Prospects for recovery of different lithologies

Table T1 and the attached site summaries summarize the expected penetration at each of the Shatsky Rise sites. In general, the objective of Expedition 324 is to core igneous rock, so sediment coring is secondary. Many of the expedition drill sites were chosen at locations where sediment cover is thick enough to provide stability for the BHA when it contacts the igneous rock interface. In such locations the sedimentary section is often condensed and large parts of the sedimentary history found elsewhere are missing. Among the primary sites, the greatest sediment penetration is predicted at proposed Site SRNH-2 (194 m) and the least at proposed Site SRSH-8 (155 m). The sediment column has been divided into two parts that are generally recognizable in seismic sections: an upper, Cenozoic ooze section and a lower, Cretaceous section that likely consists of chalk with stringers and nodules of chert and porcellanite. The estimation of the thickness of the Cretaceous chalk is difficult because it is highly dependent on the exact, but unknown, percentage of high-velocity chert and porcellanite within the otherwise much lower seismic velocity chalk.

Actual recovery is likely to be greatly different from the predicted penetration thicknesses. In the sedimentary section, recovery of the ooze section should be high, perhaps close to 100%. However, because all Expedition 324 coring will be done with RCB bits, unconsolidated sediments will be highly deformed. Furthermore, because this upper ooze section has been cored in many places and is likely to be deformed by coring, we propose to drill through most of the sediments without coring in order to save time for coring deeper in the section. In the Cretaceous chalks, recovery is typically very low because of the interbedded chert and porcellanite. Whereas the RCB bit grinds these hard sedimentary rocks, the much softer chalk is usually washed away and the resulting recovery is very low (see Brawlower, Premoli Silva, Malone, et al., 2002).

Recovery in the igneous section is also likely to be much less than the penetrated length. Typical average recovery rates in submarine lava flows is 35%–50%. This number represents an average and recovery can be much higher or lower. Cores from submarine igneous sections provide a biased sample of the true igneous section (Tominaga et al., 2009). Usually it is the interiors of massive flows that are preferentially recovered, whereas fractured rocks are usually poorly recovered. Indeed, this bias is one of the reasons that downhole logging data are important for the scientific party’s understanding of the true lithologic succession.