DIAGENESIS AND POROSITY EVOLUTION OF SUBMARINE-FAN ANDBASIN-PLAIN SANDSTONES. MARNOSO-ARENACEA FORMATION.NORTHERN APENNINES. ITALY

Sandstones of the Miocene Marnoso-arenacea Formation, a complex of slope, submarine-fan, and basin-plain deposits, were derived from several different Alpine and Apennine sources and have a remarkably diverse framework composition. Sandstones produce hydocarbons in the northern Apennines along the southern margin of the Po River valley. Reservoir quality is a function of both compaction, from burial load and tectonic compression, and cementation chiefly by ferroan calcite; secondary porosity is trivial in occurrence. The most severely compacted sands are those that: l) had the greatest quantity of clay matrix introduced either during slumping (chiefly proximal facies) or during other kinds ofdeposition (rare), 2) had the greatest quantity ofductile grains (claystone clasts,micas, some phyllite and schist grains), and 3) were deeply buried or compressed tectonically.Calcite in hemipelagic beds (up to 50%by weight) and locally as detrital grains provided a ready source of intraformationally derived cement. As a consequence, thin sands interbedded with mudstone and thick calcarenite beds, after compaction, became tightly cemented by ferroan calcite. Conversely, thick, stacked, channel-fill sands with minor carbonate detritus and few interbedded mudstones, however,avoided cementation and preserved modest primary porosity.Exploration in submarine-fan complexes similar in geometry and composition to the Marnoso-arenacea should be in the thick, stacked sandstones of channel-fill and lobe facies; turbidites rich in carbonate detritus, even if thick (> 3 m), are likely to have had all porosity occluded by cement.