A GEOCHEMICAL REVIEW OF CARBONATE SOURCE ROCKS IN ITALY

Field sampling of potential source rock units in Italy, and detailed geochemical characterization of Italian petroleum (surface seeps and reservoired oils), have provided insights into the origins of the resource base. Organic-rich, oil-prone intervals have been identified in Middle to Upper Triassic, lower Toarcian, Lower Cretaceous, Upper Cretaceous and Oligo-Miocene strata. The hydrocarbon generation potential of these intervals can be significant. For example, the weighted mean generation potential of the Upper Triassic in southern Italy is ~47 mg HC/g rock, with the net source rock representing about 20% of the exposed stratigraphic sequence. Comparable richnesses exist in the other intervals noted above, although the Toarcian and Late Cretaceous intervals, including those associated with the "Bonarelli Event" (a brief period straddling the Cenomanian-Turonian which is thought to represent an oceanic anoxic event) appear to be much more volumetrically restricted. There is sedimentological evidence that these organic-rich intervals were deposited in both shallow-water intraplatformal settings, and in deep-water interplatformal basinal settings. The stacking of sequences as a result of evolving platform bathymetry and changes in sea level enhanced the volumetric importance of intraplatformal source rocks, and may explain some of the minor geochemical differences observed among the region’s oils.