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) 

Sandra J. Lindquist 

Province Boundary and Geographic Setting
The 115,000-sq-km Po Basin Province of southern Europe is located between 43° -46° north latitude and 7° -15° east longitude (Figure 1). It includes northern Italy’s Lombardy, Po River, and Veneto plains, as well as the northern Adriatic Sea with an adjacent portion of onshore Croatia (formerly Yugoslavia) called the Istrian Peninsula. Approximately two-thirds of the area is onshore (98% Italy, 2% Croatia) and one-third offshore (65% Italy, 35% Croatia). A very small portion of the Istrian offshore might technically belong to Slovenia. The province is surrounded by thrusted terrain of the southern Alps Mountains to the west and north, the northern Apennine Mountains to the south, and the Dinaride Mountains to the east. The province contains limited south-verging structural deformation on the Adriatic and Pedealpine homoclines; significant north-verging deformation in the Monferrato, Emilia, Ferrara, Romagna and Adriatic fold systems; and limited west-verging deformation from the Dinaride thrust system. The small country of San Marino is included in the southern part of the province.

Political Entities
Italy has favorable regulations for industry and has just recently opened its prospective lands to leasing by public and private foreign companies. Croatia and the other former-Yugoslavian republics are involved in continuing disputes amongst themselves. The most recent Croatian wells were completed in 1990 (Petroconsultants, 1996).

Geologic Setting
Prior to the Late Paleozoic Hercynian orogeny and creation of the Pangea megacontinent, the Po Basin Province was on the northwestern edge of the Gondwana (Africa) subcontinent (Pieri and Flores, 1996; Dercourt and others, 1993; Anelli and others, 1986; Mattavelli and others, 1983). Deformation associated with Hercynian suturing has resulted in pre-Hercynian, Paleozoic rocks having poor reservoir potential, and they are referred to as "basement."

A post-Hercynian tensional regime affected the central Mediterranean area with the opening of the NeoTethyan and Mesogean oceanic basins. The Po Basin Province was part of the Adriatic microplate system developed between Africa and Europe. Early Mesozoic rocks record this breakup of Pangea – as well as an overall Mesozoic rising sea level – with pre-rift, Permo-Triassic continental siliciclastic rocks grading upward into Triassic rift sandstones, siltstones, shales, evaporites and carbonates (Anelli and others, 1996). North-south trending, early Mesozoic shallow-water carbonate platforms were laterally separated by extensional troughs, including the Olenus-Pindos, Sub-Pelagonian, Lagonegro, S. Alpine, Austroalpine and Piedmont rifts, in which organic-carbon rich sediments accumulated. The most anoxic basins with the richest organic matter were established in the Late Triassic Period. Platform and trough locations were relatively consistent throughout the Mesozoic era.

Jurassic and Cretaceous strata contain evidence for a continuous transition to a passive continental margin with shallow-water carbonates grading upward into generally deeper-water limestones and cherts. Regional stress regimes changed as the central and northern Atlantic Ocean began to open. Late Cretaceous to Cenozoic subduction of Tethys oceanic crust and continental collision between Africa and Europe partly deformed, rotated, overrode and consumed the Adriatic microplate, but a significant portion of that plate was preserved as a minimally deformed foreland region under the central portion of the Adriatic Sea and under the Po River basin. The area evolved into a deep-water basin characterized by siliciclastic turbidite and hemipelagite deposition that was impacted by continued compressional events and Tertiary sea level fluctuations.

The earliest evidence for Alpine compression is in the northern Alps Mountains, where northward thrusting is recorded in Upper Cretaceous Lombardo flysch (Mattavelli and others, 1983). By Oligocene time, most of the intervening part of the Tethys Ocean had been subducted, and the Adria continental microplate was being compressed from several directions. Change in relative plate motion was causing southward thrusting in the southern Alps and westward thrusting in the Dinarides, the latter attaining their present location at the eastern coast of the Adriatic Sea (Figure 3a,  3b, Figure 3c, 3d). Also beginning in the Oligocene Epoch in westerly distal locations was an eastward compressional stacking of carbonate platforms and their associated deeper basins to form the earliest Apennines (Haan and Arnott, 1991). This deformation would reach the west side of this province 10-15 m.y. later by Middle Miocene time.

A Late Miocene (Messinian) drop in sea level, combined with the effects of continued Euro-African collision, resulted in periodic isolation of the Mediterranean basin from open marine circulation. Local unconformities developed, and widespread evaporites were deposited. The Apennine orogeny peaked in Pliocene time, with the deformed zone reaching its northeasternmost expression across much of Italy, although some deformation continued into the Pleistocene Epoch. Substantial foredeep subsidence resulted in the deposition of at least 6 km of Pliocene sediment (Haan and Arnott, 1991), the subsequent filling of the foredeep, and the establishment of the Po River drainage system. Transpressional deformation continues, but certain foreland areas – especially along the Croatian Adriatic Sea and Istrian Peninsula – still remain relatively undeformed.

Exploration History
Hydrocarbon shows in the Italian Apennines have been known since the 1700s, and some shallow wells there were drilled as early as 1860. The first exploratory wells in the Po Basin Province of Italy were drilled in 1890 and 1907, followed by four more in the 1920s. Generally, several to ten Italian exploratory wells were drilled annually during the 1930s and 1940s, and both exploration and development operations intensified in the 1950s as reflection seismic data were refined. First Italian offshore exploration began in 1960 and first Croatian offshore exploration in 1967. From the early 1950s until the mid 1990s, AGIP SpA – the Italian state oil company – had exclusive exploration rights in most of the Italian Po Basin Province.

Early onshore exploration was for shallow, Tertiary-reservoired structural anticlines identified with seismic data. Deeper Mesozoic folded and faulted reservoirs were targeted in the 1970s and 1980s, with improved success rates achieved through the use of 3-D seismic techniques.

A total of 2,670 wells are in the Po Basin Province, 97% in Italy and 3% in Croatia (Petroconsultants, 1996). Of Italy’s wells, 42% are classified as wildcats (22% success rate), 41% as development (78% success rate) and 17% as outposts (43% success rate). Of Croatia’s wells (all offshore), 53% are wildcats (9% success rate) and 47% are outposts (28% success rate).

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