Regional stress alignments in the Kutai Basin, East ...docshare01.docshare.tips/files/24299/242999696.pdf · separates this basin from the Tarakan Basin on its northern boundary.

Regional stress alignments in the Kutai Basin, East Kalimantan,Indonesia: a contribution from a borehole breakout study

N. Syarifuddin *, I. Busono

Exploration Department, VICO Indonesia Co., 9th Floor, Kuningan Plaza South Tower, Jl. H.R. Rasuna Said, Kav. C11-14, Jakarta-12940,

Indonesia

Received 12 August 1997; accepted 10 September 1998

Abstract

Borehole breakout data from 134 wells located within the Kutai Basin region, East Kalimantan were analyzed to determine

the present day regional horizontal stress alignments. The data were extracted from various types of dipmeter logs. The studyreveals that the majority of the data give a coherent picture of breakout orientation. The mean azimuths for the entire-unweighted, ellipticity-weighted and magnitude-weighted data sets are preferentially aligned in the regional-mean direction of48.98 N or 128.98 N. Most of the data have a low dispersion value (So) and the ranking of reliability in Zoback's classi®cation

is À'. There is no signi®cant azimuth variation with depth. These relatively consistent alignments of breakout azimuths indicatethat the maximum regional stress direction in the study area is NW±SE. This regional-mean of breakout azimuths deviates fromthe axis of the anticlinorium trends and from the strike of the thrust±fault patterns in the region. It is believed that these

structural patterns are in¯uenced by reactivation of weak zones related to sediment loading (structural inversion). # 1999Elsevier Science Ltd. All rights reserved.

1. Introduction

The study area is mostly located onshore, at the geo-graphic center and geological depocenter of the LowerKutai Basin, East Kalimantan, Indonesia (Fig. 1).

The Kutai Basin is known as a major producer ofboth oil and gas in Indonesia. In terms of oil pro-duction, the basin ranks second behind CentralSumatra and contains up to 14% of Indonesia's oilreserves (Paterson and Syarifuddin, 1995). In gas pro-duction, the basin ranks second behind the Natuna gasreserve, containing up to 27% of Indonesia's gasreserves. The basin also has six of the 300 (2%) worldgiant ®elds (sized of 0.5±5 BBOE), and seven of the1,000 (0.7%) large ®elds (sized of 50±500 MMBOE).The reserves in the basin are found mainly along NNEanticlinal trends.

Wells drilled in the study area consistently showborehole ovalizations known as borehole breakouts.The aims of this study were to determine the regionalpattern of the breakout orientations and their relation

to the regional stress system. To achieve the studyobjectives, breakout data from 134 wells located withinthe Kutai Basin region, East Kalimantan were ana-lyzed to determine their main preferred orientation.

2. Geological setting

The Kutai Basin covers an area of approximately165,000 km2. It is the largest Tertiary Basin inIndonesia. The basin is underlain by a basement ofcontinental crust, part of Sundaland Craton, andaccreted microplates. The basin is bounded by theNW±SE Adang Flexure (also known as PaternosterBoundary Fault), separating it from the Barito Basinto the south (Fig. 2). The W±NW Mangkalihat Archseparates this basin from the Tarakan Basin on itsnorthern boundary. To the east the basin is open tothe Makassar Straits, while to the west it is boundedby the Kuching High. The Basin is divided into theUpper and Lower Kutai Basin. The Upper Basin con-sists of Paleogene sediments and is characterized by astrong NW±SE structural grain. The Lower Basin hasdominantly Miocene sediments and NNE structures.

Journal of Asian Earth Sciences 17 (1999) 123±135

1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0743-9547(98 )00049 -X

PERGAMON

* Corresponding author.

The basement beneath the Lower Kutai Basin isinterpreted to be continental in origin (Asikin, 1995).The rafted segments docked with the earlier basementin the Late Cretaceous to Paleocene (70±60 Ma).Subsequent Tertiary basin formation occurred acrossthese accreted docking zones and the Lower KutaiBasin is located over the most easterly suture zone.

During Paleocene±Eocene times, the Kutai Basinwas initially extensional with the formation of E-facingrift grabens. This was followed by a quiescent periodduring the Middle Eocene to Late Oligocene. Thepaleogeography was controlled mainly by the NNEMeratus trend and NW±SE Cretaceous transformfault trend.

The uplift and erosion on the NW Kalimantan mar-gin (Kuching High) during the Late Oligocene±Early

Miocene provided the sediment supply to the basin.Sedimentation was accommodated by continuation ofPaleogene ¯exural subsidence and minor extensionalfaulting.

The basin history from Middle Miocene to the pre-sent day is one of progressive west to east basin inver-sion. It is believed that basin inversion was due tomultiple collisions of microplates with a subductingboundary along the northwestern margin ofKalimantan.

Structurally the Sanga±Sanga PSC (Production-Sharing Contract) area is characterized by a series oftightly folded synclinal and anticlinal folds with aNNE±SSW trend, forming the Samarinda anticlinor-ium system (Fig. 3). Various structural models, includ-ing gravity gliding and contractional thrusting have

Fig. 1. Location map of the Sanga±Sanga PSC showing the main producing ®elds in the Kutai Basin.

N. Syarifuddin, I. Busono / Journal of Asian Earth Sciences 17 (1999) 123±135124

been proposed by various authors (Ott, 1987; Roseand Hartono; 1978, Asikin, 1995). The latest model isthat proposed by Ferguson and McClay (1997) as partof an integrated basin study conducted by VICOIndonesia Co. It is believed that the formation of theSamarinda anticlinorium is due to a two-stage defor-mation mechanism: early di�erential sedimentary load-ing, resulting in delta-top graben controlled byregional and counter-regional faults; followed by con-traction, causing reactivation of the graben boundingfaults to form detachment-style inversion structures.

Further contraction resulted in faulted folds, due tothe buttressing e�ect of the hanging wall on the faultwall. Hence the structural kinematics of Sanga±SangaPSC was most likely a detachment-style of inversion.

3. Stress around boreholes

As a borehole is drilled, the horizontal stress isrelieved. The borehole contracts to obtain an equili-brium between the radial stresses and the pressure of

Fig. 2. Regional tectonic map of Kalimantan. The Kutai Basin is bounded by the NW±SE Adang Flexure (south), the WNW±ESE Mangkalihat

Arch (north) and the Kuching High (west).

N. Syarifuddin, I. Busono / Journal of Asian Earth Sciences 17 (1999) 123±135 125

the mud column ( pm), minus formation pressure ( pf ).If the compressive strength of the formation is toolow, and the hydrostatic pressure of the mud columnis not large enough to overcome the imbalance, bore-hole breakout occurs in the direction of least principalstress (Fig. 4). Borehole breakout can occur in anylithology.

Well grouped orientations of breakouts have beenreported in compressive (Bell and Gough, 1979;Springer, 1987), tensile (Stock et al., 1983; Bell andBabcock, 1986) and strike-slip (Cox, 1983; Fordjor etal., 1983; Zoback and Zoback, 1989) stress regimes. Inall areas where reliable in-situ stress measurements areavailable, the mean of the breakout orientation can beshown to be parallel to sHmin (minimum horizontal

stress). This relationship is well established empirically,and strongly supported by theoretical explanations(Bell and Gough, 1979; Zoback et al., 1985).

Bell and Gough (1979) presented a theoretical hy-pothesis based upon the Kirsch equation, as proposedby Hubert and Willis (1957), to support their theorythat breakouts are caused by symmetrical spalling ofthe borehole wall, where stress concentrations exceedthe shear strength of the formation. Development ofthis hypothesis by Zoback et al. (1985), based on aborehole televiewer study, provides a clearer expla-nation of the origin of breakout.

If the stress components are expressed in polar coor-dinates, at point (r, y) exterior to the hole, then (referto Fig. 5 for notations):

Fig. 3. Structural trends in the Sanga±Sanga PSC. The outcropping fault patterns are drawn on top of SAR imagery. Note the overall NNNE±

SSW trend.


sr � 1=2�sHmax � sHmax��1ÿ a2=r2�� 1=2�sHmax ÿ sHmin��1ÿ 4a2=r2 � 3a4=r4�cos 2y� a2=r2�pm ÿ pf� �a�

sy � 1=2�sHmax � sHmin��1� a2=r2�ÿ 1=2�sHmax ÿ sHmin��1� 3a4=r4� cos 2yÿ a2=r2�pm ÿ pf� �b�

tr; y �1=2�sHmax � sHmin��1� 2a2=r2 ÿ 3a4=r4� sin 2y �c�

At the borehole wall sy has the maximum value of{(3sHmaxÿsHmin)ÿ a 2/r 2( pmÿpf )} at the ends of thediameter perpendicular to the sHmax. Consequently atthe ends of the diameter aligned with the sHmax, sywill be the minimum value. The sHmax is ampli®edaround the hole perimeter and the sy is not evenly dis-tributed around the borehole and tends to accumulateparallel to the least horizontal stress (sHmin) due to therealignment of stress trajectories near the borehole.

Recently, aligned breakouts have been reported inmany areas. Most authors agree that the horizontalprincipal stress azimuth can be deduced from breakout

orientations (Bell and Gough, 1979; Fordjor et al.,1983; Zoback et al., 1985; Evans and Brereton, 1990;Hillis and Williams, 1992).

3.1. Breakout identi®cation

The best tools for identifying breakouts in boreholesare imaging devices that record the shape and orien-tation of the breakout: borehole televiewer, acousticborehole televiewer (Zemanek et al., 1970; Plumb andHickman, 1985), and resistivity-measuring formationmicro-scanner (Ekstrom et al., 1987). However, themost economical, and hence most readily availabledata, come from four-arm dipmeter logs. Although thedesign of these tools varies, most contain electrodepads mounted on hydraulically extendible calliper padsspaced 908 apart (Fig. 6). The reference point, pad 1,is magnetically oriented. Two independent callipersmeasure the borehole diameter between pad 1±3 andpad 2±4, as well as its azimuth, as the tool is pulled upthe hole. The orientation of the tool is recorded by aninclinometer and magnetometer. Fig. 7 shows typicaldipmeter calliper logs and the associated interpretationof the borehole shape.

Since borehole shape can be in¯uenced by phenom-ena other than breakouts (washouts, asymmetric mud-caking, fracture intersection, hydraulic fractures),breakout identi®cation criteria for dipmeter log analy-sis have been proposed by several authors (Babcock,1978; Fordjor et al., 1983; Springer, 1987). These aresummarized in Table 1.

Zoback and Zoback (1989) proposed criteria forranking the reliability of breakout data (Table 2).There are four ranks: A, B, C and D, with the most re-

Fig. 4. An illustration of a borehole breakout and its stress interpret-

ation. Note the curvilinear shear failure zones which tend to be

oriented in the direction of minimum stress.

Fig. 5. Conventional signs and notation for stress conditions around

a borehole, as used for the equations given in the text.


liable data being A and the least reliable being D. Theranking system provides a quantitative means of deter-mining the relative reliability of di�erent data set.

4. Breakout analysis

A total of 4876 examples of borehole breakout data,from 134 wells located within study area, were used inthis analysis. The data were extracted from varioustypes of dipmeter logs: HDT, SHDT, FMS, by usingthe criteria previously described (Table 1). The studiedwells can be clustered geographically into eight areas(Fig. 8).

Recorded breakout-parameters include: depth ofbreakout midpoints, short and long calliper readings,long-calliper azimuths, as well as type of lithology.The results were tabulated and two additional par-ameters were then computed: calliper ellipticity (ratioof long-calliper reading over short-calliper reading)and breakout magnitude (di�erence between long-calli-per reading and bit-size). The ellipticity variable (ELL)measures degree of borehole elongation and it alsore¯ects the `clarity' of breakout. The magnitude vari-

able (MAG) represents depth of borehole breakout/enlargement in the long-axis direction.

Statistical analyses were carried out using a methodfor directional data (Mardia, 1972). Microsoft Excelsoftware version 5.0 was employed for calculation.

The breakout azimuths were linearly weightedaccording to the value of ellipticity and magnitude.This approach gives equal prominence to both thevery large numbers of low ellipticity or magnitudevalues, and to the much smaller number of highervalues. Since cross-sectional elongation detected on thefour-arm dipmeter log can be assumed to be symmetri-cal, azimuth data were assigned to the range of 08 to1808 during calculation. The circular mean and dis-persion of the breakout azimuth were then computed.Criteria developed by Zoback (Table 2) were used torank the reliability of the results.

The majority of the boreholes analyzed give a coher-ent picture of breakout orientation in the study area(Table 3). The mean azimuths for the entire-unweighted, ellipticity-weighted and magnitude-weighted data sets mostly lie in the range of 35.5±56.18N or 215.5±236.18 N, except for data from the westernarea which show a di�erent mean azimuth of 93.7/273.78 N (Fig. 9). Most of the data have a low dis-persion value (So) about the mean orientation, with adispersion range of 9.7±23.58. Data from the Sambojaarea have the most consistent breakout orientation.Breakouts were recorded between depths of 488 and10,622 ft. The 205 breakouts observed in that area,result in mean azimuth of 48.9/128.98 N and the dis-persion only 9.78. Hence the ranking of reliability isÀ'.

The borehole breakouts in the studied wells wererecorded over a depth range of 100±15,000 ft. There isno signi®cant azimuth variation with depth (Fig. 10).Clearly the breakout azimuths are aligned in the re-gional-mean direction. However, breakout azimuthsobserved in deeper section indicate a tighter clusterthan those occurring in the shallower section. It is con-sidered that with increasing lithi®cation, as indicatedby burial depth or age, formations are less likely todevelop breakouts with a `scatter' azimuth (seeDiscussion). Deeper data are considered to yield morereliable breakout-azimuth groupings.

More con®ned breakout azimuth grouping in thedeeper section can also be seen from vertical pro®lesof ellipticity and magnitude variables. Plots of bore-hole ellipticity show a slight increase with depth. Thisindicates that the borehole breakout is `clearer' atgreater depth, as expressed by relatively higher ellipti-city values. The absence of signi®cant breakout magni-tude variation with depth supports this argument.

Lithological analyses of breakout parameters are il-lustrated in Fig. 11. It can be seen that breakoutoccurs in all type of lithologies. A large number of

Fig. 6. Typical dipmeter tool. Two independent calliper measure-

ments between pads 1±3 and 2±4 can be acquired simultaneously,

together with their orientation.


breakout observations have been made in shale, coal

Fig. 7. Calliper arrangements which may be obtained by the dipmeter tool and their interpretation.

Table 1

Breakout identi®cation criteria for diameter log analysis

(a) The logging tool must stop rotating

(b) Signi®cant changes in conductivity should be recorded by all

four pads

(c) There should be no conductivity anomaly recorded by one or

two pads

(d) The larger caliper curve should exceed hole gauge. The

smaller should not be less than hole gauge and its trace

should be straight

(e) The larger diameter of hole elongation should not coincide

with azimuth of hole deviation

(f) The direction of the hole elongation should not coincide with

azimuth of hole deviation

(g) The large-diameter curve must show a sudden, rather than a

gradual increase

Table 2

Ranking criteria of breakout reliability (after Zoback and Zoback,

1989)

Rank Criteria

A Breakout for single well with dispersion less than 158.Average of breakouts in two or more wells in close

geographic proximity.

B Breakout in a single well with dispersion 15±308.C Less than four distinct breakouts, which have consistent

orientation in single well.

D Breakout in a single well with dispersion more than 308. Asingle breakout in a well.


and sandstone. The study area is characterized by adeltaic sequence and the formation is dominated bythose types of lithologies.

Interesting phenomena can be observed from litho-logical-ellipticity and magnitude curves. Both curvesindicate high average values in coal zones. These may

be related to the brittle behavior of coal. Thick coalbeds were frequently encountered during drilling wellsin the area.

In contrast, the curves show lowest average valuesin shales. As indicated previously, shale is more likelyto undergo plastic, rather than brittle deformation.

Fig. 8. Location map to show the areas from which samples used to compile the rose diagrams in Fig. 9 were obtained.


Hence, at the time of logging, the breakouts may notbe well developed, resulting in relatively low breakoutmagnitudes.

4.1. Stress interpretation

Borehole breakout data, taken from four-arm dip-meter logs in the study area, show a generally pre-ferred NE±SW direction, and with almost E±Wdirection in the western area. Within a given well, thedirection of maximum borehole elongation (breakout)is relatively consistent, regardless of depth or lithology.The data are most consistent at greater depth.Considering these facts, a relationship between bore-hole instability and regional stress acting in the studyarea is proposed.

If the tightly-grouped azimuths of breakout in thestudy area indicate orientation of the horizontal stresscomponents, following Bell's hypothesis, they indicatethe orientation of the minimum horizontal principalstress (sHmin) to be NE±SW in all but the westernarea. The inferred stress orientation also indicates thatthroughout the study area the direction of the maxi-mum horizontal stress sHmax is NW±SE. This argu-ment is supported by the insigni®cant variation ofbreakout-azimuth with depth.

The breakout analyses do not identify the verticalprincipal stress component. However, the breakout azi-muths nearly coincide with the trends of the fold axesin the anticlinorium (Fig. 3). The azimuths are alsonearly parallel to the strike of the thrust-fault pattern.Therefore, the breakout azimuths are perpendicular tothe axis of basin shortening. These indicate stronglythat a compressive stress system is still active and is re-sponsible for the borehole breakouts. The stress com-ponents of s1 and s2 are horizontal, oriented NW±SE

and NE±SW respectively, while s3 is most likely verti-cal.

5. Discussion

Breakout statistical analysis indicates a relativelyconstant NE±SW azimuth trend. However, breakoutparameter plots also show a subsidiary azimuth trendwhich is orthogonal to the principal trend. Small num-bers of orthogonally oriented observations are present.

Orthogonal azimuth trends have been ascribed tothe generation of true breakout, drilling-inducedhydraulic fractures or borehole intersection with stee-ply dipping fractures. Bell and Babcock (1986)suggested that high drilling mud pressures, which areproduced by using dense additives to stabilize bore-holes drilled in partially consolidated rocks, not onlyminimize spalling in the minimum stress direction butmay also initiate localized hydraulic fracturing in themaximum stress direction. Shallower rock units havebeen subjected to smaller con®ning pressures for ashorter period of time than deeper ones. They are alsoless consolidated and more prone to hydraulic-frac-turation for a given excess mud-weight. This allows agreater percentage of the shallow breakout-azimuthmean to be distorted. The higher dispersion in shallowobservations is, therefore, not surprising. In addition,BluÈ mling et al. (1983) indicated that tensile fracturingin the maximum stress direction was dependent uponthe maximum to minimum horizontal stress ratio. Aspreviously suggested by McGarr and Gay (1978), thecontrast between two horizontal principal stress mag-nitudes is not large enough in shallow rocks to ade-quately constrain the direction of spalling.Additionally, it was also observed that there is anincrease in the e�ective horizontal stress di�erence

Table 3

Breakout statistical summary for unweighted, ellipticity-weighted and magnitude weighted data

Unweighted Ellipticity Weighted Magnitude Weighted

Area n Mean

Azimuth (deg)

Dispersion

(deg)

Zoback's

class

Mean

Azimuth (deg)

Dispersion

(deg)

Zoback's

class

Mean

Azimuth (deg)

Dispersion

(deg)

Zoback's

class

O�shore 286 47.94 23.50 B 47.82 23.16 B 47.86 23.31 B

Delta 2501 43.16 14.30 A 42.97 13.65 A 42.52 11.69 A

Interior 158 35.54 16.90 B 35.34 16.17 B 34.83 18.32 B

Semberah 219 56.05 12.13 A 55.41 11.97 A 54.82 12.46 A

Mutiara 615 38.92 12.22 A 38.35 12.27 A 38.77 12.67 A

Samboja 205 48.92 9.68 A 48.78 9.40 A 49.30 9.04 A

South 700 54.74 14.60 A 54.41 14.32 A 53.23 15.40 B

West 192 93.72 12.39 A 93.97 12.42 A 93.11 12.26 A


(contrast) with depth. As an alternative, Babcock

(1978) indicates that the subsidiary, orthogonal azi-

muth group may be due to the borehole intersecting

existing steeply dipping fractures. A borehole intersect-

ing such fractures would encounter a strength aniso-

tropy, favorable to the development of borehole-enlargement parallel to the fracture orientation.

It is impossible to di�erentiate conclusively betweenthe real breakout and drilling-induced hydraulic frac-ture using only borehole calliper-enlargement data.However, most authors agree that minimum horizontal

Fig. 9. Map showing the mean azimuth for borehole breakout orientations plotted for the areas shown on Fig. 8. Note consistent NE±SW pre-

ferred orientation.


stress direction can be determined from the regional-mean azimuth. Whatever the mechanisms, it seemsthat orientations which correlate orthogonally with theregional-mean breakout orientation may be alignedparallel to the maximum horizontal stress direction.

6. Conclusions

The study reveals that the majority of the data givea coherent picture of breakout orientation. Mean azi-muths for the entire-unweighted, ellipticity-weightedand magnitude-weighted data sets are approximatelyaligned in the regional-mean direction of 48.9 or128.98 N. Most of the data have a low dispersion

value (So) and the ranking of reliability in Zoback'sclassi®cation is À'.

There is no signi®cant azimuth variation with depth.Breakout azimuths are nearly constantly aligned in theregional-mean direction. Data from greater depths givea tighter cluster than those from shallower sections,due to compaction processes. Lithological analysis ofthe breakout data indicate that breakout occurs in alltypes of lithology.

The breakout azimuths show a relatively consistentNE±SW alignment. This indicates that the maximumregional stress direction in the study area is in a NW±SE direction. This regional-mean of breakout azimuthsmakes a small angle with the axes of folds in the anti-clinorium and thrust-fault trends in the region.

Fig. 10. Depth pro®les for breakout data from the Delta region (left) and the Samboja area (locations on Fig. 8). Note the consistent preferred

orientation of the data through depth (ft), although shallow data are more scattered than deeper data.


Therefore, the breakout azimuths are perpendicular tothe axis of basin shortening. These relationships indi-cate strongly that a compressive stress system is stillactive and is responsible for the borehole breakouts.The deviation of the breakout azimuths from the anti-clinorium axes and the thrust fault trends may bebecause these structural trends were in¯uenced by thereactivation of weak zones related to the earlier exten-sion and sediment loading (structural inversion).

Acknowledgements

The authors wish to convey their sincere gratitudeto colleagues in the Exploration Department VicoIndonesia, especially to Steve G. Corbin (currentlyExploration Manager), for encouragement and valu-able advice concerning the presentation of this paper,and to Angus J. Ferguson for discussion and input.We wish to thank the Management of VICOIndonesia and its partners, as well as Pertamina, forpermission to publish this paper.

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