Geochemical characterization of oils and their correlation with Jurassic source rocks from the Lusitanian Basin (Portugal)

doi:10.1016/j.marpetgeo.2017.05.010

Marine and Petroleum Geology

Volume 85, August 2017, Pages 151-176

https://doi.org/10.1016/j.marpetgeo.2017.05.010 Get rights and content

Highlights

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Coimbra Formation is the source rock of the northern sector oils.
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Cabaços and Montejunto formations are the source rocks of the central sector oils.
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Cabaços Formation is the source rock of the southern sector oil shows.

Abstract

Seeking to identify the oils groups accumulated in the Jurassic of the Lusitanian Basin and the source rock of each group, stable carbon isotope and gas chromatography coupled with mass spectrometry analyses were performed in oils and oil shows from the main discoveries, and on representative organic extracts from the potential source rocks, selected based on previous works and data obtained by total organic carbon and Rock-Eval pyrolysis techniques. The geochemical comparison between the oils, and between the oils and the organic extracts, allowed the identification of three oil groups, whose differences depend on their source rocks: oils generated at the Coimbra Formation (lower-upper Sinemurian) and accumulated in the same formation and in the Água de Madeiros Formation (upper Sinemurian-lower Pliensbachian) in the northern sector of the basin; oils originated from the top of the Cabaços Formation (middle Oxfordian) and accumulated in the Montejunto (middle-upper Oxfordian) and Abadia (lower-upper Kimmeridgian) formations, in the central and southern sectors of the basin; and oil generated and accumulated at the base of the Montejunto Formation in the central sector of the basin. The geochemical correlations between the oils and the organic extracts allowed the identification of the source rocks of the different accumulations of the Jurassic succession, allowing further guidance to the petroleum exploration in the Lusitanian Basin.

Introduction

In the Lusitanian Basin, located in western-central Portugal (Fig. 1), non-commercial oil accumulations were discovered in wells drilled in the north and central sectors, and oil shows were found in wells drilled in the southern sector (Baptista, 2002).

Except for data in unpublished reports (e.g. Core Lab, 1983, Beicip-Franlab, 1996), there is only one work available (Spigolon et al., 2010) about oil-source rock correlation in the Lusitanian Basin. According to that study, the oils recovered from Lower Jurassic reservoirs in the northern sector were generated from organic matter from the Lower Jurassic, whereas organic matter from Cabaços Formation (Fm.) (middle Oxfordian; Kullberg and Rocha, 2014a) was the source rock for oils found in Upper Jurassic reservoirs in the central sector of the basin (Fig. 2). As to the origin of the oil shows of the southern sector, recovered in Upper Jurassic reservoirs (UPEP Archives; Fig. 2), nothing can be said, since they were never studied.

Prior published works (Oliveira et al., 2006, Silva et al., 2010, Silva et al., 2011, Silva et al., 2012, Silva et al., 2015, Duarte et al., 2010, Duarte et al., 2012, Duarte et al., 2013, Spigolon et al., 2011, Correia et al., 2012, Poças Ribeiro et al., 2013, Gonçalves et al., 2015, Silva and Duarte, 2015) recognized that the potential source rocks of the oils found in the Jurassic of the Lusitanian Basin occur as follows: in the northern sector, in the Coimbra Fm. (lower-upper Sinemurian; Duarte et al., 2014a), Polvoeira Member (Mb.) of Água de Madeiros Fm (upper Sinemurian-lower Pliensbachian; Duarte et al., 2014b) and in the Marly limestones with organic-rich facies (MLOF) Mb. of the Vale das Fontes Fm. (lower-upper Pliensbachian; Duarte and Soares, 2002); and in the central sector, in levels equivalent to the Moleanos Mb. of Santo António-Candeeiros Fm. (Callovian; Azerêdo, 2007), Cabaços Fm. and Montejunto Fm (middle-upper Oxfordian; Kullberg and Rocha, 2014a) (Fig. 2). In the southern sector (e.g. Barreiro region), despite geochemical studies (Spigolon et al., 2011, Gonçalves et al., 2014), no source rocks have been identified yet.

In order to contribute towards recognizing the oil groups accumulated in the Jurassic of the Lusitanian Basin, stable carbon isotope and gas chromatography coupled with mass spectrometry analyses were performed on nine oil samples recovered in Coimbra Fm., Água de Madeiros Fm., Montejunto Fm. and Abadia Fm (lower-upper Kimmeridgian; Kullberg and Rocha, 2014b) and two oil shows samples recovered in Montejunto Fm. (Fig. 2), gathered from the main discoveries reported in Baptista (2002). Representative samples of the different potential source rocks were collected from sections with oil source potential, according to previous works. Total organic carbon (TOC) and Rock-Eval pyrolysis analyses were performed on twenty-five rock samples from Coimbra, Água de Madeiros, Vale das Fontes, Santo António-Candeeiros, Cabaços and Montejunto formations (Fig. 2).

After this procedure, stable carbon isotope and gas chromatography coupled with mass spectrometry analyses were performed on the organic extracts of the potential source rocks. The results obtained with these techniques were compared with those from the analyzed oils using the cluster analysis, in order to establish the oil-source rock correlations of the various admitted petroleum systems in the Lusitanian Basin.

Section snippets

Geological setting

The tectono-stratigraphic evolution of the Lusitanian Basin reflects major controlling factors and multi-stage history typical of ocean margin rift-related basins, in this case in relationship with the opening of the North-Atlantic Ocean (Wilson et al., 1989, Pinheiro et al., 1996, Rasmussen et al., 1998, Pereira and Alves, 2012). In this framework, the Jurassic of the Lusitanian Basin can be characterized by the succession of two 1st-order cycles (Fig. 2) (Wilson et al., 1989): Upper

Oil and oil shows samples

Oil samples of 14A-1 and MR-W9 wells, located in northern sector of the Lusitanian Basin, of Bf-1, TV-4, Ba-1, Se-1, Ald-6, FR-1 and Ab-1a wells, located in central sector of the Lusitanian Basin, and oil show samples of Br-1 and Br-4 wells, located in southern sector of the Lusitanian Basin (Fig. 1) were studied.

The 14A-1 oil was recovered in fractured marly limestones from the top of the Coimbra Fm., between 2375.00 and 2404.00 m (UPEP Archives) (Table 1).

The MR-W9 oil was recovered in

Biodegradation

The absence of linear alkanes, pristane and phytane in Ald-6, FR-1 and Ab-1a (Fig. 3D) oils, indicate that they were biodegraded after accumulation (Chosson et al., 1992), attaining at least level 5 of biodegradation in the Peters and Moldowan (1993) scale. The C29ααα(S/S + R) steranes ratio > 0.56 in FR-1 and Ab-1a oils (Fig. 4B–C and Table 3) also provide further evidence of biodegradation, level 6 in the Peters and Moldowan (1993) scale. The presence of biodegradation in Ald-6, FR-1 and

Conclusions

In the northern sector of the Lusitanian Basin, the oil of 14A-1 well was generated and accumulated in the Coimbra Fm., while that of MR-W9 well had its origin in that unit, but migrated and accumulated in the Água de Madeiros Fm. Of the oils belonging to this sector, the one from 14A to 1 was formed under a higher maturation level.

In the central sector, the oil of Bf-1 well occurs in the unit (base of Montejunto Fm.) where it was generated. The other oils, TV-4, Ba-1, Se-1, Ald-6, FR-1 and

Acknowledgements

The authors thank Galp Energia, for its financial support; Mohave Oil and Gas Corporation; UPEP/ENMC and LNEG, for allowing access to the data on exploratory wells; LGQM/UERJ for the geochemical analyses; Maria Virgínia Alves Martins by the technical support during the cluster analysis; and two anonymous reviewers for the helpful comments that improved the manuscript. Marco Brito also thanks Capes for the doctoral scholarship received.

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Strong local control on OMPIs during most of the Sinemurian is suggested. Regionally widespread organic-rich facies are associated with the most negative δ¹³C values of the broad Sinemurian–Pliensbachian negative carbon isotopic trend recorded in organic matter (including the Sinemurian–Pliensbachian Boundary Event). Pliensbachian OMPIs are expressive in the areas bordering the proto-Atlantic Ocean and are often linked with positive δ¹³C excursions and short-lived warm intervals, but OMPIs are also defined for the Late Pliensbachian cool interval. Early Toarcian superregional OMPIs are associated with some of the most pronounced δ¹³C excursions of the Mesozoic. Toarcian maximum TOC content occurs with the positive δ¹³C (recovery) trend following the δ¹³C negative shift typically linked with the Early Toarcian Oceanic Anoxic Event (T-OAE), supporting the notion that peak carbon sequestration/ocean anoxia post-dated the main phase of carbon input into the atmosphere, as also suggested by recent modelling efforts. However, additional superregional OMPIs predate and postdate the T-OAE, indicating that conditions favouring preservation of organic matter (increased productivity and/or enhanced preservation) during the Early Toarcian were not restricted to the T-OAE interval.
The compilation of Sinemurian–Toarcian OMPIs presented in this paper demonstrates that organic-rich intervals of regional and superregional expression in the Lower Jurassic sedimentary record are ubiquitous and may even be more numerous than in the Cretaceous. Considering the association of some of the Sinemurian, Pliensbachian, and Toarcian (not taking into account the T-OAE related OMPIs) regional and superregional OMPIs with well-defined carbon isotopic excursions, it is here suggested that these hold the same relevance as the secondary OAEs of the Cretaceous, such as the Valanginian OAE (Weissert Event), Hauterivian OAE (Faraoni Event), and Late Aptian–Early Albian OAE (OAE 1b cluster).
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The global radiometric dataset obtained for K, U and Th is similar between field and laboratory gamma spectrometry measurements, which are correlated. The root-mean-square deviation was determined for K, U and Th, and the error of the field measurements was estimated at 0.6%, 1.4 and 2.5 ppm, respectively. We calculated the relative error (RE) between laboratory and field data. If we use the interquartile range of the RE to distinguish outliers in each radionuclide dataset, approximately 8%, 6% and 10% of the K, U and Th field measurements correspond to outliers. Conversely, 92%, 94% and 90% of the samples offer reliable field data for K, U and Th, respectively. The RE is similar between K and U, with a mean of 0.73 and 0.69, respectively, and is higher for Th (with a mean of 1.29). Higher RE values are also observed in dolostones (D), dolomitic limestones (DL), limestones (L) and marly limestones (ML). Lower RE values are linked to sandstones (S) and fine-grained siliciclastic (F-g.S) lithologies, with a mean value of 0.19, 0.33 and 0.25 for K, U and Th, and 100% of reliable data.
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Origin of crude oils from the paleogene Xingouzui formation in the Jiangling depression of Jianghan basin, central China
2020, Journal of Petroleum Science and Engineering
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Both drimane and tricyclicterpane are derived from the carbocyclic ring fracture of hopanoid. The final carbon number of compounds depends on the break which bond of hopanoid, controlled by oxidation-reduction property and the catalytic property of the mineral matrix (Brito et al., 2017). As shown in Fig. 6, the 8β(H)- homodrimane contents are lower than 8β(H)-drimane contents in all oil samples.
The Paleogene Xingouzui Formation is one of the exploration targets in the Jiangling Depression from Jianghan Basin, Central China. However, the origin and source of crude oils remains controversial. In the present work, a total of 16 fresh source rocks and 23 oil samples were collected and geochemically investigated to clarify the source of organic matters, sedimentary environment and evolution degree. The Xingouzui Formation source rock samples are mainly of fair−good source rocks, with the TOC content, S₁ and S₂ of 0.36%–1.36% (mean of 0.74%), 0.14–0.38 (mean of 0.27 mg HC/g rock) and (mean of 1.12 mg HC/g rock), respectively. And these rocks are dominated by type Ⅱ₂ kerogen within “oil window” maturity stage, with the Ro value of 0.6%–1.2% (mean of 0.9%). The biomarker results indicate that the source rocks of the Xingouzui Formation are mainly deposited in anoxic reducing environment, which main source materials are the mixture of terrestrial higher plants and lower marine organisms. The Pr/Ph values of the oils and source rocks range of 0.47–0.64 and 0.23–1.07, respectively. And the δ¹³C values of the oils and source rocks range from −34.93‰ to −26.41‰ and from −28.3‰ to −23.8‰, respectively. Family-I and family-Ⅱ oils were separated based on the results of biomarkers and stable carbon isotope, indicating that family-I oils, mainly occurring in the Wancheng fault zone and Gonggan slope zone, are characterized by relatively lower homohopane series, higher mature, and less input of terrestrial higher plants. Comparatively, family-Ⅱ oils mainly occur in the Jingzhou anticlinal zone, with the characterization of small amount of homohopane series, less mature and more terrigenous organic matter input. Family-I oils mainly originate from of the Ⅲ and Ⅱ groups of the lower Xingouzui Formation (Ex_1-Ⅲ and Ex_1-Ⅱ) source rocks in the Wancheng fault zone and less contribution from Ex_1-Ⅱ source rocks of the Shiqiao monoclinal zone; while family-Ⅱ oils are mainly derived from the Ex_1-Ⅲ source rocks in the north of Jingzhou anticlinal, and a small amount comes from the Ex_1-Ⅱ source rocks of the Shiqiao monoclinal zone. In addition, Ex_1-Ⅱ and Ex_1-Ⅲ source rocks in the Wancheng fault zone show some contribution to family-Ⅱ₂ oils. This work could provide scientific theoretical guidance for petroleum exploration and exploitation in the Jiangling depression.
Tracing bottom-water redox conditions during deposition of Lower and Upper Jurassic organic-rich sedimentary rocks in the Lusitanian Basin (Portugal): Insights from inorganic geochemistry
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The very strong lateral and vertical facies variation denotes conspicuous variations of depositional environments, ranging from fresh-to brackish-water lacustrine and marginal-marine settings at the base to mostly restricted lagoonal and shallow-marine to the top of the unit (Wilson, 1979; Azerêdo et al., 2002a,b; Azerêdo and Cabral, 2004; Azerêdo and Wright, 2004). This formation contains some organic-rich shale levels showing significant hydrocarbon source-rock potential (Spigolon et al., 2010; Silva et al., 2014; Brito et al., 2017). This unit was deposited during the Late Sinemurian to earliest Pliensbachian and is subdivided into two members: the Polvoeira Member (Mb) at the base, and the Praia da Pedra Lisa Mb at the top (Duarte and Soares, 2002; Duarte et al., 2014).
The Lower Jurassic Água de Madeiros (AM Fm) and Vale das Fontes (VF Fm) Formations, and the Upper Jurassic Cabaços Formation (Cab Fm) are considered the most important units with petroleum source-rock potential in the Lusitanian Basin, and the likely sources for its widespread hydrocarbon occurrences. Nonetheless, the bottom-water redox conditions during their deposition remain debatable. To resolve this controversy, we investigated whole-rock inorganic geochemical data of the most significant organic-rich levels of the AM, VF and Cab Fms cropping out at basin scale. Significant variations in TOC, TS and redox-sensitive elements contents and their ratios are evident within and between the organic-rich levels of the studied units. A positive covariation between primary productivity proxies, TOC enrichment and paleo-redox proxies for the Lower Jurassic units clearly suggest an interdependence between enhanced primary productivity, organic matter accumulation and bottom-water redox conditions. For the AM Fm, the paleo-redox proxies indicate that the black shales deposition took place beneath bottom-waters that ranged from suboxic to strongly euxinic conditions, with a clear predominance of the latter. A TOC threshold of ~6–7 wt% clearly separates samples deposited under suboxic to anoxic conditions from those deposited under true euxinic conditions, and is interpreted as the minimum amount of organic matter (OM) accumulation within sediments required to sustain H₂S production at rates that outpaced its consumption through precipitation of sulphides and/or OM sulphurization, allowing the diffusion of H₂S to the bottom-waters. For the Vale das Fontes Fm, the paleo-redox proxies suggest a prevalence of suboxic to anoxic bottom-waters, despite all black shales displaying TOC contents greater than the above threshold. The less reducing conditions relative to the AM Fm euxinic black shales are attributed to: i) a larger proportion of continental OM; ii) lower reactivity of the OM reaching the sediment-water interface (decreasing oxygen consumption); and iii) higher availability of reactive iron (increasing the buffer effect to H₂S produced through OM remineralization by sulphate-reducing bacteria). Both units display Mo-EF vs U-EF covariation patterns that deviate from the field of sediments deposited under unrestricted open-marine conditions, towards higher Mo enrichment relative to that of U, indicating an accelerated transport of aqueous Mo to the sediments, consistent with the operation of a metal-oxyhydroxide particulate shuttle linked to Mn and Fe redox cycling within the water column in a weakly-restricted basin. The Mo-COT regression-line slope indicates a moderate degree of bottom-water mass restriction, with [Mo]_aq concentration of ~60–70% of the present-day seawater value and bottom-water renewal times of ~10–20 years, akin to the present-day Cariaco Basin. The redox conditions of the Cab Fm organic-rich levels are dependent on the location of the depositional environment in the basin, with deposition taking place beneath oxygenated bottom-waters at Cabo Mondego, and under suboxic conditions at Vale de Ventos and Pedrógão, the latter denoting a larger degree of oxygen depletion than the former. In this context, the overall potential for OM preservation should increase from Cabo Mondego to Vale de Ventos and Pedrógão.
Carbonate reservoir outcrop analogues with a glance at pore-scale (Middle Jurassic, Lusitanian Basin, Portugal)
2020, Marine and Petroleum Geology
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Oil-source correlation in the slope of the Qikou Depression in the Bohai Bay Basin with discriminant analysis
2019, Marine and Petroleum Geology
The commonly-used tools for oil-source correlation, such as mass chromatograms of biomarkers and bivariate cross-plots of geochemical parameters, cannot deal with multiple geochemical parameters and plenty of samples simultaneously, leading to uncertainties in the results and even failures sometimes. In this paper, the discriminant analysis (DA) is selected from supervised machine learning algorithms, as it is superior to the commonly-used tools as well as other multivariate statistical methods, with the accumulation of geochemical data of source rocks and oils. In the slope of the Qikou Depression, the main source rocks in the third (Es₃) and first members (Es₁) of the Paleogene Shahejie Formation were deposited in similar depositional environments. The source rocks cannot be distinguished with the commonly-used tools. We firstly extended geochemical parameters and then used stepwise DA to select informative parameters and to develop a discriminant model for oil-source correlation. The 22 selected parameters are supported by geochemical characteristics of the source rocks in the study area. The DA of these parameters for oil-source correlation achieved a high correct rate of original validation (96.8%) and leave-one-out cross-validation (89.4%), indicating a sufficient discriminatory power. The oil-source correlation results with high posterior probabilities, showing strong similarity between the sources and oils, coincide with geological conditions and illustrate that there is still much exploration potential in the study area, especially for the Es₃ petroleum system. All these illustrate that DA is one of the most useful tools for oil-source correlation with the accumulation of geochemical data of source rocks and oils.

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Research paperGeochemical characterization of oils and their correlation with Jurassic source rocks from the Lusitanian Basin (Portugal)

Highlights

Abstract

Introduction

Section snippets

Geological setting

Oil and oil shows samples

Biodegradation

Conclusions

Acknowledgements

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Research paper
Geochemical characterization of oils and their correlation with Jurassic source rocks from the Lusitanian Basin (Portugal)

Configuração sequencial em ciclos (2^a ordem) de fácies transgressivas-regressivas do Jurássico Inferior e Médio da Bacia Lusitânica (Portugal)