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

No abstract

This work aims to present a review of dielectric logging, including physical principles, petrophysical evaluation, and applications. In addition, we will present a history of its use in Petrobras oilfields. The dielectric properties are generated by the charge alignment created by an excitation provided by an electric field. In this sense, polar characteristics of water molecules allow quantification of volume of water present in the reservoir, independently of salinity. Owing to the shallow depths of investigation, dielectric tools are useful in evaluating residual oil in high uncertainty salinity conditions, especially in mature fields, where uncertainties occur due to injection of water and/or steam. Although this logging tool has a long history of use in the petroleum industry, it was rarely used in Petrobras oilfields due to the high specificity, limitations of the technique, and/or high cost. Considering high frequencies used and proximity between electrodes, the tool provides very shallow measurements, which makes it impossible to evaluate the virgin zone in most reservoirs. Due to these characteristics, the tool was widely employed in low mobility hydrocarbon formations, where mud filtrate invasion tends to be smaller. The dielectric tool was used in Petrobras to evaluate reservoirs with very high viscosity and fresh water, in which it showed good results in the quantification of water saturation and hydrocarbon mobility. Additionally, the tool was used in Pre-salt in order to evaluate residual oil saturation to diminish the uncertain of microresistivity logs. Besides evaluating water saturation, many works have shown other applications for dielectric logging, such as to determine conductivity, salinity, wettability, Archie's "m" and "n" electric parameters, CEC, and evaluation of laminated reservoirs.

No abstract

No abstract

No abstract

The hydrogeochemical characterization of Coxilha das Lombas Aquifer, Brazil, was studied. Chemical characteristics from 23 groundwater samples and meteoric waters data from public registers were used. The ionic speciation and mineral dissolution/precipitation was calculated by EQ3/6 package software. The results showed low total dissolved solids (TDS) values and slightly acidic pH. The concentration of most abundant ions usually followed this trend Cl − >HCO 3 + >SO 4 2− >Na + . The characteristics of groundwaters and the chemistry similarity with the meteoric waters reflect their short residence time, due to high hydraulic conductivity, and low degree of water/rock interactions, due to sands composed mainly of quartz.