Fault reactivation, leakage potential, and hydrocarbon column heights in the northern north sea

Abstract

Abstract To investigate the question of how faults affect the migration of fluids in petroleum reservoirs, we evaluated the state of stress and pore pressure acting on the major faults in four oil and gas fields in the northern North Sea. Many of the faults bound hydrocarbon reservoirs. Our goal was to test the hypothesis that faults that are being reactivated in the current stress field are permeable and thus tend to leak, whereas those that are not (i.e. faults that are inactive in the current stress field) are likely to seal. To address this question, we utilize a detailed analysis of the magnitude and orientation of all three principal stresses in a number of wells in each field. These data, along with information on pore pressure, allowed us to resolve the shear and effective normal stress acting on distinct 100 m × 100 m elements of individual fault planes. By comparing the stress state resolved on each fault element to expected stress at failure (using a Coulomb failure criterion) we created color-shaded maps showing the proximity to fault slip (and hence leakage) along each fault. Fault reactivation and hydrocarbon leakage in this area appears to be caused by three factors: (1) locally elevated pore pressure due to buoyant hydrocarbons in reservoirs abutting the faults, (2) fault orientations that are nearly optimally oriented for frictional slip in the present-day stress field, and (3) a relatively recent perturbation of the compressional stress caused by postglacial rebound. We demonstrate that the combination of these three factors may have recently induced fault slippage and gas leakage along sections of previously sealing reservoir-bounding faults in some fields, whereas in others, the stress and pore pressure are not sufficient to cause fault reactivation. We show that only in cases where reservoir-bounding faults are not potentially active, the pore-pressure difference across faults can become quite high. Hence, the leakage potential of reservoir-bounding faults appears to exert an important influence on potential hydrocarbon column heights.