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The Buçaco Basin (Portugal): Organic petrology and geochemistry study
International Journal of Coal Geology 81 (2010) 281–286
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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / i j c o a l g e o
The Buçaco Basin (Portugal): Organic petrology and geochemistry study D. Flores a,⁎, L.C. Gama Pereira b, J. Ribeiro a, B. Pina b, M.M. Marques a, M.A. Ribeiro a, I. Bobos a, A. Pinto de Jesus a a b
Departamento e Centro de Geologia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal Departamento de Ciências da Terra e Centro de Geofísica, Universidade de Coimbra, 3000-272 Coimbra, Portugal
a r t i c l e
i n f o
Article history: Received 20 January 2009 Received in revised form 4 July 2009 Accepted 6 July 2009 Available online 18 July 2009 Keywords: Buçaco Basin Portugal Organic petrology and geochemistry Gas-prone
a b s t r a c t The Buçaco Basin includes a Carboniferous/Permian sequence that comprises, from bottom to top, the following stratigraphic sequence: Algeriz Formation, Vale da Mó Formation and Monsarros Formation. Algeriz Formation is constituted by deposits of basal breccia with variable thickness and alternating layers of siltstone, mudstone and conglomerate. Vale da Mó Formation corresponds to lacustrine deposits comprising massive red beds in the base, that pass alternatively to silty-mudstones, shales and grey mudstones with organic matter; this formation also includes a thin coal seam. Monsarros Formation includes fluvial conglomeratic deposits prevalent in the base and top, separated by layers of siltstone and mudstone, redder to the top. The Buçaco Basin opens as a pull-apart basin, into the Porto-Coimbra-Tomar shear zone and later is affected by the clockwise (dextral) movement of this N10°W shear zone. The petrographic study of samples from Vale da Mó Formation showed that the organic matter corresponds to a type III kerogen, derived from higher land plants (gas-prone). The thermal maturation level of the strata was determined using random vitrinite reflectance, with values ranging from 0.72% to 0.80% (%Rr). Rock-Eval pyrolysis showed that Vale da Mó Formation is the only one with potential to generate hydrocarbons, an observation consistent with the petrographic characteristics. TOC ranges from 0.08% to 1.52%. Monsarros Formation reported the highest values for S1/TOC index. © 2009 Elsevier B.V. All rights reserved.
1. Introduction and objectives The Buçaco Basin is one of the terrestrial Carboniferous basins in Portugal. It is a N–S aligned basin alongside the Porto–Tomar–Ferreira do Alentejo shear zone, which includes a sequence of Carboniferous/ Permian age (Upper Pennsylvanian [Upper Stephanian C]–Early Autunian). Two tightly compressed, faulted synclines were identified: Algeriz Syncline to the east and north, and Santa Cristina Syncline to west and south (Lemos de Sousa and Wagner, 1983). Wagner et al. (1983) considered that Buçaco Basin comprises, from bottom to top, the following Formations: Algeriz, Vale da Mó, and Monsarros. The results presented in this paper form part of a multidisciplinary investigation where the revision of the stratigraphy and structure of this basin was carried out, together with the petrography, clay mineralogy, and lithogeochemistry of the sedimentary sequence to establish the genetic and evolutionary model for Buçaco Basin. Organic petrology and geochemistry studies complemented the suite of analyses and are considered indispensable to determine the maturation of the basin and the potential to generate gaseous hydrocarbons. Vitrinite reflectance is both a sensitive parameter to establish the maturity of the organic matter, and a palaeogeotherm-
⁎ Corresponding author. Tel.: +351 220 402 468; fax: +351 220 402 490. E-mail address: dfl[email protected] (D. Flores). 0166-5162/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2009.07.004
ometer for identifying the low temperature metamorphic domains used in basin modelling studies. Rock-Eval pyrolysis, used to evaluate rocks as potential hydrocarbon generator, also provides information on the amount of free hydrocarbon (S1) (liquid and gaseous) that are released during the pyrolysis of the sample, and the amount of hydrocarbon compounds produced/released after maturation of organic matter (S2). S2 depends on the kerogen type, the TOC and the maturity of the rocks. S3 corresponds to the decomposition of oxygen compounds present in the samples. In addition, the S2 and S3 permit the calculation of hydrogen and oxygen indexes, whereas the parameter Tmax is used to measure the maturity of the organic matter (Espitalié et al., 1986). The main objectives of this study were: i) to characterize the kerogen type; ii) to assess the maturation of the strata; iii) to estimate the maximum palaeotemperature of the basin; and, iv) to evaluate potential gaseous hydrocarbon generator of Buçaco Basin.
2. Geological setting The Buçaco Basin is a Permo-Carboniferous intramontane basin (Domingos et al., 1983; Lemos de Sousa and Wagner, 1983; Wagner and Lemos de Sousa, 1983; Wagner et al., 1983; Wagner, 1983, 2004). Geographically it occurs along Porto-Coimbra-Tomar shear zone, northeast of Coimbra (Portugal) and morphologically corresponds to
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a narrow band oriented NNW–SSE, with a variable width (0.5–2 km) and approximately 30 km in length. The surface mapping of Buçaco Basin (Fig. 1) recognized three formations, from bottom to top: Algeriz Formation, Vale da Mó Formation and Monsarros Formation. The initial filling of the basin corresponded to torrential sedimentation, with transport from the substratum, proceeding from east and channeled into alluvial fans with a basal breccia with variable thickness alternating with siltstone, mudstone and conglomerate showing a strong vinous shade. Conglomeratic breccias (or fanglomerates) are recurring and erosive on the levels already deposited. To the west, the continuity of these formations to the central part of the basin shows sometimes channeled deposits but essentially floodplain and fluviolacustrine deposits, which become brighter and positive graded-bedded. The lagoon environments are characterized by deposits of even finer particle size, with siltstones and mudstones, in bright grey tones, with levels of abundant plant remains that permitted to attribute to these deposits a latest Pennsylvanian [Upper Stephanian C]–Early Autunian age. This set is covered by medium-grain conglomerates, not too heterogeneous, brighter, with predominance of quartz of fluvial origin, erosive over thin sandstone, siltstones and mudstones also of bright colours, sometimes vinous but in light tones. Conglomeratic beds are recurring to the top of the sequence. They have a vinous clay–sandstone matrix, with more intense tones, as well as clay–sandstone beds.
Aside conglomerate facies, deposits from Buçaco Basin were assembled into two main facies: the red rocks, which are predominantly quartz wacke and lithic wacke with iron cement; and, the grey rocks, with organic matter, which are, predominantly, siltstones. All samples are mineralogically mature. Clay mineralogy identified mica, illite, kaulinite, chlorite and illite/smectite. Buçaco Basin shows abrupt margins, more identifiable in its east border, where the basal torrential deposits from Algeriz Formation are erosive over deposits comprising of vinous silty-sandstone beds, showing a positive graded-bedding and revealing a more stable sedimentation which is associated to less torrential periods. The structure and sedimentation of these deposits, originating in the east, from the substratum side, show that the depressed region where the sediments were deposited is tightly constrained by the formation of a pull-apart basin (Fig. 1), into the Porto–Coimbra–Tomar shear zone, and later affected by the clock-wise (dextral) movement of this N10°W shear zone.
3. Sampling and methodology A total of 24 samples were collected from the three lithostratigraphic units. Attempts were made to cover the entire area, with the objective of having each set of samples representing each lithostratigraphic unit.
Fig. 1. A. Geologic Map of the North sector of the Buçaco Basin (Algeriz Syncline). C — Canelas; VM — Vale da Mó; Al — Algeriz; VNM — Vila Nova de Monsarros. B. General features of the pull-apart Buçaco Permo-Carboniferous Basin limits.
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This task was not always easy due to lack of access to certain areas of the basin. Whole rock polished blocks were prepared according to the techniques described in Alpern et al. (1993) and petrographic characterization was carried out using a microscope equipped with both reflected and fluorescent light. The terminology used to identify and describe the organic matter particles is the one proposed by the International Committee for Coal and Organic Petrology (ICCP 1971, 1998, 2001) as well as Taylor et al. (1998), Alpern (1980) and Hutton et al. (1999). Random vitrinite reflectance was carried out using a MPV2 Leitz microscope. The petrographic composition and random vitrinite reflectance were performed following standard procedures as well as ICCP recommendations (ISO 7404-2 (1985); ISO 7404-3 (1994); ISO 7404-5 (1994)). Proximate analysis was performed in accordance with ISO standards. Ultimate analysis was determined by a CHN elemental analyzer LECO 600, and total sulphur was determined using a LECO SC32 apparatus. The major and minor elements were determined by X-Ray Fluorescence (XRF) using Siemens SRS3000 equipment subsequent to the application of the ASTM D4326, (2004) procedure, on ash samples obtained by high temperature ashing in accordance with ISO 1171 (1997). Pyrolysis experiments were conducted using a Rock Eval 6 unit equipped with a Total Organic Carbon (TOC-weight percent) analysis module. Prior to pyrolysis, the samples were hand-pulverized in an agate mortar. Hydrogen (HI) and Oxygen (OI) indices were calculated using HI = S2/TOC × 100, and OI = S3/TOC × 100, respectively. For a detailed explanation of Rock Eval 6 instrumentation, methodology, and applications, refer to Behar et al (2001). 4. Results and discussion 4.1. Organic petrology The results of the petrographic study of coal samples from Vale da Mó Formation are shown in Table 1 and in Fig. 2A–H. It includes the results of samples B6 and B26, and for comparison, a reference coal sample (BB) representing the same basin and formation, taken from the collection in the Department of Geology, Faculty of Sciences, University of Porto. Samples B6 and B26 were collected from outcrops and the petrographic study identified weathering effects that have affected the vitrinite reflectance. Vitrinite is the most abundant maceral and liptinite occurs very scarcely. The vitrinite is present as collotelinite (Fig. 2A), but Table 1 Maceral composition of coals (vol.%, mmf), qualitative analysis of dispersed organic matter (DOM) of carbonaceous shales and mean random vitrinite reflectance (%), number of measurements (N) and standard deviation (σ). Samples Coal B6 B26 BB
Vitrinite (vol.%, mmf)
Liptinite (vol.%, mmf)
Inertinite (vol.%, mmf)
Rr %
N
σ
69 87 92
1 0 6
30 11 2
0.65 0.64 0.77
100 100 100
0.13 0.10 0.03
+ + + + + + +
+ + ++ + + + +
0.77 0.72 0.80 0.73 0.79 0.72 0.76
10 50 20 81 41 80 69
0.15 0.15 0.15 0.13 0.15 0.13 0.12
Carbonaceous shales B4 +++ B5 +++ B9 +++ B18 +++ B19 +++ B20 +++ B23 +++ mmf – mineral matter free. Rich. Moderately rich. + Rare. +++ ++
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detrovitrinite and gelinite were also identified. Inertinite is present, as fusinite (Fig. 2A), semi-fusinite and secretinite (Fig. 2F). The liptinite is very rare, appearing in the form of cutinite and sporinite. Vitrinite random reflectance (VRr) ranges from 0.64% to 0.65%. In comparison, in reference sample BB, the vitrinite and liptinite (mainly cutinite and sporinite) contents are higher, and inertinite content lower. The reflectance of this coal sample is 0.77%. VRr of the Buçaco coals places these in the category of Bituminous C (ISO 11760, 2005), whereas the coals from the Douro Basin (Upper Pennsylvanian [Lower Stephanian C]), with the same geological structural characteristics (Pinto de Jesus, 2001), belong to Anthracite C to A categories (Lemos de Sousa, 1978; Marques et al., 2009) and as Anthracite A according to (ISO 11760, 2005). In the carbonaceous shale samples, dispersed organic matter (DOM) was identified in six samples from the Vale da Mó Formation (B5, B9, B18, B19, B20 and B23) and in one from Algeriz Formation (B4). Only qualitative analysis was possible for these samples (see Table 1). Microscopically, the organic particles were mostly in the large to medium size range (150 to 50 µm) (Fig. 2B), with some small and fine particles (b30 µm) also observed (Fig. 2H), the latter exhibiting a lamination. DOM is mainly represented by vitrinite, but liptinite and inertinite are also present in small amounts. The vitrinite occurs mainly as collotelinite and telinite (Fig. 2B). Gelinites have characteristic desiccation cracks and detrovitrinite (Fig. 2H) occurs in samples where the DOM is dominated by small particles. Cutinite (Fig. 2D) is the most common liptinite, but the sporinite and resinite (Fig. 2E) are also present in traces amount. Inertinite is represented by inertodetrinite (Fig. 2H) and fusinite (Fig. 2G). Occasionally bimaceral organic particles were observed, particularly clarite and vitrinertite. Such organic composition, impart a kerogen type III to the organic matter of the Vale da Mó Formation, resulting from higher land plants. Mineral matter in the samples comprises essentially detrital minerals such as clays, quartz clasts with rounded form as well as syngenetic framboidal pyrite and iron oxides. Reflectance values range from 0.72% to 0.80% (Table 1) with a wide distribution in the histograms, which seemed to indicate the presence of two populations (Fig. 3 see also Fig. 2C). The analysis of the histogram shows that the population exhibiting a lower reflectance (population 1) range is 0.72%, and for population 2 VRr is 0.97%. The two populations identified appear to be indigenous since both occur in the form of particles of large to medium size. The organic matter allocated to population 1 has a maturity near the reflectance measured in the coal sample. Although Population 2 reports higher reflectance than the coal sample, it is not considered to be reworked organic matter because the organic material present in the black shales of the Middle/Late Devonian and Early Carboniferous metasedimentary sequence, which occurs in the Porto-Tomar-Ferreira do Alentejo shear zone, presents a higher maturity of VRr = 1.30% (Chaminé et al., 2003). Vitrinite reflectance is the best parameter for assessing the organic matter maturation, and an excellent parameter to determine the thermal maturity of sedimentary basins (Taylor et al., 1998). Thus, the vitrinite reflectance obtained from the organic matter of Vale da Mó Formation allows to estimate the maximum palaeotemperature for the Buçaco Basin using the methods described by Bostick et al. (1979), Barker and Pawlewicz (1986), Barker and Goldstein (1990) and Barker and Pawlewicz (1994). The method proposed by Bostick is a graphic one based on the VRr and the effective time of heating for the basin. Considering the vitrinite reflectance of the organic matter indicated in Table 1 and the effective heating time of 40 My (Permian duration), the maximum paleotemperature for Buçaco Basin is estimated to be in the region of 120–130 °C. The other methods, proposed by Barker and others, are based on correlations between palaeotemperature studies of basins and the VRr. The Barker and Pawlewicz (1986) method is based on data from more than 600 basins all around the world. Barker and Goldstein
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Fig. 2. Photomicrographs showing different aspects of samples from Vale da Mó Formation. All photos were taken in reflected white light and with oil immersion objective. A — Coal sample where it is possible to identify fusinite (F), a band of collotelinite (CT), collodetrinite (CD), cutinite (c), inertodetrinite (id) and sporinite (sp); B — Telinite with clearly recognizable cell walls of a plant tissue; C — Vitrinite (Collotelinite) showing two different reflectances; D — Vitrinite and cutinite (c); E — Detrovitrinite with resinite (r) and sporinite (sp); F — Detrovitrinite with secretinite (sc); G — Fusinite; H — Detrovitrinite (V) and inertodetrinite (id).
(1990) and Barker and Pawlewicz (1994) used correlations between the VRr and the homogenization temperature of fluid inclusions. The formulae for the calculation of the maximum temperature proposed by the authors mentioned above and the values obtained for Buçaco Basin are: Barker and Pawlewicz, 1986: ∘ T max = ðlnðRrÞ + 1:19Þ = 0:00782 = 124 C
Barker and Goldsteins, 1990: ∘ T max = ðlnðRrÞ + 1:26Þ = 0:00811 = 128 C
Barker and Pawlewicz, 1994: ∘ T max = ðlnðRrÞ + 1:68Þ = 0:0124 = 117 C:
This way, considering the VRr determined in the organic matter of lithologies from Vale da Mó Formation (0.72% to 0.80%) it was possible to estimate a maximum palaeotemperature that ranges from 120 to 130 °C for the Buçaco Basin. 4.2. Organic geochemistry Tables 2 and 3 summarise the proximate and ultimate analyses of the Buçaco coal samples and the results ash composition (major and minor oxides), respectively. Buçaco coals have a high ash yield
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Table 3 Composition (% wt) of coal ash samples (HTA). Sample SiO2 % Al2O3 % Na2O % K2O % CaO % MgO % Fe2O3 % TiO2 % SO3 % B6 B26
Fig. 3. Histogram of vitrinite random reflectance (%) showing the distribution of two populations (sample B19). Vitrinite random reflectance of the sample Rr = 0.79%; population 1 Rr = 0.72%, and population 2 Rr = 0.97%.
(N29 wt.%) and sample B26 is even considered to be a carbonaceous shale. Results from proximate and ultimate analyses are in accordance with the rank of these coals. However, the variations in volatile matter and carbon contents are due to the ash content of the studied samples. The atomic H/C and O/C ratios calculated are also in agreement with the nature and origin of these coals. The total sulphur content is lower than 0.57% (Table 2). Very little data was available for the reference coal sample (BB), and the ash content reported for this coal is extremely low, hence the volatile matter (daf) for this vitrinite-rich coal is considered to be a reliable rank parameter, confirming the rank attributed by the VRr. In addition, the volatile matter (daf) for coal BB is slightly lower than that for B6, confirming the same rank relation identified by the VRr, i.e. although BB and B6 can be considered to be in the same rank category (Bituminous C) B6 is marginally lower in rank (higher volatiles, slightly lower VRr). SiO2 is the dominant oxide followed by aluminium oxide in the coal ashes (Table 3). In sample B6 silica is lower but Fe2O3 has the highest content. Results from Rock-Eval pyrolysis are shown in Table 4. TOC is very low (between 0.08% and 1.52%), except for B6 and B26 samples which have values of 45.73% and 14.88%, respectively. S1 is also very low (b0.08 mg/g) and the highest value was found in the Monsarros Formation which corresponds to the top of the sequence. S2 is also very low (b0.06 mg/g), except in B6 and B26 samples which have a slightly higher values of 2.52 mg/g and 1.68mg/g, respectively. Most of the samples have a zero value for S2. It was only possible to obtain results of Tmax for the coal samples, which values are 439 °C and 457 °C, due to low S2 values obtained to the other samples. These values are in accordance with the rank established by VRr. Fig. 4 shows that the obtained values fit the curve defined for a sequence of coals of increasing rank. As expected, PI values are low ranging from 0.01 to 1.00. This parameter is related to the free hydrocarbons present in the samples
Table 2 Proximate and ultimate analyses of coal and atomic H/C and O/C ratios. Sample
Ash %, d
VM %, daf
C %, daf
N %, daf
H %, daf
O %, daf
St %, d
H/C
O/C
B6 B26 BB
29.0 76.3 1.2
35.9 45.4 33.3
72.1 59.0 –
1.2 0.9 –
3.3 5.5 –
23.1 33.9 –
0.23 0.17 0.57
0.55 1.11 –
0.24 0.43 –
VM: volatile matter. St: total sulphur. d: dry basis. daf: dry ash-free basis. –: no data available.
52.29 74.55
25.06 21.49
0.16 0.12
2.07 2.65
0.52 0.47
0.80 0.36
17.73 0.82
0.32 0.32
0.57 0.42
(S1), which are also very low. HI ranges from 1 to 11, exclusively determined for samples from Vale da Mó Formation. These values are very low and agree with the petrographic results. Monsarros Formation presents the highest S1/TOC index (sample B12), despite comprising red beds. Samples from the other formations reported values ranging from 1 to 20. Gaseous hydrocarbons were generated during coalification of coal seams and coaly matter in other rocks, beginning at rock temperatures of about 80 °C in the rank range of sub-bituminous coals (Taylor et al., 1998). The bulk of methane generation starts at Rr= 1.2–1.4% and is specially high at Bituminous A and Anthracite stages (Taylor et al., 1998). Considering that Buçaco Basin contains coaly organic matter, the generation of gaseous hydrocarbons is expected. However, their maturation is yet insufficient for maximum generation of such hydrocarbons. Rock-Eval pyrolysis and petrographic analyses show that Vale da Mó Formation is the only one that could be considered productive, or the source rock, for gaseous hydrocarbons. The preservation of gas generated depends on the porosity and the seal of reservoir rocks, and any methane already generated could be stored in the upper formation of the sequence considered to have sufficient porosity to capture it. Therefore, the conglomeratic deposits that prevail in the base and top, separated by layers of siltstone and mudstone, from Monsarros Formation could be the reservoir rocks. 5. Conclusions The Permo-Carboniferous Buçaco Basin opens as a pull-apart basin, into the Porto–Coimbra–Tomar shear zone and was later affected by a clock-wise (dextral) movement of this N10°W shear zone. In this Basin three sedimentological formations were established from bottom to top: Algeriz, Vale da Mó and Monsarros Formations. They are composed of a set of varied sediments of continental facies. Algeriz Formation is constituted by deposits of basal breccia with variable
Table 4 Rock-Eval pyrolysis results of studied samples. Sample TOC wt.% S1 mg/g S2 mg/g S3 mg/g Monsarros Formation B12 0.09 0.01 B13 0.15 0.08 Vale da Mó Formation B5 0.58 0.00 B6 45.73 0.06 B8 0.19 0.02 B9 0.17 0.00 B18 0.63 0.03 B19 1.06 0.00 B20 1.52 0.01 B21 0.25 0.00 B22 0.11 0.00 B23 0.75 0.00 B26 14.88 0.02 Algeriz Formation B3 0.07 0.00 B4 0.22 0.00 B7 0.08 0.00 B10 0.08 0.00 B11 0.12 0.01 B14 0.10 0.02 B17 0.37 0.00
0.00 0.00
0.28 0.30
0.01 2.52 0.00 0.00 0.00 0.01 0.06 0.00 0.00 0.00 1.68
0.48 13.53 0.32 0.19 0.51 1.04 1.08 0.31 0.24 0.64 1.80
0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.18 0.32 0.30 0.23 0.26 0.42 0.44
Tmax °C HI OI
S1/TOC
306 11 202 53
439
457
2 6
82 30 171 11 114 81 5 1 98 4 71 1 124 216 86 11 12 255 148 370 287 216 8 415 20 120
PI 1.00 1.00
0.02 1.00 1.00 0.14
0.01
1.00 1.00
TOC: Total organic carbon; HI: Hydrogen Index; OI: Oxygen Index; PI: Production Index.
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Fig. 4. The variation of Tmax with thermal evolution of a sequence of coals indicated by VRr using data from Amijaya & Littke (2006) and the studied samples.
thickness and alternating layers of siltstone, mudstone and conglomerate; Vale da Mó Formation corresponds to lacustrine deposits that comprises massive red beds in the base, that pass alternatively to silty mudstones, shales and grey mudstones with organic matter; this formation also includes a thin coal seam; Monsarros Formation includes fluvial conglomeratic deposits prevalent in the base and top, separated by layers of siltstone and mudstone, redder to the top. The petrographic study of samples from Vale da Mó Formation showed that the organic matter corresponds to a type III kerogen, originating in higher land plants. The thermal maturation of the basin was determined by random vitrinite reflectance, which attributed a rank of Bituminous C (VRr 0.72%–0.80%). Such values permitted to estimate the palaeotemperature for Buçaco Basin to be in the region of 120–130 °C. The geochemical study (Rock-Eval pyrolysis), showed that Vale da Mó Formation is the only productive one and the parameters are concur with those from the petrographic analyses. TOC is very low (0.08%–1.52%) and most samples reported nil S2. Monsarros Formation reported the highest values for S1/TOC index, although its lithology comprises essentially red beds. Although maturity of the organic matter present in the formations of Bucaco Basin is yet insufficient for maximum generation of gaseous hydrocarbons, it seems as if the Vale da Mó Formation is the only one that could be considered productive, or the source rock, for the gaseous hydrocarbons already produced. In addition, any generated hydrocarbon could be those found in the conglomeratic deposits from the Monsarros Formation. Acknowledgements This research was funded by “Fundação para a Ciência e Tecnologia-FCT”, project “Buçaco Basin: a new approach on stratigraphy, sedimentary basin analysis, structure, lithogeochemistry and organic petrology” Ref: POCI/CTE-GEX/60084/2004. The authors J. Ribeiro e B. Pina benefited a BI scholarship financed by FCT within this project. References Alpern, B., 1980. Pétrographie du kérogène. In: Durand, B. (Ed.), Kerogen: Insoluble organic matter from sedimentary rocks, pp. 339–383. Édtions Technip, Paris. Alpern, B., Lemos de Sousa, M.J., Pinheiro, H.J., Zhu, X., 1993. Detection and evaluation of hydrocarbons in source rocks by fluorescence microscopy. Org. Geochem. 20 (6), 789–795.
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Port. 63, 179–365. Lemos de Sousa, M.J., Wagner, R.H., 1983. General description of the terrestrial carboniferous basins in Portugal and history of investigations. In: Lemos de Sousa, M.J., Oliveira, J.T. (Eds.), The Carboniferous of Portugal: Mem. Serv. Geol. Portugal, no. 29, pp. 117–169. Marques, M., Suárez-Ruiz, I., Flores, D., Guedes, A., Rodrigues, S., 2009. Correlation between optical, chemical and micro-structural parameters of high-rank coals and graphite. Internat. J. Coal Geol. 77, 377–382. Pinto de Jesus, A., 2001. Génese e evolução da Bacia Carbonífera do Douro (Estefaniano C inferior, NW de Portugal): Um Modelo. 2 Volumes (Texto 232pp., 4 anexos; Atlas 71 pp.). Universidade do Porto. (PhD Thesis). Taylor, G.H., Teichmüller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic petrology. Gebrüder Borntraeger, Berlin, Stuttgart. 704 pp. Wagner, R.H., 1983. The palaeogeographical and age relationships of the Portuguese Carboniferous floras with those of other parts of the Western Iberian Peninsula. In: Lemos de Sousa, M.J., Oliveira, J.T. (Eds.), The Carboniferous of Portugal: Mem. Serv. Geol. Portg., Lisboa, no. 29, pp. 153–177. Wagner, R.H., 2004. The Iberian Massif: a carboniferous assembly. Journal of Iberian Geology 30, 93–108. Wagner, R.W., Lemos de Sousa, M.J., 1983. General description of the terrestrial Carboniferous basins in Portugal and history of investigations. In: Lemos de Sousa, M.J., Oliveira, J.T. (Eds.), The Carboniferous of Portugal: Mem. Serv. Geol. Portg., Lisboa, no. 29, pp. 117–126. Wagner, R.H., Lemos de Sousa, M.J., Silva, F.G., 1983. Stratigraphy and fossil flora of the upper Stephanian C of Buçaco, North of Coimbra (Portugal). Contributions to the Carboniferous Geology and Palaeontology of the Iberian Peninsula, pp. 127–156.
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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / i j c o a l g e o
The Buçaco Basin (Portugal): Organic petrology and geochemistry study D. Flores a,⁎, L.C. Gama Pereira b, J. Ribeiro a, B. Pina b, M.M. Marques a, M.A. Ribeiro a, I. Bobos a, A. Pinto de Jesus a a b
Departamento e Centro de Geologia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal Departamento de Ciências da Terra e Centro de Geofísica, Universidade de Coimbra, 3000-272 Coimbra, Portugal
a r t i c l e
i n f o
Article history: Received 20 January 2009 Received in revised form 4 July 2009 Accepted 6 July 2009 Available online 18 July 2009 Keywords: Buçaco Basin Portugal Organic petrology and geochemistry Gas-prone
a b s t r a c t The Buçaco Basin includes a Carboniferous/Permian sequence that comprises, from bottom to top, the following stratigraphic sequence: Algeriz Formation, Vale da Mó Formation and Monsarros Formation. Algeriz Formation is constituted by deposits of basal breccia with variable thickness and alternating layers of siltstone, mudstone and conglomerate. Vale da Mó Formation corresponds to lacustrine deposits comprising massive red beds in the base, that pass alternatively to silty-mudstones, shales and grey mudstones with organic matter; this formation also includes a thin coal seam. Monsarros Formation includes fluvial conglomeratic deposits prevalent in the base and top, separated by layers of siltstone and mudstone, redder to the top. The Buçaco Basin opens as a pull-apart basin, into the Porto-Coimbra-Tomar shear zone and later is affected by the clockwise (dextral) movement of this N10°W shear zone. The petrographic study of samples from Vale da Mó Formation showed that the organic matter corresponds to a type III kerogen, derived from higher land plants (gas-prone). The thermal maturation level of the strata was determined using random vitrinite reflectance, with values ranging from 0.72% to 0.80% (%Rr). Rock-Eval pyrolysis showed that Vale da Mó Formation is the only one with potential to generate hydrocarbons, an observation consistent with the petrographic characteristics. TOC ranges from 0.08% to 1.52%. Monsarros Formation reported the highest values for S1/TOC index. © 2009 Elsevier B.V. All rights reserved.
1. Introduction and objectives The Buçaco Basin is one of the terrestrial Carboniferous basins in Portugal. It is a N–S aligned basin alongside the Porto–Tomar–Ferreira do Alentejo shear zone, which includes a sequence of Carboniferous/ Permian age (Upper Pennsylvanian [Upper Stephanian C]–Early Autunian). Two tightly compressed, faulted synclines were identified: Algeriz Syncline to the east and north, and Santa Cristina Syncline to west and south (Lemos de Sousa and Wagner, 1983). Wagner et al. (1983) considered that Buçaco Basin comprises, from bottom to top, the following Formations: Algeriz, Vale da Mó, and Monsarros. The results presented in this paper form part of a multidisciplinary investigation where the revision of the stratigraphy and structure of this basin was carried out, together with the petrography, clay mineralogy, and lithogeochemistry of the sedimentary sequence to establish the genetic and evolutionary model for Buçaco Basin. Organic petrology and geochemistry studies complemented the suite of analyses and are considered indispensable to determine the maturation of the basin and the potential to generate gaseous hydrocarbons. Vitrinite reflectance is both a sensitive parameter to establish the maturity of the organic matter, and a palaeogeotherm-
⁎ Corresponding author. Tel.: +351 220 402 468; fax: +351 220 402 490. E-mail address: dfl[email protected] (D. Flores). 0166-5162/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2009.07.004
ometer for identifying the low temperature metamorphic domains used in basin modelling studies. Rock-Eval pyrolysis, used to evaluate rocks as potential hydrocarbon generator, also provides information on the amount of free hydrocarbon (S1) (liquid and gaseous) that are released during the pyrolysis of the sample, and the amount of hydrocarbon compounds produced/released after maturation of organic matter (S2). S2 depends on the kerogen type, the TOC and the maturity of the rocks. S3 corresponds to the decomposition of oxygen compounds present in the samples. In addition, the S2 and S3 permit the calculation of hydrogen and oxygen indexes, whereas the parameter Tmax is used to measure the maturity of the organic matter (Espitalié et al., 1986). The main objectives of this study were: i) to characterize the kerogen type; ii) to assess the maturation of the strata; iii) to estimate the maximum palaeotemperature of the basin; and, iv) to evaluate potential gaseous hydrocarbon generator of Buçaco Basin.
2. Geological setting The Buçaco Basin is a Permo-Carboniferous intramontane basin (Domingos et al., 1983; Lemos de Sousa and Wagner, 1983; Wagner and Lemos de Sousa, 1983; Wagner et al., 1983; Wagner, 1983, 2004). Geographically it occurs along Porto-Coimbra-Tomar shear zone, northeast of Coimbra (Portugal) and morphologically corresponds to
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a narrow band oriented NNW–SSE, with a variable width (0.5–2 km) and approximately 30 km in length. The surface mapping of Buçaco Basin (Fig. 1) recognized three formations, from bottom to top: Algeriz Formation, Vale da Mó Formation and Monsarros Formation. The initial filling of the basin corresponded to torrential sedimentation, with transport from the substratum, proceeding from east and channeled into alluvial fans with a basal breccia with variable thickness alternating with siltstone, mudstone and conglomerate showing a strong vinous shade. Conglomeratic breccias (or fanglomerates) are recurring and erosive on the levels already deposited. To the west, the continuity of these formations to the central part of the basin shows sometimes channeled deposits but essentially floodplain and fluviolacustrine deposits, which become brighter and positive graded-bedded. The lagoon environments are characterized by deposits of even finer particle size, with siltstones and mudstones, in bright grey tones, with levels of abundant plant remains that permitted to attribute to these deposits a latest Pennsylvanian [Upper Stephanian C]–Early Autunian age. This set is covered by medium-grain conglomerates, not too heterogeneous, brighter, with predominance of quartz of fluvial origin, erosive over thin sandstone, siltstones and mudstones also of bright colours, sometimes vinous but in light tones. Conglomeratic beds are recurring to the top of the sequence. They have a vinous clay–sandstone matrix, with more intense tones, as well as clay–sandstone beds.
Aside conglomerate facies, deposits from Buçaco Basin were assembled into two main facies: the red rocks, which are predominantly quartz wacke and lithic wacke with iron cement; and, the grey rocks, with organic matter, which are, predominantly, siltstones. All samples are mineralogically mature. Clay mineralogy identified mica, illite, kaulinite, chlorite and illite/smectite. Buçaco Basin shows abrupt margins, more identifiable in its east border, where the basal torrential deposits from Algeriz Formation are erosive over deposits comprising of vinous silty-sandstone beds, showing a positive graded-bedding and revealing a more stable sedimentation which is associated to less torrential periods. The structure and sedimentation of these deposits, originating in the east, from the substratum side, show that the depressed region where the sediments were deposited is tightly constrained by the formation of a pull-apart basin (Fig. 1), into the Porto–Coimbra–Tomar shear zone, and later affected by the clock-wise (dextral) movement of this N10°W shear zone.
3. Sampling and methodology A total of 24 samples were collected from the three lithostratigraphic units. Attempts were made to cover the entire area, with the objective of having each set of samples representing each lithostratigraphic unit.
Fig. 1. A. Geologic Map of the North sector of the Buçaco Basin (Algeriz Syncline). C — Canelas; VM — Vale da Mó; Al — Algeriz; VNM — Vila Nova de Monsarros. B. General features of the pull-apart Buçaco Permo-Carboniferous Basin limits.
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This task was not always easy due to lack of access to certain areas of the basin. Whole rock polished blocks were prepared according to the techniques described in Alpern et al. (1993) and petrographic characterization was carried out using a microscope equipped with both reflected and fluorescent light. The terminology used to identify and describe the organic matter particles is the one proposed by the International Committee for Coal and Organic Petrology (ICCP 1971, 1998, 2001) as well as Taylor et al. (1998), Alpern (1980) and Hutton et al. (1999). Random vitrinite reflectance was carried out using a MPV2 Leitz microscope. The petrographic composition and random vitrinite reflectance were performed following standard procedures as well as ICCP recommendations (ISO 7404-2 (1985); ISO 7404-3 (1994); ISO 7404-5 (1994)). Proximate analysis was performed in accordance with ISO standards. Ultimate analysis was determined by a CHN elemental analyzer LECO 600, and total sulphur was determined using a LECO SC32 apparatus. The major and minor elements were determined by X-Ray Fluorescence (XRF) using Siemens SRS3000 equipment subsequent to the application of the ASTM D4326, (2004) procedure, on ash samples obtained by high temperature ashing in accordance with ISO 1171 (1997). Pyrolysis experiments were conducted using a Rock Eval 6 unit equipped with a Total Organic Carbon (TOC-weight percent) analysis module. Prior to pyrolysis, the samples were hand-pulverized in an agate mortar. Hydrogen (HI) and Oxygen (OI) indices were calculated using HI = S2/TOC × 100, and OI = S3/TOC × 100, respectively. For a detailed explanation of Rock Eval 6 instrumentation, methodology, and applications, refer to Behar et al (2001). 4. Results and discussion 4.1. Organic petrology The results of the petrographic study of coal samples from Vale da Mó Formation are shown in Table 1 and in Fig. 2A–H. It includes the results of samples B6 and B26, and for comparison, a reference coal sample (BB) representing the same basin and formation, taken from the collection in the Department of Geology, Faculty of Sciences, University of Porto. Samples B6 and B26 were collected from outcrops and the petrographic study identified weathering effects that have affected the vitrinite reflectance. Vitrinite is the most abundant maceral and liptinite occurs very scarcely. The vitrinite is present as collotelinite (Fig. 2A), but Table 1 Maceral composition of coals (vol.%, mmf), qualitative analysis of dispersed organic matter (DOM) of carbonaceous shales and mean random vitrinite reflectance (%), number of measurements (N) and standard deviation (σ). Samples Coal B6 B26 BB
Vitrinite (vol.%, mmf)
Liptinite (vol.%, mmf)
Inertinite (vol.%, mmf)
Rr %
N
σ
69 87 92
1 0 6
30 11 2
0.65 0.64 0.77
100 100 100
0.13 0.10 0.03
+ + + + + + +
+ + ++ + + + +
0.77 0.72 0.80 0.73 0.79 0.72 0.76
10 50 20 81 41 80 69
0.15 0.15 0.15 0.13 0.15 0.13 0.12
Carbonaceous shales B4 +++ B5 +++ B9 +++ B18 +++ B19 +++ B20 +++ B23 +++ mmf – mineral matter free. Rich. Moderately rich. + Rare. +++ ++
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detrovitrinite and gelinite were also identified. Inertinite is present, as fusinite (Fig. 2A), semi-fusinite and secretinite (Fig. 2F). The liptinite is very rare, appearing in the form of cutinite and sporinite. Vitrinite random reflectance (VRr) ranges from 0.64% to 0.65%. In comparison, in reference sample BB, the vitrinite and liptinite (mainly cutinite and sporinite) contents are higher, and inertinite content lower. The reflectance of this coal sample is 0.77%. VRr of the Buçaco coals places these in the category of Bituminous C (ISO 11760, 2005), whereas the coals from the Douro Basin (Upper Pennsylvanian [Lower Stephanian C]), with the same geological structural characteristics (Pinto de Jesus, 2001), belong to Anthracite C to A categories (Lemos de Sousa, 1978; Marques et al., 2009) and as Anthracite A according to (ISO 11760, 2005). In the carbonaceous shale samples, dispersed organic matter (DOM) was identified in six samples from the Vale da Mó Formation (B5, B9, B18, B19, B20 and B23) and in one from Algeriz Formation (B4). Only qualitative analysis was possible for these samples (see Table 1). Microscopically, the organic particles were mostly in the large to medium size range (150 to 50 µm) (Fig. 2B), with some small and fine particles (b30 µm) also observed (Fig. 2H), the latter exhibiting a lamination. DOM is mainly represented by vitrinite, but liptinite and inertinite are also present in small amounts. The vitrinite occurs mainly as collotelinite and telinite (Fig. 2B). Gelinites have characteristic desiccation cracks and detrovitrinite (Fig. 2H) occurs in samples where the DOM is dominated by small particles. Cutinite (Fig. 2D) is the most common liptinite, but the sporinite and resinite (Fig. 2E) are also present in traces amount. Inertinite is represented by inertodetrinite (Fig. 2H) and fusinite (Fig. 2G). Occasionally bimaceral organic particles were observed, particularly clarite and vitrinertite. Such organic composition, impart a kerogen type III to the organic matter of the Vale da Mó Formation, resulting from higher land plants. Mineral matter in the samples comprises essentially detrital minerals such as clays, quartz clasts with rounded form as well as syngenetic framboidal pyrite and iron oxides. Reflectance values range from 0.72% to 0.80% (Table 1) with a wide distribution in the histograms, which seemed to indicate the presence of two populations (Fig. 3 see also Fig. 2C). The analysis of the histogram shows that the population exhibiting a lower reflectance (population 1) range is 0.72%, and for population 2 VRr is 0.97%. The two populations identified appear to be indigenous since both occur in the form of particles of large to medium size. The organic matter allocated to population 1 has a maturity near the reflectance measured in the coal sample. Although Population 2 reports higher reflectance than the coal sample, it is not considered to be reworked organic matter because the organic material present in the black shales of the Middle/Late Devonian and Early Carboniferous metasedimentary sequence, which occurs in the Porto-Tomar-Ferreira do Alentejo shear zone, presents a higher maturity of VRr = 1.30% (Chaminé et al., 2003). Vitrinite reflectance is the best parameter for assessing the organic matter maturation, and an excellent parameter to determine the thermal maturity of sedimentary basins (Taylor et al., 1998). Thus, the vitrinite reflectance obtained from the organic matter of Vale da Mó Formation allows to estimate the maximum palaeotemperature for the Buçaco Basin using the methods described by Bostick et al. (1979), Barker and Pawlewicz (1986), Barker and Goldstein (1990) and Barker and Pawlewicz (1994). The method proposed by Bostick is a graphic one based on the VRr and the effective time of heating for the basin. Considering the vitrinite reflectance of the organic matter indicated in Table 1 and the effective heating time of 40 My (Permian duration), the maximum paleotemperature for Buçaco Basin is estimated to be in the region of 120–130 °C. The other methods, proposed by Barker and others, are based on correlations between palaeotemperature studies of basins and the VRr. The Barker and Pawlewicz (1986) method is based on data from more than 600 basins all around the world. Barker and Goldstein
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Fig. 2. Photomicrographs showing different aspects of samples from Vale da Mó Formation. All photos were taken in reflected white light and with oil immersion objective. A — Coal sample where it is possible to identify fusinite (F), a band of collotelinite (CT), collodetrinite (CD), cutinite (c), inertodetrinite (id) and sporinite (sp); B — Telinite with clearly recognizable cell walls of a plant tissue; C — Vitrinite (Collotelinite) showing two different reflectances; D — Vitrinite and cutinite (c); E — Detrovitrinite with resinite (r) and sporinite (sp); F — Detrovitrinite with secretinite (sc); G — Fusinite; H — Detrovitrinite (V) and inertodetrinite (id).
(1990) and Barker and Pawlewicz (1994) used correlations between the VRr and the homogenization temperature of fluid inclusions. The formulae for the calculation of the maximum temperature proposed by the authors mentioned above and the values obtained for Buçaco Basin are: Barker and Pawlewicz, 1986: ∘ T max = ðlnðRrÞ + 1:19Þ = 0:00782 = 124 C
Barker and Goldsteins, 1990: ∘ T max = ðlnðRrÞ + 1:26Þ = 0:00811 = 128 C
Barker and Pawlewicz, 1994: ∘ T max = ðlnðRrÞ + 1:68Þ = 0:0124 = 117 C:
This way, considering the VRr determined in the organic matter of lithologies from Vale da Mó Formation (0.72% to 0.80%) it was possible to estimate a maximum palaeotemperature that ranges from 120 to 130 °C for the Buçaco Basin. 4.2. Organic geochemistry Tables 2 and 3 summarise the proximate and ultimate analyses of the Buçaco coal samples and the results ash composition (major and minor oxides), respectively. Buçaco coals have a high ash yield
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Table 3 Composition (% wt) of coal ash samples (HTA). Sample SiO2 % Al2O3 % Na2O % K2O % CaO % MgO % Fe2O3 % TiO2 % SO3 % B6 B26
Fig. 3. Histogram of vitrinite random reflectance (%) showing the distribution of two populations (sample B19). Vitrinite random reflectance of the sample Rr = 0.79%; population 1 Rr = 0.72%, and population 2 Rr = 0.97%.
(N29 wt.%) and sample B26 is even considered to be a carbonaceous shale. Results from proximate and ultimate analyses are in accordance with the rank of these coals. However, the variations in volatile matter and carbon contents are due to the ash content of the studied samples. The atomic H/C and O/C ratios calculated are also in agreement with the nature and origin of these coals. The total sulphur content is lower than 0.57% (Table 2). Very little data was available for the reference coal sample (BB), and the ash content reported for this coal is extremely low, hence the volatile matter (daf) for this vitrinite-rich coal is considered to be a reliable rank parameter, confirming the rank attributed by the VRr. In addition, the volatile matter (daf) for coal BB is slightly lower than that for B6, confirming the same rank relation identified by the VRr, i.e. although BB and B6 can be considered to be in the same rank category (Bituminous C) B6 is marginally lower in rank (higher volatiles, slightly lower VRr). SiO2 is the dominant oxide followed by aluminium oxide in the coal ashes (Table 3). In sample B6 silica is lower but Fe2O3 has the highest content. Results from Rock-Eval pyrolysis are shown in Table 4. TOC is very low (between 0.08% and 1.52%), except for B6 and B26 samples which have values of 45.73% and 14.88%, respectively. S1 is also very low (b0.08 mg/g) and the highest value was found in the Monsarros Formation which corresponds to the top of the sequence. S2 is also very low (b0.06 mg/g), except in B6 and B26 samples which have a slightly higher values of 2.52 mg/g and 1.68mg/g, respectively. Most of the samples have a zero value for S2. It was only possible to obtain results of Tmax for the coal samples, which values are 439 °C and 457 °C, due to low S2 values obtained to the other samples. These values are in accordance with the rank established by VRr. Fig. 4 shows that the obtained values fit the curve defined for a sequence of coals of increasing rank. As expected, PI values are low ranging from 0.01 to 1.00. This parameter is related to the free hydrocarbons present in the samples
Table 2 Proximate and ultimate analyses of coal and atomic H/C and O/C ratios. Sample
Ash %, d
VM %, daf
C %, daf
N %, daf
H %, daf
O %, daf
St %, d
H/C
O/C
B6 B26 BB
29.0 76.3 1.2
35.9 45.4 33.3
72.1 59.0 –
1.2 0.9 –
3.3 5.5 –
23.1 33.9 –
0.23 0.17 0.57
0.55 1.11 –
0.24 0.43 –
VM: volatile matter. St: total sulphur. d: dry basis. daf: dry ash-free basis. –: no data available.
52.29 74.55
25.06 21.49
0.16 0.12
2.07 2.65
0.52 0.47
0.80 0.36
17.73 0.82
0.32 0.32
0.57 0.42
(S1), which are also very low. HI ranges from 1 to 11, exclusively determined for samples from Vale da Mó Formation. These values are very low and agree with the petrographic results. Monsarros Formation presents the highest S1/TOC index (sample B12), despite comprising red beds. Samples from the other formations reported values ranging from 1 to 20. Gaseous hydrocarbons were generated during coalification of coal seams and coaly matter in other rocks, beginning at rock temperatures of about 80 °C in the rank range of sub-bituminous coals (Taylor et al., 1998). The bulk of methane generation starts at Rr= 1.2–1.4% and is specially high at Bituminous A and Anthracite stages (Taylor et al., 1998). Considering that Buçaco Basin contains coaly organic matter, the generation of gaseous hydrocarbons is expected. However, their maturation is yet insufficient for maximum generation of such hydrocarbons. Rock-Eval pyrolysis and petrographic analyses show that Vale da Mó Formation is the only one that could be considered productive, or the source rock, for gaseous hydrocarbons. The preservation of gas generated depends on the porosity and the seal of reservoir rocks, and any methane already generated could be stored in the upper formation of the sequence considered to have sufficient porosity to capture it. Therefore, the conglomeratic deposits that prevail in the base and top, separated by layers of siltstone and mudstone, from Monsarros Formation could be the reservoir rocks. 5. Conclusions The Permo-Carboniferous Buçaco Basin opens as a pull-apart basin, into the Porto–Coimbra–Tomar shear zone and was later affected by a clock-wise (dextral) movement of this N10°W shear zone. In this Basin three sedimentological formations were established from bottom to top: Algeriz, Vale da Mó and Monsarros Formations. They are composed of a set of varied sediments of continental facies. Algeriz Formation is constituted by deposits of basal breccia with variable
Table 4 Rock-Eval pyrolysis results of studied samples. Sample TOC wt.% S1 mg/g S2 mg/g S3 mg/g Monsarros Formation B12 0.09 0.01 B13 0.15 0.08 Vale da Mó Formation B5 0.58 0.00 B6 45.73 0.06 B8 0.19 0.02 B9 0.17 0.00 B18 0.63 0.03 B19 1.06 0.00 B20 1.52 0.01 B21 0.25 0.00 B22 0.11 0.00 B23 0.75 0.00 B26 14.88 0.02 Algeriz Formation B3 0.07 0.00 B4 0.22 0.00 B7 0.08 0.00 B10 0.08 0.00 B11 0.12 0.01 B14 0.10 0.02 B17 0.37 0.00
0.00 0.00
0.28 0.30
0.01 2.52 0.00 0.00 0.00 0.01 0.06 0.00 0.00 0.00 1.68
0.48 13.53 0.32 0.19 0.51 1.04 1.08 0.31 0.24 0.64 1.80
0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.18 0.32 0.30 0.23 0.26 0.42 0.44
Tmax °C HI OI
S1/TOC
306 11 202 53
439
457
2 6
82 30 171 11 114 81 5 1 98 4 71 1 124 216 86 11 12 255 148 370 287 216 8 415 20 120
PI 1.00 1.00
0.02 1.00 1.00 0.14
0.01
1.00 1.00
TOC: Total organic carbon; HI: Hydrogen Index; OI: Oxygen Index; PI: Production Index.
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Fig. 4. The variation of Tmax with thermal evolution of a sequence of coals indicated by VRr using data from Amijaya & Littke (2006) and the studied samples.
thickness and alternating layers of siltstone, mudstone and conglomerate; Vale da Mó Formation corresponds to lacustrine deposits that comprises massive red beds in the base, that pass alternatively to silty mudstones, shales and grey mudstones with organic matter; this formation also includes a thin coal seam; Monsarros Formation includes fluvial conglomeratic deposits prevalent in the base and top, separated by layers of siltstone and mudstone, redder to the top. The petrographic study of samples from Vale da Mó Formation showed that the organic matter corresponds to a type III kerogen, originating in higher land plants. The thermal maturation of the basin was determined by random vitrinite reflectance, which attributed a rank of Bituminous C (VRr 0.72%–0.80%). Such values permitted to estimate the palaeotemperature for Buçaco Basin to be in the region of 120–130 °C. The geochemical study (Rock-Eval pyrolysis), showed that Vale da Mó Formation is the only productive one and the parameters are concur with those from the petrographic analyses. TOC is very low (0.08%–1.52%) and most samples reported nil S2. Monsarros Formation reported the highest values for S1/TOC index, although its lithology comprises essentially red beds. Although maturity of the organic matter present in the formations of Bucaco Basin is yet insufficient for maximum generation of gaseous hydrocarbons, it seems as if the Vale da Mó Formation is the only one that could be considered productive, or the source rock, for the gaseous hydrocarbons already produced. In addition, any generated hydrocarbon could be those found in the conglomeratic deposits from the Monsarros Formation. Acknowledgements This research was funded by “Fundação para a Ciência e Tecnologia-FCT”, project “Buçaco Basin: a new approach on stratigraphy, sedimentary basin analysis, structure, lithogeochemistry and organic petrology” Ref: POCI/CTE-GEX/60084/2004. The authors J. Ribeiro e B. Pina benefited a BI scholarship financed by FCT within this project. References Alpern, B., 1980. Pétrographie du kérogène. In: Durand, B. (Ed.), Kerogen: Insoluble organic matter from sedimentary rocks, pp. 339–383. Édtions Technip, Paris. Alpern, B., Lemos de Sousa, M.J., Pinheiro, H.J., Zhu, X., 1993. Detection and evaluation of hydrocarbons in source rocks by fluorescence microscopy. Org. Geochem. 20 (6), 789–795.
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