zone, mixed vadose zone, mixed phreatic zone, and seawater phreatic zone. The mottled dolomitic limestone is developed in the mixed vadose zone and the crystalline dolomite in the mixed phreatic zone. This study provides important evidences for the formation mechanism of dolomite and a reference to the study of ancient dolomite genesis and reservoirs of the Middle Permian Qixia formation in the northwest Sichuan Basin.



中文翻译:

REEs行为表明川西北地区中二叠统栖霞组白云岩成因和独特的白云石化模式

白云岩特征及其形成机制的研究对白云岩化理论和白云岩储层勘探具有重要意义。近年来,四川盆地白云岩储层勘探已进入瓶颈状态。本研究以川西北盆地中二叠统栖霞组为例,通过露头和岩心观测取样、电子扫描等手段,分析了白云岩的特征、分布、成岩流体、成岩环境以及白云岩的形成机制。显微镜 (SEM),以及稀土元素 (REE)、碳同位素、氧同位素和锶同位素的测量。结果表明,白云石以两种形态存在,P-dol(斑驳白云质灰岩中白云岩部分的白云石)、和 C-dol(结晶白云石)。核心照片显示,C-dol可分为白云石-A(dol-A)和白云石-B(dol-B)两种。样本为低 ∑REEs + Y,13 C (1.32‰–4.32‰)、负δ 18 O (- 5.87‰ to - 1.99‰) 和 δCe, 高 δEu 和87 Sr/ 86 Sr 比值 (0.70724–0.70923)。样品中稀土元素的分配模式表现出LREE损失、La富集、Ce弱损失和Gd富集的海水特征。这些特征表明,研究区白云岩成岩流体为海水和淡水混合体,成岩环境处于较高温度。此外,在扩展 Folk 提出的“Mg 笼效应”理论后,发现 Mg 2+在同成岩阶段从被淡水冲刷的沉积物中去除,促进了白云岩的形成。根据地质背景,构建了川西北地区中二叠统栖霞组独特的白云石化模式。该模型中的地层从上到下可分为四个区域:淡水包气带、混合包气带、混合潜水带和海水潜水带。混合渗流带发育斑驳白云质灰岩,混合潜水带发育结晶白云岩。该研究为川西北地区中二叠统栖霞组古白云岩成因及储层研究提供了重要依据,为白云岩的形成机制提供了参考。

更新日期:2022-04-15
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