A geochemical review of carbonate source rocks in Italy

Field sampling of potential source rock units in Italy, and detailed geoche mical characterization of ltalian petroleum (surface seeps and reservoired oils), have provided insights into the origins of the resource base. Organi c-rich, oil-prone intervals have been identified in Middle to Upper Triassi c, lower Toarcian, Lower Cretaceous, Upper Cretaceous and Oligo-Miocene str ata. The hydrocarbon generation potential of these intervals can be signifi cant. For example, the weighted mean generation potential of the Upper Tria ssic in southern Italy is similar to 47 mg HC/g rock, with the net source r ock representing about 20% of the exposed stratigraphic sequence. Comparabl e richnesses exist in the other intervals noted above, although the Toarcia n and Late Cretaceous intervals, including those associated with the "Bonar elli Event" (a brief period straddling the Cenomanian-Turonian which is tho ught to represent an oceanic anoxic event) appear to be much more volumetri cally restricted. There is sedimentological evidence that these organic-rich intervals were d eposited 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 volumetr ic importance of intraplatformal source rocks, and may explain some of the minor geochemical differences observed among the region's oils. There is a sharp contrast in the stable isotope composition of the organic matter contained in the different source intervals. The Triassic and Toarci an organic matter is isotopically light (delta C-13(bitumen) lighter than - 26 parts per thousand), while Cretaceous and Oligo-Miocene OM is isotopical ly heavy (delta C-13(bitumen) heavier than -26 parts per thousand). Although the oils share characteristics associated with carbonate source ro ck systems, two distinct populations exist. These ave an isotopically light population (delta C-13(oil) lighter than -26 parts per thousand, typically lighter than -275 parts per thousand), which includes the oils at Rospo Ma re, Malossa and the degraded oil at Maiella; and an isotopically heavier (d elta C-13(oil) heavier than -26 parts per thousand) population, including t he oils at Monte Alpi, Bagnolo and at the Tramutola seep. The stable carbon isotopic compositions provide the strongest evidence for stratigraphic aff inity. Biomarker data permits further refinement of these two "mega-familie s".