Induced Microearthquake Patterns and
Oil-Producing Fracture Systems in the Austin Chalk
W. Scott Phillips, Nambe Geophysical, Inc.
Thomas D. Fairbanks, Nambe Geophysical, Inc.
James T. Rutledge, Nambe Geophysical, Inc.
David W. Anderson, Los Alamos National Laboratory
Tectonophysics, April 1998, vol. 289, pp 153-169,
Special Issue on Induced Seismicity
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Microearthquakes collected during hydraulic stimulation allowed us to study
fracture zones in Austin chalk oil reservoirs at two sites in the Giddings field,
Texas. We deployed three-component, downhole geophone tools in production
wells at depths of 2100 m and greater, one near Cook's Point, and two on the
Matcek lease near Caldwell. At Cook's Point, we collected 482 microseismic
events during a 4000 m3 (25,000 bbl) hydraulic stimulation in an offset well. We
collected 770 events during a similar operation on the Matcek lease. Many
seismograms contained reflected phases that constrained location depths to the
production zone at the base of the Austin chalk. By restricting all
microearthquake locations to production depths, we located 20% of the Cook's
Point events and over 60% of the Matcek events. At both sites we observed only
the fracture wing closest to the observation stations. Locations formed
elongated patterns extending up to 1 km from the stimulation well and trending
N60E, parallel to the known, regional fracture trend. The Cookıs Point seismic
zone measured over 100 m in width, while long stretches of the Matcek seismic
zone narrowed to 30 m or less. We believe the width of the seismic zone
reflected the density of conductive fractures and thus, the volume of the
reservoir accessed by the stimulation. Indeed, production rates in the first year
following stimulation were much higher at Cookıs Point, where we observed the
wider of the two seismic zones.