Granath, J. W., and P. Casero, 2004, Tectonic setting of the petroleum systems
of Sicily, in R. Swennen, F. Roure, and J. W. Granath, eds., Deformation, fluid
flow, and reservoir appraisal in foreland fold and thrust belts: AAPG Hedberg
Series, no. 1, p. 391 – 411.
Tectonic Setting of the
Petroleum Systems of Sicily
James W. Granath
Granath and Associates, Houston, Texas, U.S.A.
Consultant, Rome, Italy
etroleum systems in Sicily are divided between the Neogene fold and thrust belt
of the Sicilian accretionary prism and its foreland. By far, more production has
been established in the foreland on the Ragusa platform, where generally heavy
oil from Triassic and Jurassic source rocks has been trapped in similarly aged fractured
carbonate mound reservoirs. Traps are structures recurrently rejuvenated since the
Cretaceous. Although models of source maturation and migration history emphasize
Pliocene–Pleistocene charge related to subsidence in the foreland, occurrences of light
oil and exploration data suggest intermittent periods of charge since the Mesozoic. Hydrocarbon deposits in the immediate vicinity of the thrust front, in the narrow foredeep
depression, are also heavy and have characteristics in common with those more remote
in the foreland from the front; they do not appear to have been significantly influenced
by or mixed with oils from within the fold belt, although variations in their chemistry
have yet to be investigated in terms of source correlation. Thrust sheets in the fold and
thrust belt carry similar Mesozoic stratigraphy to the foreland, namely, a series of carbonate platforms and basins with potentially similar source rocks, and can reasonably be
expected to have undergone a similar pre-Neogene history.
Commercial hydrocarbons in the fold belt, however, are limited to thermo- and
biogenic gas-charged Tertiary reservoirs and questionably Tertiary-sourced oil offshore
to the west. These all currently are viable exploration plays. Numerous seeps and tar
deposits; indicate that at least one petroleum system was active in the fold belt, but
only one tar occurrence links that system to the Mesozoic carbonate section. Recent
structural studies have indicated that a two-phase history of thrust emplacement and
its redeformation by duplexing on lower detachments characterizes much of the Sicilian fold and thrust belt and affords exploration targets that have yet to be satisfactorily tested. Furthermore, structural petrological and fluid-inclusion studies in
Copyright n2004 by The American Association of Petroleum Geologists.
Granath and Casero
Neogene-thrusted rocks of the lower portions of the frontal fold belt indicate that a
light hydrocarbon system used Neogene detachment surfaces as a preferential migration route in at least part of the accretionary prism.
Insofar as the foreland may give an insight into the early history of the fold belt
platforms and basins, a general sense of the hydrocarbon migration history in the fold
belt might include the following: (1) early maturation and migration of a variety of
oils into carbonate mound reservoirs during a long period of time in the latter half
of the Mesozoic and Paleogene (like in the foreland plays), bolstered by (2) a charge of
immature oil generated during rapid depression in the foredeep immediately adjacent
to the moving thrust front (yet to be demonstrated), and (3) generation of higher
maturity oil and gas in the thrust wedge during stacking of thrust sheets and remigration and mixing with the earlier generated fluids (an untested play).
Sicily lies astride a segment of the Apennine–Maghreb
Mountain chain (Figure 1) that stretches from the Alps
in Europe to the Atlas Mountains in North Africa and
records most of the tectonic events involved in the
complex interaction of Europe and Africa prior to and
during the opening and closing of western Tethys. The
island is surrounded by three seas (Figure 1) that frame
the geology of Sicily. To the south, the western Mediterranean Sea is underlain by the Pelagian platform that
includes onshore Sicily as well as Malta. Paleomagnetic
data show no difference in apparent polar wandering
paths between southeast Sicily and North Africa (Meloni
et al., 1997, and references therein; Speranza et al., 2000,
and references therein), thus firmly tying the geology of
Sicily to that of North Africa, a situation that has persisted since the Permian (Stampfli et al., 2001). Mesozoic
carbonates dominate the geology of the Pelagian platform and are built on stretched North African continental crust. A useful analogy to visualize the pre-Alpine situation might be a setting similar to and comparable in
size to the modern Bahama Banks (adjusted of course
for the biotic assemblage) adjacent to Tunisia and Libya.
The Moho under the Pelagian platform is as shallow as
20 km (13 mi), and the Moho under Sicily ranges from
25 km (16 mi) in depth along the southern coast outside the fold belt to 30 km (19 mi) under the fold belt
and 35 km (22 mi) along the northern edge of the island
(Dal Piaz and Nicolich, 1991; Nicolich, 2001). Given
something like 10 km (6 mi) for the thickness of the
thrust stack (Catalano et al., 1993) and an allowance for
pre-Permian sediments, it is evident that the continental crust of Sicily has undergone considerable thinning.
The Ionian Basin lies to the southeast of Sicily. It is
the last remnant of Mesozoic oceanic crust in the central Mediterranean (Catalano et al., 2001), where it is
subducting under the Calabrian Arc (Figure 1). The abyssal plain of the Ionian Sea (Finetti, 1981, 1985; Morelli,
1985) is separated from Sicily by a continent-ocean tran-
sition at the Malta escarpment. To the north, the Tyrrhenian Sea is composed of Neogene oceanic crust developed in a back-arc environment (Kastens et al., 1988).
The Pelagian crust, with its carbonate-dominated
cover, has been colliding with a north-dipping subduction zone since the Miocene, and its northern fringe
is deformed in the Sicilian thrust belt. Thus, although
Sicily lies in the Apennine–Maghreb accretionary prism,
its tectonic character is that of a foreland fold and thrust
belt. It is composed of a complex stack of thrust sheets
involving the Mesozoic platform and elements of Alpine
crust, of the precollision forearc region, and of various
syntectonic deposits. As a result, the prism is much more
similar with retroarc fold and thrust belts around the
world in terms of its architecture than it is to oceanic
accretionary prisms, but its hydrocarbon systems are
heavily influenced by details peculiar to Sicily’s setting,
particularly factors that isolate the foreland area from
the fold belt proper.
In recent years, major discoveries have been made
at the Val d’Agri fields in Basilicata on mainland Italy
(La Bella et al., 1996), said to total nearly 1 billion bbl
of oil recoverable (Holton, 1999), in environments that
are similar to the fold and thrust belt in Sicily. Obviously, the sparsely explored fold and thrust belts of
Italy may harbor major hydrocarbon potential, but the
geology is difficult. Here, we review that geological history of Sicily, with a special emphasis on how it relates
to hydrocarbon occurrences and thereby more generally to the behavior of fluids in the system.
HISTORY OF HYDROCARBON
Hydrocarbons have been known in Sicily for many
centuries from a variety of surface manifestations. For
example, bitumen-impregnated limestones were used for
building purposes in southeastern Sicily, largely because
Tectonic Setting of the Petroleum Systems of Sicily
FIGURE 1. Tectonic sketch map of the Sicilian region. S = Segesta fault zone; C = Cefalù Basin; R = Ragusa plateau;
pe = Peloritani nappe; E = Mt. Etna; P = Pantelleria Island; M = Malta; 1 = basal thrust carrying Peloritani Mountain
sheet, including fundamentally European basement into the Sicilian accretionary prism; 2 = nappes associated with
Neotethyan and northern platformal sections; 3 = foreland uplifts; 4 = deformation front of the Sicilian and Calabrian
accretionary prisms and frontal front in Tunisia; 5 = extensional fault systems; 6 = oceanic crust in the Tyrrhenian and
Ionian basins. Modified after Pepe et al. (2000) and Casero and Roure (1994).
of their ease of workability, and as a medium to fashion
tubing for aqueducts, because of their impermeability
(Kafka and Kirkbride, 1959; Mattavelli et al., 1993).
Surficial asphalt deposits were exploited during the
19th and early 20th centuries, and several shallow wells
were drilled before World War II. J. E. Thomas, a
founder and the first president of the AAPG, recognized the southeastern dip of the Ragusa plateau away
from its western-bounding normal faults (Figure 2) and
the possibility that the structural culmination there afforded a focus for migrating hydrocarbons (A.W. Bally,
2003, personal communication). He initiated surface
geological studies in the areas in the 1940s that predated the first modern exploration permits, which were
signed beginning in the 1950s. Those studies eventually led to the discovery of Ragusa field in 1953 at the
Ragusa-1 well (Figure 3). This was the first major oil
discovery in Mesozoic reservoirs. Other discoveries followed both in Tertiary rocks and in the Mesozoic: Gela
in 1956, Cammarata-Pozzillo in 1959, and the first offshore discovery in Gela-21, also in 1959. Onshore ex-
ploration peaked in 1959 and tapered off in 1970, in
response to the heavy quality of oil discovered, difficult geology and marginal seismic quality, low oil
prices, and the passage of a stricter Sicilian hydrocarbon law.
After 1969, part of the Sicilian offshore was opened
to exploration, with the discovery of light oil at Nilde
in 1972 and at Mila in 1978. The price boom of the late
1970s and the advent of digital seismic acquisition in
Sicily spurred activity, and heavy crude was discovered
at Perla in 1979, Vega in 1980, and Prezioso in 1983. A
modest onshore resurgence of activity in the 1980s followed adjustments to the hydrocarbon law and better
seismic acquisition and processing. Several small discoveries were made. Other parts of the offshore were
opened at that time, but little success was realized.
Since 1992, activity has been dormant both onshore and
offshore, although the hydrocarbon law has once again
been modified (this time to be similar to Italian law).
Currently, several permits are held by several operators,
but activity is minimal.
Granath and Casero
FIGURE 2. Geological map of Sicily generalized from Bigi et al. (1983).
Altogether, some 350 wells have been drilled, of which
only 21 are offshore. Reserves are some 500 MMBO, with
about half already produced. Gas totals about 500 bcf,
of which just under two thirds has been produced. The
principal fields are listed in Table 1, and the geological
aspects of the various plays linked to those discoveries
are tabulated in the Appendix in this chapter.
Tectonic Setting of Sicily in the
The entire island of Sicily north of the Gela nappe
front (Figure 1) is a strongly arcuate Neogene fold and
thrust belt in the active accretionary prism that stretches
continuously from the Apennine fold belt and Calabrian
Arc in Europe to the Maghreb system of North Africa. It
lies above a north-dipping seismogenic (Gasparini et al.,
1985) subduction zone, along which most of the rem-
nants of Tethys have been consumed and into which
African lithosphere is currently being subducted (Amato
and Cimini, 2001). The volcanic arc associated with the
Sicilian sector is located to the north of the island itself
in the Aeolian Islands, beyond the extensional Cefalù
Basin. Synorogenic extension in the Cefalù Basin began
synchronously with shortening in the accretionary prism,
initially as a subaerial wedge-top basin, but it was much
amplified later by extension and back-arc spreading in
the Tyrrhenian Sea (Pepe et al., 2000). Extension in the
Tyrrhenian Sea dates from rifting and clockwise rotation of Sardinia in the Tortonian to back-arc oceanic
spreading in the late Pliocene to Quaternary.
Tectonic Elements in the
The Sicilian accretionary prism is thrust upon the
Ragusa plateau, exposed in southeast Sicily, and bordered on its north by the chiefly volcanic Hyblean Plateau (Figures 2, 3). The Ragusa plateau is the major
subaerial exposure of the Pelagian platform (Figure 1)
Tectonic Setting of the Petroleum Systems of Sicily
FIGURE 3. Map showing the sequential southward progression of foredeep (flysch) basins that envelope the Mesozoic
hydrocarbon-bearing sequences. All units north of the Gela nappe front are allochthonous. Outcrop of the Mesozoic rocks
and the hydrocarbon fields are shown in patterns. 1 = Peloritan crystalline terrain of European origin; 2 = Mt. Soro flysch,
Neotethyan seafloor section; 3 = inner Numidian flysch and related Panormide platformal rocks; 4 = outer Numidian flysch
and related Imerese domain basinal deposits; 5 = inner Messinian foredeep; 6 = central Messinian foredeep; 7 = outer
Messinian foredeep; 8 = Gela nappe; 9 = Gela foredeep and the Carla trough (Tunisian trough). Areas mentioned in the
text: C = Caltavuturo; MS– MJ = Monte Scalpello– Monte Judica; MB = Monte Bubonia; Mso-T = Monte Soro and Troı̈na.
Modified after Roure et al. (2002, their figure 2).
outside of the island of Malta. Several of the important
hydrocarbon deposits in Sicily are trapped in structures
in the Ragusa plateau, i.e., Vega, Mila, Ragusa, and Irminio fields (Figure 3; Table 1).
Since about 0.5 Ma, the northern part of the Pelagian platform, including Malta, has been decoupled
from the rest of Italy, from the Pelagian block of North
Africa, and the Ionian Basin by recurrent tectonic activity of two major tectonic zones. This triangular-shaped
area is bounded on the east by the Malta escarpment
and the southwest by the Pantelleria and Malta graben
(Figure 1), otherwise known as the Medina wrench system ( Jongsma et al., 1987).
At the east end of the island, just south of Mount
Etna, the thrust front intersects the northern extent of
the Malta escarpment, locally known as the Siracusa escarpment, to connect to the Calabrian Arc at a major
inflection in the Neogene thrust front (Figure 1). The
Malta escarpment extends to the south-southeast for
some 250 km (150 mi) to the Malta graben, separating
the Pelagian platform from the Ionian abyssal plain to
the east. The ocean-continent transition at the escarpment dates from pre-Alpine time and could have suffered inversion in the Alpine collision. Late Miocene to
Quaternary block faulting is documented on the escarpment (Groupe Escarmed, 1983). Contemporary
volcanism is concentrated at the junction of the Siracusa escarpment and the thrust belt in the Mount Etna
area. Pre-Etnan basaltic volcanism began at about 5 Ma
and was later superceded by the trachyandesites and
Granath and Casero
TABLE 1. Principal oil and gas fields of Sicily.
API (88 )
onshore; fold belt
onshore; fold belt
7 – 10
trachytes of Etna itself (Behncke, 2001). The bimodal
volcanic progression suggests that rifting, in conjunction with the arc setting of Etna, is fundamentally important (Serri et al., 2001), but the details of the magma
genesis are controversial. Flexure-related reactivation
at the ocean-continent transition may take the form of
a tear or a rift in the subducting lithosphere that
permits differential flexure or rollback of the slab and
access of magma generated in the surrounding mantle
and downgoing slab to the surface (Di Geronimo et al.,
1978; Gillot et al., 1994; Continisio et al., 1997; McGuire
et al., 1997; Monaco et al., 1997; Gvirtzman and Nur,
1999; Doglioni et al., 2001).
Transecting the Pelagian platform south of Sicily
are the Pantelleria and Malta graben (Figure 1), which
intersect the Malta escarpment southeast of Malta itself
and eventually are masked by the accretionary prism at
the Mediterranean Ridge some 200 km (120 mi) to the
east. This is a complicated mosaic of thrust, normal, inversion, and transcurrent faults, generally forming a
right-lateral system whose activity dates from the late
Aquitanian to Pliocene in Malta (Dart et al., 1993). Quaternary sedimentary infill and associated volcanic rocks
(Illies, 1981; Groupe Escarmed, 1983; Cello, 1987; Calanchi
et al., 1989) indicate that this zone has recently been
active. Jongsma et al. (1987) have suggested on plate kinematic grounds that this broad zone of deformation
forms a transform plate boundary marking as much as
100 km (62 mi) of right-lateral motion between Africa
and a Hyblean microplate (Mazzoli and Helman, 1994).
The relative offset in the internal nappes of the Maghrebs
vs. Sicily (Figure 1) is of comparable size, but the motion
may be overestimated. The offset may represent a lateral
ramp, however, as the structures may not be perfect
piercing points. Indeed, Casero and Roure (1994) have
traced facies patterns across the zone with little offset.
1600 – 2200
Possibly more important to Sicily itself but poorly
documented in the literature is the Segesta lineament,
a north-northeast trend bisecting the major fold belt
syntaxis in western Sicily (Figures 1, 3). This feature juxtaposes differing facies and structures across strands of a
(presumably) tear fault in the allochthonous units. Within the allochthons, the Segesta zone probably functions
as a lateral ramp to the thrust sheets and aids the internal kinematics of the fold belt, which involves significant rotations (see below). Its projection into the foreland
on the Pelagian platform is subtle; it displaces pre-Tertiary
sedimentary facies (Casero and Roure, 1994), presumably through reactivating older crustal-scale structures.
Heat Flow, Volcanism, and Recent Uplift
Heat flow is generally elevated across Sicily (Figure 4),
especially so along the Malta and Pantelleria graben
and in the Aeolian Islands, but also in a trend across the
Ragusa and Hyblean plateaus north through Mount
Etna. Mount Etna has been mentioned in connection
with its relationship to the Malta escarpment (Figure 1),
but all three of these areas are characterized by young
volcanism, generally basaltic in character (Serri et al.,
2001, and references therein). Della Vedova et al. (2001)
emphasize water circulation in connection with the
southeastern Sicily anomalies, but the area has experienced recurrent volcanism since the Jurassic (Figure 5).
Basalts and tuffs occur in the Jurassic Streppenosa,
Modica, and Buccheri formations and prominently in
the Late Cretaceous Amerillo Formation (Patacca et al.,
1979; Scandone et al., 1981, Rocchi et al., 1996). Scandone et al. (1981) report similar rocks from dredge sites
to the east on the Malta escarpment. The same area of
southeastern Sicily also shows high and variable magnetic anomalies (Chiappini et al., 2000), which, if caused
Tectonic Setting of the Petroleum Systems of Sicily
FIGURE 4. Heat flow and
Bouguer anomaly sketch map
of Sicily, also showing distribution of in-situ volcanic
rocks, mostly basalts, and
Mt. Etna. Heat flow in milliwatts per square meter after
Della Vedova et al. (2001).
Bouguer anomaly in milligal
after Carrozzo et al. (1986).
Locations of volcanics after
Serri et al. (2001).
by volcanics distributed in
the section, suggest that a
very large area of southeastern Sicily experienced volcanism for a major part of
the Mesozoic. Insofar as that
volcanism may have been associated with periodic extension, heat flow is likely to have been at least mildly elevated
for the majority of the post-Triassic history of the area.
Elsewhere in Sicily, heat-flow anomalies occur mainly in areas where carbonates occur at or near the surface
and where structural studies suggest that stacking of carbonates characterizes the fold belt. The coincidence
of a large area of uniform normal heat flow with a
normal Bouguer signature in the Caltanisetta trough
in central Sicily (Figure 4), where carbonate stacking is
minimal, may indicate that the high heat flow is a
ramification of the near-surface lithologies and associated water circulation.
GEOLOGY OF SICILY ITSELF
Stratigraphy of Sicily
The stratigraphy of Sicily has been approached
from many vantage points. The most convenient, in
view of the complexity of the physical stratigraphy, is
to divide the coverage into that of the Ragusa plateau
outside the fold belt (columns 7 and 8 in Figure 5) and
that in the fold belt (columns 1 – 6). Figure 5 is offered
here to summarize the general distribution of units
in Sicily and to link them to the occurrences of hydrocarbons. In the southeast, on the Ragusa plateau,
Patacca et al. (1979) remains the standard reference
and is used here in column 7 of Figure 5. It supercedes
the nomenclature developed by Rigo and Barbieri (1959)
and Schmidt Di Friedberg (1965), which used formational names from disparate and tectonically separated
localities. Nevertheless, this older nomenclature has
persisted in the petroleum industry and is shown in
column 8 of Figure 5 for reference only.
The Sicilian terranes of African origin (columns
3 – 8 in Figure 5) are broadly similar to sequences elsewhere in Italy and are composed of thick, platformal
sequences of shallow-water carbonates with intervening
deep-water, basinal sections of cherty limestones and
radiolarian cherts, both superceded by open-marine
and pelagic limestones and argillites. This tectonostratigraphic arrangement developed during and after Triassic and Early Jurassic crustal thinning and intracontinental rifting along the northern African margin. The
platform-basin system persisted through the Jurassic
and the Cretaceous into the Paleogene, with the filling
of the basinal region and the development of shoaling
indicators such as bauxitic horizons and rudist reefs on
the platforms. In the southeasternmost parts of Sicily,
the Ragusa plateau represents the exposed portion of
the autochthonous Pelagian platform (Patacca et al.,
1979), separated from the thrust front by the narrow
Gela foredeep of middle to upper Pliocene and Pleistocene sediments (Figure 2). From Oligocene onward,
the stratigraphy is dominated by clastics and marls that
herald the impending deformation, molasse deposited
in the foredeep of the southward-advancing thrust belt,
and suprawedge piggyback basins.
Tectonostratigraphic Components of
the Sicilian Accretionary Prism
Sicily is composed of several tectonostratigraphic
provinces telescoped together during the Neogene orogeny between the Serravelian and the present (Figure 6).
The bulk of the island has clear affinities with African
geology, but in the northeastern corner of Sicily, the
Peloritani Mountains are composed of fundamentally
Granath and Casero
FIGURE 5. Stratigraphy of Sicily and associated source and reservoir rocks. Columns correspond to the various lithostratigraphic domains. Formation names in italics, rock types in standard font. Columns 1 – 6 for the fold belt after
Catalano et al. (1993, 2000) and Ronchi et al. (2000). Usage of formation names for the Ragusa platform (column 7)
from Patacca et al. (1979) and in column 8 as is commonly used in the subsurface, e.g., in well logs, after Rigo and
Barbieri (1959) and Schmidt Di Friedberg (1965). Shades of blue denote various important carbonate environments:
dark blue = basinal facies, medium blue = platforms, and light blue = various open-marine and pelagic facies, including
the Streppenosa basin. Miocene stages: M = Messinian, T = Tortonian, S = Serravalian, L = Langhian, B = Burdigalian,
A = Aquitanian. Timescale from Harland et al. (1989). lms = limestones.
European Hercynian basement and tectonically juxtaposed Mesozoic sedimentary rocks, both overlain by late
Oligocene and younger coarse terrigenous strata, Messinian evaporitic, marly, and calcarenitic strata (Maccarrone et al., 2000). The Peloritani Mountains lie in thrust
contact above a Late Jurassic, Cretaceous, and Paleogene
sequence (Figures 2, 3, 6) of flyschlike turbidites (especially well exposed in the Monte Soro and Troı̈na areas)
(Figure 3) that, by reason of a major late Eocene unconformity, is commonly interpreted to have been deformed
during late Alpine events (Roure et al., 1990, and references therein). The Monte Soro and Peloritani lie in
thrust contact over a Cretaceous to lower Miocene assemblage of flysch units, the Sicilide nappe, apparently de-
posited in the Tethyan ocean and thrust onto the fundamentally African Sicilian section. Hence, the northeast
appears to be Tethyan and European in origin and is foreign to the African terranes over which it has been thrust.
Within the fold and thrust belt, the Mesozoic is represented by several platformal and intervening basinal
units that are now interdigitated in the allochthons
(Figure 6). The Panormide platformal unit outcrops in
the north along the coast (Figures 2, 3), where it lies
structurally above all other African terranes and below
the Tethyan underpinnings of the Peloritani. In western
Sicily, to the south and west of Palermo, the Trapanese–
Saccense platformal units lie structurally below the
Panormide and intervening basinal facies, where they
Tectonic Setting of the Petroleum Systems of Sicily
FIGURE 6. Tectonostratigraphic relationships between the various allochthonous units in Sicily and the Ragusa foreland
and occurrences of hydrocarbons. See Figure 5 for more conventional stratigraphic detail. Inset shows relationship
in the northeast: Peloritan (European nappe emplaced over Tethyan Sicilide, which, in turn, are emplaced over Sicani
terrane). In central Sicily, the Panormide and Imerese are thrust over the Sicani. Emplacement ages especially of the
more internal units are a matter of controversy (see De Capoa et al., 2000, and references therein). Emplacement of the
Peloritani and Sicilide units over the Panormide, for example, is commonly considered Aquitanian, but De Capoa et al.
(2000) argue for an age no older than Burdigalian. The more external units were emplaced in the Messinian, and the
Gela nappe is commonly considered Pliocene – Pleistocene in age. Elements of figure after Roure et al. (1990).
are imbricated and thrust above their foreland equivalents in the subsurface (Catalano et al., 2000).
Two major basinal units are involved in the thrust
belt. The Imerese unit is the major Mesozoic basinal
unit southwest of Palermo to the south of the city of
Cefalù (Figure 2) in structural windows through overlying thrust sheets containing the Panormide unit, Numidian flysch, and Neogene synorogenic sediments. A
similar unit occurs south of the Imerese and in eastcentral Sicily 25 km (16 mi) southwest of Mount Etna,
the Sicani unit. Both are composed of deep-water carbonate sections with important occurrences of alkali
basaltic rocks that are related to the Triassic – Jurassic
fragmentation of the carbonate platform (Lucido et al.,
1978; Guarnieri et al., 2000).
Associated with the Panormide and Imerese units
in the north is a nappe of Numidian flysch, which represents distal offshore African Neotethyan deposition
that was caught up in the accretionary prism in the
early phases of deformation in Sicily. It lies in thrust
contact over both the platform and basinal units and in
apparent depositional relationship to upper Miocene
units in the fold belt. Thus, it represents a sedimentary
unit deposited into and tectonically emplaced onto the
northern side of the Sicilian passive margin before Neogene imbrication of the Mesozoic section itself. Presumably, it was derived from the Tethyan ocean floor, accretionary complex, or the trench immediately north
of Sicily prior to the arrival of the northern reaches of
the Pelagian platform.
Paralleling the north-to-south progression of Neogene deformation is a continuous sequence of foredeep
deposits ranging in age from Aquitanian in the north to
middle Pliocene in the Gela foredeep (Figure 3). Presently, they are either involved in the thrust sheets with their
substrate or form piggyback basins carried southward
with the progression of the thrust front. They form a large
part of the outcrop in Sicily (Figure 2). These units record
the systematic deepening of deposition as the Pelagian platform and its northern equivalents entered
the foredeep and approached the leading edge of
the Sicilian accretionary prism and
synorogenic wedge-top deposition in piggyback basins during the southward progression of the thrust
Granath and Casero
Emplacement of the Thrust
Sheets of Sicily
Outside the Ragusa plateau in the southeast, all the
platforms and basinal units alike are entirely allochthonous (Figure 3), being detached along surfaces in the
Triassic and locally in the Permian (Roure et al., 1990)
much like the mainland Apennine chain (Bally et al.,
1986). Highly deformed red and green argillites in the
Cretaceous and Paleogene section mark the base of many
of the thrust sheets. Although previously thought to be
olistostromes (Rigo de Righi, 1957; Broquet et al., 1984),
these ‘‘argille varicolore’’ are more probably tectonic in
origin and are the sole representatives of melange in the
Sicilian accretionary prism (Roure et al., 1990).
Relationships among the thrust sheets are complex.
Paleomagnetic data clearly indicate that the sheets have
undergone large differential rotation with respect to the
foreland and North Africa, generally a clockwise rotation
(Channell et al., 1980, 1990; Oldow et al., 1990; Speranza
et al., 1999), thus contributing to the strong arcuate
character of the Gela front. Also contributing to the
rotational history of sheet emplacement and the asymmetry of the Gela salient are several strike-slip faults,
functioning primarily as lateral ramps during thrust
emplacement, but also locally facilitating escape blocks
from west to east (Monaco et al., 1997) and linking
extensional elements near the north coast (Abate et al.,
1998; Renda et al., 2000). The allochthonous structures
onshore are pervasively overprinted by normal faults
that presumably contributed to the maintenance of a
critical taper as the fold belt has developed and underwent uplift (Roure et al., 1990).
The surface geology of Sicily is exceedingly well
mapped, particularly by programs at the universities
in Palermo and Catania [e.g., in the Ragusa plateau by
Lentini (1984) and Grasso (1999) and in key areas of
the fold belt, such as those by Di Stefano and Vitale
(1993) and Catalano et al. (1978)]. The Structural Model
of Italy (Bigi et al., 1983) summarized those efforts up
to 1983 and stands as a standard small-scale reference.
What is clear from the mapping are both the complexity of the structural geology and the shallow level
of exposure, especially in the fold belt. This is a subaerial fold belt with much of its young carapace intact.
Neogene units dominate exposure in Sicily (Figure 2),
and although they are intimately involved in the deformation, in many cases, they obscure the deeper structure. The lateral discontinuity of the thrust systems in
Figure 2 illustrates the point. Note also that there are
several cases in Figure 2 where thrust sheets that are
stacked in standard older-over-younger relationships
are cut by thrust faults that emplace younger units over
older units. Seismic imaging is consequently difficult
and further hampered by the rough terrain and the dominantly carbonate character of much of the section.
Structural Styles and Traps
in the Foreland
Structures in the southeastern Sicily harbor the overwhelming bulk of the discovered hydrocarbon deposits
in Sicily, especially the liquid ones. The discoveries on
the Ragusa plateau are located on a north-northeasttrending high bounded by normal faults (Figure 2) that
attracted Thomas’ attention to Sicily in the 1940s. Local
dip reversals on the otherwise southeast-dipping plateau that have survived from the Oligocene to the present seem to afford the best traps.
The traps throughout the southeast are intimately
related to the stratigraphic architecture of the Triassic
and Jurassic carbonate section in the Streppenosa basin
(Figures 3, 5), isolated and modified by recurrent normal faulting that initiated during the Tethyan regional
extension. Most of the fields are related in some way to
different facies of carbonate buildups in the otherwise
subsiding Streppenosa intraplatformal basin (Patacca
et al., 1979). Ragusa, Gela, Irminio, Mila, and possibly
Perla fields sit on reef mound highs in the basin. Cammarata field appears to be developed in resedimented
carbonate derived marginally from a nearby high, and
Vega appears to be near a particularly sharp edge of a
carbonate buildup. Insofar as these are Mesozoic, rampstyle biohermal mounds (as opposed to true reefs) in
the carbonate platform, Pedley (1990) has argued that
the shelf-edge fields are localized and sharpened by syndepositional faulting. Indeed, Ragusa and Gela both have
undergone repeated rejuvenation, evidenced through
the many unconformities and thinned section across
the structure both in the Early Jurassic and intermittently since (Kafka and Kirkbride, 1959; Rocco, 1959).
Lindsey et al. (2002) described the structural setting of
Irminio in terms of a reverse fault that separates the
field from Ragusa, presumably a Neogene reactivation
of an earlier normal fault.
No commercial deposits of oil and gas have been
discovered outside the Streppenosa basin in the southeastern Sicily foreland (Figure 3), that is, elsewhere
on the more stable platform in areas characterized by
the development of Siracusa formation (see column 7;
Figure 5). Only gas and oil shows have been encountered in the handful of wells involved, which has led
Pedley (1990) to surmise that cap rocks have not developed to seal traps and/or that the traps are isolated
Tectonic Setting of the Petroleum Systems of Sicily
from hydrocarbon sources by low-permeability facies, thus affording no carrier horizons to charge the
Architecture in the Fold and Thrust Belt
A complete review of the structure of the Sicilian
fold and thrust belt is beyond the scope of this chapter,
but several points are germane to a discussion of the
petroleum systems for two major reasons. The details of
the structure, of course, dictate the traps in the petroleum systems, but in addition, palinspastic restorations of the Neogene deformation would define the
distribution of potentially source-bearing rocks in basinal sections relative to traps as the system evolved.
Insofar as the Streppenosa basin is the major source
region for the Pelagian platform hydrocarbon occurrences (see below), similar potential source intervals are
of major interest in the thrust belt.
Block rotations aside, the Sicilian fold and thrust
belt conforms to the forward-vergent, piggyback style
of deformation familiar from fold belts worldwide. Its
architecture is defined by several dominant detachment levels. Ignoring the intracrystalline detachment
under the Peloritani, horizons near the base of the Mesozoic carbonate section form the basal detachment of
the whole system (Figure 6) as in the Apennines. The
decollement typically ramps across the Mesozoic carbonate section into one or more horizons in the Cretaceous and Paleogene argillites or into the Miocene and
Pliocene evaporates and shales, the latter especially
near the thrust front. The thrust front is commonly a
triangle zone that developed above the detachment
localized in the Miocene or Pliocene.
Tectonic inversion of the older Mesozoic normal
faults is evident in the Gela trough, and their preservation into the Neogene is an important trapping mechanism. Within the fold and thrust belt itself, pre-Neogene
inversion structures have probably been a significant
factor in migration and trapping, but the Neogene overprint has, for the most part, precluded their recognition. Major unconformities in the fold belt, however,
where Paleogene sections lie over older Jurassic basinal
facies as in the Monte Scalpello – Monte Judica area
(Broquet, 1968; Pion-Leflaive et al., 1990; Larroque,
1993) and in the Imerese domain near Caltavuturo
(Figure 3), may in fact represent Alpine inversion features (Casero and Roure, 1994).
As evident in Figure 2, there is considerable complication in the thrust sheets because of both duplexing of
the Mesozoic and numerous minor detachment levels,
which give rise to the most obvious exploration targets.
The Gagliano gas-condensate field (Figure 3; Table 1),
for example, with approximately 700 bcf initially in
place, is the largest and most important onshore field
in the fold and thrust belt. Production is from the
Miocene Collesano formation (Figure 5), in thrust slices
composed of the section from the Mesozoic carbonates
to Oligocene – Miocene (Numidian) flysch, yet the field
lies below minor folds in the flysch associated with
The map pattern of the Sicilian fold and thrust belt
(Figure 2) is a patchwork of klippen, piggyback basins,
breached anticlines, and tectonic windows, suggesting
that deeper structure has caused redeformation of the
now-exposed nappes and had a strong influence on
outcrop pattern. Bello et al. (2000) have combined twoand three-dimensional seismic data, well information,
and gravity and magnetic data to develop a comprehensive picture of the deformational history of the fold
belt in eastern Sicily (Figure 7A). A two-phase picture of
accretionary wedge development resulted from their
study, with an initial emplacement of allochthonous
units over the now-buried northern reaches of the Pelagian platform in the latest Oligocene and Miocene.
Forward progression of the wedge on sub- and intraMesozoic detachments cut southward redeforming higher thrust sheets and duplicating the Mesozoic section
at depth as far south as the Numidian flysch belt in the
Gagliano area (Figures 3, 7). Windows occur where the
shallower, older thrusts have been exposed in folds
above buried ramps that branch from the sub-Mesozoic
detachment (Figure 7). Pliocene–Pleistocene piggyback
deposition has been preserved south of the tip of the
deeper second-generation Mesozoic wedge, where backthrusts have facilitated its emplacement, and north of
the younger south-vergent ramps. Because the syntectonic basins appear to occupy flats in the younger
deformational system, they date the southward progression of the basal detachment. This younger phase
of deformation presumably facilitated thickening of
the thrust wedge and its southward advance through
a process of tectonic underplating of younger paraautochthonous thrust wedges under older, higher thrust
sheets. The outcrop of Sicani basinal section (Figures 2, 3)
in east-central Sicily is particularly important in that it
implies the exposure of the deeper structure containing
potential source in the fold belt.
Similar structural relationships occur in the west,
where Catalano et al. (2000) report a superstructure of
basinal carbonate facies Mesozoic rocks (the Imerese
and Sicani domains of Figures 2, 3) along with Numidian flysch, which was emplaced during the Miocene
and redeformed above younger thrust sheets that are
composed of the platformal Trapanese and Saccense
carbonates during the latest Miocene to the Pleistocene
The deeper parts of Figure 7B differ from their interpretation, which deserves some comment. Their plates
I and II show three major detachments. The highest
emplaces the basinal Imerese – Sicani section over the
platformal Trapanese – Saccense with an undetermined
Granath and Casero
FIGURE 7. Generalized crustal-scale structure sections across Sicily. The major oil and gas fields indicated in the figure
are shown in analogous positions to their true locations. Key indicates the emplacement sequence: para-autochthonous
sheets of platform rocks of the Hyblean domain refold higher sheets during emplacement. Section A redrawn and
modified from Bello et al. (2000). Section B reinterpreted as described in the text from Catalano et al. (1996, 2000). Section B
is also extended into the offshore to the south and to the north where the Panormide domain is thrust on Imerese. Late
normal faulting overprint is neglected. P = Panormide, I = Imerese, T = Trapanese, and S = Sicani domains (terranes).
distance. The underlying Trapanese – Saccense, in turn,
is thrust some 50 km (30 mi) to the south on a detachment overlying Oligocene – Miocene terrigenous
rocks, carrying the basinal section piggyback style.
Finally, the deepest detachment redeforms these two
upper nappes on a sequence of para-autochthonous
thrust slices that stretches to the south coast. Hence,
the overall shortening of Catalano et al. (2000) telescopes the section to about a third of its original
length and implies that the basinal section originally
lay outboard (to the north) of both slabs of Trapanese –
Saccense platform. Figure 7B accepts the shallow seismic and well-controlled interpretation from Catalano et al. (2000) but takes a more conservative point
of view on the shortening, similar to the sections in
Catalano et al. (1996). The shallow deformation is attributed to the duplexing of the platformal under-
pinnings of the allochthonous basinal section in a
similar fashion to Bello et al. (2000), which is generalized here in Figure 7A. The net effect is to more
intimately associate the basinal sections with the platforms, thus also taking a more optimistic viewpoint
on the exploration potential of the west, especially
in light of the many hydrocarbon occurrences there
The younger phase of the fold-belt development
generated the most attractive exploration play in the
Sicilian fold belt. Duplexes, ramp anticlines, and refolded higher sheets can generate closures in the buried
structure that constitute an as-yet unrealized play in
both the carbonate Mesozoic and its overlying siliciclastic cover. As Bello et al. (2000) demonstrate, newer,
better seismic data are necessary over much of Sicily
to test this play.
Tectonic Setting of the Petroleum Systems of Sicily
SOURCE ROCKS AND
Proven thermogenic source rocks throughout the
central Mediterranean are associated with Mesozoic
basinal facies and the margins of those basins. The best
developed occur in Triassic to Jurassic anoxic intraplatformal basins and troughs (Zappaterra, 1994) that
developed on the margins or wholly in platformal areas,
where relatively straightforward source-to-reservoir migration routes are available to charge traps. In Sicily, the
Streppenosa basin (Figure 8 and column 7 of Figure 5)
is an archetypical example, where the bulk of Sicily’s
oil fields lies (Figure 3).
Tertiary source facies have been established in the
fold belt (Figure 5). However, the distribution of Mesozoic source facies is problematic. Although the Sicani
and Imerese basinal facies outcrop in the fold belt,
their generative potential is yet to be established. The
Mufara Formation is a putative source interval but
yet to be typed to any hydrocarbon. The subsurface
distribution of basinal facies under late orogenic cover
is unmapped. For example, Catalano et al. (2000) infer
a clear physical connection between these basinal units
and the Streppenosa basin itself, whereas Zappaterra
(1994) draws a distinction in the tectonic evolution of
the two and an intervening platformal barrier between
them. Better seismic imaging north of the Gela nappe
front is clearly needed here to constrain the subsurface
structural geometry well enough to allow realistic palinspastic restorations of the
fold belt and, in turn, specific
between the various Mesozoic facies belts.
FIGURE 8. Seeps and asphalt
occurrences in Sicily. Dashed
lines are isopachs (in meters)
on Streppenosa Formation,
outlining the Streppenosa
(source rock) Basin. Isopachs
after Patacca et al. (1979);
seep and asphalt locations
after Rigo de Righi (1957)
and Colombo and Sironi
(1959). A major issue in
source rock distribution for
Sicily is if the Streppenosa
basin closes off as shown or
is connected in some way
with the basinal sections to
the north (Figures 2, 3).
By far, the greatest focus of geochemical studies
in Sicily has been on the Mesozoic systems of the
Hyblean Plateau in southeastern Sicily, primarily on
the Noto and Streppenosa formations. The oils of this
area are largely heavy (API < 208), with high sulfur contents of 3 – 9%, except for two notable accumulations at
Mila and Irminio (Figure 3), which have API values in
excess of 308. The heavy oils are generally water undersaturated, with varying salinities. At Cammarata –
Pozzillo and Ponte Dirillo fields, for example, salinities
range from 80,000 to 90,000 ppm, and at Ragusa, salinities are 100,000 ppm, whereas at Gela, they vary considerably more, from 20,000 to 100,000 in different horizons (Dalla Casa et al., 1981). Gas is associated at Gela
and Ragusa, with as much as 80% of the gas being CO2.
Pieri and Mattavelli (1986) linked the detailed chemistry of the heavy oils to early, immature generation
instead of biodegradation, as did Novelli et al. (1988),
who typed them to sources primarily in the Noto formation (Rhaetian in age). The Noto contains primarily
types I and II kerogen that are sometimes in excess of
1% by volume, whereas the Streppenosa is considerably
leaner and dominated by type III. Brosse et al. (1988)
studied kerogen both in the Noto and Streppenosa
formations distributed between black shales and clayey
laminites and, alternatively, in a variety of laminated
limestones, micrites, wackestones, packstones, dolomitic breccias, and their recrystallized equivalents. They
found that the best oil potential was in clay-rich facies
in the Noto formation, and that the kerogens have a
Granath and Casero
variety of kinetic characteristics. However, all appeared
to have early generative capabilities, so that the occurrence of light oil in the Ragusa foreland appears to be
a ramification of the geological history that is as yet
The maturation modeling studies of Novelli et al.
(1988) emphasized rapid Pliocene – Pleistocene burial
and heating during the Gela foreland subsidence in
connection with the generation of the heavy oils, a
conclusion corroborated by Brosse et al. (1989) specifically for the Gela area. Brosse et al. (1989), however,
contrasted Ragusa with Gela and found that variations
in the pre-Tertiary subsidence history and in the kerogen composition from different facies could lead to
maturation, hydrocarbon generation, and migration
as early as the Jurassic in the deepest depocenters of
the Streppenosa basin (Figure 8). Ocampo et al. (1993)
found that biomarkers uniquely typed a Noto source
rock to oils of the small Ponte Dirillo field (very near
Gela), but that significant differences in Gela oils suggested a different generation and/or source history.
Exploration data would also suggest that multiple
periods of hydrocarbon trapping have occurred. Multiple migration events are evident in the reservoir in
Irminio with the first migration as early as the middle
Cretaceous (Lindsey et al., 2002). Isopachs on various
stratigraphic levels of the Ragusa field show a thin section above the reservoir (Kafka and Kirkbride, 1959),
indicating recurrent rejuvenation of the trap well
before the depression in the Gela foredeep. Furthermore, flattening exercises on various horizons in connection with exploration studies seem to suggest that
Oligocene structural highs that have survived through
the foreland basin flexure and associated faulting to
remaining highs today are the most successful exploration targets, implying that a major period of petroleum migration occurred prior to the emplacement
of at least most of the fold and thrust belt. Numerous
dry holes have been drilled on Pliocene – Pleistocene
traps in the Ragusa foreland. Although there may be
many reasons for the dry holes, the occurrences of
traps charged with light oil, as well as the structural
history of successful traps, would suggest that a more
complicated maturation-migration history is involved
throughout the Ragusa plateau. In any case, the simple model attributing maturation and migration to
subsidence in the fold-belt foreland is insufficient to
account for the distribution of oil fields.
Fold-belt Oils and Tars
Although many hydrocarbon seeps and asphalts
occur in the fold belt (Figure 8), no modern geochemical studies have been made to type them to sources.
No discoveries in the Mesozoic sequence have been
recorded in the onshore fold belt, although one tar
south of Palermo at Marineo (Figure 8) occurs in Triassic
dolomite. Most of the occurrences seem to be similar to
the heavy oils in the foreland, but several have more
paraffinic characteristics. Heavy-metal concentrations
vary widely. The source of these hydrocarbons is uncertain but is likely to be similar in depositional and tectonic setting to the source in the Streppenosa basin.
Because rich Mesozoic source rocks, such as the Noto
formation, are of limited distribution outside the Streppenosa basin (Figure 5), other candidates have been proposed. The Triassic Mufara Formation is common in
the north, but of limited total organic carbon. Likewise,
younger formations have some carbon content, but
their total organic carbon and maturation levels have
received little study. A systematic study of the chemistry of these occurrences in connection with their structural setting would be of value in assessing the petroleum
potential of the fold and thrust belt. Again, better seismic
imaging would contribute much to the geographic aspects of such a study.
Melange, Mud Diapirs, and Fluid Flux
through the Fold and Thrust Belt
Connections between overpressure generation, dewatering processes, and deformation in accretionary
prisms and fold and thrust belts are documented in a
rich literature (e.g., Langseth and Moore, 1990; Shipley
et al., 1995) and are intimately related to hydrocarbon
migration. Prisms with at least partially subaerial histories, such as Sicily, are similar to many retroarc fold
and thrust belts and differ from intraoceanic prisms in
that meteoric water factors into the system, providing
hydrodynamic recharge otherwise unavailable and an
additional diagenetic influence over potential reservoir rocks. Both detachment-related melange and mud
diapirs indicate that Sicily is no departure from the
norm, but also that meteoric recharge, overpressure,
and deformation in the wedge are linked to at least
one period of petroleum migration.
Guilhaumou and Larroque (1993), Larroque (1993),
and Guilhaumou et al. (1994) have studied the argille
varicolore that are associated with detachment surfaces
and mud diapirs in the Monte Scalpello and Monte
Bubonia areas (Figure 2). Detachment surfaces in the
duplex at Monte Scalpello that carry the Tertiary section are characterized by a sheared Late Cretaceous
and Tertiary argillaceous matrix containing blocks of
argillite. Calcite veins in the blocks have low-salinity
fluid inclusions with low homogenization temperatures (<1008C). Basal detachment surfaces, i.e., those
carrying the Mesozoic carbonate succession, are marked
by mélange of sheared argillitic matrix and brecciated
blocks of mudstones, sandstones, and radiolarian cherts
carrying Lower Cretaceous fossils. The blocks contain
Tectonic Setting of the Petroleum Systems of Sicily
several generations of quartz and calcite veins with two
types of fluid inclusions. One set containing lowsalinity water exists in a range of veins, from quartz
veins with inclusions that have homogenization temperatures of 1508C to overprinted veins of calcite with
inclusions homogenized at more than 2008C. Significantly, the calcite veins also have high-temperature
pyrobitumen inclusions. A second set of water-depleted
inclusions in the calcite veins carry light oil with no gas.
The surrounding melange is in thermal disequilibrium
with the veins, recording only a lower thermal overprint in the form of clay transformation. The authors
concluded that the detachment channelized several
periods of fluid flux through the melange. Overpressure
resulted in naturally hydrofractured rocks with passing
hot fluids that, in at least one case, were associated with
hydrocarbons from a deeper source.
Similar lithologies to the upper detachment melange at Monte Scalpello occur in a body that crosscuts
stratigraphy at Monte Bubonia (Figure 3) and breaches
the core of an anticline in the Gela nappe (Larroque,
1993). The matrix is sheared red and green argillites,
with blocks of Nummulites-bearing Eocene calcarenites,
Numidian flysch, and various elements of the Mesozoic
and Paleogene section. Importantly, carbonate blocks
containing superimposed crack-and-seal calcite veins
are also present. Isotopic studies indicate that cements
in the carbonates are normal-marine cements, but the
veins relate to hydrothermal fluids. No oils were encountered in these upper detachment rocks, suggesting
the possibility that, at least locally, the hydrocarbon
source rocks lay deeper in the section than that accessed
by the upper fluid pathways. Presumably, the source
rocks were in the Triassic – Jurassic part of the section,
which was represented in blocks of the lower detachment and not of the upper detachment.
Tertiary-reservoired oils in the offshore west of Sicily
at Nilde and Narciso (Figure 3) likewise have not been
conclusively typed to sources but are assumed to be derived from associated Oligocene–Miocene source rocks.
Gas and Gas-condensate
Biogenic gas has been produced at two fields in the
fold belt. At the Lippone-Mazara field (Figure 3) in
western Sicily, gas that was produced from the Tortonian Terravecchia sandstone (Figure 5) is presumed
to derive from surrounding shales, whereas at Catania,
south of Mount Etna, Pleistocene Ribera sands reservoir gas again sourced from the surrounding shales. Thermogenic gas and associated condensate derived from
the surrounding Miocene Collesano formation charge
the Gagliano field.
Sicilian reservoirs include a variety of complex fractured carbonates in the Triassic and Jurassic of the Hyblean platform. They have complex episodes of charge
and diagenetic alteration. Lindsey et al. (2002) describe
a sequence of events in Irminio field involving the emergence of the reservoir in the Lias, a period of oil migration in middle Cretaceous time, followed by an influx
of hot water and bitumen baking. A second period of
charge began in the Paleogene and continued well into
the Neogene, when a period of stylolitization and dedolomitization also occurred.
The distribution of these reservoirs is controlled by
the Mesozoic paleogeography, in that the best reservoirs
are concentrated in and fringe the carbonate mound
buildups in the Streppenosa basin, where cyclic effects
of dolomitization, karstification and subaerial dissolution, emersion, burial, and fracturing that promote reservoir quality are coupled with periods of fine-grained
deposition that afford suitable seals. Presumably, similar reservoir rocks occur elsewhere in Sicily, where the
Neotethyan extended terranes afford similar histories.
At locations in the Gela nappe, for example, Mesozoic
carbonates exhibit cataclastic fabrics with marine dolomitic cements.
Cretaceous units similar to those elsewhere in
Italy (Mattavelli et al., 1993) are potential reservoirs,
such as debris flows and turbiditic units in the basinal facies, but these have yet to be tested in Sicily.
Tertiary terrigenous sandstones that are related to the
time-transgressive advance of the fold and thrust belt
provide the other major play in Sicily, as at Nilde or
Catania (Figure 3). These are units deposited in the migrating foredeep basin or in the piggyback basins. Reservoir quality depends on cementation history, of course,
and on the provenance of the sands, either carbonate or
SUMMARY AND CONCLUSIONS
Hydrocarbons occur in two main megasystems
in Sicily: in the Ragusa plateau and in the fold and
thrust belt. The Ragusa plateau is separated from the
fold and thrust belt by a very young and narrow foredeep (Figure 2). Currently, the plateau carbonates dip
southeastward away from a strong normal fault system
along its western edge, thus setting up a high along
the main Ragusa–Irminio–Mila oil field trend. The plateau is partially underlain by the intraplatformal Streppenosa basin of Triassic and Jurassic age, which extends
to the south offshore well away from the fold belt. It contains the well-established Noto and Streppenosa source
rocks, which have been typed to the predominantly (but
Granath and Casero
not exclusively) heavy-liquid hydrocarbons in most of
the fields. Several lines of evidence suggest that a multiphase maturation and migration was involved in charging the fields:
multiple ages of mild deformation, as reflected in
thinning isopachs over structures
long history of potentially elevated heat flow
petrographic studies of the diagenetic history of reservoir rocks
observations that Oligocene structures that have
survived to the present are more likely to contain
variations among the oils involved
Although there would seem to potentially be some
charge into heavy-oil fields in the narrow foredeep,
notably Gela and Perla, from petroleum systems in the
frontal parts of the fold and thrust belt, oil-bearing fluid
inclusions from basal detachment-related melange are
much lighter and do not suggest that heavy oils were a
component of the flux. Other fluid inclusions suggest
that fresher water would also be generated and potentially flush foredeep reservoirs as easily as they may be
charged. Consequently, the bulk of the oils appear to
have been sourced from within the foreland itself, and
the Ragusa plateau has served as its own kitchen, transport, and trap system using the large fetch area of the
Streppenosa source basin.
Light oils occur in Irminio and Mila, but it is unlikely that these light oils originated outside the foreland for the reasons above. All of these deposits are
located in the zero isopach of the Streppenosa basin,
so it appears that the foreland oils are sourced from the
deeper parts of the Mesozoic dominantly carbonate section of the intraplatformal basin during multiple phases
of generation and migration. Modeling studies suggests
that maturation and migration were certainly aided
by flexure of the foreland and Neogene burial, but the
deeper parts of the Streppenosa basin may have generated hydrocarbons as early as the Jurassic. Cretaceous
charge has also been proposed for Irminio (Lindsey et al.,
Unsuccessful tests are common in the Ragusa plateau and its offshore. Many of them are located on
young structural highs and poorly positioned to test
older closures. Structures in the foreland have undergone several stages of rejuvenation, with the distinct
possibility that many of them have been either ruptured or spilled either during Alpine structural influences on the area or during the emplacement of the
Sicilian fold and thrust belt. Hence, the hydrocarbon
system in the foreland is a complex one but appears to
be unlinked, for the most part, to the fold belt, except
as a potential analog for the early prefold belt phases
of hydrocarbon generation elsewhere in Sicily.
Hydrocarbons in the fold and thrust belt are
limited to biogenic and thermogenic gas onshore,
numerous seeps and outcropping tar deposits, and oil
of questionable source in offshore fields west of Sicily.
All of these deposits are confined to Tertiary reservoirs
(with the exception of one occurrence of tar in Triassic
rocks) largely because few valid tests of pre-Tertiary
traps have been performed in the fold belt. Nevertheless, several plays are possible, as outlined in the Appendix. That a live petroleum system was active at one
time is indicated by the fluid-inclusion studies in the
Monte Scalpello area and numerous seeps and tars in
the outcrop; but whether liquids have survived in commercial quantities remains a question. Relatively new
concepts of the structural architecture in the fold belt
offer the possibility that future tests of buried closures
would confirm or deny the potential of the fold belt.
Establishing sedimentological similarities and/or physical connections between the rich Streppenosa intraplatformal basin and the Jurassic basinal sections in the
fold belt would be a great step forward in demonstrating an effective Mesozoic source in the fold belt. Better
seismic imaging in a systematic survey would also be a
J. W. Granath thanks Forest Oil International for
providing the opportunity to become familiar with
the geology of Sicily, Luigi Albanesi for his mentorship
in Italian petroleum exploration, and A. W. Bally for
his thoughtful review of an early form of the manuscript. Both of us thank the conveners of the conference Deformation, Fluid Flow, and Reservoir Appraisal in
Foreland Fold and Thrust Belts held in Palermo in May
2002 for requesting this contribution to the proceedings volume.
Sicilian petroleum systems can be classified according
to their structural and geodynamic province (Figure 2),
starting from the hinterland toward the foreland. Efficient petroleum systems, as indicated by the main oil
and gas fields, are characterized first, followed by
other, underexplored and potential plays. Only plays
that have been successful in discovering hydrocarbons have been listed. The Mesozoic carbonates in the
fold and thrust belt have been disappointing to date.
This should be a major play, but a realistic test of structures in the thrust stack has not yet been made. Focus
Tectonic Setting of the Petroleum Systems of Sicily
has largely been on the shallower, surface anticlines and
not those involved in the deeper, younger sheets
described by Bello et al. (2000) and Catalano et al. (2000).
Petroleum Systems in the Fold Belt
Petroleum System of the Outer Numidian
Domain, Exemplified by the Casalini,
Bronte-San Nicola, and Gagliano and
1) Reservoir: Argillaceous sandstone of the Collesano
formation [Tortonian(?)]; porosity with microfractures 6 – 15%; low permeability
2) Trap: Imbricated thrust folds that are low amplitude,
south verging, and involving well-bedded carbonate
substratum (basinal Imerese units); structural closure
by bedding dips on three flanks and by fault toward
the south; vertical closure exceeding 200 m (660 ft)
3) Seal: Argillites from the paleo-oceanic Sicilides units
4) Source rock: Argillites from the Collesano formation
or siliceous shales of the Crisanti Formation (Liassic)
5) Maturation: At regional scale, these two formations
become increasingly buried toward the north and
laterally enter the gas window
6) Migration: Lateral and updip from the north
7) Hydrocarbon type: Gas (90% methane) and condensate (628 API) with gas-oil ratio averaging 7000
Petroleum System of the Inverted Inner
Messinian Foredeep, Exemplified by Nilde,
Norma, and Narciso Fields
1) Reservoir: Fractured and most likely karstified
bioclastic Serravalian – lower Tortonian(?) carbonates at the top of former passive-margin sequence
(Nilde limestone or lateral equivalent of the Fortuna Formation)
2) Trap: Steeply dipping, imbricated thrust folds verging toward the southeast
3) Seal: Shales at the base of the upper Tortonian Terravecchia Formation
4) Source rock: Carbonate source according to oil analyses made by Agip; no potential source rock has yet
been evidenced in the former passive-margin sequence at Nilde, but a lateral contribution of outer
Numidian sources is also possible
5) Maturation: During Messinian times, at the onset
of inversion episodes
6) Migration: Lateral and updip from the west, where
Numidian series become increasingly thick, both
stratigraphically and by tectonic duplication
7) Hydrocarbon types: Light oil in Nilde (388 API),
becoming progressively heavier toward the east
(21 – 258 API in Narciso); farther east, away from the
oil kitchen, only heavy oil and CO2 occur (Nora Nl,
Orlando 1 and 2)
Petroleum System of the Outer
Messinian Foredeep, Exemplified by
1) Reservoir: Argillaceous turbiditic sandstones of the
central part of the lower Messinian Terravecchia
2) Traps: East-trending thrust folds and pop-up structures of the Sicani domain resulting from inversion
of high-angle structures of the former African passive margin
3) Seal: Argillites from the Terravecchia Formation
4) Source rock: The same argillites
5) Maturation: Biogenic gas
6) Migration: Synchronous with folding (i.e., Messinian)
7) Hydrocarbon type: Gas/methane = 99%
Petroleum Systems of the
Gela-type Play: Gela, Ragusa, Ponte Dirillo,
and Prezioso Fields
1) Reservoir: Porous and fractured dolomites of the
Gela Formation (Norian – Rhaetian)
2) Traps: Multistage faulted anticlines, with late-stage
transpersonal reactivation during Apennine compressional episodes (upper Pliocene)
3) Seal: Black shales of the Streppenosa Formation
4) Source rocks: Black shales and bituminous limestones of the Noto formation (Rhaetian) and Streppenosa Formation (Hettangian)
5) Maturation: Deep burial already affected source rocks
during the Mesozoic southwestward of the Hyblean
Plateau, resulting in early oil generation and expulsion
6) Migration: The Noto and Streppenosa formations
constitute in the south lateral basinal equivalents of
shallow-water platform dolomites of the Gela Formation. This global architecture of the foreland allows a direct connection between very thick source
rocks and reservoir sequences. During the Late Jurassic, Cretaceous, and Cenozoic, continuous subsidence led to the burial of source rocks into the oil
window. Since the Messinian, the progressive uplift
of the Hyblean Plateau has resulted in a regional
tilting of the margin, thus inducing long-range migration pathways from the oil kitchen toward the
traps. Vertical migration is also locally controlled
by high-angle faults, thus allowing a remigration
of hydrocarbons from Mesozoic toward Miocene
Granath and Casero
reservoirs (i.e., in the Ragusa area, where bituminous calcarenites are cropping out and mined).
7) Hydrocarbon types: Low-maturity heavy oil with sulfur; oil gravity increases with the distance of migration
8) Special cases: Mila and Irminio fields are members
of this same petroleum system, but they are located
near the facies transition between the Streppenosa–
Noto and Gela formations; for instance, the Mila
reservoirs are rather reefal and thus distinct from
those of the nearby platform; hydrocarbons are light
(308 API), and the reserves appear to be the product
of multiple periods of migration (Lindsey et al., 2002)
Perla-type Play: Perla and Vega Fields
1) Reservoir: Partially dolomitized and fractured oolitic
limestones and algal pelsparites from open-platform
environment (Siracusa and Inici formations); intergranular and intercrystalline and vuggy porosity
2) Traps: Mixed, stratigraphic, and structural; a facies transition from platform carbonates of the Siracusa formation and shales from the Villagonia (Modica) formation
occurs toward the northeast; this facies boundary
has been reactivated by numerous north-verging reverse faults during Late Cretaceous inversion episodes,
thus increasing further the stratigraphic closure
3) Seal: Pelagic marly limestones of the Buccheri (Giardini) formation
4) Source rock: Mainly black shales of the Noto and
Streppenosa formations; because of regional migration trends, a contribution of lateral equivalents
of the Siracusa formation in a back-reef position
should be also considered
5) Maturation: In case the source rock belongs to the
deep Noto – Streppenosa basin, maturation would
be rather old, as in the Gela play; instead, if source
rocks are located in the back-reef area, they would
have entered the oil window more recently, i.e.,
during the deposition of the Pliocene – Quaternary
6) Migration: Vertical along high-angle fractures (in
case the source is located in the Noto – Streppenosa
basin) or lateral and updip from a hypothetical oil
kitchen located toward the southwest
7) Hydrocarbon type: Moderately mature and not biodegraded heavy oil (12 – 168 API) with sulfur
Catania and Cisina Gas Fields in the
Pliocene – Pleistocene Foredeep of Gela
1) Reservoir: Multilayered Pleistocene sandstones
2) Traps: Trapping anticlines and/or recent transpressional reactivation of fractures
3) Seals: Interbedded argillites
4) Source rocks: Interbedded argillites
5) Maturation: Biogenic gas
6) Migration: From the argillites toward nearby
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