The relationship between silica diagenesis and physical properties in the East/Japan Sea: ODP Legs 127/128

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1 School of Ocean and Earth sciences The relationship between silica diagenesis and physical properties in the East/Japan Sea: ODP Legs 127/128 Journal of Asian Earth Sciences 30 (2007) Gil-Young Kim, Dong- Geun Yoo, Ho-Young Lee, Young-Joo Lee, Dae-Choul Kim Reported by: 阮文情 NGUYEN VAN TINH Student Number: GENERAL Electrical resistivity logs have been used extensively in a qualitative way to correlate formations penetrated by the drill in the exploitation of oil and gas reservoirs and to provide some indication of reservoir content Electrical resistivity and physical properties of marine sediments and rocks are important variables for understanding the geological events of depositional environments, the effects of mechanical and chemical diagenesis with burial depth after deposition, and seafloor investigation of ocean engineering and naval applications. A major change in sediment properties by silica diagenesis opal-a, opal-ct, Quartz can cause an abrupt change of acoustic impedance, which in turn is responsible for the presenceofseismic reflectors. To characterize electrical resistivity of sediments and rocks sampled for comparison with physical properties, and to present their changes related to sediment diagenesis (opal-a fi opal- CT fi Quartz). 1

2 STUDY AREA TheEast/JapanSeaislocatedonthe eastern margin of the Eurasian plate and is separated from the Philippine, Pacific, and North American plates by a complex border Deep basinal areas (Japan, Yamato, and Ulleung basins);block-faulted ridges such as Yamato Rise and the Korea plateau;an eastern margin Methods The four electrode technique was used to measure electrical resistivity of sediment samples Velocities in horizontal and vertical directions with respect to the core axis at ambient temperature and pressure were measured by a pulse transmission technique on the split cores. Physical properties (wet bulk density, grain density, and porosity) were measured on discrete samples taken from the closest depth interval of each section to provide a uniform sampling. 2

3 Electrical resistivity values are highly variable from approximately 0.2 ohm-m at Site 795 to 62 ohm-m at Site 794, and characterized by an increasing pattern with depth The value increases drastically at the boundary of opal-a/opal-ct and opal-ct/quartz at most sites Physical properties such as velocity, wet bulk density, grain density, and porosity are clearly different in the opal-a, opal-ct, and Quartz zones. Velocity and wet bulk density increase with depth, whereas, porosity decreases, showing abrupt changes at each boundary Variations in the physical properties with depth in most cases can be attributed to a number of factors: degree of compaction, changes in lithology, mineralogical composition, grain size and grain type (hollow or solid), as well as by diagenetic changes Average values of electrical resistivity, velocity, wet bulk density, grain density, and porosity at the opal-a/ct and Quartz zones of each site 3

4 o Profiles of velocity, wet bulk density, grain density, and porosity, wet bulk density and porosity are significantly increased at each boundary The prominent feature of the electrical resistivity is the sharp change across the opal-a/opal Velocity and wet bulk density increase with depth, whereas, porosity decreases, showing abrupt changes at each boundary Variations in the physical properties with depth in most cases can be attributed to a number of factors: degree of compaction, changes in lithology, mineralogical composition, grain size and grain type, diagenetic changes The transformation of biogenetic silica from opal-a to opal-ct is known to occur with increasing burial depth Silica diagenesis profoundly affects pore geometry and permeability, which in turn significantly changes CONCLUSION Electrical resistivity and physical properties data show significant change at the opal-a/opal- CT and the opalct/quartz boundaries, suggesting the effects of silica diagenesis. The transformation boundary is easily recognized in the electrical and physical properties. Electrical resistivity increases with increasing depth of burial, caused by changes in physical properties, these patterns are distinct at the opal-a/opal-ct and opal-ct/quartz boundaries, characterized by significant silica diagenesis. Electrical resistivity and physical properties data show the effects of silica diagenesis and can be used to locate the diagenetic boundary in a seismic model, and to estimate the reservoir character of sediments and rocks. 4

5 Thank you for your attention! 5