Structural setting of the Adriatic basin and the main related petroleum exploration plays

Marine and Petroleum Geology 42 (2013) 135e147

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Structural setting of the Adriatic basin and the main related petroleum exploration plays P. Casero a, *, S. Bigi b a b

Via Enrico di San Martino Valperga 57, 00147 Rome, Italy Dipartimento di Scienze della Terra, Università “La Sapienza”, P.le A. Moro 5, 00183 Roma, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 February 2012 Received in revised form 23 July 2012 Accepted 27 July 2012 Available online 29 August 2012

Most of the oil and gas resources located within the Adriatic domain are genetically linked to the flexure of the Adria continental margin and to the evolution of the Apennines fold and thrust belt. The source rocks contained in the pre-flexure epi-continental successions reached the maturity window during the flexural subsidence or, alternatively, the flexural accommodating siliciclastic flysch themselves generated and stored hydrocarbons. The petroleum exploration plays of the Adriatic domain are tentatively classified in this paper, according to their geological evolution with respect to the Apennines fold and thrust belt. The description of the geological evolution of these structures and related petroleum plays are described, including plays set in undeformed or poorly deformed foreland areas. A new isochrones map showing the structural setting of the substratum at the level of the Fucoidi Fm. is presented. Several different groups of structures can be recognized in the Adriatic domain, that can be connected to the final phases of deformation of the Apennines, or to the interaction with the Dinarides fold and thrust belt front to the east. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Petroleum play Fold-and-thrust-belt Adriatic Sea Foredeep

1. Introduction A good variety of structural and stratigraphic plays occur in the Adriatic Sea, ranging from fault-related anticlines, developed in Plio-Miocene times, connected to the main Apennine thrust chain, and deeper carbonate structures developed in the south, to very shallow structure in Late Pliocene to Quaternary times in the central area. Since the 1950’s and increasingly in the last fifteen years, many papers concerning the geological evolution of the Adriatic Sea have been published, although few of them concerned to the hydrocarbon exploration (Pieri and Groppi, 1975; Royden et al., 1987; Zappaterra, 1990; De Alteriis, 1995; Ori et al., 1991; Argnani et al., 1997; Bertotti et al., 2001; Di Bucci and Mazzoli, 2002; Bigi et al., 2003; Battaglia et al., 2004; Ford, 2004; Zoetemeijer et al., 1993 among many others). Papers dealing with petroleum systems (i.e. Anelli et al., 1996; Lindquist, 1999; Bertello et al., 2010), define mainly the conditions (e.g., reservoir, source rock, maturity, seal, etc.) that must coexist to generate a petroleum accumulation. Our approach is to illustrate

* Corresponding author. Tel.: þ39 06 5504360. E-mail address: [email protected] (P. Casero). 0264-8172/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2012.07.006

the main petroleum plays in the central Adriatic domain, describing several different groups of structures defined with respect to their geological evolution within the Apennines fold and thrust belt, considering that the definition of a petroleum play should include both local field characters and the more general geological context. In this way, it should be possible to define a petroleum play referring not only to the source rock (i.e. Burano Petroleum System), but also to the different kind of hydrocarbon-bearing field structures and to their different ages. This work has the aim to provide a general picture of the geological setting of the most significant oil and gas fields of Adriatic domain, based on the geological relationships of the source rocks vs. the reservoir/trap. 2. Regional geological setting of the Adriatic domain The Adriatic petroleum province belongs to the North African continental margin (Anderson, 1987; De Alteriis, 1995; Channel, 1996; Royden, 1988; Battaglia et al., 2004; Piccardi et al., 2011). Throughout the Mesozoic and the Early Paleogene the epi-continental sedimentation was predominantly carbonatic resulting from a complex paleogeographic configuration of indenting deep water basins and open shallow platforms. In general the sedimentation was more continuous, but with low accretion rate, in the deep waters domains, and more

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discontinuous with long emersion/erosion periods (Albian, uppermost Cretaceous, Paleogene) and much higher rates in the carbonate shelf domains (Zappaterra, 1990; Casero and Roure, 1994). During Mesozoic times both extensional tectonic phases (i.e. Middle Liassic) and compressional paleoinversions (i.e. Lowermost Cretaceous) occurred (Ziegler, 1987; Ziegler et al., 1995). Moreover, Cretaceous basin sedimentation records pulses of accelerated subsidence (Marchegiani et al., 1999) that could be also related with Late Cretaceous extensional tectonics involving the carbonate platform domains (e.g. Shiner et al., 2004). Starting from the Middle Eocene onwards the African continental margin was involved in the orogenic processes responsible for the development of the Alps and the Apennines (Doglioni, 1991; Bertotti et al., 2001; Faccenna et al., 2003; Doglioni et al., 2006; Patacca et al., 2008). The flexure of the lithosphere belonging to the Adria margin started from the most internal areas and migrated eastward

through time, forming foredeep basins oriented sub-parallel to the belts and filled by large quantities of terrigenous (siliciclastic) sediments, derived from the erosion of the incipient inverted margin (orogen and former foredeep). Each flexural phase was accommodated either by the sedimentation of a flysch wedge, or by the sub marine gravitational emplacement of large rock masses detached from the inverted margin sequence. This development has been extensively described by several authors, who highlighted the peculiar characteristics of the Apennines within the framework of the evolution of a foreland fold and thrust belt (Zoetemeijer et al., 1993; Ori et al., 1991; Patacca and Scandone, 1989; Mazzoli et al., 2001, 2005; Ford, 2004; Tozer et al., 2006; Patacca et al., 2008, among many others) (Fig. 1). The Adriatic domain corresponds to the youngest part of the belt, strictly connected to the evolution of the Apennines fold and thrust belt and to the interaction with the Dinarides, which are subparallel orogenic belts with opposing vergences. Its development

Figure 1. Kinematic model of the Apennines (modified from Casero, 2004). In the map are indicated the main tectonic units and the ages of the main thrust fronts.

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covers the Pliocene-Pleistocene time span and represents the result of the Pliocene flexures (Ori et al., 1986; Patacca and Scandone, 1989; Ori et al., 1991; Casero, 2004) (Fig. 1). Although some of the geodynamic evolution features are beyond the aim of this paper, nevertheless some observations relevant to the petroleum exploration can be made. For example, the different structural style of the northern arc of the Apennines with respect to the central and the southern sectors, seems to suggest a different geodynamic evolution. In fact, in the central and southern sectors of the Apennines, there are evidences of the occurrence of an older PaleogeneeMiocene belt hidden by the subsequent involvement in the Pliocene flexure and thrust propagation (Casero and Roure, 1994). The flexural history is of great importance with respect to the hydrocarbon generation and accumulation, since: a) about three quarters of the Italian biogenic gas is related to Pliocene foredeep series, b) most of the thermogenic gas and condensate is probably issued from Miocene flysch series, and c) in many oil accumulations the source rock series entered the maturity window during the flexural subsidence. 3. The petroleum exploration plays and the main structure of the Adriatic Sea Most of the production of gas in Italy (about 10 billion m3 per year), comes from the Northern Adriatic Sea. These occurrences are

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associated with two important source rocks deposited in the foredeep terrigenous units of the foreland basins formed during the Apennines orogenesis. The older source is thermogenic gas-prone whereas the younger source is biogenic gas and is situated mostly in the outer Plio-Pleistocene foredeep domain. The most important gas fields of Italy, located in the eastern Po Plain and Northern Adriatic Sea, have originated from this source (Bertello et al., 2010 and references therein). The most significant structures and petroleum fields within the Adriatic domain have been selected not only for their economic importance, but rather for their geological interest and also on the basis of the quality of the available information (mostly published in the literature by AGIP geologists), which is very variable. A new time-structure, showing the structural setting of the substratum of the Adriatic Sea is here presented (Fig. 2). The reconstruction, based on the public seismic dataset from Videpi (2009) and on the well logs reaching the Fucoidi Formation (Aptian e Albian) highlights the main positive structures in the area. There are several parameters that characterize these different groups of structures, as the structural trend, the time of the main deformation and the detachment level. The main structural trends recognizable in the Adriatic domain are the Apennine trend, NWeSE, and the transversal one, SWeNE, that characterize several structures in the southern sector of the basin (Fig. 2 and supplementary material). The time of deformation comprises different episodes, starting from: i) the Early Jurassic extensional phase (Santantonio and Carminati, 2011 and references

Figure 2. Time-structure map of the central Adriatic Sea. The isochrones (every 100 ms TWT) are referred to the top of the Fucoidi Marls Formation (Aptian e Albian) and equivalent stratigraphic units. List of the significant wells used for this reconstruction is also included (See also the supplementary material).

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therein; ii) the contractional (?)/transtensional phase during the Late Cretaceous (Ziegler, 1987; Casero and Roure, 1994; Ziegler et al., 1995; Winter and Tapponnier, 1991), iii) the contractional phases consisting both of the thrust propagation during Middle and Late Pliocene (Casero, 2004; Bigi et al., 1997; Patacca et al., 2008 and references therein), and of the inversion and/or reactivation of the more external preexistent structures (Argnani and Gamberi, 1995; Gambini et al., 1997). Different detachment levels characterize the structural building of this area. The contractional structures related to the Middle and Late Pliocene thrust belt show mainly two detachment level: a deeper one, locate at the bottom of the Triassic-Miocene carbonate succession, within the Triassic evaporites, and a shallower one, located within or at the top of the Messinian evaporites level. The two detachment levels are connected by steeper thrust ramp cutting the carbonate succession. The upper ramps develop anticlines involving the loweremiddle Pliocene siliciclastic sequences. The structures in the more external area, instead, which are likely to be the results of inversion and/or reactivation related to the same contractional phase, show generally a deep detachment level, involving the Meso-Cenozoic carbonate successions and the Triassic evaporites (Argnani and Gamberi, 1995; Gambini et al., 1997; Casero, 2004). The classification of structures proposed in this paper comprises: 1) The middle Pliocene thrust folds (middle Pliocene thrust activity, Apenninic trend, northern sector of the Italian side of the Adriatic Sea, thrust flat within the Messinian evaporites) (Figs. 2 and 3), 2) The upper Pliocene thrust folds (Late Pliocene thrust activity, Apenninic trend, northern e central sector of the Italian side of the Adriatic Sea, in the footwall of the previous belt, thrust flat within the Messinian evaporites) (Figs. 2, 4 and 5), 3) The Middle Adriatic Ridge and the Pliocene inversion structures (contractional positive anticlines, deformed likely during the Tortonian, trending NNE-SSW, located along the central sector of the Adriatic Sea) (e.g. Gargano Mare 1) (Figs. 2 and 6a,b and 7). In some cases salt domes occur, as in the case of the

Mizar structure, located along the central axis of the Adriatic Sea, involving the whole stratigraphic succession up to the quaternary deposits (Fig. 8); 4) The Cretaceous extensional structures and the Apulia Talus (Cretaceous, mainly EeW, located in the southern sector of the basin) (Figs. 1, 2 and 9), 5) The Quaternary basins (in between the positive structures mentioned above, are present several stratigraphic trough, trending NWeSE, parallel to the main structures and filled by the younger portion of the stratigraphic succession).

3.1. The Middle Pliocene structures During the latest Early PlioceneeMiddle Pliocene the internal part of the Lower Pliocene foredeep was progressively thrusted and folded (Figs. 2 and 3). This sector corresponds to the present day northern and central Apennines foothills, and to the Costiera thrust front, located partly onshore (to the south) and offshore (to the north) (Fig. 1). The development of these thrusts completely reorganized the physiography of the basin, generating coeval thrust top basins and a new wide foredeep to the east (Patacca and Scandone, 1989, 2004; Bigi et al., 1997). In the thrust top basins some significant fields producing biogenic gas from shallow marine sands were discovered. The most important ones, however, are by far the accumulations of the foredeep basin, in the footwall of this main trend, perhaps the most prolific gas basins in Italy (e.g. Squalo Centrale, etc.) (Fig. 3). The biogenic gas is stored in multiple turbiditic sandy levels more or less gently folded according to their setting. Both the source rock and the seal are provided by clays alternating with sands. In the central Adriatic offshore a number of commercial, middlesized fields (Sarago Mare 1, Mormora Mare 1, S. Giorgio Mare 1, David 1, Emilio 1, Piropo 1 wells, etc.) (Fig. 2) produce oil and/or gas from Upper Cretaceous-Paleocene resedimented, fractured bioclastic limestones intercalated in a dominant pelagic mudstone series (Scaglia Formation). These beds are interpreted to come from a nearby carbonatic shelf margin (talus sediments). However, as they have proximal characters (even coarse breccia facies) and are distributed along narrow, elongated trough with no evidence of carbonatic shelf in the area, it seems most likely that these

Figure 3. Line drawing of a seismic line showing the Middle Pliocene thrust-related fold involving in deformation the flexural Lower Pliocene deposits and the syn-orogenic sequence of Middle Pliocene. Upper Pliocene deposits passively cover the thrust front. To the East the coeval structures of Sarago and Mormora (see text for further explanation). Note the different detachment level for the Costiera structure and the easternmost ones. Location in Figure 2.

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Figure 4. Line drawing of a seismic line showing that the Costiera structure (crossed by the Atri and Savini 1 wells) is active until the late Pliocene time. The detachment level is located in the messinian evaporites, whereas gas fields are located in the middle Pliocene foredeep deposits. The Emma wells penetrate a more external inversion structure coeval to the Costiera one. Location in Figure 2.

resedimented beds come from unstable intra-pelagic ridges that have reached, at times, the photic zone and have been subsequently eroded (Colacicchi and Baldanza, 1986; Casero et al., 1990; Casabianca et al., 2002). The traps (Figs. 2 and 3) are double vergence, up thrust like, inversion folds, bounded by high angle faults; being essentially of Middle Pliocene age they probably reactivate old features. The producing structures lie clearly on a NWeSE oriented trend. The seal to the producing levels is provided by the overlying mudstones. The source rock is uncertain, possibly the

Upper Triassic Burano evaporitic formation or a kitchen deeply seated to the SW. 3.2. The Upper Pliocene structures A deformation of Late Pliocene age generated a regional unconformity above the uppermost Middle Pliocene deposits, which is evident on the flanks of the thrust top basins but very gentle in the outer foredeep basins. In the northern and central Apennines

Figure 5. The Upper Pliocene belt comprises structures that grew up until the Late Pliocene deforming previous onlapping sequences. An example are the anticlines penetrated by the Cornelia and Pesaro Mare wells, located offshore to the north of Ancona. Basal decollement in the Triassic evaporites. Location of the two geological cross sections in Figure 2. Data gently provided by RSE (Research on Energy Systems e RSE Italy Spa).

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Figure 6. Line drawing of two seismic lines showing two different structural style of the Middle Adriatic Ridge. a) Symmetric, positive fold and related thrusts, detached in the Triassic evaporites. b) salt diapir located along the core of the Middle Adriatic Ridge. To the east of both the sections, two unconformities evidence the Pyrenees (Eocene) and Sud Alpine deformational phases related to the Dinarides evolution (Casero and Roure, 1994). Location in Figure 2.

foreland basin, the Upper Pliocene fine grained siliciclastic series are much less prolific than the Middle Pliocene ones. The biogenic gas pools are mostly found in the transgressive basal levels (Figs. 2 and 3). The traps are gently refolded onlap surfaces on both the inner and

outer limbs of late Middle Pliocene thrust-related folds (e.g. Cornelia e Pesaro structure) (Fig. 5) or are either onlapping sequences on the flanks of the thrust top basins, or, more frequently, sequences draping on previous folds in the foreland basins.

Figure 7. Line drawing of a seismic line showing the Gargano Est Marine well crossing a deeply detached fold-related thrust. To the west, a number of normal faults evidencing the extensional Cretaceous deformation. Location in Figure 2.

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Figure 8. Line drawing of a seismic line showing the Mizar diapir occurrence. Location in Figure 2.

3.3. The Middle Adriatic Ridge At the eastern border of the investigated area, corresponding to the central axis of the Adriatic Sea, the occurrence of positive structures involving the TriassiceMiocene succession, and buried under the Quaternary deposits, is largely recognized by many authors (Scrocca, 2006 and references therein). It was called Mid-Adriatic Ridge by Finetti (1982) and Central Adriatic Deformation Belt by Argnani and Gamberi (1995); it consists of an array of structural highs along a main NWeSE trend (Figs. 2 and 6a). Many authors interpreted these structural highs as east-verging thrust-related folds (e.g., Bally et al., 1986; Ori et al., 1991; De Alteriis, 1995; Coward et al., 1999; Calamita et al., 2003; Scrocca, 2006), related to the development of the Apennines, although evidence of salt diapirism has been also recognized (Fig. 6b). In some cases, the ridge is constituted by the diapir itself (e.g. the Mizar structure) (Bally et al., 1986) (Fig. 8). Other Adriatic ridges caused by folds show a symmetric geometry, suggesting to be the result of tectonic inversion along preexisting extensional faults developed during the end of the Early Cretaceous and the Tertiary. The origin of the extensional Cretaceous events has been related to the onset of the convergence between Europe and Africa, and is evidenced by several structures

in the Adriatic Sea (Argnani et al., 1993; Scisciani and Calamita, 2009) (e.g. Gargano Mare 1 structure, Figs. 2 and 7). The superposed Pliocene tectonic inversion is due to the Apenninic compression or to diapirism as evidenced by the occurrence of Pliocene folded deposits and growth strata (Argnani et al., 1993; De Alteriis, 1995; Gambini et al., 1997; Bertotti et al., 2001); some authors suggest a thick-skinned tectonic inversion process for these structures (e.g., Argnani and Frugoni, 1997; Bertotti et al., 2001; Calamita et al., 2003; Bertello et al., 2010). 3.4. The Apulia carbonate platform As in other sectors of Italy, some important petroleum systems are fully contained in pre-flexural carbonatic series and do not depend on flexural subsidence. One of them is the huge heavy oil Rospo field, located in the northwestern sector of the Apulia carbonate shelf (Figs. 2 and 9), discovered in the mid-seventies. The stratigraphic section of this part of the Apulia shelf is made up of Upper Triassic alternations of dolomites and anhydrites (Burano Formation), thick Jurassic dolomitic series of inner shelf environment (Ugento Dolomites Formation) and Lower Cretaceous mudstones and bioclastic packstones (Cupello Limestones Formation). Seismic and well data show that the massive shelf series change laterally in facies towards the NE to well-bedded mudstone

Figure 9. Line drawing of a seismic line crosses the Apulia carbonate platform of Jurassic age, its slope-to-basin sequence and the pelagic deposits of the same age. See text for explanation. Rospo field is the main oil field in the Adriatic domain. Location in Figures 2 and 10.

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series (Fig. 9) of deeper water platform and euxinic environment (Emma Limestones). Probably during the early Late Cretaceous the shelf area emerged, in the meantime, a gently tilting to the NE occurred, and the shelf was deeply eroded, while the sedimentation continued in the deeper platform area (Casero et al., 1990). A large topographic high zone, strongly karstified, evidences the shelf emersion. During the Early Miocene the high was progressively transgressed by shallow marine, thin, glauconitic grainstones series followed by Messinian evaporites and Lower Pliocene marls (Andrè and Doulcet, 1991). Heavy immature oil (11 API), sulfur rich,

impregnates the karstified Lower Cretaceous and Lower Miocene limestones on the large paleotopographic high. The oil extends to the west up to the erosional limit of the karstified formation and the oil/water surface sensibly rises in the same direction. A good seal is provided by both the Messinian anhydrites and the Pliocene marls. The source of this oil and the migration path have been discussed for long time. It is in general assumed that the source is the oil prone Upper Triassic Burano Formation (Andrè and Doulcet, 1991). The migration should be relatively recent and subvertical, along faults. An alternative hypothesis would be that the source

Figure 10. Location of the main gas and oil field in the Adriatic Sea (data from Videpi, 2009).

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rock was the Emma Limestones, the migration being lateral up dip from the NE (Mattavelli et al., 1993; Casero, 2004). 3.5. Quaternary basins During the Pleistocene an important regional relative sea level fall occurred. A full set of regressive sandy/clay beds was deposited (Ori et al., 1986; Bigi et al., 1997; Patacca and Scandone, 2004). In the

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Northern Adriatic Sea, out of the area considered in this paper, in the basal sandy levels of this cycle, several biogenic gas pools were found, some of which (e.g. Barbara field, Iannello et al., 1992) of very large size (Fig. 10). The traps are everywhere gentle anticlines, in most cases draping previous features of various origins. The Quaternary deposits in the central Adriatic Sea are mainly undeformed and testify the regressive cycle that started during the Pleistocene (Figs. 9 and 10). These data suggest that thrusting activity ended in the early

Figure 11. Adriatic Sea petroleum systems (data from Videpi, 2009).

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Pleistocene time, whereas a generalized uplift has taken place (Di Bucci and Mazzoli, 2002; Di Bucci et al., 2003).

system, b) mixed oil and gas system, and c) oil system (Figs. 10 and 11). A fourth, not exploited, system includes very heavy oil from very exploration wells.

4. The Adriatic Sea petroleum systems 4.1. Biogenic gas system Based on the public dataset related to the exploration activity in this area during the last fifty years, three main petroleum systems have been recognized and commercially exploited a) biogenic gas

This system is characterized by the occurrence of commercial biogenic gas in: a) turbiditic sands of MiddleeUpper Pliocene,

Table 1 The main characters of the oil and gas fields in the Adriatic sea. Field name

Hydrocarbon type

Reservoir

Trap (age of deformation)

Trap (type) edetachment level

Generation, expulsion (age) and migration

Source rock

San Giorgio Mare

Gas and condensate (mixed)

Scaglia Fm. (turbiditic limestones)

Double vergence, up thrust like inversion folds e Triassic

Piropo

Gas and oil (15 e22 API)

Scaglia Fm. (turbiditic limestones)

Thermogenic/Pliocene/ lateral and vertical migration (to NE) Thermogenic/Pliocene/ lateral migration

Gas and condensate (mixed)

Scaglia Fm. (turbiditic limestones)

Gas and condensate (mixed)

Bolognano Fm. (Miocene)

Pre-apenninic phase/upper Pliocene Pre-apenninic phase/upper Pliocene Pre-apenninic phase/middle Pliocene Pre-apenninic phase

Gas and condensate (mixed)

Scaglia Fm.

Gas and condensate (mixed)

Scaglia Fm. (turbiditic limestones)

Gas

Scaglia Fm.

Emma lmst (Hettangian Burano Fm. Emma lmst (Hettangian Burano Fm. Emma lmst (Hettangian Burano Fm. Emma lmst (Hettangian Burano Fm. Emma lmst (Hettangian Burano Fm. Emma lmst (Hettangian Burano Fm. Emma lmst (Hettangian Burano Fm.

Mormora

Ombrina

Emilio

Sarago

David

David

Heavy oil (4 e5 API)

Liassic limestone

Emma 1

Oil_heavy sulfur oil

Gianna

Oil_heavy sulfur oil

Rospo Mare

Oil (11 API)

Santa Maria

Oil

Scaglia Fm. (turbiditic limestones) Scaglia Fm. (turbiditic limestones) Bolognano Fm. (Miocene) Scaglia Fm. (turbiditic limestones)

Barbara

Gas

Sands

Bonaccia

Gas

Sands

Calpurnia

Gas

Sands

Camilla

Gas

Sands

Emma W Clara Est

Gas Gas

Flavia

Gas

Giovanna

Gas

Squalo centrale

Gas

S. Stefano Mare Fratello Est

Gas

Sands Multilayer thin sand beds Multilayer thin sand beds Multilayer thin sand beds Pools in multilayer thin sand beds e Middle Pliocene Sand beds

Fulvia

Gas

Pennina

Gas

Eleonora

Gas

Gas

Multilayer sand beds Multilayer sand beds Multilayer sand beds Multilayer sand beds

Pre-apenninic phase/upper Pliocene Pre-apenninic phase/middle Pliocene Pre-apenninic phase/upper Pliocene

Double vergence, up thrust like inversion folds e Triassic Anticline (outer most Ap. Trend)

Stratigraphic

e Rethic)/

Double vergence, up thrust like inversion folds e Triassic

Thermogenic/Pliocene/ lateral migration

Double vergence, up thrust like inversion folds e Triassic

Thermogenic/Pliocene/ lateral migration

Stratigraphic

Thermogenic/e/lateral migration or generation in situ Thermogenic/Pliocene/ lateral migration Thermogenic/Pliocene/ lateral migration Thermogenic/Pliocene/ lateral migration Thermogenic/Pliocene/ lateral migration

Emma limestone? (Lower Lias e Rethic) Emma limestone (Lower Lias e Rethic) Emma limestone (Lower Lias e Rethic) Emma limestone (Lower Lias e Rethic)

Gentle fold/stratigraphic

In situ

Biogenic

Gentle fold/stratigraphic

In situ

Biogenic

Gentle fold/stratigraphic

In situ

Biogenic

Gentle fold/stratigraphic

In situ

Biogenic

Stratigraphic Anticline (outer most Ap. Trend) e Messinian Anticline (outer most Ap. Trend) e Messinian Gentle fold/stratigraphic

In situ In situ

Biogenic Biogenic

In situ

Biogenic

In situ

Biogenic

Stratigraphic

In situ

Biogenic

Gentle fold/stratigraphic

In situ

Biogenic

Stratigraphic

In situ

Biogenic

In situ

Biogenic

In situ

Biogenic

In situ

Biogenic

Pre-apenninic phase/middle Pliocene Middleeupper Pliocene Middleeupper Pliocene Middleeupper Pliocene Middleeupper Pliocene

Thrust related fold e Messinian

thin

e Rethic)/

Thermogenic/Pliocene/ lateral migration

Double vergence, up thrust like inversion folds e Triassic Double vergence, up thrust like inversion folds e Triassic Stratigraphic

Middle Pliocene

Thermogenic/Pliocene/ lateral migration

e Rethic)/

Double vergence, up thrust like inversion folds e Triassic

Pre-apenninic phase/Pliocene Pre-apenninic phase/Pliocene

Middleeupper Pliocene Middleeupper Pliocene Middleeupper Pliocene

Thermogenic/Pliocene/ lateral migration

e Rethic)/

thin

Middle Pliocene

thin

Middle Pliocene

Anticline (outer most Ap. Trend) e Messinian Gentle fold/stratigraphic

thin

Middle Pliocene

Gentle fold/stratigraphic

e Rethic)/

e Rethic)/

e Rethic)/

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involved in thrust-related folds belonging to the Apennines deformation (Flavia and Fulvia fields, Struttura Costiera thrust related fold, Figs. 3 and 10); b) in Lower Pleistocene sands involved in gentle passive fold related to older positive structures (Emma W field, Fig. 4). These latter are usually characterized by multiple pools in multilayer thin sand beds (Squalo Centr. field, Figs. 4 and 10). On seismic lines, the occurrence of these pools are highlighted by seismic anomalies as bright spots and pull down. The biogenic gas was generated in situ by bacterial action on the immature organic-rich clays interbedded with the reservoir sands. The main gas fields belonging to this system are located in the northern part of the central Adriatic Sea, where they have high potential reserves values and, as a consequence, great economic interest (for example Barbara, Bonaccia and Clara Est, Fig. 10 and Table .1). 4.2. Mixed oil and gas system (far traveling system) These system is characterized by volumes of light, low sulfur oil and/or condensate, associated to thermogenic gas, located mainly in the positive structures of the most external thrust front of the buried Apennines or in the inversion structures located in its footwall (Table 1 and Fig. 10). The reservoir consists of resedimented calcareous bioclastic breccias, interbedded in the Cretaceous/Paleocene portion of the Scaglia Formation, involved in symmetric anticlines related to thrust. The source rock is the Emma limestones, Lower Liassice Upper Triassic, consisting of carbonate deposits belonging to confined, exinic, pelagic intra-carbonate platform seaways (Zappaterra, 1994). It entered the oil window during the Pliocene, as a consequence of high subsidence due the flexure of the Adriatic plate. This generated thermogenic gas and oil that migrated laterally and upward to the traps (toward Nord Est) consisting of double vergence, up thrust like inversion folds, bounded by high angle faults. The oil and gas fields of Emma W, Mormora, Piropo and David belong to this system (Fig. 10 and Table 1). 4.3. Oil system (short traveling) This system, which provides important resources in term of reserves, comprises volumes of heavy, immature and sulfur rich oil in Mesozoic and Cenozoic limestones, involved in poorly deformed inversion structure of the foreland and/or in stratigraphic traps. The primary porosity of these rocks is low and the volumes are obtained by fractures. The source rock is still the Emma limestones, that has here a lower maturity due to the fact that it is still located in the foreland domain. Migration was vertical or lateral, across the fracture systems in the limestones. One of the most important field of this system is the Rospo Mare field, in the south of the study zone (Figs. 8 and 10). In this case, the southeastern border of the Emma limestone basin is located in correspondence of the margin of Apula carbonate platform: from the central part of the basin, oil should migrate laterally to south west across the porous carbonate platform Mesozoic limestone (Fig. 10). 4.4. Heavy oil The heavy oil of this system, although is know, is not already exploited. The hydrocarbons consist of 4e5 API bitumen, that was discovered in very deep stratigraphic exploration wells, and is located together of other petroleum systems as in the case of the David gas field. The reservoir is made by the Lower Jurassic limestone

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whereas the source rock is suspected to be the same Emma limestones, with an “in situ” generation or short laterally migration. 5. Discussion Most of the petroleum exploration plays of the Adriatic occur in the foreland and foothills domains; as a general rule, the gas pools are associated with flexural and post-flexural petroleum systems, whereas liquid hydrocarbons are linked to pre-flexural substratum sources (Table 1 and Fig. 11). Biogenic gas pools were found in: i) thrust top, shallow marine sands in mixed traps (Fig. 3); ii) foredeep turbiditic multi-layers sands, involved in thrust-related folds, either four-way-dip closed or erosionally truncated (Fig. 4); iii) foreland basin marine sands in mixed traps (refolded onlaps) (Fig. 5), stratigraphic traps (onlaps/ shale outs) or in structural traps (reactivated thrust-related folds or forced folds) (Figs. 3 and 4). The source is in the interbedded Pliocene clays. Thermogenic gas pools are in turbiditic sands involved in thrust-related folds in foothills areas. The gas, generated at great depth in the Pliocene siliciclastic deposits, migrated laterally-up dip along the inner flank of the folds. Liquid hydrocarbons, often associated with condensates and/or thermal gas, are stored in pre-flexural carbonatic series either in foothills or foreland domains (Figs. 3, 7 and 9). In the foothills belts, as the periadriatic basin, the traps are thrust-related folds (Figs. 2 and 3). The hydrocarbons were generated by intra-shelf euxinic basin series, pushed into the maturity window by the superposed flexural/ tectonic subsidence and migrated, as for thermal gas, laterally-up dip along the inner limb of the folds. In the foreland, as the central part of the Adriatic domain, the oils are stored in carbonates involved in paleo-structures of different nature (Figs. 7 and 9). They were also generated by intra-self euxinic series, that reached the maturity thanks to the recent passive margin subsidence. The oils migrated laterally-up dip, across the facies change screen. 6. Conclusions The Apennine thrust/fold belts and, more in detail, the Adriatic basin, are the results of a complex geological evolution due to the superimposition of several tectonic phases. This remarkable variability, at regional and local scales, is the scenario for the development of petroleum systems having a considerable economic importance. The potential petroleum exploration plays of the Adriatic domain are tentatively classified in this paper, according to their geological evolution with respect to the Apennines fold and thrust belt (Fig. 2). The main parameters for classification are: age of the main deformation, structural trend and decollement level. In this way is possible to distinguish five main group of structures. Three groups consist of contractional structure of different ages (Middle and Upper Pliocene) developed from different detachment levels. Thrust planes in some cases reactivate older structures, and are associated to diapirisms (central axes of the Adriatic Sea) (Figs. 3e7). Another group is characterized by Cretaceous normal faults controlling the margins of the Apulia carbonate platform, whereas the last group comprises small and isolated Quaternary basins, elongated NWeSE (Figs. 7e9). This classification highlights that the main petroleum plays of the area are genetically linked to the flexure of the Adria continental margin and to the evolution of the Apennines fold and thrust belt (Figs. 2 and 10). The source rocks contained in the preflexure epi-continental successions reached the maturity window during the flexural subsidence or, alternatively, the flexural accommodating siliciclastic flysch themselves generated and stored hydrocarbons. As in the other main province of the Italian

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peninsula, even in the Adriatic domain is possible to recognized three main petroleum systems associated with the above described structure: biogenic gas in terrigenous Plio-Pleistocene deposits, mixed oil and gas systems in terrigenous deposits and in Cretaceous slope-to-basin deposits, oil in meso-cenozoic carbonate deposits (Fig. 11). The central Adriatic Sea has been the target of the hydrocarbon exploration since the 1970’s, and the exploration history indicates for that period a main activity, when the main oil and gas fields in the area were discovered. From the eighties to these days gas exploration is mature in this area, and activities are mainly focused close to the existing fields; nevertheless, an interesting upside remains, mainly for the complex and deep seated plays which are still poorly understood. Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.marpetgeo.2012. 07.006. References Anderson, H., 1987. Is the Adriatic an African promontory? Geology 15, 212e215. Andrè, P., Doulcet, A., 1991. Rospo Mare Field e Italy, Apulian Platform, Adriatic Sea. In: Foster, N.H., Beaumont, E.A. (Eds.), Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, Stratigraphic Traps II, pp. 29e54. Anelli, L., Mattavelli, L., Pieri, M., 1996. Structural stratigraphic evolution of Italy and its petroleum systems. In: Ziegler, P.A., Horvath, F. (Eds.), Peri-Thethys Memoir 2: Structure and Prospects of Alpine Basins and Forelands, vol. 170. Memoir Museum National History Natural, pp. 455e483. Argnani, A., Frugoni, F., 1997. Foreland deformation in the Central Adriatic and its bearing on the evolution of the Northern Apennines. Annales Geofisicae 40, 771e780. Argnani, A., Gamberi, F., 1995. Stili strutturali al fronte della catena appenninica nell’Adriatico centro-settentrionale. Studi Geologici Camerti Special vol. 1995/1, 19e27. Argnani, A., Favali, P., Frugoni, F., Gasperini, M., Ligi, M., Marani, M., Matietti, G., Mele, G., 1993. Foreland deformational pattern in the Southern Adriatic. Annales Geofisicae 36, 229e247. Argnani, A., Bernini, M., Di Dio, G.M., Papani, G., Rogledi, S., 1997. Stratigraphic record of crustal-scale tectonics in the Quaternary of the Northern Apennines (Italy). Il Quaternario 10 (2), 595e602. Bally, A.W., Burbi, L., Cooper, C., Ghelardoni, R., 1986. Balanced sections and seismic reflection profiles across the Central Apennines. Memorie Società Geologica Italiana 35, 257e310. Battaglia, M., Murray, M.H., Serpelloni, E., Burgmann, R., 2004. The Adriatic region: an independent microplate within the AfricaeEurasia collision zone. Geophysical Research Letters 31, L09605. http://dx.doi.org/10.1029/ 2004GL019723. Bertello, F., Fantoni, R., Franciosi, R., Gatti, V., Ghielmi, M., Pugliese, A., 2010. From thrust and fold belt to foreland: hydrocarbon occurrences in Italy. In: Vining, B.A., Pickering, S.C. (Eds.), Petroleum Geology: From Mature Basin to New Frontiers. Proceedings of the 7th Petroleum Geology Conference, Geol. Soc., pp. 113e126. Bertotti, G., Picotti, V., Chilovi, C., Fantoni, R., Merlini, S., Mosconi, A., 2001. Neogene to quaternary sedimentary basins in the South Adriatic (Central Mediterranean): foredeeps and lithospheric buckling. Tectonics 20 (5), 771e787. Bigi, S., Cantalamessa, G., Centamore, E., Didaskalou, P., Micarelli, A., Nisio, S., Pennesi, T., Potetti, M., 1997. The periadriatic basin (Marche-Abruzzi sector, Central Italy) during the Plio-Pleistocene. Giornale di Geologia 59 (1e2), 245e259. Bigi, S., Lenci, F., Doglioni, C., Moore, J.C., Carminati, E., Scrocca, D., 2003. Decollement depth vs accretionary prism dimension in the Apennines and the Barbados. Tectonics 22, 1010. http://dx.doi.org/10.1029/2002TC001410. Calamita, F., Paltrinieri, W., Pelorosso, M., Scisciani, V., Tavernelli, E., 2003. Inherited mesozoic architecture of the Adria continental paleomargin in the Neogene central Apennines orogenic system, Italy. Bollettino Società Geologica Italiana 122, 307e318. Casabianca, D., Bosence, D., Beckett, D., 2002. Reservoir potential of Cretaceous platform-margin breccias, central Italian Apennines. Journal of Petrology and Geology 25 (2), 179e202. Casero, P., Roure, F., 1994. Neogene deformation at the Sicilian-North African plate boundary. In: Roure, F. (Ed.), Pery-Tethyan Platforms. Editions Technip, Paris, pp. 27e45. Casero, P., Rigamonti, A., Iocca, M., 1990. Paleogeographic relationships during Cretaceous between the northern Adriatic area and the Eastern Southern Alps. Memorie Società Geologica Italiana 45, 807e814.

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