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46682-G8
Sediment Stabilization and Anomalous Porosities Resulting from Minor Silica Cementation

Glenn A. Spinelli, New Mexico Institute of Mining and Technology

Cementation affects mechanical properties that control sediment strength and deformation. A small volume of grain coating cement can increase sediment strength and inhibit consolidation. Burial of cemented sediment can generate anomalous porosities. I am examining silica cementation and the generation of anomalous porosity in potential reservoir seal rocks, and the controls on its spatial distribution.

A small amount of grain-coating silica cement affects sediment properties and deformation at Ocean Drilling Program (ODP) Sites 1173 and 1177 offshore Japan. The cemented sediment has anomalous porosity, seismic velocity, and rigidity. With burial and increasing temperature, failure of the cement leads to collapse of the sediment framework and dramatic porosity loss. Previous studies suggest that a minor amount of opal-CT cement inhibits consolidation of the sediment at Sites 1173 and 1177. I am examining additional ODP and Deep Sea Drilling Project (DSDP) sites offshore Japan (Sites 297, 442, 443, 444, 582), Alaska (Site 178), California (Site 1020), and Guatemala (Site 495) to determine if similar cementation of hemipelagic sediment is common and what effects this cement has on sediment strength and deformation.

Sediment samples for these circum-Pacific sites were collected from archived sediment cores the Integrated Ocean Drilling Program core repositories. The amorphous silica content of sediment samples from the sites is determined using an alkaline leaching method. The nature and distribution of the cementing material is detected through secondary and backscattered electron (SEM and BSE) images. To date, these analyses have focused on the sites offshore Japan. Data from these sites indicate that sediments throughout Shikoku Basin are affected by silica cementation. The amorphous silica content of the upper Shikoku Basin facies is higher (~1 wt% of the sediment) than in the lower Shikoku Basin facies (<0.25 wt%). The position of the upper/lower Shikoku Basin facies boundary appears to be controlled by silica diagenesis. In the upper Shikoku Basin facies, amorphous silica is visible as a coating at grain contacts, filling voids in zones of clastic material, and as altered material in contact with volcanic glass shards. The observed amorphous silica appears to be sourced from these volcanic glass shards. Electron microprobe analyses indicate that the amorphous silica is nearly identical in composition to adjacent volcanic glass shards, but with much higher porosity. Mass balance calculations indicate the amorphous silica phase is mobile; it is disseminated throughout the surrounding pore space and may be the material strengthening the sediment. Future work will focus on the Alaska, California, and Guatemala sites. Understanding this cementation process in a variety of environments will aid in the interpretation of porosity within reservoir seal rocks.

This grant is supporting a Native American graduate student (M.S.) at New Mexico Tech, Mr. Russell White. In Fall 2008, he will present preliminary results of his research at both an NSF-MARGINS sponsored workshop and at the American Geophysical Union meeting.

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