The Lusitanian Basin (Portugal) – litostratigraphic and geodynamic

http://metododirecto.pt/CM2010
ISBN: 978-989-96923-1-2
Volume III – p. 274 - 278
II CENTRAL & NORTH ATLANTIC CONJUGATE MARGINS CONFERENCE
The Upper Jurassic Petroleum System: evidence of secondary
migration in carbonate fractures of Cabaços Formation, Lusitanian
Basin
Spigolon, A.L.D.(1); Bueno, G.V.(1); Pena dos Reis, R. (2); Pimentel, N. (3) & Matos,
V.G.A.E.(3)
(1) PETROBRAS R&D Center, Rua Horácio Macedo, 950, Cidade Universitária, Ilha do Fundão, 21941915, Rio de Janeiro, Brazil, [email protected]
(2) Dep.Ciências Terra FCTUC, C.Geociências UC. Lg.Mq.Pombal,3049 Coimbra. [email protected]
(3) Dep. Geologia e Centro Geologia, FCUL. Campo Grande C-6. 1749-016 Lisboa. [email protected],
[email protected]
ABSTRACT
The Lusitanian Basin has important source rocks intervals deposited mainly during the
Jurassic. In the central sector, occur oil impregnations in carbonate fractures of Cabaços
Formation. The aim of this work is to characterize geochemically the origin and thermal
maturation of this oil and to correlate with the source rock. The geochemical results obtained
for this oil indicate low thermal maturation and an anoxic marine carbonate source, which is
compatible with the composition of organic extracts from Cabaços Formation (Oxfordian). The
good oil-source rock correlation corroborates the existence of an Upper Jurassic Petroleum
System in the Turcifal Sub-basin domain.
KEYWORDS: Lusitanian Basin, Upper Jurassic Petroleum System, marine carbonate
source rock.
1. Introduction
The Lusitanian Basin located along the western Iberian margin, is one of a family
of Atlantic margin Mesozoic rift basins which formed as a response to Pangea
fragmentation in the Late Triassic and subsequent opening of the North Atlantic
(Wilson et al., 1989; Pinheiro et al., 1996; Pena dos Reis et al., 1999; Pena dos Reis et
al., 2009). In its sedimentary record, occur marine source rocks with high organic
content deposited predominantly in the Jurassic, during the rifting phase and under rapid
subsidence.
The petroleum exploration in the Lusitanian Basin began in the last century,
especially after 1938, when several geological and geophysical data were acquired.
Occurrences of petroleum impregnating surface rocks (seeps), and other hydrocarbon
shows in the subsurface indicate the presence of active petroleum systems, in both the
northern and southern parts of the basin (Source: DPEP).
Two main source rock intervals are recognized in Lusitanian Basin, both having
reached oil zone and gas zone: in the North part, occur marls and shales of Lower
Jurassic (Sinemurian and Pliensbaquian), whereas in the South part, occur mainly
organic rich black limestones of Upper Jurassic (Oxfordian) (BEICIP-FRANLAB,
1996; Spigolon, 2007).
The aim of this work is to characterize geochemically the origin and thermal
maturation of oil impregnated in carbonate fractures of Cabaços Formation present in
outcrops in the Torres Vedras region (Turcifal Sub-basin; FIG. 1) and then to associate
the oil to a petroleum system based on oil-source rock correlation.
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2. Analytical Methods
The oil samples collected from carbonate fractures were analyzed using traditional
geochemical techniques, including whole oil stable carbon isotopic composition (δ13C MS Finnigan MAT 252), liquid chromatography (MPLC), whole oil gas
chromatography (GC), and saturate biomarkers by gas chromatography coupled with a
mass spectrometer (GC-MS, m/z 191 e m/z 217).
FIG. 1 – Regional and local contexts of the Torres Vedras oil impregnation, central sector of
Lusitanian Basin.
3. Results and Discussions
The geochemical results of MPLC show a predominance of resins and asphaltenes
with 51%, associated to 22% of saturates and 27% of aromatics. The values of δ13C for
whole oil are around -24.9‰ (FIG. 2).
According to gas chromatography data, the organic composition represents a
typical signature of non-biodegraded oil, characterized by the abundance of n-alkanes
and low pristane/n-C17 and phytane/n-C18 ratios. Sometimes, the absence of light nalkanes and isoprenoids can be attributed to evaporation under conditions of prolonged
exposure. Saturate biomarkers weren’t affected by processes of secondary alteration.
The relatively more abundant compounds among terpanes (m/z 191) are C29 17α
21β(H)-30-norhopane (H29), C30 17α(H), 21β(H)-hopane (H30), 17α(H)-22,29,30trisnorhopane (Tm) and C24 tetracyclic (TET24). Moreover, the distribution of steranes
275
(m/z 217) displays a predominance of C29 steranes, pregnane (C21) and homopregnane
(C22) (FIG. 2).
In agreement with Peters et al. (2005), the high relative abundances of H29 and
TET24 compounds associated with the low concentration of diasteranes (DIA) and the
high relative intensity of C35 17α
β(H) homohopane (22S+22R) (H35)
between the homohopanes (H31-H35), represent a typical composition of source rocks
deposited in an anoxic carbonate marine environment.
FIG. 2 – Composition and organic geochemistry parameters of Torres Vedras oil
impregnation.
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The organic geochemistry parameters of this oil, like low pristane/phytane and
diasteranes/regular steranes ratios associated with high H29/H30, TET24/26 Tri, H35/H34
ratios and carbon isotopic signature enriched in 13C, are compatible with the
composition of extracts from Cabaços Formation, represented mainly by marine
organic-rich black limestone intervals (Oxfordian) (FIGS. 2 and 3).
The good oil-source rock correlation corroborates the presence of an Upper
Jurassic Petroleum System in the Turcifal Sub-basin domain, central sector of
Lusitanian Basin. It is important to mention that the geochemical characteristics of the
Upper Jurassic Petroleum System are easily recognizable when compared to Lower
Jurassic Petroleum System (FIG. 3).
FIG. 3 – Oil-Source Rock and Oil-Oil correlations based on molecular and isotopic parameters
of the Petroleum Systems from Lusitanian Basin.
The low thermal maturation found for this oil, based on low Ts/(Ts+Tm),
C29Ts/H29 and C29 S/(S+R) ratios, indicates that the kitchen area reached at least the
beginning of the oil window (FIG. 2).
This type of occurrence suggests secondary migration via fractures probably
associated with fissured carbonate reservoirs of Cabaços Formation. Similar reservoirs
were tested in some wells drilled in the Torres Vedras region which recovered oil in
fractures at the same formation (DPEP). Due to the short distance between source rock
and reservoir, and also the network of fractures and faults observed in the outcrops, this
petroleum system can be considered of high generation efficiency and drainage.
Acknowledgements
The authors thank to PETROBRAS for authorization to publish this paper as well as Mário
Duncan Rangel, José Roberto Cerqueira and Eugênio Vaz dos Santos Neto for the suggestions
about the text.
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