Continued Apennine folding until the Pleistocene Epoch resulted in the Apennine foredeep area being a good location for anticlinal traps, especially the region around the coastline where subsidence was greatest and there was optimal alternation of sandstone and shale for future reservoir and source rock (Figure 1 and Figure 4). In contrast, the Pedealpine region of biogenic gas near Milan (Figure 1 and Figure 4) did not experience such concurrent deformation, and its traps are more stratigraphic. A limited area of Miocene biogenic-gas source rocks from the Alpine foredeep has charged six anticlinal fields in the Veneto Plain of onshore northeastern Italy (Figure 4).

Neogene and Pleistocene bathyal marine shales have total organic carbon (TOC) content ranging from trace amounts to 1.0 wt % and kerogen composition of >80% terrigenous material (Riva and others, 1986; Tissot and others, 1990). These shales are thermally immature (vitrinite reflectance < 0.5% R_o) to depths exceeding 5 km in the central Po valley. An events chart illustrates the geological recency for all the events and components of this biogenic total petroleum system ( Figure 7a).

Marnoso Arenacea: Miocene Source Rock for Thermal Gas, Oil and Condensate
Miocene bathyal marine shales (and local marls) are thermally immature along the Pedealpine Homocline even to depths of 6 km (Riva and others, 1986). In the Apennine subsurface fold belt, the oil window is locally as shallow as 4 km, so Miocene thermal hydrocarbon accumulations are largely restricted to the onshore Apennine trend (Figure 4). It is common for Marnoso Arenacea shales to be thermally immature on the tops of anticlines. The formation attains thickness exceeding 3 km and has average TOC content of 0.68 wt% and a hydrogen index (HI) of 180 mg HC/g TOC (Riva and others, 1986). At 10th-largest Cortemaggiore field, maximum TOCs of 1.5 wt % and type III kerogens characterize the 2-km thick formation, which generated hydrocarbons in Plio-Pleistocene time (Pieri, 1992). Correlation of oils and source rock extracts (using pristane/phytane ratios, carbon isotopes, and GC/MS data) confirm the Marnoso Arenacea sourcing of numerous fields. An events chart shows a Late Miocene emphasis and importance for events and components of this small Tertiary thermal petroleum system (Figure 7c).

Meride / Riva di Solto: Triassic Source Rock for Thermal Oil, Gas and Condensate
There are published studies of Italian Triassic outcrops across the Southern Alps in a west-to-east trend from Lake Maggiore to the Slovenian border (Figure 1) (Stefani and Burchell, 1993). Triassic shale and carbonate source rocks of variable maturity (immature to highly overmature) crop out in the western half of the province between Lake Lugano (west) and Lake Garda (east). They were deposited in north-south trending, transtensional anoxic basins, several hundred meters deep, interspersed with shallow carbonate platforms. The persistent, Mesozoic platform/graben depositional setting resulted in variable local burial and expulsion histories for the source rock (Figure 7b and Figure 8). A Mesozoic overall rise in sea level resulted in the troughs becoming more widespread through time (maximum 1000 sq km; Stefani and Burchell, 1990), such that Rhaetian-age (uppermost Triassic) shales and marls are generally the thickest (> 2 km) and most organic rich of the Triassic source rocks. Source rock thickness can vary by an order of magnitude over a 10 km distance (Mattavelli and Margarucci, 1992).

Thickest and most basinal Upper Triassic source-rock outcrops are located on the west side of Lake Iseo (Figure 4) where they are highly overmature (R_o = 4%, Stefani and Burchell, 1990). Triassic-sourced production closest to Milan (approximately 9.2° east longitude, including 16th-largest Malossa field) is geochemically tied to Rhaetian euxinic, basinal source rock cropping out directly north of there; whereas the four fields around 11° east longitude are thought to be charged from organic-rich facies of a postulated small intraplatform depression on the Rhaetian shallow-water carbonate platform cropping out east of and at Lake Garda (Figure 4) (Stefani and Burchell, 1993). If the latter is true, there might be potential for the presence of local Triassic source rocks in the eastern half of the province where only platform carbonates have been identified in outcrop.

Westernmost Triassic-sourced production from Trecate, Villafortuna (2nd largest province field) and Gaggiano fields is associated with older, Middle Triassic Meride argillaceous carbonate source rocks that crop out in a narrow band between Lake Lugano and Lake Como and that have been penetrated in a Gaggiano well (Riva and others, 1986).

Riva di Solto source-rock extracts are characterized by abundant diasterane content, both marine and continental kerogen types II and III (13-21% amorphous, 34-59% herbaceous, 28-45% woody), and a pristane/phytane ratio near 1 (Stefani and Burchell, 1990, 1993). Land-derived kerogen is probably over-represented in outcrop samples because of high levels of thermal maturity in the most basinal facies. TOC ranges from 0.5 to 5 wt %. Pyrolysis S₂ values from several samples range from 1-3 kg/ton, with a carbon preference index (CPI) of 1. Riva di Solto and Meride source rocks can be distinguished from each other by carbon isotopes and biomarkers. Riva and others (1986) compare the Triassic source rocks as follows: Meride TOCs average 0.8 wt %, with a sulfur content of 4.5% and HI of 513 mg HC/g rock; Riva di Solto TOCs average 1.3 wt %, with a sulfur content of 3.1% and HI of 251 mg HC/g rock.

An events chart shows the emphasis and importance for components and events ranging from Jurassic through Neogene age for this Triassic thermal petroleum system (Figure 7b).