Abstract: Recent breakthroughs in the exploration of dolomite reservoirs in the Middle Permian Maokou Formation of Sichuan Basin have sparked debates over the types and genesis of the dolomite. This study focuses on the dolomite from typical wells and outcrop sections in the Member 2 of Maokou Formation in western Sichuan Basin. By employing techniques such as rock thin-section identification, major/trace element analysis, carbon/oxygen/strontium isotope measurements, rare earth element (REE)analysis, and fluid inclusion thermometry, the paper classifies the dolomite types in the study area and establishes two categories and three phases of dolomitization modes. The results indicate that based on occurrence and structure, the dolomite in the Member 2 of Maokou Formation in western Sichuan can be classified into the silty crystalline dolomite with limestones in laminated/spotted patterns, silty crystalline dolomite with siliceous nodules/bands, thin-layered silty crystalline dolomite, medium-layered fine crystalline dolomite, thick-layered massive fine-medium crystalline dolomite, and medium-coarse crystalline dolomite. For the silty crystalline dolomite with limestone, silty crystalline dolomite with siliceous materials, and thin-layered silty crystalline dolomite, δ¹³C and δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios are consistent with those of the Middle Permian seawater, positive Ce anomalies are observed in REEs, and the fluid inclusion homogenization temperatures are in the range between 111.0 ℃ and 136.9 ℃. Compared with sea water in the same stage, medium-layered silty crystalline dolomite and thick-layered massive fine-medium crystalline dolomite display larger negative δ¹⁸O values, higher ⁸⁷Sr/⁸⁶Sr ratios, and more obvious positive Ce anomalies, and the fluid inclusion homogenization temperatures are in the range of 121.9 ℃ to 147.6 ℃. The medium- coarse crystalline dolomite show more significantly negative δ¹³C and δ¹⁸O values, markedly higher ⁸⁷Sr/⁸⁶Sr ratios, more obvious positive Eu anomalies, and the fluid inclusion homogenization temperatures are in the range of 153.2 ℃ to 189.2 ℃. The dolomite of the Member 2 of Middle Permian Maokou Formation in western Sichuan is originated from dolomitization caused by diagenetic transformation water of the facies-controlled sepiolite, dolomite was mainly developed in high-energy grain shoal facies areas, and different types of dolomites were formed from diagenetic transformation water released by sepiolite at different diagenetic transformation stages. The aforementioned three types of silty crystalline dolomites were formed from minor diagenetic transformation waters released during the early stages of sepiolite diagenesis. The two types of fine-medium crystalline dolomites formed from abundant diagenetic transformation water was released during the peak period of sepiolite diagenesis. The medium-coarse crystalline dolomite was formed from diagenetic transformation water altered by hydrothermal activity during tectonic-hydrothermal events. The dolomitization mode caused by diagenetic transformation water of facies-controlled sepiolite provides a reference for predicting dolomite reservoirs in Maokou Formation.

Key words: western Sichuan Basin, Maokou Formation, dolomitization, diagenetic transformation water of sepiolite, geochemical characteristics

摘要： 近期,四川盆地中二叠统茅口组白云岩储层勘探不断取得突破,但其白云岩的类型与成因备受争议。以川西地区典型钻井和露头剖面的茅口组二段(茅二段)白云岩为研究对象,基于岩石薄片鉴定,主量/微量元素、碳/氧/锶同位素和稀土元素测定及流体包裹体测温等技术分析,划分出白云岩类型,建立了2类3期白云石化作用模式。研究结果表明:根据产状和结构,川西地区茅二段可分为与灰岩呈层状/斑状伴生的粉晶白云岩、与硅质结核/硅质条带伴生的粉晶白云岩、薄层状粉晶白云岩、中层状细晶白云岩、厚层块状细晶—中晶白云岩和中晶—粗晶白云岩。与灰岩伴生的粉晶白云岩、与硅质伴生的粉晶白云岩和薄层状粉晶白云岩的δ¹³C、δ¹⁸O、⁸⁷Sr/⁸⁶Sr比值处于中二叠世海水范围内,稀土元素具有Ce正异常,流体包裹体均一温度为111.0~136.9℃;相较于同期海水,中层状细晶白云岩、厚层块状细晶—中晶白云岩的δ¹⁸O偏负,⁸⁷Sr/⁸⁶Sr比值偏高,稀土元素具有Ce正异常,流体包裹体均一温度为121.9~147.6℃;中晶—粗晶白云岩的δ¹³C、δ¹⁸O显著偏负,⁸⁷Sr/⁸⁶Sr比值显著偏高,稀土元素具有明显Eu正异常,流体包裹体均一温度为153.2~189.2℃。川西地区中二叠统茅二段白云岩为相控型海泡石成岩转化水白云石化作用成因,高能颗粒滩相区是白云石化作用的主要发育区域,不同类型的白云岩形成于海泡石不同成岩转化阶段释放的成岩转化水。前述3类粉晶白云岩形成于海泡石成岩转化初期释放的少量成岩转化水,2类细晶—中晶白云岩形成于海泡石成岩转化高峰期释放的足量成岩转化水,中晶—粗晶白云岩形成于构造-热液活动期受热液改造的成岩转化水。相控型海泡石成岩转化水白云石化作用模式可为茅口组白云岩储层的预测提供参考。

关键词: 川西地区, 茅口组, 白云石化作用, 海泡石成岩转化水, 地球化学特征