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

Pena dos Reis, Rui Pimentel, Nuno Centro de Geociências Centro de Geologia Faculdade de Ciências e Tecnologia da Faculdade de Ciências da Universidade Lisboa Universidade de Coimbra Campo Grande C-6 Lg Marquês de Pombal 1749-016 Lisboa, 3000-272 Coimbra, Portugal e-mail: [email protected] e-mail: [email protected]

Abstract A synthesis of the knowledge about the Lusita- and Tertiary seals and traps. Related pro- nian Basin is presented here, focusing on its cesses, such as organic matter maturation and stratigraphic record, sedimentary infill, evolution, and hydrocarbons migration are also discussed. The charac- petroleum systems. Petroleum system elements are teristics of these elements and processes are analysed characterized, including Palaeozoic and Mesozoic and implications for deep offshore exploration are dis- source rocks, siliciclastic and carbonate reservoirs, and cussed. Introduction GCSSEPM The Lusitanian Basin is one of the Western Ibe- results. However, exploration continues and a good rian Margin sedimentary basins related with the understanding of these basins’ evolution and character- opening of the North Atlantic (Fig. 1) (Wilson et al., istics is crucial to enhance the chances of future 1989). These basins have their counterparts in the east- success. This paper deals with the evolution of the ern Canada Jeanne D’Arc and Whale basins, as part of mainly onshore Lusitanian Basin and its petroleum sys- the Iberia-Newfoundland conjugate margins complex tems, in order to establish an analog for other nearby (eg., Wilson et al., 1989; Pinheiro et al., 1996;2014 Peron- offshore basins, aiming to contribute to a better Pinvidic and Manatchal, 2009). The evolution of all regional framework for exploration in this region (Fig. these basins are defined by the same geodynamic con- 1). trols but also by specific local constraints, explaining In this paper we present a summary of this different characteristics and success in exploration. In approach, based on an overview of the basin evolution the Canadian basins, the intense exploration has led to and an analysis of the related petroleum systems and several good production and development results, but elements, with implications on the deep offshore on- the Iberian basins have not had so far similar positive going exploration.

Lusitanian Basin’s Evolution and Infill The Lusitanian Basin extends for about 250 km basins in southwestern Iberia (PenB, AB in Fig. 1) and north-south and 100 km east-west, facing the Atlantic the Gulf of Cadiz Basin in southern Iberia (Gb in to the west,Copyright representing the inner and most proximal Fig. 1). margins of much larger basins extending towards the The Lusitanian Basin resulted from the initial shallow and deep offshore (LB in Fig. 1). The geologi- extension of the Pangea’s continental crust and later cal record of the Western Iberian Margin’s evolution is opening of the North as a result of - present in several nearby basins sharing similarities, ing and seafloor spreading. The evolution of the such as the offshore and Galicia basins in north- Paleozoic basement and the Mesozoic extension cre- ern Iberia (PB, GB in Fig. 1), the Peniche and Alentejo ated a complex succession of events and sedimentary

Sedimentary Basins: Origin, Depositional Histories, and Petroleum Systems 1 infill (Figs. 2 and 3). The influence of the basement on Since the Late Carboniferous and during Perm- the basin’s evolution may be addressed along two main ian times, gradual uplift and erosion brought these lines: (i) its lithologies, including the presence of units rocks to more shallow structural domains and predomi- having source-rock potential; (ii) its structures, particu- nantly gentle deformation, commonly known as “late- larly the presence of important regional faults and their Variscan faulting.” These north-northeast/south-south- movement during the Mesozoic and Tertiary (e.g., Pena west sinistral and northwest-southeast dextral dos Reis et al., 2012 Dinis et al., 2008; Pena dos Reis movements conditioned the origin and configuration of et al., 2000) the Lusitanian Basin from the , in the The complex structure of the Paleozoic basement global context of the Pangaea break-up and intraconti- of western Iberia resulted mainly from the collision and nental troughs in western Europe and eastern America deformation of two terranes (e.g., Ribeiro et al., 1979, (Wilson et al., 1989) (Fig. 2). In western Iberia, strong 1990, 2007; Matte, 1991): (i) the Iberian Terrane with subsidence resulted in north-northeast/south-south- the Central Iberian Zone (CIZ) and the Ossa Morena west trending asymmetric that rapidly filled by Zone (OMZ); and (ii) the Southern Portuguese Terrane alluvial-fan siliciclastic deposits passing to sabkha and its Zone (SPZ). The joint deformation of all this clays and salts, under arid climatic condi- basement is related with the Ibero-Armorican Arch tions. This sedimentary infill corresponds to the Silves developed in the Late Paleozoic during the Variscan Group (Palain, 1976) and is composed mainly of coarse . siliciclastic red-beds forming two fining-upward mega- The Central Iberian Zone includes Silurian units sequences (Fig. 4). Its total thickness is up to 400 having organic matter in pelitic layers, sometimes sev- meters and thee paleogeographic reconstructions point eral hundred meters thick (Romão et al., 2005). to the development of north-northwest/south-southeast However, metamorphism may have over maturated -bounded half-grabens, separated by intra-rift those units and most (if not all) of the hydrocarbons basement blocks (Uphoff, 2005). These deposits may have been lost. Late Carboniferous deposits are show GCSSEPMgood reservoir potential and have been targeted more prospective, considering their post-orogenic age for gas as part of a pre-salt petroleum system (Uphoff, and therefore not so high maturation. They have been 2005). deposited in narrow intra-mountain lacustrine basins as These Upper Triassic grabens became gradually fining-upward siliciclastic deposits, including black filled-up and sabkha-like environments became pre- shales and coal seams at the top (Domingos et al., dominant, promoting the accumulation of red clays and 1983). 2014evaporites in the most subsiding parts of the basin The Ossa Morena Zone includes several units (Dagorda Formation) (Palain, 1976). The resulting containing organic-rich layers, affected by low-grade shaly deposits, some hundreds of meters thick, include metamorphism that has caused intense maturation significant amounts of gypsum and halite, which would (Chaminé et al., 2003). However, some outcropping be fundamental for the tectonic deformation of the Silurian graptolitic black-shales show highly variable Meso-Cenozoic units, acting as “decollement” level Ro% equivalent values, ranging from the late oil-win- and also as diapiric masses with kilometric-scale verti- dow to the gas-window, probably controlled by the cal movements, locally piercing the Mesozoic cover proximity to major fault zones (Uphoff, 2005; Mach- (Kullberg, 2000) (Figs. 2 and 3). ado et al., 2011). In the Sinemurian, the Hettangian sabkha-like to The Southern Portuguese Zone includes Devo- coastal environments are replaced by open marine, pre- nian to Late Carboniferous metasedimentary rocks, dominantly carbonate environments, such as the including some black-shales having source rock poten- Coimbra Group (Soares et al., 2007) (Fig. 4). This unit tial, suchCopyright as the fine-grained turbidites of the Baixo is about 200 m thick and present throughout the basin Alentejo Flysh Group (Oliveira, 1983). Although they as a result of the paleogeographic coalescence of the are affected by low-grade metamorphism and are gen- initial troughs and an expansive onlap of a carbonate erally over mature (McCormack et al., 2007; ramp over the basement towards the East. Dolomitic Fernandes et al., 2012), preliminary data (Barberes, (São Miguel Formation) are present in the 2013) suggests that there may have been some places northern and eastern areas, whereas the marly lime- where the group is preserved within the gas-window. stones are present in central and western areas of the basin (Água de Madeiros Formation) (Duarte and

Pena dos Reis and Pimentel 2 Soares, 2002; Azerêdo et al., 2003). The later includes related with a rifting event, recognized in a lower unit only a few tenths of meters in thickness, subsidence curves and related with the beginning of the deposited in open marine environments but having Atlantic opening to the south of Iberia (Wilson et al., very good organic content of upper Sinemurian–lower- 1989; Rasmussen et al., 1998). most Pliensbachian age (Polvoeira Member; Duarte The Abadia Formation corresponds to the rift- and Soares, 2002; Duarte et al., 2004, 2010, 2012). climax, and its deposits have been the classical target During the , sedimentation took of oil exploration in the Lusitanian Basin during the place in a carbonate ramp depositional system, giving 20th century (DPEP, 2013). Prograding continental place to a thick sequence of marly limestones, known siliciclastics continue to cover the basin during the in exploration and the 20th century literature as the Tithonian, resulting in the accumulation of almost 1 km Brenha Group (Witt, 1977). The sedimentation began of fluviodeltaic sands and clays (Lourinhã Formation; with the deposition of about one hundred meters of Hill, 1988). Late Tithonian to early Berriasian sedi- alternating centimeter-thick layers of marls and lime- ments are generally named as the “Purbeck Facies” and stones of Pliensbachian age (Vale das Fontes include fluvial to coastal siliciclastics. Formation; Rocha et al., 1996; Duarte and Soares, The evolution of the Lusitanian 2002; Azerêdo et al., 2003), also having very good Basin is closely related with the opening of the North generation potential (e.g., Oliveira et al., 2006; Silva et Atlantic Ocean. This opening has been developed in al., 2010; Duarte et al., 2010; Spigolon et al., 2011). three steps which are well identified in the geological The open marine marly sedimentation gradually record as break-up unconformities (Dinis et al., 2008). gave place to a predominance of limestones every- The beginning of the Early Cretaceous is also marked where in the basin–Cabo Mondego Formation in the by an important magmatic event indicated by several North (Azerêdo et al., 2003) and Candeeiros Group in dykes mainly associated with diapiric piercing struc- the South (Witt, 1977). An overall regression gradually tures (Martins et al., 2010). Lower Cretaceous promoted shallower sedimentation, reaching emersion sedimentsGCSSEPM are known only in the south and central sec- and depositional hiatus in the eastern border of the tors of the Lusitanian basin, indicating an important basin during the Callovian (Azerêdo et al., 2002, uplift of the north sector during most of this time 2003). period (Fig. 4). Late Jurassic sedimentation started in mid- The sedimentary record is composed mainly of times, following a Callovian forced regres- fluvial to coastal fine-grained siliciclastics and impure sion and emersion (Azerêdo et al., 2002) (Fig.2014 4). This limestones, grouped into two cycles, separated by situation is related with an important geodynamic re- unconformities associated to distinct segments of the organization of the basin (Wilson et al., 1989; Hiscott North Atlantic opening (Rey et al., 2006; Dinis et al., et al., 1990), resulting in a Late Jurassic depositional 2008) (Fig. 4). The late Berriasian to early Barremian trough elongated northeast-southwest and open to the cycle is interpreted as associated to the opening of the southwest. Tagus segment, whereas the early Barremian to mid- The first Oxfordian sediments correspond to a Aptian cycle is associated to the opening of the Iberian few hundred meters of laminated (mm scale) marly segment (Dinis et al., 2008). The late Aptian unconfor- limestones deposited in coastal to transitional environ- mity is present along the whole basin, resulting in ments (Cabaços Formation; Azerêdo et al., 2002) abundant coarse siliciclastics ( Forma- containing organic-rich layers having very good gener- tion) in top of Early Cretaceous sequences in the South ation potential (Spigolon et al., 2011; DPEP, 2013).A or Jurassic uplifted and eroded sequences in the North rapid marine invasion resulted in the accumulation of a (Santos et al., 2010). Abundant siliciclastics covered few hundredCopyright meters of compact grey marine limestones the basin during the Albian (Rey et al., 2006). containing marly intercalations towards the top (Mon- evolution corresponds to the tejunto Formation; Atrops and Marques, 1988). This development of a classical passive margin, controlled marine carbonate sedimentation was suddenly inter- both by the uplift of the continental areas and the rupted by a major input of coarse siliciclastics all over eustatic level variations. During the Albian and Ceno- the basin (Abadia Formation), reaching more than manian, a global eustatic sea-level rise result in the thousand meters thick in the basin depocenters (Pena marine invasion of most of the basin and the gradual dos Reis et al, 2000). This thick Kimmeridgian infill is development of a carbonate (rudist and coral-buildups)

Pena dos Reis and Pimentel 3 platform, known as Cacém Formation. These condi- The evolution of the Lusitanian Basin basically tions continue until the Turonian (Callapez, 2008). ended during the Late Cretaceous. However, during the In late Turonian times, the first signs of inversion Tertiary, the area occupied by the basin was subjected in the passive margin are indicated by prograding silici- to inversion and uplift, mainly along its northeast- clastics in the northern sector and emersion/erosion in southwest central axis; i.e., the axis of the Late Jurassic the southern sector. Inversion continues until the end of depocenter. Tertiary basins developed on each side of the Late Cretaceous, and sedimentation gradually this mainly carbonates mountain-chain–the Mondego becomes restricted to smaller areas in the north. Mag- Basin to the northwest and the Tejo Basin to the south- matic intrusions are known from this age, closely east (Figs. 2 and 3). related with the instability caused by the evolution of the Biscay (Martins et al., 2010).

Petroleum Systems’ Elements and Processes

Source rocks The Lusitanian Basin contains several forma- total thickness around 75 m in the coastal outcrops of tions having source-rock potential (Figs. 3 and 5), Peniche. The upper member is the “Marly Limestones which have been identified and studied since the begin- with organic rich facies" having a thickness of about 30 ning of exploration in Portugal (DPEP, 2013), m (Duarte et al., 2010) and TOC values range from 0– including Silurian deep-marine black-shales, Carbonif- 25%. Both units are part of the Lower Jurassic succes- erous turbiditic shales, Lower Jurassic shaly marls, and sions which extend over the basin. However, due to Upper Jurassic marly limestones (Oxfordian). Other paleogeographic conditions, it is expected the facies formations having source-rock potential include the havingGCSSEPM higher potential is in the deeper parts of the Dagorda Formation (Hettangian), the Abadia Forma- basin’s homoclinal ramp, towards the northwest (Pena tion (Kimmeridgian) and the Cacém Formation dos Reis et al., 2011; Duarte et al., 2012). (Cenomanian-Turonian). The Late Jurassic source-rock is composed of The Lower Jurassic source rock is composed of marly limestones deposited in lacustrine, lagoonal, and marly black shales deposited in a fully open marine coastal environments and have been studied by several environment (Duarte and Soares, 2002; Duarte2014 et al., authors (e.g., Spigolon et al., 2011; Silva et al., 2013). 2010, 2012; Silva et al., 2010, 2011). Total thickness of Total thickness of this unit is around 200 meters and these deposits is about 100 m. Although there are sev- TOC values in darker layers usually range from 2 to 5 eral organic-rich layers, TOC values are highly %, with restricted layers reaching up to 10-30%. Kero- variable. Kerogen is mainly of type II, III, although gen types are variable; Type III predominates, Type I some intervals are Type I (Duarte et al., 2010; 2012; and IV are also present. Organic matter accumulation Spigolon et al., 2010). Accumulation and preservation and preservation has taken place in restricted environ- of organic matter occurred in this setting particularly ments developed in most areas of the basin, related close to the “maximum flooding surfaces” of two dis- with coastal regions having continental inputs and tinct second-order sequences (Duarte et al., 2010). ephemeral marine incursions. Overall, regional varia- Those two organic-rich units have been studied in tions point to an important input of terrestrial plant- detail, regarding TOC, isotopes, palynofacies, etc. debris in the northern areas and to predominant algal (Duarte et al., 2010, 2012; Silva et al., 2010, 2011; mats development in the southern areas (Spigolon et Poças Ribeiro et al., 2013) al., 2011), although the heterogeneity of the deposits TheCopyright lower organic-rich unit is the upper Sinemu- may eventually suggest a wider lateral variability rian to lower Pliensbachian Água de Madeiros (Silva et al., 2013). Although the richest layers are not Formation, which is about 42 m in the coastal outcrops strictly contemporaneous, depending on local highs of São Pedro de Muel (Duarte et al., 2012). TOC val- and lows controlling the marine incursions, this source- ues are mostly over 7 wt. %, reaching up to 22 wt. % rock could be considered basin-wide and having a large (Duarte et al., 2012). The upper organic-rich unit is the variation of organic matter type (Silva et al., 2013). Pliensbachian Vale das Fontes Formation, having a

Pena dos Reis and Pimentel 4 Maturation Both Jurassic source-rocks can be in the hydro- rifting phase. This phase is responsible for an increase carbon generation window, although not everywhere in in heat flow and, at the same time much overburden. In the basin, as a result of the highly heterogeneous most places maturation has been attained in the Late basin’s subsidence and overburden, especially in the Jurassic (Kimmeridgian to Tithonian) and it has been Late Jurassic. most prominent in the Oxfordian depocenters, namely Non-mature Lower Jurassic source-rocks are the Central Sector’s sub-basins of Arruda, Bombarral, known in outcrop, namely at the Peniche, Montemor-o- and Freixial (Teixeira et al., 2012, 2014). Velho, and São Pedro de Muel sections (Oliveira et al., As a general statement considering vitrinite 2006; Silva et al., 2010; Spigolon et al., 2011; Duarte reflectance data (BEICIP-FRANLAB, 1996) and ther- et al., 2012), but preliminary maturation modelling mal basin modelling studies (Teixeira et al., 2012, may suggest that the units have reached maturity in 2014), it may be considered that the Lower Jurassic several exploration wells in the basin (Teixeira et al., source-rock is mature for oil in the north sector of the 2012, 2014). These non-mature Late Jurassic source- basin and mature for gas in the south sector, whereas rocks are also present in different outcrops, such as the Upper Jurassic source rocks may be not mature in Cabo Mondego or Montejunto (Spigolon et al, 2011), the north sector and are mostly mature in the south sec- whereas they reached the oil-window in nearby wells tor. However, local depocenters are areas of increased such as SB-1, FX-1 and CP-1 (Teixeira et al., 2012, overburden and maturation of the Lower Jurassic 2014). This situation points to a very important role of source-rock in the northern sector, which may be the differential subsidence along the basin, both in time case close to São Pedro de Muel (Porto Energy, 2012). and space. The same kind of situation may have promoted matura- Thermal modeling in different locations point to tion of Upper Jurassic source-rocks in northern a crucial role played by the Late Jurassic intense silici- depocenterGCSSEPM areas. clastic input into the basin, related to the Oxfordian

Migration Well and field data include non-commercial alpine uplift; Ribeiro et al., 1980), feeding the Creta- occurrences of oil and gas in many wells and a few oil ceous . shows in outcrops, proving the existence of maturation2014 Another pattern of migration corresponds to and subsequent migration of hydrocarbons. Biomarkers hydrocarbons of an Oxfordian provenance (Cabaços studies (Spigolon et al., 2011) point to a potential for Formation) in Upper Jurassic reservoir units. This oil two different oil systems related to the migration of system is very frequently identified in oil-shows hydrocarbons towards different reservoir units. (DPEP, 2013) and also in some outcrops (Spigolon et A Lower Jurassic source rock provenance is al., 2010). This play is only evident in the southern sec- identified in oil shows occurring in several outcrops tor, where post-Oxfordian successions are thick enough around diapir walls in the northern sector of the basin. to have the necessary overburden. The source rocks are The oil is believed to have migrated from the Lower mid-Oxfordian marly limestones (Cabaços Formation) Jurassic marls along the faults created (or used) during and the reservoir units include late Oxfordian fractured the salt uplift, feeding a major reservoir of carbonates (Montejunto Formation) and Kimmeridgian Aptian age (Figueira da Foz Formation) (Fig. 4). These turbiditic siliciclastics (Abadia Formation). This sys- pathways have been active probably since the Late tem comprises only Upper Jurassic units and its JurassicCopyright and should have been enhanced at major stratigraphical proximity explains the frequency of this halokinetic events related with compression, namely in kind oil shows in the Lusitanian Basin (Pena dos Reis the Late Cretaceous (with documented extrusion; Pena and Pimentel, 2011). dos Reis, 2000) and the late (with intense

Pena dos Reis and Pimentel 5 Reservoirs The Lusitanian Basin has a thick sedimentary reefal build-ups, both with good reservoir potential infill and a very distinct facies, including a wide range (Uphoff et al., 2010). Besides interparticle and vuggy of siliciclastic, carbonate, and mixed deposits (Fig. 3). porosity, these units may also be fractured reservoirs. Granular reservoirs are present in siliciclastic continen- The Upper Jurassic limestones of the Montejunto tal, transitional, and marine facies; porous reservoirs in Formation may also contain porous (Uphoff et al., some coastal carbonates; and fractured reservoirs in 2010) fractured reservoirs, enhanced by its strati- several shallow marine to deep marine carbonates of graphic and geometric proximity to the Upper Jurassic different ages (Pena dos Reis and Pimentel, 2010a, source-rock of the Cabaços Formation (Pena dos Reis 2010b). Oil seeps and oil shows have been observed in and Pimental, 2011). This fact is particularly important siliciclastic and carbonate units of different ages, close to diapiric structures promoting fractures and including Late Triassic, Jurassic, and Cretaceous trapping, as seen in the Torres Vedras oil-seep (Spigo- (DPEP, 2012). However, a comprehensive and system- lon et al., 2010). atic study of the several reservoir units hass yet to be The fine- to coarse-grained turbiditic deposits of produced and published, and most of the following the Abadia Formation contains abundant sandy layers considerations are based on sparse bibliographic refer- having reservoir potential. However, once again, car- ences and in personal observations. bonate cementation, especially where calciclastic The sedimentary record of the Lusitanian Basin particles are present, partially obliterates the intergran- includes abundant siliciclastic deposits related to ular porosity (Garcia et al., 2010). The following phases of intense tectonic activity and subsequent ero- prograding fluviodeltaic sequence (Lourinhã Forma- sion and accumulation. Many of those deposits were tion) is compositionally immature, resulting in initially very porous, but in many cases diagenesis interesting inter- and intragranular porosities, with val- obliterated a significant part of it. However, is may be ues aroundGCSSEPM 10 to 15%. Early diagenetic carbonate assumed that the basin included enough volume of cementation has been incomplete, inhibiting fill com- siliciclastic rocks to contain all the oil that had been paction and preserving most of the primary porosity generated during the Mesozoic. (Atlantis, 2010). The basal deposits of the basin are the Silves Lower Cretaceous sedimentation includes fluvial Group siliciclastics of Late Triassic age. Proximal to and transitional to coastal deposits having some reser- distal alluvial-fan red-beds have intergranular porosity voir potential. The presence of infiltrated clays in partially filled by carbonate cementation (Atlantis,2014fluvial units and irregular carbonate cementation in 2010), with values ranging from 16% to 23%, locally coastal units has diminished its intergranular porosity, up to 70%. resulting in an average around 5% (Atlantis, 2010). The Lower Jurassic sequence includes thick, Locally, some coastal high energy or biohermal car- compact, and dolomitized carbonate units, especially at bonates have good primary porosity (Dinis et al., the bordering areas of the basin, known in outcrop 2008). Cenomanian transgression resulted in the close to the basement to the east. The Dagorda Forma- marine flooding of the basin and deposition of shallow tion includes dolomites with up to 20% primary marine and reefal carbonate units that also have good porosity (Uphoff, 2005), whereas at the Coimbra For- reservoir properties (Dinis et al., 2008). mation intercrystalline and vuggy porosity with The Cenozoic inversion resulted in basins uplift brecciation related to early meteoric diagenesis are and locally to some intracontinental basins containing important. some hundreds of meters of mainly siliciclastic infill From the Pliensbachian onward, the monotonous (Pais et al., 2012). Paleogene and Miocene infill alternationCopyright of limestones and marls of the Brenha include immature and matrix-rich clays that have low Group have low reservoir potential, although some reservoir potential, but also transgression related bio- fracture-related porosity may be present. Middle Juras- clastic limestones that have good intergranular and sic units correspond to the Candeeiros Formation, moldic porosities. Cleaner sands were deposited in the which is predominantly shallow marine carbonates basin during the late Miocene and Pliocene that could having high to moderate energy textures and some be reservoir properties provided a seal is present.

Pena dos Reis and Pimentel 6 Traps Several stratigraphic traps may be present in the different source-rocks, reservoirs, and seals have been Lusitanian basin. The first one is the direct superposi- essential to promote migration and trapping of hydro- tion of Hettangian clays and over Late carbons (Fig. 4). The main tectonic events promoting Triassic sandstones, promoting accumulation of hydro- the intense structuration of the basin took place during carbons both from underlying Palaeozoic units and the Late Jurassic (rift-climax) and Late Cretaceous from down-thrown Jurassic source-rocks (Figs. 3 and (beginning of the alpine inversion), frequently involv- 4). Another favourable situation is the occurrence of ing the Hettangian plastic evaporites, affecting the biohermal build-ups in several places and during sev- petroleum systems and creating multiple traps (Fig. 4). eral times, namely the (e.g., at the Alpine inversion during the Tertiary has also Pataias cement quarry), Upper Jurassic (Amaral For- been important for deformation and creation of struc- mation) and Upper Cretaceous (Cenomanian reefs). tural traps in many places of the basin (Ribeiro et al., Structural traps seem to have a predominant role 1980). However, the probable fracture network devel- in the Lusitanian Basin as a consequence from the opment may have also destroyed many traps’ integrity, intense tectonic deformation during the Mesozoic enhancing leakage and even total loss of prior hydro- extension and Tertiary compression. Up-lift and sub- carbon accumulations. sidence of tectonic blocks bringing side-by-side

Seals The knowledge about the characteristics of geo- with sandy layers, as is the case of the Upper Jurassic logical units having seal potential in the Lusitanian and Lower Cretaceous units. However, Upper Creta- Basin is well behind what has been presented for the ceous and Tertiary shaly units may have acted as other petroleum system elements. The Dagorda Forma- importantGCSSEPM seals, particularly in the more subsiding or tion is probably the main effective seal in the basin, distal areas of the basin. The behaviour of carbonate consisting of compact red clays containing variable units as seals is highly dependent on the presence or amounts of gypsum and halite having thicknesses up to absence of dense fractures that promote leakage. Most hundreds of meters. This thick shaly package is virtu- of the carbonate units of the Lower-Middle Jurassic ally impermeable and its plasticity confers the ability to and Upper Cretaceous may act as seals provided they deform and to pierce higher stratigraphic levels,2014 con- have not been affected by intense fragile deformation, trolling vertical migration and sealing different as is usually seen (for example) associated with dia- reservoirs. There are several other shaly units, but they piric outcropping structures. cannot be perfect seals due to frequent intercalations

Petroleum Systems and Plays From the analysis of the petroleum system ele- further gas expulsion during the Mesozoic reburial ments and its articulation in space and time, three main (Uphoff, 2005). The same line of thought may be petroleum systems may be considered in the Lusitanian developed regarding other Palaeozoic units having Basin (Pena dos Reis and Pimentel, 2010a, 2010b) source-rock potential, such as the Carboniferous tur- (Fig. 5). bidites of the Southern Portuguese Zone (Fernandes et A pre-salt petroleum system may be defined as al., 2012; McCormack et al., 2007; Barberes, 2013). sourced by meta-sedimentary Paleozoic rocks feeding The main conditioning factor seems to be the (over) Upper TriassicCopyright siliciclastic reservoirs and sealed by the maturation of such Lower and Upper Palaeozoic rocks. Hettangian evaporitic clays. This kind of play has been However, highly variable results have been reported, initially described for the Silurian black-shales and the sometimes in outcrops a few meters apart (Uphoff, Silves Group sandstones (Uphoff, 2005). Those grapto- 2005; McCormack et al., 2007). This situation points to litic black-sales have reached the oil and gas-window the role of late Variscan thin-skinned tectonics, result- in Paleozoic times, before late Variscan uplift and ero- ing in thrust sheets bringing side-by-side units from sion and may have kept some generative potential for different depths and structural settings.

Pena dos Reis and Pimentel 7 A second petroleum system is related with the impregnated the overlying Montejunto Formation Lower Jurassic source-rocks, namely the Sinemurian limestones and the Abadia Formation turbidites and Pliensbachian organic-rich marls (Água de (Spigolon et al., 2010; Uphoff et al., 2010; Pena dos Madeiros and Vale das Fontes formations). Its geo- Reis and Pimentel, 2011). The seal for this kind of play chemical characteristics point to good oil generation could be Cretaceous and/or Tertiary clays and potential (Duarte et al., 2010, 2012; Spigolon et al., siltstones. 2011); in highly subsided areas of the basin, it has A fourth petroleum system may be present, cor- probably reached the oil-window and even the gas- responding to the accumulation of hydrocarbons in the window (Teixeira et al., 2012, 2014). The intense Tertiary cover of the Lusitanian basin. Several source movement of basement blocks affecting the Mesozoic rocks might be involved depending on the sector of the and Tertiary infill and cover is responsible for many sit- basin, and the Tertiary deposits would act both as reser- uations in which the Lower Jurassic units appear voirs and seals. Due to the generally thin sedimentary geometrically beside or above many of the potential cover in the inversion-related Tertiary continental reservoir units. Therefore, the Lower Jurassic source basins (a maximum of 800 meters) south of rocks may have laterally and vertically fed Jurassic and (Ribeiro et al., 1979), this system should naturally be Cretaceous siliciclastic and carbonate units, as well as more prominent in the offshore areas, where the Ter- Upper Triassic siliciclastics. tiary prograding accumulations can reach greater From the observation and analysis of unpub- thicknesses (Alves et al., 2003). lished geochemical data of several diapir-related oil- To synthesize, we may consider that several seeps (e.g., Paredes de Vitória and ; Atlantis, petroleum systems and plays have been active and are 2010), it seems that Lower Jurassic source-rocks were proven in the Lusitanian basin, involving source-rocks the main feeders, using the associated vertical faulting and reservoirs of different ages and lithological charac- and brecciation as migration conduits along the diapir- teristics (Fig. 5). Preliminary modelling attempts walls. It may be speculated that in non-outcropping suggestGCSSEPM that maturation has been promoted by intense diapirs, the same situations led to oil accumulations, subsidence and burial, mainly in Late Jurassic times, at sealed by an unconformable Tertiary cover. different intensities in different sectors of the basin A third petroleum system corresponds to the (Teixeira et al., 2012, 2014). As a general pattern, we maturation and oil generation of the transitional to may consider that in the North sector only the Lower coastal marine Oxfordian Cabaços Formation. These Jurassic units have reached maturity for oil and eventu- marly rocks have been buried under kilometers2014 of ally for gas, whereas in the Central and South sectors Oxfordian and Kimmeridgian rift-related siliciclastics the Lower Jurassic entered the gas-window and the (Abadia and Lourinhã formations), entering the oil Upper Jurassic entered the oil-window (Teixeira et al., window in most of the basin since the Early Cretaceous 2012, 2014). (Teixeira et al., 2012, 2014). This oil has abundantly

Implications for Deep Offshore Exploration From the compared analysis of onshore and off- Peniche basin. These include (i) the pre-salt play, shore petroleum systems in the Lusitanian and Peniche sourced by Silurian and/or Carboniferous rocks, feed- basins, many analogies may be used to approach these ing Upper Triassic redbeds, and sealed by Hettangian plays, but differences are also important. Both basins evaporites; (ii) the diapiric-related play, sourced by have in common its geodynamic framework and evolu- Lower Jurassic deep marine shales and marls, feeding tion, but the specific proximal position of the Cretaceous fluvial sands, and sealed by Upper Creta- LusitanianCopyright Basin and the outer position of the Peniche ceous marls; (iii) the turbidite play, sourced by Upper basin, regarding crustal stretching and North Atlantic Jurassic transitional marls and known in the Lusitanian opening (Alves et al., 2006), has resulted in diachronic Basin to have fed Upper Jurassic turbiditic sands (Aba- elements that must be addressed and understood dia Formation) as well as fractured limestones. (Table 1). However, due to diachronic geodynamic evolu- Most of the plays that have been identified in the tion of the proximal and outer parts of this Atlantic Lusitanian Basin may also be considered valid for the margin, it may be speculated that in the Peniche Basin

Pena dos Reis and Pimentel 8 the turbidite facies (equivalent to the Abadia Forma- Migration was highly dependent on faulting, tion) have been deposited during the Lower Cretaceous related both to extension and compression. Considering as thick rift-climax related sediments (Alves et al., the modelled late maturation timings, compression 2003, 2009). Reservoir potential of these deposits is structures were probably more important as conduits, therefore considered to be high, including deep sea fan although many of them corresponded to the re-activa- geometries with channelized over-bank deposits and tion of previously extension structures. also re-sedimentation as contourites. Mesozoic traps seem to be predominantly struc- A fourth play, absent in the onshore areas, may tural, although there are a few stratigraphic traps be considered in the deep offshore related to the accu- associated with bioherms. On the other hand, Tertiary mulation of thick deep marine deposits corresponding traps may include many stratigraphic accumulations to a Tertiary play: Jurassic source rocks feeding Upper related to the presence of coarse-grained turbiditic Cretaceous to early Tertiary sands and sealed by Neo- channels and fine-grained over-bank deposits. Main gene clays. Based on recently acquired seismic lines of unconformities may also act as traps, namely the one the Peniche Basin (Consortium Petrobras/GALP/Par- between deformed Mesozoic reservoir units and flat- tex; DPEP, 2013), its seismic stratigraphic analysis and lying Tertiary sealing units. thickness modelling in pseudo-wells show that Meso- The influence of inversion issues must be Cenozoic overburden has been sufficient to promote stressed on any approach to the Peniche basin. both maturation of the Jurassic source rocks and also Although it helped in creating folds and faulted migra- sealing of the Upper Cretaceous to early Tertiary reser- tion pathways, its impact on seal integrity may have voirs (Sagres, 2013). All these plays have their own been critical. This pattern is quite evident in outcrop risks and many of the considerations previously pre- analogues in the Lusitanian basin, such as the Paredes sented about the relations between the Lusitanian and de Vitória and Monte Real diapirs. Therefore, anticlinal the Peniche basins are important to reduce the risk of closures associated with significant inversion features the deep offshore exploration activities (Table 1). shouldGCSSEPM be avoided or, at least, carefully looked Lower and Upper Jurassic source-rock identifi- examined. cation is rather confident, but its areal distribution is Understanding the onshore basins is a crucial difficult to predict because paleogeographic recon- starting point to approach the offshore basins. Due to structions are, for the moment, incomplete at a its remarkable exposures and geological detailed stud- regional-scale distribution. However, the patterns in the ies, the Lusitanian Basin is well known and may be Lusitanian Basin (Pena dos Reis et al., 2011)2014 may be used as an analog to the Peniche Basin (Alves, 2009; extrapolated to larger areas, and probably both Jurassic Sagres, 2013). The same depositional packages and source rocks are present in most of the areas of the Pen- unconformities may be recognized in both basins and iche basin. Moreover, both units most probably the different petroleum systems identified at the Lusita- attained maturation for oil, although at different times nian Basin may prove to be present at the Peniche (Sagres, 2013) (Table 1). Basin (Fig. 6). Farther south on the Western Iberian It is crucial to understand the timing and location Margin, the outcrops of the southwestern Alentejo and of maturation of petroleum systems as there are import- western Algarve may also be used as analogues for the ant differences between both basins. For example, rift- offshore Alentejo basin. climax subsidence and related salt withdrawal at the The “analog approach” must be based on under- Peniche Basin (Alves et al., 2003, 2009) occurs 10-15 standing the basin’s evolution—how each geodynamic My (?) later than the landward Lusitanian basin, it context generates each petroleum system element and seems likely that in the Peniche Basin the kitchen areas how they are articulated in space and time. De-risking and theCopyright migration pathways have been controlled strategies for the West Iberian margin offshore basins mainly by the Cretaceous subsidence and diapirism. must include basin and petroleum systems analysis based on detailed outcrop studies.

Pena dos Reis and Pimentel 9 Acknowledgments This paper collects data and ideas from many to important historical exploratory data and unpub- years of research, during which several institutions and lished reports. Petrobras Portugal, namely Rudy colleagues were most helpful. We thank CENPES/ Ferreira and Marisa Calhôa are also thanked for the Petrobras and particularly Edison Milani, Adriano access to data about the Peniche Basin and for their Viana, and Gilmar Bueno for two financed research collaboration with the SAGRES project. Finally, indi- projects supporting this work (ATLANTIS, 2007-2010 vidual collaborations must also be thanked, including and SAGRES, 2011-2013) and for their permanent col- Antônio Garcia for the coordination of the Atlantis laboration. We also thank DPEP (the Portuguese oil Project and Ramón Salas, Hugo Matias, and Ricardo agency) and Director Teresinha Abecasis for the access Pereira for many fruitful discussions.

References

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Pena dos Reis and Pimentel 12 Spigolon, A.L.D, R. Pena Dos Reis, N. Pimentel, and V. Uphoff, T. L., 2005, Subsalt (pre-Jurassic) exploration play Matos, 2011, Geoquímica organica de rochas poten- in the northern Lusitanian basin of Portugal: AAPG cialmente geradoras de petróleo no contexto evolutivo Bulletin v. 89, no. 6, p. 699-714. da Bacia Lusitânica: Bol.Geociências da Petrobras, v. Uphoff, T.L., D.P. Stemler, and R.J.McWhorter, 2010, Juras- 19, nos. 1/2, p. 131-162. sic reef exploration play in the southern Lusitanian Teixeira, B.A., N. Pimentel, and R. Pena dos Reis, 2012, basin, Portugal: II Central & North Atlantic Conju- Regional variations in Source Rock maturation in the gate Margins Conference, Lisbon 2010. Lusitanian Basin (Portugal) – the role of rift events, Wilson, R.C.L. R.N. Hiscott, M.G. Willis, and F.M. Grad- subsidence, sedimentation rate, uplift and erosion: stein, 1989, The Lusitanian Basin of West Central Abstracts III Atlantic Conjugate Margins Conference, Portugal: Mesozoic and Tertiary tectonics, Stratigra- p. 91-92. phy and Subsidence History, in A.J. Tankkard and H. Teixeira, B. A., Pimentel, N. and Pena dos Reis, R., 2014 Balkwill, eds., Extensional tectonics and Stratigraphy (acc.). Thermal modelling and maturation of Jurassic of the North Atlantic margins: AAPG Memoir 46, p. source rocks in the Lusitanian Basin (Northeast 341-361. Atlantic, Portugal). Journal of Petroleum Geology. Witt, W.G., 1977, Stratigraphy of the Lusitanian Basin: Shell Prospex Portuguesa Unpublished Report, 61 p.

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Pena dos Reis and Pimentel 13 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 Processes

Traps & Seal Integrity Inversion (Antiforms and Faulting) Migration Uplift / Folding COMPRESSION / COLLISION CYCLE

Post-rift b Drift Maturation & Seal (Overburden & Thermal evolution) Subsidence POST-RIFT CYCLE Source Rock & Reservoir Syn-rift (Sedimentary environments + Depositional thickness) Extension Basement blocksGCSSEPM RIFT CYCLES Pre-Rift Thermal history Lithologies Basement Tectonic terranes2014 COMPRESSION/ COLLISION CYCLE

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Pena dos Reis and Pimentel 14 Ireland Great Atlantic Britain Ocean

France

Portugal

Spain GB PB

Bb PenB P GCSSEPMO LB SPAIN Bb R T U 100 km 2014 G A L AB

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

Pena dos Reis and Pimentel 15 Figure 2. Geological framework of the Lusitanian Basin (onshore and offshore). Mesozoic based on LNEG; Paleozoic GCSSEPMbased 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. 2014

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Pena dos Reis and Pimentel 16 GCSSEPM

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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,Copyright 1990; Azerêdo et al., 2003; Rey et al., 2006).

Pena dos Reis and Pimentel 17 L CRET U JURA

LM JURASSIC TJ Salt

U TRIAS

A

SEISMIC HORIZONS

Lourinhã H10 - Top Tithonian (Lourinhã) Abadia H9 - Top Kimmeridgian (Abadia) Cab+Montej H8 - Top Oxfordian (Montejunto) TJ Salt LM JURASSIC H6 - Top Callovian (Candeeiros) U TRIAS H2 - Top Hettangian B (Dagorda) H1 - Top Upper Triassic GCSSEPM (Silves) Figure 4. Interpreted seismic lines across the Lusitanian Basin (in Carvalho, 2013); see Figure 2 for location.

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Pena dos Reis and Pimentel 18 P A L E O Z O I C M E S O Z O I C C E N O Z O I C T r i a s s i c J u r a s s i c C r e t a c e o u s Sil Dev Carb Perm E M L E M L Paleogene Neogene E L SOURCE ROCK

RESERVOIR

SEAL

OVERBURDEN

HALOCINESIS TRAP

MAT/MIGR/ACC

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

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Pena dos Reis and Pimentel 19 Seismo-stratigraphic Units LusitanianLusitanian BasinBasin Upper JURA QUATERN. L-M JURA Upper CRET Upper TERT U. TRIAS Lower CRET Lower TERT Rasmussen et al. 1998 PenicPenichehe BasiBasinn B

Unconformities

Campanian

Aptian

Callovian TGS-NOPEC seismic line courtesy from PETROBRAS A GCSSEPM 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. 2014

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Pena dos Reis and Pimentel 20