University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln
USGS Staff -- Published Research
US Geological Survey
1982
The Distribution of Natural Fractures and Joints at
Depth in Crystalline Rock
Donald A. Seeburger
Stanford University
Mark D. Zoback
U.S. Geological Survey
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Seeburger, Donald A. and Zoback, Mark D., "The Distribution of Natural Fractures and Joints at Depth in Crystalline Rock" (1982).
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. B7, PAGES 5517-5534,JULY 10, 1982
The Distribution
of Natural
Fractures and Joints
at Depth in Crystalline Rock
DONALD A. SEEBURGER
Geophysics
Department,StanfordUniversity,Stanford,California94305
MARK D. ZOBACK
U.S. GeologicalSurvey,Menlo Park, California 94025
This paperpresentsthe resultsof studiesof the natural fracturedistributionencounteredin 10 testwells
drilledin threeareasof the UnitedStates.Sevenof the wellsweredrilledto depthsof 200-250m, while
threeweredrilled to depthsof about 1 kin. Usingan ultrasonicboreholeteleviewer,fracturedepths,strikes,
and dips were determined.Steeplydipping fractureswere found throughout each of the wells, and in
general,few horizontalfractureswere observed.Statisticallysignificantfracturepole concentrations
were
found for each well which were basicallyinvariant with depth, although some variation of fracture
orientationwith depthwasfoundin two wells.The significantfractureorientationswere not found to be
the samein wellsonly severalkilometersapart in a givenregion.In noneof the wellsdid the numberof
observablefracturesdecreasemarkedlywith increasingdepth.No simplerelationshipof fractureorientation or fracturedensitywith major structuralfeaturessuchasthe SanAndreasfault wereobserved,and no
simplerelationbetween
thesignificant
fractureorientations
andeitherpastor presentregionalstress
fields
could be determined.
INTRODUCTION
existenceand the orientation of the fractures were determined;
The uppercrustof the earth is composedof fracturedrock.
Fractures
are found on all scales from microfractures
with
most boreholestudieshave beenconcernedwith only the presence of fractures.
In this paper we presentthe resultsof in situ fracturestudies
performed in test wells drilled in three different regions' five
cal propertiesof rock. Laboratory and theoreticalfracture wells were in the Mojave Desert near Palmdale, California;
threewerenear Limekiln Valley, southof Hollister,California;
studies have been extensive. Fractures have been found to affect
the strengthof rock [e.g., Brace, 1960], the velocityof elastic and two wells were at Monticello Reservoir, northwest of
waves[e.g.,Nut, 1971;Hadley,1976],and the effectiveelastic Columbia,SouthCarolina.The wellswerecontinuouslylogged
moduli [e.g.,Walsh,1965;BudianskyandO'Connell,1976].The usingan ultrasonicboreholeteleviewer(describedbelow), and
permeability
anddistribution
ofporefluids
incrystalline
rocks the fractureorientationsand distributionas a functionof depth
are primarilydetermined
by the densityanddistribution
of were determined.This data was evaluated to determine (1)
fractures[e.g.,Snow,1968;Brace,1980].A knowledgeof the in whetherstatisticallysignificantfracture orientationsare to be
situ fracture distribution is thus of great importance for found in the total fracturepopulation,(2) whetherthe number
characterizationof the upper crust.But systematicsubsurface of fractures or the orientation of significantconcentrations
varieswith depth,(3) whetherthe fracturefrequencyand orienobservations of fractures have been few. Most such observationshavebeenmadein tunnelsandmineswherethe stress tation vary from locationto locationin a givenregion,and (4)
field and fracturepattern may have beenconsiderablyaltered what relation, if any, can be found betweenthe observedfracduringoperations.For example,McGarr [1971] and McGarr turesand what is known about the regionalstressfield and the
et al. [1979] studied the relation of fracture occurrenceand geologichistory.
rock burstsin deepmines.Fractureorientationswereobserved
TECHNIQUE
to be affectedby the presenceof the tabular excavations,and
The boreholeteleviewcr(manufacturedby SimplecManufacburst fractures were observed to occur where natural fractures
dimensionsof microns ,to lineamentswith dimensionsof kilometers.Thesefracturesexert a profoundeffectupon the physi-
were unusuallysparse.Overbeyand Rough [1971] compared
fractureorientationsmeasuredat surfaceoutcropsand from
aerial photographswith those found in oriented cores and
impressionpacker surveysof hydraulicallyfracturedintervals
in easternOhio. They found a fair degreeof correlation between natural and inducedfracturesat depth and major fracture trendsmeasuredat the surface.The data discussedby
Overbeyand Rough [1971] are significantin that both the
• Now at ChevronOverseasPetroleumInc., SanFrancisco,California 94105.
This paperis not subjectto U.S. copyright.Publishedin 1982by the
AmericanGeophysicalUnion.
Papernumber2B(gO3.
turing Co. under licenseof Mobil Oil Corp.) is a rotating
acoustictransduceremitting pulsesfocusedin a 3ø beam at a
rate of 180 timesper second[Zernaneket al., 1970]. The transducer rotates at three revolutionsper secondand movesverti-
callyin the hole at a speedof 2.5 cm/s.The amplitudeof the
reflectedsignalis plotted as brightnesson a three-axisoscilloscopeas a functionof the beamazimuth and verticalpositionin
the hole. The scopetrace is triggeredat magneticnorth by a
flux gatemagnetometerin the tool. Essentially,the smoothness
of the boreholewall is mapped.Where the smoothnessof the
borehole wall is perturbed by a planar featuressuch as a
fracture, a dark sinusoidalpattern is seen(seeFigure 1). Resolution of the tool dependsupon hole diameter, wall conditions,reflectivityof the formation,and acousticimpedanceof
the well bore fluid. The wall conditionis the most important
5517
This article is a U.S. government work, and is not subject to copyright in the United States.
5518
SEEBURGER
AND ZOBACK: DISTRIBUTIONOF NATURAL FRACTURES
the surface.Only thosefeaturesfor whichthe sinusoidalsignature couldbe resolvedwerepickedasfractures.
There are two major limitationsin the analysisof fracture
-D 2
orientationfrom televiewerdata. First, with the televiewer,only
the orientationof a smallportion of a fractureplaneis actually
observed.On a large scale,many fracturesmay appearplanar,
but whenrestrictedto viewingonly smallportionsof the entire
fracture,variationsof strike and dip by as much as 10ø may be
apparent.Thus,in tryingto determine
preferredfractureorien-
hie
N
E
S
B
H T
V
W
tation, data from televiewersurveysmay resultin more scatter
and lower levelsof statisticalsignificance
for preferredorientations than would be found in surveyswhich considerlarger
portionsof the fractures.The secondproblemis that nearly
vertical fractures are not often intersectedby vertical wells.
Thus, there is a bias in the method as vertical fracturesare not
sampled.
N
LOG
To evaluate the distribution of fracture orientations,poles to
fractureplanesfor all dippingfracturesin eachwell wereplotted on a lower hemisphere,
equal-areaprojection.To obtain an
estimateof the statisticalsignificance
of thesepole groupings
(and thus to arrive at an estimateof the preferredfracture
Strike: Orientation of midpoint
between peak and trough
(at h/2)
_
Dip: tan• h/d
Fig. 1. Isometricsketchof fractureor beddingplaneintersecting
boreholeat moderatedip angle and corresponding
BoreholeTeleviewerlog I-afterZemaneket al., 1970,Figure7].
factor as a rough well bore makes detection of fine features
quite difficult. Except for highly fractured intervals,the conditions in the Mojave and Monticello wells were nearly ideal,
and all fractureswere aperturesof more than a few millimeters
wereprobablydetected.The conditionsin the Limekiln Valley
wells were highly variable. In heavily fractured,poor picturequality intervals,only a subsetof the total fracturepopulation
could be analyzed.All of the wells were drilled as undeviated,
vertical holes. Deviation surveysrun in the later wells confirmed that the wellswereessentiallyvertical.
As an example, Figure 2 presentsteleviewer pictures of a
15-m vertical sectionof a well in the Mojave desert.The interval from about 168 to 172 m in Figure 2 is one in which
numeroussinglesubparallelfracturesare found. From about
173 to 175 m thereis a relativelyunfracturedinterval, although
severalsmall fracturesmight be presentnear 174.5 m. From
about 175 to 178 m is an interval in which a few distinct,
parallelfracturesare foundwith largeapparentaperture.From
178 to 183 m, two more fracturessimilarto thosejust aboveare
seenas well as severalfiner scalefeatureswhich also appear to
be fractures.
Knowing the well diameterthe dip of thesefracturesmay be
calculatedby measuringthe peak to trough amplitude of the
sinusoids. The fracture strike is taken to be in the direction
of
the midpoint betweenpeak and trough (Figure 1). The test
wells were drilled with a diameterof approximately15 cm so
that the circumference(horizontal scale)is about 50 cm. Thus,
there is greater than 3:1 horizontal exaggerationin the picturesand as a result,fractureswith dips of lessthan 5ø appear
to be horizontal. Apparently low-dipping fractures (such as
those in the first few meters of Figure 2) actually dip at least
10ø-15 ø'
Televiewer surveyswere run in each well from total depth
(TD) to the top of the water column or the bottom of casing,in
either case,usuallyto within severalmetersor tensof metersof
orientations), orientation-density diagrams were prepared
usinga methoddescribedby Karnb[1959]. The pole densities
were contoured in intervals of 2a, where a is the standard
deviation of the total number of points in a given area under
random sampling.The expecteddensity E for no preferred
orientationis 3a. The standarddeviationand the samplingarea
usedin preparingthesediagramsare both determinedby a
statisticalrelationbasedupon the numberof polesplotted.The
numberof polesN, the sampleareaA, givenasa fractureof the
total area of the hemisphere,the expecteddensityE, and the
standarddeviationa are givenfor eachplot. Observeddensities
that differ from E(3a) by more than 2 or 3 timesthe standard
deviation(i.e., >6a) are likely to be significant,particularlyif
the higherdensitiesare clusteredin onesectionof the diagram.
An exampleof the methodis presentedin Figure 3; the number
of polesN is 165,the sampleareaA is0.05,the expecteddensity
E is 8.6,and the standarddeviationa is 2.9. As shownin Figure
3b, the only statisticallysignificantpole concentrationsin
Figure3a are two clusterswith meanstrikesand dipsof about
N20øW, 63øSW,and N52øW, 55øNE.
DATA AND INTERPRETATION
Mojave Desert Wells
Five wells were drilled in the westernMojave desertas part
of an in situ stressmeasurementprogram along the locked
portion of the San Andreasfault in southernCalifornia. The
siteslie alongthe part of the fault that last rupturedin the 1857
Fort Tejon earthquake. Throughout the western Mojave
desert,Tertiary formationsrest unconformablyupon a surface
of pre-Tertiary crystallinerocks that underwentdeep erosion
duringlate Cretaceousand early Tertiary time [Dibblee,1967].
In middleto late Miocenetime the Mojave blockwasdeformed
primarily by normal faulting along northwesttrending faults
[Dibblee, 1967]. Geologic and paleomagneticstudiesindicate
that as much as 10% north-south shortening of the wedge
betweenthe Garlock and San Andreas faults may have occurredduring the Plioceneand Quaternary [Ponti and Burke,
1979]. This shorteninghas been accommodatedby strike slip
faulting on northwest striking faults, thrust faulting on eastwest trendingfaults, folding, and possibleblock rotation. The
nature of the deformationis consistentwith an appliednorth-
SEEBURGER AND ZOBACK' DISTRIBUTION
168
OF NATURAL
FRACTURES
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E'
....................
S
W"
N
.........
1"8'3m
' N
S
Fig. 2. Televiewerpicturesfor 15.2m of well in two 7.6-msections.
Top of sequence
is at upperleft,bottomat lowerright.
Numerousfracturesare presentwhichappearsasdark sinusoidalbands.
5520
SEEBURGERAND ZOBACK' DISTRIBUTION OF NATURAL FRACTURES
LKC
N
E
N
b
E
W--
S
E=8.6
o'= 2.9
Fig. 3. (a) Lowerhemisphere
equal-areastereographic
projectionof fracturepolesencountered
in well LKC. (b) The
corresponding
orientation-density
diagram.The standarddeviation,a = 2.9, and the samplingarea, A = 0.05, are determinedby the numberof fractures,N = 165,usingthe methodof Karnb[1959]. A randomdistributionof poleswouldyield
pole densitiesbetween2a and 4a. Pole densitiesgreater than 6a representstatisticallysignificantpreferredfracture
orientations.Two significantpole clustersare shownhere: a north-northweststriking southweõtdippingclusterwith
maximumcontourof 10a and a northweststriking,northeastdippingdusterwith maximumcontourof 6a.
south compression.Geodetic measurements[Savage and
Prescott,1979;LisowskiandSavage,1979;PrescottandSavage,
1976;Thatcher,1976] indicatethat presentdeformationin the
western Mojave near Palmdale has a direction of maximum
shorteningof N10ø-lSøW and a directionof maximumextension of about N75ø-80øE.This deformationfield is basically
near Valyermo to Adobe Mountain in the westernMojave
desert(Figure4).
Well 1 wasdrilledon the north limb of a largesyncline2 km
southwest of the San Andreas fault and between the San An-
consistent with the in situ stress field orientations determined
dreasandPunchbowlfaults(Figure4).Well 1 is theonlywellin
this studywhich did not penetratecrystallinerock. The data
and analysisare presentedherefor completeness.
The a•(isof
by hydraulic fracturingin two of the wells consideredin this
study,Mojave 1 and 2 [Zobacket al., 1980].
Four of the wellsin the westernMojave desertcomprisea
north-northeasttrendingprofile running acrossthe San An-
sitethedip of thenorthlimb wasapproximately
450-50øto the
southwest.
The well wasdrilled to a depthof 245 m, entirely
throughthe upperMiocenePunchbowlformation.Here the
dreas fault from the foothills of the San Gabriel
Punchbowlformation consistsof massive,cross-bedded,
coarse
35'00'
Mountains
thesyndine
strikes
N70øWandplunges
to thewest.At thewell
118'15'
117'15'
'
ow,.os
AIR
ROS
At•M
OND
'
-,4
':'
FORCE
BASE
'•
•,
MOJAVE
•,
LANCASTER
'•'4 DESERT
PALMDALE
VE;TORVI LLE
.• MEASUREMENT
SITES
0
i
IO
i
•
20 KM
•
,
34015'
Fig. 4. Map showinglocationof wellsin the westernMojave desert,California. Well 1 is about 2 km southwestof the
SanAndreas
faultandwasdrilledentirely
through
theMiocence
Punchbowl
sandstone.
Wells2, 4, 5,andXT[,Rare4, 22,
34,and 4 km, respectively,
northeastof the SanAndreasfault.All waredrilledin Mesozoicquartzmonzonitewhichis part of
the suiteof Mesozoicintrusiveswhichformthe basementof muchof thewesternMojave.
SEEBURGERAND ZOBACK' DISTRIBUTION OF NATURAL FRACTURES
terrestrial sandstonewith large lensesof pebble and cobble
conglomerate[Noble, 1954]. At surfaceoutcropsthe formation
is relativelyunfractured,althoughthereare beddingplanefracturesspacedat 1- to 3-m intervalsand someverticaljoints with
locally varying attitudes,spacedat 3- to 7-m intervals[Sbaret
al., 1979].
Wells 2, 4, 5, and XTLR were drilled at buttes located 4, 22,
34, and 4 km, respectively,northeastof the San Andreasfault
(Figure4). Thesewellsweredrilledentirelyin quartzmonzonite
to depthsof 257, 230, 239, and 869 m, respectively.
The quartz
monzonitebuttes are part of the suite of Mesozoic intrusives
which form the basementof much of the western Mojave.
Horizontal and steeplydipping fracturesand joints were observedin outcropsat thesesites.The buttes are apparently
composedof slightly more siliceousmaterial than the surroundingbasement,and are thus more resistantto erosion(D.
B. Burke,personalcommunication,1980).
The rock penetratedby the Mojave wells was found to be
highlyfractured.Figure 5 containsfrequencyplotsshowingthe
number of fracturesper meter as a functionof depth for each
well, and Figure 6 containsplots of the cumulativenumber of
fracturesin each well as a function of depth. The number of
fracturesobservedin well 1, drilledin sandstone,
is significantly
lessthan that observedin the wellsdrilled in quartz monzonite.
The numberof fracturesper meteris not uniform throughout
eachwell. Fracturesoften occurin relativelydenseclusters,as
at about 55 m and 115-125 m in Mojave 4 (Figure 5). These
fracturedintervalsappearin Figure 6 as steeperportionsof the
cumulative
fracture curve. The cumulative
fracture curve for
Mojave 2 appearsto flatten somewhatat depthsgreaterthan
160m, implyinga decreasein the fracturedensity.In the other
wells, the curvesindicate that the fracture density may be
decreasingwith depth below 150-200 m, but there is only a
very moderatetrendto lowerfracturedensitywith depth.Also,
note that in the upper 150 m the number of fracturesencounteredin Mojave 2, 4, and 5 are similar.The data show no
NUMBER
00
I
I
5521
OF FRACTURES
60
I
I
120
I
I
180
I
,\
5O
'\
:
x\
I00
_
150
_
_
"•
i
•
_
-
:•õ0 -
\
II
xx5
4
:
Fig. 6. Cumulative number of fracturesplotted as a function of
depth for the Mojave wells.The sandstoneencounteredin Mojave 1 is
significantlylessfractured than the granite encounteredin wells 2, 4,
and 5. In general,only a slightdecreasein the numberof fractureswith
increasingdepthis observed.
tendencyfor the fracturedensityto increaseasthe San Andreas
fault is neared.
MOJAVE
1
MOJAVE
2
FRACTURES
0
2
,
!
4
,
,
0
2
4
MOJAVE
PER
0
4
5
METER
2
.__L•
MOJAVE
0
4
i
,
I
2
!
I
4
,
,
__
__
__
__
__
__
__
150-__
200_
---TD
TD-
Fig. 5. Frequencyplot of the numberof fracturesper meter as a
functionof depthin the Mojave wells1, 2, 4, and 5.
Polesto all of the dippingfracturesare shownin Figure7a.
Figure 7b containsthe orientation-density
plots derivedfrom
the pole distributions.At least two significantmaxima are
observedin the densityplots for each well. For all wells, the
secondarymaximaare significantat only the 6a level.For wells
2 and4 the primarymaximaalsobarelyexceedthe 6a level.
In Mojave 1 the predominantfractureset (> 10a) strikes
N20ø-30øW and dips 350-40ø to the southwest.A secondcluster (> 6a) with fracturesstriking slightlyeast of north is also
present.Beddingplanes,which would strike about N70øW and
dip about45ø-50øSW,werenot observed.
In Mojave 2 a broad
cluster(>6a) of northwesterlystriking, southwestdipping
(-,, 50ø)fracturesare found.In Mojave 4, fracturesetstrending
N35ø-55øE and dipping 500-60ø to the northwest and
southeastare foundat the 6a level.Most fracturesin Mojave 2
and 4 appearto be randomlyoriented,with minor significant
maxima.In contrast,in Mojave 5 a fracturesetstrikingabout
N80øEanddippingsteeply( -,,60ø)to thesoutheast
isextremely
pronounced(> 16a). A secondset of fractures(>6a) is also
observedin Mojave 5 strikingnorthwestand dipping to the
northwest.Note that the majorityof observedfracturesdipped
from 40ø to 70ø. Very few horizontalfractureswereobservedin
thesewells:3 in Mojave 1, 2 in Mojave2, 3 in Mojave4, and4
in Mojave 5.
Fracturesoccurringbelow a depth of 137 m were analyzed
separatelyto testfor depth dependencein the fractureorienta-
5522
SEEBURGER
ANDZOBACK'
DISTRIBUTION
OFNATURAL
FRACTURES
SEEBURGER
AND ZOBACK: DISTRIBUTIONOF NATURAL FRACTURES
5523
By mid-Triassicmuchof the Piedmonthadbeenworn down
to a broad peneplainwhich truncatedthe exposedcomplex
rock structures.Subsequentre-elevationof the region was accompaniedby the formationof northeast-southwest
trending
structuraltroughs.Thesetroughsare generallyparallelto regional northeast-southwest
trendingAppalachianstructures.
Further uplift in Late Triassic or Early Jurassictime was
accompaniedby the emplacementof numerousnorthwestsoutheasttrending diabasedikes intruded along preexisting
fractures.After another erosional cycle, during which the
Piedmontwasagainreducedto a peneplain,continentaluplift
elevatedthe region now occupiedby the Appalachiansand
adjacentPiedmontareas.Sincethe Jurassic,the region has
undergoneweathering,erosion,and depositionwithout any
toward unfracturedrock with depth.
One significant(> 14a) clusterof poles was found in well major tectonicdisturbances.
The two wells,Monticello 1 and 2, were drilled to depthsof
XTLR striking N5ø-25øW and dipping about 60øSW.A comparisonof Figures7 and 10 showsthat the fractureorientations 1100and 1203m, respectively,
aspart of a programto studythe
found in Mojave 2 and XTLR are nearly the same.With the stressesassociatedwith induced seismicityat Monticello Reincreaseddepth and numberof fracturesin XTLR, the level of servoir.Both wellsweredrilled in intrusivegranodioritebodies
of late Paleozoicage.The drill sitefor Monticello 1 waslocated
significance
of thispoleclusterincreasedsubstantially.
Deep measurementsof the stressfield in XTLR [Zoback et on the top of a broad ridge westof the centerof the reservoir.
tion. Polesand densityplots for the fracturesfound in these
intervalsareshownin Figure8. A comparison
of Figures7 and
8 showsthat each of the maxima presentbelow 137 m also
appearswhenfracturesin the entirewell are considered.
Thus,
the fracturepatternsin the deeperintervalsare basicallysubsetsof thepatternsfoundthroughoutthe wells.
Well XTLR was drilled at the same site as Mojave 2. The
resultsof fracturestudiesperformedin this well are shownin
Figures9 and 10.From Figure9 it canbe seenthat the quartz
monzonitewas highlyfracturedthroughout.Note that only a
slightdecreaseof fracturingwith depth was observed.The
greatestdecreasein fracturedensitywasfound at about 140 m,
as in the shallow wells. There is, however, no strong trend
a!., 1980] indicate that the minimum horizontal stressis the
Monticello
2 was drilled within
1 km downstream
of the dam
minimumprincipalstress
belowa depthof about300m. Below which impoundsthe reservoir.The two wells are about 5 km
this depth strike slip faulting would be preferred. To test
whetherthe changefrom a thrust-typeenvironmentto a strike
slip environment.affectedthe fractureorientations,the distri-
butionof thosefracturesfoundbelow300 m in XTLR was
considered(Figure 11). One significant(> 14a) cluster was
found.The orientationof this clusteris indistinguishable
from
the preferredfracture orientation found above 300 m, in the
entirewell,and from Mojave 2. The levelof significance
for this
fractureorientationis greatestwhen the deepfracturedata are
included.The primary effectseenin the near-surfaceintervals
apart. A northward projectionof severalkilometersplacesthe
north-south trending, steeplydipping, dip slip (down to the
east)Wateree Creek fault near and to the west of the location of
Monticello 2 [Secor,1980].
Boreholeteleviewerrecordswereobtainedfor the total depth
of each well. The well conditions were excellent,and the recorded data quality wasgenerallyvery good.
The resultsof the fracturesurveysare presentedin Figures13
and 14. Figure 13 showsthe number of fracturesas a function
of depthin eachwell. Figure 14 showspolesto fractureplanes
seems
to betheaddition
ofrandomly
oriented
fractures
which and orientation-densityplots.
The data showthat the stateof natural fracturingin the two
served
to decr•sethesignificance
of theshallow
intervalclusters.Therefore,the changein minimum stressorientationprod- wellsis significantlydifferent.The total number of fracturesin
ucedno noticeablechangein the observedfracturedistribution. Monticello 2 is approximately 3 times that in Monticello 1
(Figure 13). Fractures in Monticello 1 were found to occur
Monticello Reservoir, South Carolina
mostlyin discreteintervals,suchas at 140, !90, 300, and 580 m
Two approximately1-km deepwellsweredrilled near Monwithin relativelyunfracturedrock.In contrast,the granodiorite
ticello Reservoir,Fairfield County, South Carolina (Figure 12). encounteredin Monticello 2 was highly fractured,particularly
Thewellsitesarein an areaunderlain.
by a complex
series
of
from the surface to about 275 m and from about 460 to 510 m.
almandine-amPhibolite
facies,
metamorphic
rocks,andgranitic Due to the highly fractured nature of the rock, small discrete
intrusivesof the Charlotte Belt lithologiesof the Piedmont fracturezones,as found in Monticello 1, are not as apparentin
Province [Damesand Moore, 1974]. These consistof interlay- Monticello 2. The lowest fracture densityin Monticello 2 is
eredandfolded
gneisses,
amphibolite,
andschist,
all ofwhich
found from about 510 to 750 m and is similar to the fracture
havebeenintrudedby plutonsof graniteto granodioritecom-
densityfound in Monticello 1 at the samedepth.Below 750 m,
a slight increasein the fracturedensityis observedin Monticello 2. It is interestingto note that the slopeof the cumulative
position.The followingdescriptionof the geologichistoryof
the regionhasbeensynthesized
from the work of Overstreetand
Bell [1965], Fisheret al. [1970], and DamesandMoore [!974].
fracture curve for Monticello
2 in the intervals 275-460
m and
Mostof themetamorphic
rocks
presently
exposed
in the 750 m-TD are about the same and the segmentsare nearly
Piedmont were originally depositedas a thick sequenceof colinear.In the intervalfrom 460 to 750 m, strainenergyrelease
shale,siltstone,volcanictuff, etc. in the 'Appalachiangeosyn- was apparently concentratedin the denselyfractured zone at
cline' between250 and 600 m. y. ago. Episodesof folding, 460-510 m. As a result,the fracturedensityfrom 510 to 750 m is
regionalmetamorphism,and igneousintrusionapparentlyfol- the lowestfoundin this well. In neitherwell is therean apprecilowed eachof three long sedimentationintervals.At the end of abledecreasein the numberof observedfractureswith depth.
thelatePaleozoic,
thefinaldepositional
period,thepreviously Twenty-six of the 147 fracturesin Monticello 1 and 65 of the
formed metamorphic rocks and the unmetamorphosedsedi- 439 fracturesin Monticello 2 were horizontal.This is by far the
ments experiencedregional metamorphism.During this time highestdensity of horizontal fracturesof any of the data sets.
the Piedmont was uplifted, accompaniedby faulting, folding, Horizontal fractureswere found throughouteach well; howand the intrusion of discordant plutons. Also during this ever, about half of eachtotal was found in the upper 300 m of
period, northwestwarddirected overthrustingoccurred on a the well (Figure 15).In both wells,severalhorizontal fractures
major scalein the southernAppalachians.
werefoundat depthsgreaterthan 1 km. In addition,the density
5524
SEEBURGER
ANDZOBACK'
DISTRIBUTION
OFNATURAL
FRACTURES
II
II
II
II
•J
b
Ez
II
II
z<•
b
I
Ez
z
(,/3
-
<[ Lu b
Ez
_'2
z.-••'- '
Ld
II
II
z<•
z
-
b
(,/3
SEEBURGER
AND ZOBACK.'DISTRIBUTIONOF NATURAL FRACTURES
5525
XTLR
FRACTURES/METER
2
I
I
4
!
I
CUMULATIVE
6
,
0
FRACTURES
2OO
i
I
I
4OO
I
6OO
i
I
20
40
60
80
TD
TD
lOO
Fig. 9. Frequencyplot of numberof fractureplottedas a functionof depthfor well XTLR (left)and cumulativenumberof
fracturesplottedasa functionof depthfor XTLR (right).
of horizontalfracturesis relativelyhigh in Monticello2 in the
interval400-500 m. The presenceof thesehorizontalfractures
may be due to decreasedconfiningstresses
as a result of the
post-Jurassic
erosionalhistoryof the area.
The orientation-density
plot for Monticello1 (Figure 14)
showstwo smallsignificant
maxima(contours> 6•) with approximateorientationsof strike N45øE, dip 60øE and strike
N
w
XTLR
E
S
N50øW, dip 25øW. For Monticello 2, there is one extremely
densecluster of poles(maximum contour > 12•) with a strike
of about N5øE and dip 70øE.
In addition to the major differencesin fractureorientation
found within a horizontal distance of only 5 km, there were
major differencesin fracture orientation vertically in each well.
In Monticello 1 this differenceis shown in Figure 16 in which
N
W
E
S
A =0.016
E=9.0
0-=3.0
Fig. 10. Lowerhemisphere,
equal-area
plotsof polesto all dippingfractures
in wellXTLR (left)andorientation-density
diagramforall dippingfractures
(right).Themaximumcontours
(14a)indicatethatthepreferred
fractureorientation
strikes
north-northwest
anddipssteeplyto the southwest.
This poledistribution
compares
favorably,particularlyin the deeper
interval,with that foundin Mojave2, whichwasdrilledat the samesiteasXTLR. The strikeof thissignificant
fracture
orientation
isparallelto thedirectionof themaximumregionalcompressive
stress,
possibly
implyingthatthesefractures
are
beingheld openlike tensilefeatures.
5526
SEEBURGERAND ZOBACK: DISTRIBUTION OF NATURAL FRACTURES
XTLR
300 m-TD
E W
E
S
A = 0.02;5
E=9.0
o'= 3.0
Fig. 11. Lowerhemisphere,
equal-areastereographic
polediagramsfor all dippingfracturesbelow300m in XTLR.
orientation-densityplots are shown for the intervals surface-
2, but as shown in Figure 16, this fracture set is most pro-
305m, 305-610m, and610m-TD. In theupperzone(surface- nouncedat depthsgreaterthan 600 m. However, this orienta305 m) one significantclusterwith northweststrike and southwest dip is apparent, a subsetof the cluster found when all
fractures encountered in the well were considered. In the middle
zone(305-610 m), a northeaststriking,southeastdippingcluster is apparent.This clusteris alsoapparentwhen all fractures
are considered.In the pole densityplot for the bottom third of
the well (610 m-TD), the distribution is seento be essentially
random.In Monticello 2 (Figure 17) the fracturesin the upper
zoneform two significantclusters(> 6a): one strikingapproximately north-southand dippingto the east,and one striking
eastnortheastand dippinggentlyto the southeast.In the lower
interval (610 m-TD) thereis a densecluster( > 16a) of fractures
strikingnorth-southand dippingto the east.This north-south
striking set is prominent when the fracturesas a whole are
considered.
The fracture cluster found in the interval
305-610
m hasa strike which differsby about 60øfrom that foundin the
rest of the hole. This group of fracturesis the west-northwest
trending, northeast dipping set responsiblefor the extended
lobe on the densityplot for the entirewell (Figure 14).
Secor [1980] presentsthe resultsof joint studiesmade at
surfaceoutcropsnear MonticelloReservoir.At outcropswithin
a few kilometers of the well sites,fracture distributions similar
tion is similar to that of the Wateree Creek fault, where studied
several kilometers south of the Monticello
2 site.
It has been noted that the fracture distribution in the interval
305-610 m in Monticello 2 is markedly different than that
found aboveand below.The majority of the fracturesfound in
thisintervalwerelocatedin a highlyfracturedintervalcentered
at a depthof about 500 m (Figure 13).Pore pressuremeasurementsmade in the well revealeda zone of artesianpressures
from above 394 m to somewhereabove 591 m [Zoback and
Hickman,1982].The presenceof the artesianzonesuggests
that
impoundmenthas raisedthe pore pressurein an aquifer represented,perhaps,by the distinct fracture set found in the
interval 305-610
m.
Limekiln Valley, California
Fracture studieshave been performedin three wells drilled
near the creepingsectionof the San Andreasfault in central
California.Thesewellswere drilled to depthsof about 220 m in
Cretaceousquartz monzoniteof the Gabilan Range.The wells
were located 2, 4, and 14 km west of the San Andreas fault
(Figure 18).
The Gabilan Rangeis part of the Coast Rangesprovinceof
to those found in both Monticello 1 and 2 were found. Secor
centralCalifornia.(For a detailedsummaryof the geologyof
concluded that there was little regional consistencyto the thisarea,seePage [ 1966] and Compton[ 1966].)The structural
orientation of the major joint sets.The marked differencewe and stratigraphichistory of the Coast Rangesprovincehas
have observedin the two wells seemsto support this con- beenquitecomplex.In the Tertiary,folding,high-anglereverse
clusion. Surface fractures were also studied on the cleared
faulting,thrusting,and strike slip faultingin the Coast Ranges
bedrock surfaceat the site of the Virgil C. Summer Nuclear have been more or less simultaneous.All of these types of
Station [Dames and Moore, 1974]. This site is about 6 km deformationhaveaffectedPlio-Pleistocene
depositsin one area
southeast of the Monticello
1 site and 1 km east of Monticello
or another,implyingthat all havebeenat leastlocallyactivein
2. Poles to 247 fractureswith no observedsheardisplacement the Quaternary.The Gabilan Rangeitselfis a broad granitic
across their faces and to 85 fractures which exhibited either
complex enclosingnumerouslarge metamorphicrelics. Its
shear displacementor hydrothermal alteration were plotted lengthisabout75 km in a northwest-southeast
direction,with a
[Damesand Moore, 1974]. In both cases,a pole densitymaxi- maximum width of about 15 kin. It is boundedon the east by
mum occurredfor planesstrikingabout N44øE, dippingmore the San Andreas fault and on the north, west, and south by
than 60øSE. This orientation is very similar to one of the Cenozoicclasticsedimentaryrocks.As a whole, the Gabilan
maxima found in Monticello 1. However, other maxima in the
Rangeappearsrelatively undeformed,althoughthere is evisurfacedata, suchas a northeasttrending,northwestdipping denceof small-scalefoldingand lineation.
Geodetic measurements of strain across the San Andreas
set, are not apparent in data from either well. There is no
surface indication of the pronouncednorth-south trending, fault [Thatcher, 1979; Savageand Burford,1973] have been
eastwarddippingset of fractureswhichis found in Monticello modeledby the motion of relatively rigid blocks.There has
SEEBURGER
ANDZOBACK:
DISTRIBUTION
OFNATURAL
FRACTURES
5527
..... 4'-.,•'.• ,• Map
Location
"I
•
N
Belair
Belt
i
1
kilometers
Monticello
Monticello
1
Monticello
Reservoir
Monticel Io 2
0
i
5
i
I
I
[
kilometers
Fig.12. Mapshowing
location
ofwells
near
Monticello
Reservoir,
Fairfield
County,
South
Carolina.
Both
wells
were
drilled
inintrusive
granodiorite
bodies
oflatePaleozoic
ageintheCharlotte
Belt.
beenno evidenceof strainaccumulation
on eithersideof the
Televiewer
picturequalitywasgenerally
goodin wellsLKB
faultsystem,
sothattherelative
motionappears
to beaccom- and LKC, but well LKD wasso intenselyfracturedthat the
modated
byslipaccumulation
in a verynarrowzonecenteredoveralldata qualityis relativelypoor.The fracturedensity
offractures
permeter)
isgreater
in these
wellsthanin
on the San Andreasfault. Suchmotion is consistentwith a (number
(compare
Figures
6, 16,and19).
roughly
north-south
regional
compression.
However,
ananaly- anyoftheotherwellsstudied
sisof foldtrends[Page,1966]indicates
thatthedirection
of LKC, theleastfracturedof theLimekilnwellsismoredensely
thanMojave4 and Monticello2, the mostdensely
maximumshortening
is N35ø-50øE.
Also,earthquake
focal fractured
mechanisms
indicate
general
north-south
compression
butwith fracturedwellsin the othertwo data sets.Only in LKC is there
indication
thatthenumber
offractures
isdecreasing
considerable
scatterrZobackand Zoback,1980].Gawthrop a possible
proximity
to theSan
r1977],
ina study
oftheseismicity
ofthecentral
coastal
region withdepth.Thereisnorelationbetween
of California,foundthat the leasthorizontalprincipalstress Andreasfaultandthedensityof fracturingin thesewells.LKD,
was orientedaboutN60øW,This conclusionwasbasedupon the well farthestfrom the fault, encounteredthe mostfractures,
to thefault,liesbetween
LKC and
anaverage
of30events,
withapproximately
equalnumbers
of whileLKB, thewellclosest
strikeslipandthrustevents.
Thedirection
of maximum
com- LKD in fracturedensity.
Polesto dippingfractureplanesand orientation-density
pression
variedfromN10øWto N60øE,
withan average
of
in Figure20.The fractures
aboutN30øE,closeto thedirection
inferredfromfoldorienta- plotsfor eachwellarepresented
tions.
encounteredin the Limekiln wells seemto belong to a few
5528
SEEBURGER
ANDZOBACK:
DISTRIBUTION
OFNATURAL
FRACTURES
MONTICELLO 1
MONTICELLO 2
FRACTURES / METER
CUMULATIVE
I00
200
I
I
FRACTURES
300
400
I
I
I
I
200
4OO
2
E 50600
800
iooc
120C
Fig.13. Frequency
plots
ofthenumber
offractures
permeter
plotted
asa function
ofdepth
forMonticello
1 and2
plotted
asfunctions
ofdepth.
Thenumber
offractures
encountered
in Monticello
2 wasabout3 timesthatencountered
in
Monticello
1andthere
islittle
indication
inthese
wells
ofadecrease
inthenumber
offractures
withincreasing
depth.
well-developed
fracturesystems
as,in eachcase,themaximum cluster(> 10tr)hasan approximatemeanstrikeof N65øEand
contouronthedensity
plotisat least10it.In wellLKB, located dip of 55øE. There are also two clustersin the orientation-
2 km westoftheSanAndreas
fau;*,twosignificant
clusters
of densityplot for well LKC, whichis located4 km from the San
polesareobserved.
Themostsignificant
cluster
(> l&r) hasa Andreas
fault.Themostsignificant
cluster
(> 10tr)hasan apmeanstrikeofaboutN60øWanddipof60øW.
Thesecondaryproximatemeanstrikeof N20øW and dip 65øW,whilethe
MONTICELLO
1
MONTICELLO
N
2
N
E
W
S
S
N
W
E
--r--
s
W
E
=0.07
E
cr= 2.8
s
E=.O
o'= 3.o
Fig.14.Lower
hemisphere,
equal-area
diagrams
forMonticello
1 and
2.(a)Poles
toalldipping
fractures.
(b)
Orientation-density
diagrams
foralldipping
fractures.
Although
located
only5 kmapart,
thefracture
patterns
found
in
thesewellsareverydifferent.
SEEBURGERAND ZOBACK' DISTRIBUTION OF NATURAL FRACTURES
0¸
CUMULATIVE
20
HORIZONTAL
40
FRACTURES
60
are apparent on the orientation-densityplot for well LKD,
located 14 km from the fault. The most significant cluster
(> 14tr)has an approximatemean strike of N10øE and dip of
60øW.The two lesserclusters( > 8tr)haveapproximateorientations of N5øW, dip 65øE and N40øE, dip 65øE; these two
clustersmay not be statisticallydistinct. Orientation-density
plots for the upper 110 m and 110 m-TD for each well are
shownin Figure 21. A comparisonof the fractureorientations
in the shallow and deep sectionswith those of the entire well
indicatesthat the orientationsare consistentthroughout each
ß
I
I
I
I
5529
I
2OO
entire well.
400
FRACTURE ORIENTATION AND REGIONAL STRESSES
Theoriesof natural fracturegenesiscan be dividedinto four
basic groups:(1) tensilefracturesdue to applied compressive
stresses,e.g., longitudinal splitting [Jaeger and Cook, 1969]
found in planes perpendicularto the direction of least compression[seeEngelderand Gelset,1980],(2)shearfracturesdue
to compressivestresses,
(3) tensilefractureswhich accommo-
6OO
MONTICELLO
1
date extension due to a net tensile stress or to the removal
of
confiningstresses,
e.g.,fracturingdue to residualstresses[En800
gelder, 1979], and (4) natural hydraulic fracturing [Secor,
1965]. Conjugatepairs of shearfracturesmay develop,with an
angle of about 30ø betweenthe direction of maximum compressivestressand the fractureplanes;however,often only one
of the conjugatepairs becomesa well-developedfracture set
iooo [Wilcox et al., 1973].
The measuredshearstressesin thesewells are currently low
(lessthan 100 bars) [Zoback et al., 1980; Zoback and Hickman,
1982], even in unfracturedintervals. If suchlow shear stresses
Fig. 15. Cumulative number of horizontal fracturesplotted as a
are representativeof the stressstateunder which thesefracture
functionof depth for Monticello 1 and 2. About half of the horizontal
fractures in each well were found at depths above 300 m. Several setswere formed,extensivedevelopmentof shear setswould
not be expectedand most of the fractureswould be of tensile
horizontal fractureswere also found at depthsof about 1 km in both
wellsand in the highly fracturedintervalsbetweenabout 400 and 500
origin. It is interestingto note, however,that in severalcases
m in Monticello 2.
(e.g., in LKC and LKD, Figure 20) the pole clustersin the
orientation-densityplots could be interpretedas indicativeof
secondarycluster (>6a) has an approximatemean strike of the presenceof conjugate shear fracture sets.Whether these
clustersare due to the fractureshaving formed in shear,to the
N50øW and dip 55øE.The fracturepopulationpickedfor LKD
is only a subsetof the total population due to the intense resultsof shearmotion on favorably oriented,preexistingtenfracturingand resultantdata quality problems.Three clusters sile fractures, or to processesnot involving shear stresses
M
0 m-505
m
O N
T
505
N
ICE
L L 0
m - 610
I
rn
610
N
W
E
m-TD
N
W
EW
E
N=25
S
E= 6.6
0-=2.2
S
A =0.15
ß
E =7.6
S
0-=2.5
A = 0.166
E = 7.5
0-=2.5
Fig. 16. Orientation-densitydiagramsfor Monticello 1 for the intervalssurface-305m, 305-610 m, and 610 m-TD.
Different fracturedistributionsare foundin eachof theseintervals:shallowdipping,northweststrikingfracturesin the upper
interval; southeastdipping and northeaststriking fracturesin the intermediateinterval; and a random distribution in the
lower interval.
5530
SEEBURGERAND ZOBACK: DISTRIBUTION OF NATURAL FRACTURES
MONTICELLO
Om-305m
305
2_
rn-610
N
rn
610 rn -TD
N
W
>
E W
EW
86
-
-"-'-F--'-
?.(r
j
A - 0.054
S
N
A - 0.083
E -8.5
S
o- = 2.8
A - O.07Z
E - 8.3
'
S
o- = 2.8
E - 8.4
o- = 2.8
Fig. 17. Orientation-densitydiagramsfor Monticello 2 for the intervalssurface-305m, 305-610 m, and 610 m-TD. In
the upper interval two significantpole maxima are found at the 6a level.One of the maxima is part of the north-south
strikingset found when the fracturepopulationof the entire well wasconsidered(Figure 14). The secondshallowdipping
group showsup as a slight(>4a) concentrationin Figure 14. In the intermediateinterval a strongnorthweststriking,
northeastdippingset is found. Most of the fracturesin this interval are found in the denselyfracturedzone at about 500-m
depth.In the bottom intervalthe north-southstrikingfracturesetis veryprominent;thereis little evidencebelow600 m for
the northweststrikingsetof the intermediateinterval.
cannot
be determined
here. From
the televiewer
data
it is
impossibleto determine whether the fracture setsare of shear
or tensileorigin or whetherany sheardeformationhas occurred
acrossthe fracturessubsequentto their creation.
The Mojave and Limekiln Valley wellswere locatedcloseto
the San Andreasfault. The stressfield responsiblefor causing
displacementon the fault and the deformationassociatedwith
faulting might be expectedto affect the fracturedistributions.
For example,Friedman [1969] studied the macrofractureand
microfracture
distributions
from cores recovered
from
wells
drilled near the Oak Ridge fault, a major reversefault in the
Ventura basin. Friedman concludedthat conjugateshear sets
of fractureshad developed,with one set parallel to the fault
plane.He alsoconcludedthat the abundanceof microfractures
increaseswith proximity to the fault and is independentof the
depth of burial. The Mojave and Limekiln wellsclosestto the
San Andreasfault were 2 km away from it. It is possiblethat
closerto the fault the fractureorientationsmay parallel the San
Andreas(Friedman's[1969] data generallycamefrom within a
half kilometer of the mappedsubsurfacelocation of the Oak
Ridge fault). However,the data presentedhere showthat there
is no tendencyfor the fractureplanesto align themselves
with
the San Andreas
fault
nor
is there
an increase in fracture
densityasthe fault is approached.
36 ø 52.5'
Salt
Bautista
CA L A VERA S FA UI T
LKB
Salinas
0
IO
ki Iometers
121ø45.0'
'•'
MEASUREMENT
SITE
36030.0 '
121ø7.5'
Fig. 18. Map showinglocation of wells near Hollister, California. Wells LKB, LKC, and LKD are 2, 4, and 14 km
southwestof the SanAndreasfault,respectively.
Thesewellsweredrilledin Cretaceousquartz monzonite.
SEEBURGER
AND ZOBACK:DISTRIBUTION
OFNATURALFRACTURES
FRACTURES/METER
o
2
4
FRACTURE
2
0
5531
/ METER
4
i
I
50
i
i
i
iiii
ii
I
I00 E
E
__
150-
25O
LKB
LKC
250-
FRACTURES/METER
2
4
i
6
i
O
CUMULATIVE
IO
FRACTURES
20
d
30
!
5O
o150
-
2OO
•'•
200-'
LKC
250.
LKD
LKD
250-
Fig.19. (a)-(c)
Frequency
plotofnumber
offractures
permeter
plotted
asa function
ofdepth
forwells
LKB,LKC,and
LKD,respectively.
(d)Cumulative
number
offractures
plotted
asa function
ofdepth.
In general,
these
wells
encountered
moredensely
fractured
rockthananyoftheothers
inthisstudy.
Again,
there
isonlya slight
tendency
forthenumber
of
fractures
todecrease
wtihdepth.
Also,thereisnotendency
forthenumber
offractures
toincrease
neartheSanAndreas
fault.
at depthsof lessthan 1 km [Talwaniet al., 1978].The earthquakes
appearto occurin dusters
andapparently
donotdefine
linear fault planes.Eventsin the differentclustersall have
slightlydifferentcomposite
focal mechanisms.
Most earthof focalmechacello ! and 2, the differencein magnitudebetweenthe two quakesare of the thrusttype.A comparison
closeto theMonticello! site(Figure18)
horizontal
principal
stresses
isrelatively
smallexcept
at shallow nismsfor earthquakes
depthwherethegreatest
horizontal
principal
stress
issubstan- with the shallow fracture orientationsin the well showsfairly
of possible
faultplanesandsignificant
grouptiallygreaterthantheverticalstress.
Asdiscussed
by Zoback goodagreement
[ZobackandHickman,
1982].A comand Hickman,this verticalstressprofileis indicativeof con- ingsof naturalfractures
in the upperthirdof Monticello2
ditionsconducive
to thrust-typefaultingin the upper300m or parisonof poledensities
focalmechanisms
for earthquakes
soif appropriately
oriented
fractureplanesarepresent.
Seis- (34-305m) withcomposite
nearthatwellsitealsoshows
a goodcorrelation
with
mologic
studies
confirmthatthereservoir-induced
earthquakes occurring
faultplanes.Thus,theshallowearthquakes
whichhavebeenrecordedin thisareaare apparentlyoccurring oneof thepossible
At Monticello Reservoir,the current local stressfield has
beendetermined
by in situstress
measurements
usingthehydrofracture
technique[ZobackandHickman,1982]and from
earthquake
focalmechanisms
[Talwaniet al.,1978].In Monti-
5532
SEEBURGER
AND ZOBACK: DISTRIBUTION OF NATURAL FRACTURES
LKB
LKC
E
LKD
W
E
W
E
S
S
S
N
N
N
E W
E
W
E
=
s
A = 0.036
E = 8.7
o': 2.9
s
A = 0.052
E = 8.6
cr: 2.9
s
83
A = 0.031
E = 8.8
cr= 2.9
Fig.20. Lower
hemisphere,
equal-area
diagrams
forLKB,LKC,andLKD.(a)Poles
to alldipping
fractures.
(b)
Orientation-density
diagram
foralldipping
fractions.
Ineach
well,
dense
pole
concentrations
were
found.
Notethat,again,
there
isnosignificant
fracture
orientation
common
toeach
ofthepopulations.
at Monticello Reservoir seem to be associated with shear
motion on preexistingfracturessuchas thoseencounteredin
these wells.
orientations
of fractures
nearfaultsmaychangewithtimeand
deformation,and Hodgson[1971] found little correlationof'
jointswithfoldaxesin hisfieldmappingof theCombRidge
Asshownin Figures
7, 14,and20,theorientation
of signifi- area of the Colorado Plateau.
cantfracture
clusters
variesrapidlyfromwellto wellin a given
SUMMARY AND CONCLUSIONS
region.If thefractures
in anyareaaretheresultof regionally
applied stresses,this difference of fracture orientations would
In this paper, observationsof the natural fracture distri-
implythat severalfracturemechanisms
are involvedand/or butionsfoundin 10wellsfromthreedifferentregions
of North
that extensivepostfracturedeformationhas occurred.Alternatively, the varied fracture orientationscould be the result of
localvariationsor amplifications
of thestressfield.
Furthercomplications
are causedby lithologyvariations.
For example,the difference
in fracturedensityobserved
in
Mojave1,drilledentirelyin sandstone,
andtheotherMojave
wells,drilledin crystalline
rock,may be duein part to the
different thermal historiesof theserock units.
Clearly,themajorcomplication
in attempting
to analyze
the
mechanism
of fractureformationin the uppercrustis the
uncertainty
ofthegeologic
historyoftherockmass.
Rocks
may
undergo
repeated
tectonic
loadings
withresultant
folding,
fracturing,and faulting.Each tectoniccyclemay add its own
distinctfractureset,or preexisting
fracturesetsmaybemodified.Thefractures
orjointswhichareobserved
todayreflect
the
entiredeformational
historyof the rock.The existence
of fracture setswhichcannotbe explainedon the basisof the current
Americahavebeenpresented.
All but one of thesewellswere
drilled in granitic rock. Numerous fractures were found
throughouteachwell, and only a slightdecrease
of fracture
density
wasobserved
withdepth.Thus,fromthedatapresented
here,fractures
incrystalline
rockcouldprobably
beexpected
at
depthsfar in excess
of 1 km. In somewells,fracturesseemed
to
be relativelyuniformlydistributed
with depth,whilein others
theywereprimarilyconcentrated
in densely
fractured
intervals.
At leastonestatistically
significantconcentration
of fracture
planeswasfoundin everywell.In the Mojaveand Limekiln
Valley wells,thesesignificantfractureconcentrations
did not
varymuchwithdepth.However,in the Monticellowells,more
variationof fractureorientation
withdepthwasfound.In all of
thewells,mostfractures
weresteeplydigging,
andfewhorizontal fractures were observed.
The fracturedensity
in thewellswasfoundto varysignifi-
cantlywithina givenregiondue,apparently,
to bothdifferences
stressfield is not unexpected.
As examples,Tchalenkoand in lithology(in the Mojave wells)and/or local structuralor
Arnbraseys
[1970],Freund[1974],and Wilcoxet al. [1973] stressconcentration
effects(in the Monticellowells).The orienfoundfromfieldmapping
andclaymodelexperiments
thatthe tation of the mostsignificantfractureconcentration
wasalso
SEEBURGER
ANDZOBACK'
DISTRIBUTION
OFNATURAL
FRACTURES
LKB
LKC
LKD
O- I10 m
O- I10 m
N
N
w
5533
0-110
m
N
Ew
,•••-E
W
E
A= 0.052
S
E = 8.5
S
o'= 2.8
I10 m-TD
S
½=2.8
IIOm-TD
N
A = 0.083
E=8.0
IlOm - TD
N
A = 0.17
S
cr = 2.7
E= 8.6
o'= 2.9
N
N-S
E = 8.3
A = 0.072
E:7.5
S
o' = 2.5
E= •8.2
o' = 2.7
,
Fig.21. Lower
hemisphere,
equal-area
orientation-density
diagrams
fortheupper
(toprow)andlower
(bottom
row)
sections
of LKB,LKC,andLKD to showanyvariation
of fracture
orientation
withdepth.Littlevariation
of fracture
orientation
withdepthisseenin thesewells.
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(ReceivedMarch 24, 1981;
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