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Coquinas are a sedimentary rock composed mainly of shells and their fragments with a carbonate or siliciclastic matrix. These rocks exhibit a high range of porosity and permeability values that are related to depositional and diagenetic processes. A series of coquina reservoirs were discovered in the Campos and Santos Basins in the eastern margin of Brazil and the understanding of the petrophysical properties of these rocks is essential for reservoir production. In this work, samples from the Morro do Chaves Formation (Sergipe–Alagoas Basin) and the Itapema Formation (Santos Basin) were selected to identify the porosity vs permeability relationship according to the biofabric characteristics and porosity control. The biofabric textures where classified into four groups according to taphonomic patterns and diagenetic modification: (i) well-sorted and preserved biofabric, (ii) well-sorted and altered biofabric, (iii) unsorted and preserved biofabric, and (iv) unsorted and altered biofabric. The porosity control varies in all groups from depositional to diagenetic with distinct pore types, pore sizes, and pore connectivity that are located in different global hydraulic elements. The analysis of the pore space was carried out through laboratorial measurements (gas injection) and by images from high-resolution computed tomography from plug samples, resulting in a pore network model where values of pore radius, pore throat radius, and coordination number were obtained. The best values of porosity and permeability are related to well-sorted altered biofabrics, whereas the lowest values, either with unsorted biofabrics, with low pore connectivity, or biofabrics that had the porosity reduced by compaction and cementation. [ABSTRACT FROM AUTHOR]

Three-dimensional distribution of karstic zones and estimated porosity were obtained in fractured carbonate units of the Salitre Formation (Brazil) from GPR data. Low-amplitude (shadow) GPR zones could be associated with mineral dissolution halos that developed around fractures and bedding planes. GPR volumes show a 3D mazelike geometry of the vertical karstified carbonate zones, similar to the set of passages controlled by ~ N–S and E–W oriented fracture systems, observed on the surface in unmanned aerial vehicle images. Measurements in plug samples indicate that the mineral dissolution caused bulk density and dielectric permittivity decline and incipient increase in porosity from 0.95 to 1.01%. These changes in the rock properties affect the EM impedance contrasts between carbonate bedding, causing loss of the reflected GPR signal. The correlation between dielectric permittivity and porosity could be analyzed using the complex refractive index model (CRIM), which is used to convert the relative dielectric permittivity into porosity. The CRIM porosities are somewhat lower than plug sample porosities due to different estimation methodologies and measurement scales. However, the fact that porosity is slightly higher in the dissolved carbonates was maintained. In addition, GPR amplitude and frequency instantaneous attributes were used to estimate spatial variability in porosity throughout the whole GPR data volume. The crossplot of these two GPR attributes show that altered zones generally cluster in narrow ranges of instantaneous GPR amplitude and in broad range of instantaneous GPR frequency. Assuming empirical correlation between physical properties and GPR attributes two predicted porosity volumes were established for the karstified carbonates in the studied area. [ABSTRACT FROM AUTHOR]

One of the most significant problems in the characterization and recovery of Aptian carbonate reservoirs, especially the Brazilian pre-salt, has been the lack of precise estimates of permeability. The heterogeneity of the permeability of the carbonate reservoirs occurs owed great different of the pore form occurred mainly by diagenetic processes. In this study, propose a joint and integrated methodology to estimate the permeability in the reservoir. To achieve this goal, porosity-permeability core data, image logs, applying the Rock Types concepts, analysis of the results of well log Nuclear Magnetic Resonance (NMR), modeling the well logs resistivity (laterolog and induction), as well as estimating the specific surface of the rock using images of section 2D from the pugs, to quantitatively estimate the permeability of the reservoir based on the petrophysical properties of the rocks. In the study it was possible to identify different pore systems distributed in eight Hydraulic Flow Units (HFU) determined from the pore groove radii. The Nuclear Magnetic Resonance (NMR) log it served to separate area of the spectrum corresponding to the small pores from the area corresponding to the large pores. The resistivity logs were analyzed with the purpose of estimating the direction (vertical, horizontal and dipping) and thickness of the fractures, which were also modeled with the purpose of identifying the invasion of the drilling fluid. The specific surface area was obtained by image processing algorithms. The results showed an acceptable precision of this methodology to estimate the permeability in carbonate reservoirs that have in their composition fragments of stromatolites and associated bioclastics, found partially or totally dolomitized. • The Hydraulic Flow Units with the highest velocity zones coincide with the fractured. • Dissolution improves reservoir quality in both spherulites and stromatolites. • The presence of fractures does not affect the quality of the reservoir studied. • The digital image analysis allows estimation of the petrophysical parameters. • The radial resistivity image identified the drilling mud in the fracture zones. [ABSTRACT FROM AUTHOR]