Tertiary and Quaternary siliciclastic reservoir rocks are predominantly turbidite deposits of the Alpine and Apennine foredeeps and forelands, which cover most of the province. Reservoir rocks range from silty, fine-grained sandstones to local conglomerates that were commonly deposited along the long axes of syndepositional synclines. Onlaps and truncations are common. At onshore Cortemaggiore field (thermal Marnoso Arenacea total petroleum system and 10^th largest field in the province), the stacked Miocene reservoirs average 25-30% porosity and 80-180 md permeability, with individual producing zones ranging from 1.5 to 31 m in thickness (Pieri, 1992). The offshore Barbara field (biogenic Porto Garibaldi total petroleum system and 3^rd largest field in the province) produces from southeast trending stacks of Quaternary sandstones of centimeter to meter thicknesses (Ianniello and others, 1992). Best porosities are around 30%, and permeabilities range from 5 to 1000 md. Water-free production occurs from sandstones with 35-75 % water saturations and net-to-gross ratios ranging from 27% in the silty sands to 95% in the coarser sands. Cenozoic siliciclastic reservoirs in the Po Basin Province contain >78% of the total ultimate recoverable reserves (Table 3).

SEAL ROCK
Seals for Mesozoic carbonate reservoirs are at stratigraphic or fault contacts with the uppermost carbonates in the stratigraphic section – Cretaceous-aged, impervious pelagic marls and argillaceous limestones known as Scaglia and Marne di Bruntino (Figure 2). Cenozoic siliciclastic reservoirs are sealed by contiguous deep-marine shales and impermeable sandstones, generally meters to tens of meters thick. Some biogenic gas accumulations have shale seals < 1m thick and are presumed to be leaking even as they are continually replenished.

ASSESSMENT UNITS
Porto Garibaldi (Biogenic*) Total Petroleum System #406001
One established, biogenic gas assessment unit is appropriate for Porto Garibaldi (Figure 5a). (Examinations of existing field size distributions by reservoir age and by geographic and tectonic location did not distinguish unique populations for the purposes of this resource assessment.) This biogenic-gas petroleum system has an area of approximately 89,300 sq km. Sixty-one percent of the "biogenic gas" assessment unit is onshore; 39% is offshore. Thirty percent of the offshore is in Croatian waters, and all water depths are < 100 m. Dry methane gas (C₂₊ < 0.2 %) is the expected resource, and minimum field size is expected to be 1 mmboe onshore and perhaps 2 mmboe offshore. Further infrastructure development is needed offshore. Purely stratigraphic traps are underexplored within this petroleum system. *For the practical purposes of quantitative resource assessment, Porto Garibaldi is combined with volumetrically small Marnoso Arenacea (discussed below) – but still called Porto Garibaldi (#406001) – with one established assessment unit named Neogene Flysch Gas (#40600101) in the U. S. Geological Survey database. In combination, Porto Garibaldi and Marnoso Arenacea contain approximately 18.5 TCF known, ultimately recoverable reserves (87.9 % of the total province reserves, 97.8% of the total province gas, and 6.4% of the total province oil).

Meride / Riva di Solto (Thermal Triassic) Total Petroleum System #406002
One established assessment unit ( "Thermal Triassic" #40600201) is utilized for Meride / Riva di Solto (Figure 5b), even though several distinct productive areas are delineated and several different pods of source rock of variable Triassic age are known to exist. Meride / Riva di Solto has an area of approximately 31,500 sq km. The petroleum system and assessment unit are totally onshore in Italy, but the reader should be aware that a largely untested Triassic graben might exist offshore from the area of the Italian Po River delta southeastward (parallel with, but southwest of, the Adriatic axis) (Grandic and others, 1997). The expected resource for Meride / Riva di Solto is oil (possibly overpressured) with associated gas and condensate. Because of potentially great depths, this petroleum system is also underexplored. It is expected that the minimum field size will be 2 mmboe. Meride / Riva di Solto’s outline includes the westernmost field (Desana) in the province, of which little is published other than its 1954 discovery date, its abandoned status, and the fact that it has few reserves. Desana could possibly belong to the Marnoso Arenacea total petroleum system instead, so Marnoso Arenacea’s outline is in close proximity to Desana field (Figure 5c).

Marnoso Arenacea (Thermal Tertiary) Total Petroleum System
For the practical purposes of quantitative resource assessment, volumetrically small Marnoso Arenacea is combined with Porto Garibaldi (discussed above) – and called Porto Garibaldi (#406001) – with just one established assessment unit named Neogene Flysch Gas (#40600101) in the U.S. Geological Survey database. Marnoso Arenacea has an area of approximately 35,400 sq km and is entirely onshore in Italy (Figure 5c). The eastern two-thirds, east of Milan, include the Emilia, Ferrara and Romagna fold trends of the northern Apennines Mountains and contain all known production (Figure 1). These thrusted fold trends contain cores of Tertiary or Mesozoic rocks. The western one-third, west of Milan, includes the Monferrato Arc and S. Piedmont Basin that have little published information. These western folds contain allochthonous Liguride oceanic nappe in their cores (Pieri and Flores, 1996). The western area has fewer than two dozen exploratory wells and no shows reported in the Petroconsultants data base.