A Strength Profile Across the Ocean Crust-Mantle Transition: Oman Drilling Project Sites CM1 and 2

To better understand the strength profile across an ancient ocean crust-mantle transition preserved in the Samail Ophiolite, we have analysed real-time drilling parameters and undertaken scratch tests on the core samples acquired at the Oman Drilling Project Sites CM1 and 2.

Drilling parameters, such as WOB, ROP, RPM and torque, have been used to assess real-time optimization of drilling operations by engineers. Here, the use of these datasets is extended to scientific purposes to constrain the in-situ subsurface environment and physical properties of rock cut by the drill bit. Methods such as Specific Energy (SE) and Equivalent Strength (EST) can calculate drilled rock strength equivalent from drilling parameters.

We also used scratch tests on drill core from the lower oceanic crust down into the upper mantle. This method acquires a continuous high-resolution profile of rock strength by mechanically scratching the outside surface of core samples. The strength data are equivalent to the uniaxial compressive strength (UCS) values. We repeated the test several times by using the same groove on the core surface to increase the reproducibility of the results.

Our preliminary results suggest that scratch test measurements show general agreement with geophysical logging characteristics, such as sonic-derived UCS, and core descriptions, reflecting the presence of strong, relatively fresh layered gabbro and weak, nearly 100% serpentinized dunite. We are analyzing drilling parameters and will present detailed results for further discussion, combining our observations with other measurements performed on the core samples from the CM sites.