The characteristics and genesis mechanism of sandstone concretions in the Middle Devonian Jinbaoshi Formation of the Northwestern Sichuan Basin

Key words: 
• Northwestern Sichuan Basin /
• Middle Devonian Jinbaoshi Formation /
• Quartz sandstone /
• Concretions /
• Genetic mechanism

Abstract: [Objective] The origin of concretions in sedimentary rocks can not only reveal the depositional environment but also serve as key evidence for studying diagenetic evolution, holding significant importance for research on pore evolution in sedimentary rocks. [Methods] This study focuses on the Devonian Jinbaoshi Formation in the Gediba section of northwestern Sichuan Basin. Through detailed analysis of rock thin sections, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), the characteristics of concretions in the Jinbaoshi Formation were examined, and their genetic mechanisms were explored. [Results] The quartz sandstone in the Jinbaoshi Formation of the Gediba section exhibits high maturity and well-developed pores, with numerous ellipsoidal to sub-spherical concretions distributed along the bedding planes, displaying brown to dark brown surfaces. Microscopic analysis reveals that the quartz grains within the concretions are consistent with the surrounding sandstone in terms of morphology, size, sorting, roundness, and texture. Compared to the quartz sandstone, the quartz grains within the concretions are sparsely arranged, primarily in point contact, and are predominantly cemented by silica, followed by iron and clay minerals. Residual pores locally filled with crude oil were observed. The concretion-bearing intervals exhibit small-scale dome structures and bioturbation, with filamentous microbial remnants and silicified bacterial colonies identified under SEM. [Conclusion] Comprehensive analysis suggests that the formation of concretions in the Jinbaoshi Formation of the Gediba section is closely related to microbial mats. Gases produced by the decomposition of organic matter migrated upward, forming dome structures on the sediment surface. When the microbial mats were too thin to produce sufficient gas for upward migration, localized sparse distribution of detrital grains occurred, accompanied by early siliceous and ferruginous cementation. These concretions resisted compaction during subsequent burial processes, maintaining their spherical morphology, and turned brown due to oxidation after being uplifted to the surface. This study establishes a genetic evolution model of the concretions, which can provide insights into the diagenetic evolution and pore structure adjustment of the quartz sandstone in the Jinbaoshi Formation, offering important geological significance for hydrocarbon exploration.