FRACTURE CHARACTERIZATION IN THE SAN BERNARDO FOLD BELT, SAN JORGE BASIN, ARGENTINA

Laura Chiaramonte* **, Atilla Aydin*, Juan Homovc**, Gustavo Petrelli**

*Department of Geological and Environmental Sciences, Stanford University,

**Repsol-YPF, Argentina

E-mail: chiarlau@pangea.stanford.edu

Abstract The seismic sections of Perales anticline show a

coherent seismic anomaly in the frontal limb of the

structure at the level of Castillo Formation.

The outcrop characterization carried in this formation

showed that Castillo Formation is intensely fractured

and faulted at the steeper or overturned limbs of the

anticlines studied.

From the field study it was suggested that the

anomaly could be due to dilatational fractures

generated as splay fractures at the ends of the

reverse/thrust faults.

Two unsuccessful wells corroborated that Castillo

Formation was intensely fractured. Although an

important water influx confirmed the hypotheses of

an open fracture network, the confined character of

the influx might evidence a proportion of closed

fractures.

The objective of this paper is to combine the

information of the outcrops with subsurface

information to elucidate the nature of the anomaly.

The analysis of fracture directions in the outcrops and

wel-bore shows an apparent relationship between

fracture orientation and the main structures, such as

the reverse faults and the strike slips faults that

segmented the anticline.

Finally, with the information available from the

outcrops and image loggings it cannot be concluded

weather open fractures are the reason of the seismic

anomaly. Therefore a more detailed study will be

needed.

Introduction Resolving the nature of the anomaly was considered

crucial for the development of Castillo Formation as

an oil reservoir at the steeper limbs of Perales

anticline.

Relaying in the hypotheses that this anomaly is due to

fractures, an investigation of potential fractures and

faults in the Castillo Formation has been conducted.

In this report, we summarize the results of this

investigation along with the results of two wells

drilled following the surface analog study.

Geological Setting The San Jorge Basin, located in the central Patagonia

of Argentina is a Jurassic and early Cretaceous rift

basin that evolved into a sag basin (Figure 1). The

sedimentary record of the thermal stage is mainly

continental, with deposits ranging from coarse

alluvial fans in the flanks to high-sinuosity meander

belts and shallow ephemeral lakes in the central area

with significant pyroclastic supply.

250 Km

SOUTH AMERICAN PLATE

SCOTIA PLATE

MALVINAS ISLANDS

42°

45°

48°

51°

54°

54°60°66°72°78°84°90°96°

NORDPATAGONIAN MASSIF

ANTARTIC PLATE

DESEADO BASIN

SAN JORGE BASIN

CO MO D OR O RI VAD AVI A

Figure 1. Location map (after Homovc et al, 2000).

The eastern part of the basin is characterized by

extensional style structures. The San Bernardo Fault

Belt, to the west, is a NNW range characterized by

wide fault propagation anticlines, with N-S

orientation, that were reactivated during the Miocene

(due to the subduction along the western margin of

South America). The anticlines are often arranged in

echelon and fragmented by WNW strike-slip faults.

The Perales Anticline, host of one of the major

oilfields in the area (Figures 2), is the southward

continuation of the San Bernardo range in the

subsurface. The NNW orientation of this range and

the subsurface anticlines, contrast with that of the

regional extensional faults in WNW orientation.

Stanford Rock Fracture Project Vol. 13, 2002 PC-1

Seismic Anomaly LAGO

Range

San

CALETA OLIVIA

Ocean Atlantic

LASHERAS

SANTA CRUZ

C H U B U T26

18

26

12

43

40

OutcropStudy Area

Subsurface Study Area

70° 69° 68°

46° 30'

46°

PICO

20 Km.

COMODORO RIVADAVIA

45° 30'

PeralesAnticline

Bernardo1

2

Seismic sections of Perales anticline show complex

features at the frontal limb along the whole anticline

(Figure 4, 5 & 6)

Deep migration studies and velocity analysis

conferred geological character to these reflections. In

addition, the acoustic velocity of the anomaly is

similar to that of the host rock. The nature of these

features has been a matter of diverse interpretations.

These features were formerly explained as reflections

from beds either overturned or belonging to horse

blocks. Later Homovc et al. (1999) interpreted these

anomalous reflections as fractures filled with

magmatic material. Recently Aydin (2000) suggested

that the anomaly could be the effect of dilatational

fractures due to splay-fracture mechanism related to

high-angle reverse faults. These fractures could be

filled with magmatic material, hydrocarbons fluids,

or solids such as calcite.

Figure 2. Perales Anticline and San Bernardo Range locations (after Homovc et al, 2000). 1. Chenque Anticline, 2. Cerro Castillo Anticline.

Although Bajo Barreal Formation (Figure 3) is the

main reservoir of the Perales Anticline, the Castillo

Formation offers high potential when secondary

porosity is enhanced by cement dissolution and

fracturing (Strelkov et al. 1998).

Outcrop Characterization Aydin (2000) performed an outcrop characterization

of Castillo Formation, in San Bernardo Fold belt in

order to extrapolate the distribution of fractures

associated with folds and faults to the Perales

anticline.

The field data consisted of measurements of fracture

intensity in the Castillo Formation and

characterization of the nature and distribution of

fracture and faults. The area of study is located near

Canadon Matasiete, where the frontal limb of the

Cerro Castillo anticline is exposed (Figure 2), and at

Angostura and Chenque Anticline, where fracturing

is associated with a reverse fault and a strike-slip

fault, respectively

It was found that the intensity of fracturing is in part

controlled by brittleness of the rock layers and by the

magnitude of slip along faults at a high angle to the

fold axes. Fracture zones with even a small amount

of shearing provide significant fracture porosity and

permeability and thus are effective flow pathways.

However, as slip increases, a brecciated fault rock

with a low permeability develops.

Figure 3. Stratigraphic column of Perales Anticline.

Lithology The lithology of the Castillo Formation consists of

tuffs, tuffaceous sandstones and sandy and

argillaceous tuffs with intercalations of tuffaceous-

argillaceous matrix sandstones. The productive

horizons have a secondary porosity that ranges

between 9 - 12% with permeability values between 3

a 20 mD (Corveleri et al, 1996).

The siliceous tuff layers within the Castillo

Formation at the frontal limb exhibit a dense fracture

network. The number of fractures increases near the

faults from a background value of 10-15 per Meter

Square to more than twice of this amount.

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Figure 4. Seismic Section of The Perales Anticline (True Amplitude -Time). BB: Bajo Barreal Fm., CAS: Castillo Fm., D-129: Pozo D-129 Fm.

Figure 5. Top Seccion Tobacea Time Structure Map

Stanford Rock Fracture Project Vol. 13, 2002 PC-3

Figure 6. Depth-migrated seismic line of Perales anticline. Furthermore, the study showed that the Castillo/Bajo Barreal boundary is faulted. Most likely, the faults associated to this boundary are reverse faults. Although fracture patterns vary from different units inside this formation, two well-connected fractures sets commonly exist. The most common configurations includes strike-parallel and dip-parallel fracture sets but also two oblique sets. The fracture characterization showed that the majority of the faults associated with the inferred thrust fault at the core of the folded strata are either low-angle and strike-parallel or a few are high-angle faults oblique to the trend of the faults zones. Fracture orientation was measured in different locations associated with two main structural features, a reverse fault and a strike fault (Figure7). The principal fracturing directions associated with the reverse faulting are 190°, 290° and 305°. The main directions associated with the strike fault are 310° and 235°. From the field study it was concluded that the normal frontal limb of the Cerro Castillo anticline has been attenuated to about half of its thickness as measured along the eastern side of Canadon Matasiete. Faulting appears to be the main mechanism of this process as documented by the high-resolution mapping. It is believed that in the later formation of the anticline,

the faults take advantage of preexisting joint orientation. Borehole data Well A (Figure 8) was aimed to target in the conventional plays of Bajo Barreal Formation and the anomalous zone of Castillo Formation. Due to an accidental deviation in Well A, the anomaly was reached laterally. The Formation Micro Scanner log (FMS) of Well A showed a considerable amount of fractures with maximum density of 14 fractures/m. Although is not possible to discriminate the nature of the filling material, the log image differentiates between filled and opened fractures,. The principal filled-fracture directions are 240º, 195º and 280º. For open fractures, the main directions are 275º and 195º. Castillo Formation was not tested during the completion of Well A, because no significant shows of oil or gas were found. Nevertheless, during the completion of the upper Bajo Barreal Formation (which only produced formation water), oil filtrate from Castillo Formation was found. But Castillo oil potential remains doubtful due to the inconclusive tests. Well B (Figure 9) was drilled directionally in the northern part of Perales anticline. The objective of

Stanford Rock Fracture Project Vol. 13, 2002 PC-4

COLHUE HUAPILAKE

Meseta Cerro Kepi Fault

Cerro ChenqueFault

Co. Chenque

?

0 1 Km

1 orders t

2 o rdernd

310°

235°

1 orderst

3 orderth

2 ordernd

4 cm2 mm openinggenerally open

1-2mm. Opening mode fractures with 8 cms. spacing.

Sierra Silva Anticline

1

2

Figure 7. Fracture orientation in outcrop characterization.

Figure 8. Well A sketch. .

NW SE

Stanford Rock Fracture Project Vol. 13, 2002 PC-5

Figure 9. Well B sketch. .

NW SE

Well B was the center of the anomalous zone in Castillo Formation. The Formation Micro Imager (FMI) showed that the beddings dipped to the east. As a result the idea of overturned beds or horse blocks was discarded. The fracture density topped a value of around 12 fractures/m. The principal filled-fracture direction is 185º and for the open-fracture the main directions are 270º and 210º. Although the FMI log showed that the rocks were extensively fractured, oil or gas shows of hydrocarbons were even poorer than those in Well A. Fracturing was also locally confirmed by the high-rate water influx from some reservoirs of the Castillo Formation. Conclusions The imaging logs in both wells confirmed the existence of an important network of fractures in the anomalous zone. The important water influx in Well B supports that a fraction of the fracture network is conductive and perhaps responsible for anomalous water production during drilling. The operating problems in Well A leave the door open for further studies to establish the potential productive zones. On the other hand, neither of the wells exhibited a considerable amount of hydrocarbon shows, neither in the mud logging nor in the electrical logs.

Moreover, the FMI log registered in Well B was useful to discard the hypothesis that the anomaly was the result of the seismic response of overturned or rotated beds, because the dipmeter orientations show that the beds are dipping 45° to 80° to the SE, opposite to the NW dipping of the reflectors in the anomalous zone. The analysis of fracture directions in the outcrops and well-bore shows that the occurrence of more oblique fractures near the strike slip faults (235º - 240º and 310º) and more north-south fractures (195º) when the main structure is the reverse faults (Figure 10). A detailed analysis of this issue is critical for the future development of Castillo Formation in different locations of the anticline. With the information available from the outcrops and well loggings it cannot be concluded weather open fractures are the reason of the seismic anomaly. This assumption could be analyzed through an acoustic impedance modeling. The same acoustic model would allow checking the feasibility of an anomaly generated through the reflection of shear fractures or small faults at divergent angles from the main faulting.

Stanford Rock Fracture Project Vol. 13, 2002 PC-6

Finally, evaluating if the seismic anomaly is related

Figure 10. Comparison of fracture o

to seismic migration problems in the processing stage ould be of high importance.

rientation in outcrops and well-bores.

Future workhe next step in this research will be the modeling f

with synthetic seismograms. This

nts would like to thanks Repsol – YPF South Division

viding the information

s.

ydin, Atilla, 2000. Fracturing and Faulting of the Castillo anadon Matasiete: Implications for the

Corb magnetica en la

cuenca del Golfo San Jorge, XIII Congreso Geologico Argentino y III Congreso de Exploracion de Hidrocarburos, Actas I, Buenos Aires, 1996.

Cou en,

. 1998.

in st.

Unpub. Internal report, Repsol, YPF, Dec. 1999.

Hom de

Actas I, Salta 1999.

w

Top Seccion Tobacea Time Structure Map

Tthe fracture systemmodel will generate synthetic seismic responses of the fractured rock filled with different solid or fluid material. This information will be later compared with the actual seismic response of the anomaly in Castillo Formation. AcknowledgmeI and Sebastian Leoni for proused and the consent for its publication in this paper, The Rock Fracture Project and Fulbright Scolarshipfor their support. I would also like to acknowledge the students of the Rock Fracture Project at Stanford University for their help and useful suggestions. A final acknowledgment for Rene Manceda (Repsol-YPF) for all his support and insightful suggestion References A

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