Petrophysics and petrography of Aptian tight carbonate reservoir, Araripe Basin, NE Brazil

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1 Petrophysics and petrography of Aptian tight carbonate reservoir, Araripe Basin, NE Brazil Tiago Miranda (UFPE),) Antonio Barbosa (UFPE), Igor Gomes (UFPE), Agnelo Soares (UFCG), Rafael Santos (UFPE), Gabriel Matos (Petrobras), Elizabeth McKinnon (JSG- UTexas), Virgínio Neumann (UFPE), Randall Marrett (JSG-UTexas) Summary In recent decades, the exploitation of hydrocarbon reserves in carbonate reservoirs has motivated increasing numbers of investigations into their petrophysical characteristics due to their importance in fluid flow predictions. For tight reservoirs that are mainly composed of carbonate and fine-grained rocks, the determination of the petrophysical and petrographic parameters that affect the framework and their importance for fluid flow are key to determining the reservoir s exploitation potential. Analysis of porosity, permeability, grain and bulk density, elastic wave velocities, and petrographic characteristics were made on an Aptian limestone from the Araripe Basin (NE Brazil), which has been used as an analogue for naturally fractured carbonate reservoirs. This study focuses on quantification of petrophysical and petrographic parameters in order to include the results in numerical simulations to combine the fracture network permeability and porosity and the rock matrix properties (double-porosity and/or double-permeability problems) to build more reliable geomechanical models. The lithofacies analyzed were laminated limestone (matrix) and recrystallized calcite (fracture fill). Crato Formation presents an intergranular porosity and average of porosity and permeability values of 12% and 0.04 md, respectively that allowed us to classify this unit as an analogue of unconventional tight carbonate reservoir. Keywords: intergranular porosity, laminated limestone, naturally fractured reservoir.

2 Introduction The main goal of reservoir characterization is to describe the spatial distribution of petrophysical parameters, such as porosity and permeability. Studies that relate rock fabric to pore-size distribution, and thus to petrophysical properties, are key to quantifying geologic models in numerical terms for input into computer simulators (Lucia, 1995). The distribution of reservoir quality in tight carbonates depends primarily upon how diagenetic processes have modified the rock microstructure lead in to significant heterogeneity and anisotropy (Rashid et al., 2015). The Crato Formation of the Araripe Basin has large outcrops of a laminated lacustrine limestone deposited during the Aptian (Neumann, 1999). The equivalent of the Lower Cretaceous rock succession in Brazilian marginal basins is the Barra Velha Formation (Catto, 2015), in the Pre-Salt layer, which is a significant fraction of world oil reserves. We applied the petrophysical analysis in the Crato Formation focusing on following parameters: porosity, permeability, grain density, total density, and elastic wave velocities. The main goal of this study was relate the pore-size distribution and petrophysical parameters to include the results in numerical models that combine the fracture network permeability and porosity and the rock matrix properties (double-porosity and/or double-permeability problems) to build more reliable geomechanical models (Miranda et al., 2012). Material and Methods The first step in this study was the acquisition of 41 plugs (1.27 cm by 10 cm) of Crato Formation. These samples were collected via in situ core extraction or through hand sample plug preparation. Afterwards all plugs were analysed at the Laboratory of Petrophysics at the Federal University of Campina Grande. We measured porosity, permeability, grain density, total density, and elastic wave velocities, following the rationale described by Soares et al. (2015). Second, we selected suitable samples to prepare 13 high polished thin sections in order to do cathodoluminescence (CL) and scanning electron microscopy (SEM) techniques. The petrographic analyses were done at the Federal University of Pernambuco and the Bureau of Economic Geology at University of Texas at Austin. Results The principal petrographic characteristic observed in the laminated limestones of the Crato Formation was the fine grained matrix (micrite or mudstone) which consists of 1 to 4 μm-diameter crystals of low- Mg calcite, similar to most lacustrine calcareous muds (Scholle, 2003) (Figure 1). SEM analyses allowed us to observe that Crato Formation has an intergranular porosity. However, locally was identified a secondary porosity such as vugs, moldic structures, and stylolites. We observed also that Si or Fe replaced some pores due to the interaction with meteoric water, possibly during the postdepositional diagenetic stage. The opening-mode fractures (veins) are filled mainly by recrystallized calcite (neomorphism) with no porosity and permeability through these structures. Considering the effect of bedding orientation on permeability and porosity, the laminated limestone presents porosity values ranging from 4% to 22% with an average of 12% and permeability values ranging from 0.0 to 0.09 md with an average of md. Figure 2 presents air permeability versus porosity on a log-normal scale. For comparison purposes, dashed black lines indicate the three regions of grain size described by Lucia (1995) for carbonates with intergranular porosity. The majority of laminated limestone samples cluster in one region of the graph, which would be accurate for very fine grained carbonates that consist of < 20 μm particle size. Grain density values measured for all analyzed plugs (Figure 3A) show that laminated limestone and recrystallized calcite present an average grain density of 2.68 g/cm³. Nevertheless, three calcareous plugs, which were extracted from a high limestone layer (calcrete) present grain density of about 2.8 g/cm³. Figure 3B shows the relationship between bulk

3 density and porosity has a reliable linear regression fit (R² = 0.98) with the exception of a few samples composed of the calcrete lithofacies. This relationship may be useful to estimate porosity based only in the knowledge of bulk density (Soares et al., 2015). Figure 3C presents the relationship between P wave velocity (VP) and porosity for 40 MPa of confining pressure. In general, VP tends to decrease while porosity increases, but with a high dispersion (R² = 0.53). This occurs because wave velocities are dependent on other variables beyond porosity. The recrystallized calcite plugs show higher P wave velocities than the laminated limestone plugs and the calcrete plugs show the highest P wave velocity. Elastic anisotropy can be seen in Figure 3D, which shows the VS1/VS2 ratio against porosity. All samples present anisotropy lower than 5%, which allows us to classify them as of a low degree of elastic anisotropy. Figure 1 Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) images showing the main petrographic characteristics of the laminated limestone of Crato Formation. A) Backscatter image showing micrite matrix and vein filled recrystallized calcite; B) Ca element; C) Fe element; and D) Si element. Red and yellow lines represent the fracture wall.

4 Figure 2 Air permeability versus porosity for the laminated limestone of Crato Formation. Dashed black lines indicate the three regions of grain size described by Lucia (1995). The samples of the Crato Formation cluster in a region of very fine grain carbonates that consist of < 20 μm particle size. Figure 3 - A) Grain density versus 41 carbonate samples from the Crato Formation; B) Bulk density versus porosity follows a common linear distribution; C) P wave velocity versus porosity shows a high dispersion distribution; and D) Ratio of S waves versus porosity showing that Crato Formation represent a low degree of elastic anisotropy

5 Conclusions The high resolution petrographic analyses of carbonates in an analogue of naturally fractured carbonate reservoirs, in Araripe Basin, NE Brazil, allowed us to classify in the micritic matrix intergranular porosity. Most of the opening-mode fractures documented in outcrops were classified as veins completely filled by recrystallized calcite with no porosity or permeability. The laminated limestone of the Crato Formation presents average of porosity and permeability values of 12% and 0.04 md. The grain density showed to be a very useful parameter for rock typing. A reliable linear fit relating porosity and total density was established for the analyzed limestone plugs. In general, wave velocities decrease with increasing porosity, but the high dispersion of this relationship indicates that it depends on more variables than porosity. These results allowed us to classify this unit as an analogue of unconventional tight carbonate reservoirs. Acknowledgements Financial support for this study was provided by a grant from the Project Crato Petrobras/Federal University of Pernambuco (UFPE). We are grateful to the CMG Reservoir Simulation Foundation, Geosciences Graduate Program/UFPE, Fracture Research and Application Consortium, University of Texas at Austin. References Catto, B. [2015]. Laminitos microbiais no Membro Crato (Neo-Aptiano), Bacia do Araripe, NE do Brasil. Dissertação de Mestrado, Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, 111p. Lucia, F. J. [1995]. Rock-Fabric/Petrophysical Classification of Carbonate Pore Space for Reservoir Characterization. AAPG Bulletin, 79(9), Miranda, T.S., Barbosa, J.A., Gomes, I.F., Neumann, V.H., Fernandes, R., Matos, G., Queiroz, R., Guimarães, L.J. [2012]. Aplicação da técnica de scanline à modelagem geológica/geomecânica de sistemas de fraturamento nos depósitos carbonáticos e evaporíticos da Bacia do Araripe, NE do BrasilBoletim de Geociências da Petrobras, 20, Neumann, V.H.M.L. [1999]. Estratigrafía, Sedimentología, Geoquímica y Diagénesis de los Sistemas Lacustres Aptienses-Albienses de la Cuenca de Araripe (Noreste de Brasil). Tese de Doutorado. Universidade de Barcelona. Barcelona. 244p. Rashid, F., Glover, P.W.J., Lorinczi, P., Collier, R., Lawrence, J., [2015]. Porosity and permeability of tight carbonate reservoir rocks in the north of Iraq. J. Petroleum Sci. Eng. 133, Scholle, P. A. [2003]. A Color Guide to the Petrography of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis, AAPG Memoir 77. Soares, J.A., Garcia, A.J.V., Bezerra, F.H.R., Barbosa, J.A., Friedrich, A., Cazarin, C.L., Tabosa, L.D.G., Coura, R.L.C. [2015]. Petrophysics and Rockphysics of Carbonates from Brazil and Portugal. In.14th International Congress of the Brazilian Geophysical Society. Rio de Janeiro, Brazil, August 3-6.