Pressure Regimes and Pressure Systems in the Adriatic Foredeep (Italy)
Sandro Carlin, Jacopo Dainelli, 1998. "Pressure Regimes and Pressure Systems in the Adriatic Foredeep (Italy)", Abnormal Pressures in Hydrocarbon Environments, B.E. Law, G.F. Ulmishek, V.I. Slavin
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The exploration for oil and gas in the Adriatic Basin of Italy has resulted in a large quantity of pressure data. Abnormally high pressures in the area are mainly caused by compaction disequilibrium resulting from the high sedimentation rate of the Pliocene to Quaternary strata. The comparison of pressure profiles in the northern and central Adriatic basins has shown the presence of five pressure regions. Three regions are present in the post-Messinian siliciclastic succession that infills the Adriatic foredeep, and two pressure regions have been identified in the Cretaceous to Miocene carbonates of the Apulian continental margin. The boundaries of these regions are coincident with the main structural features of the Apenninic belt, indicating that the major thrusts act as pressure barriers. In the post-Messinian strata, the innermost region (with respect to Apenninic vergence) includes the inner buried thrusts in front of the Apenninic chain. This region is characterized by moderate to low overpressures, hydrostatic gradients, and good lateral hydraulic continuity. In the second region, in proximity of the outermost thrusts, overpressures are high and compartmentalization is pronounced. In the third pressure region, the undeformed foredeep of the Apennines, lateral hydraulic continuity prevails and high overpressures are present in the two principal Pliocene depocenters (the Romagna foredeep and the Pescara Basin). The fourth region, in the Cretaceous to Miocene carbonates, includes strata involved in Apeninnic thrusting; it is characterized by moderate overpressures. The fifth pressure region, in the foreland of the Apulian margin, has normal pressure conditions. Gas pools in the lower Pliocene interbedded sandstone-shale sequence in the first pressure region are mostly found below regional mudrock seals; whereas in the second and third pressure regions the overpressured shale beds in the lower Pliocene provide excellent seals for the interbedded gas-bearing sandstone reservoirs.
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Abnormal Pressures in Hydrocarbon Environments
Abnormal pressures, pressures above or below hydrostatic pressures, occur on all continents in a wide range of geological conditions. According to a survey of published literature on abnormal pressures, compaction disequilibrium and hydrocarbon generation are the two most commonly cited causes of abnormally high pressure in petroleum provinces. In young (Tertiary) deltaic sequences, compaction disequilibrium is the dominant cause of abnormal pressure. In older (pre-Tertiary) lithified rocks, hydrocarbon generation, aquathermal expansion, and tectonics are most often cited as the causes of abnormal pressure.
The association of abnormal pressures with hydrocarbon accumulations is statistically significant. Within abnormally pressured reservoirs, empirical evidence indicates that the bulk of economically recoverable oil and gas occurs in reservoirs with pressure gradients less than 0.75 psi/ft (17.4 kPa/m) and there is very little production potential from reservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Abnormally pressured rocks are also commonly associated with unconventional gas accumulations where the pressuring phase is gas of either a thermal or microbial origin. In underpressured, thermally mature rocks, the affected reservoirs have most often experienced a significant cooling history and probably evolved from an originally overpressured system.