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

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1 Journal of Asian Earth Sciences 3 (27) The relationship between silica diagenesis and physical properties in the East/Japan Sea: ODP Legs 127/128 Gil-Young Kim a, *, Dong-Geun Yoo a, Ho-Young Lee a, Young-Joo Lee a, Dae-Choul Kim b a Petroleum & Marine Resources Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 35-35, South Korea b Department of Environmental Exploration Engineering, Pukyong National University, Busan , South Korea Received 17 December 25; received in revised form 6 November 26; accepted 17 November 26 Abstract Six sites were drilled on ODP Legs 127 and 128. One of the significant results of this drilling was the detection of an opal amorphous silica (opal-a) and opal cristobalite (opal-ct) transformation at all the drill sites. Laboratory measurements of electrical resistivity and physical properties (e.g., velocity, porosity, and density) were conducted on the sediments and rocks sampled from the four sites (Sites 794, 795, 796, and 797). Electrical resistivity and physical properties data show significant change at the opal-a/opal-ct and the opal- CT/Quartz boundaries, suggesting the effects of silica diagenesis. Electrical resistivity reflects the change of physical properties (esp., velocity, wet bulk density, and porosity) well throughout the sites. The transformation boundary is easily recognized in the electrical and physical properties. Electrical resistivity increases by about 5% and 95% across the opal-a/opal-ct and opal-ct/quartz boundaries, respectively. Thus, electrical resistivity as well as physical properties may be used to identify lithology and diagenetic boundaries such as opal-a/ct, opal-ct/quartz, and to estimate the reservoir character (e.g., permeability and pore geometry) of sediments and rocks. Ó 27 Elsevier Ltd. All rights reserved. Keywords: Electrical resistivity; Physical properties; Silica diagenesis; The East/Japan Sea 1. Introduction Physical properties including 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 (Archie, 1942). Electrical resistivity is used as a primary method for reservoir characterization, being related to porosity, porosity style and the fluids in the pore space (e.g., oil, water, or gas). In the case of saturated marine sediment, the resistivity changes will reflect changes in porosity and fabric because the conduction of the current passing through sediments depends on the permeability of the sediment frame containing interstitial water and grain particles. Consequently, resistivity may be considered a lithology indicator for characterizing marine sediments. * Corresponding author. Tel.: ; fax: address: gykim@kigam.re.kr (G.-Y. Kim). Although the use of electrical resistivity measurement to characterize sediment properties both on the seafloor and in the laboratory is demonstrated in the literature, the method is poorly known in marine investigation. Traditionally, methods of measuring the electrical resistivity of cores make direct electrical contact with the sample material (Boyce, 198; Schoonmaker and others, 1985; O Brien, 199; Kim and Manghnani, 1992). This method frequently contributes to disturbance and contamination of the sample, thus, the resolution is limited by the physical size of the electrodes and their signal to noise ratio. Electrical resistivity and physical properties of marine sediments and rocks are also 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 (Hamilton, 198; Hamilton and Bachman, 1982; Silva and Brandes, 1998). In general, physical properties of marine /$ - see front matter Ó 27 Elsevier Ltd. All rights reserved. doi:1.116/j.jseaes

2 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) sediments significantly reflect the variation of sediment texture, but sediments and rocks from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill sites are slightly different due to digenesis (e.g., compaction, consolidation, alteration, and deformation of sediment) after deposition (Hamilton, 198; Hamilton and Bachman, 1982). A major change in sediment properties by silica diagenesis (e.g., opal-a, opal-ct, Quartz) can cause an abrupt change of acoustic impedance, which in turn is responsible for the presence of seismic reflectors (Wilkens et al., 1987). Wilkens et al. (1987) reported that the change of sediment structure by diagenesis has a significant effect on sediment properties. A reflector similar to the one studied by Wilkens et al. (1987) is frequently found around the margins of the north Atlantic Ocean. ODP Legs 127 and 128 completed drilling of six sites in the East/Japan Sea. Sites 795 and 796 are in the Japan Basin, Sites 794 and 797 in the Yamato Basin, Site 799 on the Yamato Rise, and Site 798 on the Oki Ridge. Basement was penetrated at Sites 794, 795, and 797 where basaltic rocks were recovered (Tamaki et al., 1992). Sediments recovered from the sites are characterized by the diagenetic transformation from opal-a to opal-ct. The purposes of this study are to characterize electrical resistivity of sediments and rocks sampled from the four sites (Sites 794, 795, 796, and 797) for comparison with physical properties, and to present their changes related to sediment diagenesis (opal-a fi opal-ct fi Quartz). 2. Geological setting The East/Japan Sea is located on the eastern margin of the Eurasian plate and is separated from the Philippine, Pacific, and North American plates by a complex border (Jolivet et al., 1989). Three structural and physiographic provinces exist and have significance for crustal structure: (1) deep basinal areas (Japan, Yamato, and Ulleung basins); (2) block-faulted ridges such as Yamato Rise and the Korea plateau; and (3) an eastern margin consisting of a complex of north-trending tectonic ridges and silled basins bounded by active thrust and reverse faults (Shipboard Scientific Party, 199). Fig. 1. Locations of sites drilled during ODP Legs 127/128. Leg 127 sites are 794, 795, 796, and 797. Leg 128 sites are 794, 798, and 799. Site 794 was drilled on both legs. NKP, North Korea Plateau; SKP, South Korea Plateau; KYR, Kita-Yamato Rise; YR, Yamato Rise.

3 45 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) The Japan Basin is the deepest and largest of the basinal areas in the East/Japan Sea (Fig. 1). The depth of the seafloor ranges from 3. to 3.7 km and the depth of the acoustic basement is km below sea level (Tamaki et al., 199). An oceanic-type crust (about km thick) underlies this basin. The Yamato Basin (Fig. 1) is the second prominent basin of the East/Japan Sea. Shallower (2 2.5 km) than the Japan Basin and much smaller, the basin floor has a relatively flat acoustic basement which lies km below sea level. In addition, the crust is much thicker (17 19 km) and laterally more variable than in the Japan Basin, and not typical of oceanic crust (Tamaki et al., 199). The Ulleung Basin is a bowl-shaped back-arc basin (Fig. 1). The basin is bounded by the continental slopes of the Korean Pensinsula, the Korea Plateau, the Oki Bank, and the Japanese Arc (Tamaki et al., 199). The continental shelf of the Korean Peninsula is narrow, less than 25 km, and flanked by a steep slope. Thick Neogene strata, uplifted and faulted by the back-arc closure, characterize the southern margin. The central part of the basin is underlain by an undeformed sedimentary section up to 5 km thick, while the acoustic basement in the southern basin lies below about 1 km (Chough, 1983). The acoustic basement consists largely of volcanic materials and is overlain by thick layers of volcanic sill/flow sediment complexes in the northern basin (Lee et al., 1999). In addition, the high-standing blocks scattered throughout the East/Japan Sea comprise rifted continental fragments (e.g., Korea Plateau, Kita-Yamato, and Yamato Rise) which are underlain by granitic and volcanic basement rocks. ODP Site 799 is located on the Yamato Rise, the most prominent of these basement highs. Samples were taken from ODP sites (794, 795, 796, and 797) on the Japan and Yamato basins of the East/Japan Sea (Fig. 1). The lithology from site to site in the East/Japan Sea is relatively simple (Fig. 2). In general, silt clay overlies diatom ooze and clay, which in turn overlie either diatom claystone or siliceous claystone, depending on the position of the opal-a to opal-ct diagenetic boundary. The claystone generally contains porcellanite and chert below the opal-a/opal-ct boundary. Basement was recovered at only three sites: 794, 795, and 797. The basement rocks vary (e.g., basaltic sills and flows, tuff), and the igneous units are frequently interbedded with sedimentary layers (Nobes et al., 1992). 3. Methods The four electrode technique (Olhoeft, 198) was used to measure electrical resistivity of sediment samples. The mea- Fig. 2. Major lithology from sites drilled during legs 127/128. The opal-a/opal-ct and opal-ct/quartz boundaries are marked from the results of XRD analyses (Nobes et al., 1992). The basement rocks were penetrated at Sites 794, 795, and 797.

4 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) surement was conducted at room temperature (23 C) in directions parallel and perpendicular to the core axis at 1 Hz. A cubic plexiglas sample holder was (Kim and Manghnani, 1992). The sampler holder consists of an outer pair of current electrodes, and an inner pair of potential electrodes. A pair of sponges separates the current from the potential electrode. The sponges are impregnated with salt by cyclic saturation with seawater (O Brien, 199; Kim and Manghnani, 1992). A thin tissue paper membrane is placed on either side of the cubic sediment sample prior to insertion of the sample into the sample holder. The membrane is used to improve contact between the electrodes and the sample. Velocities in horizontal and vertical directions with respect to the core axis at ambient temperature and pressure were measured by a pulse transmission technique (Birch, 196) on the split cores. Physical properties (wet bulk density, grain density, and porosity) of Site 795 were also measured (Boyce, 1976) on discrete samples taken from the closest depth interval of each section to provide a uniform sampling frequency. The physical properties data at other sites (Sites 794, 796, and 797) were from the data reported by Nobes et al. (1992). 4. Results and discussion Electrical resistivity values are highly variable from approximately.2 ohm-m at Site 795 to 62 ohm-m at Site 794, and characterized by an increasing pattern with depth (Figs. 3 6). These trends are very common regardless of sediment composition and geographical location (Wilkens and Handyside, 1985; Mascle et al., 1988; O Brien, 199; Kim and Manghnani, 1992). The value increases drastically at the boundary of opal-a/opal-ct and opal-ct/quartz at most sites, as shown in the figures. It is also characterized by significant differences among the boundaries (Table 1). Above the opal-a/ opal-ct (opal-a zone), electrical resistivity is in the range of ohm-m, suggesting sediments of high porosity saturated by seawater. Below the opal- CT/Quartz (Quartz zone), the value is as high as a range of ohm-m (Table 1), caused by a significant change of physical properties by silica diagenesis (Nobes et al., 1992). The opal-ct zone is intermediate between the opal-a and Quartz zones. Electrical resistivity increases by 5% and 95% across the opal-a/opal-ct and opal-ct/quartz boundaries, respectively.. Resistivity (ohm- m), Holes A and B Resistivity (ohm- m), Hole A Site 794 Hole A Site 794 Hole B Site 794 Hole C 1 Site 795 Hole A Site 795 Hole B Opal- A/Opal- CT Resistivity (ohm- m), Hole C Resistivity (ohm- m), Hole B 8 Fig. 3. Electrical resistivity profile at Site 794. The boundary of opal-a/ opal-ct and opal-ct/quartz is marked. Note that the values are significantly increased at each boundary. Fig. 4. Electrical resistivity profile at Site 795. The boundary of opal-a/ opal-ct and opal-ct/quartz is marked. Note that the values are significantly increased at each boundary.

5 452 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) Resistivity (ohm- m) Resistivity (ohm- m) Site 796 Hole A Site 796 Hole B 1 2 Opal- A/Opal- CT 2 Opal- A/Opal- CT No data 4 8 Site 797 Hole A Site 797 Hole B Fig. 5. Electrical resistivity profile at Site 796. The boundary of opal-a/ opal-ct and opal-ct/quartz is marked. Fig. 6. Electrical resistivity profile at Site 797. The boundary of opal-a/ opal-ct and opal-ct/quartz is marked. Table 1 Average values of electrical resistivity, velocity, wet bulk density, grain density, and porosity at the opal-a/ct and Quartz zones of each site Sites Properties Opal-A zone Opal-CT zone Quartz zone 794 Electrical resistivity (ohm-m) Velocity (km/s) Wet bulk density (g/cm 3 ) Grain density (g/cm 3 ) Porosity (%) Electrical resistivity (ohm-m) Velocity (km/s) Wet bulk density (g/cm 3 ) Grain density (g/cm 3 ) Porosity (%) Electrical resistivity (ohm-m) Velocity (km/s) Wet bulk density (g/cm 3 ) Grain density (g/cm 3 ) Porosity (%) Electrical resistivity (ohm-m).543 Velocity (km/s) 1.53 Wet bulk density (g/cm 3 ) Grain density (g/cm 3 ) Porosity (%) Flow characteristics of electrical current are controlled by the pore geometry more than by the rock matrix, because the current flows mainly through the electrolyte that fills the interconnected pore spaces in a saturated rock (Walsh and Brace, 1984). Brace and others (1968) reported a good relationship between permeability and electrical

6 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) Velocity (km/ s) Wet bulk density (g/cm 3 ) Grain density (g/cm 3 ) Porosity (%) Site 794 Hole A Site 794 Hole B Site 794 Hole C Fig. 7. Profiles of velocity, wet bulk density, grain density, and porosity at Site 794. Note that wet bulk density and porosity are significantly increased at each boundary. 1 Velocity (km/ s) Wet bulk density (g/cm 3 ) Grain density (g/ cm 3 ) Porosity (%) Site 795 Hole A Site 795 Hole B Fig. 8. Profiles of velocity, wet bulk density, grain density, and porosity at Site 795. Note that wet bulk density and porosity are significantly increased at each boundary.

7 454 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) Velocity (km/ s) Wet bulk 1.6 density (g/ cm3 ) Grain 2. density 2.5 (g/ 3. cm3 ) 3.5 Porosity (%) Site 796 Hole A Site 796 Hole B Fig. 9. Profiles of velocity, wet bulk density, grain density, and porosity at Site 796. Note that wet bulk density and porosity are significantly increased at each boundary. 1.2 Velocity (km/ s) Wet bulk density (g/cm 3 ) Grain density (g/cm 3 ) Porosity (%) Site 797 Hole A Site 797 Hole B Site 797 Hole C Fig. 1. Profiles of velocity, wet bulk density, grain density, and porosity at Site 797. Note that wet bulk density and porosity are significantly increased at each boundary.

8 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) conductivity in rocks. Thus, change in pore geometry (e.g., silica diagenesis) plays a primary role in the increase of electrical resistivity (Kim and Manghnani, 1992). Similarly, physical properties such as velocity, wet bulk density, grain density, and porosity are clearly different in the opal-a, opal-ct, and Quartz zones (Table 1). In the opal-a zone, velocity, wet bulk density, grain density, and porosity show the ranges of km/s, g/cm 3, g/cm 3, and %, respectively. In the opal-ct zone, velocity, wet bulk density, grain density, and porosity are km/s, g/ cm 3, g/cm 3, and %, respectively. In the Quartz zone, velocity, wet bulk density, grain density and porosity show the highest values: km/s, g/cm 3, g/cm 3, and %, respectively. Overall, velocity and wet bulk density increase with depth, whereas, porosity decreases, showing abrupt changes at each boundary (Figs. 7 1). The profiles of wet bulk density mirror the porosity, as previously reported. The velocities above the opal-a/opal-ct boundary area almost constant, whereas the velocities below the boundary tend to increase significantly with depth (Figs. 7 1), suggesting a certain degree of diagenesis such as consolidation, compaction, and alteration. In the lower section (at basement below 55 m) of Site 794 (Fig. 7) the velocity and wet bulk density are characterized by higher values, 4 6 km/s, g/cm 3, respectively, due to samples taken from igneous rocks, where the porosity is lower than 2%. These patterns are similar at sites 795, 796, and 797, reflecting the presence of basement rocks (Figs. 8 1). The opal-a/opal-ct and opal-ct/quartz boundaries are sharp increases in wet bulk density. Grain density exhibits high variability above the opal-a/opal-ct boundary, especially in the uppermost sediments (Figs. 7 1). Below the opal-a/opal-ct boundary, the value is high, caused by the dissolution of opal-a and precipitation of opal-ct (Nobes et al., 1992). This gives rise to lower porosity below the boundary. The high porosity above the opal-a/opal-ct boundary can be explained by the characteristic structure of clay minerals and the open-cellular nature of diatoms (Hamilton, 198; 1982; Nobes et al., 1992). The porosity reaches a maximum at the intervals of 5 1 m above the opal-a/opal-ct boundary, where the diatom fractions are abundant (Nobes et al., 1992). 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 (Hamilton, 198; O Brien, 199). Diagenetic processes are related to the dissolution of opal-a and biogenic calcium carbonate, and their redeposition as opal-ct and calcite cement, respectively. In this study, the critical diagenetic factor can be considered the silica progression from opal-a through opal-ct to Quartz. As discussed earlier, the prominent feature of the electrical resistivity is the sharp change across the opal-a/opal- CT diagenetic boundary defined by the X-ray diffraction (XRD) results (Nobes et al., 1992). At this boundary, there are relatively large changes in the electrical resistivity and physical properties (e.g., velocity, wet bulk density, and porosity), since there is a decrease in porosity associated with the dissolution of the amorphous, open-cellular structure of the diatoms and the precipitation of the opal-ct. But the depth of the opal-a/opal-ct boundary is slightly different in individual sites. The opal-a/opal-ct boundary occurs at or near a lithologic boundary (Fig. 2), whereas the opal-ct/quartz boundary often lies within a lithologic unit and does not appear to be as distinct as the transition from opal-a to opal-ct (Nobes et al., 1992), in part due to poor core recovery below the opal-a/opal-ct boundary. Nonetheless, the transition appears sharply and distinctly in electrical resistivity and physical properties profiles (Figs. 7 1). In general, the transformation of biogenetic silica from opal-a to opal-ct is known to occur with increasing burial depth (Iijima and Tada, 1981). The primary controlling factors of this phase change include temperature, time, surface area of test, pore water chemistry, lithology, and permeability (Lewin, 1961; Hurd, 1972; Kastner et al., 1977). Among these factors, temperature and time are the most important. Phase transition of opal-a to opal-ct is a dehydration reaction that creates an increase in the silica density and can increase the acoustic impedance (the product of density and velocity) in silica-rich sediments (Mayer et al., 1986). In these sites, the temperatures at the opal-a/ opal-ct and opal-ct/quartz boundaries are high, approximately 4 and 56 C, respectively (Nobes et al., 1992). The range of the temperature and time corresponds to the transition field of opal-a and opal-ct causing silica diagenesis (Tamaki et al., 1992). The opal-a/opal-ct diagenetic boundary produces in most cases a strong bottom simulating reflector (BSR) in seismic profiles (Kuramoto et al., 1992). The strong reflection results from an abrupt change of density, due to the silica phase change, rather than the gradual change of velocity by compaction and consolidation with burial depth. According to Kuramoto et al. (1992), the BSR is observed throughout the East/Japan Sea. In addition, silica diagenesis profoundly affects pore geometry and permeability, which in turn significantly changes electrical resistivity as well as physical properties, related to the presence of BSR. Thus, for the detailed interpretation of seismic data, diagenetic effects must be considered. 5. Conclusion Laboratory measurements of electrical resistivity and physical properties were made on split-core sections collected during ODP Legs 127/128. Electrical resistivity increases with increasing depth of burial, caused by changes in physical properties. Furthermore, these patterns are distinct at the opal-a/opal-ct and opal-ct/quartz boundaries, characterized by significant silica diagenesis.

9 456 G.-Y. Kim et al. / Journal of Asian Earth Sciences 3 (27) Thus, electrical resistivity and physical properties data show the effects of silica diagenesis and can be used to locate the diagenetic boundary (e.g., BSR) in a seismic model, and to estimate the reservoir character of sediments and rocks. Acknowledgements This work was supported by a grant from the Korea Integrated Ocean Drilling Program (KIODP) to G.Y. Kim (25). This study was partly supported by Korea Institute of Geoscience & Mineral Resources (KIGAM). This research used samples and data provided by the Ocean Drilling Program (ODP). We thank Dr. R.H. Wilkens and an anonymous reviewer for their comments. References Archie, G.E., The electrical resistivity log as an aid in determining some reservoir characteristics. Petroleum Transactions American Institute of Mining, Metallurgical, and Petroleum Engineers 146, Birch, F., 196. The velocity of compressional waves in rocks up to 1 kilobars. 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