Diagenesis of Aptian sandstones and conglomerates of the Campos Basin

• Sandstones and conglomerates rich in volcanic, feldspars and heavy mineral grains.
• Diagenesis by interaction of immature detrital composition with alkaline fluids.
• Grains dissolution, and precipitation of smectite, dolomite, calcite and zeolites.
• Mechanical compaction more effective than cementation in porosity reduction.
• Sandstones with smectitic rims show the best potential for porosity preservation.

A systematic study of 71 samples of sandstones and conglomerates of the Itabapoana Formation (Aptian), selected from the cores of 13 wells in the offshore portion of Campos Basin, eastern Brazil, allowed the recognition of the main primary and diagenetic constituents and pore types, and their space and time relations. These rocks are contemporaneous to the extensive, lacustrine “Pre-Salt” carbonate reservoirs under exploration. The rocks range from very fine-grained sandstones to conglomerates, with predominance of very coarse-grained sandstones, and their original detrital composition corresponds to lithic arkoses and feldspathic litharenites, rich in volcanic rock fragments (VFRs). Smectitic clay minerals are the most abundant diagenetic constituents, related mostly to the alteration of VRF, heavy minerals and biotite. Burial and thermal history and fluid flow also played a fundamental role on the diagenetic processes and reservoir quality. The main diagenetic processes were the dissolution of volcanic fragments, feldspars and heavy minerals, the authigenesis of smectites, hematite, dolomite, calcite, zeolites and titanium oxides in intergranular pores and replacing VRFs, feldspars, biotite and heavy minerals, mechanical and chemical compaction, and the albitization of feldspars, mostly of plagioclase. Primary intergranular porosity was strongly reduced by compaction, and by smectite, calcite, dolomite and zeolite cementation. On the other hand, secondary porosity was generated by the dissolution of primary and diagenetic constituents, as well as by fracturing. The understanding of the space and time distribution of diagenetic processes in these rocks will allow the development of predictive models of their quality, which may contribute to the reduction of risks involved in their exploration.