Po Plain Petroleum Systems: Insights from Southern Alps Outcrops (Northern Italy)

Fantoni, Roberto ¹; Galimberti, Roberto ¹; Ronchi, Paola ¹; Scotti, Paolo ¹
(1)Eni E&P, San Donato Milanese, Italy.

The present Po Plain is a foreland shared by two chains, the Southern Alps and the Appennines (Northern Italy). As well as in the bordering chains, the foreland compressional architecture is overprinted on the polyphasic framework produced by Mesozoic extensional cycles (from late Permian to early Cretaceous).

At the beginnig of the Seventies, thanks to the introduction of multiple coverage reflection seismic, the exploration of the Mesozoic carbonate succession began and soon led to the discovery of the Malossa gas and condensate field, in a compressional structure of Cenozoic age (located 30 km east of Milan and 40 km south of the southalpine outcrops). The hydrocarbon search continued with the exploration of the Mesozoic extensional structures, partially restructured during the alpine compression, leading in 1984 to the discovery of the Villafortuna-Trecate oil field, located 30 km west of Milan and 40 km south of the southalpine outcrops.

The Malossa and Villafortuna-Trecate petroleum systems include Triassic-Early Jurassic source rocks and reservoirs. The comparison between subsurface data (wells and 2D and 3D seismic surveys) and analogous successions outcropping in the Southern Alps allowed a satisfactory definition of these petroleum systems.

The trap of the Malossa field is a tectonic high related to the Mesozoic extensional phase passively involved in a SW verging Cenozoic compressional structure. The petroleum system is composed of an Early Jurassic reservoir made of carbonate platform dolomite (Albenza Formation) and Norian source rocks (Aralata Group and Riva di Solto shale). The source rocks are characterised by a type II and II/III kerogen; the original Source Potential Index (SPI) could have been higher than 3 t HC/m².

The trap of the Villafortuna-Trecate oil field consists of an alpine compressional structure involving a pre-existing Mesozoic extensional relief. Mesozoic extensional features are present within this alpine multi-kilometric structure. Reservoirs are made of dolomitized carbonate platform rocks (Monte San Giorgio Dolomite, Anisian; Dolomia Principale, Campo dei Fiori Dolomite and Albenza Dolomite, Norian-Hettangian). The hydrocarbons were produced from Upper Anisian-Ladinian source rocks (Besano and Meride Limestone formations) characterized by thin interbedded black shales often having very high TOC, and type II kerogen. The original SPI, for the thicker successions, is calculated in about 4 t HC/m².

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.