EXPLORATION Italy to open Po Valley to competitive exploration

Marco Pieri Giovanni Flores
Petroconsultants SA
Geneva

• Po Plain with main surrounding features [40684 bytes]
  
• Cross-section [41195 bytes]
  
• Generalized Geology [92713 bytes]

The broad Po-Veneto plain and the Northern Adriatic include Italy's most important gas province and the country's largest oil field discovered so far, Villa Fortuna-Trecate (1984).

This area covers approximately 72,500 sq km, the size of Virginia or Kentucky. No less than 55,000 sq km of that since 1953 has been under exclusive concession to ENI, the Italian state petroleum authority. It was therefore explored and exploited solely by AGIP, the ENI Group operating company. This virtual monopoly is now in the process of being abolished, possibly by yearend 1996, opening the area to free enterprise and competitors.

Italian production in 1994 totaled 35 million bbl of oil and 700 bcf of gas and represented about 14% of the nation's energy demand. Of this, three quarters of the gas and two thirds of the oil derive from the Po plain and northern Adriatic.

There a large number of plays exist, with Triassic to Pleistocene reservoirs, with traps of structural, stratigraphic, and combination types. These plays are at different stages of maturity. The first commercial success occurred in 1944 after the introduction of reflection seismic, which resulted in the discovery of Caviaga gas field in the western part of the Po plain.

In the 1950s and 1960s exploration was aimed mostly at the Late Tertiary clastic sequence. In the Northern Adriatic-that is, the eastern extension of the Po Pliocene-Quaternary basin-Ravenna Mare field was the first gas offshore discovery in Europe (1960). Several others followed.

In the next decade, with the advent of digital seismic surveying, exploration was pushed to deep Mesozoic carbonate plays. The rewards of this effort were the 1973 discoveries of Malossa condensate and Cavone oil fields and the 1982 discovery of Gaggiano and 1984 discovery of Villa Fortuna-Trecate oil fields.

Some 180 discoveries of various sizes were made since 1944 in the Po plain and northern Adriatic. About 25 are oil or condensate fields, the others gas fields.

According to the Italian Petroleum Law, the Exploration Permit allowing prospecting and drilling activity may be granted on a maximum 1,000 sq km, for 6 years, with two 3 year renewals. Exploitation concessions allowing production activities are limited to the size of the discovery. Royalties amount to 9% for both oil and gas onshore and 8% for oil and 5% for gas offshore.

Regional framework

The geological evolution of northern Italy coincides with that of the northern part of the Adriatic plate. This microplate originated as a "spur" of the African plate, when Africa began to separate from Europe and the gap between was gradually being occupied by the short lived Mesogean ocean.

Above a folded and metamorphosed Hercynian basement of no petroliferous interest, continental deposits (Permian to Late Triassic) were laid down, followed by paralic and lagoonal sediments and thick shallow water platform carbonates from Late Triassic to Early Jurassic.

The carbonate platform persisted in some areas throughout the Mesozoic and beyond, but in the majority of the area was replaced in the Early Jurassic by deeper sea sediments deposited in collapsed sectors of the Adria continental margin.

The subduction and closure of the Mesogean ocean (Early Cretaceous to Late Oligocene) caused the collision of the Europe and Adria continental margins, building an Europe-verging northern Alpine belt that soon emerged and was subjected to erosion.

From Late Oligocene to present the Adria plate northern and western continental margins were subducted underneath the European and Europe derived elements. This brought about the building up of the south Alpine and Apenninic fold-and-thrust belts and the accompanying foredeeps that were rapidly filled with flysch sediments. The sedimentary successions deposited in the Mesogean ocean, detached from their substratum, were in part thrust on the Apennine belt and presently form the allochthonous Liguride nappe that covers a large part of the northern Apennine.

Stratigraphy

The predominantly carbonate deposition in the widespread shallow water platform from Middle Triassic to Early Lias was controlled by the extensional tectonics of the early rifting phases. The overall thickness greatly varies, from a few hundred meters in the more stable blocks to a few thousand meters in the more subsiding areas.

Strongly subsiding minor sub-basins with restricted circulation and anoxic environments occur, wherein the most important source rocks of the Italian Mesozoic were deposited, such as the Meride limestone (Ladinian-Carnian, western-southern Alps, western Po plain), the Riva di Solto shale (Rhaetian, western-southern Alps), and the Emma limestone (Late Triassic-Early Lias, northern Adriatic).

According to their different burial history, oil from Triassic sources could be generated during the Mesozoic or later as a consequence of Neogene strong subsidence and migrated into Triassic, Early Lias, and even Late Mesozoic-Paleogene calcareous and dolomitic reservoirs.

Where the Early Mesozoic carbonate platform collapsed and deep sea pelagic basin took place, carbonate-cherty formations alternating with marly and shaly intervals were deposited. In this deepwater environment, relatively stable "pelagic carbonate platform" are characterized by stratigraphic gaps, condensed sequences. hardgrounds, and submarine erosion, while in more subsiding sub-basin the sedimentary sequence is more complete and thicker. In the transition zones between shallow water or pelagic platforms and deeper basins, turbiditic calcarenites and breccias with fair reservoir characteristics were deposited.

Tertiary sedimentation is prevailingly clastic; flysch and shales filled the foredeeps in front of the fold-and-thrust belts. The foredeep floor morphology, the sedimentary thickness, and the porosity distribution are controlled by the synsedimentary tectonics. The turbidity currents flowed with their load of sands in the synclines, whereas in the re1atively high incipient anticlines turbiditic sedimentation is scarce and shales prevail.

According to geochemical studies, the organic matter of the shales intercalated in flysch deposits generated the light oils and gases reservoired in the Miocene sands of the western Po plain (Cortemaggiore field).

Sedimentation rates increased during Pliocene-Quaternary times, when more than 190,000 cu km of sediments were deposited in the northern Apennine foredeep. The shales intercalated within the turbiditic units of this time interval contain the organic matter that generated the biogenic gas of the major Po plain and northern Adriatic fields.

Structure and traps

Three structural units of the main order are present in the southern Alps, Po-Veneto plain, and northern Adriatic: the southern Alps and Dinarides fold-and-thrust belts, the external northern Apennines, and the undeformed foreland.

The age of deformation is Cretaceous to Mid-Miocene in the western southern Alps and Late Miocene to Quaternary in the eastern southern Alps and the northern Apennines. Plastic, incompetent formations such as the Late Triassic evaporites in the northern Apennines acted as main detachment levels and caused the structuring of the overlying sedimentary se- quence in a series of thrust anticlines.

The western southern Alps extend in the subsurface of the northwestern Po plain, whereas the front of the eastern southern Alps and of the Dinarides corresponds to the limit of the Veneto plain. The external northern Apennines are almost totally buried underneath the southern Po plain and Adriatic mildly deformed Quaternary sediments.

The external structural elements of the north Apennine belt are arc-shaped. The Monferrato arc, to the west, has a Liguride core; the Emilia arc, in the center, consists of the Tertiary sequence, detached from its Mesozoic substratum; to the east, in the Eerrara-Romagna arc and in the Adriatic folds, a Mesozoic carbonate core is present.

In general, the traps in the fold and thrust belts are of the structural anticline type; however, where the porosity of the reservoir is controlled by stratigraphic factors, combination traps may be present.

In some areas, underneath the overthrust external edge of the belts, "subthrust," still untested plays may be present, either in the lowermost structural units of the belts or in the underlying foreland. This type of play has been successfully tested in the southern Apennines.

In the undeformed foreland, structural traps in the Mesozoic succession are related to the extensional synsedimentary tectonics, whereas in the Pliocene succession stratigraphic type traps prevail. In the northern Adriatic, where the Pliocene-Quaternary sediments unconformably overlie a Mesozoic carbonate eroded surface, important gas fields were found in gentle drape anticlines.

Petroleum systems

The petroleum systems in the southern Alps, Po-Veneto plain, and northern Adriatic area may be classified in three main types:

• Triassic source rocks where oil (light to heavy), condensates, and thermogenic gas were generated during Mesozoic and Neogene subsidence and migrated into Mesozoic carbonate reservoirs, in structural traps (Riva di Solto, Meride, Emma P.S.);

• Miocene source rocks (flysch), where light oil, condensates, and thermogenic gas were generated during Pliocene, probably from deep subthrust successions, and migrated into sandy Neogene reservoirs, in structural and combination traps (Cortemaggiore field); and

• Pliocene, Quaternary (and subordinately Late Miocene) source rocks, where biogenic dry gas was generated during the diagenetic stage and contemporaneously migrated into sandy Pliocene-Quaternary reservoirs, in structural, combination, and stratigraphic traps. This petroleum system includes most of the Po plain and northern Adriatic gas fields.

The oils of some fields in the eastern Po plain (Cavone and Bagnolo fields) are not correlatable to a definite source, although a Mesozoic origin is probable.

Search status, future

The large number of plays existing in the area have been defined and explored at different times, hence their exploration maturity stages also differ.

Pliocene-Quaternary plays have been explored since the 1950s; the more evident structural features were drilled first, then the stratigraphic plays. The present exploration, aimed at subtle traps and requiring sophisticated seismic acquisition and interpretation, is still significantly successful.

The poor quality of the seismic response into the Mesozoic carbonate section delayed the definition of the Mesozoic plays to the beginning of the 1970s, when the first commercial results were obtained. On a total of about 1,300 exploration wells drilled in the area, fewer than 150 penetrated Mesozoic and only about 40 of those reached Triassic. Information is therefore relatively scant if compared with the data concerning the Tertiary clastic succession. In a large part of the area deep wells, in the order of 5,000 m or more, will be necessary in order to reach the early Mesozoic section.

Analysis of the exploration history of the area leads one to conclude that, in general, future discoveries will depend on the use of up-to-date prospecting techniques and on their progress. Skilled interpretation, based on up-to-date geological models, will also be indispensible.

Bibliography

Errico, G., Groppi, G., Savelli, S., and Vaghi, G.C., Malossa field: A deep discovery in the Po Valley, Italy, AAPG Memoir 30, 1980.

Rigo, Fabrizio, Italy to open "exclusive" Po basin area in 1992, OGJ, May 27, 1991, p. 102.

The Authors