Changes in source rock and reservoir properties of biosiliceous mudstone due to the migration of biogenic silica during burial diagenesis.

Diatomaceous deposits are mixtures of diatom tests (biogenic silica)and argillaceous materials. Biogenic silica migrates in the deposits as its mineral phase transforms during burial diagenesis, concentrating and diluting organic matter and argillaceous materials, and changing pore properties. The migration is considered through a simple burial diagenesis model. As a result, analytical data related to various properties including the properties of source and reservoir rocks of the Onnagawa Formation, are explained.The migration of biogenic silica during opal-A / CT transformation emphasizes a slight compositional difference in diatomaceous deposits and forms hard (opal-CT porcelanite)-soft (opal-CT siliceous mudstone) alternating beds. The silica flows from the proto-siliceous mudstone into the proto-porcelanite, which concentrates the argillaceous materials and organic matter in the siliceous mudstone, improving its source rock potential. In addition, due to the migration, the compositional difference is emphasized even in proto-porcelanite (hard), and light-dark colored porcelanite alternating beds are formed and become distinct. Conversely, during the opal-CT / Quartz transformation, the silica flows from the light-colored porcelanite into the dark-colored porcelanite, and the porosity of light-colored porcelanite is increased, and that of dark-colored porcelanite is decreased and chertified. Since compaction during opal-CT / Quartz transformation is more advanced than during opal-A / CT transformation, the increased porosity tends to be maintained. This phenomenon forms a double porosity reservoir in which light-colored porcelanite with high porosity and dark-colored porcelanite with dense fractures, alternate with each other.The improvement of source and reservoir rock properties is promoted as biogenic silica migration increases. Therefore, better source and reservoir rocks are formed adjacent to each other by the diagenesis of diatomaceous deposits with more primary diatom test contents. The primary diatom test contents are estimated from the Al₂O₃/TiO₂ when chemical compositions of diatom tests and argillaceous materials are properly estimated. [ABSTRACT FROM AUTHOR]