Abstract: To clarify the properties of diagenetic fluid, the evolution of sedimentary environment, the source of Mg-rich fluids, the migration paths of fluids is important to explain the genesis of dolomite and understand the formation process of dolomite. This paper is a case study of the Middle-Upper Cambrian Xixiangchi Formation in central Sichuan Basin. Based on the analyses of petrological characteristics, sedimentary cycles, trace elements as well as carbon, oxygen and magnesium isotopes, this paper systematically investigates the changes of sedimentary-diagenetic environment and the migration rule of Mg-rich fluids, and explores the genesis of dolomite in Xixiangchi Formation. The dolomitization fluid of Xixiangchi Formation is dominated by Cambrian contemporaneous seawater, and dolomitization occurred in the penecontemporaneous-shallow burial stage. The Xixiangchi Formation in central Sichuan Basin is generally dominated by shallow water, brackish water and saline water marine sediments under the warm and arid subtropical climate. Sabkha dolomitization and seepage-reflux dolomitization are dominant. There is a coupling relationship between the variation trend of δ²⁶Mg value of dolomite in Xixiangchi Formation and the sedimentary cycle. When Mg-rich seawater migrates vertically, near-source seepage-reflux dolomitization occurs, and the δ²⁶Mg of dolomite gradually increases downward; when Mg-rich seawater migrates laterally, far-source seepage-reflux dolomitization occurs, and the δ²⁶Mg of dolomite does not vary vertically significantly; Sabkha dolomitization mainly occurs in an evaporation and restriction environment, and the δ²⁶Mg of dolomite gradually decreases downward. The formation of dolomite in Xixiangchi Formation is mainly controlled by sea level changes. Thin-bedded dolomite can be formed in the low sea level period, and large-scale thick-bedded dolomite reservoir can be formed by layer-by-layer superimposition of multiple dolomitization in the high sea level period.

Key words: Mg isotope, genesis of dolomite, migration of dolomitization fluid, Xixiangchi Formation, Cambrian, Sichuan Basin

摘要： 明确成岩流体性质、沉积环境演化、富Mg流体来源、流体迁移路径等因素对揭示白云岩成因、理解白云石化过程具有重要意义。以川中地区中—上寒武统洗象池组为例,基于岩石学与沉积旋回分析及微量元素和碳、氧、镁同位素特征,系统分析了沉积-成岩环境的变迁和富Mg流体的迁移规律,探讨了洗象池组白云岩的成因。洗象池组白云石化流体主要为寒武纪的同期海水,白云石化作用发生于准同生—浅埋藏阶段。川中地区洗象池组整体为温暖、干旱亚热带气候背景下的浅水、半咸水—咸水海相沉积,白云石化类型以萨布哈白云石化和渗透回流白云石化为主。洗象池组白云岩δ²⁶Mg值变化趋势与沉积旋回存在耦合关系。当富Mg海水进行垂向迁移时,发生近源渗透回流白云石化作用,白云岩δ²⁶Mg向下逐渐增大;当富Mg海水进行侧向迁移时,发生远源渗透回流白云石化作用,白云岩δ²⁶Mg垂向上无明显变化;在蒸发局限环境中,主要发生萨布哈白云石化作用,白云岩δ²⁶Mg向下逐渐减小。洗象池组白云岩的形成主要受海平面变迁控制,低海平面时期形成薄层状白云岩,高海平面时期多种白云石化作用逐层叠加可形成大规模厚层状白云岩储层。

关键词: Mg同位素, 白云岩成因, 白云石化流体迁移, 洗象池组, 寒武系, 四川盆地