Geochemistry and Habitat of the Oils in Italy (1)

Abstract

All varieties of liquid petroleum, ranging from condensates (>50 degrees API) to immature sulfur-rich heavy oils (as low as 5 degrees API), have been found in Italy. However, nonbiodegraded heavy oils account for about 70 percent of the total original oil in place. Geochemical analyses indicate that 11 oil groups are present in the Italian basins and two main types of source rocks have been identified: Triassic carbonates and Tertiary shales. About 95 percent of the oils were originated from Middle and Upper Triassic carbonates containing type II kerogen [about 1 percent 4total organic carbon (TOC) and 500 mg hydrocarbon/g TOC]. Only a relatively minor amount of oil was generated by Tertiary shales containing type III kerogen with TOC generally less than 1 percent. Timing of generation and migration and bulk properties of oils were controlled by geodynamic histories of the three main Italian geologic settings: (1) Apennine and Southern Alp thrust belts, (2) foredeep (depression bordering the thrust belts), and (3) foreland (nondeformed African continental margin). Within the Apennine thrust belts, deep burial during the Neogene resulted in the generation of substantially lighter oils, not only from deeply buried Triassic but sometimes also from Tertiary source rocks. In the late Neogene, foredeep depocenters located in the central Adriatic and southern Sicily, high subsidence (up to 1000 m/m.y.), a low geothermal gradient (22 degrees C/km), and compressional tectonics caused the generation of immature heavy oils generally at depths below 5000 m nd temperatures greater than 100 degrees C. Rapid burial and higher geothermal gradients (32 degrees C/km), which occurred since the Jurassic, resulted in the generation of light oils from the Late Cretaceous to the Oligocene in the southern sector of Adriatic foreland.