<p>Reservoir management for various subsurface applications often depends on predicting properties from laboratory experiments. This study aims to describe the elastic and plastic strain development of oil and water-saturated chalk during compaction. Hence, twelve North Sea Lower Cretaceous chalk samples with initial porosity from 31 to 44% were compacted with uniaxial strain in the laboratory, and six had creep phases included in the test design. The traditional compaction phases were identified from stress–strain curves and dynamic moduli derived from density and elastic wave velocities. The pure elastic strain was computed from applied axial stress and chalk frame modulus derived via Gassmann fluid substitution of the dynamic bulk modulus. The derived elastic uniaxial strain is smaller than the measured strain, and the residual strain is quantified as plastic. Interpretation shows a significant plastic strain in the initial compaction phase prior to pore collapse, commonly recognised as elastic. The plastic strain observed below pore collapse stress presumably originates from micro-cracks, and creep phases show that measure creep strain is predominantly plastic, so creep works as a micro-crack closure mechanism. Post&#xa0;pore collapse, measured creep strain includes both elastic and plastic strain. Yet, the onset of pore collapse is controlled by a specific pure elastic uniaxial strain in the order of 0.2% independent of porosity and saturation fluid. Quantifying elastic and plastic strain in experiments may provide insight to improve large-scale models as in situ reservoirs may only experience the elastic strain during effective stress changes and not the plastic strain from, e.g. micro-crack closure observed in experiments.</p>