Biomarkers stratigraphy of Irati Formation (Lower Permian) in the southern portion of Paraná Basin (Brazil)

Marine and Petroleum Geology 95 (2018) 110–138

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Research paper

Biomarkers stratigraphy of Irati Formation (Lower Permian) in the southern portion of Paraná Basin (Brazil)

T

Darlly E.S. dos Reisa,∗, René Rodriguesa, John M. Moldowanb,c, Cleveland M. Jonesa, Marco Britoa, Danielle da Costa Cavalcanted, Helena Antunes Portelaa a Departamento de Estratigrafia e Paleontologia, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro (UERJ), Rua São Francisco Xavier 524, 20550-013 Maracanã, Rio de Janeiro - RJ, Brazil b Biomarker Technologies, Inc., 638 Martin Avenue, Rohnert Park, CA 94928, USA c Geological Sciences Department, Stanford University, 450 Serra Mall Bldg. 320 Rm. 118, Stanford, CA 94305-2115, USA d Laboratório de Quimioestratigrafia e Geoquímica Orgânica, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier, 524, Maracanã, Rio de Janeiro - RJ CEP 20550-900, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords: Biomarker stratigraphy Irati Formation Lower Permian Paraná Basin Brazil

This study covers the application of biomarkers as stratigraphic markers in a Lower Permian marine section from the Paraná Basin, southern Brazil. The biomarker variations reveal changes in sedimentary environment that were not detected by sedimentological and paleontological methods and will contribute to high resolution stratigraphy. The Irati Formation contains two lithostratigraphic units: (1) the lower unit (Taquaral Member) is composed of shales, deposited in a marine epicontinental to restricted environment and (2) the upper unit (Assistência Member) which consists of carbonates interbedded with shales and organic-rich shale intervals deposited in a restricted marine environment. The biomarker differences observed in the Taquaral Member can be explained by two different types of organic matter. The lower and upper parts are characterized by a high relative abundance of high molecular weight n-alkanes, C27 (17α) trisnorhopane, C31 αβR homohopane, high hopanes/steranes ratio, and low hydrogen index values. This facies contains greater relative amount of terrestrial organic matter. The low hopanes/steranes ratio, together with the high hydrogen index values and amorphous organic matter relative proportion observed in the middle part of the Taquaral Member indicate a more marine organic matter contribution. In the Assistência Member based on gammacerane/C30 αβ hopane ratio, C28ααα (20R) methylcholestane/C29ααα (20R) methylcholestane ratio, total organic carbon, hydrogen index values, palynofacies and sedimentary structures it was possible to distinguish changes in salinity and redox conditions. The bituminous shales intervals were deposited in an anoxic environment (high total organic carbon, high hydrogen index values, high preserved amorphous organic matter content) with lower salinity (lower gammacerane/C30 αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane ratios) on the other hand, the limestones intervals were deposited in an aerobic environment (low total organic carbon, low hydrogen index values, high oxidized organic matter content) with higher salinity (higher gammacerane/C30 αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) methylcholestane ratios).

1. Introduction The presence of oil shale in the Irati Formation of the Paraná Basin has attracted interest from oil companies since the 1950's (Padula, 1969). Petrobras, the Brazilian national oil company, started industrial exploitation of this oil shale in the 1980's (Milani et al., 2007). The presence of tar sands and numerous oil shows, similarly related to oil generation in the shales and marls of the Irati Formation (Cerqueira and

∗

Santos Neto, 1990; Loutfi et al., 2010), led to a significant increase in the number of publications regarding aspects on the characterization of the quantity, quality and thermal evolution of the organic matter of this formation (Cerqueira and Santos Neto, 1986; Zalán et al., 1990; Hachiro and Coimbra, 1993; Santos Neto et al., 1992; Santos Neto and Cerqueira, 1993; Araújo et al., 2000; Santos et al., 2003, 2006; Corrêa and Pereira, 2005; Rodrigues et al., 2010a, 2010b; Alferes et al., 2011; Gama Jr. and Perinotto, 1992; Rocha-Campos et al., 2011, Reis and

Corresponding author. E-mail addresses: [email protected], [email protected] (D.E.S. dos Reis), [email protected] (R. Rodrigues), [email protected], [email protected] (J.M. Moldowan), [email protected] (C.M. Jones), [email protected] (M. Brito), [email protected] (D. da Costa Cavalcante), [email protected] (H.A. Portela). https://doi.org/10.1016/j.marpetgeo.2018.04.007 Received 27 March 2017; Received in revised form 8 February 2018; Accepted 9 April 2018 Available online 17 April 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. Location of Paraná Basin and the SC-20-RS well.

concepts of Vail et al. (1977), divided the basin filling history into a set of 6 s order sequences, or supersequences, limited by regional unconformities. These unconformities represent long periods of erosion and a halt in sedimentation. The supersequences consist of the following (Fig. 2): Rio Ivaí (Upper Ordovician to Lower Silurian), Paraná (Lower Devonian to Upper Devonian) and Gondwana I (Upper Carboniferous to Upper Permian), which correspond to Paleozoic transgressive and regressive cycles. Furthermore, Gondwana II (Middle Triassic to Upper Triassic), Gondwana III (Upper Jurassic to Lower Cretaceous) and Bauru (Lower Cretaceous to Upper Cretaceous), consist of continental units with important igneous rock contributions. The Rio Ivaí Supersequence is composed of fluvial sandstones of the Alto Garças Formation, diamictites of the Iapó Formation related to the glaciation of the Upper Ordovician, marine shales of the Vila Maria Formation and by the Três Lagoas basalt. The Paraná Supersequence includes the marine sandstones of the Furnas Formation and shales of the Ponta Grossa Formation. The Gondwana I Supersequence includes the continental sandstones and diamictites of the Itararé Group, related to the Carboniferous glaciation, deltaic sandstones of the Rio Bonito Formation, marine shales of the Palermo Formation, marine shales and carbonates of the Irati Formation, marine to continental siltstones of the Serra Alta, Corumbataí, Teresina and Rio do Rasto formations, fluvial and eolian sandstones of the Pirambóia, Sanga Cabral (Fig. 2). The Irati Formation is of Artinskian-Kungurian age (Santos et al., 2006; RochaCampos et al., 2011) (Fig. 2), with an average thickness of 40 m, reaching a maximum of 70 m (Mendes et al., 1966). It is characterized

Rodrigues, 2014). Although many of these publications dealt with different geochemical aspects, there still exists a wide gap concerning the use of biomarkers data to delineate the stratigraphic framework of the Irati Formation at high-resolution. In order to carry out this study, we have selected the SC-20-RS well that was drilled by Serviço Geológico do Brasil in the Southern area of the Paraná Basin. Here, we present new data that demonstrate variations on biomarker distribution due to changes in the depositional environment during the Lower Permian in the Paraná Basin. 2. Geologic setting The Paraná Basin is a wide sedimentary-magmatic sequence located in the South America that includes areas in Brazil, Paraguay, Argentina and Uruguay, totaling an area of approximately 1.5 million square kilometers (Fig. 1). The basin has an oval shape, 1750 km long by 900 km wide, with the longer axis being NNE-SSW. Its current outline is defined by erosive limits, mostly related to the Meso-Cenozoic geotectonic history of the continent (Zalán et al., 1990; Milani et al., 2007). This basin evolved during the Paleozoic and Mesozoic, and exhibits a stratigraphic record that developed from the Upper Ordovician to the Upper Cretaceous, with an accumulated maximum thickness of over 6000 m in its central portion (Zalán et al., 1990; Milani et al., 2007). The thick sedimentary package of the Paraná Basin does not represent a continuous record. It has a series of regional unconformities that span long intervals of time. Milani et al. (1998, 2007), applying the

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Fig. 2. Lithostratigraphic chart, petroleum systems and second order sequences of the Paraná Basin. (adapted from Milani and Zalán, 1999; Santos et al., 2006; Rocha-Campos et al., 2011; Pereira et al., 2012). Legend for the elements of petroleum systems: ♦ - source rock; ■ - reservoir; arrows within the circles indicate the critical moment, when the generation, migration and accumulation of most of the hydrocarbons occurred, or even the remobilization of previously existing accumulations. Simplified geological timescale of Cohen et al. (2013.).

Formation and the igneous rocks of the Serra Geral Formation. The Bauru Supersequence includes the conglomerates, as well as the alluvial-fluvial and eolian sandstones of the Bauru Group (Fig. 2).

by its regular distribution and uniform lithology, occurring in an extension of more than 2,000 km. In Brazil, it is present in the states of Mato Grosso, Goiás, São Paulo, Paraná, Santa Catarina and Rio Grande do Sul. It is subdivided into the Taquaral and Assistência members. The lower member, Taquaral, is essentially siliciclastic, deposited in a normal salinity marine environment, and the upper member, Assistência, is characterized by carbonates interbedded with bituminous shales deposited in a restricted marine environment (Santos Neto and Cerqueira, 1993; Rodrigues et al., 2010a, 2010b; Alferes et al., 2011). The Gondwana II Supersequence is represented by the fluvio-lacustrine sandstones of the Santa Maria Formation. The Gondwana III Supersequence includes the eolian sandstones of the Botucatu

3. Petroleum systems The Paraná Basin contains two petroleum systems (Zalán et al., 1990; Milani and Zalán, 1999; Pereira et al., 2012) (Fig. 2): in the first, mostly for gaseous hydrocarbons, generation occurred in the shales of the Ponta Grossa Formation and accumulation occurred in the sandstones of the Itararé Group or the Rio Bonito Formation; the second, being oil prone, involves generation in the oil shales of the Irati

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Fig. 3. Interpretation of the systems tract and the depositional sequences for the Irati Formation, based on gamma ray, lithology, sedimentary structures, resistivity, total organic carbon (TOC) and insoluble residue (IR). A-H = chemostratigraphic units.

Fig. 4. Chemostratigraphic units of the Irati Formation and fourth order sequences. TOC = total organic carbon; IR = insoluble residue; S = Sulphur; HI = hydrogen index; Tmax = maximum temperature.

4. Material and methods

Formation and accumulation in the sandstones of the Rio Bonito and Pirambóia formations. Throughout almost the entire Paraná Basin, the Ponta Grossa and Irati source rocks have not undergone sufficient burial to generate hydrocarbons (Zalán et al., 1990; Milani and Zalán, 1999; Milani et al., 2007). However, there is the possibility of an unconventional generation process due to the thermal effect of intrusive igneous rocks of the Serra Geral Formation (Cerqueira and Santos Neto, 1986; Araújo et al., 2000; Santos et al., 2003; Corrêa and Pereira, 2005; Thomaz Filho et al., 2008; Loutfi et al., 2010) (Fig. 2), that may be a possible new exploratory play in the basin.

Total organic carbon (TOC), total sulphur (S) and insoluble residue (IR = amount of sample obtained after removal of the carbonates) assays were done on 169 core samples from the Irati Formation, each one separated by approximately 30 cm, in an interval of 47 m. Based on the TOC content, 73 samples were selected for Rock-Eval pyrolysis, 33 samples for organic extraction, liquid chromatography and gas chromatography-mass spectrometry and 25 samples for palynofacies. All analyses were carried out in the Chemostratigraphy and Organic Geochemistry (LGQM) and the Palynofacies laboratories of State

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Taquaral Member

H

Irati Formation Assistência Member

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A

B

C

D

E

G F

Chemostratigraphy Unit

Litho stratigraphy

0.40 0.38 0.38 0.39 0.37 0.38 0.37 0.45 0.40 0.38 0.35 0.39 0.39 0.38 0.37 0.38 0.37 0.38 0.37 0.37 0.38 0.37 0.38 0.40 0.38 0.37 0.38 0.39 6.35 0.39 0.39 0.45 0.39 0.45 0.40

0.20 0.19 0.15 0.21 0.15 0.18 0.38 0.40 0.25 0.26 0.16 0.22 0.15 0.17 0.18 0.28 0.20 0.20 0.21 0.25 0.30 0.63 0.13 0.14 0.60 0.33 0.21 0.17 0.19 0.21 0.25 0.30 0.28 0.33 0.83

6.60 3.13 4.60 5.07 1.02 0.49 0.60 5.80 1.53 2.56 1.24 2.97 1.50 2.78 1.64 1.86 2.09 2.58 2.07 1.05 0.63 0.52 0.65 0.57 0.21 1.28 1.52 1.96 12.00 1.11 1.63 7.71 4.87 10.84 9.31

0.31 0.20 0.38 0.49 0.15 0.13 0.12 0.29 0.26 0.26 0.24 0.23 0.20 0.42 0.24 0.35 0.31 0.34 0.36 0.35 0.11 0.67 0.70 0.69 0.39 0.65 0.22 0.21 0.38 0.37 0.38 0.86 0.40 0.48 0.61

Roeq(%) 31αβ (S/R) Hp/St 28αααR/ 29αααR 0.83 0.95 0.56 1.22 1.21 1.29 0.19 0.74 0.52 0.53 0.37 0.56 0.44 0.60 0.38 0.66 0.88 1.09 0.94 0.56 0.16 2.38 0.19 0.12 0.10 1.14 0.41 0.56 0.92 0.89 0.91 1.02 0.49 0.70 1.49

27αααR/ 29αααR 0.69 0.31 0.81 1.32 1.12 0.47 0.20 0.38 0.69 0.62 0.62 0.89 1.03 0.38 0.49 0.53 0.66 0.50 0.04 0.58 0.25 0.17 0.12 0.09 0.18 0.33 0.49 0.99 0.58 0.61 0.42 0.60 0.67 0.63 0.44

31αβR/ 30αβ 0.39 0.25 0.20 0.29 0.26 0.28 0.17 0.19 0.44 0.48 0.24 0.56 0.50 0.20 0.19 0.48 0.16 0.18 0.17 0.18 0.17 0.18 0.11 0.14 0.18 0.21 0.16 0.47 0.33 0.37 0.31 0.02 0.46 0.60 1.11

29αβ/ 30αβ 1.60 0.65 0.61 0.88 0.98 0.64 0.11 0.21 0.38 0.39 0.38 0.35 0.43 0.21 0.26 0.20 0.12 0.18 0.10 0.09 0.13 0.13 0.05 0.05 0.07 0.07 0.21 0.54 0.27 0.24 0.16 0.83 0.41 0.66 0.27

27β/ 30αβ 0.50 0.31 0.30 0.46 0.48 0.43 0.26 0.43 0.55 0.61 0.39 0.59 0.58 0.58 0.35 0.49 0.19 0.19 0.20 0.40 0.29 0.46 0.26 0.25 0.23 0.33 0.25 0.74 0.45 0.39 0.25 1.21 0.65 1.08 0.72

27α/ 30αβ 0.63 1.13 0.75 1.36 0.91 0.68 0.44 0.66 0.66 1.00 0.24 0.75 0.93 0.28 0.30 0.10 0.28 0.52 0.40 0.09 0.26 0.25 0.20 1.08 1.00 0.45 0.90 0.50 0.25 0.11 0.25

4.75 9.00 0.93 5.50 11.33 7.75 12.50 0.60 0.46 0.50 16.37 0.40 0.55 18.00 18.55 58.00 40.00 18.75 14.75 2.50 5.50 47.00 40.00 44.00 8.00 0.50 0.67 0.64 0.86 1.05 0.65 0.54

0.18 0.28 0.23 0.57 0.21 0.11 0.45 0.36 0.10 0.10 0.16 0.16 0.04 0.09 0.13 0.21 0.38 0.23 0.33 0.55 0.37 0.71 1.06 0.95 0.50 0.72 0.16 0.03 0.02 0.03 0.01 0.01 0.03 0.01 0.05

93 83 84 93 93 91 34 86 90 92 92 90 90 84 85 87 84 83 80 82 82 90 76 66 55 91 87 90 91 90 87 88 87 90 92

2.34 2.00 1.43 1.19 1.60 1.02 0.53 0.67 0.67 0.71 0.78 0.64 0.93 14.90 16.90 14.60 12.10 10.90 8.74 4.09 2.68 0.78 1.94 1.03 1.49 0.57 0.41 0.67 0.61 1.42 2.74 0.70 0.52 0.46 0.43

7.10 7.40 8.00 9.60 11.10 12.00 13.20 14.30 17.90 20.20 21.70 24.30 25.60 26.20 26.80 27.10 28.00 28.60 28.90 29.50 30.40 31.00 33.50 33.80 34.70 35.90 37.40 38.60 39.50 40.10 40.70 41.30 43.10 48.80 50.10

Ph/nC 18 Pr/Ph G/30αβ IR(%) TOC (%) Depth (m)

Table 1 Biomarker ratios in samples from the Irati Formation in the SC-20-RS well. G = gammacerane; 30αβ = C30αβ hopane; Pr = pristane; Ph = Phytane; nC18 = C18 n-alkane; 27α = C27(17 α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; 31αβS = C31αβS homohopane; 27αααR = C27ααα(20R) cholestane; 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; 29αααS = 29ααα(20S) ethylcholestane; Hp = C30αβ ho pane; St = major peak between steranes; RoEq = 0,50 × [29ααα(S/R)] + 0,35 (according Sofer et al., 1993).

D.E.S. dos Reis et al.

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Fig. 5. Microphotographs of the organic content of samples from Taquaral (units A, B and C) and Assistência (Unit D) members. Left, observation under white transmitted light, right, under ultraviolet light. The values of total organic carbon (TOC) and hydrogen index (HI) for each sample are highlighted.

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Fig. 6. Data on total organic carbon (TOC), insoluble residue (IR), sulphur (S), hydrogen index (HI), kerogen type, palynofacies of the Irati Formation. AOM = amorphous organic matter.

Table 2 Data on insoluble residue (IR), total organic carbon (TOC), sulphur (S), hydrogen index (HI), kerogen type, palynofacies and biomarkers for samples from Taquaral Member. AOM = amorphous organic matter. G = gammacerane; 30αβ = C30αβ hopane; Pr = pristane; Ph = Phytane; nC18 = C18 n-alkane; 27α = C27(17 α) tristrisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; 27αααR = C27ααα(20R) cholestane; norhopane; 27β = C27(17β) 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; Hp = C30αβ hopane; St = major peak between steranes. Parameters

Taquaral Member Unit A

IR (%) TOC (%) S (%) HI (mgHC/gTOC) Kerogen Type AOM (%) Acritarchs (%) Botryococcus (%) Pollens and Spores (%) Non-Opaques Phytodasts (%) Opaques Phvtochsts (%) G/30αβ Pr/Ph Ph/nC18 27α/30αβ 27β/30αβ 29αβ/30αβ 30βα/30αβ 31αβR/30αβ 27αααR/29αααR 28αααR/29αααR Hp/St

Unit B

Unit C

50.10 m

48.80 m

43.10 m

40.7 m

40.10 m

38.60 m

37.40 m

35.90 m

91.00 0.34 0.10 ? ? 10.00 10.00 1.00 10.00 14.00 55.00 0.05 0.25 0.54 0.72 0.27 1.11 0.26 0.44 1.49 0.61 9.31

90.00 0.46 0.10 ? ? 10.00 10.00 1.00 1.00 3.00 70.00 0.01 0.11 0.65 1.08 0.66 0.60 0.60 0.63 0.70 0.48 10.84

87.00 0.52 0.13 17.48 III 75.00 10.00 1.00 1.00 4.00 9.00 0.03

87.00 2.74 0.40 213.50 II/III 40.00 1.50 1.50 7.00 11.00 39.00 0.01 0.50 0.86 0.25 0.16 0.31 0.37 0.42 0.91 0.38 1.63

90.00 1.42 0.25 93.96 III 50.00 3.00 1.00 3.00 13.00 30.00 0.03 0.90 0.64 0.39 0.24 0.37 0.47 0.61 0.89 0.37 1.11

90.00 0.67 0.12 41.60 III 70.00 0.00 0.00 0.00 4.00 26.00 0.03

87.00 0.41 0.34 ? ? 10.00 19.00 1.00 1.00 2.00 67.00 0.16 1.00 0.50 0.25 0.21 0.16 0.61 0.49 0.41 0.22 1.52

91.00 0.57 0.33 27.97 III 4.00 1.00 1.00 1.00 3.00 90.00 0.72 1.08 8.00 0.33 0.07 0.21 0.46 0.33 1.14 0.65 1.28

0.65 0.41 0.46 0.58 0.67 0.49 0.40 4.87

0.74 0.54 0.47 0.50 0.99 0.56 0.21 1.96

and oxygen index (OI) (mgCO2/gCOT) were also calculated. The organic extracts were obtained by extraction in soxlet, utilizing dichloromethane P.A. as solvent. The saturated hydrocarbons (alkanes) fraction of the organic extracts were obtained by liquid chromatography, utilizing a glass column with a solid phase composed of a mixture of silica gel (1/3) and alumina (2/3). The solvent employed was hexane. The biomarkers of the alkane fraction were analyzed in an Agilent 6890 gas chromatograph coupled to a mass spectrometer 5793

University of Rio de Janeiro (UERJ). The equipment and procedures utilized are described below. The TOC and S values of the samples were obtained with a Leco SC632 equipment, and expressed as mass percentage relative to the original sample. The Rock-Eval pyrolysis analyses were carried out in a Rock-Eval 6 analyzer from Vinci Technologies. The following parameters were determined: S1 (mgHC/gRock), S2 (mgHC/gRock), S3 (mgCO2/gRock) and Tmax (oC). Hydrogen index (HI) (mgHC/gCOT)

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qualitative and quantitative estimation. The absolute and relative proportions, state of preservation and size of organic matter was taken into consideration for palynofacies analyses. These were counted in three main groups, namely, phytoclasts, palynomorphs and amorphous organic matter, as proposed by Tyson (1995) for the classification of palynofacies.

5. Results and discussion 5.1. Sequence stratigraphy The Gondwana I Supersequence, according to Milani et al. (2007), is composed of a large transgressive-regressive 2nd order cycle, limited at the base by the Neodevonian unconformity and at the top by the Eotriassic unconformity. The maximum flooding surface (MFS) was placed by Della Fávera et al. (1992), Menezes, 1994, Etgeton (1997), Milani (1997), Milani et al. (1998), in the pelitic interval of the Palermo Formation. According to these authors, the deposits of the Irati Formation belong to the basal section of the regressive cycle of this sequence (Fig. 2). Alternatively, Lavina (1991), Perinotto (1992), Gama Jr. and Perinotto (1992), Lopes (1995) and Mendonça Filho (1999) placed the MFS of this 2nd order cycle of the upper Paleozoic of the basin in the shale oil interval of the Irati Formation. Based on Holz, 1995, 1998, Holz and Diaz, 1998 and Ade, 1999, the Irati Formation belongs to the highstand systems tract (HST). In this work, the model of Milani et al. (2007) is adopted, since the pelites of the Palermo Formation constitute a stratigraphic marker corresponding to the important marine expansion of the basin, while the oil shales of the Irati Formation are related to the restriction of water circulation between the syneclise and the Panthalassa Ocean, thus placing them at the beginning of the regressive portion of this cycle. In terms of the 3rd order cycle, the Irati Formation is considered by

Fig. 7. Van Krevelen diagram (according to Espitalié, 1984) showing hydrogen index (HI) versus maximum temperature (Tmax) data for all studied samples.

Network, monitoring total ions (TIC), for total alkanes, m/z 191, for terpanes, and m/z 217, for steranes. The ratios between biomarkers were calculated based on the peaks heights observed in the chromatograms. For palynofacies studies, the samples were treated with HCl and HF and washed thoroughly to remove all acids, and slides were prepared with Entellan as mounting media. These slides were analyzed (following the method of Batten, 1996; Batten and Stead, 2005) under transmitted white light and ultraviolet incident light (fluorescence) for

Fig. 8. Presence of acritarchs, taquaral member (Unit A) and Botryococcus in assistência member (Unit E).

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Fig. 9. Total ion chromatograms (TIC) of units A, B and C. Pr = pristane; Ph = phytane; 19 to 29 = C19 to C29 n-alkanes; St = steranes; Hp = hopanes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

with other information, such as lithology and resistivity (Fig. 3). Association of gamma ray, and resistivity logs, TOC, IR and sedimentary facies, three 4th order sequences were distinguished (according to Vail et al., 1977) (Fig. 3). These sequences are similar to those defined by Araújo et al. (2001), regarding their limits, as well as their reach, with the exception of the flooding surface of the first sequence (FS-I, Fig. 3). This surface was placed by Araújo et al. (2001) at the base of the first limestone bank, however, in this work it was placed in the Taquaral Member shale layer richest in organic matter, as described below.

Holz et al. (2010) as constituting a single sequence, while for Araújo et al. (2001), the upper limit of this cycle reaches the basal deposits of the Serra Alta Formation, the model adopted in this work. Different authors (Hachiro and Coimbra, 1993; Hachiro, 1996; Araújo et al., 2001) recognize three 4th order cycles encompassing the deposits of the Irati Formation, although with different limits and consequently with different reaches. The base of the Taquaral Member was distiguished from the Palermo Formation by the increase total organic carbon (TOC) values, together

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Fig. 10. Mass chromatograms (m/z 191) of units A, B and C. G = gammacerane; 30αβ = C30αβ hopane; 27α = C27(17 α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; Hp = C30αβ ho pane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

The sequence limit was placed at the top of the limestones layer, known for its low IR values (Fig. 3).

Each sequence is limited by the deposition of shales, which define the start of a transgressive systems tract (TST), characterized by a transgressive surface (TS) (Fig. 3). These shales are characterized by the upward increase in TOC values. The flooding surface (FS; Fig. 3), which separates these shales from those of the HST is known to represent the point of highest TOC values of the sequence. From the FS, TOC values of the shales decrease rapidly, representing the start of the HST (Fig. 3).

5.2. Chemostratigraphy The biomarkers associated with TOC, S, IR, hydrogen index (HI) and palynofacies data allowed for the identification of distinct

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Fig. 11. Mass chromatograms (m/z 217) of units A, B and C. 27αααR = C27ααα(20R) cholestane; 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; 29αααS = 29ααα(20S) ethylcholestane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

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trisnorhopane/C30αβ hopane, C27(17β) trisnorhopane/C30αβ hopane, C29αβ norhopane/C30αβ hopane, C30βα moretane/C30αβ hopane, C31αβR homohopane/C30αβ hopane, C27ααα (20R) cholestane/ C29ααα (20R) ethylcholestane, C28ααα (20R) methylcholestane/ C29ααα (20R) ethylcholestane, hopanes/steranes, C31αβS homohopane/C31αβR homohopane and C29ααα (20S) ethylcholestane/ C29ααα (20R) ethylcholestane. All these parameters can be influenced by different sources of organic matter and paleoenvironmental conditions (Peters et al., 2005). Variations in the ratios phytane/nC18, C27(17β) trisnorhopane/C30αβ hopane, C30βα moretane/C30αβ hopane, C31αβS homohopane/C31αβR homohopane and C29ααα (20S) ethylcholestane/C29ααα (20R) ethylcholestane can also reflect changes in the degree of thermal maturation (Peters et al., 2005). Values of Tmax below 430 °C (Figs. 4 and 7) indicate that the studied section is thermally immature (Espitalié, 1984). By the equation for the correlation between the vitrinite reflectance and the ratio C29ααα (20S) ethylcholestane/C29ααα (20R) ethylcholestane (RoEq. = 0.50×[C29ααα (20S)/C29ααα (20R)]+0.35), proposed by Sofer et al. (1993), it can be seen that the organic matter of the Irati Formation is thermally immature in well SC-20-RS, reaching values of 0.4% vitrinite reflectance (Table 1). Thus, the variation observed in the ratios between biomarkers must be due to different types of organic matter or different paleoenvironmental conditions, as discussed below. The Taquaral Member, is composed of shales, known by their high values of IR (86–92%, Figs. 3 and 4), with linsen and wavy stratification and a variable bioturbation intensity (CPRM, 1986). The combination of lenticular and wavy stratification characterize a hummocky crossstratification, a feature characteristic of reworking by storm waves, and consequently a shallow depositional environment rich in oxygen (Milani et al., 2007), which is reflected in the behavior of TOC, HI and palynofacies. Based on this information, this member was subdivided into three chemostratigraphic units: A, B and C (Figs. 3–6). Chemostratigraphic unit A, corresponding to the TST of sequence I, is characterized by low TOC values, usually lower than 0.5%, with predominance of opaque phytoclasts (samples 50.70 m; 50.10 m, 48.80 m and 47.00 m) or oxidized amorphous organic matter (sample 43.10 m) (Figs. 3–6; Table 2), indicating an aerobic environment (Tyson, 1987). These data explain the low IH values (< 20 mgHC/ gTOC) and the position of the samples in the diagram, indicating a type III behavior (Fig. 7). An important characteristic of this unit is the presence, although low in relative proportion and in diversity, of acritarchs (Fig. 8). This fact, together with the high relative proportion of phytoclasts (Figs. 5 and 6), indicate a proximal marine environment ( Staplin, 1961; Sarjeant, 1967). This interpretation is in agreement with the paleontological studies carried out by Cazzulo-Klepzig et al. (1989), Holz and Diaz (1998) and Chahud and Petri (2013). The predominance of high molecular weight n-alkanes and the high relative abundance of hopanes in the total ion chromatograms (Fig. 9), together with the high values of the ratios hopanes/steranes (> 4, Tables 1 and 2), are consistent with a higher contribution of terrestrial organic matter (Tissot and Welte, 1984). The moderates to high values of the ratios C27 (17α) trisnorhopane/C30αβ hopane, C27 (17β) trisnorhopane/C30αβ hopane, C30βα moretane/C30αβ hopane and C31 αβR homohopane/C30αβ hopane (Fig. 10 and Tables 1 and 2) have also been frequently observed in samples where terrestrial organic matter predominates (Henz et al., 1987; Villar et al., 1988). The values of the ratios C27ααα(20R) cholestane/C29ααα(20R) ethylcholestane are variable (Tables 1 and 2 and Fig. 11): above 1 in samples 50.10 m and 41.30 m and below 1 in samples 48.80 m and 43.10 m. The high relative proportion of C29ααα (20R) ethylcholestane in organic extracts and pre-Mesozoic oils has been interpreted as evidence that the primary production in Paleozoic oceans was dominated by green algae (Grantham and Wakefield, 1988; Schwark and Empt,

Fig. 12. Microphotographs of the organic content of samples from Assistência (units E to H). Left, observation under white transmitted light; right, under ultraviolet light. The values of total organic carbon (TOC) and hydrogen index (HI) for each sample are highlighted.

chemostratigraphic intervals, informally denominated units A, B, C, D, E, F, G, and H from the base to the top of the Irati Formation (Figs. 4–26). Among the various biomarker parameters that can be used in geochemical studies, the following ratios were selected (Tables 1–3): gamacerane/C30αβ hopane, pristane/phytane; phytane/nC18, C27(17α)

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Fig. 13. Total ion chromatograms (TIC) of units C and D. Pr = pristane; Ph = phytane; 19 to 29 = C19 to C29 n-alkanes; St = steranes; Hp = hopanes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

2006). On the other hand, according to Kodner et al. (2008), C27ααα (20R) cholestane has its origin tied to red algae. Despite the predominance of terrestrial organic matter in the studied samples from Unit A, the data on steranes indicate a small contribution of green algae in samples 48.80 m and 43.10 m and of red algae in samples 50.10 m and 41.30 m. Chemostratigraphic unit B, described as a FS corresponding to Sequence I, is characterized by TOC values between 0.43% and 2.74% (Figs. 3 and 4). In the palynofacies a mixture between partially oxidized amorphous organic matter and opaque phytoclasts was observed (Figs. 5 and 6 Table 2). When plotted on the diagram, it was classified as type II/III and III organic matter (Fig. 7). The Rock-Eval pyrolysis data are consistent with the palynofacies data (Figs. 5 and 6). Compared with the previous unit, this unit is more enriched in organic carbon, as well as in non-oxidized amorphous organic matter (Figs. 3–6), although individually these data suggest an aerobic environment (Tyson, 1987).

Unit B shows higher relative abundance of steranes and lower values of the ratio hopanes/steranes (< 2) (Fig. 9 and Table 1), indicating a greater influence of algae in the composition of the organic matter (Moldowan et al., 1985). Other molecular organic geochemical features to distinguish Unit A from Unit B are the lower C27 (17α) trisnorhopane/C30αβ hopane and C27(17β) trisnorhopane/C30αβ hopane value ratios (Fig. 10 and Tables 1 and 2). This is consistent with higher values of HI and lower contribution of terrestrial organic matter observed in palynofacies (Figs. 5 and 6). The steranes of Unit B exhibit a similar proportion between the C27ααα (20R) cholestane and the C29ααα (20R) ethylcholestane (Fig. 11 and Tables 1 and 2), indicating significant contribution of red and green algae (Grantham and Wakefield, 1988; Schwark and Empt, 2006, Kodner et al., 2008). Chemostratigraphic unit C represents the base of the HST of the Sequence I, and is characterized by low TOC values (< 0.7%) and the

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Table 3 Data on total organic carbon (TOC), insoluble residue (IR), sulphur (S), hydrogen index (HI), kerogen type, palynofacies and biomarkers for samples from Assitência Member. AOM = amorphous organic matter. G = gammacerane; 30αβ = C30αβ hopane; Pr = pristane; Ph = Phytane; nC18 = C18 n-alkane; 27α = C27(17 α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; 27αααR = C27ααα(20R) cholestane; 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; Hp = C30αβ hopane; St = major peak between steranes. Parameters

Assistência Member Unit D

IR (%) TOC (%) S (%) HI (mgHC/gTOC) Kerogen Type AOM (%) Acritarchs (%) Botryococcus (%) Pollens and Spores (%) Non-Opaques Phytodasts (%) Opaques Phvtochsts (%) G/30αβ Pr/Ph Ph/nC18 27α/30αβ 27β/30αβ 29αβ/30αβ 30βα/30αβ 31αβR/30αβ 27αααR/29αααR 28αααR/29αααR Hp/St

Unit E

Unit F

Unit G

Unit H

34.70 m

33.80 m

33.50 m

31.00 m

30.40 m

29.50 m

26.80 m

25 60 m

20.20 m

14.30 m

13.20 m

11.10 m

8.00 m

7.40 m

49.00 1.49 0.86 258.39 II/III 90.00 0.00 0.00 2.00 2.00 6.00 0.50 0.20 44.00 0.23 0.07 0.18

66.00 1.03 1.03 246.60 II/III 50.00 0.00 0.00 9.00 3.00 38.00 0.95 0.25 40.00 0.25 0.05 0.14 0.13 0.09 0.12 0.69 0.57

76.00 1.49 1.94 354.64 II/III 60.00 0.00 0.00 9.00 4.00 27.00 1.06 0.26 47.00 0.26 0.05 0.11 0.16 0.12 0.19 0.70 0.65

90.00 0.78 0.46 43.76 III 20.00 0.00 7.00 7.00 2.00 64.00 0.71 0.09 5.50 0.46 0.13 0.18 0.22 0.17 2.38 0.67 0.52

90.00 2.68 2.79 343.66 II/III 80.00 0.00 0.00 1.00 1.00 18.00 0.37 0.40 2.50 0.29 0.13 0.17 0.20 0.25 0.16 0.11 0.63

82.00 4.09 2.27 345.48 II/III 70.00 0.00 0.00 2.00 1.00 27.00 0.55

85.00 16.90 4.07 428.52 II 98.00 0.00 0.00 1.00 0.00 1.00 0.13 0.30 18.55 0.35 0.26 0.19 0.68 0.49 0.38 0.24 1.64

90.00 0.93 0.18 55.02 III 5.00 0.00 5.00 8.00 26.00 56.00 0.04 0.93 0.55 0.58 0.43 0.50 0.47 1.03 0.44 0.20 1.50

49.00 0.57 0.16 22.17 III 1.00 0.00 5.00 4.00 17.00 73.00 0.10 1.00 0.50 0.61 0.39 0.48 0.39 0.62 0.53 0.26 2.56

94.00 0.67 0.30 10.46 III 0.00 0.00 1.00 1.00 11.00 87.00 0.36 0.66 0.60 0.43 0.21 0.19 0.27 0.38 0.74 0.29 5.80

34.00 0.53 0.21 132.08 III 20.00 0.00 0.00 30.00 7.00 43.00 0.45 0.44 12.50 0.26 0.11 0.17 0.19 0.20 0.19 0.12 0.60

93.00 1.60 0.96 285.00 II/III 80.00 0.00 0.00 1.00 2.00 17.00 0.21 0.91 11.33 0.48 0.98 0.26 0.44 1.12 1.21 0.15 1.02

84.00 1.43 0.66 105.59 III 60.00 0.00 0.00 23.00 1.00 16.00 0.23 0.75 0.93 0.30 0.61 0.20 035 0.81 0.56 038 4.60

83.00 2.00 0.38 179.50 II/III 60.00 0.00 0.00 22.00 2.00 16.00 0.28 1.13 9.00 031 0.65 0.25 0.41 031 0.95 0.20 3.13

0.18 0.10 0.39 0.21

14.75 0.40 0.09 0.18 0.58 0.58 0.56 035 1.05

C30βα moretane/C30αβ hopane and C31αβ R homohopane/C30αβ hopane and lower values of the ratios phytane/n-C18, gamacerane/C30αβ hopane, C27ααα (20R) cholestane/C29 ααα (20R) ethylcholestane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane. The higher relative abundance of n-alkanes, hopanes, C27(17α) trisnorhopane, C27(17β) trisnorhopane, C30βα moretane and C31αβ R homohopane at the base of Unit C may indicate that this section was deposited during a period of greater influence of higher plants (Tissot and Welte, 1984; Henz et al., 1987; Villar et al., 1988). The increase in relative abundance of steranes, especially for C27ααα (20R) cholestane and C28ααα (20R) methylcholestane, which are derived from algae, specifically from halophillic algae, for the later compound (Volkman et al., 1998; Zhu et al., 2005), indicates a greater contribution of these organisms to the organic matter and a possibly higher salinity during deposition of the top of Unit C. The higher values of the ratios phytane/ n-C18 and gammacerane/C30αβ hopane (Figs. 9 and 10, Tables 1 and 2) are also consistent with a higher salinity (Kates, 1978; Ten Haven et al., 1988). According to Sinninghe Damsté et al. (1995), the presence of a high relative abundance of gammacerane constitutes evidence of stratification of the water column. However, based on low values of TOC and HI, in conjunction with the high oxidation of the organic matter present (Figs. 5 and 6) it is not possible to postulate this interpretation for the top of Unit C. The boundary between Taquaral (Unit C) and Assistência members (Unit D) is marked by the appearance of the first limestone layers interbedded with marls (identified by IR values between 14 and 76%), as opposed to the siliciclastic characteristic (identified by high IR values, around 90%) of the Taquaral Member (Figs. 3 and 4). Chemostratigraphic unit D represents the top of the HST of Sequence I, with TOC values between 0.24 and 1.94% (Figs. 3 and 4). On the diagram this unit is characterized by type II/III to III organic

predominance of oxidized amorphous organic matter (sample 38.60 m) or opaque phytoclasts (samples 37.40 m and 35.90 m) (Figs. 3–6), indicating an aerobic environment (Tyson, 1987). In the diagram, Unit C is characterized by the predominance of type III organic matter (Fig. 7), which is consistent with the palynofacies data. Another important characteristic of this unit is the presence, although low in relative proportion and in diversity, of acritarchs (Fig. 6). This fact, together with a representative presence of phytoclasts suggests that this unit was deposited in a proximal marine environment under continental influence (Cazzulo-Klepzig et al., 1989; Holz and Diaz, 1998; Chahud and Petri, 2013). The lower total organic carbon contents and HI values, the higher relative abundance of phytoclasts and lower relative proportion of preserved amorphous organic matter, compared to those of Unit B (Figs. 5 and 6; Table 2), are due to the fact that Unit C represents a deposit related to the HST, thus, shallower and more proximal to the area of influx of terrestrial organic matter. Compared to Unit B, the relative abundance of hopanes in the total ion chromatograms and the values of the ratios hopanes/ steranesC27(17α) trisnorhopane/C30αβ hopane, C27(17β) trisnorhopane/C30αβ hopane, C30βα moretane/C30αβ hopane and C31αβ R homohopane/C30αβ hopane are substantially higher at the base of Unit C (Figs. 9 and 10; Tables 1 and 2). This is consistent with the evidence of palynofacies and Rock-Eval pyrolysis data, of a possible greater contribution of higher plants (Tissot and Welte, 1984; Henz et al., 1987; Villar et al., 1988) in the organic matter composition of this interval. The samples from the base (samples 38.60 m and 37.40 m) and the top (sample 35.90 m) of Unit C reveal distint biomarker distributions and ratios (Figs. 9–11; Tables 1 and 2). Comparatively, samples at the base show predominance of n-alkanes and a higher relative abundance of hopanes, higher values of the ratios hopanes/steranes, C27 (17α) trisnorhopane/C30αβ hopane, C27 (17β) trisnorhopane/C30αβ hopane,

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Fig. 14. Mass chromatograms (m/z 191) of units C and D. G = gammacerane; 30αβ = C30αβ hopane; 27α = C27(17 α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

mgHC/gTOC and, consequently, the position of the samples in van Krevelen diagram (Fig. 7). The contrast between units C and D is striking based on their lithological, geochemical and palynofacies features. The highest values of TOC and HI and the greater proportion of preserved amorphous organic matter indicate that the base of Unit D was deposited in a more reducing environment, although according to Tyson (1987), the data obtained still indicate a predominantly aerobic environment (Figs. 3–6). The higher relative abundance of steranes, n-alkanes and iso-alkanes, together with the very low values of the ratios hopanes/steranes (< 1, Fig. 13, Tables 1 and 3), indicate a strong influence of marine algae (Moldowan et al., 1985) in the composition of the organic matter of Unit D. Samples from the base of Unit D reveal lower pristane/phytane

matter (Fig. 7). The highest values of TOC and HI occur at the base, which is composed of marls (interval between 35.00 m and 33.50 m), and the lowest values are in the top, which is composed of limestones (interval between 32.00 m and 31.30 m) (Figs. 3 and 4). The marl layers, richer in organic matter, coincide with humid periods and higher water levels, while the dry phases produced limestones deposited in shallow water and poor preserved organic matter (Holz et al., 2010). The presence of structures characteristic of aerial or subaerial exposure (mud cracks and teepee, CPRM, 1986) in limestones, indicates that Unit D is a sequence boundary (Figs. 3 and 4) (Vail et al., 1977). In the marl samples examined in the microscope (34.70 m, 33.80 m and 33.40 m), a predominance of partially oxidized amorphous organic matter was observed (sample 34.70 m), or a mixture between partially oxidized amorphous organic matter and opaque phytoclasts (samples 33.80 m and 33.40 m) (Figs. 5 and 6), resulting in HI values below 400

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Fig. 15. Mass chromatograms (m/z 217) of units C and D. 27αααR = C27ααα(20R) cholestane; 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; 29αααS = 29ααα(20S) ethylcholestane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

of the sample from the top of Unit C (Figs. 14 and 15 and Table 1), indicating that both samples were deposited in a hypersaline environment (Ten Haven et al., 1987, 1988; Volkman et al., 1998; Zhu et al., 2005). In the North sector of the basin there is evidence of evaporites (Milani et al., 2007) in the limestone layers, which confirms this assertion. The higher salinity together with the presence of desiccation structures, indicate an environment of very shallow waters. This interpretation is consistent with the study of Oelofsen and Arajo (1983),

ratios than those from Unit C (Fig. 13 and Table 1). In addition to this fact, TOC, HI and palynofacies data provides evidence that the base of Unit D was deposited in an environment more prone to preservation of organic matter. Additionally, based on Fu Jiamo et al. (1986), low values pristane/phytane ratios constitute evidence of strongly reducing and hypersaline conditions. However, the samples from the base of Unit D exhibit values of the ratios gammacerane/C30αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane similar to those

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Fig. 16. Total ion chromatograms (TIC) of Unit E. Pr = pristane; Ph = phytane; 19 to 29 = C19 to C29 n-alkanes; St = steranes; Hp = hopanes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

(< 0.5%) and HI (< 50 mgHC/gTOC), and the organic matter is predominantly composed of opaque phytoclasts (Figs. 3, 4, 6 and 12), indicating an aerobic environment (Tyson, 1987). At the middle and upper part of Unit E, between 30.40 m and 26.20 m, the shales are laminated and sometimes bituminous, with no indication of bioturbation (CPRM, 1986), exhibit moderate to high values of TOC (between 1% and 17%), S (between 2% and 4%) and HI (between 200 and 500 mgHC/gTOC), and amorphous organic matter with different intensities of fluorescence predominate in their composition (Figs. 3, 4, 6 and 12), which characterizes a dysaerobic to anoxic environment (Tyson, 1987). According to the van Krevelen diagram, the samples from the base behave as type III, and those from the middle and upper part behave as type II/III to II (Fig. 7), which is consistent with the palynofacies (Figs. 6 and 12; Table 3). The base of the unit is characterized by the presence, although in low relative proportion, of Botryococcus algae (Fig. 8). This in addition to a significant presence of phytoclasts (Figs. 6 and 12), indicate that this section was deposited in a proximal environment, under fluvial influence (Tyson, 1995). In the upper part of Unit E, the higher values

which identified the presence of mesosaurus from Stereosternun and Brazilossauros genera that inhabited shallow waters. On the other hand, Sinninghe Damsté et al. (1995) propose that the presence of gammacerane in a high relative abundance constitutes evidence of stratification of the water column. However the TOC, HI and palynofacies data do not corroborate this interpretation for the samples from Unit D. Another difference between the top of Unit C and the base of Unit D is the significantly lower values of the ratio C27ααα (20R) cholestane/ C29ααα (20R) ethylcholestane in the samples from Unit D (Fig. 15 and Table 1), indicating, according to Grantham and Wakefield (1988), Volkman (2003), Schwark and Empt (2006) and Kodner et al. (2008), a greater influence of green algae in the composition of the organic matter of those samples. Chemostratigraphic unit E represents the TST of Sequence II, and is composed of shales known for their high values of IR (between 79% and 90%), which differentiates it lithologically from Unit D (Figs. 3 and 4). At the base, between 31.00 m and 30.70 m, the shales are massive and exhibit bioturbation (CPRM, 1986), low values of TOC (< 1%), S

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Fig. 17. Mass chromatograms (m/z 191) of Unit E. G = gammacerane; 30αβ = C30αβ hopane; 27α = C27(17α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane. The predominance of steranes and the high relative abundance of C27ααα (20R) cholestane, which derived from algae, specifically red algae for the latter compound (Kodner et al., 2008), indicate a greater contribution of these organisms in the composition of the organic matter of the samples from the base of this unit. The higher values of the gammacerane/C30αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane ratios in the samples from the base of Unit E, deposited in an aerobic environment, demonstrate that this section was deposited during a period of higher salinity (Ten Haven et al., 1988; Volkman et al., 1998; Zhu et al., 2005). The greater influence of bacteria in the composition of the organic matter in the samples from the mid and upper part, in relation the samples from the base, is easily characterized by the predominance of phytane and the larger relative proportion of hopanes in the total ion chromatograms, by the increasing values of the ratio hopanes/steranes and by the increase in values of HI (Tissot and Welte, 1984). Chemostratigraphic unit F constitutes the greater part of the HST of

of TOC and a better quality of organic matter (Figs. 6 and 12), compared to those from the base, are due to the fact that they represent deposit related to the marine flooding surface, thus deeper and more distal from the area of continental influence (Tyson, 1995). This interpretation is consistent with the published data by Oelofsen and Araújo (1983), who identified the presence of mesosaurus from Mesosaurus genus that lived in relatively deeper waters. Based on biomarker data it is also possible to differentiate the samples from the base from those of the middle and upper part (Figs. 16–18; Tables 1 and 3): at the base, it can be observed the predominance of steranes, lower values of the ratio hopanes/steranes and higher values of the ratios gammacerane/C30αβ hopane, C27ααα (20R) cholestane/C29ααα (20R) ethylcholestane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane; and in the mid and upper part, the predominance of phytane and a greater relative proportion of hopanes in the total ion chromatograms, larger values of the ratio hopanes/steranes and lower values of the ratio gammacerane/C30αβ hopane, C27ααα (20R) cholestane/C29ααα (20R) ethylcholestane and

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Fig. 18. Mass chromatograms (m/z 217) of Unit E. 27αααR = C27ααα(20R) cholestane; 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; 29αααS = 29ααα(20S) ethylcholestane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

Tables 1 and 3), is also consistent with a larger contribution of higher plants (Tissot and Welte, 1984; Henz et al., 1987; Villar et al., 1988) in the composition of the organic matter from Unit F. Despite all samples from Unit F being predominantly composed of terrestrial organic matter, the lower values of the ratios hopanes/steranes and C27ααα (20R) cholestane/C29ααα (20R) ethylcholestane in the samples from the lower part of Unit F (25.60, 24.30, 21.70, 20.20 and 17.90 m; Fig. 21; Tables 1 and 3) still indicate a greater influence of green algae (Grantham and Wakefield, 1988; Moldowan et al., 1985; Volkman, 2003; Schwark and Empt, 2006; Kodner et al., 2008), in the organic matter composition from these samples. The gradual upward increase of gammacerane/C30αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane ratios in Unit F (Fig. 20; Tables 1 and 3), deposited in an aerobic environment, suggest a modification in the depositional system, most probably as a

sequence II and is composed of silty shales, recognized by higher values of IR (around 90%), with bioturbation, wavy and linsen stratification (CPRM, 1986), TOC values below 1% and predominance of opaque phytoclasts (Figs. 3, 4, 6 and 12), indicating an aerobic environment dominated by storm waves (Tyson, 1987; Milani et al., 2007). The very low values of HI (< 50 mgHC/gTOC) and, consequently, the position of the samples in a diagram, behaving as type III (Fig. 7), are a consequence of these paleoenvironmental conditions. The very low values of TOC and HI, and the higher relative abundance of phytoclasts, when compared to those from the top of Unit E (Figs. 6 and 12), are due to the fact that they represent deposits related to the HST, thus shallower and more proximal to the area of influx of terrestrial organic matter. The increasing proportion of high molecular weight n-alkanes and values of C27 (17β) trisnorhopane/C30 αβ hopane ratio and C31 αβR homohopane/C30 αβ hopane ratio (Figs. 19 and 20;

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Fig. 19. Total ion chromatograms (TIC) of units E, F, G and H. Pr = pristane; Ph = phytane; 19 to 29 = C19 to C29 n-alkanes; St = steranes; Hp = hopanes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

indicating an aerobic environment (Tyson, 1987). Permian palynofacies with a high contribution of bisaccate pollen grains generally indicate dry climates (Wilson, 1962; Dino et al., 2002). In a diagram, this unit is characterized by the predominance of organic matter of type III (Fig. 7), which is consistent with the palynofacies data. Despite the greater contribution of higher plants, the predominance of steranes in the total ion chromatogram, the very low values of the ratios hopanes/steranes and C27ααα (20R) cholestane/C29ααα (20R)

result of evolution from normal saline water to hypersaline water (Ten Haven et al., 1988; Volkman et al., 1998; Zhu et al., 2005). Chemostratigraphic unit G represents the top of the HST of Sequence II, and is composed of laminated limestones identified by low IR values between 24% and 34%, which distinguishes it lithologically from Unit F (Figs. 3 and 4). The TOC values are low (< 0.50%) and the organic matter is composed of a mixture of opaque phytoclasts and partially oxidized bisaccate pollen grains (Figs. 6 and 12; Table 3),

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Fig. 20. Mass chromatograms (m/z 191) of units E, F and G. G = gammacerane; 30αβ = C30αβ hopane; 27α = C27(17 α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

climate during deposition of Unit G was arid, it is reasonable to infer that the higher relative proportions of phytane and gammacerane are tied to a higher salinity of the environment (Kates, 1978; Ten Haven et al., 1988). Chemostratigraphic unit H in this well was only partially preserved. Unit H represents the base of TST of Sequence III, and is composed of shales known for their elevated values of IR (between 71% and 93%), which differentiates it lithologically from Unit G (Figs. 3 and 4). The TOC values range between 0.40% and 2.00%, those for S vary between 0.20% and 2.00%, and those for HI vary between 50 and 300 mgHC/

ethylcholestane (Figs. 19 and 21; Table 3), indicate an influence of green algae (Grantham and Wakefield, 1988; Moldowan et al., 1985; Volkman, 2003; Schwark and Empt, 2006; Kodner et al., 2008) in the composition of the organic matter of this unit. Unit G can be distinguished from the former based on these geochemical characteristics (Figs. 19 and 21; Tables 1 and 3). Other biomarker characteristics that differentiate units F and G are the higher values of the ratios phytane/nC18 and gammacerane/C30αβ hopane for Unit G (Figs. 19, 20 and 22; Tables 1 and 3). Considering that both units were deposited in aerobic environments and that the

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Fig. 21. Mass chromatograms (m/z 217) of units E, F, G and H. 27αααR = C27ααα(20R) cholestane; 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; 29αααS = 29ααα(20S) ethylcholestane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

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Fig. 22. Mass chromatograms (m/z 191) of units E, F, G and H. G = gammacerane; 30αβ = C30αβ hopane; 27α = C27(17 α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

intervals indicates a more arid climate (Wilson, 1962; Dino et al., 2002). Compared to Unit G, it can be observed that at the base of Unit H there is an increase in the values of the ratios pristane/phytane, C27 (17α) trisnorhopane/C30αβ hopane, C27 (17β) trisnorhopane/C30αβ hopane, C30βα moretane/C30αβ hopane and C31αβ R homohopane/ C30αβ hopane, and a reduction in the values of the ratios gammacerane/C30αβ hopane (Figs. 19, 21 and 22; Tables 1 and 3). Considering that both sections were deposited in aerobic environments and that the climate during deposition of Unit G was more arid (presence of bisaccate pollen grains, Figs. 6 and 12), it is reasonable to infer that the modifications observed are due to the fact that the base of Unit H was deposited in a period of lower salinity, as observed in other sedimentary basins (Rullkotter et al., 1984; Connan et al., 1986 Ten Haven et al., 1988). In relation to Unit G, it can be observed that at Unit H there is an increase in values of the ratios C27ααα (20R) cholestane/C29 ααα (20R)

gTOC, with the higher values in the sections from 11.10 m to 10.50 m, 9.60 m to 9.30 m and 7.70 m to 7.10 m (Figs. 3 and 4). In the diagram, samples from the intervals richer in organic matter exhibit predominance of type II/III organic matter, while samples from intervals poorer in organic matter show predominance of type III organic matter (Fig. 7). Utilizing palynofacies, it can be observed that the samples of type II/III are composed of partially oxidized amorphous organic matter (interval 11.10 m–10.50 m) or a mixture between amorphous organic matter and partially oxidized bisaccate pollen grains (interval 7.70 m–7.10 m), while the samples of type III are composed of a mixture between opaque phytoclasts and bisaccate partially oxidized pollens (interval 9.10 m–8.00 m) (Figs. 6 and 23). According to Tyson (1987), the association of these data indicate that the environment during deposition of this unit varied between aerobic (intervals 12.60 m–11.40 m; 10.20 m–9.90 m and 9.10 m–8.00 m) and dysaerobic (interval 11.10 m–10.50 m, 9.60 m–9.30 m and 7.70 m–7.10 m). On the other hand, the presence of bisaccate pollen grains in the upper

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Fig. 23. Microphotographs of the organic content of samples from Assitência Member (Unit H). Left, observation under white transmitted light; right, under ultraviolet light. The values of total organic carbon (TOC) and hydrogen index (HI) for each sample are highlighted.

trisnorhopane/C30αβ hopane, C30βα moretane/C30αβ hopane, gammacerane/C30αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane and decrease values of the ratio C27ααα (20R) cholestane/C29 ααα (20R) ethylcholestane (Figs. 24–26; Tables 1 and 3), which, together with the greater relative proportion of pollen grains towards the top, are good evidence of the influence of higher plants

ethylcholestane (Fig. 21), indicating a greater influence from red algae in the composition of the organic matter (Kodner et al., 2008). The samples of Unit H exhibit different biomarker distributions among them: from the 12.60 to the 7.10 m, there is a trend towards an increase in the relative proportion of hopanes in the total ion chromatograms, of the values of the ratios hopanes/steranes, C27 (17β)

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Fig. 24. Total ion chromatograms (TIC) of Unit H. Pr = pristane; Ph = phytane; 19 to 29 = C19 to C29 n-alkanes; St = steranes; Hp = hopanes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

higher relative amounts of non-opaque phytoaclasts (Unit F) or polens and spores (Unit H). Biomarker data together with total organic carbon data, rock-eval pyrolysis and palynofacies allowed us to recognize three 4th order sequences. Each sequence has two systems tracts, one lower, transgressive, and another upper, a highstand systems tract. At the base of the transgressive systems tracts, the depositional sequences exhibit characteristics of deposits laid down in aerobic and proximal environments, such as low values of TOC and HI and the abundance of phytoclasts, whose effect on biomarker data are the predominance of high molecular weight linear alkanes in the total ion chromatograms and the high values of the ratios hopanes/steranes, C27 (17α) trisnorhopane/C30αβ hopane, C27 (17β) trisnorhopane/C30αβ hopane, C30βα moretane/ C30αβ hopane and C31αβ R homohopane/C30αβ hopane. At the top of the transgressive systems tracts, which correspond to the marine flooding surface, the samples exhibit dysaerobic to anoxic and more distal environmental indicators, such as moderate to high TOC and HI values, as well as a greater relative proportion of marine algal origin, amorphous organic matter, whose effect on biomarkers are the low values of the ratios hopanes/steranes. At the base of the highstand

(Tissot and Welte, 1984; Moldowan et al., 1985; Henz et al., 1987) in this direction. 6. Conclusions In the study of the biomarkers of the Lower Permian of the Paraná Basin, specifically the Irati Formation, a greater relation of gammacerane with salinity variations of the sedimentation environment was evidenced, than with the stratification of the water column. It was also observed that the Irati Formation intervals with hypersaline deposition characteristics (top of Unit C, Unit D and base of Unit E) presented higher values of the C28/C29αααR steranes ratios in relation to the other intervals deposited in the condition of lower salinity. In the carbonate intervals (units D and G) the lower values of the C27/C29αααR steranes and hopanes/steranes ratios were observed, suggesting a greater contribution of green algae, possibly algae mats. In the interval that presented higher relative amount of opaque phytoplasts (Unit A), the highest values of the C27Tm/C30αβ hopanes and hopanes/steranes ratios were observed. On the other hand, a higher relative proportion of C31αβ hopane was observed in the intervals with

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Fig. 25. Mass chromatograms (m/z 191) of Unit H. G = gammacerane; 30αβ = C30αβ hopane; 27α = C27(17 α) trisnorhopane; 27β = C27(17β) trisnorhopane; 29αβ = C29αβ norhopane; 31αβR = C31αβR homohopane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

systems tracts there are evidences of aerobic, proximal environments, such as low values of TOC and HI, phytoclasts in greater relative proportion, the predominance of high molecular weight linear alkanes in the total ion chromatograms and the high values of the ratios hopanes/ steranes, C27 (17α) trisnorhopane/C30αβ hopane, C27 (17β) trisnorhopane/C30αβ hopane, C30βα moretane/C30αβ hopane and C31αβ R homohopane/C30αβ hopane. At the top of these systems tracts there are

indicators of aerobic, hypersaline environments with subaerial exposure, in particular: low values of TOC and HI, high relative proportion of oxidized organic matter, low relative proportion of n-alkanes, higher values of the ratios phytane/nC18, gammacerane/C30αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane and limestones with mud crack and teepee structures.

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Fig. 26. Mass chromatograms (m/z 217) of Unit H. 27αααR = C27ααα(20R) cholestane; 28αααR = C28ααα(20R) methylcholestane; 29αααR = C29ααα(20R) ethylcholestane; 29αααS = 29ααα(20S) ethylcholestane; Hp = C30αβ hopane; St = major peak between steranes. The values of total organic carbon (TOC), insoluble residue (IR), generation potential (S2) and hydrogen index (HI) for each sample are highlighted.

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Acknowledgements

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