The genesis and evolution of lacustrine pre-salt carbonate reservoirs, which contain giant hydrocarbon accumulations along the South Atlantic margins, has attracted major research interest. The huge extension and volume, and unusual textural and compositional features, are key elements for understanding the tectonic, structural, stratigraphic, and sedimentological generation and early evolution of the region, as well as potentially of other lacustrine carbonate systems. A systematic petrographic and petrophysical study has been performed on the Lower Cretaceous lacustrine carbonate reservoirs from northern Campos Basin, Offshore Brazil, in order to unravel the main controls on the origin and evolution of pore systems. The main lithologic types recognized in the rift section are bivalve-gastropod grainstones and rudstones, arenites constituted by ooids of syngenetic magnesian clay minerals (mostly stevensite), and dolostones, while stevensitic claystones with calcite spherulites, fascicular calcite crusts, intraclastic rudstones and grainstones, and dolostones are the main lithologic types in the sag section. The eogenetic evolution of bioclastic reservoirs was controlled by the balance between dissolution and neomorphism of the aragonitic bivalve and gastropod bioclasts, favoring either the generation of poorly connected moldic porosity or the preservation of well-connected interparticle porosity. The stevensitic arenites were strongly affected by meteoric dissolution and replacement by dolomite and silica, related to regional uplift and erosion after the rift phase, which generated highly heterogeneous pore systems with moldic, intercrystalline, vugular, and microcrystalline pores. Stevensitic claystones that are replaced by calcite spherulites and dolomite normally show low porosity, but locally constitute reservoirs, where secondary porosity was generated by stevensite dissolution. The precipitation of crystal shrubs of fascicular-optic calcite in coalescent crusts generated growth-framework primary porosity, which was reduced mostly by dolomite cementation, or enlarged by dissolution, enhancing their permeability. Non-coalescent calcite crusts contain abundant syngenetic magnesian clay minerals. Their porosity is related to dissolution of these clays, which generated poor permeability. Intraclastic grainstones and rudstones are compacted and cemented, or rich in clay matrix (''hybrid packstones''). Where they display preserved interparticle primary porosity or matrix dissolution, they may have good porosity and permeability. The heterogeneous dolomitization of both the rift and the sag deposits either destroyed their primary or early diagenetic porosity, or generated high porosity and permeability values in the dolostones. Relationships between replacement and compaction indicate that most of the diagenetic processes occurred during eodiagenesis, controlled mostly by the instability of the aragonite in the bioclastic reservoirs and of the stevensite in the claystones, ooidal arenites, and fascicular calcite c...

No abstract

Fluvial and estuarine sandstones of the Aptian Mucuri Member of the Mariricu Formation are the main reservoir rocks in the onshore portion of the Espírito Santo Basin, Brazilian eastern margin. These sandstones are contemporaneous with the major lacustrine pre-salt reservoir rocks which are currently the target of exploration offshore. The sandstones are texturally and compositionally immature, rich in feldspars, biotite, garnets and other heavy minerals, and are interpreted to have been derived from the erosion of igneous and medium-high grade metamorphic-plutonic terrains under dry climatic conditions. Early diagenetic processes have significantly impacted the reservoir qualities of the sandstones. Detrital feldspars were dissolved and replaced by kaolinite. Biotite was replaced and expanded by pyrite, calcite, kaolinite and/or smectite. The growth of authigenic rims and coatings of smectitic clays was the most important diagenetic process in most of the sandstones, except for those cemented by abundant poikilotopic calcite. Coarse-crystalline pyrite replaced primary constituents, particularly biotite and mudstone intraclasts, and all other diagenetic constituents. Mechanical compaction, although relatively limited, was the main process of porosity reduction, except in sandstones with abundant calcite cement. Primary intergranular porosity was commonly strongly reduced by calcite or smectite cementation and by compaction, but intragranular and mouldic secondary pores were generated by the dissolution of detrital feldspars and heavy minerals. Characterizing the diagenetic patterns of the Mucuri sandstones is important for increasing the recovery from producing oilfields and also for the development of analogous reservoir rocks elsewhere.

2 rigoti@; 3 claudia lima@; 4 gabriella.fontaneta@; 5 neilma.lima@; 6 eveline.zambonato@ 7 jubahniuk@ 8 saalima@; 9 milenefigueiredo@; 10 cpirescpf@; 11 rosimenezes@ 11 rgontijo@