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U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY


Petroleum Systems of the Po Basin Province of Northern Italy and the Northern Adriatic Sea: Porto Garibaldi (Biogenic), Meride/Riva di Solto (Thermal), and Marnoso Arenacea (Thermal) 

by 
Sandra J. Lindquist 

PETROLEUM OCCURRENCE AND CHARACTER
There are 263 fields in the Po Basin Province (Table 1), with 3.54 BBOE (48% produced) known, ultimately recoverable reserves (Petroconsultants, 1996) in three total petroleum systems. Ninety-seven percent of those ultimately recoverable reserves are in Italy; 3% are in Croatia. Onshore, 160 fields are in Italy and none in Croatia. The westernmost Italian onshore area and the Italian Veneto Plain north of the Adriatic Sea contain few fields (Figure 1). Offshore, 100 fields are distributed with 13 in Croatia and 87 in Italy. (Another three Italian fields discussed in the literature are not in the Petroconsultants data base of "fields.") Eighty-eight percent of the total province reserves are gas (nearly 19 TCF), and 50% of the gas has been produced. Eleven percent of the total province known or discovered reserves are oil (nearly 400 MMBOE), with 24% of the oil having been produced. One percent of the total province reserves is condensate, and 83% of that has been produced. Reservoir rocks range in age from Triassic to Quaternary.

Porto Garibaldi : Plio-Pleistocene (and minor Miocene) Biogenic Gas
Conditions necessary for the formation and entrapment of gas interpreted to be biogenic include adequate total organic carbon (TOC) content, high sedimentation rates, synsedimentary tectonics, adequate reservoir presence, and a cool thermal regime (<60° C) where bacteria can still exist and where organic matter does not degrade significantly (Tissot and others, 1990). Biogenic gas accumulations are commonly found in close proximity to their organic source material, with negligible migration required. Such conditions were met in the Po Basin Province during deposition of Neogene (especially Pliocene) and Pleistocene siliciclastic turbidites. The optimal location was the Apennine foredeep, northeast of and parallel with the present Apennine thrust front. Although biogenic gas has been confirmed within the entire Po Basin Province from as far west as Milan (Figure 4 and Figure 5a), the best combination of factors for its entrapment is in the eastern half of the province near the coast and offshore. Because of the cool Cenozoic thermal regime within the Po Basin Province, confirmed biogenic gas is present at depths to 4,500 m (15,000 ft) (Mattavelli and others, 1983).

Italian gas of biogenic origin is distinguished from that of thermal origin by the following geochemical criteria incorporating carbon and hydrogen isotopes and C2+ content (Tissot and others, 1990; Mattavelli and others, 1991):
 

Type of Gas
 C isotopes (d13C1)
H isotopes (d D)
C2+ hydrocarbons
Biogenic
Mixed
Thermal
-75 to -60 ‰
-60 to -50 ‰
-50 to -30 ‰
-180 to -210 ‰
-180 to -210 ‰
-150 to -180 ‰
< 0.2 %
0.1 - 5 %
0.3 - 10 %

Based on the Petroconsultants (1996) data base of known, ultimately recoverable reserves, nearly 85% (16 TCF) of the total gas (i.e., 75% of the total hydrocarbons) in the Po Basin Province is gas of biogenic origin (Table 1 and Table 2). Another 8-10% (1.8 TCF) of the province hydrocarbon totals and gas totals is mixed biogenic and thermal gas, with the remaining gas thermally generated. Previous reports attribute a comparable allocation of province gas-in-place reserves to biogenic sources and thermal sources (80% biogenic, 10% thermogenic and 10% mixed; Mattavelli and Novelli, 1983).

For this report, all field accumulations have been classified as to a unique or combined hydrocarbon source (Table 1). For the purposes of resource assessment, they are further assigned into just two major petroleum systems. When published designation or compositional information (isotopes or C1 vs. C2+ content) was lacking for a field, the following hierarchy of criteria (data from Petroconsultants, 1996) was used to assign it to a source-rock family:
 

Hydrocarbon
Source
Commodity
(G=gas,
C=condensate) 
Field
Location
Reservoir
Depositional
Environment
 Reservoir
Age
Definite biogenic
Probable biogenic
Definite mixed
Probable mixed
Tertiary thermal
Triassic thermal
G (plus 2 of following 3):
" (plus 1 of following 3):
G or GC (plus all of following 3):
      "                      "
      "                      "
GC or heavier      "
offshore
"
onshore
 "
 "
 "
deep
"
other than deep
"
"
"
Plio/Pleistocene
"
"
Mio/Plio/Pleistocene
Tertiary
mostly pre-Tertiary

Gas in Miocene reservoirs that is interpreted to be biogenic has been discovered in six fields on the Veneto Plain (Figure 4), but it constitutes <1% of total province reserves compared to >70% for Plio-Pleistocene biogenic gas (Table 2 and Table 3). Five of the six fields produce from Miocene reservoir rocks and one from Pliocene reservoir rocks.

Solely biogenic gas is in 123 fields with Pliocene reservoirs nearly 50% of the known recoverable reserves in the province (Table 3) extending from the Milan area (west) to the Croatian offshore (southeast) (Figure 4 and Figure 5a). The majority of Pliocene reserves (derived from Petroconsultants (1996) data) are in Upper Pliocene rocks, hence the total petroleum system name "Porto Garibaldi" (Figure 2) for both the primary source rock and the primary reservoir rock. Pleistocene biogenic gas reservoirs contain about 21.6% of the ultimately recoverable province reserves in 74 fields, mostly offshore but also trending intermittently westward to a location near 10° longitude (65 km east of Milan). A histogram of field sizes for all biogenic gas accumulations is compared with populations for thermogenic and mixed biogenic/thermogenic accumulations in Figure 6. (There were no significant distinctions in field size distribution between Pliocene and Pleistocene reservoirs.) Size data are derived from Petroconsultants (1996).

Mixed Biogenic and Thermal Gas
Biogenic gas probably mixed with thermal gas occurs in 21 fields (Table 1), generally where tectonism created suitable fault conduits for upward migration of thermal gas (Mattavelli and others, 1983). Fields with potentially mixed accumulations are not areally distinct from those with unmixed accumulations (Figure 4). Nor are size distributions (Figure 6). Potentially mixed accumulations comprise approximately 8.5% of the total province recoverable reserves (Table 2) and occur primarily in Miocene and younger reservoirs. Statistically, this mixed population is combined with the dominant biogenic gas total petroleum system of the province for the purposes of resource assessment (Figure 5a).

Marnoso Arenacea: Thermal Miocene Gas, Oil and Condensate
Thermal Miocene hydrocarbons thus far have been discovered solely onshore, mostly in a relatively continuous trend associated with the Apennine foredeep and fold front (Figure 1, Figure 4 and Figure 5c) where the thermal gradient is highest in the province. The source rock is the Upper Miocene Marnoso Arenacea Formation (Figure 2). Miocene hydrocarbons are mostly within Tertiary reservoirs in 30 fields (Table 1), hence the total petroleum system name "Marnoso Arenacea" for both the source and reservoir rocks. One field contains a presumed-Tertiary oil of unique geochemical character within a Mesozoic carbonate reservoir (Bagnolo in Piano field; Riva and others, 1986). The Tertiary source rock for that one field is postulated to be older Miocene Gallare marls (Figure 2).

Migration occurred laterally updip from thermally mature synclinal areas and perhaps along faults from thermally mature footwall locations. Volumetrically, thermal Miocene hydrocarbons are mostly gas (Table 2), with C2+ content generally higher than in most biogenic gas (> 0.2%). Thermal Miocene hydrocarbons constitute just 4% of the total province recoverable reserves (Table 2), and field sizes are smallest in the province (Figure 6).

Miocene-sourced oils from Cortemaggiore field (10th-largest province field on Figure 4) are described as napthenic, with API gravities (34° - 44° ), sulfur content (maximum 0.14%), vanadium content and nickel content similar to thermal oils from Triassic source rocks (Riva and others, 1986; Mattavelli and Novelli, 1990; Pieri, 1992). Miocene oils are distinguished from Triassic oils by unique carbon isotope signatures, high pristane/phytane ratios (>2), the presence of oleanane, and different sterane and hopane distributions. For Miocene oil saturates, d 13C ranges from 22.6 to 23.1 o/oo and for aromatics from
22.8 to 24.4 o/oo. The unique Bagnolo-in-Piano (Gallare) oil is heavy (16° API) and high in sulfur (4.9%). It has d 13C values comparable to Miocene Marnoso Arenacea oils but a pristane/phytane ratio intermediate (1.5) between typical Tertiary and Triassic sourced oils. Those parameters correlate well with Gallare source-rock extracts.

Meride / Riva di Solto: Thermal Triassic Oil, Gas and Condensate
Thermal Triassic-sourced hydrocarbons also have been discovered solely onshore in several distinct areas (Figure 4 and Figure 5b) near or projected from outcrops of organic-rich, Middle to Upper Triassic source-rock facies deposited in anoxic depositional troughs. Triassic hydrocarbons are mostly, but not exclusively, within Mesozoic reservoirs in 12 fields (Table 1). One field (Cernusco) produces known Triassic-sourced hydrocarbons from Pliocene reservoirs. The total petroleum system name reflects the hybrid of 2 major Triassic source rocks the Middle Triassic (Ladinian) "Meride" Limestone and the Upper Triassic (Rhaetian) "Riva di Solto" Shale (Figure 2). To avoid a cumbersome name length, the largely Mesozoic carbonate reservoirs are excluded from the total petroleum system name.

Lateral updip migration, fault migration, and vertical migration were necessary to charge local structural highs from the areally restricted source rock centers. Volumetrically, Triassic hydrocarbons are mostly oil, and they comprise approximately 12% of the total province recoverable reserves (Table 2). This thermal Triassic petroleum system has the widest range of field sizes and the flattest distribution (Figure 6).

Oil data have been published for three Triassic-sourced fields that represent the three distinct productive areas the westernmost Villafortuna-Gaggiano complex (#2 on Figure 4), Malossa field (#16 on Figure 4), and Cavone (one of the easternmost Triassic group of four on Figure 4). The oils have distinctions regionally, particularly in terms of API gravity and sulfur content. Villafortuna-Gaggiano oils are overpressured and in a liquid phase at depths of 4600 to 6200 meters (amongst the deepest oil fields in the world). API gravities range from 34° -40° , and sulfur content is low (Mattavelli and Novelli, 1990). They are similar to Malossa Triassic oils in carbon isotope values and in isoprenoid distribution, but differ by vanadium/nickel ratio and the presence of dibenzothiophene.

Malossa oils are also overpressured and have 47° -53° API gravities; low sulfur content (0 - 0.6%); vanadium, nickel and polar compound presence; a pristane/phytane ratio of 1-1.2; and d 13C values of 29.2 to 30.9 o/oo for saturates and 28.5 to 30.3 o/oo for aromatics (Riva and others, 1986; Mattavelli and Novelli, 1990; Stefani and Burchell, 1990). They contain significant quantities of diasteranes and have abundant peaks in C27-C35 hopanes and C29-C30 moretanes (Stefani and Burchell, 1990).

Oils from Cavone field have API gravities of 20° -22° ; high sulfur content (to 4%); abundant polar compounds (20-36%), C20+ content, and biomarkers; negligible diasterane and low Ts/Tm ratios (Riva and others, 1986; Mattavelli and Novelli, 1990).


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U. S. Geological Survey Open-File Report 99-50M