The Lagoa Feia Formation, buried in excess of 3,000 m, is the exploration frontier of the prolific Campos basin. It contains the source beds of

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all the basin's oil in addition to having its own potential carbonate reservoirs. The faulted margins of the basin fed a system of alluvial fans, sand flats, and mud flats. Alternating dry and rainy periods regulated the size and nature of contemporaneous basinal alkaline lakes. Dry periods corresponded to contracted playa lakes with ostracod carbonates and euxinic shales; rainy periods corresponded to expanded pluvial lakes with pelecypod banks. Subaqueous intrusions of basaltic magma generated hyaloclastites with kerolitic ooids and hyalotuffs.

Petrographic analysis reveals 5 diagenetic stages: (1) syndepositional alteration of lithoclasts to trioctahedral smectites; (2) early dolomitization, early silicification, and cementation by bladed-rim calcite and zeolites; (3) freshwater-vadose dissolution of bioclasts and lithoclasts, freshwater-phreatic sparite cementation, and neomorphism; (4) mixed saline-freshwater silicification; and (5) burial with compaction, late dolomitization, and partial conversion of smectites to illite.

Pelecypod limestones with primary interparticle, secondary intraparticle, moldic, and moldic-enlarged porosities are the potential reservoirs. Ideal conditions for porosity generation and preservation were subaerial exposure followed by rapid lake expansion and burial.

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