Caracterização petrofísica multiescalar de tufas carbonáticas da formação Jandaíra, bacia Potiguar.

Abstract

The Jandaíra Formation consists of carbonate rocks of Cretaceous age that occur in much of the Potiguar Basin. Among the rocks present in this formation are the carbonaceous tufas resulting from several diagenetic processes, characteristic of the studied area. In this work the petrophysical, compositional and petrographic properties of two samples of carbonate tufa are analyzed, in scales varying from a few tens of centimeters up to a few micrometers. The tufas studied were collected in two outcrops of the municipality of Felipe Guerra, in Rio Grande do Norte. The carbonates, and in particular the carbonate tufas, are very heterogeneous rocks whose measured properties depend on the scale of investigation. This research aims to quantify the dependence of these properties on the scale of investigation through several characterization methods to indicate if it is appropriate to extrapolate a given property on a scale to another scale of interest. For this, the properties were investigated in a tufa cube with 30 cm of edge, originating from the outcrop Cachoeira Roncador (CR). One of the samples was suitable in cube form and soon after divided in divided into 8 cubic blocks with 15 cm of edge. From each block a cylindrical plug was extracted, for conventional petrophysical analysis and X-ray tomography, a thinning blade, and a portion for XRD analysis. The other sample originated from the outcrop next to the Nossa Senhora do Pérpetuo Socorro Church (TFG) was analyzed by X-ray microtomography and by conventional petrophysical tests. The petrophysical properties were investigated through conventional laboratory tests and tomography and microtomography images. These analyzes allowed the complete characterization of the matrix and porous structure of the sample, revealing the porosity and the permeability present. Characteristic processes were observed in the formation of tufas such as polysynthetic twinning and characteristic structures such as stilllites, geo - tetals and ankeritization processes. The crossing of the information generated by the analysis of the thin films and XRD was of fundamental importance for the correct determination of the mineral composition. The analysis of the porosity of the samples, by the various methods and scales, showed a great variation in the measured values, confirming the great heterogeneity presented by the carbonate tufa. The porosity estimated by thin slice analysis and conventional petrophysical analysis (gas pycnometry) presented values in the same order of magnitude, while the porosity estimated by the tomography in the blocks and in the plugs presented values in a smaller order of magnitude. The porosity estimated by the microtomography was up to an order of magnitude higher than the values measured conventionally. This indicates that thisproperty in carbonates is very dependent on the scale of observation. In tomography images only large vugular pores are recognized, which results in an underestimation of porosity by loss of information on microporosity. On the other hand, microporosity is captured by microtomography, but, due to the small sample size, large volute pores are not captured. The observed differences between the various scales and methods further suggest that the pores are essentially poorly connected. The permeability measured by all methods indicates very low values, which confirms the suspicion of low pore connectivity already indicated by the porosity values measured at the various scales.