Exploratory well to start in Lusitanian basin, Portugal

Dec. 8, 1997
The Lusitanian basin of Portugal covers the on- and offshore regions of the west-central Portuguese coast between the capital of Lisbon and the northern city of Porto (Fig. 1 [174,729 bytes]) . The basin extends from the outcropping Hercynian terrains of the Central Iberian massif on the east into the offshore beneath the Portuguese continental shelf. In total, the basin covers over 30,000 sq km.
Thomas Uphoff, George Allen, Michael Stearns, Patric Monteleone
Mohave Oil and Gas Corp.
Houston
The Lusitanian basin of Portugal covers the on- and offshore regions of the west-central Portuguese coast between the capital of Lisbon and the northern city of Porto (Fig. 1 [174,729 bytes]). The basin extends from the outcropping Hercynian terrains of the Central Iberian massif on the east into the offshore beneath the Portuguese continental shelf. In total, the basin covers over 30,000 sq km.

Mohave Oil and Gas Corp. commenced an in-depth study of the oil and gas potential of onshore Portugal in early 1992. Positive results of this evaluation led to Mohave's acquisition of three onshore exploration licenses in the northern part of the basin in 1993 (Fig. 1). In the fall of 1997 Mohave acquired two additional licenses, contiguous with the original leasehold, both on- and offshore. Altogether, Mohave's present leasehold comprises over 3,000 sq km.

Mohave and partners (including Doreal Energy Corp., Gaelic Resources plc, Capex S.A., and EDC Portugal Ltd., a Samedan Oil Corp. subsidiary, were preparing to spud their first exploratory well, Aljubarrota-1 (Fig. 1).

Exploration history

Surface oil seeps and asphalt occurrences in the basin attest to the presence of working petroleum systems, and during the mid- to late 1800s asphalt and bitumen were mined at several locations.1

The basin has been the site of periodic exploration activity since the late 1930s but is still largely underexplored. Drilling density of significant exploratory tests within Mohave's area of interest in the northern part of the basin is only one well per 600 sq km.

Early exploration (pre-1963) focused on shallow drilling around salt-related surface structures associated with oil seeps. Oil shows and minor oil recoveries were reported, but no commercial production was established. During 1969, exploration interest turned to the offshore with major international company participation in regional seismic, gravity, and magnetic surveys. Offshore concessions were awarded in the early 1970s, leading to the drilling of 19 wells through 1982. Oil and gas shows were recorded and live oil (33-37° API) was recovered from 2 wells,1 but again no production was established.

Onshore drilling ceased in 1963 and did not resume until 1981. Five more onshore wells have been drilled and abandoned, mainly in the southern half of the basin. The most recent exploration well was P&A'd in 1990, and a salt dome gas storage appraisal well was drilled in 1997.

Mohave's effort targets two plays:

  1. A pre-salt Triassic gas play where giant structures can yield reserves in the 1-50 tcf gas range, and
  2. A post-salt Jurassic oil play supported by indirect hydrocarbon indicators on seismic data. Mohave's first well is a Triassic gas test.

Regional framework

Paleozoic: The Lusitanian basin is a Mesozoic rift basin, initiated as part of Late Triassic opening of the Tethys seaway (Fig. 2 [155,522 bytes]). North to north-northeast trending rift faulting was superimposed upon an older, eroded Hercynian fold belt with northwest- and northeast-trending structural fabrics.2, 3 This pre-rift Hercynian terrain subcrops Triassic syn-rift strata along the eastern flank of the basin where the subcropping section ranges from Silurian to Permo-Carboniferous age.

Significant source rocks in the pre-rift sequence have been sampled and analyzed by Mohave and others.1 These include Silurian graptolitic, black shales and Carboniferous and Permian coals.4, 5, 6 Maturity measurements on these samples range from lower oil/upper gas window to overmature for the Silurian, and immature to oil window mature for the Permo-Carboniferous. These values indicate significant, post-Hercynian, hydrocarbon-generative potential for the pre-rift sequence upon subsequent reburial during Mesozoic rifting.

Mesozoic: Early syn-rift sedimentation is recorded by continental clastic deposits of the Late Triassic Silves formation. Outcrop studies of the Silves along the eastern basin margin7 suggest point sources of coarse clastic input into the basin from westward flowing streams draining old Hercynian higlands of the Iberian massif to the east. Upon entry into the rift valley, these streams would be deflected southward along rift half-graben axes, thereby concentrating sandier facies of rift fill in a fairway along the eastern side of the rift system. Silves outcrop samples exhibit good reservoir characteristics, particularly in well-developed braided stream facies of the upper Silves.

The formation has been penetrated in 6 wells in the northern Lusitanian basin, 5 in the offshore and 1 onshore (Fig. 3 [152,662 bytes]). All wells were drilled high on the rift-margin shoulders, out of the primary sandstone fairway as shown by the Esso Fa-1 and Mobil SM-1 wells (Fig. 4 [60,002 bytes]).

Late syn-rift marine influx appears to be marked by a black shale member of the upper Silves observed in the outcrop belt. This shale has sufficient TOC to be considered a potential source rock given sufficient burial. Isolation of the syn-rift seaway led to development of a transition from supratidal dolo- mite and anhydrite into a substantial evaporite section (including massive halite) of the Late Triassic/Early Jurassic Dagorda formation.

Evaporite deposition continued during early post-rift thermal subsidence, with renewed marine influx developing a transition zone of tidal flat dolomite and anhydrite capping the Dagorda. Continued deepening of marine waters established the region as a broad, lower Jurassic carbonate shelf consisting of basal, shallow water Coimbra formation limestones overlain by deeper water, shaley carbonates of the Brenha formation. Lateral, nearshore equivalents of the Brenha are the Candieiros formation This Lias/Dogger carbonate cycle is capped by a late Middle Jurassic regional unconformity.

Good, oil-prone source rock facies are documented for the Brenha formation1 and are considered the source of the basin oil seeps and oil shows recorded in Jurassic and younger strata.

Carbonate sedimentation continued into late Jurassic time, developing a second carbonate cycle of Malm age. Shallow water limestones of the Montejunto formation unconformably overlie the Lias/Dogger cycle and grade shoreward into coastal plain to nearshore, restricted carbonate shelf equivalents named the Cabo Mondego formation.

Continued subsidence led to deposition of deeper water, open shelf carbonates of the Abadia formation in the basin center, with a laterally equivalent section of Alcobaca formation shoreface clastics and shallow shelf carbonates at the basin margin. Major influx of siliciclastics occurred during the Kimmeridgian, resulting in pro- gradation over most of the carbonate platform by a wedge of continental, siliciclastic facies termed the Gres Superiores formation.

During Late Jurassic and extending into Early Cretaceous, igneous dikes locally intruded the central and southern parts of the basin.2 This igneous activity was probably coincident with creation of oceanic crust as the Atlantic Ocean opened north of the Gibraltar fracture zone. A regional unconformity marks the top of the Jurassic section.

Early Cretaceous sedimentation continued to be continental, as coarse grained and clay-rich, marly terrigenous facies of the Torres Vedras formation were deposited. Marine flooding of the basin then occurred and limestones of the Cacem formation were deposited across the region. This carbonate interval lies with apparent conformity on the Torres Vedras and is an important seismic marker horizon. Regional uplift established an unconformity at the top of the Early Cretaceous section.

Late Cretaceous strata lie unconformably on the Cacem. These rocks are lower coastal plain sandstones and shales, which grade westward into sandy and dolomitic limestones penetrated in offshore wells. Late Cretaceous igneous extrusive and intrusive activity was concentrated in the Lisbon-Sintra area,2 in the southern part of the basin.

Cenozoic: Offshore wells have penetrated lower Tertiary sandy and argillaceous dolomites termed the Espadarte formation. These are overlain by nearshore and lower to upper coastal plain sands and shales of the Benfica and Moreia formations.

Tectonics: Paleozoic Hercynian orogenesis resulted in structural trends whose reactivation during later tectonic phases had significant impact on localizing play fairways in the Lusitanian basin. Northeast-trending, late Hercynian strike-slip faults3 may have served as loci for Triassic rift faults, as is interpreted for the Nazare fault (Fig 3). The Nazare fault underwent wrenching and inversion during subsequent orogenic periods. Primary Hercynian fold axes trend NW-SE,3 oblique to the later rifting. Subsequent, episodic reactivation of these positive features created timely cross-rift arches which focused hydrocarbon migration during the Mesozoic.

Tethyan rifting initiated in the late Triassic and marked the separation of North America and Africa.8 The Lusitanian basin developed as part of this extensional tectonic phase. Some rift faults and rift polarity transfer zones were localized along older Hercynian trends. Seismic data indicate significant fault reactivation and salt pillow development may have begun as early as late Lias/ Dogger time.

During early Malm (Oxfordian), the Lusitanian basin was subjected to regional uplift, possibly reflecting crustal bulging prior to North Atlantic seafloor spreading. Accelerated subsidence and influx of coarse clastics during Middle Malm time mark a period of renewed fault movement and salt diapirism and reflect basin response to the opening of the North Atlantic basin via seafloor spreading to the west.3

Prior to Cretaceous deposition, Upper Jurassic sediments were deeply eroded over salt pillows, as at San Pedro de Muel and Obidos. The axis of the Triassic Lusitanian rift basin remained isolated, craton-ward, of the eastern rift shoulder of the younger North Atlantic Ocean basin, as described by Dumestre and Carvalho.9 Hence, it received primarily continental sediments during the Early Cretaceous (Torres Vedras formation).

During Cenozoic Alpine orogenesis the Lusitanian basin was subjected to regional N-S directed compression resulting from the convergence of Africa and Europe/Iberia.2 Wrench-reactivation of older rift faults occurred and local transpressional and transtensional regimes developed depending on local fault geometry. Halokinesis was reactivated, and shallow piercement salt domes were formed.

Systems, play types

The thick evaporite section of the Dagorda formation divides the Lusitanian stratigraphic succession into two petroleum systems.

The older, subsalt petroleum system relies primarily on pre-rift source rocks of Silurian and Permo-Carboniferous age. These rocks exhibit significant hydrocarbon-generative potential in outcrop within the old Hercynian fold belt and would expel hydrocarbons (primarily gas) upon reburial beneath the Mesozoic rift axis.

Triassic Silves formation sandstones comprise the reservoir section for this system and would be concentrated within the eastern half of the rift complex as noted previously. This explains the relative absence of Silves reservoirs in the existing wells which are all located up on the rift shoulders. Dagorda formation evaporites provide an excellent regional seal to the system.

Primary traps are horst blocks within the rift system. Most prospective are those which appear to be upthrown along inverted rift faults, such as the Nazare fault, because they have a high probability of containing Silves coarse-sand facies. Original intra-rift horst blocks may be bald with regard to Silves sandstones.

Mohave and partners are currently drilling a 25,000+ acre prospect on a pop-up horst block along the Nazare fault (Fig. 5 [141,116 bytes]). The structure appears to be related to transpression across the fault due to left-lateral slip at a confining bend in the Nazare fault trace. Seismic data indicate a lower to mid-Jurassic age for the structure, pre-dating significant hydrocarbon migration. Reserves potential of this prospect is several trillion cubic feet of natural gas. Several similar-sized features have been identified.

The younger, suprasalt petroleum system is entirely Mesozoic, and based on proven source rocks of the Liassic Brenha formation. Reservoirs may be Early to Middle Jurassic carbonates and Late Jurassic sandstones. Porous carbonates of the Lias and Dogger section may develop as primary, high energy facies such as reefs and oolite or bioclastic shoals, as tidal flat dolomites, and as detrital carbonate grainstone aprons eroded from growing salt pillows. Late Jurassic Malm reservoirs can develop in numerous sandstones facies of the nearshore Alcobaca formation or Gres Superiores prograding clastic wedge.

Traps for this petroleum system may be structural or stratigraphic. Seals are more problematic than in the older petroleum system. In the various carbonate depositional environments, anhydrite, shale, and tight carbonate strata can provide good quality, subregional seals. Within the prograding Malm clastic wedge, seals will be of more local nature. The presence of effective seals, and hence working traps, is supported by numerous seismic amplitude anomalies and "flat spots" observed on seismic data in the basin (Fig. 6 [206,693 bytes],Fig.7 [224,298bytes]).

These indirect hydrocarbon indicators are a focus of ongoing evaluation and are Mohave's main target in the offshore. They occur in both the Jurassic carbonate (Lias/ Dogger) and clastic (Malm) sequences, creating two distinct play fairways. Evaluation of limited seismic data to date indicates anomalies to be concentrated along a well-defined, cross-rift arch inherited from the Hercynian. This and other similar cross trends may provide a first order play fairway delineation for this petroleum system. Individual traps in this system could contain oil reserves in the 25-75 million bbl range based on current limited seismic coverage.

No indirect hydrocarbon indicators were noted in association with any of the previously drilled wells. The offshore exploration phase appears to have focused primarily on Alpine-age salt features. These failed due to post-migration timing, and/ or lack of significant reservoir development.

Conclusions

Proven and probable hydrocarbon systems have been identified in the Lusitanian basin of Portugal. These systems were poorly evaluated by past exploration drilling.

Distinct gas and oil play fairways can be delineated in the northern part of the basin. The subsalt gas play is a frontier play offering significant reserves upside of multiple trillion cubic feet of gas in large fields. The overlying, suprasalt oil play is anticipated to have smaller individual field sizes but will be of considerably less risk if ongoing evaluation validates observed seismic amplitudes and "flat spots" as indirect hydrocarbon indicators.

References

  1. Gabinete Para A Pesquisa E Exploracao (G.P.E.P.), The petroleum potential of Portugal, Comissao Reguladora dos Produtos Quimicos e Farmaceuticos, Portugal, 1993.
  2. Pinheiro, L.M., Wilson, R.C.L., Pena dos Reis, R., Whitmarsh, R.B., and Ribeiro, A., The western Iberia margin: A geophysical and geological overview, in proceedings of the Ocean Drilling Program, Scientific Results, Vol. 149, 1996, pp. 3-23.
  3. Wilson, R.C.L., Hiscott, R.N., Willis, M.G., and Gradstein, F.M., The Lusitanian basin of west-central Portugal, Mesozoic and Tertiary tectonic, stratigraphic, and subsidence history, in AAPG Memoir 46, 1989, pp. 341-361.
  4. Robardet, M., and Gutierrez Marco, J.C., Sedimentary and faunal domains in the Iberian peninsula during Lower Paleozoic times, in Pre-Mesozoic geology of Iberia, Springer-Verlag, Berlin, 1990, pp. 383-395.
  5. Gutierrez Marco, J.C., de San Jose, M.A., and Pieren, A.P., Post-Cambrian Paleozoic stratigraphy, in Pre-Mesozoic geology of Iberia, Springer-Verlag, Berlin, 1990, pp. 160-171.
  6. Quesada, C., Robardet, M., and Gabaldon, V., Synorogenic phase [Ossa-Morena zone] (Upper Devonian-Carboniferous-Lower Permian), in Pre-Mesozoic geology of Iberia, Springer-Verlag, Berlin, 1990, pp. 273-279.
  7. Palain, C., Une serie detritique terrigene les "Gres De Silves," Trias et Lias Inferieur du Portugal, Servicos Geologicos de Portugal, Memoria 25, 1976, 377 p.
  8. Emery, K.O., and Uchupi, E., The geology of the Atlantic Ocean, Springer-Verlag, New York, 1984, 1,050 p.
  9. Dumestre, M.A., and Carvalho, F.F., Petroleum geology of the Lusitanian basin, OGJ, Aug. 3, 1987, pp. 54-58.

The Authors

Thomas L. Uphoff is chief geologist for Mohave Oil and Gas Corp., having joined the company in January 1997. He has over 23 years' domestic and international experience, including 14 years with Sohio/British Petroleum and 6 years with BHP Petroleum (Americas) Inc. He has worked on and directed exploration projects in the U.S. Gulf Coast, Midcontinent, and Rockies, as well as in Canada, Latin America, and the Middle East. He has BS and MS degrees in geology from the University of Texas at El Paso.
George Allen is an explorationist who has worked with Mohave since 1993. He was previously with Heritage Exploration and Production Co., Mobil Oil Corp., and Samedan Oil Corp. His areas of experience include Paleozoic basins of the western U.S. and Mesozoic-Cenozoic basins of Colombia, Nigeria, and western Europe. He received BS and MS degrees in geology from Baylor University.
Michael J. Stearns is vice-president of exploration for Mohave. He holds a BS degree in geology/geophysics from Indiana University and an MBA from the University of Houston. He worked during 1970-90 in various exploration line and staff positions at Texaco, Tenneco, and Valero Production Co., focusing on the U.S. Gulf Coast, East Coast, Rockies, and Michigan basin. In 1990 he and two associates founded Trexpro Inc. to provide exploration services to industry clients in the U.S. and Canada. He joined Mohave in October 1993.
Patric H. Monteleone is chairman and president of Mohave, which he founded in August 1993. He has a BS degree in geology from Northern Arizona University and PhD in geology from Leicester University, England. He has worked as an explorationist and in exploration management for British Petroleum, Sohio, Occidental, Tenneco, and Heritage E&P. He has extensive exploration experience on the Alaskan North Slope, U.

S. Gulf Coast, and Latin America.

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