Diagenetic study on the Neogene sedimentary basin as paleoenvironmental proxy data for an offshore CO2 storage project in Pohang Basin, South Korea

Abstract

As the importance of reducing greenhouse gas emissions has attracted much attention, Pohang Basin has been identified as a CO2 geological sequestration test site in Korea. This study aims to provide a better understanding of the diagenetic history of Pohang Basin through mineralogical and microstructural analyses, with a focus on tectonic evolution in the reservoirs offshore of the Pohang Basin during the East Sea (Sea of Japan) opening in the Miocene. We focused on two sections in the core that are considered to be target layers for CO2 storage. Samples were analyzed for texture observation and chemical analysis. The major mineral components include quartz, feldspar, illite, chlorite, kaolinite and calcite, and the minor mineral components include pyrite, siderite, iron oxide, and titanium oxide. The detrital grains are composed of quartz and feldspar, and the primary pore is cemented with kaolinite (I), calcite, pyrite and chlorite. The analyzed chlorites are differentiated into five types by their textural characteristics, such as pore-filling (PF), pore-filling with kaolinite (PF-k), pore-lining (PL), replaced (RD), and detrital (DA) chlorites, and they were all categorized as either brunsvigite, diabantite, or ripidolite. Generally, the dissolution of feldspar is observed for all samples. The diagenetic temperature is estimated by chlorite geothermometry to be 170 °C on average, which is higher than the present temperature of Pohang Basin (55–60 °C). Considering all results together, the target layers have undergone diagenetic processes from early diagenesis (eogenesis) to late diagenesis (mesogenesis) and uplift-related diagenesis (telogenesis). After sedimentation of the Pohang Basin, kaolinite (I), calcite, and pyrite were cemented in the pore, and detrital mica was replaced with chlorite. As the burial depth increased, the dissolution of feldspar advanced. The high temperature resulting from the chlorite geothermometry could be influenced by the burial heat as source. Also, the Neogene volcanism and related tectonics (known as Eoil Orogeny) could become a heat source capable of high temperatures, and a high geothermal gradient and tectonically compressional environment may have influenced fluid flow. After the uplift, kaolinite (II) was precipitated in both an intergranular pore and intragranular pore as a result of dissolution. The effects of diagenesis, such as cementation and dissolution, in CO2 reservoir rocks, could be an important factor in assessing the appropriacy of predicting potential charge after injection in carbon capture and storage project.