Quantifying
Fault
Stability in the Fort Worth Basin, Texas
Abstract
From 1970 through 2005 there were 2 Mw ≥2.5 earthquakes in the Ft. Worth Basin (FWB) and from 2006 through mid-2018 there have been 125. Evidence from structural geological analysis, site-specific pore-pressure modeling, temporal and spatial correlation between seismicity and subsurface injection activities, and proximity of seismicity to known or suspected faults, leads to a general scientific consensus that this increase has been induced by increases in pore fluid pressure from waste water injection and from cross-fault
pore pressure imbalance due to oil and gas related injection and production. Shallow crustal earthquakes result from the reactivation of and slip on a pre-existing
fault
or faults. The larger the
fault
surface area that slips, and the stronger the rock involved, the larger the magnitude of the resulting seismic event. There are two important corollaries of this: (i) earthquake magnitude scales with
fault
size and rock strength, and (ii) earthquakes can only occur if there is a pre-existing
fault
appropriately oriented within the ambient stress state for reactivation if sufficient pore fluid pressure increase or imbalance should occur. Here we consider three factors that control the likelihood that a
fault
will be reactivated by an increase in pore fluid pressure: ambient stress state, orientation and shape of
fault
(s), and frictional strength of the
fault
(s). We use newly developed 3D models of pore pressure evolution and
fault
geometries, together with stress data from a variety of sources, including wells and seismicity, to characterize
fault
stability within the FWB.
Fault
stability is assessed on the basis of
fault
area combined with its proximity, in stress terms, to reactivation, here expressed as pore pressure perturbation required for slip (ΔPf crit ). In a fully 3-dimensional stress state, faults that have large areas with low values of ΔPf crit are less stable than faults that have small areas with low values of ΔPf crit . The stability of
fault
populations as well as individual faults can be evaluated, and the likely effects of different stress models can be compared.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019