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Volume III – p. 284 - 288
II CENTRAL & NORTH ATLANTIC CONJUGATE MARGINS CONFERENCE
Jurassic reef exploration play in the southern Lusitanian Basin,
Portugal
Uphoff, T. L.(1); Stemler, D.P. (2) & McWhorter, R.J. (3)
(1) Mohave Oil and Gas Corp., 24 Waterway Ave, Suite 350, The Woodlands, TX, USA 77380
tuphoff@mohaveog.com
(2) Forest Oil Corporation, 707 17th St, Suite 3600, Denver, CO, USA 80202 dpstemler@forestoil.com
(3) Mohave Oil and Gas Corp., 24 Waterway Ave, Suite 350, The Woodlands, TX, USA 77380
rmcwhorter@mohaveog.com
ABSTRACT
A well-developed shelf-edge reef trend is observed on reprocessed 2-D and newly
acquired 3-D seismic data from the western margin of the Turcifal sub-basin. Mohave Oil and
Gas drilled a 1358m reef test based on the limited 2-D data and encountered Upper Jurassic
lime mudstones and reef debris grainstones. Verification of reef facies led to acquisition of the
3-D seismic program which shows three stacked reef complexes ranging in age from Dogger to
Oxfordian. Oxfordian and Kimmeridgian source rocks within the sub-basin range up to 6%
TOC, and 25º API oil has been recovered from fractured, non-reef Oxfordian carbonates on the
east side of the sub-basin. The Oxfordian reef tract is coeval with the productive Abenaki reef
reservoir at Panuke Deep gas field, offshore eastern Canada.
KEYWORDS: Lusitanian, Portugal, Jurassic, reef, exploration.
1. Introduction
Mohave Oil and Gas Corporation identified a Jurassic reef complex on limited 2D seismic data along the western margin of the Turcifal sub-basin of the southern
Lusitanian Basin, Portugal (FIG. 1). Mohave drilled a 1358m exploration well based on
the 2-D data which encountered lime mudstones and tight reef debris grainstones in the
Upper Jurassic section, but did not penetrate reef core facies. This verification of reef
environment led to the proposed 117 km2 3-D seismic program over the reef trend
currently in progress.
FIG. 1 - Location of reef trend, significant wells and simplified stratigraphy
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2. Regional Stratigraphy and Structure
The region is divided into three sub-basins by the tripartite intersection of the
Torres Vedras Fault, Montejunto Thrusted Anticline, and Sobral Fault (FIG. 1). The
Bombarral sub-basin lies to the north of this “Y” intersection, the Arruda sub-basin to
the southeast, and the Turcifal sub-basin to the southwest. The juncture of the three
trends is marked by the Matacães Salt Structure, a probable diapir with Liassic Dagorda
Fm. evaporites at the surface.
Lower and Middle Jurassic stratigraphy in this area is comprised of a basal Lias
evaporite-bearing sequence overlain by Lias and Dogger carbonate rocks. The Malm is
characterized by Oxfordian carbonates overlain by Kimmeridgian and Tithonian
siliciclastics. Dogger-age reef facies have been described from outcrops in the Serra de
Porto do Mós, north of the study area (Seifert, 1958) and from well samples from the
Abadia-2 well at Abadia Dome (FIG.1). Upper Jurassic reefs are well documented from
outcrops along the eastern margin of the Arruda sub-basin (e.g. Nose and Leinfelder,
1997) and have been penetrated in wells along the Montejunto Thrusted Anticline and
in the Bombarral sub-basin.
3. Upper Jurassic Source Rocks
TOC values of both Oxfordian argillaceous carbonates and Kimmeridgian shales
range up to 6%, with the maximum values for each occurring in the Turcifal sub-basin.
Vitrinite reflectance values for outcrop and shallow well samples in the northeast
Turcifal indicate early oil window maturity. Oil with ~25o API gravity has been
recovered from Kimmeridgian sands and fractured, Oxfordian non-reef limestones in
this area. No % Ro data are available from the Turcifal basin axis or western margin.
Whole oil chromatograms for oils collected from the northeast Turcifal sub-basin and
along the Montejunto Thrusted Anticline indicate two separate oil families, suggesting
two petroleum systems have been active in the region.
4. Reef Reservoirs
The Campelos-1 well (Cp-1) and Bombarda-1 well (Bb-1) in the Bombarral subbasin (FIG.1) both logged significant reef porosity with modern wireline tools. Cp-1
penetrated over 100m of water-bearing Oxfordian reef reservoir with porosity ranging
from 10% to 18%. Bb-1 drilled 75m of oolitic and bioclastic grainstone/packstone with
reef debris. The well penetrated a net 25m of porosity ranging from 10% to 14%. The
Pragança-1 well (Pr-1) was drilled in 1959 on the northeast plunge of the Montejunto
Thrusted anticline trend (FIG. 1). This well is located down dip of extensive tar
accumulations in outcrops of Oxfordian reef and associated carbonate and overlying
sandstone facies. Pr-1 penetrated 450m of interbedded Oxfordian reef boundstone,
grainstone and mudstone with significant saturation of tar and heavy oil that became
more liquid with depth.
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FIG. 2 - Aldeia-2 reef facies core and analysis
On the Montejunto Anticline at Abadia Dome (FIG. 1), the Aldeia-2 well (Ald-2)
cored 1m of oil-saturated Oxfordian reef facies (FIG. 2). Whole core analysis indicated
23% porosity and horizontal and vertical permeability of 1 Darcy. Additionally, the
Abadia-2 well (Ab-2), the only deep well at Abadia Dome, penetrated upper Dogger
carbonates described as reefal, algal and oolitic, with abundant sample oil shows.
Microlog separation indicates over 100m of net porous and permeable zones within the
upper 160m of the Dogger reefal facies.
5. New 3-D Seismic Data over the Turcifal Reef Trend
At present, 3-D seismic acquisition is complete over the northern 40% of the
Turcifal shelf-edge reef trend (FIG. 1). An E-W inline from the current 3-D dataset that
highlights important aspects of the Turcifal reef complex is shown as FIG. 3.
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FIG. 3 - 3D inline 80 over the Turcifal sub-basin Jurassic reef trend. Interpreted reef bodies in
blue and possible mass-wasting deposit in brown. Formation tops are indicated.
The seismic line shows how reefal development initiated in the Lower Jurassic,
localized over a major rift fault. After the Dagorda evaporites were deposited,
continuing post-rift sag draped them over the rift margin fault. The initial stage of
carbonate deposition then occurred, including parallel trends of patch reefs and what
appear to be shoaling episodes. The next stage of reefing appears to have been
significantly exposed afterward, resulting in an interpreted mass-wasting deposit
downdip. The final reefing episode took place through the end of Oxfordian
(Montejunto Fm.). This reef has a greater areal extent than any of the previous episodes
(FIG. 4), as well as the greatest height.
Overlying Kimmeridgian Abadia Fm. onlaps the reef front and covers the top.
Where penetrated in the Mohave TVR G-1 well, the basal Abadia consists of 80m of
intercalated, thin beds of silty shale, siltstone and silty, argillaceous micrite,
characterized by high Gamma Ray values, largely in the 50 to 75 API unit range.
Compaction folding over the reef bodies is well-exhibited, as is faulting in the Abadia
clastics that does not appear to affect the underlying Montejunto carbonates. The
essentially flat nature of the post-reef Upper Jurassic section indicates the reef trend has
not been subjected to significant Cretaceous and Tertiary deformation which could
breach reef reservoirs.
The recovery of live oil from fractured, non-reef Montejunto carbonates and
overlying Abadia fine-grained sandstones at the northeast margin of the Turcifal subbasin (FIG. 1) indicates the sub-basin has generated hydrocarbons. The reef trend along
the western margin of the sub-basin is in an excellent position to receive a hydrocarbon
charge and its structural history suggests little risk of subsequent breaching.
Data acquisition in the southern part of the 3-D survey which will tie the Mohave
TVR G-1 well has not yet been completed.
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FIG. 4 - Time structure map on the top of the Montejunto Fm. showing shelf-edge reef
development.
Acknowledgements
The authors thank Porto Energy Corp., Mohave Oil and Gas Corporation and Divisão Para a
Pesquisa e Exploracão de Petróleo (D.P.E.P.) for permission to present this data.
References
Nose, M. & Leinfelder, R.R. (1997) - Upper Jurassic Coral Communities within Siliciclastic Settings
(Lusitanian Basin, Portugal): Implications for Symbiotic and Nutrient Strategies: Proc. 8th Int.
Coral Reef Symp. P.1755-1760
Seifert, H. (1958) - The North Candieiros Anticline. Company Report: Companhia dos Petróleos de
Portugal & Mobil Exploration Portugal Inc., Archives, Divisão Para a Pesquisa e Exploracão de
Petróleo (D.P.E.P.), Lisboa
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