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

Abstract

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.

Keywords

• geochemical characteristics
• genetic mechanism
• dolomite
• maokou formation
• middle permian
• sichuan basin