Use of Tmax as a thermal maturity indicator in ...

Clay Minerals, (2010) 45, 115–130

Use of T max as a thermal maturity indicator in orogenic successions and comparison with clay mineral evolution F. DELLISANTI1,*, G. A. PINI1 1

AND

F. BAUDIN2

Dipartimento di Scienze della Terra e Geo-Ambientali, Universita` di Bologna P. Porta S. Donato, 1 I-40126, Bologna, Italy, 2 UPMC ! Universite´ de Paris 06, CNRS-UMR 7193, ISTEP, Equipe Evolution et Mode´lisation des Bassins Se´dimentaires, case 117, 4 place Jussieu 75252 Paris Cedex 05, France

(Received ; revised )

AB ST R ACT : The relationship between three parameters, the Tmax given by Rock-Eval pyrolysis, the illite content in illite-smectite mixed layers (I-S) and the Ku¨bler Index (KI) has been investigated in the Cretaceous!Neogene, sedimentary syn-orogenic successions in the Northern Apennines (Italy). A strong relation was found between maturity stages of kerogen, illite content in I-S and KI. The oil formation zone for continental organic matter (Type III), delimited by Tmax between 434 and 465ºC, corresponded to rocks with short-range ordering R1, I-S with illite content between 60 and 85% and KI values in the range 0.85!0.65 (ºD2y). Over-mature rocks were characterized by Tmax >465ºC, a long-range ordered I-S with an illite content >85% and KI in the range 0.65!0.45 (ºD2y). The relationship permits use of both mineralogical parameters and T max to estimate palaeotemperatures in sedimentary successions and it can be exploited in hydrocarbon research to evaluate the petroleum potential.

KEYWORDS: Rock-Eval pyrolysis, Tmax, Ku¨bler Index, illite!smectite, thermal maturity, palaeotemperatures, petroleum potential. Tmax is defined as the pyrolysis temperature at which the maximum amount of hydrocarbon is released by kerogen (Espitalie´ et al., 1977). As a general rule, Tmax increases linearly with the maturation degree of the organic matter (Barker, 1974; Espitalie´ et al., 1977), thus giving a rapid estimate of the thermal maturity of sedimentary basins. Tmax is dependent on the cracking kinetics of the organic matter and is correlated with the type of organic matter: lacustrine (Type I), marine (Type II) and continental (Type III). The relationship between Tmax and different stages of oil and

* E-mail: [email protected] DOI: 10.1180/claymin.2010.045.1.115

gas formation zones varies with the type of organic matter; however Type III is the most reliable in estimating the maturation degree (Espitalie´, 1986). The variation of Tmax with the organic matter maturity has been compared to the change in vitrinite reflectance (%Ro) (Teichmu¨ ller & Durand, 1983; Espitalie´ et al., 1984) and, for the kerogen of Type III, the beginning of the oil formation is characterized by Ro of 0.5% and Tmax in the range 430!435ºC, whereas the transition oilgas zone is fixed at Ro of 1.35% and Tmax ~465ºC (Espitalie´, 1986). The progressive conversion of smectite to illite and the ordering of the illite structure, indicated by the Ku¨bler Index (‘illite crystallinity’) (Ku¨bler, 1967; Guggenheim et al., 2002), are a function of the burial temperature (see Kisch, 1983; Frey, 1987; Frey & Robinson, 1999). The illitization reaction

# 2010 The Mineralogical Society

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and the illite Ku¨bler Index are widely used as thermal indicators in sedimentary successions of low- and very low-grade metamorphism and, associated with the vitrinite reflectance, are used to determine a scale of palaeotemperatures (Jaboyedoff & Thelin, 1996; Merriman & Frey, 1999). However, the relationship between clay minerals and Tmax evolution is not well defined. Burtner & Warner (1986), in a preliminary study using selected samples from wells, reported a correlation for the oil window, indicating Tmax values in the range 435!460ºC and an illite content in illite-smectite mixed layers (I-S) between 60 and 90%, although they did not find a linear relationship. On the contrary, a poor correlation between the illite content in I-S and kerogen maturity was found by Scotchman (1987), who pointed out that the illitization reaction was strongly influenced by either the pore-water chemistry or K+ availability, suggesting the need to adopt extreme caution in using the clay mineral evolution as an indicator of the thermal maturity of sedimentary basins when the chemical factors are not well constrained. Velde & Espitalie´ (1989) have found regular trends between the illite content in I-S and Tmax values in wells from six sedimentary basins, suggesting a range of correlation between the two parameters. However, the relationship between the kerogen maturity and clay transformation applied only to sediments at the end of the illitization reaction (about 90% of illite content in I-S) whereas, between immature kerogen and more smectitic I-S mixed layers, the correlation was not clear or failed because of the different reaction kinetics. Furthermore, the data analysed in previous work corresponded only to late Tertiary and older sequences, while the kerogen maturity and clay mineral evolution could also be dependent on the age of the rocks (Velde & Espitalie´, 1989). A well defined correlation between Tmax and mineralogical data can be useful both to enable the use of Tmax as a geothermometer and to support petroleum research in calculating maturity organic parameters. Thus, to overcome the uncertainty mentioned above, the present work reports a study of the relationship between Tmax and clay mineral evolution, examining sedimentary syn-orogenic successions covering a wide timespan from the early Cretaceous up to the late Pliocene. In addition, as a novel approach, the correlation between Tmax and the Ku¨bler Index in the same sedimentary successions is also reported.

GEOLOGICAL SETTING The Northern Apennines of Italy are an orogenic, northeast-verging wedge composed of stacked nappes and thrust-bounded units (Elter, 1975; Barchi et al., 2001; Carmignani et al., 2001; 2004; Cerrina Feroni et al., 2004). The more extensive and distant allochthonous nappe, the Ligurian nappe, consists of several structural units, which can in turn be grouped into the Ligurian and Subligurian units (Bortolotti et al., 2001; Cerrina Feroni et al., 2004, and references therein) (Fig. 1). The Ligurian units derive from the middle Jurassic to late Cretaceous-Eocene sedimentary cover and basement of the Ligurian ocean, a part of the oceanic seaways of the Neo-Tethys, and from the thinned margin of a conterminous, easterlyplaced, continental domain (African promontory, Channell et al., 1979, or Adria microplate, Dercourt et al., 1986). The so-called Internal Ligurian units actually display complete sequences of oceanic crust beneath the sedimentary cover (Bortolotti et al., 2001; Marroni & Pandolfi, 2001). The oceanic crust is only present as blocks and slabs in mass wasting deposits at the stratigraphic base of the external Ligurian units, together with fragments of continental crust (Marroni et al., 2001). The Subligurian units are considered to represent the sedimentary cover of the Adria microplate originally positioned to the East of the Ligurian units (Bortolotti et al., 2001). The Ligurian units, or part of these, participated in a late Cretaceous-Eocene accretionary wedge (e.g. Vai & Castellarin, 1993; Pini, 1999, Vescovi et al., 1999; Daniele and Plesi, 2000; Marroni & Pandolfi, 2001), mainly related to the subduction of oceanic crust, which was part of a single Alpine orogen (e.g. Laubscher et al., 1992; Doglioni et al., 1999; Castellarin, 2001; Mantovani et al., 2008), together with the Alpine units of Corsica (Molli, 2008, and references therein). The diverse structural units experienced different burial depths during this phases of deformation (hereafter defined as ‘Alpine phases’), with the Internal units that were stacked westward by thrust faults and, in turn, overthrust by the External units (see Antola unit in Fig. 1) (e.g. Marroni, 1994; Marroni et al., 1999, 2001; Levi et al., 2006). The estimated palaeotemperatures for the Internal Ligurian units increase gradually from E to W, until reaching the metamorphic condition of the Cravasco-Voltaggio unit and the Western Alps (Reutter et al., 1980, 1983; Leoni et al., 1996,

Relationship between Tmax and clay minerals

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FIG. 1. Structural-geological map of the Northern Apennines and location of the sampling sites.

1998; Ducci et al., 1995; Ellero et al., 2001; Leoni, 2001). Deformation also occurred in the External Ligurian units during this stage, possibly related to retrovedge kinematics, changes of subduction direction or to transpressions (Marroni & Treves, 1998; Marroni et al., 2001; Molli, 2008, and references therein). The Ligurian nappe as a whole has been subsequently involved in the Oligocene-Plio/ Pleistocene intracontinental deformations on the western margin of the Adria microplate (Apenninic phases: Castellarin et al., 1992; Cerrina Feroni et al., 2004). During these phases, the Ligurian nappe always maintained the highest position on top of the stack of nappes and units, since it progressively moved NE above the Adria margin (Boccaletti et al., 1990, and reference

therein; Castellarin et al., 1992). The age of emplacement of the Ligurian nappe above the sediments of the Adria microplate progressively changes from early Oligocene on the western side (close to the Tyrrhenian Sea), to early-middle Miocene along the Apennines main divide, to the middle-upper Miocene and Pliocene towards the NE border of the chain (Ricci Lucchi, 1986; Boccaletti et al., 1990; Lucente & Pini, 2008, and references therein). The translation of the nappe mainly occurred in submarine conditions, so that middlelate Eocene to Pliocene sedimentary successions deposited in satellite basins were deposited on top of the Ligurian nappe (Epiligurian basins: Ori & Friend, 1984; Ricci Lucchi, 1986). Deformation related to this stage also occurred inside the nappe, reactivating and/or offsetting the

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tectonic contacts inherited by the Alpine phases. The main structures have been referred to thrust faults propagating from the base of the nappe (Castellarin & Pini, 1989), normal faults (Bertotti et al., 1997; Capozzi & Picotti, 2002) and/or diapiriclike reactivation of stratally-disrupted units (me´langes) from the base/lower part of the nappe (Castellarin & Pini, 1989; Borgia et al., 2006; Festa et al., 2010). The translation of the Ligurian nappe and of the overlying Epiligurian basins coincides with the onset of thrust-bounded structural units from the successions of the western margin of Adria, such as Tuscan metamorphic units, the Tuscan nappe, the Cervarola-Falterona unit and the Umbria-RomagnaMarche fold-and-thrust belt (Fig. 1). Deformation of the Adria-related successions occurred at the front and below the lip of the advancing Ligurian nappe. The more recent sediments of the Adriarelated successions are thick, turbiditic sequences considered to be the infill of a migrating foredeep (the Macigno, Mt. Falterona, Mt. Cervarola and Marnoso-arenacea Formations) and finer-grained,

slope deposits (‘draping muds’; Ricci Lucchi, 1986). Tectono-sedimentary me´langes due to the deformation of coalescent bodies from submarine landslides and debris flows are interbedded between the top of the foredeep successions and the base of the Ligurian nappe (Lucente & Pini, 2008; Vannucchi et al., 2008; Camerlenghi & Pini, 2009). The maximum thickness of the Ligurian nappe, containing the Epiligurian sediments and the subnappe me´ langes, is 4!4.5 km, inferred from industrial wells and shown in some recent reconstructions (Cerrina Feroni et al., 2002; Boccaletti et al., 2004).

MATERIALS AND METHODS Sampling A suite of 71 samples was collected from the Epiligurian, External and Internal Ligurian, Subligurian and Cervarola-Falterona structural units cropping out in the Italian Northern Apennines (Fig. 1, Table 1). The units studied

TABLE 1. Mineralogical parameters (KI and % illite in I-S), total organic carbon (%TOC), Tmax and hydrogen index data, and vitrinite reflectance data (%Ro) for different structural units. (Tmax) indicates anomalously low values due to oil impregnation. Sample

Kubler Index (~º2y)

Illite in I-S (%)

Ordering I-S

Tmax (ºC)

Hydrogen Index (mgHC/gTOC)

TOC (%)

Ro (%)

569 !362 !351 !358 !331 !362 !353

15 54 45 57 21 22 56

0.97 0.21 0.34 0.66 0.58 0.47 0.59

n.m.

R1

476

60

0.21

R1 R3 R3 R3 R3 R3 R3 R3 R3 R3 R0 R0

465 438 468 456 499 563 !327 !358 !361 !353 419 420

45 38 20 106 17 22 63 51 193 112 80 80

0.29 0.55 0.54 0.65 0.64 2.22 0.36 0.24 0.14 0.16 0.30 0.31

Internal Ligurian (early Cretaceous–early Paleocene) T184 0.65 100 R3 T220 0.55 90 R3 T157 0.46 100 R3 T223 0.54 100 R3 T151 0.48 100 R3 T155 0.39 100 R3 T225 0.48 100 R3 Subligurian (middle Eocene) T189 0.8

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Epiligurian (late Eocenelate Pliocene T180 0.7 85 T192 0.47 100 T178 0.54 100 T160A 0.54 100 T193A 0.48 100 T176 0.54 100 T191 0.39 100 T195 0.69 88 T162 0.54 100 T145 0.53 100 Slr28 n.m. 22 Slr5pt n.m. 22

n.m. n.m.

0.91 1.68 1.57 0.73 n.m.

0.3

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Relationship between Tmax and clay minerals

TABLE 1. (contd.) Sample

Kubler Index (~º2y)

Illite in I-S (%)

Ordering I-S

Slr5apt Slr6 Af1c Slr5asp Slr43pt Slr28sp Slr25pt Slr6sp Slr39sp Slr27pt Slr39pt Slr48pt Af1ept Slr41pt Slr27sp T209

n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.

22 25 44 31 33 20 24 25 30 35 45 50 39 33 19 n.m

R0 R0 R0 R0 R0 R0 R0 R0 R0 R0 R0 R0 R0 R0 R0 n.m

External Ligurian (early Cretaceous–middle Eocene) T88 n.m. 54 R0 T113 n.m. 60 R0 T1pt n.m. 26 R0 T122 n.m. 60 R0 T110 n.m. 38 R0 T111 n.m. 60 R0 T29 0.77 76 R1 T114 n.m. 47 R0 T116 n.m. 51 R0 T168 0.87 67 R1 T92 0.78 84 R1 T133 n.m. 46 R0 T85 0.72 82 R1 T62in 0.82 69 R1 T66df 0.69 85 R1 T36 0.68 81 R1 T127 0.78 70 R1 T136 0.64 90 R3 T137 0.67 92 R3 T138 0.64 82 R1 T124 n.m. 54 R0 T213 0.66 89 R3 T221 0.65 86 R3 T205 0.68 86 R3 T204 0.59 92 R3 T143 0.78 84 R1 T144 0.63 91 R3 T146 0.6 92 R3 T159 0.7 91 R3 T172 0.51 90 R3 T187 0.68 84 R1 T201 0.74 83 R1 T219 0.63 90 R3 T222 0.59 90 R3 T106 n.m. n.m n.m n.m.: not measurable

Hydrogen Index (mgHC/gTOC)

TOC (%)

420 420 420 422 422 423 425 425 425 425 425 429 430 431 432 424

73 78 70 70 87 33 73 110 100 93 68 90 103 83 100 67

0.79 0.40 0.32 0.41 0.29 0.59 0.30 0.20 0.20 0.32 0.61 0.39 0.30 0.31 0.20 0.42

424 428 430 430 431 431 433 434 434 435 436 437 437 438 440 443 457 466 496 502 512 477 483 520 532 !330 !353 !359 !354 !363 !338 !349 !359 !361 418

68 61 95 80 76 90 170 48 38 111 20 42 166 137 160 2 73 54 18 37 11 58 47 13 11 59 56 35 49 126 142 86 114 91 68

0.37 1.24 0.18 3.42 0.84 0.37 0.20 2.10 0.73 0.63 0.85 0.81 0.50 0.31 0.13 1.98 0.41 0.77 0.56 1.08 0.92 0.33 1.15 0.96 1.20 0.29 0.35 0.32 0.27 0.36 0.25 0.26 0.50 0.45 0.36

Tmax (ºC)

Ro (%)

0.47 0.49 0.42 1 0.83 0.39

1.11 0.69

1.48 n.m. 0.76

0.39

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covered a wide range of thermal maturity, from a shallow diagenetic zone up to anchizone (Leoni et al., 1996; Botti et al., 2004; Dellisanti & Pini, 2007; Dellisanti et al., 2008). Since geothermal anomalies or short periods of intense heating due to magmatic intrusions were not present, a mean geothermal gradient of about 30ºC km!1 was assumed for the last, Oligocene-Pliocene to recent stages of intracontinental deformation (Reutter et al., 1980; Ventura et al., 2001; Zattin et al., 2002). High-pressure, low- temperature conditions (20!25ºC km !1 ) should be expected for the Cretaceous-Eocene accretionary wedge (Marroni, 1994; Molli, 2008). All the samples were analysed by Rock-Eval pyrolysis and by X-ray powder diffraction (XRD); in a few samples the vitrinite reflectance was also determined.

Analytical methods Rock-Eval pyrolysis is used to determine the petroleum potential and the thermal maturity of the kerogen occurring in a rock, as proposed by Espitalie´ et al. (1977; 1985a,b). The procedure consists of progressive heating (25ºC min!1) of the whole rock from 25ºC to 650ºC by using the Rock-Eval 6 analyser (VINCI Technologies) which measures the hydrocarbons released during the heating over the temperature range 300!650ºC. The amount of hydrocarbon compounds (HC) given in mg g!1 of dry rock (S2 parameter) indicates the oil not yet released from the rock by natural processes and represents the residual petroleum potential. The temperature at which the maximum amount of HC is generated is indicated as Tmax (culmination of S2 peak) and is used as an indicator of the thermal maturity of the rock (Espitalie´, 1986). The total organic carbon (TOC) can also be determined using the Rock-Eval 6 apparatus. The hydrogen index (HI) has been obtained from the ratio of S2/TOC. Because the pyrolysis of coals and kerogens of various origins has shown a good correlation between HI and the elemental composition of organic matter (Espitalie´ et al. 1985a), the correlation with Tmax permits determination of the type of organic matter in the rocks. Standard samples (IFP 160000 and FB 2330) and a blank were used as reference materials to calibrate the Tmax data. Only samples with S2 >0.1 mgHC g!1 were considered reliable in estimating the Tmax values (Espitalie´ et al. 1985a,b). Mineralogical and crystallographic data were determined on 60%. In samples from shallow diagenetic zones, characterized by the presence of a disordered R0-type I-S (illite content