Four geologic settings dominate oil, gas fields of Italy, Sicily

Dec. 6, 1999
The oil and gas fields of Italy and Sicily produce a variety of oil and gas types from a wide range of traps.
The oil and gas fields of Italy and Sicily produce a variety of oil and gas types from a wide range of traps. The major oil and gas fields of Italy can be divided into four general structural/stratigraphic settings. These zones, which extend from west to east in the Apennine system and from north to south in the Maghrebian, consist of the:
  1. Internal zone of the Apennine thrust system;
  2. External zone of the Apennine and Maghrebian thrust systems;
  3. Plio-Pleistocene foredeep; and
  4. Mesozoic carbonate foreland.

1. Internal zone-Apennine thrust system.The western (internal) portion of the Apennine system is composed of stacked, imbricate thrust sheets containing rocks ranging in age from Late Triassic to Upper Miocene. Oil and gas fields have been discovered in allochthonous Mesozoic carbonates throughout the length of the system. The northernmost fields of the internal zone are Gaggiano and Villafortuna, located just south of the intersection of the Apennine system with the Alps. Gaggiano and Villafortuna produce light oil (34-42°) from thrusted Middle Triassic dolomites at depths of 4,650-6,200 m. Villafortuna, discovered in 1984, is currently Italy's most productive oil field (60,000 b/d).

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During the last decade attention has also been focused on the internal zone of the southern Apennines. The discovery of several fields containing light to medium gravity oil during the 1970s sparked initial interest in the area. These fields, including Castelpagano (31°, 1971) and Benevento (46°, 1973), are productive from stacked sections of allochthonous Mesozoic carbonates (Figs. 4, 5). Hampering further progress was the poor quality of seismic data in the region, the result of rough topography and the structural complexity of the subsurface.

Improved seismic techniques, including 3D, led to the first discovery in the prolific Val D'Agri area in 1988. To date at least four major fields have been discovered in the area, Tempa Rossa (1988), Monte Alpi (1988), Monte Enoch (1994), and Cerro Falcone (1992). Characterizing these fields are high flow rates (3,000-12,000 b/d) and large oil columns (600-1,000 m). The total recoverable reserves for the four fields are estimated at 1.02 billion bbl of oil equivalent. The Val D'Agri fields are discussed in more detail later in this, the second part of this three-part article.

Allochthonous Miocene (flysch) sandstones are also productive within the internal Apennine system. They form reservoirs for light oil, gas, and condensate fields in the Emilian and Romagna nappes along the southern edge of the Tertiary Po basin of northern Italy.

2. External zone-Apennine and Maghrebian thrust systems Fields of the external zone are dominantly gas productive but also include several oil and condensate fields. The main reservoirs are allochthonous Miocene flysch and Lower-Middle Pliocene foredeep turbidite sandstones.

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The largest field of the external zone is Luna (Fig. 6), a gas field that lies along the west side of the Gulf of Taranto in southernmost Italy. The field contains large reserves of dry gas in sandstones that lie at or immediately above the top of the allochthonous Miocene flysch section. The structure is the product of both Upper Miocene and Lower Pliocene movements. A thin section of Messinian evaporites is present on the flanks but absent over the crest of the Luna structure, the result of Early Pliocene uplift and erosional truncation of the Messinian. Lower Pliocene shales, possibly olistostromal, provide the seal.

The gas from Luna is dry (99% methane) and biogenic in origin. The field was discovered in 1971 and currently includes 35 wells. Initial recoverable gas reserves were estimated at 1.3 tcf. During the period 1994-97 gas production from offshore Zone D (which consists only of Luna and several small nearby fields) was 348 bcf and remained stable during 1998 at 70 bcf.

Gas fields with allochthonous Miocene reservoirs of the external zone of the Apennine thrust system are common throughout the length of the Italian peninsula. The gas from most of these fields is dry and biogenic, although the presence of heavier hydrocarbons in a few areas probably indicates limited mixing with thermogenic gases.

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The Apennine system of peninsular Italy is replaced by the regional Maghrebian wrench-thrust system on Sicily. The Maghrebian system extends southwest from Sicily through Tunisia and westward across northern Africa to the Atlantic. The external zone of the Maghrebian thrust belt contains several significant, structurally trapped fields in north-central Sicily, as well as offshore to the west of the island. The largest onshore field is Gagliano, which contains gas, condensate, and light oil in allochthonous Miocene flysch sandstones at depths of 2,800-3,000 m (Fig. 7). Offshore, Nilde field produces 39° gravity oil from highly productive Middle Miocene organic limestones at rates of up to 10,000 bo/d.

3. Pliocene-Pleistocene foredeep. liocene and younger sandstones of the foredeep contain the majority of Italy's proved gas reserves. Most of these reserves are currently located in the north-central Adriatic and Po basin of northern Italy. During 1998, production from four of the larger offshore fields (Barbara, Angela, Porto Garibaldi, and Agostino) totaled more than 270 bcf. The dry, biogenic gases lie in both stratigraphic and structural traps. Reservoirs range from turbidite sandstones to gravel. Several of the larger fields of the north-central Adriatic and central Italy are described below.

The large Barbara, North Barbara, and Northeast Barbara gas field complex was discovered in 1971 in the north-central part of the Adriatic. A total of 112 productive wells have been drilled to date. Gas production is from an Upper Pliocene sandstone at an average depth of 1,400 m. The trap is a simple low relief (less than 100 m) structure draped over a broad uplift in the underlying Mesozoic foreland.

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Onshore in central and southern Italy, South Salvo, Torrente Vulgano, and Pisticci fields produce gas from Lower and Middle Pliocene foredeep sandstones. South Salvo is stratigraphically trapped by the updip pinch out of multiple Pliocene sandstones that are overlain by approximately 800 m of allochthonous Miocene flysch. The field includes 74 productive wells with an average depth of 1,200 m. In addition to Pliocene gas, Pisticci field also contains heavy oil in autochthonous foreland carbonates along the western side of the field (Fig. 8).

At least two fields, Torrente Tona and Candela, produce both light oil (40°) and gas from Middle and Upper Pliocene foredeep reservoirs along the east coast of central Italy. At Torrente Tona, sandstones and limestones are stratigraphically trapped by updip olistostromes derived from the east flank of the southern Apennines.

4. Mesozoic carbonate foreland.
Several large oil fields have been discovered in the relatively undisturbed platform carbonates of the Mesozoic foreland of southern Italy and Sicily, both on and offshore. Onshore, various thicknesses of allochthonous Miocene flysch and in some cases a thin section of Plio-Pleistocene sediments overlie the fields. Offshore the allochthonous Miocene section is generally absent and the Mesozoic foreland sequence is overlain by sediments of the Plio-Pleistocene foredeep.

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Sicily's largest oil field, Gela, is located along the southwestern coast of the island on a broad anticline in the autochthonous foreland section (Fig. 9). Productive Upper Triassic shelf carbonates are overlain and sealed by organic-rich black shales of the Triassic Streppenosa formation and Cretaceous-Eocene basinal limestones. The southern feather-edge of the allochthonous, olistostromal Gela nappe and a thin interval of Plio-Pleistocene sediments cap the section.

Gela was discovered in 1956 and includes 104 productive wells. Depth to the reservoir averages approximately 3,300 m. Gela oil is the heaviest (10°) produced from the foreland fields of southern Sicily. All of the oils of the area are characterized by relatively low gravity oils (15-21°) and a significant sulfur content. Ragusa field, second to Gela in size, produces 19° gravity oil with a sulfur content of 2%. Studies indicate that the relatively heavy oils of southern Sicily are immature, the result of a low thermal gradient and the early expulsion of hydrocarbons from organic rich Upper Triassic shales. Only Gela oils are believed to be additionally biodegraded.

Perla field is located offshore just south of Gela. The reservoir lies at the top of an interval of Lower Jurassic shelf limestones (Siracusa formation) at depths of slightly less than 3,000 m. The reservoir is overlain and sealed by marls and evaporite and underlain by rich source rocks of the Streppenosa shale. Farther to the south Vega field produces similar oils (15-21°) from the same stratigraphic level.

Val D'Agri oil play

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The Val D'Agri fields Tempa Rossa, Monte Alpi, Monte Enoc, Cerro Falcone, and Costa Molina (Fig. 4) rank as some of the largest in Europe. The total proved recoverable reserves for the four fields are estimated at 1.02 billion BOE.3 Production from Monte Alpi, Monte Enoc, Cerro Falcone, and Tempa Rossa fields is expected to reach a combined 165,000 bo/d by 2002. Oil will be transported via a new pipeline (to be completed in 2001) to a refinery and marine oil terminal at Taranto.

Val D'Agri wells are productive from multiple zones in a variety of carbonate facies, with oil columns of 600-1,000 m. Although the specific type and character of the zones varies, in general the reservoir properties of the productive limestones and dolomites are excellent. The reservoirs include dolomites and leached, sometimes karstic, vuggy limestones. The section is also characterized by large, open fracture systems. Flow rates in the recent South Apennines oil discoveries range from 3,000-12,000 bo/d plus associated gas. The produced oils display a wide range of gravities, from 17-46°, but most commonly cluster from 32-37°.

Major Val D'Agri fields

Tempa Rossa field
Tempa Rossa was discovered in 1988 by a group of companies headed by Petrex. To date six wells have been drilled in the field, including Tempa Rossa 1, 1A, and 2; Tempa D'Emma 1; Gorgolione 1; and Perticara 1.

All of the wells are oil productive from Miocene through Cretaceous limestones and dolomites. The oil column in the field is estimated at 1,000 m gross and 700 m net. Proved recoverable reserves are estimated at 420 million BOE, with associated gas forming about 7% of the total.4 Production is expected to reach 45,000 bo/d by 2002.

Test rates range up to 8,038 bo/d, and long term test results from at least two wells have been published. The No. 2 well produced a total of 116,000 bbl of oil (17°) during a 135-day test, at an average rate of 1,220 bo/d. The test produced no water and reflected no decrease in reservoir pressure. A 1992 sidetrack of the No. 1 well tested oil rates of over 7,600 bo/d (17-22°) and had produced over 500,000 bbl of oil through December 1996 during "test" production. As of the end of 1996 the well was producing at a rate of 3,700 bo/d.

Monte Alpi field

Monte Alpi was discovered in 1988 by a group headed by Fina Italiana. The field currently consists of nine wells (all productive), including the Monte Alpi 1, 2, 3, 4, 5; Monte Alpi Est, Nord and Ovest wells; and the Volturino 1.

Oil (30-37°), with associated gas, is produced from Upper Cretaceous limestone and dolomite reservoirs. The oil column is 1,060 m gross and 702 m net. Oil flows (constrained by gas flaring restrictions) range up to 6,793 bo/d. The production of 45,000 bo/d is projected for the field by 2000.

Monte Enoc field

Monte Enoc was discovered in 1994 and currently consists of 8 wells, all of which are oil productive (30-37°), including Monte Enoc 1, 2, 3 and 9; Monte Enoc NW 1 and 1A; Monte Enoc Ovest 1; and Al* 1A). Oil flows (constrained by gas flaring restrictions) range up to 7,717 b/d.

Cerro Falcone field

Cerro Falcone consists of 6 wells, all oil productive, drilled between 1992-97. Oil (32-36°) has been tested at rates of up to 6,600 b/d along with significant volumes of associated gas (up to 4.4 MMcfd). The field produces from Miocene to Cretaceous carbonate reservoirs, with an oil column of 760 m.

A total of 42 production wells are currently planned for Monte Alpi, Monte Enoch, and Cerro Falcone fields. Eighteen of these have been completed, with four wells currently on production. Production is expected to peak at 120,000 bo/d in 2002. Total recoverable reserves for the three fields are estimated at 600 MMBOE, of which approximately 12% is associated gas.5

Rock, hydrocarbon types

The oil and gas fields of southern Italy and Sicily contain a wide variety of hydrocarbon types, all of which are believed to have been generated within three general source rock intervals.

  1. Mesozoic oil-prone carbonates.
  2. Miocene oil and wet gas prone clastics of the syntectonic flysch sequence.
  3. Pliocene-Quaternary foredeep clastics.

The large volumes of heavy oil discovered in carbonate reservoirs of the Mesozoic foreland can be traced to the development of organic-rich carbonates in the narrow troughs between the Mesozoic carbonate platforms. These include black shales and marls of the Upper Triassic-Lower Jurassic Streppenosa shale and Noto formation of Sicily and the Upper Triassic Meride limestone of northern Italy. The oils generated from these intervals are relatively heavy (11-22°), contain significant amounts of sulfur and are thought to be thermally immature.

Moderate to light oils (25-40°, no significant sulfur) have been found in Miocene and Pliocene reservoirs in scattered fields of the Italian peninsula, and in the allochthonous Mesozoic carbonates of northern Italy (Villafortuna field) and the Val D'Agri trend of the southern Apennines. The characteristics of these oils indicate that they originated in source rocks within the Miocene flysch section, although specific intervals have yet to be identified.

Analysis of the gases found in Miocene, Pliocene, and Quaternary reservoirs, including the fields of the Po basin and Adriatic, indicates that over 80% are biogenic in origin, derived from terrestrial organic matter in the abundant clays of the foredeep section.