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Outcrop gamma-ray spectrometry: applications to the Sinemurian--Pliensbachian organic-rich facies of the Lusitanian Basin (Portugal)/Espectrometria de rayos gamma: aplicaciones a las facies organicas carbonatadas del Sinemuriense-Pliensbachiense de la Cuenca Lusitanica (Portugal).

1. Introduction

Organic-rich deposits, particularly with dark and laminated marls (commonly named as black shales), are of great importance for petroleum exploration, since they may represent major source rock intervals (e.g., Bodin et al., 2011; Riediger, 2002; Liming et al., 2000). The outcrop Gamma-ray logs allows the characterization of source rocks and their organic content estimation, being also a very important correlation tool to better understand the subsurface data (Bessa and Hesselbo, 1997). Gammaray (GR) spectrometry measures the total gamma radiation and the individual contribution from the three main radioactive elements: potassium (K), thorium (Th) and uranium (U) (see a detailed discussion in Ellis, 1987). In carbonate sequences (the focus of this work), the K and Th are mainly concentrated in the minerals that constitute the insoluble residue (e.g., Lucia, 2007). The U also occurs in the detrital clay fraction however, and unlike Th, it is also partly carried in solution as uranyl carbonate complexes (U[O.sub.2][(C[O.sub.3]).sub.2.sup.2-]) (Langmuir, 1978). Under oxygen depleted conditions, it may be precipitated to enrich the sediment in authigenic (nondetrital) U, fixed at the sediment-water interface accumulating together with the organic matter (Wignall and Myers, 1988). In this systems, it is possible to obtain a linear relationship between the total organic carbon (TOC) and U (Lining and Kolonic, 2003). This technique, when associated with the direct analysis of the rocks (biostratigraphy, sedimentology, petrography, geochemistry) can be very helpful, not only to characterize a potential source rock (e.g., Bodin et al., 2011; Lining et al., 2005; Lining and Kolonic, 2003; Myers and Wignall, 1987), but also as an important tool for basin analysis, particularly in sequence stratigraphy studies (e.g., Catuneanu, 2006; Collins et al., 2006; Pawellek and Aigner, 2003; Parkinson, 1996), lithostratigraphic characterization (e.g., Raddadi et al., 2005; Martinius et al., 2002; Hadley et al., 2000; Bessa and Hesselbo, 1997) and correlation with subsurface data (e.g., Aigner et al., 1995; Leslie et al., 1993).

Here we studied the Lower Jurassic organic-rich units deposited during the Late Sinemurian-Pliensbachian time interval, one of the main potential source rock intervals of the Lusitanian Basin (e.g., Duarte et al., 2012; Oliveira et al., 2006). More than one hundred wells have been drilled and thousands of kilometres of seismic lines were obtained in the Lusitanian Basin. However, no significant oil or gas field with commercial interest was found in this basin or in Portugal.

Both field and laboratory spectral GR techniques were applied in different key-sections of the western sector of the Lusitanian Basin near S. Pedro de Moel and Peniche (Fig. 1), supported by stratigraphic, geochemical and mineralogical investigations. One of the main targets of this data set is to correlate the U content with the organic richness and test their relationship, in order to use the U results as a proxy to estimate the TOC. The geochemical analysis, normally more complex and expensive, can be replaced using indirect measurement of U content by GR spectrometry in wells and outcrops. This work also highlights the importance of these methods in the delimitation of the 2nd order Transgressive-Regressive (T-R) facies cycles (sensu Jacquin and de Graciansky, 1998) and the identification of maximum flooding intervals (maximum flooding surfaces in Galloway, 1989) already presented for the Lusitanian Basin (e.g., Duarte et al., 2010). Finally, the obtained data are also used to characterize the radiometric signature of the different lithologies and lithostratigraphic units in outcrop and to correlate the results with the natural GR data collected in some exploratory wells drilled in the surrounding areas.

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2. Geological Setting

The Lower Jurassic is particularly well represented in the Lusitanian Basin, one of the Atlantic basins bordering the Variscan Iberian Massif (Fig. 1A). It comprises a thick carbonate succession consisting of up to 550 m of mostly marl-limestone alternations characterizing a great part of the Late Sinemurian-Toarcian series (Duarte et al., 2010; Duarte, 2007; Duarte and Soares, 2002). These facies were deposited in the Upper Triassic-Callovian 1storder cycle (Azeredo et al., 2003; Soares et al., 1993; Wilson et al., 1989), resulting in the progressive flooding of the basin, controlled by an epicontinental sea and sustained by a low-gradient carbonate ramp dipping NW (Duarte et al., 2010; Duarte, 2007; Dommergues and Mouterde, 1987).

The studied late Sinemurian-Pliensbachian interval is characterized by the occurrence of organic-rich facies (Duarte et al., 2011a, b, 2010; Oliveira et al., 2006). This study is focused on the Agua de Madeiros and Vale das Fontes formations, defined by Duarte and Soares (2002), although other units were partially analysed, for example, the Coimbra and Lemede formations (Fig. 2). As a result from the ramp morphology and controlled by regional tectonics and sea level changes, the thickness of these units increases towards the west, which are biostratigraphically well constrained due to the abundance of benthic and nektonic fauna, namely ammonites and calcareous nannofossils (Silva et al., 2011; Duarte et al., 2010; Oliveira et al., 2007; Dommergues and Mouterde, 1987; Mouterde, 1953, 1967) (Fig. 2).

The top of the Coimbra Formation (Fm), consists of a series of thick limestone beds with some argillaceous interbeds with abundant benthic macrofauna (e.g., Duarte and Soares, 2002). The age equivalent Agua de Madeiros Fm, belonging to the Oxynotum-Jamesoni Zone, consists of marl-limestone alternations with frequent dark and laminated marls and is very well exposed in the S. Pedro de Moel coastal area, where its type-section is defined (Duarte and Soares, 2002). This unit consists of two members: the Polvoeira Member (Mb), which contains organic-rich dark, laminated marls (mostly black shales), and the overlying Praia da Pedra Lisa (PPL) Mb, with a predominance of limestones, especially at the base of the succession.

The overlying Vale das Fontes Fm represents the return to a marly sedimentation, which characterizes almost the entire Pliensbachian (Jamesoni-Margaritatus Zone) of most of the Lusitanian Basin. This formation is subdivided into three members, but only the uppermost one was analysed, the Marl-Limestone with Organic-Rich Facies (MLOF) Mb, due to the abundance of dark marls. This interval belongs to the Davoei-Margaritatus Zone and was previously also referred to an Organic Matter Preservation Interval (Late Pliensbachian OMPI sensu Silva et al., 2011).

The uppermost Pliensbachian is represented by the Lemede Fm (Duarte, 2007; Duarte and Soares, 2002) which comprises a carbonate dominated sequence.

3. Materials and Methods

3.1. Field Procedure

Field work concentrated on the two mentioned stratigraphic units which were studied in three sections where recent geological studies had been conducted and detailed stratigraphic logs constructed (used in this study): Polvoeira (Duarte et al., 2011a; 2012), Agua de Madeiros (Duarte et al., 2010, 2012) and Portinho da Areia do Norte (Silva et al., 2011). A high resolution spectral GR survey was carried out with a portable GR spectrometer (model GR-130G from Exploranium[R]) equipped with a NaI (Tl) detector 38 mm x 51 mm in size, integrating a 256 channels system and with a spectral range of 0-3 MeV. This equipment measures the character of about 0.3 [m.sup.3], in a hemispheric shape, behind the outcrop face. Each section was logged by measuring the total gamma-radiation every 20-30 cm (or each bed), over a time integration of 10 s. Around 700 valid readings were obtained, covering a stratigraphic interval of approximately 85 m. This equipment also allows the quantification of the major radioactive elements: U, Th and K. Based on their sedimentary and radiometric characteristics, a total of 60 different beds were GR-measured. The obtained U, Th and K concentrations are the average result of three valid measurements, each one with a duration of 100 s. This methodology follows the recommendations of the manufacturers and has been widely used in other works with similar logging procedures (e.g., Bodin et al., 2011; Pereira et al., 2003; Svendsen and Hartley, 2001). Field tests carried out at the beginning of the study, using measurements with higher time integration intervals (20 and 30 s for the total GR and 200 s for the U, Th and K), do not show a significant difference, indicating that the used measurement duration resulted in an acceptable data reproducibility. The portable equipment was calibrated after each hour of use with a [sup.137]Cs radioactive source. As a cross-check, several test analyses were undertaken using an alternative portable scintillation counter, Saphymo SPP2, equipped with a NaI (Tl) detector that is 38 x 25 mm in size. All the gamma-radiation data were measured in counts per second (cps), U and Th in parts per million (ppm) and K in percentage (%). All measurements followed a strict procedure to avoid significant errors mainly related with the heterogeneity of the outcrops and the sample volume. A detailed description of these procedures can be found in Svendsen and Hartley (2001).

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3.2 Laboratory procedure

During the field work, 45 rock samples with an average weight of 1 kg were collected for later laboratory analysis. All parameters (gamma-ray spectrometry, geochemical and mineralogical) were obtained from the same portion of rock avoiding errors associated with lateral and vertical variation. For each sample, the U, Th and K were quantified in the Natural Radioactivity Laboratory (University of Coimbra) using a GammaRay spectrometer Ortec[R], with a 3x3" NaI (Tl) detector, integrating a 1024 Multichannel system with a NaI spectral analysis software ScintiVision-32. The higher precision of this equipment is related to the fact that all these measurements were performed in standardized conditions, allowing the cross-check and validation of the radiometric data obtained with the field instrument. As mentioned earlier, quantification of the different components of the U is important when studying organic-rich intervals. For this purpose, and taking into account that the minimum Th/U ratio of normal mudstones where U is assumed to be entirely detrital is 3, the formula introduced by Wignall and Myers (1988) to estimate the detrital and authigenic U fractions in mudstones was used (Eq. 1):

[U.sub.authigenic] = [U.sub.total] - Th/3 (Eq.1)

The mineralogical analyses were performed by X-ray diffraction (XRD--model PW3710, with a PW1830 generator, Cooper ampoule and PW1050 goniometer) in the X-Ray Laboratory of the Earth Sciences Department of the University of Coimbra. The data was grouped and semi-quantifications were projected in the graphic logs as carbonates (calcite and dolomite) and siliciclastics (quartz, feldspar and muscovite/biotite mica). Due to the difficulty of accurately semi-quantifying the abundance of the clay minerals, these were not incorporated in the data set.

The TOC and insoluble residue content were determined with a SC-144DR LECO[R] analyser in the LAFO Laboratory of the Geosciences Institute of the Federal University of Rio de Janeiro with an analytical precision of [+ or -] 0.1%.

Taking into account that the GR curve obtained in outcrop can be used to calibrate and better understand the subsurface GR-character, the logs of exploration wells 14 A-1 and Alj-2 drilled in the Lusitanian Basin were acquired from the Research and Petroleum Exploration Division of Portugal for correlation (see location in Fig. 1B).

4. Results

4.1. Polvoeira section

This section is situated 4 km south of the S. Pedro de Moel village, in the cliffs of the Polvoeira beach (WGS84 coordinates: 39[degrees] 43' 26.76"N; 09[degrees] 02' 51.66"W) (Fig. 1B). A total of 30 m were logged (interrupted only by one small inaccessible sector), belonging to the Oxynotum-Raricostatum Zone, Late Sinemurian (Duarte et al., 2011a, 2012; Duarte and Soares, 2002) (Fig. 3A). The analysed section includes the last 4 m of the Coimbra Fm and the transition to the Agua de Madeiros Fm, where a big portion of the Polvoeira Mb was logged, in a sequence with a great predominance of the organic-rich levels towards the top (Fig. 3B).

Regarding the total GR values, this section reveals a generalized tendency to increase towards the top, from 30 to 60 cps registered in the Coimbra Fm to 180-190 cps obtained in some black shales of the Povoeira Mb (Fig. 4). The highest GR value (210 cps) is associated with one of the thickest black shale horizons. The spectral GR data collected in the field and in the laboratory are quite similar, although it is emphasized that in the field method, the U concentrations are normally higher and the Th and K lower when compared with the laboratory data. Specially the U and partly also the Th values, show a similar trend to the total GR, where the low values (< 1 ppm of U and Th) observed in the Coimbra Fm, contrast with the highest values observed in the black shales of the Polvoeira Mb (11.6 ppm of U, 7.6 ppm of Th and 210 cps of total GR). On the other hand, the K values did not show any particular trend using the field equipment, ranging from 0.5% to 2%. In contrast, in the laboratory measurements K has a similar trend as the Th, ranging from 1 to 3.1% (Fig. 4).

The variability of TOC generally follows the U trend, where the highest TOC value (20.4 wt.%) corresponds to the highest U concentration (11 ppm). Insoluble residue always shows a good correlation with the mineralogy, where the lowest value (35%) corresponds to the highest carbonate/siliciclastic ratio and to the highest concentrations of Th and K. The XRD analysis also registered the presence of dolomite and the highest peaks of pyrite or uraninite were observed in this section, coincident with the black shale levels with high U content. The uraninite and pyrite share the same spectral interval becoming very difficult to distinguish them.

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4.2. Agua de Madeiros section

This sector is located 2 km north to the previous section (Coordinates: 39[degrees] 44' 28.66"N; 09[degrees] 02' 21.52"W). A total of 32 m were logged, in a sequence that belongs to the Raricostatum-base of Jamesoni Zone (Duarte et al., 2010). The base of this section is stratigraphically equivalent to the top of the Polvoeira section described previously. This section includes the last 13 m of the Polvoeira Mb and almost the whole Praia da Pedra Lisa (PPL) Mb (15.5 m) (Fig. 3C). The transition between these two members is very expressive and visible in outcrop, materialized by a thick limestone bed deposited over a decimetre thick black shale bed (Fig. 3D). It also contains a 3 m section of the Marly Limestone with Uptonia and Pentacrinus (MLUP) Mb of the Vale das Fontes Fm with a clearly predominance of grey marls (Fig. 5).

The lithological transition between the two members of the Agua de Madeiros Fm is also well marked in the radiometric data, characterized by the abrupt decrease from 150 to 54 cps, which can be considered a good benchmark in the correlation with the well logs. From the 22 m of the profile towards the top, the GR values start a positive and irregular incursion (33 to 120 cps) coincident with the more frequent and expressive occurrence of dark/argillaceous levels (Fig. 5).

Regarding the concentration of the different radioactive elements, the abrupt transition mentioned before is also observed, namely in the U concentrations (9.3 to 1.7 ppm). From the bottom of the PPL Mb to the top of this section, including the small analysed interval correspondent to the MLUP Mb, the content in U, Th and K increases gradually. Comparing the field with the laboratory data, the U values are similar but the Th and K content were higher in the laboratory analyses. Nevertheless, the variability associated to both techniques is quite similar (Fig. 5).

Regarding the geochemical data, the levels analysed in the top of the Polvoeira Mb show high TOC percentages (2.0 to 9.6 wt.%), although slightly lower when compared with the top of the Polvoeira section. For the insoluble residue, the highest amounts (> 60%) were obtained in the levels with the most important argillaceous fractions. The two samples easily observed at the 14 m and 28 m of the log by their high carbonate/siliciclastic ratio correspond to thick limestone beds characterized by low GR intensities (< 50 cps).

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4.3 Portinho da Areia do Norte section

This section is located in the cliffs of the Portinho da Areia do Norte beach, in the Peniche Peninsula (Coordinates: 39[degrees] 22' 06.60"N; 09[degrees] 22' 42.98"W). The logged section covers the last 20 m of the MLOF Mb from the Vale das Fontes Fm and the transition to the Lemede Fm (Fig. 3E), all dated from the Margaritatus Zone (Silva et al., 2011). This sequence is the most argillaceous and organic-rich of the entire Vale das Fontes Fm (Oliveira et al., 2006) (Fig. 3F). The transition to the Lemede Fm is easily observed by the colour change of the rock (yellow), announcing a predominant calcareous interval.

The GR data reveal high intensities, increasing progressively towards the top (100 to 181 cps), with two main peaks observed at 3 m (173 cps) and 13 m (181 cps) of the log (Fig. 6) (see also Parkinson, 1996). Comparing with the S. Pedro de Moel section, the obtained curve is more regular and concordant with a marly sedimentary sequence. From the 18 m of the profile (top of the MLOF Mb), the GR-values decrease rapidly from 160 cps to 90 cps, characteristic of the carbonate-dominated Lemede Fm.

The radioactive elements, namely the U, show a slight increase to the top of the section accompanying the GR-response, ranging between 8 and 12 ppm. The Th and K concentrations do not reveal any particular trend. Some differences were found in the laboratory analysis. First, the U content is much lower (around 1.5-5.3 ppm), but the trends defined between both analyses continue to be quite similar. The Th readings are similar (1.6-7.8 ppm) and the K, contrarily to the U, show higher values, ranging between 1-3% (Fig. 6).

Regarding the geochemical data, the average TOC is 3 wt.%, far from the 7 wt.% observed in the Agua de Madeiros Fm at S. Pedro de Moel. It must be highlighted the occurrence of dolomite in trace amounts in some samples, and pyrite/uraninite in the richest U samples.

5. Discussion

5.1 Implications in the lithological characterization

The obtained GR values range between 26 to 210 cps (Fig. 7), which can be considered a large variation considering that only carbonated rocks were analysed (e.g.Pawellek and Aigner, 2003, 2004; Boyle, 1982). Those differences can be easily observed, not only when distinguishing different formations and members, but also within a single member (e.g. Polvoeira and PPL members).

The units with the lowest GR values and U, Th, K content, correspond to the sections with the highest carbonate content and lower clay fractions, with a predominance of limestone sequences such as the Coimbra Fm and the first meters of the PPL Mb (average values < 50 cps). The GR data in these units normally assumes blocky patterns, proved by the lower standard deviations ([delta] < 10). On the other side, the highest peaks (> 150 cps) were obtained in the organic rich sections such as the Polvoeira and MLOF members (Fig. 7), mainly supported by the precipitation of authigenic U (lowest Th/U and K/U ratios, Fig. 8). The curves of those members are characterized by irregular patterns due to the marl and limestone alternations, showing higher standard deviations ([delta] > 20). The highest Th concentrations were observed in the MLOF Mb and are consistent with a more abundant siliciclastic fraction and higher Th/U ratio (2.2) when compared to the Polvoeira Mb (Fig 8). This statement is also confirmed by their higher quartz, mica and feldspar content (obtained by XRD).

A correlation matrix for total GR and the three radioactive elements measured in outcrop is shown in Table 1. The total GR values are dominated by the U content shown by the strong correlation between the U log and the total GR counts (r = 0.77). The Th and K values show lower, but still significant correlation (r = 0.64 and 0.46 respectively). Most of the correlations with K values were not significant due to the restrictions of the portable spectrometer.

It is possible to observe in Table 2 significant differences between the outcrop and laboratory measurements. An example is observed in the Polvoeira log at 17.5 m (Fig. 4). To the highest U concentration (11 ppm) obtained among all the laboratory analysis corresponds a significantly lower radiometric signature in the outcrop survey (GR = 125, U = 6.9, Th = 1.4). This is a consequence of the lithological sequence and the outcrop geometry where a marl horizon 11 cm thick occurs between two limestone beds (with 14 and 30 cm). As the used portable equipment measures the character of approximately 0.3 [m.sup.3] of rock, the obtained value in the marl horizon will be depleted by the limestone beds with lower GR values. Another example is the Vale das Fontes Fm with big discrepancies between the U values obtained in the field and in the laboratory. Again, the lithological sequence seems to be the main reason. For instance, the Vale das Fontes Fm is mainly a marly sequence characterized by high GR values. The expected trend is to obtain super-estimated values. On the contrary, the Agua de Madeiros Fm consists of marl-limestones alternations. The measurements performed in the marly horizons are frequently affected and depleted by the proximity of the limestone beds. Also the internal calibrations of both equipments are different. Nevertheless, we emphasize the similar curves/trends between field and laboratory data. These differences highlight the importance of the laboratory measurements performed in standardized conditions (resulting in significantly stronger correlations), and the restrictions input to the GR logging in wells and outcrops.

A correlation matrix for the radiometric, mineralogical and IR data measured in laboratory (except the GR) is shown in Table 3. The majority of the correlations are statistically significant (see Sachs, 1984). The total GR shows a good correlation with the calcite (r = 0.71) higher than with the carbonate fraction (r = 0.56) mainly because the few samples with dolomite disturbe this relation. The radioactive elements, in particular Th and K, show a highly significant correlation between them (r = 0.87). These two elements are also strongly correlated with the mineral fraction, increasing together with the siliciclastic fraction and decreasing with the precipitation of carbonate confirming their tight relationship with a continental source materialized by clays, feldspar and micas (see Boyle, 1982). Based in highly significant correlations, the GR, U, Th and K values can be used as a proxy for the carbonate or siliciclastic content (see Pawellek and Aigner, 2004). However, in the studied interval, the U is the exception without any significant correlation with the different elements, due to the precipitation of authigenic U.

The carbonate and siliciclastic fractions show highly significant correlations between them (r = 0.85-0.96), except the mica where the decrease of the correlation quality is substantial. However, when the samples from the Agua de Madeiros and Vale das Fontes formations are analysed and correlated separately, two different and highly significant relationships become clear (see the example between the mica and the Th in Fig. 9). This is a consequence of different sediment supply sources for the Agua de Madeiros and Vale das Fontes formations and also independent depositional environments between S. Pedro de Moel and Peniche regions, previously suggested by Duarte et al. (2010). The less significant correlations between the siliciclastic fraction (specially the mica) and the Th and K observed in Peniche (r = 0.54 and 0.49 respectively) when compared to S. Pedro de Moel (r = 0.74 and 0.90), suggest different and intermittent sediment supplies from the eastern mainland and from the granitic Berlengas Horst to the west.

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5.2 Sequence stratigraphy and subsurface data correlation

The studied interval corresponds to two 2nd order T-R facies cycles (SUS and SP in Duarte et al., 2010; Duarte, 2007) and its correlation with the surface GR log is illustrated in the figure 10. In addition, two well logs were selected (14 A-1 and Alj-2) for further analysis and correlation with the surface data (see location in Fig. 1B). The GR spectrometry data collected at the surface allowed the subdivision of the lower Brenha Group (Witt, 1977) into four units (numbers 2, 3, 4 and 5, with the first one corresponding to the top of the Coimbra Fm), providing a better understanding of these data set (Fig. 10). The Brenha Group, described in the well reports, comprises a thick (> 1500 m) marine sequence deposited during the late Sinemurian-late Callovian time interval (see Alves et al., 2002) which is subdivided into several equivalent units at the surface (Azeredo, 2007; Azeredo et al., 2003; Duarte and Soares, 2002).

Starting from the top of the Coimbra Fm (unit 1) it is possible to observe a deepening-upward phase in the surface and subsurface GR logs, indicating a decrease in the energy of the depositional environment, linked with the development of a transgressive phase (Catuneanu, 2006; Pawellek and Aigner, 2003), that continues through a great part of the Polvoeira Mb (unit 2). The Maximum Flooding Interval (MFI) of the upper Sinemurian T-R facies cycle occurs around the 30 m of the Polvoeira section, where the high GR values (180 to 190 cps) coincide with high Th and K content, indicating maximum condensation (minimum sand input) (Fig. 4 and 10). This interval is also coincident with expressive thick black shale layers very rich in organic matter (TOC > 10% and high U). Using only total GR and U can be risky because these parameters indicate anoxia, not necessarily sea level change or condensation. This GR peak can be also correlated with the 2350 m deep of the 14 A-1 well. From this point, the regressive phase of the upper Sinemurian T-R facies cycle is characterized by the gradual decrease of the GR, suggesting an increased availability of oxygen in the depositional system, or, as suggested by Catuneanu (2006), a gradual increase in the energy. After the abrupt GR-decrease between the Polvoeira and PPL members (limit between unit 2 and 3), the GR assumes a typical blocky trend associated with a well stratified limestone sequence. This trend suggests constant (and probably low) energy levels during the sedimentation (Catuneanu, 2006; Pawellek and Aigner, 2003), although higher than in the Polvoeira Mb.

The increase of the GR and radioactive elements content starting from the 22 m of the Agua de Madeiros log (Fig. 5 and 10) dated from the uppermost Sinemurian indicates the base of the Pliensbachian T-R cycle. This trend continues in the few surface measurements done in the MLUP Mb of the Vale das Fontes Fm. In the well data the deepening-upwards trend is well expressed. The transgressive phase corresponds to almost the entire Vale das Fontes Fm (unit 4) as suggested by Duarte et al. (2010), and is also observed in the two well logs. At Peniche, Parkinson (1996) suggests two cycles of upwards-increase in clay content during the Pliensbachian interval, with the most clay-rich sediments being found at the base of the Ibex Zone and at the top of the Margaritatus Zone. A similar pattern is also observed in the GR well logs, even if not so pronounced, with the limit between the two cycles of upwards-increase being found at 2260 and 2615 m of the 14 A-1 and Alj-2 wells respectively. The MFI of this T-R facies cycle is observed at 14 m of the Portinho da Areia do Norte log, where the highest GR, U, Th and K values were obtained (Fig. 6 and 10). This interval corresponds to the last black shale package coincident with a TOC peak of 10% (see also Oliveira et al., 2006; Silva et al., 2011 and references therein). In subsurface, the correlative GR peak occurs at the 2185 m and 2555 m of the 14 A-1 and Alj-2 wells, respectively. From this point onwards, all the determined parameters start to decrease, characteristic of the regressive phase that will last until the extreme base of the Toarcian. This phase, mainly coincides with the deposition of the Lemede Fm (unit 5), which is characterized by a regular radiometric background (70 to 90 cps), represented by well stratified limestone beds.

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Besides the 2nd order T-R facies cycles, 3rd order cycles could be also tentatively identified along the studied sections. For example, the GR peaks obtained at 14 m of the Polvoeira log (figure 4) and between the 3-4 m of the Portinho da Areia do Norte section (figure 6) probably correspond to 3rd order MFI. However, for a more detailed analysis of these 3rd order cycles, a dataset with a higher resolution and a better understanding of the lateral facies variation and bathymetric changes across the basin is needed.

5.3. The uranium as a proxy for the total organic carbon content

The studied interval (upper Sinemurian-Pliensbachian), is currently regarded as one of main potential source rocks of the Lusitanian Basin. Based on its geochemical characteristics this thick marl-limestone sequence has a great potential for hydrocarbons generation. However, thermal maturation studies concluded that in these regions and in outcrop, the organic matter is immature or very early mature (Duarte et al., 2011b, 2012; ; Silva et al., 2011; Ferreira et al., 2010; Oliveira et al., 2006). Nevertheless, in the offshore sectors of the Lusitanian Basin and in the Peniche Basin (deep offshore) the petroleum system may work because the studied interval is buried more deeply.

The TOC curve shows a similar pattern to the U content (Fig. 10). The Polvoeira Mb at S. Pedro de Moel is characterized by the highest average TOC (7%) and U (6.5 ppm) values while the MLOF Mb analysed in Peniche, represents the second most important organic rich section (TOC - 3% and U - 3.3 ppm).

The U peaks of the entire interval are in part decoupled from the Th and K curves. It is also possible to observe that the Th/U ratio drops below 2.2, particularly in the Polvoeira Mb where this ratio is 0.8 (Fig. 8). After applying the formula introduced by Wignall (1994) to the U content measured mainly in marl and black shale levels, it is evident that the authigenic U assumes an important percentage in the total U content. In the Agua de Madeiros Fm at S. Pedro de Moel, the authigenic U is the main component of U (4.8 ppm of authigenic U and 1.7 ppm of detrital U). In the Vale das Fontes Fm at Peniche, the detrital U persists as the main component (1.9 ppm of detrital U against 1.3 ppm of authigenic U). The reduction and precipitation of authigenic U, especially in the Agua de Madeiros Fm, is responsible for the significant increase of the total U content in the sediments, being also the main reason for the increasing total GR counts (r = 0.77 in Table 1). Several authors (e.g., Bodin et al., 2011; Liming and Kolonic, 2003; Wignall and Myers, 1988) assume that the precipitation of authigenic U suggests that oxygen depleted conditions prevailed during the time of deposition. This constitutes one more indicator of a possible deep depositional environment that leads to an oxygen depleted period, more severe during the upper Sinemurian in the S. Pedro de Moel region than in the Pliensbachian of Peniche.

[FIGURE 10 OMITTED]

In table 4, the correlation coefficients between the total U, authigenic U, Th and the TOC are listed. In the Agua de Madeiros Fm and MLOF Mb it is noticiable a strong [U.sub.total]/TOC correlation (r = 0.72 and r = 0.64 respectively). Specifically in the Polvoeira Mb, the Th/TOC ratio also shows a significant correlation (r = 0.72, Fig. 11B). In the 44 rock samples analysed in both sections the correlation is highly significant (r = 0.72) (see Correia et al., 2011). If only the calculated authigenic U is taken into account, the correlation [U.sub.authigenic]/TOC becomes stronger, confirming the preferred precipitation of the authigenic U with the organic matter (Lining and Kolonic, 2003; Wignall and Myers, 1988). In Figure 11A, it is possible to observe three samples with anomalous U/TOC relationships that diminish the quality of the correlations. If these values are excluded, the quality of the [U.sub.authigenic]/TOC correlation increases significantly (r = 0.88). The XRD analyses of the mineral fractions show that both samples are characterized by their high content in carbonate, which Lining and Kolonic (2003) refers as one of the main disturbing factors in the U/TOC correlations.

In both sections, the U/TOC ratios are low (Fig. 11A), although slightly lower in the Vale das Fontes Fm (0.3 ppm/wt.%TOC, Fig. 11D) when compared with the same ratio in the Agua de Madeiros Fm (0.7 ppm/wt.%TOC, Fig. 11C). These results are similar to others obtained by different authors (compiled by Lining and Kolonic, 2003) in formations dated from the Lower Jurassic of Western Europe. These low ratios are consistent with carbonate-dominated systems, but also with the thermal immaturity of the source rocks as suggested by Lining et al. (2005).

6. Conclusions

1. The GR spectrometry performed in the laboratory had a great importance in all correlations established in this study due to their accuracy. Even so, the portable equipment is a quick, simple and low cost technique, with multiple applications.

2. The spectral GR logs show great similarity to the observed sedimentological changes in the studied units. The Th and K values show a strong correlation with the siliciclastic fraction. In the particular case of the mica, the correlation indicates different sediment supply sources between S. Pedro de Moel and Peniche. During the Pliensbachian, the influence of the Berlengas Horst was already present in the Peniche sectors.

3. The outcrop gamma-ray techniques are in good agreement with the Sinemurian-Pliensbachian T-R cycles helping in the identification of maximum flooding intervals. After the characterization of the radiometric signature of each unit and lithotype, these techniques are also a powerful tool to integrate subsurface and surface geology, making these techniques an important tool in oil exploration.

4. Part of the Late Sinemurian-Pliensbachian interval of the Lusitanian Basin, namely the Polvoeira and the MLOF members, have a high potential as a source rock. This is in good agreement with the high estimated contents of authigenic U (average Th/U ratio of 0.8 and 2.2, respectively) and TOC (average values of 7 wt.%TOC and 3 wt.%TOC, respectively), suggesting the deposition under oxygen depleted conditions.

5. For the entire studied interval a highly significant U/TOC correlation (r = 0.72) was obtained allowing the U content to be used as a proxy to predict the TOC in these sectors of the Lusitanian Basin. Time and resources consuming geochemical analysis can be replaced using indirect measurement of the U by GR spectrometry, not only in these outcrops, but in other sedimentary basins where similar depositional systems occur. The quality of the correlation increases (r = 0.79) if only the authigenic U component is considered. It was also confirmed that the high carbonate content can interfere with the development of a stable U/TOC relationship.

6. The U/TOC ratio of the studied samples is low (0.3-0.7 ppm/wt.%TOC), similarly to other case studies of Western Europe basins. The low U/TOC ratios are consistent with Jurassic carbonate systems, but also with the thermal immaturity of the source rocks.

[FIGURE 11 OMITTED]

http://dx.doi.org/10.5209/rev_JIGE.2012.v38.n2.40464

Acknowledgments

The authors are especially grateful to Professor Joao Graciano Mendonca-Filho for the TOC analysis undertaken at LAFO laboratory (UFRJ). The constructive comments of the two anonymous reviewers are also highly appreciated. This work is a contribution to the project "High resolution stratigraphy of the Lower Jurassic organic-rich marine series in the Lusitanian Basin", PTDC/CTE-GIX/098968/2008 (FCT-Portugal and COMPETEFEDER).

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G. G. Correia (1) *, L. V. Duarte (1), A. Pereira1, R. L. Silva (1)

(1) Departamento de Ciencias da Terra and IMAR-CMA, Faculdade de Ciencias e Tecnologia, Universidade de Coimbra, 3000-272 Coimbra, Portugal

* Corresponding autor: gil.correia@gmail.com / Tel: +351 963305338 /Fax: +351 239823603

Received: 15/02/2012 / Accepted: 04/09/2012
Table 1--Correlation matrix for radioactive elements determined by
gamma-ray spectrometry in outcrop. At 99% confidence level a
correlation coefficient of 0.339 (n = 57) is considered to be a
significant result (Sachs, 1984).

Tabla 1--Matriz de correlaciones de los elementos radiactivos
determinados por espectrometria de rayos gamma en afloramiento. Un
coeficiente de correlacion de 0.339 (n = 57) es un resultado
significativo para un nivel de confianza del 99% (Sachs, 1984).

r               [U.sub.field]  [Th.sub.field]  [K.sub.field]  Total GR

[U.sub.field]   1
[Th.sub.field]  0.50           1
[K.sub.field]   0.08           0.19            1
Total GR        0.77           0.64            0.46           1

Table 2--Relationship between lithology and the radioactive
elements concentrations obtained through field and laboratory
measurements.

Tabla 2--Relation entre la litologia y las
concentraciones de elementos radiactivos obtenidos mediante
mediciones de campo y laboratorio.

Lithologies    Formations            [U.sub.   [Th.sub.   [K.sub.
                                     field]    field]     field]

Limestone      All                   1.9       0.9        0.18
Marly          All                   6.3       2.2        0.20
  limestone
Marls          Agua de Madeiros Fm   6,3       2.9        1.00
               Vale das Fontes Fm    9.6       4.8        0.43

Black Shales   Agua de Madeiros Fm   7.6       3.6        1.06
                                     10.1      4.2        0.25
               Vale das Fontes Fm

Lithologies    n    [U.sub.   [Th.sub.   [K.sub.   n
                    lab]      lab]       lab]

Limestone      4    1.7       0.8        0.40      2
Marly          4    2.4       3.1        1.29      5
  limestone
Marls          16   6.7       5.2        2.37      11
               9    2.8       7.0        2.52      5

Black Shales   19   6.4       4.9        2.04      15
               5    3.7       4.7        1.66      5

Table 3--Correlation matrix for radioactive elements determined by
gamma-ray spectrometry (laboratory), total gamma-ray counts
obtained in the field and other elements determined by whole rock
XRD and geochemistry. At 99% confidence level a correlation
coefficient of 0.386 (n = 44) is considered to be a significant
result (Sachs, 1984).

Tabla 3--Matriz de correlaciones de los elementos radiactivos
determinados por espectrometria de rayos gamma (en laboratorio), el
total de rayos gamma obtenidos en el campo y otros elementos
determinados por difraccion de rayos X y geoquimica. Para un nivel
de confianza del 99%, un coeficiente de correlacion de 0.386 (n =
44) es un resultado significativo (Sachs, 1984).

r              Total   [U.sub.   [Th.sub.   [K.sub.   Carbonate
               GR      lab]      lab]       lab]

Total GR       1
[U.sub.lab]    0,47
[Th.sub.lab]   0.56    0.02
[K.sub.lab]    0.64    0.20      0.87
Carbonate      0.56    0.06      0.84       0.79      1
Calcite        0.71    0.18      0.74       0.84      0.86
Quartz         0.55    0.04      0.83       0.72      0.96
Mica           0.50    0.18      0.57       0.49      0.71
IR             0.55    0.20      0.65       0.78      0.62

r              Calcite   Quartz   Mica   IR

Total GR
[U.sub.lab]
[Th.sub.lab]
[K.sub.lab]
Carbonate
Calcite        1
Quartz         0.85
Mica           0.64      0.53
IR             0.75      0.69     0.30   1

Table 4--[U.sub.total]/TOC, [U.sub.antigenic]/TOC and
Th/TOC correlations obtained total authigenic for the entire
studied interval and for the Agua de Madeiros and Vale das Fontes
formations separately. At 99% conidence level a correlation
coeficient of 0.386 (n = 44), 0.471 (n = 29) and 0.641 (n = 15) is
considered to be a signiicant result (Sachs, 1984).

Tabla 4--Correlaciones [U.sub.total]/COT, [U.sub.antigenico]/COT y
Th/COT obtenidas total autigenico para todo el intervalo estudiado
y para las formaciones de Agua de Madeiros y Vale das Fontes por
separado. Para un nivel de conian- za del 99%, coeicientes de
correlacion de 0.386 (n = 44), 0.471 (n = 29) y 0.641 (n = 15) son
resultados signiicativos (Sachs, 1984).

Formations            [U.sub.        [U.sub.authigenic]/   Th/     N
                      total]/TOC r   TOC r                 TOC r

All                   0.72           0.79                  0.60    44
Agua de Madeiros Fm   0.72           0.79                  0.64    29
Vale das Fontes Fm    0.64           0.80                  0.45    15
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Author:Correia, G.G.; Duarte, L.V.; Pereira, A.; Silva, R.L.
Publication:Journal of Iberian Geology
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