Analysis of the Petroleum Systems of the Lusitanian Basin (Western Iberian Margin)—A Tool for Deep Offshore Exploration

Pena dos Reis, RuiCentro de GeociênciasFaculdade de Ciências e Tecnologia da

Universidade de CoimbraLg Marquês de Pombal3000-272 Coimbra, Portugale-mail: penareis@dct.uc.pt

Pimentel, NunoCentro de GeologiaFaculdade de Ciências da Universidade LisboaCampo Grande C-61749-016 Lisboa, Portugale-mail: Pimentel@fc.ul.pt

Abstract

A synthesis of the knowledge about the Lusita-nian Basin is presented here, focusing on itsstratigraphic record, sedimentary infill, evolution, andpetroleum systems. Petroleum system elements arecharacterized, including Palaeozoic and Mesozoicsource rocks, siliciclastic and carbonate reservoirs, and

Mesozoic and Tertiary seals and traps. Related pro-cesses, such as organic matter maturation andhydrocarbons migration are also discussed. The charac-teristics of these elements and processes are analysedand implications for deep offshore exploration are dis-cussed.

Introduction

The Lusitanian Basin is one of the Western Ibe-rian Margin sedimentary basins related with theopening of the North Atlantic (Fig. 1) (Wilson et al.,1989). These basins have their counterparts in the east-ern Canada Jeanne D’Arc and Whale basins, as part ofthe Iberia-Newfoundland conjugate margins complex(eg., Wilson et al., 1989; Pinheiro et al., 1996; Peron-Pinvidic and Manatchal, 2009). The evolution of allthese basins are defined by the same geodynamic con-

results. However, exploration continues and a goodunderstanding of these basins’ evolution and character-istics is crucial to enhance the chances of futuresuccess. This paper deals with the evolution of themainly onshore Lusitanian Basin and its petroleum sys-tems, in order to establish an analog for other nearbyoffshore basins, aiming to contribute to a betterregional framework for exploration in this region (Fig.1).ht 2

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Sedimentary Basins: Origin, Depositional Histories, and Petroleum Systems 1

trols but also by specific local constraints, explainingdifferent characteristics and success in exploration. Inthe Canadian basins, the intense exploration has led toseveral good production and development results, butthe Iberian basins have not had so far similar positive

In this paper we present a summary of thisapproach, based on an overview of the basin evolutionand an analysis of the related petroleum systems andelements, with implications on the deep offshore on-going exploration.

Lusitanian Basin’s Evolution and Infill

The Lusitanian Basin extends for about 250 kmnorth-south and 100 km east-west, facing the Atlanticto the west, representing the inner and most proximalmargins of much larger basins extending towards theshallow and deep offshore (LB in Fig. 1). The geologi-cal record of the Western Iberian Margin’s evolution ispresent in several nearby basins sharing similarities,such as the offshore Porto and Galicia basins in north-ern Iberia (PB, GB in Fig. 1), the Peniche and Alentejo

basins in southwestern Iberia (PenB, AB in Fig. 1) andthe Gulf of Cadiz Basin in southern Iberia (Gb inFig. 1).

The Lusitanian Basin resulted from the initialextension of the Pangea’s continental crust and lateropening of the North Atlantic Ocean as a result of rift-ing and seafloor spreading. The evolution of thePaleozoic basement and the Mesozoic extension cre-ated a complex succession of events and sedimentary

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infill (Figs. 2 and 3). The influence of the basement onthe basin’s evolution may be addressed along two mainlines: (i) its lithologies, including the presence of unitshaving source-rock potential; (ii) its structures, particu-larly the presence of important regional faults and theirmovement during the Mesozoic and Tertiary (e.g., Penados Reis et al., 2012 Dinis et al., 2008; Pena dos Reiset al., 2000)

The complex structure of the Paleozoic basementof western Iberia resulted mainly from the collision anddeformation of two terranes (e.g., Ribeiro et al., 1979,1990, 2007; Matte, 1991): (i) the Iberian Terrane withthe Central Iberian Zone (CIZ) and the Ossa MorenaZone (OMZ); and (ii) the Southern Portuguese Terraneand its Zone (SPZ). The joint deformation of all thisbasement is related with the Ibero-Armorican Archdeveloped in the Late Paleozoic during the VariscanOrogeny.

The Central Iberian Zone includes Silurian unitshaving organic matter in pelitic layers, sometimes sev-eral hundred meters thick (Romão et al., 2005).However, metamorphism may have over maturatedthose units and most (if not all) of the hydrocarbonsmay have been lost. Late Carboniferous deposits aremore prospective, considering their post-orogenic ageand therefore not so high maturation. They have beendeposited in narrow intra-mountain lacustrine basins asfining-upward siliciclastic deposits, including blackshales and coal seams at the top (Domingos et al.,1983).

The Ossa Morena Zone includes several unitscontaining organic-rich layers, affected by low-grade

Since the Late Carboniferous and during Perm-ian times, gradual uplift and erosion brought theserocks to more shallow structural domains and predomi-nantly gentle deformation, commonly known as “late-Variscan faulting.” These north-northeast/south-south-west sinistral and northwest-southeast dextralmovements conditioned the origin and configuration ofthe Lusitanian Basin from the Late Triassic, in theglobal context of the Pangaea break-up and intraconti-nental troughs in western Europe and eastern America(Wilson et al., 1989) (Fig. 2). In western Iberia, strongsubsidence resulted in north-northeast/south-south-west trending asymmetric grabens that rapidly filled byalluvial-fan siliciclastic deposits passing to sabkhaevaporite clays and salts, under arid climatic condi-tions. This sedimentary infill corresponds to the SilvesGroup (Palain, 1976) and is composed mainly of coarsesiliciclastic red-beds forming two fining-upward mega-sequences (Fig. 4). Its total thickness is up to 400meters and thee paleogeographic reconstructions pointto the development of north-northwest/south-southeastfault-bounded half-grabens, separated by intra-riftbasement horst blocks (Uphoff, 2005). These depositsshow good reservoir potential and have been targetedfor gas as part of a pre-salt petroleum system (Uphoff,2005).

These Upper Triassic grabens became graduallyfilled-up and sabkha-like environments became pre-dominant, promoting the accumulation of red clays andevaporites in the most subsiding parts of the basin(Dagorda Formation) (Palain, 1976). The resultingshaly deposits, some hundreds of meters thick, includeht 2

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metamorphism that has caused intense maturation(Chaminé et al., 2003). However, some outcroppingSilurian graptolitic black-shales show highly variableRo% equivalent values, ranging from the late oil-win-dow to the gas-window, probably controlled by theproximity to major fault zones (Uphoff, 2005; Mach-ado et al., 2011).

The Southern Portuguese Zone includes Devo-nian to Late Carboniferous metasedimentary rocks,including some black-shales having source rock poten-tial, such as the fine-grained turbidites of the BaixoAlentejo Flysh Group (Oliveira, 1983). Although theyare affected by low-grade metamorphism and are gen-erally over mature (McCormack et al., 2007;Fernandes et al., 2012), preliminary data (Barberes,2013) suggests that there may have been some placeswhere the group is preserved within the gas-window.

significant amounts of gypsum and halite, which wouldbe fundamental for the tectonic deformation of theMeso-Cenozoic units, acting as “decollement” leveland also as diapiric masses with kilometric-scale verti-cal movements, locally piercing the Mesozoic cover(Kullberg, 2000) (Figs. 2 and 3).

In the Sinemurian, the Hettangian sabkha-like tocoastal environments are replaced by open marine, pre-dominantly carbonate environments, such as theCoimbra Group (Soares et al., 2007) (Fig. 4). This unitis about 200 m thick and present throughout the basinas a result of the paleogeographic coalescence of theinitial troughs and an expansive onlap of a carbonateramp over the basement towards the East. Dolomiticlimestones (São Miguel Formation) are present in thenorthern and eastern areas, whereas the marly lime-stones are present in central and western areas of thebasin (Água de Madeiros Formation) (Duarte and

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Soares, 2002; Azerêdo et al., 2003). The later includesa lower unit only a few tenths of meters in thickness,deposited in open marine environments but havingvery good organic content of upper Sinemurian–lower-most Pliensbachian age (Polvoeira Member; Duarteand Soares, 2002; Duarte et al., 2004, 2010, 2012).

During the Early Jurassic, sedimentation tookplace in a carbonate ramp depositional system, givingplace to a thick sequence of marly limestones, knownin exploration and the 20th century literature as theBrenha Group (Witt, 1977). The sedimentation beganwith the deposition of about one hundred meters ofalternating centimeter-thick layers of marls and lime-stones of Pliensbachian age (Vale das FontesFormation; Rocha et al., 1996; Duarte and Soares,2002; Azerêdo et al., 2003), also having very goodgeneration potential (e.g., Oliveira et al., 2006; Silva etal., 2010; Duarte et al., 2010; Spigolon et al., 2011).

The open marine marly sedimentation graduallygave place to a predominance of limestones every-where in the basin–Cabo Mondego Formation in theNorth (Azerêdo et al., 2003) and Candeeiros Group inthe South (Witt, 1977). An overall regression graduallypromoted shallower sedimentation, reaching emersionand depositional hiatus in the eastern border of thebasin during the Callovian (Azerêdo et al., 2002,2003).

Late Jurassic sedimentation started in mid-Oxfordian times, following a Callovian forced regres-sion and emersion (Azerêdo et al., 2002) (Fig. 4). Thissituation is related with an important geodynamic re-organization of the basin (Wilson et al., 1989; Hiscott

related with a Late Jurassic rifting event, recognized insubsidence curves and related with the beginning of theAtlantic opening to the south of Iberia (Wilson et al.,1989; Rasmussen et al., 1998).

The Abadia Formation corresponds to the rift-climax, and its deposits have been the classical targetof oil exploration in the Lusitanian Basin during the20th century (DPEP, 2013). Prograding continentalsiliciclastics continue to cover the basin during theTithonian, resulting in the accumulation of almost 1 kmof fluviodeltaic sands and clays (Lourinhã Formation;Hill, 1988). Late Tithonian to early Berriasian sedi-ments are generally named as the “Purbeck Facies” andinclude fluvial to coastal siliciclastics.

The Cretaceous evolution of the LusitanianBasin is closely related with the opening of the NorthAtlantic Ocean. This opening has been developed inthree steps which are well identified in the geologicalrecord as break-up unconformities (Dinis et al., 2008).The beginning of the Early Cretaceous is also markedby an important magmatic event indicated by severaldykes mainly associated with diapiric piercing struc-tures (Martins et al., 2010). Lower Cretaceoussediments are known only in the south and central sec-tors of the Lusitanian basin, indicating an importantuplift of the north sector during most of this timeperiod (Fig. 4).

The sedimentary record is composed mainly offluvial to coastal fine-grained siliciclastics and impurelimestones, grouped into two cycles, separated byunconformities associated to distinct segments of theNorth Atlantic opening (Rey et al., 2006; Dinis et al.,ht 2

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et al., 1990), resulting in a Late Jurassic depositionaltrough elongated northeast-southwest and open to thesouthwest.

The first Oxfordian sediments correspond to afew hundred meters of laminated (mm scale) marlylimestones deposited in coastal to transitional environ-ments (Cabaços Formation; Azerêdo et al., 2002)containing organic-rich layers having very good gener-ation potential (Spigolon et al., 2011; DPEP, 2013).Arapid marine invasion resulted in the accumulation of afew hundred meters of compact grey marine limestonescontaining marly intercalations towards the top (Mon-tejunto Formation; Atrops and Marques, 1988). Thismarine carbonate sedimentation was suddenly inter-rupted by a major input of coarse siliciclastics all overthe basin (Abadia Formation), reaching more thanthousand meters thick in the basin depocenters (Penados Reis et al, 2000). This thick Kimmeridgian infill is

2008) (Fig. 4). The late Berriasian to early Barremiancycle is interpreted as associated to the opening of theTagus segment, whereas the early Barremian to mid-Aptian cycle is associated to the opening of the Iberiansegment (Dinis et al., 2008). The late Aptian unconfor-mity is present along the whole basin, resulting inabundant coarse siliciclastics (Figueira da Foz Forma-tion) in top of Early Cretaceous sequences in the Southor Jurassic uplifted and eroded sequences in the North(Santos et al., 2010). Abundant siliciclastics coveredthe basin during the Albian (Rey et al., 2006).

Late Cretaceous evolution corresponds to thedevelopment of a classical passive margin, controlledboth by the uplift of the continental areas and theeustatic level variations. During the Albian and Ceno-manian, a global eustatic sea-level rise result in themarine invasion of most of the basin and the gradualdevelopment of a carbonate (rudist and coral-buildups)

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platform, known as Cacém Formation. These condi-tions continue until the Turonian (Callapez, 2008).

In late Turonian times, the first signs of inversionin the passive margin are indicated by prograding silici-clastics in the northern sector and emersion/erosion inthe southern sector. Inversion continues until the end ofthe Late Cretaceous, and sedimentation graduallybecomes restricted to smaller areas in the north. Mag-matic intrusions are known from this age, closelyrelated with the instability caused by the evolution ofthe Biscay (Martins et al., 2010).

The evolution of the Lusitanian Basin basicallyended during the Late Cretaceous. However, during theTertiary, the area occupied by the basin was subjectedto inversion and uplift, mainly along its northeast-southwest central axis; i.e., the axis of the Late Jurassicdepocenter. Tertiary basins developed on each side ofthis mainly carbonates mountain-chain–the MondegoBasin to the northwest and the Tejo Basin to the south-east (Figs. 2 and 3).

Petroleum Systems’ Elements and Processes

Source rocks

The Lusitanian Basin contains several forma-tions having source-rock potential (Figs. 3 and 5),which have been identified and studied since the begin-ning of exploration in Portugal (DPEP, 2013),including Silurian deep-marine black-shales, Carbonif-erous turbiditic shales, Lower Jurassic shaly marls, andUpper Jurassic marly limestones (Oxfordian). Otherformations having source-rock potential include theDagorda Formation (Hettangian), the Abadia Forma-tion (Kimmeridgian) and the Cacém Formation(Cenomanian-Turonian).

The Lower Jurassic source rock is composed ofmarly black shales deposited in a fully open marineenvironment (Duarte and Soares, 2002; Duarte et al.,2010, 2012; Silva et al., 2010, 2011). Total thickness ofthese deposits is about 100 m. Although there are sev-eral organic-rich layers, TOC values are highly

total thickness around 75 m in the coastal outcrops ofPeniche. The upper member is the “Marly Limestoneswith organic rich facies" having a thickness of about 30m (Duarte et al., 2010) and TOC values range from 0–25%. Both units are part of the Lower Jurassic succes-sions which extend over the basin. However, due topaleogeographic conditions, it is expected the facieshaving higher potential is in the deeper parts of thebasin’s homoclinal ramp, towards the northwest (Penados Reis et al., 2011; Duarte et al., 2012).

The Late Jurassic source-rock is composed ofmarly limestones deposited in lacustrine, lagoonal, andcoastal environments and have been studied by severalauthors (e.g., Spigolon et al., 2011; Silva et al., 2013).Total thickness of this unit is around 200 meters andTOC values in darker layers usually range from 2 to 5%, with restricted layers reaching up to 10-30%. Kero-ht 2

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variable. Kerogen is mainly of type II, III, althoughsome intervals are Type I (Duarte et al., 2010; 2012;Spigolon et al., 2010). Accumulation and preservationof organic matter occurred in this setting particularlyclose to the “maximum flooding surfaces” of two dis-tinct second-order sequences (Duarte et al., 2010).Those two organic-rich units have been studied indetail, 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 Sinemu-rian to lower Pliensbachian Água de MadeirosFormation, which is about 42 m in the coastal outcropsof São Pedro de Muel (Duarte et al., 2012). TOC val-ues are mostly over 7 wt. %, reaching up to 22 wt. %(Duarte et al., 2012). The upper organic-rich unit is thePliensbachian Vale das Fontes Formation, having a

gen types are variable; Type III predominates, Type Iand IV are also present. Organic matter accumulationand preservation has taken place in restricted environ-ments developed in most areas of the basin, relatedwith coastal regions having continental inputs andephemeral marine incursions. Overall, regional varia-tions point to an important input of terrestrial plant-debris in the northern areas and to predominant algalmats development in the southern areas (Spigolon etal., 2011), although the heterogeneity of the depositsmay eventually suggest a wider lateral variability(Silva et al., 2013). Although the richest layers are notstrictly contemporaneous, depending on local highsand lows controlling the marine incursions, this source-rock could be considered basin-wide and having a largevariation of organic matter type (Silva et al., 2013).

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Maturation

Both Jurassic source-rocks can be in the hydro-carbon generation window, although not everywhere inthe basin, as a result of the highly heterogeneousbasin’s subsidence and overburden, especially in theLate Jurassic.

Non-mature Lower Jurassic source-rocks areknown in outcrop, namely at the Peniche, Montemor-o-Velho, and São Pedro de Muel sections (Oliveira et al.,2006; Silva et al., 2010; Spigolon et al., 2011; Duarteet al., 2012), but preliminary maturation modellingmay suggest that the units have reached maturity inseveral exploration wells in the basin (Teixeira et al.,2012, 2014). These non-mature Late Jurassic source-rocks are also present in different outcrops, such asCabo Mondego or Montejunto (Spigolon et al, 2011),whereas they reached the oil-window in nearby wellssuch as SB-1, FX-1 and CP-1 (Teixeira et al., 2012,2014). This situation points to a very important role ofdifferential subsidence along the basin, both in timeand space.

Thermal modeling in different locations point toa crucial role played by the Late Jurassic intense silici-clastic input into the basin, related to the Oxfordian

rifting phase. This phase is responsible for an increasein heat flow and, at the same time much overburden. Inmost places maturation has been attained in the LateJurassic (Kimmeridgian to Tithonian) and it has beenmost prominent in the Oxfordian depocenters, namelythe Central Sector’s sub-basins of Arruda, Bombarral,and Freixial (Teixeira et al., 2012, 2014).

As a general statement considering vitrinitereflectance data (BEICIP-FRANLAB, 1996) and ther-mal basin modelling studies (Teixeira et al., 2012,2014), it may be considered that the Lower Jurassicsource-rock is mature for oil in the north sector of thebasin and mature for gas in the south sector, whereasthe Upper Jurassic source rocks may be not mature inthe north sector and are mostly mature in the south sec-tor. However, local depocenters are areas of increasedoverburden and maturation of the Lower Jurassicsource-rock in the northern sector, which may be thecase close to São Pedro de Muel (Porto Energy, 2012).The same kind of situation may have promoted matura-tion of Upper Jurassic source-rocks in northerndepocenter areas.

Migration

Well and field data include non-commercialoccurrences of oil and gas in many wells and a few oilshows in outcrops, proving the existence of maturationand subsequent migration of hydrocarbons. Biomarkersstudies (Spigolon et al., 2011) point to a potential for

alpine uplift; Ribeiro et al., 1980), feeding the Creta-ceous sandstones.

Another pattern of migration corresponds tohydrocarbons of an Oxfordian provenance (CabaçosFormation) in Upper Jurassic reservoir units. This oilht 2

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two different oil systems related to the migration ofhydrocarbons towards different reservoir units.

A Lower Jurassic source rock provenance isidentified in oil shows occurring in several outcropsaround diapir walls in the northern sector of the basin.The oil is believed to have migrated from the LowerJurassic marls along the faults created (or used) duringthe salt uplift, feeding a major sandstone reservoir ofAptian age (Figueira da Foz Formation) (Fig. 4). Thesepathways have been active probably since the LateJurassic and should have been enhanced at majorhalokinetic events related with compression, namely inthe Late Cretaceous (with documented extrusion; Penados Reis, 2000) and the late Miocene (with intense

system is very frequently identified in oil-shows(DPEP, 2013) and also in some outcrops (Spigolon etal., 2010). This play is only evident in the southern sec-tor, where post-Oxfordian successions are thick enoughto have the necessary overburden. The source rocks aremid-Oxfordian marly limestones (Cabaços Formation)and the reservoir units include late Oxfordian fracturedcarbonates (Montejunto Formation) and Kimmeridgianturbiditic siliciclastics (Abadia Formation). This sys-tem comprises only Upper Jurassic units and itsstratigraphical proximity explains the frequency of thiskind oil shows in the Lusitanian Basin (Pena dos Reisand Pimentel, 2011).

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Reservoirs

The Lusitanian Basin has a thick sedimentaryinfill and a very distinct facies, including a wide rangeof siliciclastic, carbonate, and mixed deposits (Fig. 3).Granular reservoirs are present in siliciclastic continen-tal, transitional, and marine facies; porous reservoirs insome coastal carbonates; and fractured reservoirs inseveral shallow marine to deep marine carbonates ofdifferent ages (Pena dos Reis and Pimentel, 2010a,2010b). Oil seeps and oil shows have been observed insiliciclastic and carbonate units of different ages,including Late Triassic, Jurassic, and Cretaceous(DPEP, 2012). However, a comprehensive and system-atic study of the several reservoir units hass yet to beproduced and published, and most of the followingconsiderations are based on sparse bibliographic refer-ences and in personal observations.

The sedimentary record of the Lusitanian Basinincludes abundant siliciclastic deposits related tophases of intense tectonic activity and subsequent ero-sion and accumulation. Many of those deposits wereinitially very porous, but in many cases diagenesisobliterated a significant part of it. However, is may beassumed that the basin included enough volume ofsiliciclastic rocks to contain all the oil that had beengenerated during the Mesozoic.

The basal deposits of the basin are the SilvesGroup siliciclastics of Late Triassic age. Proximal todistal alluvial-fan red-beds have intergranular porositypartially filled by carbonate cementation (Atlantis,2010), with values ranging from 16% to 23%, locallyup to 70%.

reefal build-ups, both with good reservoir potential(Uphoff et al., 2010). Besides interparticle and vuggyporosity, these units may also be fractured reservoirs.

The Upper Jurassic limestones of the MontejuntoFormation may also contain porous (Uphoff et al.,2010) fractured reservoirs, enhanced by its strati-graphic and geometric proximity to the Upper Jurassicsource-rock of the Cabaços Formation (Pena dos Reisand Pimental, 2011). This fact is particularly importantclose to diapiric structures promoting fractures andtrapping, as seen in the Torres Vedras oil-seep (Spigo-lon et al., 2010).

The fine- to coarse-grained turbiditic deposits ofthe Abadia Formation contains abundant sandy layershaving reservoir potential. However, once again, car-bonate cementation, especially where calciclasticparticles are present, partially obliterates the intergran-ular porosity (Garcia et al., 2010). The followingprograding fluviodeltaic sequence (Lourinhã Forma-tion) is compositionally immature, resulting ininteresting inter- and intragranular porosities, with val-ues around 10 to 15%. Early diagenetic carbonatecementation has been incomplete, inhibiting fill com-paction and preserving most of the primary porosity(Atlantis, 2010).

Lower Cretaceous sedimentation includes fluvialand transitional to coastal deposits having some reser-voir potential. The presence of infiltrated clays influvial units and irregular carbonate cementation incoastal units has diminished its intergranular porosity,resulting in an average around 5% (Atlantis, 2010).ht 2

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The Lower Jurassic sequence includes thick,compact, and dolomitized carbonate units, especially atthe bordering areas of the basin, known in outcropclose to the basement to the east. The Dagorda Forma-tion includes dolomites with up to 20% primaryporosity (Uphoff, 2005), whereas at the Coimbra For-mation intercrystalline and vuggy porosity withbrecciation related to early meteoric diagenesis areimportant.

From the Pliensbachian onward, the monotonousalternation of limestones and marls of the BrenhaGroup have low reservoir potential, although somefracture-related porosity may be present. Middle Juras-sic units correspond to the Candeeiros Formation,which is predominantly shallow marine carbonateshaving high to moderate energy textures and some

Locally, some coastal high energy or biohermal car-bonates have good primary porosity (Dinis et al.,2008). Cenomanian transgression resulted in themarine flooding of the basin and deposition of shallowmarine and reefal carbonate units that also have goodreservoir properties (Dinis et al., 2008).

The Cenozoic inversion resulted in basins upliftand locally to some intracontinental basins containingsome hundreds of meters of mainly siliciclastic infill(Pais et al., 2012). Paleogene and Miocene infillinclude immature and matrix-rich clays that have lowreservoir potential, but also transgression related bio-clastic limestones that have good intergranular andmoldic porosities. Cleaner sands were deposited in thebasin during the late Miocene and Pliocene that couldbe reservoir properties provided a seal is present.

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Traps

Several stratigraphic traps may be present in theLusitanian basin. The first one is the direct superposi-tion of Hettangian clays and evaporites over LateTriassic sandstones, promoting accumulation of hydro-carbons both from underlying Palaeozoic units andfrom down-thrown Jurassic source-rocks (Figs. 3 and4). Another favourable situation is the occurrence ofbiohermal build-ups in several places and during sev-eral times, namely the Middle Jurassic (e.g., at thePataias cement quarry), Upper Jurassic (Amaral For-mation) and Upper Cretaceous (Cenomanian reefs).

Structural traps seem to have a predominant rolein the Lusitanian Basin as a consequence from theintense tectonic deformation during the Mesozoicextension and Tertiary compression. Up-lift and sub-sidence of tectonic blocks bringing side-by-side

different source-rocks, reservoirs, and seals have beenessential to promote migration and trapping of hydro-carbons (Fig. 4). The main tectonic events promotingthe intense structuration of the basin took place duringthe Late Jurassic (rift-climax) and Late Cretaceous(beginning of the alpine inversion), frequently involv-ing the Hettangian plastic evaporites, affecting thepetroleum systems and creating multiple traps (Fig. 4).

Alpine inversion during the Tertiary has alsobeen important for deformation and creation of struc-tural traps in many places of the basin (Ribeiro et al.,1980). However, the probable fracture network devel-opment may have also destroyed many traps’ integrity,enhancing leakage and even total loss of prior hydro-carbon accumulations.

Seals

The knowledge about the characteristics of geo-logical units having seal potential in the LusitanianBasin is well behind what has been presented for theother petroleum system elements. The Dagorda Forma-tion is probably the main effective seal in the basin,consisting of compact red clays containing variableamounts of gypsum and halite having thicknesses up tohundreds of meters. This thick shaly package is virtu-ally impermeable and its plasticity confers the ability todeform and to pierce higher stratigraphic levels, con-trolling vertical migration and sealing differentreservoirs. There are several other shaly units, but they

with sandy layers, as is the case of the Upper Jurassicand Lower Cretaceous units. However, Upper Creta-ceous and Tertiary shaly units may have acted asimportant seals, particularly in the more subsiding ordistal areas of the basin. The behaviour of carbonateunits as seals is highly dependent on the presence orabsence of dense fractures that promote leakage. Mostof the carbonate units of the Lower-Middle Jurassicand Upper Cretaceous may act as seals provided theyhave not been affected by intense fragile deformation,as is usually seen (for example) associated with dia-piric outcropping structures.ht 2

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cannot be perfect seals due to frequent intercalations

Petroleum Systems and Plays

From the analysis of the petroleum system ele-ments and its articulation in space and time, three mainpetroleum systems may be considered in the LusitanianBasin (Pena dos Reis and Pimentel, 2010a, 2010b)(Fig. 5).

A pre-salt petroleum system may be defined assourced by meta-sedimentary Paleozoic rocks feedingUpper Triassic siliciclastic reservoirs and sealed by theHettangian evaporitic clays. This kind of play has beeninitially described for the Silurian black-shales and theSilves Group sandstones (Uphoff, 2005). Those grapto-litic black-sales have reached the oil and gas-windowin Paleozoic times, before late Variscan uplift and ero-sion and may have kept some generative potential for

further gas expulsion during the Mesozoic reburial(Uphoff, 2005). The same line of thought may bedeveloped regarding other Palaeozoic units havingsource-rock potential, such as the Carboniferous tur-bidites of the Southern Portuguese Zone (Fernandes etal., 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 tothe role of late Variscan thin-skinned tectonics, result-ing in thrust sheets bringing side-by-side units fromdifferent depths and structural settings.

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A second petroleum system is related with theLower Jurassic source-rocks, namely the Sinemurianand Pliensbachian organic-rich marls (Água deMadeiros and Vale das Fontes formations). Its geo-chemical characteristics point to good oil generationpotential (Duarte et al., 2010, 2012; Spigolon et al.,2011); in highly subsided areas of the basin, it hasprobably reached the oil-window and even the gas-window (Teixeira et al., 2012, 2014). The intensemovement of basement blocks affecting the Mesozoicand Tertiary infill and cover is responsible for many sit-uations in which the Lower Jurassic units appeargeometrically beside or above many of the potentialreservoir units. Therefore, the Lower Jurassic sourcerocks may have laterally and vertically fed Jurassic andCretaceous siliciclastic and carbonate units, as well asUpper Triassic siliciclastics.

From the observation and analysis of unpub-lished geochemical data of several diapir-related oil-seeps (e.g., Paredes de Vitória and Leiria; Atlantis,2010), it seems that Lower Jurassic source-rocks werethe main feeders, using the associated vertical faultingand brecciation as migration conduits along the diapir-walls. It may be speculated that in non-outcroppingdiapirs, the same situations led to oil accumulations,sealed by an unconformable Tertiary cover.

A third petroleum system corresponds to thematuration and oil generation of the transitional tocoastal marine Oxfordian Cabaços Formation. Thesemarly rocks have been buried under kilometers ofOxfordian and Kimmeridgian rift-related siliciclastics(Abadia and Lourinhã formations), entering the oil

impregnated the overlying Montejunto Formationlimestones and the Abadia Formation turbidites(Spigolon et al., 2010; Uphoff et al., 2010; Pena dosReis and Pimentel, 2011). The seal for this kind of playcould be Cretaceous and/or Tertiary clays andsiltstones.

A fourth petroleum system may be present, cor-responding to the accumulation of hydrocarbons in theTertiary cover of the Lusitanian basin. Several sourcerocks might be involved depending on the sector of thebasin, and the Tertiary deposits would act both as reser-voirs and seals. Due to the generally thin sedimentarycover in the inversion-related Tertiary continentalbasins (a maximum of 800 meters) south of Lisbon(Ribeiro et al., 1979), this system should naturally bemore prominent in the offshore areas, where the Ter-tiary prograding accumulations can reach greaterthicknesses (Alves et al., 2003).

To synthesize, we may consider that severalpetroleum systems and plays have been active and areproven in the Lusitanian basin, involving source-rocksand reservoirs of different ages and lithological charac-teristics (Fig. 5). Preliminary modelling attemptssuggest that maturation has been promoted by intensesubsidence and burial, mainly in Late Jurassic times, atdifferent intensities in different sectors of the basin(Teixeira et al., 2012, 2014). As a general pattern, wemay consider that in the North sector only the LowerJurassic units have reached maturity for oil and eventu-ally for gas, whereas in the Central and South sectorsthe Lower Jurassic entered the gas-window and theUpper Jurassic entered the oil-window (Teixeira et al.,ht 2

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window in most of the basin since the Early Cretaceous(Teixeira et al., 2012, 2014). This oil has abundantly

2012, 2014).

Implications for Deep Offshore Exploration

From the compared analysis of onshore and off-shore petroleum systems in the Lusitanian and Penichebasins, many analogies may be used to approach theseplays, but differences are also important. Both basinshave in common its geodynamic framework and evolu-tion, but the specific proximal position of theLusitanian Basin and the outer position of the Penichebasin, regarding crustal stretching and North Atlanticopening (Alves et al., 2006), has resulted in diachronicelements that must be addressed and understood(Table 1).

Most of the plays that have been identified in theLusitanian Basin may also be considered valid for the

Peniche basin. These include (i) the pre-salt play,sourced by Silurian and/or Carboniferous rocks, feed-ing Upper Triassic redbeds, and sealed by Hettangianevaporites; (ii) the diapiric-related play, sourced byLower Jurassic deep marine shales and marls, feedingCretaceous fluvial sands, and sealed by Upper Creta-ceous marls; (iii) the turbidite play, sourced by UpperJurassic transitional marls and known in the LusitanianBasin to have fed Upper Jurassic turbiditic sands (Aba-dia Formation) as well as fractured limestones.

However, due to diachronic geodynamic evolu-tion of the proximal and outer parts of this Atlanticmargin, it may be speculated that in the Peniche Basin

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the turbidite facies (equivalent to the Abadia Forma-tion) have been deposited during the Lower Cretaceousas thick rift-climax related sediments (Alves et al.,2003, 2009). Reservoir potential of these deposits istherefore considered to be high, including deep sea fangeometries with channelized over-bank deposits andalso re-sedimentation as contourites.

A fourth play, absent in the onshore areas, maybe considered in the deep offshore related to the accu-mulation of thick deep marine deposits correspondingto a Tertiary play: Jurassic source rocks feeding UpperCretaceous to early Tertiary sands and sealed by Neo-gene clays. Based on recently acquired seismic lines ofthe Peniche Basin (Consortium Petrobras/GALP/Par-tex; DPEP, 2013), its seismic stratigraphic analysis andthickness modelling in pseudo-wells show that Meso-Cenozoic overburden has been sufficient to promoteboth maturation of the Jurassic source rocks and alsosealing of the Upper Cretaceous to early Tertiary reser-voirs (Sagres, 2013). All these plays have their ownrisks and many of the considerations previously pre-sented about the relations between the Lusitanian andthe Peniche basins are important to reduce the risk ofthe deep offshore exploration activities (Table 1).

Lower and Upper Jurassic source-rock identifi-cation is rather confident, but its areal distribution isdifficult to predict because paleogeographic recon-structions are, for the moment, incomplete at aregional-scale distribution. However, the patterns in theLusitanian Basin (Pena dos Reis et al., 2011) may beextrapolated to larger areas, and probably both Jurassicsource rocks are present in most of the areas of the Pen-

Migration was highly dependent on faulting,related both to extension and compression. Consideringthe modelled late maturation timings, compressionstructures were probably more important as conduits,although many of them corresponded to the re-activa-tion of previously extension structures.

Mesozoic traps seem to be predominantly struc-tural, although there are a few stratigraphic trapsassociated with bioherms. On the other hand, Tertiarytraps may include many stratigraphic accumulationsrelated to the presence of coarse-grained turbiditicchannels and fine-grained over-bank deposits. Mainunconformities may also act as traps, namely the onebetween deformed Mesozoic reservoir units and flat-lying Tertiary sealing units.

The influence of inversion issues must bestressed on any approach to the Peniche basin.Although it helped in creating folds and faulted migra-tion pathways, its impact on seal integrity may havebeen critical. This pattern is quite evident in outcropanalogues in the Lusitanian basin, such as the Paredesde Vitória and Monte Real diapirs. Therefore, anticlinalclosures associated with significant inversion featuresshould be avoided or, at least, carefully lookedexamined.

Understanding the onshore basins is a crucialstarting point to approach the offshore basins. Due toits remarkable exposures and geological detailed stud-ies, the Lusitanian Basin is well known and may beused as an analog to the Peniche Basin (Alves, 2009;Sagres, 2013). The same depositional packages andunconformities may be recognized in both basins andht 2

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iche basin. Moreover, both units most probablyattained maturation for oil, although at different times(Sagres, 2013) (Table 1).

It is crucial to understand the timing and locationof maturation of petroleum systems as there are import-ant differences between both basins. For example, rift-climax subsidence and related salt withdrawal at thePeniche Basin (Alves et al., 2003, 2009) occurs 10-15My (?) later than the landward Lusitanian basin, itseems likely that in the Peniche Basin the kitchen areasand the migration pathways have been controlledmainly by the Cretaceous subsidence and diapirism.

the different petroleum systems identified at the Lusita-nian Basin may prove to be present at the PenicheBasin (Fig. 6). Farther south on the Western IberianMargin, the outcrops of the southwestern Alentejo andwestern Algarve may also be used as analogues for theoffshore Alentejo basin.

The “analog approach” must be based on under-standing the basin’s evolution—how each geodynamiccontext generates each petroleum system element andhow they are articulated in space and time. De-riskingstrategies for the West Iberian margin offshore basinsmust include basin and petroleum systems analysisbased on detailed outcrop studies.

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Acknowledgments

This paper collects data and ideas from manyyears of research, during which several institutions andcolleagues were most helpful. We thank CENPES/Petrobras and particularly Edison Milani, AdrianoViana, and Gilmar Bueno for two financed researchprojects supporting this work (ATLANTIS, 2007-2010and SAGRES, 2011-2013) and for their permanent col-laboration. We also thank DPEP (the Portuguese oilagency) and Director Teresinha Abecasis for the access

to important historical exploratory data and unpub-lished reports. Petrobras Portugal, namely RudyFerreira and Marisa Calhôa are also thanked for theaccess to data about the Peniche Basin and for theircollaboration with the SAGRES project. Finally, indi-vidual collaborations must also be thanked, includingAntônio Garcia for the coordination of the AtlantisProject and Ramón Salas, Hugo Matias, and RicardoPereira for many fruitful discussions.

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Wilson, R.C.L. R.N. Hiscott, M.G. Willis, and F.M. Grad-stein, 1989, The Lusitanian Basin of West CentralPortugal: Mesozoic and Tertiary tectonics, Stratigra-phy and Subsidence History, in A.J. Tankkard and H.Balkwill, eds., Extensional tectonics and Stratigraphyof the North Atlantic margins: AAPG Memoir 46, p.341-361.

Witt, W.G., 1977, Stratigraphy of the Lusitanian Basin: ShellProspex Portuguesa Unpublished Report, 61 p.

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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

Syn-rift

Post-rift � Drift

Pre-Rift

Source Rock & Reservoir (Sedimentary environments + Depositional thickness)

Inversion

Uplift / Folding COMPRESSION / COLLISION CYCLE

Extension

Subsidence POST-RIFT CYCLE

RIFT CYCLES

Traps & Seal Integrity (Antiforms and Faulting)

Migration

COMPRESSION/ COLLISION CYCLE

Processes

Tectonic terranes Basement Lithologies

Thermal history

Basement blocks

Maturation & Seal (Overburden & Thermal evolution)

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LB PenB

GB

Bb

Bb

PB

SPAIN P O R T U G A

100 km

Atlantic Ocean

Ireland Great Britain

France

Spain

Portugal

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Figure 1. Study area showing the location of the Western Iberian Margin’s onshore and offshore basins;abbreviations are defined in the text.

AB

AlgB

L

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Figure 2. Geological framework of theLusitanian Basin (onshore and offshore).Mesozoic based on LNEG; Paleozoicbased on Pena dos Reis et al., 2012. AF–Arrife fault; LCF–Lousã-Caldas fault;VFF–Vila Franca fault. A and B dashedlines are seismic lines in Figure 4.

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Figure 3. Lithostratigraphic chart of the Lusitanian Basin including geodynamic events, cyclicity, seismichorizons, and main petroleum system elements (adapted from Pena dos Reis et al., 2011; partially based onWilson, 1990; Azerêdo et al., 2003; Rey et al., 2006).

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Figure 4. Interpreted seismic lines across the Lusitanian Basin (in Carvalho, 2013); see Figure 2 for location.

U TRIAS

TJ Salt LM JURASSIC

U JURA

SEISMIC HORIZONS

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)

A

B

L CRET

Cab+Montej

U TRIAS

TJ Salt LM JURASSIC

Abadia

Lourinhã

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Figure 5. Petroleum system events chart of the Lusitanian Basin (adapt. from Pena dos Reis and Pimentel, 2010b).

SOURCE ROCK

RESERVOIR

SEAL

OVERBURDEN

TRAP

MAT/MIGR/ACC

CRITICAL MOMENT

Sil Dev Carb Perm E M L E M L J u r a s s i c C r e t a c e o u s

C E N O Z O I C

Paleogene Neogene

P A L E O Z O I C T r i a s s i c

E L

M E S O Z O I C

HALOCINESIS

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Figure 6. Seismic stratigraphic correlation between Peniche and Lusitanian basins. Both lines are presented at the same horizontal andvertical scales. The main unconformities are underlined. Note the significant thickness of the Tertiary sediments in the Peniche Basin.

A

B Peniche Basin

Lusitanian Basin

A

BPeniche Basin

Lusitanian Basin

Campanian

Aptian

Callovian TGS-NOPEC seismic line courtesy from PETROBRAS

Rasmussen et al. 1998

Unconformities

Upper JURA L-M JURA U. TRIAS

Upper CRET Lower CRET

QUATERN. Upper TERT Lower TERT

Seismo-stratigraphic Units

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