Oil distribution in outcropping carbonate-ramp reservoirs (Maiella Mountain, Central Italy): Three-dimensional models constrained by dense historical well data and laboratory measurements

Abstract

Heavy oil and bitumens have been exploited in Italy during the past, in particular over the Maiella Mountain’s northwest flank (Central Italy), where relatively undeformed, hydrocarbon-bearing carbonate-ramp reservoirs of the Bolognano Formation crop out. These reservoirs represent the exhumed analog of a wider petroleum system that has been investigated also in the subsurface by exploration activities, both onshore and offshore (Central Adriatic). Here we present, for the first time, a historical data set composed of 180 shallow wells drilled over the area in 1942 by a company named Azienda Lavorazione Bitumi Asfalti; we digitally reconstructed this data set, integrating it with field observations, laboratory measurements, and thin-section analysis. Using Petrel (mark of Schlumberger) three-dimensional software, we modeled the reservoir and the hydrocarbon distribution, also calculating volumes of hydrocarbons in place. In our work, we demonstrate the presence of these quite homogeneous, porous carbonate reservoirs over the whole region, and we identify internal reservoir geometries characterized by clinoforms, likely resulting from a large-scale depositional cross-bedding and progradation. We also infer a different function of faults in the hydrocarbon accumulations: major faults were able to create independent compartments during migration, whereas minor, more recent elements did not control hydrocarbon accumulation, likely postdating a main lower-Pliocene migration phase. We believe that the observations derived from our work can represent a good analog for carbonate-ramp reservoirs elsewhere, may help in identifying appropriate modeling solutions, and can be used as calibration for future renewed exploration efforts in the region, both onshore and offshore.