Application of petrophysical methods to estimate total organic carbon in Lower Jurassic source rocks from the offshore Lusitanian Basin (Portugal)

Journal of Petroleum Science and Engineering 180 (2019) 1058–1068

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Application of petrophysical methods to estimate total organic carbon in Lower Jurassic source rocks from the offshore Lusitanian Basin (Portugal)

T

Sérgio L.R. Sêcoa,∗, Ricardo L. Silvaa,b, Neil Watsonc, Luís V. Duartea, Alcides J.S.C. Pereirad, Grant Wachb a

MARE – Marine and Environmental Sciences Centre, Department of Earth Sciences, University of Coimbra, Portugal Basin and Reservoir Lab, Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada c Atlantic Petrophysics Limited, Halifax, Nova Scotia, Canada d CITEUC – Center for Earth and Space Research, Department of Earth Sciences, University of Coimbra, Portugal b

ARTICLE INFO

ABSTRACT

Keywords: Lusitanian Basin Lower Jurassic Well-logs Passey method Source rock

Lower Jurassic outcrops in the westernmost part of the Lusitanian Basin (São Pedro de Moel area, Portugal) reveal three stratigraphic units with source rock potential: i) Unit F from the Coimbra Formation, ii) the Polvoeira Member of the Água de Madeiros Formation, and iii) the Marly Limestones with Organic-rich Facies Member of the Vale das Fontes Formation. Despite decades of research, these source rocks have not yet been characterised in the offshore areas of the Lusitanian Basin. As a consequence of the discrepancy between organic matter contents in correlative surface-subsurface intervals, this paper investigates the applicability of petrophysical methods to semi-quantitatively estimate total organic carbon (TOC) and delineate source rock intervals in Lower Jurassic (Sinemurian–Pliensbachian) units in the offshore Lusitanian Basin. This study is based on the i) high-resolution stratigraphic correlation between well 14A-1 and the São Pedro de Moel section and ii) calibration of the TOC curve calculated for well 14A-1 using the high-resolution TOC datasets from São Pedro de Moel. Sonic transit time vs resistivity cross-plot shows that several intervals of the aforementioned lithostratigraphic units deviate from the (organic) lean sediment baseline, interpreted to indicate the presence of hydrocarbons and low-density organic matter. Thermal maturity from cuttings samples were used to estimate vitrinite reflectance for each well point measurement. These values vary between 0.51 and 0.87, indicating that the studied interval has reached the oil generation window. The level of organic metamorphism is in accordance with the hydrocarbon generation window interval, varying between 9.49 and 9.76. Neutron/resistivity, density/resistivity, and sonic/resistivity cross-plots and the petrophysical-based method for TOC calculation outlined in Passey et al. (1990) demonstrate the close similarity between TOC curves calculated for well 14A-1 and TOC measurements from the São Pedro de Moel outcrops. This comparison highlights the organic-rich nature of Unit F, the Polvoeira Member and the Marly Limestones with Organic-rich Facies Member in the offshore areas of the Lusitanian Basin and their potential as hydrocarbons source intervals. This work demonstrates a feasible solution to semi-quantitatively estimate TOC in subsurface source rocks when there is large uncertainty regarding sample quality. Delineation of source rock intervals in the offshore wells of the Lusitanian Basin reduces uncertainty, minimises risk, and may aid in new exploration concepts for hydrocarbons offshore Portugal. On a broader scale, this work reinforces the importance of outcrop control to correlate and improve the current knowledge of Portugal's offshore areas.

1. Introduction One of the main risks in hydrocarbon exploration is the presence or absence of a source rock capable of generating commercially viable amounts of hydrocarbons. Source rocks are defined in the specialized

∗

literature as a rock unit containing enough organic matter of suitable chemical composition to generate and expel hydrocarbons via biogenic or thermal processes, irrespectively of its maturity (e.g. Tissot and Welte, 1984; Miles, 1994; Suárez-Ruiz et al., 2012; see Law, 1999 for classification of source rocks according to its maturity).

Corresponding author. Department of Earth Sciences, University of Coimbra, Pole II, Rua Sílvio Lima, 3030-790, Coimbra, Portugal. E-mail address: [email protected] (S.L.R. Sêco).

https://doi.org/10.1016/j.petrol.2019.05.065 Received 18 February 2019; Received in revised form 24 May 2019; Accepted 27 May 2019 Available online 28 May 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Geological setting of the Lower Jurassic in the Lusitanian Basin, Portugal. Location of the offshore well 14A-1 and the Sinemurian–Pliensbachian outcrop: 1) São Pedro de Moel (modified from Duarte et al., 2010).

et al., 2013; Silva and Duarte, 2015; Brito et al., 2017; Bruneau et al., 2018), geochemical data from several wells, kindly provided by ENMC/ UPEP (National Entity for the Fuel Market, E.P.E./Research and Exploration Unit of Petroleum Resources), indicate that offshore Lower Jurassic successions have low total organic carbon (TOC) and low generation potential (i.e. low S2 and Hydrogen Index). Discrepancies between outcrop and well geochemical datasets result from biostratigraphic uncertainty and the nature of the well samples (cuttings) used in geochemical studies. Common issues with geochemical data derived from cuttings are dilution (dilution of organic-rich facies by organic-lean lithologies), contamination by drilling fluids, and cavings. All these processes can result in underestimated TOC and RockEval values (i.e. S2 and Hydrogen Index) (e.g. Waples, 1985; Bordenave, 1993; Jarvie, 2012). As aforementioned, several Lower Jurassic organic-rich intervals and source rocks are exposed onshore in the LB, i.e. Unit F of the Coimbra Fm, the Polvoeira Member (Mb) of the Água de Madeiros Fm, and the Marly Limestone with Organic-Rich Facies Mb of the Vale das Fontes Fm (Oliveira et al., 2006; F. Silva et al., 2010; Duarte et al., 2010, 2012; 2013, 2017; Silva et al., 2011, 2012; 2013, 2015; Correia et al., 2012; Poças Ribeiro et al., 2013; Silva and Duarte, 2015; Brito et al., 2017). Recently, Sêco et al. (2018) presented for the first time a high-resolution stratigraphic framework for the Lower Jurassic in several offshore wells, based on a detailed comparison between lithologic, biostratigraphic, and gamma-ray attributes. Before Sêco et al. (2018) study, these units were assumed to be included in the (seismic scale) Coimbra and Brenha informal formations (e.g. Witt, 1977; Alves et al., 2002; Pereira and Alves, 2012), with no stratigraphic detail on such a potentially prolific stratigraphic source rock interval.

Fig. 2. Onshore and offshore lithostratigraphic chart for the Upper Triassic–lowermost Toarcian series of the Lusitanian Basin, modified from Witt (1977), Duarte and Soares (2002) and Duarte et al. (2010, 2012, 2014a, 2014b).

In Portugal, the occurrence of hydrocarbons in several wells, outcrops, and seeps prove that petroleum systems exist in the Lusitanian Basin (LB) (sensu Magoon and Beaumont, 1999). Brito et al. (2017) demonstrated that oil recovered from Lower Jurassic limestones during drillstem tests (1.8 barrels of oil) in the offshore 14A-1 well could be correlated with the organic-rich Lower Jurassic Coimbra Formation (Fm), which crops out at São Pedro de Moel (Fig. 1). However, and despite indication of Lower Jurassic source rocks onshore (e.g. Oliveira et al., 2006; F. Silva et al., 2010; Duarte et al., 2010, 2012; 2013, 2017; Silva et al., 2011, 2012; 2013, 2015; Correia et al., 2012; Poças Ribeiro 1059

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Fig. 3. TOC distribution and gamma-ray records of surface-subsurface correlation between São Pedro de Moel type section and 14A-1 well-log (modified from Sêco et al., 2018). Stratigraphic chart and cycles modified from Duarte et al. (2010, 2014a, 2014b). Detail TOC analysis of the units by Duarte et al. (2010, 2012, 2013, 2017), Correia (2011), Correia et al. (2012), Poças Ribeiro et al. (2013) and Brito et al. (2017). 1060

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Fig. 4. Sonic transit time vs resistivity cross-plot showing the background delimitation of the lean sediment baseline.

Stapel et al., 1996). The Triassic–lowermost Jurassic evaporitic deposits of the Dagorda formation (e.g. Zbyszewski, 1959; Palain, 1976) are overlain by the Sinemurian to Callovian carbonate succession (e.g. Azerêdo et al., 2003). The westernmost outcrops of Lower Sinemurian–Pliensbachian age (Fig. 2) are formally defined, corresponding to the Coimbra, Água de Madeiros, Vale das Fontes and Lemede formations (see Duarte and Soares, 2002; Duarte, 2007). The São Pedro de Moel sections (the focus of this work) comprise the most complete succession of the Coimbra Formation (Fm) (Units A to H; Azerêdo et al., 2010; Duarte et al., 2014b). This formation is represented at the base by a dolomitic series interfingering vertically with marl-limestone successions and several organic-rich levels (e.g. Azerêdo et al., 2010; Duarte et al., 2013, 2014b, 2017). The Água de Madeiros Fm comprises two members: the Polvoeira Member (Mb), dominated by marl–limestone alternations rich in organic matter, overlain by the microsparitic limestones of the Praia da Pedra Lisa (PPL) Mb (Duarte et al., 2010, 2014a). The Vale das Fontes Fm is represented by a thick marl–limestone succession with some organic-rich intervals (Duarte et al., 2010; Silva et al., 2012). This unit is subdivided into three members: i) Marls and limestones with Uptonia and Pentacrinus (MLUP) Mb, ii) Lumpy marls and limestones (LML) Mb, and iii) Marly Limestone with Organic-Rich Facies Mb, which include abundant organicrich facies. The Lemede Fm is characterised by limestone-dominated successions. Above, the Toarcian is generally characterised by marl–limestone successions of the São Gião Fm (see Duarte, 1997; Duarte and Soares, 2002).

In this paper and based on the high-resolution correlation between the São Pedro de Moel section and well 14A-1 (Sêco et al., 2018), we study the applicability of petrophysical methods of TOC estimation to the investigation of the Lower Jurassic (Sinemurian–Pliensbachian) succession in the offshore LB. Many studies have used wireline logs to quantify TOC in subsurface organic-rich intervals, and are mainly based on one of the following methods: i) Schmoker (1981), determined from gamma-ray log; ii) Schmoker and Hester (1983), determined from density log; iii) Passey et al. (1990), the estimation is based on overlaying of a properly scaled porosity log (sonic, density and neutron) on a resistivity curve, and iv) Carpentier et al. (1991), based on sonic and resistivity cross-plot. Petrophysical data in conjunction with conventional geochemical analysis is a very powerful technique used to predict TOC, organic matter type and maturity from wells (e.g. Issler et al., 2002; H. Zhao et al., 2007; Pemper et al., 2009; LeCompte and Hursan, 2010; Lashin and Mogren, 2012; Liu et al., 2012; Combez et al., 2014; Euzen et al., 2014; Xuanjun et al., 2015; Rouse and Houseknecht, 2016; Shi et al., 2016; Wang et al., 2016; P. Zhao et al., 2016, 2017; Nie et al., 2017). In this study, the following methodology will be used in the Lower Jurassic interval of well 14A-1: i) qualitative petrophysical analysis to select and quantify organic-rich intervals using a sonic vs resistivity cross-plot; ii) detailed investigation of maturity level; iii) semi-quantitative estimation of TOC contents in the selected intervals by Passey et al. (1990) method, calibrated with the São Pedro de Moel type section for TOC variation. The main goal of this paper is to identify in well 14A-1 the known organic-rich and source rock lithostratigraphic units defined in the onshore LB (i.e. Duarte and Soares, 2002; Duarte et al., 2010). Clear delineation of source rock intervals in the offshore wells of the LB reduces uncertainty, minimises risk, and may aid in new exploration concepts for hydrocarbons offshore Portugal. Of a broader significance, this work reinforces the importance of outcrop control to correlate and improve our knowledge of offshore prospects.

2.1. The Lower Jurassic interval in well 14A-1: a synthesis The wildcat well 14A-1 was drilled in 1975 (N 4420710 m; E 497360 m, Fig. 1), about 25 km north of São Pedro de Moel. Well 14A-1 reached a total depth of 2862 m measured from the drill floor (mDF) and drilled ∼781 m of Lower Jurassic sediments (drilling report). Drill stem tests recovered about 1.8 barrels of oil (33–37° API) from a Lower Jurassic limestone between 2332 and 2404 mDF. The entire Lower Jurassic exhibits variable gas concentration. According to the well completion report, stratigraphy was based on chronostratigraphy and comparisons with the general seismic-scale informal lithostratigraphic chart of Witt (1977) (Fig. 2). A geological description and the scarce biostratigraphy data are referenced in both

2. Geologic background The origin of the LB is related to the opening of the North Atlantic Ocean (e.g. Montenat et al., 1988; Wilson et al., 1989; Alves et al., 2002). Basin infill comprises the interval between the Upper Triassic and Lower Cretaceous (e.g. Montenat et al., 1988; Wilson et al., 1989; 1061

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Fig. 5. Sinemurian–Pliensbachian offshore 14A-1 well. Track 1: 14A-1 depth, chrono and lithostratigraphic correlation. Track 2: Gamma-ray, Caliper and Spontaneous Potential. Track 3: Tmax; Track 4: Vitrinite Reflectance (Ro: minimum, mean and maximum), Vitrinite Reflectance Line (Ro Line) and Jarvie's equation application (VRo). Track 5: Geochemical data (Hydrogen Index, Oxygen Index, Production Index, S1, S2 and S3). Track 6: Sonic/Resistivity (LLD) overlay. Track 7: Sonic/Resistivity (LLS) overlay. Track 8: Density (ρb)/Resistivity (LLD) overlay. Track 9: Neutron porosity/Resistivity (LLD) overlay. Note: red colour fill is the limit between the minimum and maximum Ro at each analysis point (Track 4). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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2012, 2013, 2017) in São Pedro de Moel reference section allows for a new approach for the TOC calibration of possible organic-rich intervals in well 14A-1.

Table 1 Geochemical data from 14A-1 drilling report and vitrinite reflectance from Jarvie's formula. Geochemical data n = 48 n = 49

Tmax (°C) S1 (mg HC/g rock) S2 (mg HC/g rock) S3 (mg CO2/g rock) HI (mg HC/g TOC) OI (mg CO2/g TOC) PI [S1/(S1+S2)] n > 14 Ro (%) %VRo (Jarvie's equation)

Min.

Max.

426 0.01 0.01 0.01 37 50 0.06 0.72 0.51

446 0.16 1.75 2.30 156 589 1.00 1.84 0.87

3.2. TOC estimation 3.2.1. Testing sonic transit time vs resistivity In order to improve the identification of possible resource plays (see Bowman, 2010), we apply the concept of sonic transit time vs resistivity to calculate a new pseudo-sonic dataset based on resistivity curvilinear regression and highlight units or stratigraphic intervals with “excess resistivity”. An assessment of the usefulness of wireline data to estimate TOC contents in well 14A-1 was conducted by using petrophysical proprieties of sonic and resistivity methods. In a sonic transit time vs resistivity cross-plot, deviation from the lean sediment baseline suggests “excess resistivity” (presence of hydrocarbons in pore spaces) and “excess slowness” (presence of low velocity, high transit time organic matter). Comparison with gamma-ray allows differentiation between potential reservoirs (low gamma-ray) and organic-rich intervals (high gamma-ray). In the sonic transit time vs resistivity cross-plot, the lean sediment baseline (curvilinear regression equation) is estimated using a threeparameter model (a, b and c) as follows (Eq. (1)):

Min. – Minimum; Max. – Maximum; HI – Hydrogen Index; OI – Oxygen Index; PI – Production Index.

Y = a + b/X + c x ln(X)

(1)

where X is the resistivity-data and Y is the sonic transit time estimated through resistivity-data. These parameters are fit to a set of data with the Excel Solver add-in, to setting up the objective and adjusting the parameter values to minimise the sum of squared errors. This first observation level allows to quickly select intervals for further analysis using the Passey et al. (1990) method.

Fig. 6. Plot of vitrinite reflectance calculated for each well-depth point (VRo(eq)) vs. level of organic metamorphism (LOM).

drilling and completion reports; a summary is presented here (see Sêco et al., 2018): The top of Dagorda fm (Dolomite mb; Hettangian) consists of interbedded dolostones, anhydride and argillaceous facies between 2527 and 2552 mDF. The Coimbra fm (Sinemurian) (between 2376 and 2527 mDF) comprise a thick calcareous interval interbedded by marls and shales intervals. Argillaceous dolostone and dolomitic limestone alternations characterise the base of the unit. Occasionally lignite and pyrite are also observed in the uppermost part of the Coimbra fm, coinciding with an increase in argillaceous levels. The interval between 2376 and 1771 mDF is assigned to the Brenha fm. This unit presents a considerable lithological variation and consists of thick marl and shalelimestone alternations (see also Pereira and Alves, 2012). Some lignite and disseminated pyrite are observed and coincide mostly with marls and/or shales intervals or increase in argillaceous content.

3.2.2. Adapting Passey et al. (1990) method to estimate TOC Estimation of TOC in well 14A-1 was performed using the Passey et al. (1990) method. In this method, a scaled porosity log (e.g. sonic transit time curve) is overlain on a resistivity curve. According to Passey et al. (1990) interpretation, in water-saturated organic-lean shale intervals, these two logs are parallel and lay to top of each other and together comprise the baseline. When baselined this fashion in lean water-saturated intervals, organic-rich intervals (or hydrocarbon reservoirs) are characterised by a separation between these two curves (the ΔLogR separation, Equation (1)) as the porosity curve responds to the presence of low-density and low-velocity kerogen and the resistivity curve responds to the formation pore fluid. In immature source rocks (i.e. absence of generated hydrocarbons), the curve separation is due to the porosity curve. In mature source rocks, in addition to the porosity curve response, the resistivity increases because of the presence of hydrocarbons in pore spaces. Following Passey et al. (1990), ΔLogR is calculated from the sonic/ resistivity overlay using the following equation (Eq. (2)):

3. Material and methods 3.1. Incorporating previous and current lithostratigraphic controls The drilling report and geochemical (TOC, Rock-Eval Pyrolysis, and vitrinite reflectance) and petrophysical datasets from well 14A-1 were provided by the ENMC/UPEP. Based on the detailed onshore lithostratigraphic nomenclature of Duarte and Soares (2002), Sêco et al. (2018) presented a high-resolution lithostratigraphic and sequence stratigraphic framework for the Sinemurian–Pliensbachian successions offshore of the LB. Supported by the gamma-ray correlation between the São Pedro de Moel outcrops and well 14A-1 (Figs. 1–3, see also Sêco et al., 2018), a petrophysical analysis was undertaken. Wireline logs, geochemical analysis, and petrophysical analysis curves were incorporated into a Techlog project (Techlog software was provided by Schlumberger to the Basin and Reservoir Laboratory, Dalhousie University, Canada). High-resolution source rock potential studies of Duarte et al. (2010,

ΔLogR = log10 (R/Rbaseline) + 0.02 x (Δt-Δtbaseline)

(2)

where R (ohm-m) is the value of the resistivity curve, Rbaseline (ohm-m) is the value of the resistivity curve at the baseline, 0.02 is based on the ratio of −50 μs/ft per one resistivity cycle, Δt (μs/ft) is the value of the sonic curve, and Δtbaseline (μs/ft) is the value of the sonic curve at the baseline. Combining the ΔLogR response (see Passey et al., 1990) with the gamma-ray curve, organic-rich intervals are identified and selected for further analysis. As in Passey et al. (1990), neutron and density were also used to calculate ΔLogRNeu and ΔLogRDen. These TOC profiles are calculated based on ΔLogR (Passey et al., 1990) using the following equation (Eq. (3)): 1063

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Fig. 7. Calculated total organic carbon (wt.%) from sonic/resistivity, density/resistivity and neutron/resistivity cross-plots in well 14A-1.

TOC = (ΔLogR) x 10(2.297

– 0.1688 x LOM)

(3)

where TOC is the total organic carbon content in wt.%, and LOM is the level of organic metamorphism. In the absence of a reliable and detailed %VRo dataset, the equation (Eq. (4)) of Jarvie et al. (2001, 2005) can be used to calculate %VRo from Tmax. %VRo(eq) = 0.0180 x Tmax – 7.16

(4)

where %VRo(eq) is the estimated vitrinite reflectance value and Tmax is derived from rock-Eval Pyrolysis (in °C). Estimation of %VRo(eq) from LOM and vice-versa is carried out LeCompte and Hursan (2010) equation (Eq. (5)): %VRo(eq) = −0.0039 LOM3 + 0.149 LOM2 – 1.5688 LOM + 5.5173 (5) where X and Y is the LOM and %VRo(eq), respectively. 4. Results and discussion

Fig. 8. Outcrop photographs of the São Pedro de Moel region described. A) Praia dos Pescadores section, showing the Unit F with an expressive marly interval and the transition to unit G of Coimbra Fm. B) Polvoeira Mb of the Água de Madeiros Fm in the Praia da Pedra do Ouro section exhibiting black shales levels. C) The top of Polvoeira Mb and basal part of PPL Mb transition at Água de Madeiros section. D) The top of Marly Limestones with Organic-rich Facies Mb of Vale da Fontes Formation displaying an expressive and thick marly interval in the Praia da Pedra do Ouro section.

4.1. Evaluating wireline response to source rock intervals Sonic transit time vs resistivity cross-plot shows data points with excess resistivity (Fig. 4). The lean sediment baseline for each depth increment of resistivity curve (LLD) in well 14A-1 was estimated (see section 3.2.1) by the following equation (Eq. (6), see section 3.2.1.):

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Table 2 Optimal correction factors applied for the total organic carbon calculation in the sonic/resistivity, density/resistivity and neutron/resistivity cross-plots. Correction factor Formation

Member

Depth (m)

Vale das Fontes Água de Madeiros Coimbra

MLOF Polvoeira U.H U.G U.F U.E

2174.2 2338.2 2376.2 2471.0 2474.4 2479.8 2482.0 2487.6 2488.8 2490.0

U.D

to to to to to to to to to to

2190.0 2376.0 2380.2 2474.2 2479.6 2481.8 2487.4 2488.6 2489.8 2490.6

ΔLogRSon

ΔLogRDen

ΔLogRNeu

5.0 2.75 0.73 2.38 1.85 1.4 0.1 1.57 0.1 2.85

2.5 8.3 0.8 3.6 3.8 1.86 0.1 3.59 – –

2.1 5.25 1.4 6.03 3.65 2.65 0.1 3.69 – –

MLOF – Marly Limestones with Organic-rich Facies Member; U. – Unit.

lean sediment baseline +0.0010 x LLD2

=

logarithmic resistivity cycles (0.06, equation (2)). Based on Passey et al. (1990) method, baselines were established in lean shale intervals (Fig. 5 and Appendix A). Regarding LOM calculation, a derived equation from LeCompte and Hursan (2010, see section 3.2.2.) was estimated (Eq. (8)):

48.449980491–331.592123643/LLD (6)

The obtained lean sediment baseline curve highlights the “excess resistivity” and “excess slowness” of Unit F, the Polvoeira Mb, and the Marly Limestones with Organic-rich Facies Mb (Fig. 4 and Supplementary Material – Appendix A). The theoretical deviation from the lean sediment baseline curve due to the presence of mature organic matter (higher resistivity and transit time) is consistent with the results in Fig. 4 and is more prominent in the Polvoeira Mb. The results in Fig. 4 indicate that organic-rich intervals in well 14A-1 can be differentiated from organic-lean intervals using petrophysical methods.

LOM = (0.8725 x %VRo(eq)3)–(4.359 x %VRo(eq)2)+(10.065 x % VRo(eq))+4.09 (8) Calculated LOM vary between 9.49 and 9.76 (Appendix A) and agree with the hydrocarbon generation window interval (see Passey et al., 1990). LOM data are within the reliable interval determined for the calculation according to the %VRo(eq)/LOM ratio (Fig. 6). Based on calculated ΔLogRSon, ΔLogRDen and ΔLogRNeu, the empirical equation (Eq. (3)) of Passey et al. (1990) was used to calculate TOC curves for each studied interval (Fig. 7). Because of the low resolution and mismatch between geochemical data from well 14A-1 and outcrop, and the similarity (thickness and gamma-ray response) between the well and São Pedro de Moel outcrops (Figs. 3 and 8; Sêco et al., 2018), calibration of calculated TOC curves was performed using outcrop TOC data in Duarte et al. (2010, 2012, 2013, 2017), Correia et al. (2012), Poças Ribeiro et al. (2013), and Brito et al. (2017) (Fig. 7 and Table 2). Since TOC data are not from the well, the calibrated TOC curve will only be considered to be a semi-quantitative solution. The overall shapes of the TOC curves for the well 14A-1 per studied unit are in good agreement with the shapes of the TOC curves in the high-resolution datasets from São Pedro de Moel (Figs. 7 and 9 and Appendix A), suggesting similarity regarding the stratigraphic distribution of TOC. Adding to Brito et al. (2017) findings (i.e. that Unit F is a source rock in the offshore), we postulate that despite the semiquantitative nature of the calculated TOC curves for the well 14A-1, these curves highlight the organic-rich nature of Unit F, the Polvoeira Mb, and the Marly Limestones with Organic-rich Facies Mb lithostratigraphic units in the offshore and their potential as source rocks.

4.2. Estimation of thermal maturity level Tmax values from the well 14A-1 drilling report were used for the estimation of the level of organic metamorphism (LOM) (section 3.2.2.). Rock-Eval Pyrolysis Tmax varies between 426 and 446 °C; the few measured %VRo samples exhibits range between 0.72 and 1.84 (unpublished geochemical report; Beicip-Franlab, 1996). Based on the calculation from Tmax data from well 14A-1, %VRo(eq) varies between 0.51 and 0.87 (Fig. 5 and Table 1), suggesting that the studied interval has reached the oil generation window (Peters, 1986; Jarvie et al., 2005). The %VRo(eq) linear regression (Fig. 5, Table 1, and Eq. (7)) was used to calculated %VRo for each wireline well point measurement (Supplementary Material – Appendix A) and transformed to LOM using LeCompte and Hursan (2010) equation (see section 3.2.2.). %VRo(eq) = (0.0001464427 x Depth + 0.4190316)

(7)

4.3. Integrating well-log data and semi-quantitative TOC determinations ΔLogRSon, ΔLogRDen, and ΔLogRNeu for Unit F, Polvoeira Mb, and Marly Limestones with Organic-rich Facies Mb in well 14A-1 are presented in Fig. 5 (calculations and dataset are shown as Supplementary Material in Appendix A). For the studied intervals, sonic transit-time and resistivity curves relative scaling is 200 us/ft and 1000 Ω-m with five (5) logarithmic resistivity cycles (0.025, equation (2)). Both resistivity curves, LLD and LLS, are shown to highlight significant differences in the ΔLogRSon response. The only significant difference in the studied intervals is the slight decrease of the electrical resistivity recorded in the LLS (Fig. 5), which may be due to the inclusion of an increased component of well drilling fluid conductivity in the tool response function. The resistivity curve scale for ΔLogRDen and ΔLogRNeu is 10000 Ω-m. For the bulk density curve (ρb), the scaling value is 1.6 g/ cm3 and six (6) logarithmic resistivity cycles (3.75, equation (2)). The fractional porosity scale for the neutron curve (ϕN) is 100% and six (6)

4.3.1. Unit F of the Coimbra Formation The organic-rich nature of Unit F in well 14A-1 is demonstrated using the method of Passey et al. (1990). Figs. 7 and 9 illustrates the calculated TOC curves using ΔLogRSon, ΔLogRDen and ΔLogRNeu and show a comparable pattern with the TOC profile from São Pedro de Moel. Unit F of the Coimbra Fm (Fig. 8 A) is recognised as a source rock (Duarte et al., 2013, 2014b; Poças Ribeiro et al., 2013; Brito et al., 2017). Outcrop kerogen assemblages in this unit are dominated by Amorphous Organic Matter (showing a high abundance of microbial mat-related particles) (Poças Ribeiro et al., 2013). HI above 600 mg HC/g TOC (Brito et al., 2017) and the abundance microbial mat-related kerogen particles in Unit F suggests the presence of Type I kerogens and 1065

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Fig. 9. Comparison between TOC profiles and gamma-ray records of surface-subsurface correlation between the São Pedro de Moel type section and the well 14A-1 (modified from Sêco et al., 2018). Stratigraphic chart is modified from Duarte et al. (2010, 2014a, 2014b).

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a potential source of light crude oil (Peters and Cassa, 1994; Duarte et al., 2013). Brito et al. (2017) demonstrated that oil recovered (33–37° API) between 2332 and 2404 m in well 14A-1 correlates with the Coimbra Fm outcrop at São Pedro de Moel. Our study shows a good agreement between the TOC curves for Unit F from onshore and offshore, confirming that a Coimbra Fm source rock exists in the offshore and sourced the oil recovered from well 14A-1.

Natural Endogenous Resources of the Centro Region (Centro 2020, Centro-01-0145-FEDER-000007, Portugal), and the Basin and Reservoir Lab of the Department of Earth Sciences, (Dalhousie University, Canada, GW - Principal Investigator). LVD was supported by FCT, through the strategic project UID/MAR/04292/2019 granted to the Marine and Environmental Sciences Centre (MARE, Portugal). We also kindly acknowledge the Executive Editor Tahar Aïfa, Tiago M. Alves, and an anonymous referee for their contributions that significantly improved the manuscript.

4.3.2. The Polvoeira Member of the Água de Madeiros Formation The Polvoeira Member (Fig. 8 B and 8 C) of the Água de Madeiros Fm is similar in thickness to the São Pedro de Moel section (Fig. 3). The sonic transit time vs resistivity cross-plot recognised excess resistivity in the Polvoeira Mb (Fig. 4). Based on TOC contents, this unit is mainly divided into two subunits, Interval I and II (sensu Duarte et al., 2012), with a decrease in absolute TOC at the top of the unit (see Duarte et al., 2010, 2012; and Fig. 7). The response of ΔLogRSon, ΔLogRDen and ΔLogRNeu in the Polvoeira Mb display the expected positive separation (Fig. 5), and a good correlation is observed between log-derived TOC curve with measured field TOC pattern, i.e. Interval I and II (Figs. 7 and 9).

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.05.065. References Alves, T.M., Gawthorpe, R.L., Hunt, D.W., Monteiro, J.H., 2002. Jurassic tectono-sedimentary evolution of the northern Lusitanian Basin (offshore Portugal). Mar. Pet. Geol. 19, 727–754. Azerêdo, A.C., Duarte, L.V., Henriques, M.H., Manuppella, G., 2003. Da dinâmica continental no Triásico aos mares do Jurássico Inferior e Médio. Cadernos da Geologia de Portugal. Instituto Geológico e Mineiro 43 pp. Azerêdo, A.C., Silva, R.L., Duarte, L.V., Cabral, M.C., 2010. Subtidal stromatolites from the sinemurian of the Lusitanian Basin (Portugal). Facies 56, 211–230. Beicip-Franlab, 1996. Geochemical evaluation of Lusitanian and Porto basins. Internal report of Unidade de Pesquisa e Exploração de Recursos Petrolíferos, Entidade Nacional para o Mercado de Combustíveis. Ministério da Economia, Lisboa, Portugal (unpublished report). Bordenave, M.L., 1993. Screening techniques for source rock evaluation. Appl. Petrol. Geochem. 217–278. Bowman, T., 2010. Direct method for determining organic shale potential from porosity and resistivity logs to identify possible resource plays. In: AAPG Annual Convention, New Orleans, LA, pp. 11–14. Brito, M., Rodrigues, R., Baptista, R., Duarte, L.V., Azerêdo, A.C., Jones, C.M., 2017. Geochemical characterization of oils and their correlation with Jurassic source rocks from the Lusitanian Basin (Portugal). Mar. Pet. Geol. 85, 151–176. Bruneau, B., Chauveau, B., Duarte, L.V., Desaubliaux, G., Moretti, I., Baudin, F., 2018. 3D numerical modelling of marine organic matter distribution: example of the early Jurassic sequences of the Lusitanian Basin (Portugal). Basin Res. 30, 101–123. Carpentier, B., Huc, A.Y., Bessereau, G., 1991. Wireline logging and source rocks – estimations of organic carbon content by the CARBOLOG method. Log. Anal. 32 (3), 279–297. Combez, V., Rohais, S., Baudin, F., Euzen, T., Petrovic, M., 2014. Organic content variations and links to sequence stratigraphy in the montney and doig formations (alberta/British columbia). GeoConvention. Focus 1–4. Correia, G., 2011. Aplicações da Espectrometria de Raios Gama no Estudo de Séries Carbonatadas. O caso do Jurássico Inferior da Bacia Lusitânica. Master Thesis in Geosciences,unpublished,. Departamento de Ciências da Terra da Universidade de Coimbra, Coimbra, pp. 97. Correia, G., Duarte, L.V., Pereira, A., Silva, R.L., 2012. Outcrop spectral gamma-ray: applications to the sinemurian-pliensbachian organic-rich facies of the Lusitanian Basin (Portugal). J. Iber. Geol. 38 (2), 373–388. Duarte, L.V., 1997. Facies Analysis and Sequential Evolution of the Toarcian-Lower Aalenian Series in the Lusitanian Basin (Portugal), vol. 83. Comunicações do Instituto Geológico e Mineiro, pp. 65–94. Duarte, L.V., 2007. Lithostratigraphy, Sequence Stratigraphy and Depositional Setting of the Pliensbachian and Toarcian Series in the Lusitanian Basin (Portugal), vol. 16. Ciências da Terra (UNL), Lisboa, pp. 17–23. Duarte, L.V., Soares, A.F., 2002. Litostratigrafia das séries margo-calcárias do Jurássico inferior da Bacia Lusitânica (Portugal), vol. 89. Comunicações Instituto Geológico e Mineiro, pp. 135–154. Duarte, L.V., Comas-Rengifo, M.J., Silva, R.L., Paredes, R., Goy, A., 2014a. Carbon isotope stratigraphy and ammonite biochronostratigraphy across the SinemurianPliensbachian boundary in the western Iberian margin. Bull. Geosci. 89 (4), 719–736. Duarte, L.V., Silva, R.L., Azerêdo, A.C., Paredes, R., Rita, P., 2014b. A Formação de Coimbra na região de S. Pedro de Moel (Oeste de Portugal). Caracterização litológica, definição litostratigráfica e interpretação sequencial. Comunicações Geológicas 101, 421–425 Especial I. Duarte, L.V., Silva, R.L., Mendonça Filho, J.G., 2013. Variação do COT e pirólise RockEval do Jurássico Inferior da região de S. Pedro de Moel. Potential de geração de hidrocarbonetos. Comunicações Geológicas 100, 107–111 Especial I. Duarte, L.V., Silva, R.L., Mendonça Filho, J.G., Azerêdo, A.C., Paredes, R., 2017. Total organic carbon content and carbon stable Isotopes in the Sinemurian shallow-water carbonates (Coimbra Formation) of the Lusitanian Basin. In: Portugal. 33rd International Meeting of Sedimentology, Book of Abstracts, Toulouse 10-12 October, pp. 255. Duarte, L.V., Silva, R.L., Mendonça Filho, J.G., Ribeiro, N.P., Chagas, R.B.A., 2012. Highresolution stratigraphy, palynofacies and source rock potential of the Água de Madeiros formation (lower jurassic), Lusitanian Basin, Portugal. J. Pet. Geol. 35 (2),

4.3.3. The marl-limestone with organic-rich Facies Member of the Vale das Fontes Formation The organic-rich intervals of Marly Limestones with Organic-rich Facies Mb are expressive in the top of the unit (see Duarte et al., 2010, 2013; Silva et al., 2012) (Fig. 8 D). The TOC content of this unit, when compared with the Polvoeira Mb, is generally lower. Although a good response is obtained from the application of the Passey et al. (1990) method in this unit, as regards ΔLogRSon and ΔLogRNeu, the log-calibrated TOC for the studied interval presents a better correlation with the TOC profile measured in São Pedro de Moel than ΔLogRDen response (Figs. 7 and 9). 5. Conclusions This paper presents a new concept to estimate semi-quantitative TOC in subsurface source rocks based on outcrop-subsurface correlation, outcrop TOC calibration, and improvements in both geochemical and petrophysical techniques. Petrophysical methods identify Unit F, the Polvoeira Mb and the Marly Limestones with Organic-rich Facies Mb as organic-rich intervals in the Sinemurian–Pliensbachian succession of the offshore well 14A-1 from the LB. The overall shapes of the TOC curves calculated for the well 14A-1 per studied unit are in good agreement with the shape of the TOC curve determined in the São Pedro de Moel outcrop (Figs. 7 and 9 and Appendix A), thus suggesting marked similarities regarding the stratigraphic distribution of TOC contents. Adding to Brito et al. (2017) findings, we suggest that, despite the semi-quantitative nature of the calculated TOC curves for well 14A-1, this same dataset highlights the organic-rich nature of Unit F, the Polvoeira Mb, and the Marly Limestones with Organic-rich Facies Mb in the offshore LB and their potential as source rocks. The importance of the stratigraphic and organic geochemistry studies from the São Pedro de Moel outcrop to i) produce a high-resolution correlation with well 14A-1, ii) estimate the semi-quantitative TOC contents in the selected intervals in well 14A-1, and iii) understand the main potential marine source rocks in the offshore of the Lusitanian Basin is also demonstrated here. Acknowledgements The authors acknowledge the financial and technical support provided by the ENMC/UPEP (Portugal), the Natural Radioactivity Laboratory of the Department of Earth Sciences (Universidade de Coimbra, Portugal), the Project ReNATURE - Valorization of the 1067

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