David J Wiprut¹, Mark D Zoback¹
(1) Stanford University, Stanford, CA

Abstract: Fault reactivation and hydrocarbon leakage along a previously sealing normal fault in the northern North Sea

Detailed seismic imaging, in situ stress and pore pressure measurements are used to analyze reverse-fault reactivation of a long-dormant reservoir-bounding normal fault in the northern North Sea. We have used observations of drilling-induced tensile wellbore failures (in conjunction with pore pressure, density and leak-off test measurements commonly conducted in exploration wells) to determine the magnitudes and orientations of all three principal stresses in five wells that surround and penetrate the fault. The magnitudes of the three principal stresses (Sv, Shmin, and SHmax) are consistent with depth and reflect a strike-slip to reverse faulting stress regime. Fault reactivation is caused by three factors: i) a recent increase in the compressional stress in the area associated with post-glacial rebound, ii) locally-elevated pore pressure due to the presence of natural gas in a hydrocarbon reservoir on the footwall side of the fault, and iii) a fault orientation which is nearly optimally-oriented for frictional slip in the present-day stress field. By assuming a coefficient of friction of 0.6 we are able to calculate the critical pore pressure at which fault slip and gas leakage are expected. We demonstrate that gas leakage occurs along sections of the previously sealing reservoir-bounding fault at measured values of pore pressure within 2 MPa of the calculated critical pore pressure. We argue that similar pore pressure triggering of fault slip in the crust may occur due to the static accumulation of gas columns of CO2, He, etc. in the vicinity of tectonic faults.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana