Diagenesis and Porosity Evolution of Submarine-Fan and Basin-Plain Sandstones, Marnoso-Arenacea Formation, Northern Apennines, Italy

ABSTRACT 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: 1) had the greatest quantity of clay matrix introduced either during slumping (chiefly proximal facies) or during other kinds of deposition (rare), 2) had the greatest quantity of ductile 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. LA DIAGENESE ET L'EVOLUTION DE LA POROSITE DANS LES GRES DE CONES DE DEJECTION SOUS-MARINS ET DE PLAINES DE BASSIN, FORMATION MARNOSO-ARENACEA, APENNINS SEPTENTRIONALS, ITALIE RESUME Les gres du Miocene de la formation Marnoso-arenacea, complexe de depots de pente, de cones de dejection sous-marins et de plaine de bassin, proviennent de maintes sources alpines et apennines, et affichent une diversite de composition et de texture remarquable. Ces gres produisent des hydrocarbures dans les Apennins septentrionals, le long de la bordure sud de la vallee du Po. La qualite du reservoir depend de la compaction resultant de la charge d'enfouissement et de la compression tectonique, ainsi que de la cimentation, principalement ferro-sparitique: l'incidence de porosite secondaire est negligeable. Les sables les plus tasses sont ceux qui: 1) detenaient une surabondance de matrice argileuse, introduite lors de glissements (principalement dans les facies proximaux) ou durant d'autres genres de sedimentation (rare), 2) detenaient une surabondance de grains plastiques (fragments d'argilite, micas, grains de phyllite et de schiste), et 3) ont ete profondement enfouis ou soumis a la compression tectonique. La calcite des couches hemipelagiques (jusqu'a 50 pour-cent au poids) et celle que l'on retrouve sous forme detritique par endroits, ont constitue de toute evidence une source intraformationelle de ciment. Ainsi, suite a la compaction, les fines intercalations de sable et d'argile avec d'epaisses calcarenites se sont pourvues d'un ciment ferro-sparitique. Au contraire, les epais chenaux sableux, pauvres en debris calcaires et en intercalations argileuses, demeurent non-cimentes et preservent une porosite primaire modeste. L'exploration de complexes de cones de dejection semblables en geometrie et composition a ceux de la Marnoso-arenacea visera donc les epais sables superposes des chenaux et des facies de lobes; les turbidites riches en debris calcaires auront vraisemblablement toute leur porosite obstruee par du ciment et ce, en depit de leur epaisseur (>3 metres). Traduction: Marc Charest et Jean Pelletier End_Page 313------------------------