Use of Borehole Image Logs as a Planning Tool in Direction

8/10/2019 Use of Borehole Image Logs as a Planning Tool in Direction

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VI Congreso de Exploracin y Desarr ollo de H idrocarburos Ampli ando las FronterasI APG, Mar del Plata, Argentina, 2005.

Simposio Evaluacin de Formaciones, Nuevas Soluciones para viejos problemas I APG & SPWLA.

USE OF BOREHOLE IMAGE LOGS AS A PLANNING TOOL IN DIRECTIONAL

DRILLING

Christian Hofmann(1)

[email protected] Paris (2)

[email protected]

(1)Pluspetrol S.A.(2)Baker Atlas

Keywords: image logs, fractured reservoirs, directional drilling.

Resumen:LAS IMGENES DE POZO COMO HERRAMIENTA EN LA PLANIFICACIN DE

PERFORACIONES DIRECCIONALES.

La evolucin de la tecnologa en la perforacin, desde los pozos verticales hasta los direccionales fueacompaada de una evolucin acorde en lo que se refiere a tcnicas y herramientas de perfilaje, en especialen lo que corresponde a imgenes, resistivas y/o acsticas a pozo abierto. En el yacimiento Ramos fueronutilizadas las imgenes a pozo abierto como una herramienta muy importante en un principio, para ladeteccin de fracturas, siendo utilizadas en la actualidad para la determinacin de la trayectoria ptima del

pozo dentro del reservorio con el objetivo de atravesar la mayor cantidad de fracturas abiertas. En el presente

trabajo se describe en detalle esta metodologa.

GENERAL

The Ramos Field is located in the thrusted and folded belt of the Subandean System of NorthwesternArgentina and Southern Bolivia. It consists in a NNE-SSW trending anticline structure of large longitudinalextension, which may be observed in the attached images. Fig. 1 show Ramos Field located in NorthwesternArgentina thrust and fold belt.

In this basin, which was interpreted as a foreland basin (Uliana, Legarreta et al. 1999), Early Silurian-Jurassic sediments deposited overlying a Cambrian-Ordovician basement (Starck 1999).

Stratigraphy

The Silurian-Devonian sediment cycle in the stratigraphic column (Fig. 2) is the most important one from thepetroleum point of view, since it contains the source rock and major reservoirs, consisting in the gas-and-

condensate bearing fractured quartzites of the Huamampampa, Icla and Santa Rosa formations, mentionedin increasing order of importance. These rocks underlie the Carbonic sediments represented in the RamosField by the oil-bearing Tupambi and Tarija Formations. These formations were exploited at the beginningof the 20thcentury, but turned out to be of no commercial interest.

The main Formations of the stratigraphic column penetrated by Ramos wells are described below:

Kirusillas Lipeon Fms.: predominantly Silurian shales, with hydrocarbon source rock characteristics,currently in the gas window (Cruz et al. 2001). It remains undrilled within the field area.

Santa Rosa Fm.: predominantly psammitic rocks of the Lower Devonian, which forms a thicking-up andnormal grading bed sequence. It has been reached by a total of 6 wells, two of which are currently productive.


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Ramos Area

Fig. 1: Ramos Field satellite image and location map.

Icla-Huamampampa Fms:. Middle Devonian shales culminating in quartzitic sandstones (Huamampampa),

which make up a perfect thicking-up and normal grading bed sequence. The Icla Fm. has been penetrated by9 wells -one producing from these levels. The Huamampampa Fm., which has been drilled through in allcases, is the main reservoir of this field.

Los Monos Fm.: 600-m package of Middle-to-Upper Devonian shales interbedded with sandstones of littlecontinuity and thickness. It is the main source rock of the HC basin, and is overlaid by the Carbonic Tarija

and Tupambi Fms. The seal rocks and reservoirs are influenced by the events of the Upper-PaleozoicGondwanic glaciation (Schulz et al. 1999).

This study is limited to the Huamampampa, Icla and Santa Rosa Formations, which represent the Ramos

Field reservoirs.


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Fig. 2: Ramos Field generalized stratigraphic column.

General Characteristics of Reservoir Rock

Considering these are naturally fractured reservoirs, Structural Geology is of paramount importance toreservoir analysis and crucial to field prospecting and development. The structural style definition and the

angle relationship among limbs, plunges and anticline crests are important to establish a distribution modelof the fracture systems existing in the anticline structure.

After establishing the structure

deformation pattern, the naturalfractures that affect the rocks may

be classified (Figure 3, Stearns etal., 1972) considering the

predominant trend of fracture

planes and their relationship withstress diagrams. This also helps in

planning field development, i.e.determining well locations,horizontal wells preferred

direction, relation with aquifer, etc.

The complex system of the rock

fracture and microfracturecontrols the reservoir drainagemechanism and remarkably

improves the low primary porosity,as well as permeability and

Fig. 3: Fold-associated fracture models (Stearns and Friedman 1972).


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transmissibility, mainly along the main structure axis.

The microfracture system greatly enhances the storage capacity of the Devonian reservoirs, which generally

present a significant degree of diagenesis bysiliceous cementation and recrystallization. The hydrocarbon-in-place volumes present in this structure would otherwise be uncommercial.

These characteristics are very important and make our Devonian reservoirs a single case. The compoundsystem, i.e. fractures and matrix, depends on the scale; in both cases consider rocks where primary porosityhas almost disappeared, as mentioned before. This is the reason why this type of reservoirs is so difficult tocharacterize.

With reference to log interpretation, normalization and calibration to core data was required for each well.Image logs become highly useful when reservoir characterization is required, because they are the only logs

that effectively resolve reservoir fractures, which is essential to reservoir characterization.

Ramos Field Drilling History

The first deep well in Ramos Field was drilled in 1977. It was a vertical well reaching deep levels into the

Santa Rosa Formation.

Vertical drilling took place in this field till 1996, when the first high-angle horizontal well (R.e-15H) was

successfully drilled by deviating at depth from a pre-existing vertical well (R.e-15).

The success of this well as a productive well encouraged the use and optimization of this technique, and notonly were high angle directional wells drilled, but also underbalance drilling was applied in the last drilling

projects.

Drilling technology evolution was parallel by the evolution of open-hole logging tools and techniques. The

first open hole logs were limited to traditional tools such as Dual Laterolog, Acoustic, Density and Neutronlogs. Interpretation based on these logs has always been an issue difficult to solve. As previously described,

the reservoir had little or no primary porosity, and its main porosity system was provided by fractures,macrofractures and microfractures. Considering that no logging tool detects these features accurately, coredata and well tests were only the reliable sources.

In 1996, when R.-1005 well was drilled, image logging was introduced by Western Atlas, which ran the firstacoustic image log, called CBIL. Image logging was incorporated to the standard set of logging tools to be

run in the reservoir, and was the first log to show visually identifiable and measurable fractures (fracturezones, orientation and density).

In other words, it was the first attempt to characterize reservoirs using image logs. From this moment

onwards, the advance in image logging was followed by the improvement of logging interpretation achievedby resorting to the commercially available methods (Archie, Aguilera, etc.).

When drilling R.-1010 westbranch, a new methodology emerged including borehole imaging as a

directional well planning tool. It consisted in developing a structural model in advance, in order to predictfracture set behavior and calculate the direction of the well to reach as many fractures as possible. This plan

was monitored while drilling by means of intermediate logs which made it possible to reorientate the welldirection based on the orientation of the fracture sets to be detected, which could depart from their theoretical

behavior. This method required interpreting image logs at wellsite because it was necessary to know thefracture pattern to plan the next section of the hole. This project also confirmed the convenience of drilling a

pilot well that made it possible to know the structural position of landing at the top of the reservoir

(proximity to axis) as well as the pattern of the fracture sets, in order to determine the initial well direction.Finally, this technology incorporated the in-situ stress analysis by means of the interpretation of the borehole

Breakout derived CBIL. Table 1 summarizes the main elements considered when planning and drilling ahorizontal well in Ramos Field.


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Structural analysis ofneighboring wells and existing

theoretical model.

Development of a theoretical

model for the new drilling

well.

Adjustment of model to actual

data taken while drilling.

Development of a reservoir

model to be used in reservoir

characterization.

Use of available data:

- Core data

- Well tests

- Borehole images and other logs

- Structural model in use

Detail model for the new drilling

well :

- Structural position

- Bedding inclination

- Fracture sets attitude.

- In-situ stress model.

Model Adjustment to actual

data:

- Image logs

- Wellsite interpretation.

- Fracture orientation rose plots

- Well path trajectory

modification.

Development of final model :

- Fracture and bedding attitude

- Structure adjustment

- Measurement of fracture

opening, in-situ stress, etc.

Table 1: Flowchart used to develop new drilling projects.

BOREHOLE IMAGE LOGS

Borehole images have been given multiple applications.

In this particular case, because of the reservoir characteristics, images have been regularly used since 1996,

in order to accurately define the structure (structural dip, faults) and, specially, to characterize fractures (type,density and orientation).

Considering the fact that the orientation of natural fracture sets is used to define and adjust the direction of

the following section of the hole, most image interpretations were performed at wellsite while logging.


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Image logs are the main logging tools in Ramos field development and are complemented by full waveacoustic logs (MAC) as secondary tool.

Whereas the resistivity image provides a unique image, the CBIL provides two images -amplitude and traveltime. Fig. 4 shows a comparison of the three images. The fractures observed are subvertical and with

irregular planes; these features have also been found in cores.

The authors believe the resistivity image by itself is optimistic in the number of open fractures it detects.When compared to travel time, some conductive fractures observed in the resistivity image may have little

or no expression on travel time, and they should consequently be interpreted as partially open or closedfractures. Even when travel time is a lower quality image, it may be greatly useful to characterize fractures.On the other hand, the electrical image makes it possible to confirm the continuity of the fractures detectedon the borehole wall by the CBIL, within the formation.

Another feature of this reservoir is the existence of large fractures. Fig. 5 shows an example of these

fractures which are confirmed by the acoustic log (MAC) full waveform attenuations. Even when Stoneleywave permeability indexes were also computed, the examination of the full waveform raw data was also very

useful.

From the borehole breakout analysis, the minimum horizontal stress direction was determined at five verticalborehole sections from 151 to N-S azimuth, and is consistently with a WSW stress, probably related to the

present-day Nazca plate thrust.

FRACTURE ANALYSIS

The relationship between the fractures and the structure must be considered to properly understand the

reservoir.

Fig. 4: Resistivity image & Acoustic image inamplitude and travel time presentation.

Fig. 5: Resistivity image (STAR), Acousticimage (CBIL) & MAC (full wave).


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Based on Stearns and Friedman classification (1972), fractures in anticline reservoirs may be grouped intodifferent genetically related sets (Fig. 3).

The main natural fractures identified in PP.St.R-1003 for the Huamampampa and Icla formations were sortedbased on this classification. Fig. 6 provides a comparison between the theoretical models and the

open/partially open fracture rose plots.Type I fractures include two sets of shear fractures and one set of tension fractures. These fractures are

parallel to the regional stress that generated the structure. The big fractures shown in Fig. 5 are within thiscategory.

Type II fractures also comprise two sets of shear fractures and one tension fracture set. They areperpendicular to the regional stress and may be observed on Figure #10.

The fact that Type I fractures more commonly develop on fold limbs whereas Type II fractures are morecommon on anticline crests is a generally accepted paradigm.

However, no simplifications should be made on this type of reservoirs since fracture distribution may bedetermined by a great number of factors.

Fig. 7 presents Fig. 6 data set considering depth and hole sections. Type I fractures prevail in the lowersection, where the bedding thickness is relatively low, and Type II fractures are predominant in the middlesection, where the bedding is the thickest. In the upper section, where the bedding thickness is intermediate,

both types of fractures coexist. As a conclusion, bedding thickness should also be considered when fracturedistribution is being analyzed.

Fig. 6 Fracture classification for Huamampampa

Fm., R.-1003 well.Fig. 7 Variation of fracture orientation in depth

for Huamampampa Fm., R.-1003 well.

CASE STUDIES

There follow three case studies showing the application of the above described methodology and the way

borehole-image-based fracture interpretations were used to optimize well path trajectory. The structuralmap on Fig. 8 shows well locations and cross-sections.


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VI Congreso de Exploracin y Desarrollo de H idrocarburos Ampli ando las F ronterasI APG, Mar del Plata, Argenti na, 2005.

Simposio Evaluacin de Formaciones, Nuevas Soluciones para viejos problemas I APG & SPWLA .

Fig. 8: Structural map at the top of Huamampampa Fm., the main Ramos Field reservoir, with the location

of each cross section.

Huamampampa Formation: Well R-1010

To better understand the text, refer to Fig. 8 (Structural Map), Fig. 9 (Cross-section and Plain View) and


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Fi . 10: Well R-1010 acoustic ima e lo .

Fig. 9: Well R.-1010 structural cross-section with west and east branches, and fracturedistribution rose diagrams along the well path.

A bilateral wellbore was projected for the HuamampampaFm. in well R-1010. With the purpose of determining thedirection of both branches, a pilot well was drilled in the

western limb of the structure, near the crest.Fig. 10 presents and acoustic image of the pilot well. Openfractures detected are Type II, being the East and Westdirections the most suitable directions to drill the sidetracks.

Following the drilling chronology, run #4 was drilled in an

initial NW direction, and after 653 meters a CBIL waslogged. Compared to the pilot well, a relative increase in

Type I fracture is observed in this run (4.2% vs.37.3%); thiswas logical since on the anticline western limb, from aninitial position near the axis drilling directed to the limb (see

Fig. 13a).

It must be remembered that Type I fractures are morefrequent on limbs.

Considering drilling would continue towards the western

limb, an increase of Type I fractures and a reduction of TypeII fractures were expected. For this reason, drilling directionwas changed NNW so as to reach as many fractures as

possible, and 305 meters of horizontal drilling werecompleted.

After concluding the drilling, a CBIL was logged. Theanalysis of the fractures identified in this run (#5) did notconfirm the prediction since Type II fractures were more

relatively frequent (87.1%).


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WNW ESE

Run #5 Run #4 Pilo t Well

AllOpenFr.

Total Open Frac.:85 Total Open Frac.:306 Total Open Frac.:24

TypeI

Fractures Nr 11 (12.9 %) Fractures Nr 114 (37.3 %) Fractures Nr 1 (4.2%)

TypeII

Fractures Nr 74 (87.1 %) Fractures Nr 192 (62.7 %) Fractures Nr 23 (95.8%)

Western Limb

0

8 %

0

3 %

0

38 %

0

38 %

0

14 %

0

6 %

0

14 %

0

25 %

0

25 %

WNW ESE

Pilo t Well Run #6

AllOpenFr.

Total Open Frac.:24 Total Open Frac.:57 Total Open Frac.:60 Total Open Frac.:48

Nodiffe

rentiat

edOpenFr

acturesT

ypeI

Fractures Nr 1 (4.2%) Fractures Nr 5 (8.8%) Fractures Nr 26 (54.2%)

TypeII


Nodiffe

rentiat

edOpenFr

actures

Axial

ZoneEastern

Limb

Western Limb (near

Axial Zone)

0

8 %

0

7 %

0

5 %

0

19 %

0

5 %

0

17 %

0

15 %

0

25 %

0

17 %

0

25 %

Fi . 11b: Rose dia rams with fracture distribution in R.-1010 East branch.

Fig. 11a: Rose diagrams with fracture distribution in R.-1010 west branch. The figure shows Type Ifractures in red and Type II fractures in green, depending on the structure location along the well path foreach logging run. All Open fractures are printed in blue.


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Once the West branch had been finished (runs #4 and # 5), the East branch was drilled (run #6).

844 meters of subhorizontal drilling were completed in a SE direction, drilling through part of the western

limb, the anticline axis and the whole Huamampampa Fm. on the eastern limb.

Fig. 11b shows the location of fracture sets considering their position in the anticline structure.

The first section of the horizontal well is similar to the pilot well; both are located in the western limb near

the axial plane and show the predominance of Type II fractures.

The fractures encountered when drilling through the axial zone could not be classified because of the

chaotic fracturing pattern.

The final section of run #6 investigates the anticline eastern limb. At this flank bedding dip east 58 onaverage.

Type I fractures are slightly more frequent (54.2%) on this limb, which is consistent with the theory.

Santa Rosa Formation

Well R-1010 transversely drilled through the Ramos structure. In the Huamampampa Fm., the anticline has

a vast extension.

Consequently, it could be subhorizontally drilled, with deviations in the order of 75 from the vertical,completing a good length of drilling (1802 meters).

The Ramos structure for the Santa Rosa Fm. is narrower, and may transversely have amplitude estimated of50 meters on the crest.

For this reason, it is convenient to plan inclined wells instead of horizontal wells to obtain a reasonablewellbore length.

Well R-1003

To better understand the text, refer to Fig. 12 (Cross-section & Plain View) and the different runs on Fig.13 (Rose Plots).

Well R-1003 was drilled on the eastern limb of the structure. Runs #4 and #5 subvertically tested the IclaFm. and the top of Santa Rosa Fm. as a pilot well.

A sidetrack (run #6) was initiated in the Icla Fm in a WSW direction up to Santa Rosa Fm at a 64inclination. Drilling continues through the Santa Rosa Fm in the same direction with a horizontal well (run#7) across the axial plane and part of the anticline western limb.

After drilling each section, an image was logged and interpreted to monitors the evolution of fracturedistribution.

The fracture distribution analysis reveals that Type I fractures are abundant (70%) in the pilot well, anddecrease relatively in number towards the structure axis: 67.6% in run #6, 43.7% in run #7, still on theeastern limb (near the axial zone), and 48.6% on the axial zone.

Finally, in run #7 section, which tests the western limb, the number of Type I fractures remarkablyincreases representing 77% of the total number of open/partially open fractures.


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WSW ENE

Run #7 Run #6 Pilo t Wel l

AllOpenFr.

Total Open Frac.:148 Total Open Frac.:105 Total Open Frac.:112 Total Open Frac.:68 Total Open Frac.:140

TypeI

Fractures Nr 114 (77%) Fractures Nr 51 (48.6%) Fractures Nr 49(43.7%) Fractures Nr 46 (67.6%) Fractures Nr 98 (70%)

TypeII

Fractures Nr 34 (23 %) Fractures Nr 54 (51.4%) Fractures Nr 63 (56.3%) Fractures Nr 22 (32.4%) Fractures Nr 42 (30%)

Axial

ZoneEastern LimbWestern

Limb

0

15 %

0

12 %

0

15 %

0

16 %

0

16 %

0

9 %

0

13 %

0

13 %

0

13 %

0

17 %

0

5 %

0

17 %

0

9 %

0

9 %

0

8 %

Fig. 13: R-1003 Rose plots showing fracture distribution.

Fig. 12: Structural cross section with well path for R.-1003 well, with the horizontal branch in Santa

Rosa Fm. The figure shows the rose diagrams for fracture distribution along the well path.


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Well R-1011

Well R-1011 was drilled in the anticline eastern limb, towards the structure axis. The different sections ofthe hole were drilled at about 40 of deviation (see Cross-section and Plain View on Fig. 14).

Fig. 15 presents the evolution of the different sets of fractures as the drilling proceeds to the structure axis.

Figure 14: Structural cross-section with R.-1011 well path. The figure shows the rose diagrams for fracturedistribution along the well path.

Run #4 drilled the Icla Fm. and the Santa Rosa Fm. upper section, on the structure limb, in a WNWdirection. Type I fractures clearly prevail over Type II fractures (70.6% versus 29.4%).

In run #5, the hole was gradually NW deviated because of the number of Type I fractures encountered in

the previous run. However CBIL analysis reveals at this section a gradual increase of Type II fractures.

The last section (run #6) of the drilled hole and the subsequent analysis of its image confirmed this trend.

The analysis of the different sections of hole shows that as we move from a limb location (run #4) to alocation nearer the structure axis (run #6), the number of Type II fractures (29.4%, 41.5% y 79.7%)

gradually increases over Type I fractures.

The comparison of wells R-1011 and R-1003 interestingly reveals some similarities with reference to theirdrilling trajectory. R-1011, in its closest position to the structure axis (run #6) shows a clear predominance

of Type II fractures (79.7%).

In the case of R-1003 (run #7), the percentage of Type II fractures in the same structural position reaches56.3%, and 51.4% in the axial zone.

The relatively greater number of Type II fractures (in theory, more numerous in the axial zone) in R-1011is presumably due to the fact that it is near the structure culmination (maximum curvature), whereas R-

1003, which is more towards the North, is located on the plunge.


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WNW ESE

Run #6 Run #5 Run #4

AllOpenFr.

Total Open Frac.:202 Total Open Frac.:142 Total Open Frac.:327

TypeI

Fractures Nr 41 (20.3 %) Fractures Nr 83 (58.5%) Fractures Nr 231 (70.6%)

TypeII


Axis Direction Eastern Limb

0

15 %

0

15 %

0

6 %

0

11 %

0

7 %

0

11 %

0

14 %

0

5 %

0

14 %

Figure 15: R-1011 Rose plots with fracture distribution showing all open fractures in blue, Type I fractures

in red and Type II fractures in green, related to structure location along the well path for each logging run.

CONCLUSIONS

Borehole image logs proved to be the main logging tool to drill wells directionally. The combination ofresistivity images with acoustic images is considered to be optimum. However, whenever the operationrestricted logging to one image, CBIL was preferred.

The full wave acoustic log (MAC) verified fractured zones and provided fracture permeability indexes, butdid not replace borehole imaging because the proposed methodology is based on the knowledge on fracture

orientations.

In general terms, the paradigm that Type I fractures are more frequent on anticline limbs and Type IIfractures on the axial zone was confirmed throughout the study. However, as some exceptions and

variations have been encountered, it should not be taken as law. This is where the validity of this methodlies -the evolution of the different sets of fractures is monitored while drilling the successive sections of awellbore.

Whenever horizontal drilling was planned, the information provided by the pilot well with reference toimage interpretation was crucial to determine the direction of the branch.

ACKNOWLEDGMENTS

The author thank to PLUSPETROL S.A. and BAKER ATLAS for permission to publish the paper.


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BIBLIOGRAPHY

Aguilera R.:Naturally Fractured Reservoirs, 2nd Edition,1995, Pennwell.

Cruz C., Sylwan C. and Villar H.: La Cuenca de Tarija, Bolivia y Noroeste de Argentina: Sistemapetrolero nico o mltiples sistemas petroleros?

Disalvo A. and Villar H.: Los Sistemas Petrolferos del Area Oriental de la Cuenca Paleozoica Noroeste,Argentina. IV Congreso de Exploracin y Desarrollo de Hidrocarburos, Mar del Plata, Argentina, 1999, pp.83-100.

Gomez Omil R. and Luquez J.: Ramos Field, A Giant Gas Field in a Subandean Belt (NW Argentina).AAPG International Conference & Exhibition, Ro de Janeiro, 1998, pp. 986-987.

Quiroga H., Luquez J., Hamdan V., Sobbi F., Shaw D., Bennion D. and Poon D.: Numerical Simulation ofthe Ramos Gas Condensate Reservorir. INGEPET, Lima, Peru, 2000.

Stark D.: Silurian-Jurassic Stratigraphy and Basin Evolution of Northwestern Argentina. In PetroleumBasins of South America. AAPG Memoir 62, 1995, pp. 241-267.

Stark D.: Los Sistemas Petroleros de la Cuenca de Tarija. IV Congreso de Exploracin y Desarrollo deHidrocarburos, Mar del Plata, Argentina, 1999, pp. 63-82.

Stearns D. and Friedman M.: Reservoirs in Fractured Rock. AAPG Memoir (1972), pp. 82-106.

Ulliana M., Legarreta L., Laffitte G. and Villar H.: Estratigrafa y Geoqumica de las Facies Generadoras deHidrocarburos en las Cuencas Petrolferas de Argentina. IV Congreso de Exploracin y Desarrollo de

Hidrocarburos, Mar del Plata, Argentina, 1999, pp. 1-61.

Use of Borehole Image Logs as a Planning Tool in Direction

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