32- 2009 Khoshbakht -Image Log - JPSE J

Journal of Petroleum Science and Engineering 67 (2009) 65–74

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Journal of Petroleum Science and Engineering

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Comparison of Asmari, Pabdeh and Gurpi formation's fractures, derived fromimage log

F. Khoshbakht a,⁎, H. Memarian b, M. Mohammadnia a

a National Iranian oil Company (NIOC)-Research Institute of Petroleum Industry (RIPI), Tehran, Iranb University of Tehran, Tehran, Iran

⁎ Corresponding author.E-mail address: [email protected] (F. Khoshbakht)

0920-4105/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.petrol.2009.02.011

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 January 2008Accepted 25 February 2009

Keywords:image logopen and filled fracturesorientationfracture density

Small scale fractures around the wellbore, strongly affect the hydrocarbon production. Identification offracture plays a main role in production and development plans of oil fields. Image logs are one of thepowerful tools in fracture study in wells. Image log is a high resolution “pseudo picture” of borehole wall.Using these logs help to identify the exact orientation, depth and type of natural fractures. In this study,image logs of a well in a naturally fractured reservoir were interpreted in order to extract fractures.Statistically analysis has been done to identify the fracturing pattern. Present study shows that pattern offracturing in Asmari and Pabdeh is similar but, the Gurpi formation is completely different. Fractures densityin thin bedded intervals is high hence the maximum fractures density observed is in the upper Asmari andPabdeh formations. Fracture density log shows two descending trends that are related to mechanism offolding.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In naturally fractured reservoirs, fractures control behaviors of thereservoir. When fractures are open, they act as pathways forhydrocarbon production and may even transform a very lowpermeability reservoir into highly productive zones. When cemented,they act as barriers to hydrocarbon flow hindering the motion ofhydrocarbons toward the well (Haller and Porturas, 1998). In almostall of reservoirs in the south west of Iran fractures play a main role inpetroleum production hence the recognition of fracture pattern andtheir distributions are vitally important in exploration, production anddevelopment of oilfields. In all phases of production and developmentof fields, knowledge of fracture properties is essential in decisionmaking of selecting perforation location, deviated and horizontalwells trajectory determination and recovery plans.

Core study, well test, flow-meter, thermal survey, pressure test,production rate, mud loss, seismic data, well logs, stoney wave andimage log are used for fracture detection in various scales. Core studyis the ordinary method for small scale fracture determination inboreholes. Using cores include 3 serious limitations in fracturedreservoirs: low recovery factor in fractured zones, lack of orientationand high cost. Because of these restrictions using image logs is moreadvantageous.

In the present study interpreted fractures in image log of a wellwhich passed entirely through three formations (Asmari, Pabdeh and

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Gurpi) statistically analyzed to identify fracture patterns and dis-tribution in aforementioned formations.

There are two main categories of factors affect fracture generationin folded structures: The first category is geomechanical properties ofrock such as lithology, porosity, grain size and fabric of beds and thenext category is geometrical factors such as: bedding thickness andstructural setting (Nelson, 2001). The well which was considered inthis study is a slightly deviatedwell and three formations are drilled inthis well, thus structural setting affects could be omitted. The study ofsystematic tectonic related fractures leads to the recognition offractures pattern aroundwellbore. If adequate data from other sourcessuch as cores, outcrop and seismic data will be available; fracturemodeling of a reservoir could be done with low uncertainty. Theresults of this study provide a reliable source of data that could beused in fracture modeling.

2. Background

A few years after oil discovery in the Middle East carbonates,geologists claimed that the most possible reason for extraordinaryproduction of carbonate reservoirs is developing of fractures in theseformations (Lees, 1933, 1938; O'Brien 1953). During the late 1960s till1979 the National Iranian oil company employed lots of geologists tostudy Asmari fractures. These researchers published their findings inmany private reports (Sangree et al., 1961; Twerenbold andMcQuillan,1962; Halsey, in press; Barker and Speers, 1978, 1978;…) and somepapers (McQuillan, 1973, 1974, 1985 Colan-Sadd, 1978). After thisgolden stage of fracture study in Iranian fields, for two decades therewas no serious motion in Asmari fractures study. In recent years with

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Fig. 1. Structural setting of the Zagros fold-thrust belt showing the major fault zones, distribution of oil and gas fields (Sepehr and Cosgrove, 2004).

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the developing of various well logging tools such as borehole imagelogs, DSI and seismic imaging, a large number of fracture data wereobtained from the subsurface. Image logs arewidely used for studying

Fig. 2. Mesozoic–Cenozoic stratigraphy correlati

small scale fractures. Most of image logs studies are restricted toidentification of fractures in single wells. There is very limited numberof studies which used the interpretation of image log for integrated

on chart of the Zagros basin (Motiei 1995).

68 F. Khoshbakht et al. / Journal of Petroleum Science and Engineering 67 (2009) 65–74

fracture study. Rezaie and Nogole-Sadat (2004) established a relationbetween the curvature of the Asmari Formation and fracturing systemby using the FMS logs. Results of new techniques confirm someprevious studies and reject others.

2.1. Geological setting

Zagros fold-thrust belt extends about 2000 km from southeasternTurkey through northern Syria and Iraq to western and southern Iran(Alavi, 2004). Structurally, Zagros basin is placed in the north of theArabian plate. The studied field is located in Dezful embayment ofZagros basin in south west of Iran (Fig. 1). Zagros basin is a part of theTethys Ocean and is one of the most important petroleum basins in allover the world (Alavi, 1994). This basin is located in the south-west ofIran and north of Iraq. Geological history of this basin include longtime subsidence and deposition which interrupted by short timeuplift. Folding process of this basin occurred in Miocene and Plioceneand continued till now which formed long anticlines (Mottie, 1995).These anticlines constitute most of oil traps in this basin.

2.2. Studied formations description

Alavi (2004) collects all lithostratigraphic units of the Zagros fold-thrust belt data and published the perfect description of formations.The summary of three studied formations is mentioned below:

Asmari (Oligocene to lowermost Miocene): medium-bedded tothick-bedded, locally shelly or oolitic, nummulites-bearing lime-stones (grainstone, packstone, wackestone) shoaling upwardabove a thin basal conglomerate from fine-grained (low-energy)deep-marine marly limestone to high-energy shallow-marineskeletal grainstone; composed of a number of sequences; aunconformity-bounded, highly prolific reservoir; interpreted astransgressive–regressive foredeep facies of the proforeland basin(Fig. 2).Pabdeh (Upper Paleocene to lowermost Oligocene): thin-beddedGlobigerina-bearing deep-marine hemipelagic–pelagic calcareous

Fig. 3. Edited UGC map of field a

shale, marl, and limemudstone with subordinate argillaceouslimestone containing fish fossils; foredeep facies grading towardthe northeast into sandy Shahbazan dolomitic limestone andtoward the southwest into sabhka-type Jahrum carbonates. Gurpiformation is comprised of pelagic marl with calcareous marl andshale (Fig. 2).Gurpi (Upper Cretaceous; Santonian toMaastrichtian): dark bluishgray, thin-bedded, deep-marine, Globigerina-bearing pelagic marl(and marly limestone) and hemipelagic claystone; foredeep facies(resting unconformably on Sarvak and onlapping Ilam) resultingfrom prograde southwestward migration of proforeland depo-zones (Fig. 2).

2.3. Specification of the studied well

The studied well is located in south western flank of a carbonatefield in Iran (Fig. 3). The field consists of a slightly asymmetricalanticline with the axis oriented approximately northwest to south-east. The southwest flank is steeper. Structural dip and azimuth are 37and 145.6°. Asmari, Pabdeh and Gurpi formations are entirely drilledand image logs are acquired from the three formations. Asmari wasdrilled from 1730 m, after that Pabdeh is reached in 2235 m and Gurpiis located from 2451 to 2571 m drilling depth. This well is drilled bywater basemud so imager tool usedwas the full-bore formationmicroscanner (FMI). Totally the quality of FMI is well, except for somelimited small distortion because on some occasions the tool gets stuck.

2.4. Image logs

Image log is a “pseudo-picture” of borehole wall which image thephysical property of borehole wall such as electrical resistivity oracoustic impedance. In the first method resistivity of borehole wall isconverted into high resolution resistivity image of wall. Resistivityimage is acquired by pad base imager tool, 16–32 buttons beingmounted in each pad. There are 2, 4, 5 or 8 pads on an imaging tool.Each pad covers limited part of borehole wall (Serra, 1989). Acousticimager records amplitude and travel time of reflected ultra-sonic

nd location of studied well.

Fig. 4. After processing, digital resistivity converted into colorful image log, top row is nominal resistivity and bottom raw is a colorful spectrum. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)


pulses. In different processing steps, the raw measured resistivity oramplitude and travel time were converted to a colorful image (Fig. 4).

3. Data sources and procedures

This study is executed in three phases; processing and interpreta-tion of image log, classification and analysis of data and finallyinterpretation of results. The first acquired raw FMI digital data wasloaded into professional software. Speed correction, eccenteringcorrection, normalization and image filtering had been done beforestarting interpretation. Next generated image log was interpreted inorder to recognize structural and sedimentological features. Apparentorientation of interpreted features converted into true orientation bytaking into account hole azimuth and deviation. At last the resultswere interpreted and compared with previous findings about Asmarifractures.

Structural features were divided into two main parts; fracturingand compactional features. Open and filled fractures, fault and flexuralslip were fracturing subcategory while structural and compactional

Fig. 5. a) Medium thickness bedding in Asmari, b)vague b

stylolites, solution seams and bedding plane were compactionalsubcategories. The main aim of image log interpretation wasidentification of these two major parts of structural features. In thisstudy compactional features weren't included and only the fractureswere taken into account. Summaries of interpreted features in studiedwell are described below.

3.1. Bedding

In an image log, bedding boundary is defined as a plane which twosides of it show different resistivity or acoustic impedance; thereforein image log bedding appears as abrupt change in image color whichusually could be seen in all pads.

In the upper Asmari very thin shaly layers and anhydrite/limestonecontact is abundant. Very thin lamination in upper Asmari's carbonateanddense streakwere indicators of bed boundary. In themiddle part ofAsmari the quantity of bed boundaries was very limited and formationchanges to massive limestone with anhydrite nodules (Fig. 5a). In thispart, variation of texture formed vague bed boundaries (Fig. 5b). In

edding boundary in Asmari (depth:1707.8 to 1709 m).


some depths, changing in texture caused variation in macroscopicporosity such as vuggy porosity. In some short intervals (b0.5 m)flattening of anhydrite nodules creates pseudo-layers.

The one third upper part of Pabdeh formation consists of verydense thick carbonates hence in the two third lower part there werevery thin lamination in carbonates.

There was no sharp bedding plane in Gurpi and this formationconsists of very compacted carbonates. Diagenesis process influenceGurpi and destroyed sharp bedding plane. Vuggy porosity, vuggyfracture and lots of filled fracture were some signs of diagenesis.

With increasing of depth, bedding dip does not show significantchange. Dip of bedding changes from 28.8 to 49.5 and beddingazimuth varies from 115.2 to 163.3°. There may be no main faultcrossing this well which would significantly change the beddingorientation. UGC map and geophysical data verify this point.

3.2. Fractures

3.2.1. Open fracturesAperture of this kind of fractures filled with mud and if the mud is

conductive, open fracture trace shows a dark (conductive) appearancein image log (Serra, 1989). For the reason that resistivity of matrix ismore than conductive mud, open fracture appears as a darkcontinuous or discontinuous sinusoidal wave in the image log.

The majority of Asmari's open fractures were discontinuous withsmall openings. In some fractured intervals there were lots of mediumdip open fractures with openings of about a few millimeters (Fig. 6a).Most of the open fractures exist in the Pabdeh formation (Fig. 6b)which solution changes some of them into vuggy fractures. In a few

Fig. 6. a) Open fractures in Asmari, Themajor set (longitudinal set) cross cutminor set (shear

intervals there were some sinusoidal open fractures where plenty ofdiscontinuous small non-oriented fractures crossed them. There werevery limited numbers of open fractures in the Gurpi formation. Mostof them were vuggy fractures and some of them were partially filled.

3.2.2. Filled fractureMaterials that fill the opening space of fractures usually are

crystalline minerals. Because of denseness and lack of porosity,resistivity of these materials is too high. Filled fractures appear as awhite sinusoidal wave in image log. Additionally, if resistivity offracture filling material is higher than the matrix, a “Halo effect” effectappears around the filled fracture trace. Halo effect is a common signfor filled fracture recognition in carbonate.

Almost all filled fracture in the studied well show halo effect andwere discontinuous. In Asmari there were some discontinuous filledfractures which showed halo effect. In Pabdeh formation filledfractures were abundant and halo effect appears in peak and troughpoint of sinusoidal trace of fractures (Fig. 7a). Gurpi formation's filledfractures have medium to high dip and showed strong halo effect.

3.2.3. Discontinuities parallel to beddingFlexural slips are themovement along bedding and usually occur in

the lamb of the fold at the time of folding. Someof the interpreted openfractureswere parallel to the bedding planes. These kinds of features incore which displacement could be seen are named flexural slip, but inimage logs it is impossible to see the displacement along beddingplane. Comparison of cores and image logs of other wells of this fieldproved that discontinuities parallel to bedding in image log areflexuralslip in core. “Discontinuities parallel to bedding” sometimeswere used

set), (b) Clear bedding plane and open fracture in Pabdeh Formation (depth: 2333.5–35).

Fig. 7. a) Filled fracture in Pabdeh formation recognized by “Halo effect” (depth: 2416.5–17.8 m), b) An example of discontinuity parallel to bedding.


to name this type of fractures. Flexural slip formed where thesuccession of competent layer such as limestone and incompetentsuch as shaly layer existed. This conditionwas present in some parts ofAsmari and Pabdeh where the formations are well bedded, in contrastto Gurpi formation which almost of it is completely incompetent thatthere was no discontinuity parallel to bedding. Correlation betweenbedding plain density and flexural slip density clearly shows thisphenomenon.

3.2.4. FaultFault is a type of fracture that displacement occurred along the

fault plane. If displacement is more than thewell diameter, movementwill not be visible in image log and in this situation indirect evidencesshow the location of fault. Some of these factors are:

1. abrupt change in dip and azimuth of bedding;2. abrupt change in well trajectory;3. changing of in situ stress orientation;4. abrupt cutting in bedding;5. high concentration of fracture in a limited interval and brecciated

zone;6. unexpected change in wireline logs response;7. abrupt pressure change in well.

Twominor faults were interpreted in this well. Both of them causeabrupt change in bed texture. There is no significant change neither inbedding orientation nor brecciating in the place of faults.

4. Results and discussion

For the reason of precise analysis of the obtained data from FMI,bedding, open fractures, filled fractures, flexural slips were consideredseparately. These features in each formationwere divided as well. Poleprojections of each category for individual formation were sketched.Having the orientation of individual feature helped to study theaccordance of each set. Also the density log of each feature is drawnversus depth. Comparing the density log of each feature would rely on

Table 1Feature statistics, number of each feature and orientation of possible open and filled fractu

All formation Asmari F.

Features(no.) Set 1 Set 2 Set 1

All fractures(727) 238/41 42/30 235/43Open fractures (523) 57/40 12/30 235/42Filled fractures (37) 168/41 241/50 235/44Bedding (215) 47.1/53.8 48.8/53.5Flexural slip (167) 48.9/54.6 50 3/55

the frequency distribution and relationship between them. Fromstructural geology point of view this classification helps to separatevarious fracture sets, determine relative age of fracture sets and timeof filling material precipitation episode. For reservoir engineers openfractures are more interesting than other types of fractures, sorecognition of open fracture's exact depth and characteristics are themost attractive. Also for studying fracture patterns and the relativetime of fracture generation, open and filled fracture in Asmari, Pabdehand Gurpi formationwere separated. The orientation of possible openand filled fracture sets, bedding plane and flexural slip of eachformation are in Table 1. The division of fracture sets is based on highconcentrative area of pole on pole projection diagrams.

4.1. Analysis of all fractures

560 open and filled fractures were interpreted in this well.Generally two sets of fractures could be recognized. The main setorientation is 238/41; the orientation of the next set is 043/30.Separating each formation's fractures (Fig. 8) explicit that these twosets exist in Asmari and Pabdeh formations but in Gurpi theorientation and quantity of fractures are completely different. InGurpi formation fractures orientation is scattered and it is difficult todetermine any fracture set in the pole diagram.

In the Zagros thrust-fold belt concentric folding formed the majorfold type. The type of folding was formed by two mechanisms offlexural slip and neutral surface (Barker and Speers, 1977; Colan Sadd,1978). During the growth of fold, crestal zone of fold enduredtensional stress and longitudinal fractures existed. By the develop-ment of folding, competent layers began to slip and shear fractures arecreated during slip in the limb of folds.

The main fracture set is approximately parallel to anticline axis, sothis set is longitudinal fractures. Generally longitudinal fracturesexisted in the hinge zone of a fold and the concentration of this type offracture in hinge zone is higher than in other part of an anticline. Butthis well is not located very close to the anticline axes where thecurvature of fold is at maximum. The possible explanation for this

re sets, bedding plane and flexural slip in each formation.

Pabdeb F. Gurpi F.

Set 2 Set 1 Set 2 Set 1 Set 2

42/31 268/58 – 322/35 59/842/33 268/58 – 322.35 60.8170/35 185/44 255/60 – –

46.1/54.1 44.8/49.946/54 –

Fig. 8. Lower-hemisphere pole projection for all fracture (row 1), open fracture (row 2), filled fracture (row 3), discontinuity parallel to bedding (row 4), bedding (row 5), Asmari(column 1), Pabdeh (column 2), Gurpi (column 3).


phenomenon is the displacement of the maximum curvature zoneduring folding process. The south west limb of the field is steeper andmaximum curvature zone extended toward south west.

Second set of fracture is the shear fracture. In the beginning stageof folding shear fractures are dominant because the structure is totallyunder compressional forces (Gholipour, 1998). Shear fractures usuallywere formed in the limb and continuity of them cross cut the anticlineaxis. Shear fractures were formed before the longitudinal fractures. Insome interval two sets of fractures cross cut each other and a smalldisplacement occurred in cutting point (Fig. 6a).

Comparison of fracture density with bedding density that isinverse of bedding thickness showed that fracture density in thinbedded intervals are greater than in the thick bedded zones (Fig. 6b).In the upper part of Asmari and Pabdeh formations which are wellbedded, the density of fractures is high and vice versa, in the middle

part of Asmari, which consists of massive and thick bedded limestone,fracture density is too low. Thickness of bedding is not the only factorwhich determines number of fracture in that bed. In a 10 cm densecarbonate more than 4 fractures are visible in the image whereas in a4 cm layer above it there is no fracture (Fig. 6a). Thereupon in thesame circumstances fracture density is related to the thickness ofbedding.

Density log of all fracture shows two descending trends (Fig. 9).First one has a slight direction and located in the upper part of Asmariand the next one start near the Asmari–Pabdeh contact and continueto middle of Gurpi formation. These trends could be explained bymechanism of folding and neutral surface. In a fold, the area above theneutral surface is under extension and below it is under compression.The degree of deformation in the peak of fold is at maximum andtoward the neutral surface decreases, while at the place of neutral

Fig. 9. Density (fractures per 0.5 m) logs of bedding (track 3), discontinuity parallel to bedding (track 4), all fracture (track 5), filled fracture (track 6), and open fracture (track 7).


surface is zero. Deformation at the upper part of a set of layer is higherthan the lower part so at the top of Asmari, fractures developed morecompared in the lower part. At the time of bending fracture developedmore in the upper part of the set compared to the lower part (Pabdeh+upper half of Gurpi).

4.2. Analysis of open fracture

In the Asmari formation two sets of open fractures withorientations 235/43 and 042/31 are observable whereas there isonly one fracture set in Pabdeh and its orientation is 268/58 (Fig. 8).These sets in Asmari and Pabdeh are similar and possibly the origin ofthese sets is the same. The second set of open fractures in Asmari didnot exist in Pabdeh, thus the origin of this set is different from the firstset. It is possible that the second set was not created in folding processand local tectonic events such as fault generated this fracture set.

Open fractures of Gurpi were completely different from Asmariand Pabdeh's open fractures. Because of limited number of fracturesand wide dispersion of fractures in Gurpi formation, it is difficult to

separate fracture sets, nevertheless possibly two sets of open fracturecould be separated. The orientation of these sets are 322/35 and 059/08. These sets do not exist in Asmari and Pabdeh formations. Thesesets might be generated before that of Asmari and Pabdeh's fractureand prior to folding. The other explanation is that because ofgeomechanical properties of Gurpi and wide dispersion of Gurpi'sfracture orientation, these fractures are nonsystematic. Precisedecision about age and generation of Gurpi fractures need morefield investigation and studying of nearby wells' cores and image logs.

Open fractures are suitable pathway for hydrocarbons to move inthe reservoir and are vital for hydrocarbon production. Zones withhigh density of open fractures usually have very high permeability. Bytaking into account the high density of open fractures, four highfractured zones could be recognized.

4.3. Analysis of filled fracture

General orientation of filled fractures confirms that these two setsof filled fracture are similar to two sets of open fractures, so open and


filled fracture generation and ages are the same. Gurpi formation'sfilled fractures like open fractures do not show any relationship to thefractures of the other two studied formations. And also open and filledfracture sets of Gurpi do not have any obvious relationship. Thesecould be the reasons that the fractures in the Gurpi formation arenonsystematic.

5. Conclusion

Fracture pattern in Asmari and Pabdeh formations is similar butthe Gurpi formation is completely different. The maximum frequen-cies of fractures existed in the upper Asmari and the entire Pabdehformation. Majority of Asmari and Pabdeh formation's fractures in thestudied well are longitudinal fracture parallel to fold axis. Thesefractures formed in the south western limb of anticline near hingezone of the fold. Displacement of maximum curvature zone of thestructure caused abundance of longitudinal fracture set. The minor setof fractures in Asmari and Pabdeh are shear fracture.

Correlation between bedding and different fracture density logs,abundance of flexural slips and dominancy of longitudinal fractureswith evidences from other sources indicates that the two majormechanism of folding in this structure is flexural slip and neutralsurface.

Most of fractures in this well were open. Zones with high density ofopen fractures usually have very high permeability. By taking intoaccount the high density of open fractures, four high fractured zonescould be recognized.

This study also shows that for fracture study especially in fracturedreservoirs, image logs are a powerful alternative for core.

Acknowledgements

In particular, I would like to express my most sincere gratitude toDr. M. Azizzadeh for his helpful advice and guidance. Special thanks goto M. Saemi for the helpful discussions held during the preparation ofthe final manuscript. Y. P. Mazaheri reviewed an earlier version of thispaper; his comments and suggestions are gratefully acknowledged.This work has been supported by Research and Technology Directo-rate/National Iranian Oil Company.

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32- 2009 Khoshbakht -Image Log - JPSE J