By Mark Zoback, Stephen Hickman, William Ellsworth, and the safod science Team doi: 10. 2204/iodp sd. 11. 02. 2011

Scientific Drilling, No. 11, March 2011  

19

Science Reports

design, the samples and physical property measurements of 

the fault zone obtained during Phase 2 were not the only 

information available to us to guide Phase 3 coring opera-

tions. Due to accelerated fault creep following the 2004 

earthquake, the casing deployed across the fault zone follow-

ing Phase 2 was deformed at specific places which directly 

indicated the active strands of the San Andreas at depth. 

Phase 1 and 2 Operational Overview. As mentioned above, 

Phases 1 and 2 were rotary drilled. In order to obtain as 

much scientific information as possible during drilling, a 

comprehensive real-time sampling of drill cuttings, drilling 

fluid, and formation gases in the drilling mud was carried 

out. Following each phase, a suite of geophysical measure-

ments was obtained, and a limited amount of coring was 

done at each depth where casing was set. 

As can be seen in Fig. 3, the Main Hole starts vertically 

and at approximately 1.5 km depth; directional drilling tech-

niques were employed to slowly deviate the borehole (even-

tually at an angle ~60° from vertical) in order to intersect the 

San Andreas Fault in the vicinity of the repeating target 

earthquakes. A wide range of information is available online 

including that related to real-time operations (Table 1). One 

source of information that provides a convenient overview of 

Phases 1 and 2 are the Commercial Mud Logs, which deliver 

also lithologic descriptions of the drill cuttings. Numerous 

faults were observed in all of the rock units drilled through 

(Boness and Zoback, 2006). Bradbury et al. (2007) de- 

scribed the mineralogy of drill cuttings in terms of fault zone 

composition and geologic models.

The first geologic surprise that occurred during Phase 1 

was that soon after deviating the borehole toward the San 

Andreas Fault, we drilled through a major fault zone at a ver-

tical depth of 1.8 km (interpreted to be the Buzzard Canyon 

Fault, see Fig. 3) as we passed out of the Salinian granitic 

basement rocks and into previously unknown arkosic sand-

stones and conglomerates, with some interbedded shales 

(Boness and Zoback, 2006; Solum et al., 2006). In general, 

these are strongly cemented rocks that are likely derived 

from weathering of Salinian granites and granodiorites. 

Draper Springer et al. (2009) described this section in some 

detail and pointed to at least a dozen significant faults within 

it. While they argued for this being a depositional unit 

formed proximal to the Salinian granite, they suggested that 

it may have been translated along strike by as much as 

~300 km. One reason this unit had not been identified by geo-

physical surveys through the site area is that these rocks are 

so strongly cemented that their seismic velocities and resis-

tivity do not vary significantly from the fractured Salinian 

granites and granodiorites (Boness and Zoback, 2006). 

At a measured depth along the borehole of 1460 m (while 

still drilling in the granite/granodiorite), a planned pause in 

drilling took place to run steel casing into the hole before 

further drilling. Prior to casing the hole, a suite of geophysi-

cal logs was run. After running the casing into the hole and 

cementing it in place, 7.9 meters of fractured and faulted 

hornblende-biotite granodiorite core were obtained. In addi-

tion, fluid samples were taken at this depth, and a small-scale 

hydraulic fracturing experiment was done to constrain the 

magnitude of the least principal stress. 

After drilling resumed, Phase 1 continued to a total verti-

cal depth of 2507 m. As shown in Fig. 3, Phase 1 drilling 

ended in the arkosic sandstone/conglomerate section. At the 

end of Phase 1 drilling a second suite of geophysical logs was 

run. Boness and Zoback (2006) presented a summary of the 

Phase 1 lithologies and geophysical logs. After cementing 

steel casing into the wellbore, an 11.6-m core—composed of 

fractured and faulted arkosic sandstone and conglomerate—

was obtained, and fluid sampling was then performed.

One mishap that occurred during Phase 1 was a collision 

between the Main Hole and the Pilot Hole at 1.1 km depth. 

Because of the respective layouts of the drilling equipment 

used for the Pilot and Main Holes, the wellheads of the two 

boreholes were located only 6.75 m apart. In an attempt to 

avoid collision of the two holes at depth, repeated gyroscopic 

surveys of both holes and directional drilling were used. 

This is commonplace in the oil industry where dozens of 

wells are often drilled from the same platform or drill site. 

After the incident, we learned that the collision was caused 

by poor calibration of one set of the gyroscopic survey in-

struments. The lasting impact of the hole collision is loss of 

access to the lower part of the Pilot Hole, as the casing is 

severely damaged at 1.1 km depth. The Pilot Hole seismic 

Table 1.

 Accessing SAFOD Data Online.

Description

URL

EarthScope Data Portal – Information about and access to all SAFOD EarthScope data 

and samples

http://www.earthscope.org

IRIS DMC –  SAFOD seismological data archive including assembled data sets

http://www.iris.edu/hq

Northern California Earthquake Data Center – Earthquake catalogs and seismograms for 

all local networks including SAFOD, High-Resolution Seismic Network (HRSN) and NCSN

http://www.ncedc.org/safod/

ICDP Web site – Direct access to all data obtained as drilling, logging and coring 

operations were underway. Bibliography of SAFOD papers.

http://safod.icdp-online.org

Online Core Viewer – Photographs of all cores and samples taken for scientific study

http://www.earthscope.org/data/safod_core_

viewer

Phase 3 Core Atlas – High-resolution images of Phase 3 cores as well as preliminary 

lithologic and microstructural descriptions

http://www.icdp-online.org/upload/projects/safod/

phase3/Core_Photo_Atlas_v4.pdf

General information about the Parkfield Experiment

http://earthquake.usgs.gov/research/parkfield/

index.php

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