11_ Release faults, associated structures and their control on petroleum trends in the Recôncavo...

AUTHORS

Nivaldo Destro � Petrobras ResearchCenter, Ilha do Fundao, Quadra 7, 20179-900,Rio de Janeiro, Brazil;[email protected]

Nivaldo Destro received his degree in geologyand an M.Sc. degree in structural geology fromthe Escola de Minas of the Federal Universityof Ouro Preto, where he is also conducting aPh.D. project. He joined Petrobras in 1986 as anexploration geologist. Currently, he is a struc-tural geologist at Petrobras Research Centerin Rio de Janeiro, Brazil. His work has con-centrated on the sealing properties of faults.

Peter Szatmari � Petrobras ResearchCenter, Ilha do Fundao, Quadra 7, 20179-900,Rio de Janeiro, Brazil

Peter Szatmari received his diploma in geol-ogy from the Eotvos University in Budapest,Hungary and his Ph.D. from the University ofEdinburgh, United Kingdom. After a few yearsas a visiting fellow at Princeton University,United States and working as a consultant, hejoined Petrobras Research Center in 1980,teaching and organizing research groups intectonics. His main interests are the role ofFischer-Tropsch synthesis in petroleum originand salt tectonics.

Fernando F. Alkmim � Departmentof Geology, Federal University of Ouro Preto,Morro do Cruzeiro, 35400-000, Ouro Preto,Minas Gerais, Brazil

Fernando F. Alkmim received his degree ingeology from the Escola de Minas of the Fed-eral University of Ouro Preto (1978) and hisDr.rer.nat. degree in geology from the Tech-nical University of Clausthal, Germany (1985).Alkmim is currently a professor at the FederalUniversity of Ouro Preto, teaching field geologyand tectonics. His research focuses on faultdynamics and Precambrian geology.

Luciano P. Magnavita � PetrobrasExploration and Production Department,Antonio Carlos Magalhaes Avenue, 1113,41856-900, Salvador, Bahia, Brazil

Luciano Magnavita received a degree in geologyfrom the University of Brasılia, Brazil, in 1976.He joined Petrobras in 1978, where he workedin the exploration department in Salvador.He obtained a Ph.D. in geology in 1992 from

Release faults, associatedstructures, and their controlon petroleum trends in theReconcavo rift, northeast BrazilNivaldo Destro, Peter Szatmari, Fernando F. Alkmim,and Luciano P. Magnavita

ABSTRACT

Release faults are rift cross faults, which develop to accommodate

the variable displacements of the hanging-wall block along the strike

of normal faults. Release faults are nearly perpendicular or obliquely

oriented to the strike of the normal fault they are related to. They

have maximum throws adjacent to the parent normal fault and die

out in the hanging wall away from it. They form to release the bend-

ing stresses in the hanging wall and do not reflect the orientation of

the regional stress field in a basin. Commonly, they show normal-

oblique displacements and are preferentially located along the strike

ramps. Release faults may also act at the scale of an entire basin,

reaching displacements of thousands of meters. Joints, shale, and

salt diapirs may develop in association with release faults. Because

all these structures represent domains of stress release, they may

work as conduits for oil migration and oil traps in extensional basins.

This is the case of the Reconcavo basin in northeastern Brazil, a Cre-

taceous failed rift, connected to the eastern Brazilian continental

margin basins. In the Reconcavo basin, two large-scale release faults,

with displacements in the order of 3 km, developed in the hanging

wall of the rift border faults and control the location of the main oil

fields.

INTRODUCTION

According to Morley et al. (1990), rift cross faults are faults formed

at high angles to the rift axis. Because of the seminal work on

transfer faults by Gibbs (1984), rift cross faults have been variously

interpreted in recent extensional tectonics literature as transfer faults

(Gibbs, 1984, 1990), transverse faults (Letouzey, 1986; Colletta

et al., 1988), hard-linked transfer faults (Walsh and Watterson,

Copyright #2003. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received December 14, 2001; provisional acceptance June 20, 2002; revised manuscriptreceived October 9, 2002; final acceptance February 20, 2003.

DOI:10.1306/02200300156

AAPG Bulletin, v. 87, no. 7 (July 2003), pp. 1123–1144 1123

1124 Release Faults, Associated Structures, and their Control on Petroleum Trends

1991; McClay and Khalil, 1998), release faults (Destro, 1995; Rob-

erts, 1996), and cutoff stretch accommodation faults (Stewart,

2001). The current interest stems largely from the genetic role these

faults play in the architecture of rifts and extensional basins (e.g.,

Harding and Lowell, 1979; Bally, 1981; Gibbs, 1984, 1990; Le-

touzey, 1986; Rosendahl et al., 1986; Etheridge et al., 1987, 1988;

McClay and Ellis, 1987; Colletta et al., 1988; Milani and Davison,

1988; Scott and Rosendahl, 1989; Morley et al., 1990).

The purpose of this paper is to highlight the role that release

faults, a variety of cross faults (Destro, 1995), played in both the

development of extensional systems and hydrocarbon migration

and accumulation. After a discussion on geometric, kinematic, and

dynamic aspects of release faults, we present examples of release

faults and associated fractures from the Reconcavo rift basin of

northeastern Brazil, where this peculiar type of cross structure

exerts a major control on petroleum trends. The results presented

here are based on data obtained during a structural analysis con-

ducted in the field, along with geologic data including about 5000

wells and about 30,000 km of two-dimensional (2-D) seismic lines

and 750 km2 of three-dimensional surveys. These subsurface data

are synthesized in the structural contour map of Reconcavo basin of

Aragao (1994), which is adopted here.

RELEASE FAULTS

Release faults are cross faults in general of normal character, which

develop in the hanging-wall blocks of the main components of the

rifts. Release faults were first identified in outcrops of the Sergipe-

Alagoas basin, northeast Brazil and named ‘‘falhas de alıvio’’ by

Destro et al. (1990). Later, studying the same type area, and also

based on regional seismic and well data, a more detailed inves-

tigation of release faults was carried out by Destro (1995). The term

‘‘release’’ has a genetic connotation, in the sense that these faults

allow the releasing of the bending stresses of the hanging-wall blocks

caused by the variation of displacement along the strike of normal

faults.

Mandl (1988) predicted the existence of cross faults similar to

release faults by analyzing the stress changes that may occur in the

hanging-wall block of a normal fault. Similar faults have been iden-

tified also in other areas (e.g., Souza Ferreira, 1990; Souza Ferreira

et al., 1995; Roberts, 1996; Stewart, 2001). Stewart (2001) quantified

the along-strike stretch of the hanging-wall blocks of normal faults

and, modifying Destro’s (1995) term, called these cross faults ‘‘cut-

off stretch accommodation faults.’’

Release faults form as a result of varying throws along the strike

of a listric parent normal fault (Figure 1a). The hanging wall bends as

a result of differential vertical displacements, so that cross faults and/

or fractures become geometrically and mechanically necessary to

accommodate the increase of length along strike in the hanging wall

(Figure 1a) (Destro, 1995). The release faults form to accomplish

Oxford University, England. Since then, he hasbeen working with several Brazilian basins. Hismain interests are tectonics and sedimentationin extensional basins, salt tectonics, and sealingprocesses associated with faulting.

ACKNOWLEDGEMENTS

This paper is a result of a Ph.D. project byN. Destro for Ouro Preto Federal University,Brazil. We thank Maria Alice N. F. de Aragaofor the helpful contribution in the identificationof release faults in the Reconcavo rift. Andre A.Bender made thoughtful review and sugges-tions on an earlier version of this manuscript.Walter B. Maciel and Carlos Eduardo B. deSalles Abreu are thanked for their thoughtfulcomments on the influence of release faultsin the formation of turbidity systems. John H.Shaw, John Lorenz, and an anonymous refereeare thanked for their thorough, helpful, andconstructive reviews. We thank Petrobras forprovision of financial support and permissionto publish. F. F. Alkmim received support fromCNPq (Brazilian Council for the Scientific andTechnological Development) grant #300833/99-7.

this local stretching, because faulting is the most active

deformation mechanism in the brittle upper crust (Kuz-

nir and Park, 1987; Morley et al., 1990), although duc-

tile deformation also occurs (Larsen, 1988).

Release faults do not connect distinct normal faults,

but die out in an individual hanging wall (Figure 1a).

Because release faults form to accommodate differen-

tial downdip movements of the hanging wall, they do

not cut the normal fault planes or detachment surfaces

at depths (Figure 1a, c). Because release faults are a

result of differential downdip displacements of the

hanging-wall blocks of the parent faults, they always

present maximum vertical displacements smaller than

the maximum vertical displacements of the parent faults

to which they are related. Generally, release faults do

not reveal strike-slip movements in seismic sections

and in structural contour maps (e.g., flower structures,

en echelon folds, and Riedel-type geometries) (Destro,

1995).

The cross section of Figure 1b shows extension

parallel to the transport direction of the parent normal

fault. Because the increase in length of the hanging wall

is caused by bending that resulted from differential

vertical displacements and not by horizontal extension

along the strike of the parent normal fault, the net

extension along this direction is zero (Figure 1c). Thus,

release faults are compatible with regional plane strain

deformation perpendicular to the normal faults during

extension and do not necessarily indicate regional three-

dimensional strain. However, between the terminations

of a parent fault, three-dimensional strain deformation

must occur in the hanging wall (Destro, 1995). Because

footwall uplift takes place beneath the major normal

parent faults, footwall blocks may also display small-

scale release structures, such as faults and joints.

As shown above, release faults preferentially nu-

cleate along strike ramps (Figure 2a, b). In these cases, an

alternation of dextral and sinistral senses of horizontal

displacement is expected to occur along the strike of

the parent normal fault. In the example of Figure 2b,

there is a line of neutral strike-slip movement caused by

the opposite senses of displacements along the parent

fault. In this region, compression may occur, forming

reverse faults. When release faults are positioned at the

terminations of the parent faults, their location may be

caused by preexisting weak zones or contrasting rheo-

logical interfaces (Figure 2c). In this case, only one

sense of displacement occurs along the whole length of

Destro et al. 1125

Figure 1. (a) Block diagram showing thedisplacement variation along the strikeof a normal fault (modified after Destro,1995). The rake of slickenside lineations(angle a) along the release faults is in thesame range as the dip (angle u) of thenormal fault. In (b), extension is greaterthan zero; in (c), it is equal to zero (fur-ther explanation in the text). Note thatrelease faults die out at depth on thenormal parent fault’s trace.


Figure 2. Idealized structural contour maps, block diagrams, and cross sections of some basic types of release faults (modified afterDestro, 1995). They may form on the strike ramps (a, b), at the normal fault tips (c), or distributed along the strike of the normalfaults (d). The cross sections BB0 show that release faults do not present footwall uplift. Nondimensional structural contours indicatedby numbers 1 (highest) to 5 (lowest). Arrows on map view represent the apparent lateral movements originated by the release faults.Note that in (c) and (d), preexistent weak zones control the release faults.

the parent normal fault. Release faults can also form a

system of a large number of elements showing smaller

displacements (Figure 2d), especially when the rocks

involved display a preexistent pervasive fabric.

Because bending of the hanging wall is greater along

the strike ramps, release faults are more common over

the ramps (Destro, 1995; Stewart, 2001). The greater

bending of the hanging walls is represented by the

smaller radius of curvature of contour lines, as shown in

Figure 3a and b. The release faults tend to be perpen-

dicular to the contour lines. In outcrops, they tend to be

perpendicular to the strike of the bedding (Destro,

1995).

Evidence from surface and subsurface data (e.g.,

Destro, 1995) indicates that the angular relationship

between the parent normal fault and the associated

release faults depends primarily upon the geometry of

the parent fault, whether they are curved or approxi-

mately straight in map view (Figure 3a, b). If the hori-

zontal trace of the parent normal fault is curved, then

the release faults located in the area of maximal cur-

vatures of the hanging-wall contour lines are at low

angle to the parent fault (Figure 3b). Conversely, if the

horizontal fault trace is straight, then the hanging-wall

contour lines become less curved, and the release faults

are roughly perpendicular to the parent fault (Figure 3a).

The release-faulting model predicts that the local

stress field can strongly depart from the regional one.

The stress field adjacent to a parent normal fault is

shown in Figure 3c. The stress ellipsoid in the footwall

of the normal fault reproduces the regional stresses,

where the maximum principal stress (s1) is vertical,

and the minimum principal stress (s3) is parallel to the

extension direction (A). Along the release faults, which

Destro et al. 1127

Figure 3. (a, b) Idealizedcontour maps in hangingwall showing (a) high-anglerelease faults and (b) ob-lique release faults (modi-fied after Destro, 1995).Nondimensional structuralcontours indicated by num-bers 1 (highest) to 3 (low-est). (c) Stress-field statearound normal and re-lease faults (adapted fromMandl, 1988; Destro, 1995).(A) Regional stress field.(B) Reversion in the role ofthe intermediate (s2) andthe smallest (s3) principalstresses, causing the forma-tion of a release fault. (C)Perpendicular to s3, releasefractures may develop par-allel to the release faults. (D)Reversion among the threeprincipal stresses, forming areverse fault. (d) Block dia-gram showing the influenceof a transfer zone and re-lease faults in the formationof channels and turbiditysystems in extensional ba-sins. Note the formation ofrelease faults disconnectedfrom the parent fault (seetext for further explanation).

behave as normal faults, s1 is kept vertical, but the in-

termediate principal stress (s2) and s3 switch positions

with respect to the regional stress field by rotating 90j(B). Perpendicular to the minimum principal stress (s3),

release fractures may develop parallel to the release faults

(C). Locally, where downslip was greatest, compres-

sion may occur in the hanging wall, rotating again the

regional stress field; s3 becomes close to vertical, and s1

parallel to the strike of the parent fault (D) (see also

Figure 3c).

In general, release faults are normal faults. As a

consequence of their genetic connection to the parent

normal faults, they commonly show an oblique compo-

nent of movement. However, because of their smaller

horizontal displacements, release faults are not associated

with flower structures, en echelon folds, and Riedel-

type geometries on seismic sections and structural con-

tour maps (Destro, 1995).

Release structures can occur as single faults or as

sets of elements of variable number and character, in-

cluding the typical normal-oblique release fault, as well

as reverse faults, fractures, gash veins, and diapirs, dis-

cussed further in this paper. Rosendahl et al. (1986)

discuss the effects of accommodation zones on deposi-

tional processes. Morley et al. (1990) point out the im-

portance of transfer zones to hydrocarbon exploration.

These authors analyze the effect that footwall uplift,

formed as a flexural isostatic response to displacement

on major boundary faults, has on synrift sedimentation.

Our work in extensional basins indicates that release

structures form in largest number and diversity in ac-

commodation or transfer zones, where two synthetic

normal faults overlap or approach, as shown in Figure

3d. In this case, a synthetic approaching transfer zone

(according to the classification of Morley et al. 1990)

develops. In the area between the two faults, subsidence

is enhanced, acting as a focal point of relatively higher

quality reservoir rock. In this peculiar structural setting,

the abundant release faults may play an important role

in the basin dynamics, acting as favorable oil migration

pathways. In decoupled extensional systems, like salt

basins, release faults disconnected from the parent fault

may form (Figure 3d), controlling the development of

longer channels that may allow turbidity currents to flow

to distant depocenters.

THE RECONCAVO BASIN

The Reconcavo basin, located in northeastern Brazil (Fig-

ure 4), forms the southern portion of the Reconcavo-

Tucano-Jatoba rift, a Late Jurassic to Early Cretaceous

aborted branch of the South Atlantic rift system (e.g.,

Szatmari et al., 1985, 1987; Milani and Davison, 1988;

Magnavita, 1992; Szatmari and Milani, 1999). Con-

nected to the eastern Brazilian continental margin, the

Reconcavo-Tucano-Jatoba rift is an approximately 400-

km-long and 100-km-wide system of north- to northeast-

trending half grabens (Figure 4) (Magnavita, 1992).

The Reconcavo basin is filled with strata deposited

during prerift, synrift, and postrift phases, whose aver-

age thickness along the depocenters exceeds 6 km.

Upper Paleozoic through Jurassic prerift deposits are

mainly red beds (Alianca Formation) and coarse-grained

fluvial deposits (Sergi Formation). The synrift strata were

deposited during the Neocomian, when active faulting

along the eastern margin of the basin caused the dep-

osition of fanglomerates greater than 4 km thick (Sal-

vador Formation). In the deep lake that developed,

shales of the Candeias Formation (main hydrocarbon

source rocks) were deposited, along with occasional tur-

bidity influxes and sandstone fan incursions (Netto and

Oliveira, 1985). By the end of the Neocomian, in the

Hauterivian, the subsidence rate declined, and a prograd-

ing system of delta fans filled the lake (Ilhas Group).

The rift phase terminated with the Barremian Sao Se-

bastiao fluvial sediments, which are unconformably

overlain by the postrift Aptian conglomerates of the

Marizal Formation. Late Cretaceous through Cenozoic

postrift deposits are represented in the basin by a thin

(100 m) veneer of alluvial and fluvial sandstones of

the Barreiras Formation.

The northeast-oriented and southeastward-dipping

half graben of the Reconcavo basin (Figure 4a, b) is

bordered by the major Salvador fault and contains a

series of synthetic and antithetic normal faults. The

Salvador fault is the main normal fault of the basin and

reaches displacements of as much as 6 km (Figure 4b).

Other important normal faults are the Paranagua and

Tombador faults, located at the western boundary of

the basin. The map in Figure 4a depicts a series of cross

faults, which are oriented at high angles to the rift axis.

The most prominent among them are the south and

north Mata-Catu faults, which, together with some

smaller ones, will be described in the following sections.

Figure 4 also shows other important features of the

basin, like the Camacari and Alagoinhas lows. They

represent the deepest depocenters of the basin and are

located, respectively, in the eastern and western por-

tions of the basin. In the Camacari low, the synrift se-

quence reaches a thickness of as much as 6 km. In the

Alagoinhas low, it is about 3.9 km thick. In the southern


Destro et al. 1129

Figure 4. (a) Simplified tectonic map on top of the prerift Sergi Formation for the Reconcavo rift (modified from Aragao, 1994).Release faults: the south Mata-Catu and Itanagra-Aracas release faults are related to the Salvador parent fault. The north Mata-Caturelease fault, however, is associated to the Tombador parent fault. (b) Cross sections showing the position of the main border of theReconcavo basin to the east. (c) Strike section: note in this latter section the large displacement on the south Mata-Catu and Itanagra-Aracas faults, and that they are associated to the major Salvador border fault. The Barra transfer fault is the southern limit of theReconcavo rift.

portion of the basin, the top of the prerift sequence is

about 2.5 km deep. To the northeast, the basin dies out.

RELEASE FAULTS IN THE RECONCAVO BASIN

As the Reconcavo basin is a half graben limited by a

single normal fault, the observed increase in length of

the hanging wall suggests that release faults might be

formed. The major north and south Mata-Catu faults

(Figure 4) and several smaller cross faults terminate

against the Salvador fault to the east, or against the

major Tombador and Paranagua faults to the west,

where they exhibit the maximum vertical displace-

ments. All of them die out in the hanging-wall blocks

toward the central areas of the basin. They are inter-

preted here as release faults, which formed to accom-

plish the increase in length of the hanging-wall blocks

along the strike of the major Salvador border fault

in the east and along the major faults of the western

boundary of the basin.

Our study of the geometric and kinematic prop-

erties of release faults in the Reconcavo basin is based

on the analysis of the south and north Mata-Catu faults,

which are best exposed and documented in both seis-

mic sections and well data. Figure 5a and b show seis-

mic sections located, respectively, in the footwall and in

the hanging-wall blocks of the south Mata-Catu fault.

The major normal and release faults of the Reconcavo

basin are also clearly shown in the gravimetric map of

the basin (Figure 6). The close relationship between

the release faults and their associated parent faults is

evident in Figures 4 and 6. For example, the south and

north Mata-Catu faults are related, respectively, to the

Salvador and Tombador parent faults. In these figures,

it can also be seen that the Camacari and Alagoinhas

lows are controlled by the south and north Mata-Catu

faults and are located near the intersection between

these release faults and their associated parent faults.

The south and north Mata-Catu faults (Figure 4)

have been traditionally considered to be a single fault,

called Mata-Catu fault, in which the sense of dip changes

along its strike (e.g., Milani and Davison, 1988; Mag-

navita, 1992). On the basis of regional gravimetric and

seismic data, Milani and Davison (1988) interpreted it

as a transfer fault, in the sense of Gibbs (1984). Aragao

(1994) mapped this fault zone and portrayed it as being

composed of two distinct faults (Figure 4a), separated

by a conjugate divergent collinear transfer zone, fol-

lowing the Morley et al. (1990) terminology. Based on


Destro et al. 1131

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field data, described in detail below, we have deduced

that the south and north Mata-Catu faults are normal-

oblique faults, dipping 70j to the southwest and 70j to

the northeast, respectively. The data by Aragao (1994)

and our present field data have led us to propose that

these faults are release faults, instead of transfer faults.

According to Souza Ferreira (1990) and Magnavita

(1992), the south Mata-Catu fault has experienced two

main phases of movement, the first in the early Valan-

ginian and the second in the late Barremian, being ac-

tive during the entire rifting period. According to those

authors, the north Mata-Catu fault nucleated during

the Hauterivian, and its movement climaxed during the

late Barremian. Both the south and north Mata-Catu

faults cut the sediments of the Sao Sebastiao Forma-

tion, the youngest rift phase unit, thereby indicating

that both were still active in late Barremian/early Aptian.

A unique aspect of the south Mata-Catu fault is

that it is not located over a ramp with respect to the

hanging wall of the major Salvador fault (Figure 4). This

may be caused by the existence of a preexistent weak

zone, also suggested by its strong alignment with the

north Mata-Catu fault, the latter clearly positioned

over a strike ramp in the hanging wall of the Tombador

fault (Figure 4). Although the Salvador fault is strongly

controlled by the foliation of the basement granulites

(Magnavita, 1992), outcrops in the Precambrian base-

ment adjacent to the south Mata-Catu fault are sparse,

which make it difficult to confirm the influence of a

preexistent weak zone. In addition, the Salvador border

fault zone has a complex geometry, which may have

constrained the position of release faults.

As shown previously, the release faults do not nec-

essarily indicate regional three-dimensional strain in

extensional basins. This is the case in the Reconcavo

basin, where the regional extension direction is approx-

imately perpendicular to the axis of the basin; that is,

northwest-southeast oriented (e.g., Szatmari et al., 1985,

1987; Milani and Davison, 1988; Magnavita, 1992;

Szatmari and Milani, 1999).

The South Mata-Catu Fault

The geometric and kinematic properties of the south

Mata-Catu release fault were defined by the analysis of

exposures along the fault zone, especially in the areas

between the towns of Mata de Sao Joao and Catu

(Figure 7). The south Mata-Catu fault is a superb

example of a release fault, with a very large, as much as

3 km, vertical displacement, giving rise to a large num-

ber of subordinate mesoscopic-scale release faults that

crosscut normal faults oriented parallel to the rift axis.

In outcrops along the south Mata-Catu fault, bedding

trends preferentially northwest and dips toward the

southwest (Figure 8a), controlled by the northwest-

trending fault. The dip to the southwest is caused by drag

on faults dipping southwest, and tilting of beds toward

fault planes dipping northeast.

The northwest-trending faults along the south Mata-

Catu fault zone are mainly normal and normal-oblique

faults and are marked by the development of deforma-

tion bands (Aydin and Johnson, 1978), showing milli-

metric to centimetric thicknesses (Figure 9a). They con-

sist of a series of light-colored, more resistant silicified

strands of comminuted or pulverized rock. The discrete

fault surfaces, commonly displaying slickenside stria-

tions, are observed internally or at the boundaries of the

deformation bands. Sometimes these faults join to-

gether, forming fault zones with metric thicknesses.

They are observed cutting mainly coarse-grained sand-

stones of the Sao Sebastiao Formation. They strike

about 340j and dip about 70j to northeast and south-

west (Figure 8b).


Figure 6. Bouguer map of the Reconcavo basin (modifiedafter Figueiredo et al., 1994). The relation between the releasefaults and their associated parent faults is also evidenced in thismap. The north and south Mata-Catu faults are characterizedalong a clear northwest-southeast anomaly confined in thebasin. Contours are in milligal.

The slickenside striations on the fault planes of the

northwest-trending faults plunge about 50j toward the

south-southwest and north-northeast, with relatively

high rakes (Figures 8c, 9b). Because the northwest-

trending faults strike about 340j (Figure 8b), this

indicates that these faults have both dip-slip and

oblique-slip displacements. The concentrations in the

south-southwest and north-northeast portions of the

Destro et al. 1133

Figure 7. Geologic map of the study area (adapted from Petrobras, 1969; Souza Ferreira, 1990; Magnavita, 1992). Along the southand north Mata-Catu faults, both northwest- and northeast-trending faults are observed in outcrops. The Camacari and Alagoinhaslows are located at the extremities of the south and north Mata-Catu faults. The north Cassarongongo fault (NCSF) presents reversedisplacement, whereas the south Cassarongongo fault (SCSF) shows normal displacement.

stereogram of Figure 8c represent a predominance of

normal-sinistral faults along the south Mata-Catu fault,

although normal-dextral faults are also observed (Figure

8c). The sense of displacement on the faults was de-

termined on the basis of the displacement of markers and

the drag of bedding, coupled with slickenside analysis.

The slip-linear plot (Marshak and Mitra, 1988) for

northwest-trending faults in Figure 8d shows that most

of the striations point toward the center of the stereo-

gram, indicating predominance of dip-slip and oblique-

slip faults with relative high rakes. Pure strike-slip faults

are sparse and are represented by arrows at low angles

to the equatorial circle (Figure 8d).

The northeast-trending faults are parallel to the rift

axis and show the same geometric characteristics as the

northwest-trending faults. These two systems commonly

cut and displace each other in outcrops (Figure 9a). The

northeast-trending faults strike about 30j and dip around

70j (Figure 8e). The striations on these faults are pre-

dominantly steep and are represented by heavy dots in

Figure 8f. Slickenside striations dipping less than 45jare represented by open triangles in Figure 8f. This be-

havior can be observed in the slip-linear plot (Figure

8g), where normal, strike-slip, and oblique-slip faults

are observed. As a result of the connection between the

northwest- and northeast-trending fault systems, the

strike-slip component of displacement is locally fa-

vored on both systems because of the lateral movement

between adjacent blocks.

The maximum vertical displacement of the south

Mata-Catu fault is about 3 km (Figure 4c), whereas

the displacement of the Salvador fault, its parent fault,

is about 6 km in this area (Figure 4b). This is in accord-

ance with the release-faulting model, which predicts that

the parent faults always present displacements greater

than their associated release faults. As shown previously,

in outcrops located along the south Mata-Catu fault, the

northwest- and northeast-trending faults offset each other.

Because the displacements along the south Mata-Catu

fault are much greater than the displacements along these

small-scale secondary northeast-trending faults, these

latter faults act as local transfer structures with respect

to the northwest-trending release faults and show, as a

result, low rake striations (Figure 8g).

One outcome of the complexity of the kinematic

interactions between normal faults and release faults is

suggested by Destro (1995), who pointed out that sub-

horizontal slickenside lineations may be occasionally

found on both normal and release faults (see also Figure

2). Thus, it is not necessary to invoke regional strike-slip

faulting and horizontal compressive stress to explain the

low rakes of slickenside lineations on both normal and

release fault planes, unless there is additional evidence

that this occurs, such as evidence from seismic and well

data.

The North Mata-Catu and Cassarongongo Faults

Along the north Mata-Catu fault zone, bedding dips

gently to the northeast (Figure 8h). Although exposed

in fewer outcrops, two systems of small-scale northwest-

and northeast-trending faults are observed around the

north Mata-Catu fault (Figure 8i, j), similar to the south

Mata-Catu fault. A few slickenside striations are ob-

served on the northeast-trending faults, and similar to

the south Mata-Catu fault, they show both normal and

oblique-slip displacements (Figure 8k). Slickenside stri-

ations on the northwest-trending faults are sparse.

To study the behavior of northeast-trending faults

outside the domain of the south and north Mata-Catu

faults, field work was carried out in the area of the

south Cassarongongo fault (Figure 7). Bedding planes

show gentle dips (Figure 8l). Only northeast-trending

Destro et al. 1135

Figure 8. Data from the south Mata-Catu fault: (a) equal-area lower hemisphere contoured stereogram of poles to bedding; (b)contoured stereogram of poles to faults trending northwest; faults strike about 340j and dip about 70j; (c) contoured stereogram ofplots to slickenside striations for northwest-trending faults; (d) slip-linear plots for the arrays of northwest-trending faults; the arrowspoint mainly toward the center of the stereogram, indicating predominance of normal to normal-oblique movements; (e) stereogramof poles to faults trending northeast; (f) plots to slickenside striations for northeast-trending faults; open dots represent striationsplunging less than 30j; (g) slip-linear plots for northeast-trending faults; this plot shows that strike-slip and oblique normaldisplacements are also important in this fault set. Data from the north Mata-Catu fault: (h) stereogram of poles to bedding, alsoshowing a subtle but visible influence of the north Mata-Catu fault; (i) stereogram of poles to northwest-trending faults; ( j)stereogram of poles to northeast-trending faults; the pattern of distribution of these faults is similar to the pattern of theircorrespondent faults in the south Mata-Catu fault; (k) slip-linear plots for northeast-trending faults. Data from the Cassarongongoarea (south Cassarongongo fault): (l) stereogram of poles to bedding; (m) stereogram of poles to northeast-trending faults; (n) slip-linear plots for northeast-trending faults; in this area, only northeast-trending normal faults were observed.

faults were observed (Figure 8m); they show predom-

inance of dip-slip movements (Figure 8n).

Based on the studied field data, as well as seismic

and well data, the proposed regional stress field for the

major normal faults and the local stress fields for the

main release faults are shown in Figure 10.

IMPORTANCE OF RELEASE FAULTS TOHYDROCARBON ACCUMULATIONS INTHE RECONCAVO RIFT

Commercial oil production in the Reconcavo basin

dates back to the early 1940s and resulted in the dis-

covery of as much as 80 hydrocarbon accumulations.

The main petroleum system is the Sergi/Agua Grande-

Candeias, accounting for 2.7 billion bbl (57%) of the

proven oil volume in the basin (Figueiredo et al., 1994).

The Sergi Formation is the main reservoir (eolian-fluvial

system), averaging 18% porosity and 800 md perme-

ability. In this section, we analyze the role of release

faults and several release faulting-related cross struc-

tures in the distribution of oil fields in the Reconcavo

rift, including shale diapirs, release fractures, and a reverse

fault. As presented below, these structures contribute

in particular ways to the formation of structural traps.

Oil Fields Located along the South andNorth Mata-Catu Faults

The south and north Mata-Catu faults produced the

most prolific petroleum trend in the Reconcavo basin,


Figure 9. (a) Fault zones cutting thesandstones of the Sao Sebastiao Forma-tion, trending northwest and northeast,parallel, respectively, to the south Mata-Catu fault and to the rift axis (locality 1in Figure 7). (b) Northwest-trending faultplane in outcrop shown in (a). It dipssouthwest and shows slickenside stria-tions with relatively high rakes.

with oil fields located in their footwalls blocks (Figures

11, 12). The fields along this trend produce almost only

from the prerift Sergi reservoir, filled with oil generated

from synrift source rocks. The oil kitchen coincides with

the main lows (Camacari and Miranga lows). Because the

synrift source rocks of the Candeias Formation (Lower

Cretaceous) are above the Sergi Formation (Upper

Jurassic), traps are typically structural (horsts and tilted

Destro et al. 1137

Figure 10. Block diagram showing major features of the Reconcavo basin and associated local stress fields. Legend: (A) stress ellipsoidrepresenting the regional stress field, which was responsible for the northwest-southeast extension; (B) stress fields around the mainrelease faults; in this case, there is a reversion in the role of the intermediate (s2) and the smallest (s3) principal stresses; (C) theorientation of the principal stresses are the same as in (B), but s3 becomes negative, allowing the formation of open fractures in theCandeias area; (D) reversion among the three principal stresses, which caused the formation of the north Cassarongongo reverse fault;note that the vertical displacement of the south Mata-Catu fault is smaller than the displacement of the Salvador fault, its parent fault.

blocks), and secondary hydrocarbon migration may rely

on pathways along some fault zones (Magnavita, 2000).

Release faults do not present footwall uplift that occurs

along major normal boundary faults because of flexural

isostatic response to displacement. We suggest that the

reservoirs located in the footwall blocks of both the south

and north Mata-Catu faults were preserved from erosion

by this process.

By analyzing the membrane potential sealing of a

fault zone of the Reconcavo-Tucano-Jatoba rift, Mag-

navita (2000) observed a reduction in pore diameter of

at least two orders of magnitude, the resulting differ-

ence in capillary pressure, being capable of trapping

hydrocarbons. For the Mata-Catu fault trend, he ob-

served that some faults must have leaked, and others

must have sealed, with vertical migration along faults

and lateral migration along carrier beds, in the present

case of the Sergi and Agua Grande carrier bed system.

This process would have allowed hydrocarbons to enter

the fault-reservoir system of the Buracica field, at a

distance of about 40 km from the oil kitchen.

Miranga Field

The Itanagra-Aracas fault, located at the eastern border

of the basin (Figures 11, 12), is parallel to the south and

north Mata-Catu faults. It has a maximum throw of

about 2 km near the Salvador fault (see also Figure 4c),

dying out basinward. The reservoirs correspond to del-

taic sandstones of the synrift upper Neocomian to Bar-

remian Marfim and Pojuca formations. As pointed out

above, it seems that this fault also formed as a release

fault, helping to accommodate the variation in displace-

ment along the Salvador fault. The release of stress along

it resulted from local extension subparallel to the rift axis

(see Figure 10 for the local position of s3). This allowed

the development of shale diapirs from the Candeias

Formation parallel to the Itanagra-Aracas fault (Figure

13a, b), as well as northeast-trending diapirs of the

Candeias Formation parallel to the Salvador fault (Figure

13a, c). The diapirs formed along the Itanagra-Aracas

fault are better developed than the ones formed parallel

to the Salvador fault (compare Figure 13b, c). This sug-

gests that the minimum principal stress (s3) perpendic-

ular to the release fault, i.e., parallel to the rift axis, is

smaller than the regional minimum principal stress (s3),

parallel to the extension direction, as predicted from the

release-faulting model. Figure 13a shows that the closure

of the northern portion of the Miranga field is caused by

the influence of a major northwest-trending shale diapir,

parallel to the Itanagra-Aracas release fault.

Candeias Field

The Candeias field lies in the southern portion of the

Reconcavo rift (Figure 11). Open fractures oriented

transversally to the rift axis are responsible for most of

the oil production in this field. The fractures cut cal-

ciferous shales of the synrift Candeias Formation (Fig-

ure 14a) at a depth of about 2 km. They can be as much

as 0.5 cm open, with quartz crystals lining their walls

and reaching as much as 1 cm in length and enclosing

oil bubbles. The structural description of these frac-

tures in cored well shows that they trend northwest

(Figure 14b), parallel to the south and north Mata-

Catu faults. Breakout data from the Reconcavo basin

indicate that the present maximum horizontal stress

(SHmax) is parallel to the rift axis (Lima et al., 1997)

and thus, at high angle to these open fractures. The

occurrence of open fractures where quartz crystals did

not develop shows that the change of the stress field

since rifting has not been sufficient to close them. In

Figure 10, the arrow indicating s3 in the Candeias area


Figure 11. Distribution of the oil and gas fields in the Recon-cavo basin. The south and north Mata-Catu faults, the Itanagra-Aracas fault, and the south and north Cassarongongo faults (SCSFand NCSF) constitute important petroleum trends in the basin.

Destro et al. 1139

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corresponds to a tensile stress. As shown previously,

the release faults and fractures tend to form over the

strike ramps. In Figure 10, it can be seen that the Can-

deias field is located over the ramp of a local normal

fault, as well as over the southern ramp of the entire

Reconcavo basin, adjacent to the Salvador fault.

Field and subsurface evidence in some Brazilian

rifts, like the Sergipe-Alagoas basin (Destro, 1995) and

the Reconcavo basin (this work), indicate that release

fractures oriented parallel to release faults are more

common than fractures developed parallel to the rift

axis. It would be reasonable to expect that these latter

fractures are more common because they are perpen-

dicular to the regional and overall extension direction.

However, they are scarce or practically absent in those

rifts. We believe that the release-faulting model may

clarify this unexpected observation. It has been shown

by several authors (e.g., Rosendahl et al., 1986; Morley

et al., 1990; Destro, 1995) that most of the deforma-

tion in rifts takes place along the major border faults.

Thus, in the basin, the minimum regional principal

stress (s3) parallel to the extension direction would not

easily reach negative values necessary to form tensile

fractures. In contrast, release faults form in the hanging

wall of normal faults, where deformation concentrates

so that the local minimum principal stress (s3) reaches

negative values, and consequently, tensile release frac-

tures may develop.

Brejinho Field

The Brejinho field is located in the western border of

the Reconcavo rift (Figure 11). The reservoir is also

composed of sandstones of the prerift Sergi Formation.

It lies in an anticlinal feature associated with the reverse

north Cassarongongo fault (Figure 15a, b); the south

Cassarongongo fault has normal slip motion (Figure

15). The Cassarongongo fault is located in the region of

maximum throw of the major Paranagua fault (Figure

15). The release-faulting model predicts localized

compression in this portion of the hanging-wall blocks

of the major normal faults because the portions of a

hanging-wall block located at the opposite sides of the

line of neutral strike-slip motion moved toward each

other (Figure 2b). The strong bending of the hanging

wall in this downwarped area also reflects this com-

pression. The Canabrava oil field is also located in the

reverse segment of the north Cassarongongo fault (Fig-

ure 11), whereas the Cassarongongo field is situated on

the footwall of the south Cassarongongo fault, which

presents dip-slip motion (Figure 15).

Destro et al. 1141

Figure 14. (a) Subvertical open fractures (indi-cated by arrows) observed in calciferous shalesof the Candeias Formation. These fractures areresponsible for most of the oil production in theCandeias field (see Figures 10, 11 for location). (b)Equal-area lower hemisphere contoured stereo-gram of fractures shown in (a) (modified fromMartins et al., 1997). They strike 340j and areparallel to the south and north Mata-Catu releasefaults. These fractures are preserved open atdepths as much as 2 km.

DISCUSSION AND CONCLUSIONS

To characterize release faults, it is necessary to dis-

tinguish them from transfer faults. Destro (1995)

showed that both may be present in rifts, but that they

are genetically distinct. Gibbs (1984) formally sug-

gested the term ‘‘transfer fault’’ in analogy with ‘‘tear

faults’’ in thrust systems (Dahlstrom, 1969). He pointed

out that transfer faults ‘‘allow leakage’’ between exten-

sional faults with differing rates and that ‘‘the presence

of a strike-slip component on transfer faults is important,

as such faults will have displacements much larger than

the dip-slip component apparent on a single geoseismic

line.’’ (Dahlstrom, 1969, p. 616). The cross faults in the

Reconcavo rift are distinct from transfer faults exactly in

these two aspects: (1) they are associated with individual

normal faults (the border faults), dying out in the hanging

wall before reaching other normal faults, and (2) vertical

movements predominate over horizontal motion on them.

Although this work identified only release faults in

the Reconcavo rift, in the contiguous Tucano rift, two

transfer faults were described, the Jeremoabo fault


Figure 15. Structural contour map ontop of prerift sequence for the Cassar-ongongo area (modified after Aragao,1994). The north Cassarongongo fault hasreverse-slip displacement (compare withFigure 2c), whereas the south Cassaron-gongo fault has normal-slip movement.Note smaller release faults (indicated byarrows) located on the strike ramps andat high angles to the strike of the Parana-gua fault. (b) Seismic section throughthe north Cassarongongo fault where itsreverse-slip displacement is shown (afterAragao, 1999).

(Destro et al., in press) and the Carita fault (D. V. F.

Vasconcellos, 2002, personal communication). These

faults fit Gibbs’s (1984) definition of transfer faults, in

that they connect distinct border faults and present a

predominance of strike-slip displacements. The iden-

tification of these transfer faults had the support of a

regional accurate fieldwork by Magnavita (1992) and

a structural subsurface map of the Tucano-Jatoba rift

by Aragao (1994), which resulted from the interpre-

tation of about 10,000 km of 2-D seismic lines and

114 wells.

We propose that the faults and associated structures

described here are a result of a broad, scale-independent

process named release faulting, in which release faults

and associated structures form at high angles to parent

normal faults to accommodate lateral changes in the

vertical subsidence of basins. We also emphasize that

this process may be important not only in rift basins,

but any extensional basin (e.g., passive margin and salt

basins) and that they may play a major role in the evo-

lution of petroleum systems.

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