In-situ and laboratory stress investigations using borehole data from the North Atlantic Ocean

Abstract: This doctoral thesis presents results from investigations ofin-situ and laboratory stress from borehole logging andlaboratory deformation experiments on drill-cores. The datacome from hydrocarbon exploration and production wells in theDanish Sector of the North Sea, and from boreholes drilled bythe Ocean Drilling Program (ODP). The drill-core samples werecollected from two ODP boreholes in the equatorial and easternAtlantic Ocean. The objectives for this project are: (1) to collect datafrom the North Atlantic Ocean where few or no stressmeasurements exist, and to incorporate the data into the WorldStress Map (WSM); (2) to determine the source of stress thatgenerates the mean orientation of the maximum horizontal stressin individual boreholes in ocean areas; and (3) to study themechanical responses of sediments and rocks in order tounderstand the nature of stress and stress paths that producedeformation of the ocean floor. The orientations of in-situ horizontal stresses weredetermined from analyses of borehole breakouts anddrilling-induced fractures (DIFs) as determined from four-armcaliper and Formation MicroScanner (FMS) data. Logging datawere analyzed using graphical and automated programs, usingbreakout and DIF criteria, directional statistics, and theborehole data were ranked according the WSM ranking system. Stress data from 44 oil wells and ODP boreholes in the NorthAtlantic Ocean represent important additions to the WSM fromareas where few stress data were available. Despite of theirrelatively shallow total depths, it is suggested that thestress orientation in many of the ODP boreholes are influencedby plate boundary forces. The observed variation in downholedistribution of breakouts in the oil wells and the lack ofregional trends indicate that local and regional stress sourcescontrol the stress field in these wells. Detailed laboratory stress measurements were conducted in atriaxial cell mounted in a servo-hydraulic load frame, usingreconsolidation and triaxial tests on small sub-samples fromtwo drill-core samples of different lithologies. Results from tests on a claystone from the basal section ofthe ocean-continent transition in West Iberia margin suggestthat the in-situ effective stresses are low because of the highpore pressure. The fractured nature of the underlying basementand breccia suggest that the pore fluids originate from deepersections. Results from tests on a calcarenite from about 1 kmdepth show that the elastic versus elastic-plastic behaviordiffers significantly, but that the rock behavior can bedescribed by critical state soil mechanics.