B. J. Katz*, E. I. Dittmar** and G. E. Ehret**
*Texaco EPTD, 3901 Briarpark, Houston, TX 77041, USA.
**Texaco Exploration Department, 4800 Fournace, Bellaire TX 77401, USA.
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 regions oils.
There is a sharp contrast in the stable isotope composition of the organic matter contained in the different source intervals. The Triassic and Toarcian organic matter is isotopically light (d 13Cbitumen <-26), while Cretaceous and Oligo-Miocene OM is isotopically heavy (d 13Cbitumen heavier than-26).
Although the oils share characteristics associated with carbonate source rock systems, two distinct populations exist. These are an isotopically light population (d 13Coil lighter than -26, typically lighter than -27.5), which includes the oils at Rospo Mare, Malossa and the degraded oil at Maiella; and an isotopically heavier (d 13Coil heavier than -26) population, including the oils at Monte Alpi, Bagnolo and at the Tramutola seep. The stable carbon isotopic compositions provide the strongest evidence for stratigraphic affinity. Biomarker data permits further refinement of these two "mega-families".