Distribution and genesis of the Maokou Formation dolomite in Fengdu-Shizhu area, eastern Sichuan Basin

Abstract

To investigate the development laws and genesis of the Maokou Formation dolomite in the Fengdu-Shizhu area, we analyzed core, thin section, logging, and geochemical data, and obtained the following understandings: 1. The Maokou Formation dolomite includes layered granular dolomite and leopard-spotted limy dolomite which are fine to medium, and moderately euhedral, and have intergranular pores and intergranular dissolved pores; 2. Vertically, the dolomite is the superimposition of multiple stages, 3–12 m a layer and cumulatively up to 30 m. The distribution of the dolomite is controlled by sedimentary cycles and is commonly found in the middle and upper parts of the cycle; 3. The analysis of rare earth distribution and carbon, oxygen, and strontium isotopes indicates that the dolomitizing fluid is the penecontemporaneous saline seawater, and some pore edges were affected by later hydrothermal dolomitization, resulting in recrystallization of dolomite and cementation of saddle-like dolomite; 4. The relationship between the plane distribution of the dolomite and the paleogeographic pattern during the sedimentary period indicates that the dolomite is concentrated in the granular shoals near geomorphic highlands and slope break zones. In summary, it is proposed that the overlap and migration of granular shoals and isolated seawater promoted the occurrence of reflux infiltration of dolomitizing fluid and dolomitization. Multistage granular shoals on the platform margin provide a good material foundation for the development of dolomite. Karstification is conducive to the occurrence of early dolomitization within the shoals and the preservation of pores. It is found that the early dolomite of the Maokou Formation is best developed in the highlands of faults 15 and 16. The basement faults controlled the sedimentary paleogeomorphology, thereby restricting the distribution of dolomite. This understanding provides a new idea for the exploration of dolomite in the Maokou Formation in the Sichuan Basin.