State of Stress in a Plio-Pleistocene Gulf Coast Growth Fault: Implications for Fracture Driven Fluid Flow
Buoyant forces from trapped hydrocarbons are equal to the least principal stresses in the EI-330 field (offshore Louisiana, Gulf of Mexico); we interpret that this zone is in dynamic equilibrium where hydrocarbons are contained by the stress field and where migration occurs by hydraulic fracturing of this dynamic seal. The Pathfinder well was drilled in 1993 to measure in-situ properties of a growth fault inferred to be a major hydrocarbon migration pathway. Fluid pressures rise from hydrostatic at the surface to 85% of lithostatic at 8000 feet. The minimum horizontal effective stress (Shmin-P), as measured in a series of hydrofracture experiments, first rises and then decreases with depth in the borehole. In the vicinity of the fault (8000 feet) Shmin-P is ~375 psi in the fault zone and ~575 psi in shales immediately above the fault zone.
The fault zone is thin (1-10 cm) and has micro-darcy permeability although several thousand feet of displacement occurred across it. Large vertical fractures are present immediately above the fault zone itself.
1800 foot hydrocarbon columns are present, abutted against the growth fault, in the vicinity of the stress measurements. The buoyant forces from these trapped hydrocarbons are approximately equal to the least principal effective stresses measured along the growth fault. The vertical fractures present in the hanging wall are suggestive of vertical hydrofracturing. These results imply the EI-330 growth fault zone is a dynamic system where fluid flow is dominated by fracture permeability as the pore pressure approaches the least principal stresses present.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California