Relating P-Wave Velocity to Rock Strength in High-porosity, Shallowly Buried Sediments: Implications for in Situ Stress Estimates
Olcott, Katelyn A.; Saffer, Demian
The magnitude of in situ stresses is important toward understanding fault and earthquake mechanics, as well as for hydrocarbon exploration. Wellbore failures, including compressional borehole breakouts (BO) can provide information about stress orientation and can be used to constrain in situ stress magnitude if the rock or sediment unconfined compressive strength (UCS) is known. Values of UCS used to estimate stresses from BO are typically determined from laboratory-derived empirical relations between P-wave velocity (Vp) and UCS. For many applications in sedimentary basins and tectonically active settings, UCS is estimated from relations developed for lithified shales. We seek to advance our understanding of Vp as a proxy for UCS, particularly in high-porosity (~30-60%), shallowly buried (<2 km) sediments where estimates of UCS based on relationships defined for fully lithified shales may lead to overestimates of strength. We focus on the Nankai accretionary prism offshore SW Japan, formed by subduction of the Philippine Sea Plate beneath the Pacific Plate. Breakouts have been identified from azimuthal resistivity logs at IODP Site 808, which penetrated the accretionary prism and plate boundary décollement ~3 km landward of the trench. We determine UCS from triaxial tests on core samples recovered during Ocean Drilling Program Leg 190 from ODP Site 1174, located ~1 km away from Site 808. We then compare UCS measurements to Vp data from wireline logging.
Our results indicate that directly measured values of UCS are considerably lower (>1 MPa) than those estimated from Vp using the existing relationships for shales. When applied to estimate in situ stresses from the observed wellbore failures, values of UCS determined from these relationships likely lead to significant overestimates of stress magnitude. Our results should apply to the general case of shallow, relatively high-porosity sediments, and therefore carry implications for borehole stability and assessment of shallow geohazards globally.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013