Analysis of the Petroleum Systems of the Lusitanian Basin (Western

Analysis of the Petroleum Systems of the Lusitanian Basin (Western Iberian
Margin)—A Tool for Deep Offshore Exploration
Pena dos Reis, Rui
Centro de Geociências
Faculdade de Ciências e Tecnologia da
Universidade de Coimbra
Lg Marquês de Pombal
3000-272 Coimbra, Portugal
e-mail: [email protected]
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Pimentel, Nuno
Centro de Geologia
Faculdade de Ciências da Universidade Lisboa
Campo Grande C-6
1749-016 Lisboa, Portugal
e-mail: [email protected]
Abstract
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Mesozoic and Tertiary seals and traps. Related processes, such as organic matter maturation and
hydrocarbons migration are also discussed. The characteristics of these elements and processes are analysed
and implications for deep offshore exploration are discussed.
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A synthesis of the knowledge about the Lusitanian Basin is presented here, focusing on its
stratigraphic record, sedimentary infill, evolution, and
petroleum systems. Petroleum system elements are
characterized, including Palaeozoic and Mesozoic
source rocks, siliciclastic and carbonate reservoirs, and
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Introduction
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The Lusitanian Basin is one of the Western Iberian Margin sedimentary basins related with the
opening of the North Atlantic (Fig. 1) (Wilson et al.,
1989). These basins have their counterparts in the eastern Canada Jeanne D’Arc and Whale basins, as part of
the Iberia-Newfoundland conjugate margins complex
(eg., Wilson et al., 1989; Pinheiro et al., 1996; PeronPinvidic and Manatchal, 2009). The evolution of all
these basins are defined by the same geodynamic controls but also by specific local constraints, explaining
different characteristics and success in exploration. In
the Canadian basins, the intense exploration has led to
several good production and development results, but
the Iberian basins have not had so far similar positive
results. However, exploration continues and a good
understanding of these basins’ evolution and characteristics is crucial to enhance the chances of future
success. This paper deals with the evolution of the
mainly onshore Lusitanian Basin and its petroleum systems, in order to establish an analog for other nearby
offshore basins, aiming to contribute to a better
regional framework for exploration in this region (Fig.
1).
In this paper we present a summary of this
approach, based on an overview of the basin evolution
and an analysis of the related petroleum systems and
elements, with implications on the deep offshore ongoing exploration.
Lusitanian Basin’s Evolution and Infill
The Lusitanian Basin extends for about 250 km
north-south and 100 km east-west, facing the Atlantic
to the west, representing the inner and most proximal
margins of much larger basins extending towards the
shallow and deep offshore (LB in Fig. 1). The geological record of the Western Iberian Margin’s evolution is
present in several nearby basins sharing similarities,
such as the offshore Porto and Galicia basins in northern Iberia (PB, GB in Fig. 1), the Peniche and Alentejo
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basins in southwestern Iberia (PenB, AB in Fig. 1) and
the Gulf of Cadiz Basin in southern Iberia (Gb in
Fig. 1).
The Lusitanian Basin resulted from the initial
extension of the Pangea’s continental crust and later
opening of the North Atlantic Ocean as a result of rifting and seafloor spreading. The evolution of the
Paleozoic basement and the Mesozoic extension created a complex succession of events and sedimentary
Sedimentary Basins: Origin, Depositional Histories, and Petroleum Systems
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Since the Late Carboniferous and during Permian times, gradual uplift and erosion brought these
rocks to more shallow structural domains and predominantly gentle deformation, commonly known as “lateVariscan faulting.” These north-northeast/south-southwest sinistral and northwest-southeast dextral
movements conditioned the origin and configuration of
the Lusitanian Basin from the Late Triassic, in the
global context of the Pangaea break-up and intracontinental troughs in western Europe and eastern America
(Wilson et al., 1989) (Fig. 2). In western Iberia, strong
subsidence resulted in north-northeast/south-southwest trending asymmetric grabens that rapidly filled by
alluvial-fan siliciclastic deposits passing to sabkha
evaporite clays and salts, under arid climatic conditions. This sedimentary infill corresponds to the Silves
Group (Palain, 1976) and is composed mainly of coarse
siliciclastic red-beds forming two fining-upward megasequences (Fig. 4). Its total thickness is up to 400
meters and thee paleogeographic reconstructions point
to the development of north-northwest/south-southeast
fault-bounded half-grabens, separated by intra-rift
basement horst blocks (Uphoff, 2005). These deposits
show good reservoir potential and have been targeted
for gas as part of a pre-salt petroleum system (Uphoff,
2005).
These Upper Triassic grabens became gradually
filled-up and sabkha-like environments became predominant, promoting the accumulation of red clays and
evaporites in the most subsiding parts of the basin
(Dagorda Formation) (Palain, 1976). The resulting
shaly deposits, some hundreds of meters thick, include
significant amounts of gypsum and halite, which would
be fundamental for the tectonic deformation of the
Meso-Cenozoic units, acting as “decollement” level
and also as diapiric masses with kilometric-scale vertical movements, locally piercing the Mesozoic cover
(Kullberg, 2000) (Figs. 2 and 3).
In the Sinemurian, the Hettangian sabkha-like to
coastal environments are replaced by open marine, predominantly carbonate environments, such as the
Coimbra Group (Soares et al., 2007) (Fig. 4). This unit
is about 200 m thick and present throughout the basin
as a result of the paleogeographic coalescence of the
initial troughs and an expansive onlap of a carbonate
ramp over the basement towards the East. Dolomitic
limestones (São Miguel Formation) are present in the
northern and eastern areas, whereas the marly limestones are present in central and western areas of the
basin (Água de Madeiros Formation) (Duarte and
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infill (Figs. 2 and 3). The influence of the basement on
the basin’s evolution may be addressed along two main
lines: (i) its lithologies, including the presence of units
having source-rock potential; (ii) its structures, particularly the presence of important regional faults and their
movement during the Mesozoic and Tertiary (e.g., Pena
dos Reis et al., 2012 Dinis et al., 2008; Pena dos Reis
et al., 2000)
The complex structure of the Paleozoic basement
of western Iberia resulted mainly from the collision and
deformation of two terranes (e.g., Ribeiro et al., 1979,
1990, 2007; Matte, 1991): (i) the Iberian Terrane with
the Central Iberian Zone (CIZ) and the Ossa Morena
Zone (OMZ); and (ii) the Southern Portuguese Terrane
and its Zone (SPZ). The joint deformation of all this
basement is related with the Ibero-Armorican Arch
developed in the Late Paleozoic during the Variscan
Orogeny.
The Central Iberian Zone includes Silurian units
having organic matter in pelitic layers, sometimes several hundred meters thick (Romão et al., 2005).
However, metamorphism may have over maturated
those units and most (if not all) of the hydrocarbons
may have been lost. Late Carboniferous deposits are
more prospective, considering their post-orogenic age
and therefore not so high maturation. They have been
deposited in narrow intra-mountain lacustrine basins as
fining-upward siliciclastic deposits, including black
shales and coal seams at the top (Domingos et al.,
1983).
The Ossa Morena Zone includes several units
containing organic-rich layers, affected by low-grade
metamorphism that has caused intense maturation
(Chaminé et al., 2003). However, some outcropping
Silurian graptolitic black-shales show highly variable
Ro% equivalent values, ranging from the late oil-window to the gas-window, probably controlled by the
proximity to major fault zones (Uphoff, 2005; Machado et al., 2011).
The Southern Portuguese Zone includes Devonian to Late Carboniferous metasedimentary rocks,
including some black-shales having source rock potential, such as the fine-grained turbidites of the Baixo
Alentejo Flysh Group (Oliveira, 1983). Although they
are affected by low-grade metamorphism and are generally over mature (McCormack et al., 2007;
Fernandes et al., 2012), preliminary data (Barberes,
2013) suggests that there may have been some places
where the group is preserved within the gas-window.
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related with a Late Jurassic rifting event, recognized in
subsidence curves and related with the beginning of the
Atlantic opening to the south of Iberia (Wilson et al.,
1989; Rasmussen et al., 1998).
The Abadia Formation corresponds to the riftclimax, and its deposits have been the classical target
of oil exploration in the Lusitanian Basin during the
20th century (DPEP, 2013). Prograding continental
siliciclastics continue to cover the basin during the
Tithonian, resulting in the accumulation of almost 1 km
of fluviodeltaic sands and clays (Lourinhã Formation;
Hill, 1988). Late Tithonian to early Berriasian sediments are generally named as the “Purbeck Facies” and
include fluvial to coastal siliciclastics.
The Cretaceous evolution of the Lusitanian
Basin is closely related with the opening of the North
Atlantic Ocean. This opening has been developed in
three steps which are well identified in the geological
record as break-up unconformities (Dinis et al., 2008).
The beginning of the Early Cretaceous is also marked
by an important magmatic event indicated by several
dykes mainly associated with diapiric piercing structures (Martins et al., 2010). Lower Cretaceous
sediments are known only in the south and central sectors of the Lusitanian basin, indicating an important
uplift of the north sector during most of this time
period (Fig. 4).
The sedimentary record is composed mainly of
fluvial to coastal fine-grained siliciclastics and impure
limestones, grouped into two cycles, separated by
unconformities associated to distinct segments of the
North Atlantic opening (Rey et al., 2006; Dinis et al.,
2008) (Fig. 4). The late Berriasian to early Barremian
cycle is interpreted as associated to the opening of the
Tagus segment, whereas the early Barremian to midAptian cycle is associated to the opening of the Iberian
segment (Dinis et al., 2008). The late Aptian unconformity is present along the whole basin, resulting in
abundant coarse siliciclastics (Figueira da Foz Formation) in top of Early Cretaceous sequences in the South
or Jurassic uplifted and eroded sequences in the North
(Santos et al., 2010). Abundant siliciclastics covered
the basin during the Albian (Rey et al., 2006).
Late Cretaceous evolution corresponds to the
development of a classical passive margin, controlled
both by the uplift of the continental areas and the
eustatic level variations. During the Albian and Cenomanian, a global eustatic sea-level rise result in the
marine invasion of most of the basin and the gradual
development of a carbonate (rudist and coral-buildups)
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Soares, 2002; Azerêdo et al., 2003). The later includes
a lower unit only a few tenths of meters in thickness,
deposited in open marine environments but having
very good organic content of upper Sinemurian–lowermost Pliensbachian age (Polvoeira Member; Duarte
and Soares, 2002; Duarte et al., 2004, 2010, 2012).
During the Early Jurassic, sedimentation took
place in a carbonate ramp depositional system, giving
place to a thick sequence of marly limestones, known
in exploration and the 20th century literature as the
Brenha Group (Witt, 1977). The sedimentation began
with the deposition of about one hundred meters of
alternating centimeter-thick layers of marls and limestones of Pliensbachian age (Vale das Fontes
Formation; Rocha et al., 1996; Duarte and Soares,
2002; Azerêdo et al., 2003), also having very good
generation potential (e.g., Oliveira et al., 2006; Silva et
al., 2010; Duarte et al., 2010; Spigolon et al., 2011).
The open marine marly sedimentation gradually
gave place to a predominance of limestones everywhere in the basin–Cabo Mondego Formation in the
North (Azerêdo et al., 2003) and Candeeiros Group in
the South (Witt, 1977). An overall regression gradually
promoted shallower sedimentation, reaching emersion
and depositional hiatus in the eastern border of the
basin during the Callovian (Azerêdo et al., 2002,
2003).
Late Jurassic sedimentation started in midOxfordian times, following a Callovian forced regression and emersion (Azerêdo et al., 2002) (Fig. 4). This
situation is related with an important geodynamic reorganization of the basin (Wilson et al., 1989; Hiscott
et al., 1990), resulting in a Late Jurassic depositional
trough elongated northeast-southwest and open to the
southwest.
The first Oxfordian sediments correspond to a
few hundred meters of laminated (mm scale) marly
limestones deposited in coastal to transitional environments (Cabaços Formation; Azerêdo et al., 2002)
containing organic-rich layers having very good generation potential (Spigolon et al., 2011; DPEP, 2013).A
rapid marine invasion resulted in the accumulation of a
few hundred meters of compact grey marine limestones
containing marly intercalations towards the top (Montejunto Formation; Atrops and Marques, 1988). This
marine carbonate sedimentation was suddenly interrupted by a major input of coarse siliciclastics all over
the basin (Abadia Formation), reaching more than
thousand meters thick in the basin depocenters (Pena
dos Reis et al, 2000). This thick Kimmeridgian infill is
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The evolution of the Lusitanian Basin basically
ended during the Late Cretaceous. However, during the
Tertiary, the area occupied by the basin was subjected
to inversion and uplift, mainly along its northeastsouthwest central axis; i.e., the axis of the Late Jurassic
depocenter. Tertiary basins developed on each side of
this mainly carbonates mountain-chain–the Mondego
Basin to the northwest and the Tejo Basin to the southeast (Figs. 2 and 3).
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platform, known as Cacém Formation. These conditions continue until the Turonian (Callapez, 2008).
In late Turonian times, the first signs of inversion
in the passive margin are indicated by prograding siliciclastics in the northern sector and emersion/erosion in
the southern sector. Inversion continues until the end of
the Late Cretaceous, and sedimentation gradually
becomes restricted to smaller areas in the north. Magmatic intrusions are known from this age, closely
related with the instability caused by the evolution of
the Biscay (Martins et al., 2010).
Petroleum Systems’ Elements and Processes
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total thickness around 75 m in the coastal outcrops of
Peniche. The upper member is the “Marly Limestones
with organic rich facies" having a thickness of about 30
m (Duarte et al., 2010) and TOC values range from 0–
25%. Both units are part of the Lower Jurassic successions which extend over the basin. However, due to
paleogeographic conditions, it is expected the facies
having higher potential is in the deeper parts of the
basin’s homoclinal ramp, towards the northwest (Pena
dos Reis et al., 2011; Duarte et al., 2012).
The Late Jurassic source-rock is composed of
marly limestones deposited in lacustrine, lagoonal, and
coastal environments and have been studied by several
authors (e.g., Spigolon et al., 2011; Silva et al., 2013).
Total thickness of this unit is around 200 meters and
TOC values in darker layers usually range from 2 to 5
%, with restricted layers reaching up to 10-30%. Kerogen types are variable; Type III predominates, Type I
and IV are also present. Organic matter accumulation
and preservation has taken place in restricted environments developed in most areas of the basin, related
with coastal regions having continental inputs and
ephemeral marine incursions. Overall, regional variations point to an important input of terrestrial plantdebris in the northern areas and to predominant algal
mats development in the southern areas (Spigolon et
al., 2011), although the heterogeneity of the deposits
may eventually suggest a wider lateral variability
(Silva et al., 2013). Although the richest layers are not
strictly contemporaneous, depending on local highs
and lows controlling the marine incursions, this sourcerock could be considered basin-wide and having a large
variation of organic matter type (Silva et al., 2013).
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The Lusitanian Basin contains several formations having source-rock potential (Figs. 3 and 5),
which have been identified and studied since the beginning of exploration in Portugal (DPEP, 2013),
including Silurian deep-marine black-shales, Carboniferous turbiditic shales, Lower Jurassic shaly marls, and
Upper Jurassic marly limestones (Oxfordian). Other
formations having source-rock potential include the
Dagorda Formation (Hettangian), the Abadia Formation (Kimmeridgian) and the Cacém Formation
(Cenomanian-Turonian).
The Lower Jurassic source rock is composed of
marly black shales deposited in a fully open marine
environment (Duarte and Soares, 2002; Duarte et al.,
2010, 2012; Silva et al., 2010, 2011). Total thickness of
these deposits is about 100 m. Although there are several organic-rich layers, TOC values are highly
variable. Kerogen is mainly of type II, III, although
some intervals are Type I (Duarte et al., 2010; 2012;
Spigolon et al., 2010). Accumulation and preservation
of organic matter occurred in this setting particularly
close to the “maximum flooding surfaces” of two distinct second-order sequences (Duarte et al., 2010).
Those two organic-rich units have been studied in
detail, regarding TOC, isotopes, palynofacies, etc.
(Duarte et al., 2010, 2012; Silva et al., 2010, 2011;
Poças Ribeiro et al., 2013)
The lower organic-rich unit is the upper Sinemurian to lower Pliensbachian Água de Madeiros
Formation, which is about 42 m in the coastal outcrops
of São Pedro de Muel (Duarte et al., 2012). TOC values are mostly over 7 wt. %, reaching up to 22 wt. %
(Duarte et al., 2012). The upper organic-rich unit is the
Pliensbachian Vale das Fontes Formation, having a
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Source rocks
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Maturation
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rifting phase. This phase is responsible for an increase
in heat flow and, at the same time much overburden. In
most places maturation has been attained in the Late
Jurassic (Kimmeridgian to Tithonian) and it has been
most prominent in the Oxfordian depocenters, namely
the Central Sector’s sub-basins of Arruda, Bombarral,
and Freixial (Teixeira et al., 2012, 2014).
As a general statement considering vitrinite
reflectance data (BEICIP-FRANLAB, 1996) and thermal basin modelling studies (Teixeira et al., 2012,
2014), it may be considered that the Lower Jurassic
source-rock is mature for oil in the north sector of the
basin and mature for gas in the south sector, whereas
the Upper Jurassic source rocks may be not mature in
the north sector and are mostly mature in the south sector. However, local depocenters are areas of increased
overburden and maturation of the Lower Jurassic
source-rock in the northern sector, which may be the
case close to São Pedro de Muel (Porto Energy, 2012).
The same kind of situation may have promoted maturation of Upper Jurassic source-rocks in northern
depocenter areas.
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Both Jurassic source-rocks can be in the hydrocarbon generation window, although not everywhere in
the basin, as a result of the highly heterogeneous
basin’s subsidence and overburden, especially in the
Late Jurassic.
Non-mature Lower Jurassic source-rocks are
known in outcrop, namely at the Peniche, Montemor-oVelho, and São Pedro de Muel sections (Oliveira et al.,
2006; Silva et al., 2010; Spigolon et al., 2011; Duarte
et al., 2012), but preliminary maturation modelling
may suggest that the units have reached maturity in
several exploration wells in the basin (Teixeira et al.,
2012, 2014). These non-mature Late Jurassic sourcerocks are also present in different outcrops, such as
Cabo Mondego or Montejunto (Spigolon et al, 2011),
whereas they reached the oil-window in nearby wells
such as SB-1, FX-1 and CP-1 (Teixeira et al., 2012,
2014). This situation points to a very important role of
differential subsidence along the basin, both in time
and space.
Thermal modeling in different locations point to
a crucial role played by the Late Jurassic intense siliciclastic input into the basin, related to the Oxfordian
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Migration
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Well and field data include non-commercial
occurrences of oil and gas in many wells and a few oil
shows in outcrops, proving the existence of maturation
and subsequent migration of hydrocarbons. Biomarkers
studies (Spigolon et al., 2011) point to a potential for
two different oil systems related to the migration of
hydrocarbons towards different reservoir units.
A Lower Jurassic source rock provenance is
identified in oil shows occurring in several outcrops
around diapir walls in the northern sector of the basin.
The oil is believed to have migrated from the Lower
Jurassic marls along the faults created (or used) during
the salt uplift, feeding a major sandstone reservoir of
Aptian age (Figueira da Foz Formation) (Fig. 4). These
pathways have been active probably since the Late
Jurassic and should have been enhanced at major
halokinetic events related with compression, namely in
the Late Cretaceous (with documented extrusion; Pena
dos Reis, 2000) and the late Miocene (with intense
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alpine uplift; Ribeiro et al., 1980), feeding the Cretaceous sandstones.
Another pattern of migration corresponds to
hydrocarbons of an Oxfordian provenance (Cabaços
Formation) in Upper Jurassic reservoir units. This oil
system is very frequently identified in oil-shows
(DPEP, 2013) and also in some outcrops (Spigolon et
al., 2010). This play is only evident in the southern sector, where post-Oxfordian successions are thick enough
to have the necessary overburden. The source rocks are
mid-Oxfordian marly limestones (Cabaços Formation)
and the reservoir units include late Oxfordian fractured
carbonates (Montejunto Formation) and Kimmeridgian
turbiditic siliciclastics (Abadia Formation). This system comprises only Upper Jurassic units and its
stratigraphical proximity explains the frequency of this
kind oil shows in the Lusitanian Basin (Pena dos Reis
and Pimentel, 2011).
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Reservoirs
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reefal build-ups, both with good reservoir potential
(Uphoff et al., 2010). Besides interparticle and vuggy
porosity, these units may also be fractured reservoirs.
The Upper Jurassic limestones of the Montejunto
Formation may also contain porous (Uphoff et al.,
2010) fractured reservoirs, enhanced by its stratigraphic and geometric proximity to the Upper Jurassic
source-rock of the Cabaços Formation (Pena dos Reis
and Pimental, 2011). This fact is particularly important
close to diapiric structures promoting fractures and
trapping, as seen in the Torres Vedras oil-seep (Spigolon et al., 2010).
The fine- to coarse-grained turbiditic deposits of
the Abadia Formation contains abundant sandy layers
having reservoir potential. However, once again, carbonate cementation, especially where calciclastic
particles are present, partially obliterates the intergranular porosity (Garcia et al., 2010). The following
prograding fluviodeltaic sequence (Lourinhã Formation) is compositionally immature, resulting in
interesting inter- and intragranular porosities, with values around 10 to 15%. Early diagenetic carbonate
cementation has been incomplete, inhibiting fill compaction and preserving most of the primary porosity
(Atlantis, 2010).
Lower Cretaceous sedimentation includes fluvial
and transitional to coastal deposits having some reservoir potential. The presence of infiltrated clays in
fluvial units and irregular carbonate cementation in
coastal units has diminished its intergranular porosity,
resulting in an average around 5% (Atlantis, 2010).
Locally, some coastal high energy or biohermal carbonates have good primary porosity (Dinis et al.,
2008). Cenomanian transgression resulted in the
marine flooding of the basin and deposition of shallow
marine and reefal carbonate units that also have good
reservoir properties (Dinis et al., 2008).
The Cenozoic inversion resulted in basins uplift
and locally to some intracontinental basins containing
some hundreds of meters of mainly siliciclastic infill
(Pais et al., 2012). Paleogene and Miocene infill
include immature and matrix-rich clays that have low
reservoir potential, but also transgression related bioclastic limestones that have good intergranular and
moldic porosities. Cleaner sands were deposited in the
basin during the late Miocene and Pliocene that could
be reservoir properties provided a seal is present.
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The Lusitanian Basin has a thick sedimentary
infill and a very distinct facies, including a wide range
of siliciclastic, carbonate, and mixed deposits (Fig. 3).
Granular reservoirs are present in siliciclastic continental, transitional, and marine facies; porous reservoirs in
some coastal carbonates; and fractured reservoirs in
several shallow marine to deep marine carbonates of
different ages (Pena dos Reis and Pimentel, 2010a,
2010b). Oil seeps and oil shows have been observed in
siliciclastic and carbonate units of different ages,
including Late Triassic, Jurassic, and Cretaceous
(DPEP, 2012). However, a comprehensive and systematic study of the several reservoir units hass yet to be
produced and published, and most of the following
considerations are based on sparse bibliographic references and in personal observations.
The sedimentary record of the Lusitanian Basin
includes abundant siliciclastic deposits related to
phases of intense tectonic activity and subsequent erosion and accumulation. Many of those deposits were
initially very porous, but in many cases diagenesis
obliterated a significant part of it. However, is may be
assumed that the basin included enough volume of
siliciclastic rocks to contain all the oil that had been
generated during the Mesozoic.
The basal deposits of the basin are the Silves
Group siliciclastics of Late Triassic age. Proximal to
distal alluvial-fan red-beds have intergranular porosity
partially filled by carbonate cementation (Atlantis,
2010), with values ranging from 16% to 23%, locally
up to 70%.
The Lower Jurassic sequence includes thick,
compact, and dolomitized carbonate units, especially at
the bordering areas of the basin, known in outcrop
close to the basement to the east. The Dagorda Formation includes dolomites with up to 20% primary
porosity (Uphoff, 2005), whereas at the Coimbra Formation intercrystalline and vuggy porosity with
brecciation related to early meteoric diagenesis are
important.
From the Pliensbachian onward, the monotonous
alternation of limestones and marls of the Brenha
Group have low reservoir potential, although some
fracture-related porosity may be present. Middle Jurassic units correspond to the Candeeiros Formation,
which is predominantly shallow marine carbonates
having high to moderate energy textures and some
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Traps
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with sandy layers, as is the case of the Upper Jurassic
and Lower Cretaceous units. However, Upper Cretaceous and Tertiary shaly units may have acted as
important seals, particularly in the more subsiding or
distal areas of the basin. The behaviour of carbonate
units as seals is highly dependent on the presence or
absence of dense fractures that promote leakage. Most
of the carbonate units of the Lower-Middle Jurassic
and Upper Cretaceous may act as seals provided they
have not been affected by intense fragile deformation,
as is usually seen (for example) associated with diapiric outcropping structures.
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The knowledge about the characteristics of geological units having seal potential in the Lusitanian
Basin is well behind what has been presented for the
other petroleum system elements. The Dagorda Formation is probably the main effective seal in the basin,
consisting of compact red clays containing variable
amounts of gypsum and halite having thicknesses up to
hundreds of meters. This thick shaly package is virtually impermeable and its plasticity confers the ability to
deform and to pierce higher stratigraphic levels, controlling vertical migration and sealing different
reservoirs. There are several other shaly units, but they
cannot be perfect seals due to frequent intercalations
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Seals
different source-rocks, reservoirs, and seals have been
essential to promote migration and trapping of hydrocarbons (Fig. 4). The main tectonic events promoting
the intense structuration of the basin took place during
the Late Jurassic (rift-climax) and Late Cretaceous
(beginning of the alpine inversion), frequently involving the Hettangian plastic evaporites, affecting the
petroleum systems and creating multiple traps (Fig. 4).
Alpine inversion during the Tertiary has also
been important for deformation and creation of structural traps in many places of the basin (Ribeiro et al.,
1980). However, the probable fracture network development may have also destroyed many traps’ integrity,
enhancing leakage and even total loss of prior hydrocarbon accumulations.
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Several stratigraphic traps may be present in the
Lusitanian basin. The first one is the direct superposition of Hettangian clays and evaporites over Late
Triassic sandstones, promoting accumulation of hydrocarbons both from underlying Palaeozoic units and
from down-thrown Jurassic source-rocks (Figs. 3 and
4). Another favourable situation is the occurrence of
biohermal build-ups in several places and during several times, namely the Middle Jurassic (e.g., at the
Pataias cement quarry), Upper Jurassic (Amaral Formation) and Upper Cretaceous (Cenomanian reefs).
Structural traps seem to have a predominant role
in the Lusitanian Basin as a consequence from the
intense tectonic deformation during the Mesozoic
extension and Tertiary compression. Up-lift and subsidence of tectonic blocks bringing side-by-side
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Petroleum Systems and Plays
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From the analysis of the petroleum system elements and its articulation in space and time, three main
petroleum systems may be considered in the Lusitanian
Basin (Pena dos Reis and Pimentel, 2010a, 2010b)
(Fig. 5).
A pre-salt petroleum system may be defined as
sourced by meta-sedimentary Paleozoic rocks feeding
Upper Triassic siliciclastic reservoirs and sealed by the
Hettangian evaporitic clays. This kind of play has been
initially described for the Silurian black-shales and the
Silves Group sandstones (Uphoff, 2005). Those graptolitic black-sales have reached the oil and gas-window
in Paleozoic times, before late Variscan uplift and erosion and may have kept some generative potential for
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further gas expulsion during the Mesozoic reburial
(Uphoff, 2005). The same line of thought may be
developed regarding other Palaeozoic units having
source-rock potential, such as the Carboniferous turbidites of the Southern Portuguese Zone (Fernandes et
al., 2012; McCormack et al., 2007; Barberes, 2013).
The main conditioning factor seems to be the (over)
maturation of such Lower and Upper Palaeozoic rocks.
However, highly variable results have been reported,
sometimes in outcrops a few meters apart (Uphoff,
2005; McCormack et al., 2007). This situation points to
the role of late Variscan thin-skinned tectonics, resulting in thrust sheets bringing side-by-side units from
different depths and structural settings.
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impregnated the overlying Montejunto Formation
limestones and the Abadia Formation turbidites
(Spigolon et al., 2010; Uphoff et al., 2010; Pena dos
Reis and Pimentel, 2011). The seal for this kind of play
could be Cretaceous and/or Tertiary clays and
siltstones.
A fourth petroleum system may be present, corresponding to the accumulation of hydrocarbons in the
Tertiary cover of the Lusitanian basin. Several source
rocks might be involved depending on the sector of the
basin, and the Tertiary deposits would act both as reservoirs and seals. Due to the generally thin sedimentary
cover in the inversion-related Tertiary continental
basins (a maximum of 800 meters) south of Lisbon
(Ribeiro et al., 1979), this system should naturally be
more prominent in the offshore areas, where the Tertiary prograding accumulations can reach greater
thicknesses (Alves et al., 2003).
To synthesize, we may consider that several
petroleum systems and plays have been active and are
proven in the Lusitanian basin, involving source-rocks
and reservoirs of different ages and lithological characteristics (Fig. 5). Preliminary modelling attempts
suggest that maturation has been promoted by intense
subsidence and burial, mainly in Late Jurassic times, at
different intensities in different sectors of the basin
(Teixeira et al., 2012, 2014). As a general pattern, we
may consider that in the North sector only the Lower
Jurassic units have reached maturity for oil and eventually for gas, whereas in the Central and South sectors
the Lower Jurassic entered the gas-window and the
Upper Jurassic entered the oil-window (Teixeira et al.,
2012, 2014).
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A second petroleum system is related with the
Lower Jurassic source-rocks, namely the Sinemurian
and Pliensbachian organic-rich marls (Água de
Madeiros and Vale das Fontes formations). Its geochemical characteristics point to good oil generation
potential (Duarte et al., 2010, 2012; Spigolon et al.,
2011); in highly subsided areas of the basin, it has
probably reached the oil-window and even the gaswindow (Teixeira et al., 2012, 2014). The intense
movement of basement blocks affecting the Mesozoic
and Tertiary infill and cover is responsible for many situations in which the Lower Jurassic units appear
geometrically beside or above many of the potential
reservoir units. Therefore, the Lower Jurassic source
rocks may have laterally and vertically fed Jurassic and
Cretaceous siliciclastic and carbonate units, as well as
Upper Triassic siliciclastics.
From the observation and analysis of unpublished geochemical data of several diapir-related oilseeps (e.g., Paredes de Vitória and Leiria; Atlantis,
2010), it seems that Lower Jurassic source-rocks were
the main feeders, using the associated vertical faulting
and brecciation as migration conduits along the diapirwalls. It may be speculated that in non-outcropping
diapirs, the same situations led to oil accumulations,
sealed by an unconformable Tertiary cover.
A third petroleum system corresponds to the
maturation and oil generation of the transitional to
coastal marine Oxfordian Cabaços Formation. These
marly rocks have been buried under kilometers of
Oxfordian and Kimmeridgian rift-related siliciclastics
(Abadia and Lourinhã formations), entering the oil
window in most of the basin since the Early Cretaceous
(Teixeira et al., 2012, 2014). This oil has abundantly
Implications for Deep Offshore Exploration
op
From the compared analysis of onshore and offshore petroleum systems in the Lusitanian and Peniche
basins, many analogies may be used to approach these
plays, but differences are also important. Both basins
have in common its geodynamic framework and evolution, but the specific proximal position of the
Lusitanian Basin and the outer position of the Peniche
basin, regarding crustal stretching and North Atlantic
opening (Alves et al., 2006), has resulted in diachronic
elements that must be addressed and understood
(Table 1).
Most of the plays that have been identified in the
Lusitanian Basin may also be considered valid for the
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Pena dos Reis and Pimentel
Peniche basin. These include (i) the pre-salt play,
sourced by Silurian and/or Carboniferous rocks, feeding Upper Triassic redbeds, and sealed by Hettangian
evaporites; (ii) the diapiric-related play, sourced by
Lower Jurassic deep marine shales and marls, feeding
Cretaceous fluvial sands, and sealed by Upper Cretaceous marls; (iii) the turbidite play, sourced by Upper
Jurassic transitional marls and known in the Lusitanian
Basin to have fed Upper Jurassic turbiditic sands (Abadia Formation) as well as fractured limestones.
However, due to diachronic geodynamic evolution of the proximal and outer parts of this Atlantic
margin, it may be speculated that in the Peniche Basin
8
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Migration was highly dependent on faulting,
related both to extension and compression. Considering
the modelled late maturation timings, compression
structures were probably more important as conduits,
although many of them corresponded to the re-activation of previously extension structures.
Mesozoic traps seem to be predominantly structural, although there are a few stratigraphic traps
associated with bioherms. On the other hand, Tertiary
traps may include many stratigraphic accumulations
related to the presence of coarse-grained turbiditic
channels and fine-grained over-bank deposits. Main
unconformities may also act as traps, namely the one
between deformed Mesozoic reservoir units and flatlying Tertiary sealing units.
The influence of inversion issues must be
stressed on any approach to the Peniche basin.
Although it helped in creating folds and faulted migration pathways, its impact on seal integrity may have
been critical. This pattern is quite evident in outcrop
analogues in the Lusitanian basin, such as the Paredes
de Vitória and Monte Real diapirs. Therefore, anticlinal
closures associated with significant inversion features
should be avoided or, at least, carefully looked
examined.
Understanding the onshore basins is a crucial
starting point to approach the offshore basins. Due to
its remarkable exposures and geological detailed studies, the Lusitanian Basin is well known and may be
used as an analog to the Peniche Basin (Alves, 2009;
Sagres, 2013). The same depositional packages and
unconformities may be recognized in both basins and
the different petroleum systems identified at the Lusitanian Basin may prove to be present at the Peniche
Basin (Fig. 6). Farther south on the Western Iberian
Margin, the outcrops of the southwestern Alentejo and
western Algarve may also be used as analogues for the
offshore Alentejo basin.
The “analog approach” must be based on understanding the basin’s evolution—how each geodynamic
context generates each petroleum system element and
how they are articulated in space and time. De-risking
strategies for the West Iberian margin offshore basins
must include basin and petroleum systems analysis
based on detailed outcrop studies.
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the turbidite facies (equivalent to the Abadia Formation) have been deposited during the Lower Cretaceous
as thick rift-climax related sediments (Alves et al.,
2003, 2009). Reservoir potential of these deposits is
therefore considered to be high, including deep sea fan
geometries with channelized over-bank deposits and
also re-sedimentation as contourites.
A fourth play, absent in the onshore areas, may
be considered in the deep offshore related to the accumulation of thick deep marine deposits corresponding
to a Tertiary play: Jurassic source rocks feeding Upper
Cretaceous to early Tertiary sands and sealed by Neogene clays. Based on recently acquired seismic lines of
the Peniche Basin (Consortium Petrobras/GALP/Partex; DPEP, 2013), its seismic stratigraphic analysis and
thickness modelling in pseudo-wells show that MesoCenozoic overburden has been sufficient to promote
both maturation of the Jurassic source rocks and also
sealing of the Upper Cretaceous to early Tertiary reservoirs (Sagres, 2013). All these plays have their own
risks and many of the considerations previously presented about the relations between the Lusitanian and
the Peniche basins are important to reduce the risk of
the deep offshore exploration activities (Table 1).
Lower and Upper Jurassic source-rock identification is rather confident, but its areal distribution is
difficult to predict because paleogeographic reconstructions are, for the moment, incomplete at a
regional-scale distribution. However, the patterns in the
Lusitanian Basin (Pena dos Reis et al., 2011) may be
extrapolated to larger areas, and probably both Jurassic
source rocks are present in most of the areas of the Peniche basin. Moreover, both units most probably
attained maturation for oil, although at different times
(Sagres, 2013) (Table 1).
It is crucial to understand the timing and location
of maturation of petroleum systems as there are important differences between both basins. For example, riftclimax subsidence and related salt withdrawal at the
Peniche Basin (Alves et al., 2003, 2009) occurs 10-15
My (?) later than the landward Lusitanian basin, it
seems likely that in the Peniche Basin the kitchen areas
and the migration pathways have been controlled
mainly by the Cretaceous subsidence and diapirism.
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Pena dos Reis and Pimentel
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7
Acknowledgments
to important historical exploratory data and unpublished reports. Petrobras Portugal, namely Rudy
Ferreira and Marisa Calhôa are also thanked for the
access to data about the Peniche Basin and for their
collaboration with the SAGRES project. Finally, individual collaborations must also be thanked, including
Antônio Garcia for the coordination of the Atlantis
Project and Ramón Salas, Hugo Matias, and Ricardo
Pereira for many fruitful discussions.
EP
M
This paper collects data and ideas from many
years of research, during which several institutions and
colleagues were most helpful. We thank CENPES/
Petrobras and particularly Edison Milani, Adriano
Viana, and Gilmar Bueno for two financed research
projects supporting this work (ATLANTIS, 2007-2010
and SAGRES, 2011-2013) and for their permanent collaboration. We also thank DPEP (the Portuguese oil
agency) and Director Teresinha Abecasis for the access
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(Portugal): Coimbra and Água de Madeiros formations: International Journal of Coal Geology v. 111, p.
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199-225.
Rey, J., J.L. Dinis, P. Callapez, and P.P. Cunha, 2006, Da
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Ribeiro, A., M.T. Antunes, M.P. Ferreira, R.B. Rocha, A.F.
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géologie génerale du Portugal: Serv.Geol.Portugal,
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Portugal: foreland detachment in basement and cover
rocks: Tectonophysics, v. 184, p. 357-366.
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Geology of Iberia: Springer-Verlag, p. 397-410.
Ribeiro, A., J.M. Munhá, R. Dias, A. Mateus, E. Pereira,
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C.R. Gomes, C.R., 1996, The 1st and 2nd rifting
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Uphoff, T. L., 2005, Subsalt (pre-Jurassic) exploration play
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Portugal: Mesozoic and Tertiary tectonics, Stratigraphy and Subsidence History, in A.J. Tankkard and H.
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Matos, 2011, Geoquímica organica de rochas potencialmente geradoras de petróleo no contexto evolutivo
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(acc.). Thermal modelling and maturation of Jurassic
source rocks in the Lusitanian Basin (Northeast
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Table 1. Conceptual identification of major exploration risks related with rift evolution in oceanic margins’ basins.
Basin Stage /
Main Petroleum System Risk Types
In Oceanic Margin Basins Exploration
7
Processes
Traps & Seal Integrity
Inversion
EP
M
(Antiforms and Faulting)
Migration
Uplift / Folding
COMPRESSION / COLLISION CYCLE
Post-rift Æ Drift
Maturation & Seal
SS
(Overburden & Thermal evolution)
Subsidence
C
POST-RIFT CYCLE
Source Rock & Reservoir
Syn-rift
G
(Sedimentary environments
+ Depositional thickness)
Extension
14
Basement blocks
RIFT CYCLES
Thermal history
Pre-Rift
Lithologies
Tectonic terranes
COMPRESSION/ COLLISION CYCLE
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Basement
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Ireland
Great
Britain
Atlantic
Ocean
7
France
Portugal
EP
M
Spain
GB
C
SS
PB
G
Bb
LB
14
PenB
20
Bb
C
op
yr
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100 km
N
AB
P
O
R
T
U
G
A
L
SPAIN
AlgB
Figure 1. Study area showing the location of the Western Iberian Margin’s onshore and offshore basins;
abbreviations are defined in the text.
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Figure 2. Geological framework of the
Lusitanian Basin (onshore and offshore).
Mesozoic based on LNEG; Paleozoic
based on Pena dos Reis et al., 2012. AF–
Arrife fault; LCF–Lousã-Caldas fault;
VFF–Vila Franca fault. A and B dashed
lines are seismic lines in Figure 4.
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EP
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C
Figure 3. Lithostratigraphic chart of the Lusitanian Basin including geodynamic events, cyclicity, seismic
horizons, and main petroleum system elements (adapted from Pena dos Reis et al., 2011; partially based on
Wilson, 1990; Azerêdo et al., 2003; Rey et al., 2006).
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L CRET
7
U JURA
LM JURASSIC
TJ Salt
EP
M
U TRIAS
A
Abadia
Cab+Montej
C
TJ Salt
SS
SEISMIC HORIZONS
Lourinhã
LM JURASSIC
G
U TRIAS
B
H10 - Top Tithonian
(Lourinhã)
H9 - Top Kimmeridgian
(Abadia)
H8 - Top Oxfordian
(Montejunto)
H6 - Top Callovian
(Candeeiros)
H2 - Top Hettangian
(Dagorda)
H1 - Top Upper Triassic
(Silves)
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Figure 4. Interpreted seismic lines across the Lusitanian Basin (in Carvalho, 2013); see Figure 2 for location.
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M E S O Z O I C
PALEOZOIC
Dev Carb Perm
Jurassic
M
E
CENOZOIC
Cretaceous
E
L
L
Paleogene
Neogene
SS
EP
M
Sil
Triassic
E M
L
RESERVOIR
SEAL
OVERBURDEN
TRAP
MAT/MIGR/ACC
CRITICAL MOMENT
G
C
HALOCINESIS
SOURCE ROCK
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Figure 5. Petroleum system events chart of the Lusitanian Basin (adapt. from Pena dos Reis and Pimentel, 2010b).
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Lusitanian Basin
Seismo-stratigraphic Units
QUATERN.
Upper TERT
Lower TERT
Upper CRET
Lower CRET
EP
M
Upper JURA
L-M JURA
U. TRIAS
Rasmussen et al. 1998
Peniche Basin
B
A
Campanian
Aptian
Callovian
14
TGS-NOPEC seismic line courtesy from PETROBRAS
G
C
SS
Unconformities
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Figure 6. Seismic stratigraphic correlation between Peniche and Lusitanian basins. Both lines are presented at the same horizontal and
vertical scales. The main unconformities are underlined. Note the significant thickness of the Tertiary sediments in the Peniche Basin.
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