Middle Triassic Source Rocks in North Lombardy

M. Gnaccolini, M. Gaetani, L. Mattavelli, C. Leoni, G. Poliani, A. Riva

Using molecular geochemistry techniques, we established that the Perledo-Varenna and Meride Formations (Middle Triassic, southern Alps) represent the source rocks of the Gaggiano and Villafortuna deep oil fields discovered 40 km northwest of Milan. To find the geological factors which control the areal extent thickness and organic matter distribution relative to these sequences, a sedimentological and geochemical study was undertaken.

The units are made up of well-bedded limestones and dolomites of disaerobic to anaerobic environment, deposited in troughs between the Esino or M.S. Salvatore carbonate platforms. The 300-400 m-thick intraplatform sequences consist mostly of massive (> 50%) or gently laminated (15%) mudstones and wackestones, with a consistent supply of turbiditic packstones and coarse-grained debris flow deposits (6%). The dolomitized facies have the same facies proportion of the limestones. The organic matter is mostly of terrestrial origin, with organic carbon content reaching a maximum of 2%. Thin marly intercalations within the mudstone/wackestone sequences represent the richer organic facies (mean value 0.71%), and all the other facies have a mean value below 0.25%. Deep burial and tectonics caused a strong thermal alteration of the organic matter as indicated by the R_o values, ranging approximately from 1.5% to 2.5%, and by Rock-Eval pyrolysis. Moreover, vitrinite data and Conodonts Alteration Index suggest the upper thrust sheets were affected b a stronger thermal degradation.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.