Analysis of a Multistage Dolomitization in the Early Jurassic Platform of Lombardy Southern Alps as an Analogue of Po Valley Reservoirs, Northern Italy

Ronchi, Paola¹, Andrea Ortenzi¹, Dario Sartorio¹, Paolo Scotti¹, Elisabetta Previde Massara², Chiara Maragliulo³, Flavio Jadoul⁴, simonetta Cirilli⁵ (1) ENI E&P, San Donato Milanese, Italy (2) ENI Tecnologie, San Donato Milanese, Italy (3) Pavia University, Italy (4) Milan University, Milan, Italy (5) Perugia University, Italy

In Po Valley oil reservoirs produce mainly from early Jurassic porous and fractured Conchodon Dolomite platform. The diagenetic history of the Conchodon Dolomite, the basin and the tectonic evolution have been studied both in outcrops (Iseo Lake) and subsurface (Malossa). The areas belong to Triassic-Jurassic palaeohighs that were continuously buried till Tertiary, when the Alpine onset produced thrusts and uplift with different timing, intensi­ty and effects in the two areas. The petrographic analysis evidenced a first massive dolomite replacement on limestone that increased the intercrystalline porosity and a few phases of dolomite cements precipitation which, after a corrosion phase, filled the intercrystalline pores. The geochemical data, including stable isotopes, trace elements and fluid inclusion analyses, pointed out an increase of crystallization temperature (Th from 65 up to 100°C), a depletion in oxygen isotopes (from about -4 down to -8‰) and a decrease in diagenetic fluid salinity (from seawater to brackish water) from the replacement dolomite to the subse­quent dolomite precipitation phases. These analyses lead to interpret the massive dolomiti­zation as occurred in burial environment in presence of seawater derived fluids; later dolomite cements precipitation took place from these seawater derived fluids, partially dilut­ed by the influx of fresh water. The comparison of the sedimentologic, burial and structural histories of the studied areas, and the distribution of the dolomitization in the Lombardy Basin, suggest that the areas involved in Cretaceous Alpine foredeep may have been dolomi­tized earlier that the more external ones, involved in the Neogene foredeep.