Figure 8 illustrates variability in Triassic expulsion history as related to differences in burial history, reflecting the presence of Mesozoic grabens (deep burial histories) and platforms (shallow burial histories). Graben source rocks would likely be more oil prone than source rocks from the platforms and platform margins. Mesozoic thermal gradients during rifting were probably higher than Cenozoic thermal gradients during continental collision. Much of the Triassic oil potentially generated from deepest grabens as early as Early to Middle Jurassic time either was not trapped or was subsequently lost from extensional fault-block traps during the later Alpine and Apennine orogenies. A later major episode of hydrocarbon expulsion under cooler thermal conditions occurred from Miocene and shallow-burial Triassic source rocks during the Alpine and Apennine orogenies. The highest present Cenozoic thermal gradients are associated with the Apennine mountain front. Regionally within the Po Basin Province, the top of the oil window (approximately 0.6 % R_o) ranges from about 4 km to 6 km in depth today.

TRAP STYLE
Mesozoic extensional block faults throughout the Po Basin Province might have served as early traps for Mesozoic-generated Triassic oils from paleograbens. Late Tertiary reactivation of those faults, during the Alpine and Apennine orogenies, locally compromised those pre-existing traps and influenced the trends of later deformation. Tertiary compression ceased in the western Alps before Pliocene time, but continued in the eastern Alps and northern Apennines into the Quaternary Period. Less modification of Mesozoic extensional block faults occurred in foreland regions away from the thrust fronts. Tertiary siliciclastics are locally detached from underlying Mesozoic carbonates.

Most Mesozoic fields (Meride / Riva di Solto thermal total petroleum system) are in faulted, Mesozoic-carbonate paleohighs that have been modified into basement-involved, thrusted anticlines – south-verging in the southern Alps and north-verging in the northern Apennines. Mesozoic reservoirs of this total petroleum system contain 12+ % of the ultimately recoverable province reserves (Table 2 and Table 3). Although some traps existed in Mesozoic time, the Neogene Alpine and Apennine orogenies were the critical events for trap formation/modification and for generation and migration (Figure 7b).

Tertiary (largely Miocene) reservoired fields of the thermal Marnoso Arenacea total petroleum system are in faulted, thrusted anticlines characterized with depositional lenses, onlaps and truncations. Faults provide necessary conduits for charging the reservoirs with thermal hydrocarbons generated off-structure. This Tertiary thermal hydrocarbon system contains just 4% of the province’s ultimately recoverable reserves (Table 2). Critical elements and processes for this petroleum system are Neogene in age (Figure 7c).

Tertiary and Quaternary reservoirs of the Porto Garibaldi biogenic-gas total petroleum system are in thrusted anticlines, simple drape structures and stratigraphic traps. Purely stratigraphic traps are underexplored. The presence of major faults increases the possibility that biogenic gas will be mixed with thermal hydrocarbons from below. More than 75% of the province ultimately recoverable reserves are in traps incorporating biogenic gas (Table 2). Critical elements and processes for this petroleum system are Pliocene and younger in age (Figure 7a).

DISCOVERY HISTORY
The first Italian onshore field was discovered in 1890 – followed by one in 1907 and two in the 1920s – in the Emilia fold trend of the northern Apennine Mountains. These early discoveries were shallow Tertiary anticlines < 1 km deep and sourced with thermal Miocene gas. Around 1940, the first field sourced from biogenic gas was discovered onshore near the Po River delta. Cernusco, the first Triassic-sourced field, was discovered in 1958. Regular annual Italian onshore discoveries began in 1951. The first Italian offshore field discovery was in 1960. The largest province field (Porto Garibaldi – Agostino) was discovered offshore in 1968. The first two Croatian (offshore) fields were discovered in 1977, including the largest Croatian field to date, Ivana, which ranks 20^th in the province.