PORE SYSTEM QUANTIFICATION AND CHARACTERIZATION IN VOLCANIC ROCKS: A CASE STUDY FROM THE LOWER CRETACEOUS SERRA GERAL GROUP, PARANÁ BASIN, SOUTHERN BRAZIL
M. Becker1*, E. F. Lima1, B. L. Waichel2, and I. F. Mantovani3
1 Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Prédio 43136, Caixa Postal 15001, Agronomia, 91501-970 Porto Alegre, RS, Brazil.
2 Departamento de Geociências, Universidade Federal de Santa Catarina, Campus Universitário Trindade, 88040-900, Florianópolis, SC, Brazil.
3 Departamento de Engenharia Mecânica, Universidade Federal de Santa Catarina, Campus Universitário Trindade, 88040-900, Florianópolis, SC, Brazil.
* Corresponding author, mobecker29@gmail.com.
Key words: Serra Geral Group, Paraná Basin, Lower Cretaceous, Brazil, volcanic rocks, pore system, petrophysical properties, X-ray microtomography.
Studies of pore systems in volcanic rocks are of increasing importance due to these rocks’ potential as reservoirs for hydrocarbons. For this paper, samples of basaltic pahoehoe, rubbly pahoehoe and acidic lava from the Lower Cretaceous Serra Geral Group (Paraná Basin, southern Brazil) were analysed in order to quantify and characterize the constituent pore systems. The Serra Geral Group volcanics were erupted as part of the Paraná-Etendeka Igneous Province in the Early Cretaceous (Valanginian – Hauterivian). Analyses included experimental measurements by permo-porosimeter integrated with X-ray microtomography (μ-CT) image analysis of vertical and horizontal sample plugs. In addition petrographic analyses were carried out to characterize pore types in thin section. The experimental porosity values ranged from 0.11 to 13.08% and most permeability values were generally lower than 0.0004 mD. Values varied as much within flow zones as they did between them.
Porosity and permeability values were not sufficient for the Serra Geral Group volcanics to be considered as a potential reservoir analogue. However the wide range in values was attributed to the processes which controlled the origin and development of the pore system. Primary pores observed included intracrystalline sieve, vesicular and interflow laminar types; secondary porosity, such as spongy, interclast and intra-matrix pore types, was related to dissolution and the precipitation of secondary minerals. The porosity values obtained by μ-CT (between <0.01 and 3.37%) were lower than those experimentally measured by permo-porosimeter. This was attributed to the presence of multi-scale pores in the volcanic rocks sampled, and also to limitations with image resolution. Even so, the use of μ-CT allowed the visualization of porosity variations and was useful in characterizing the pore system.
The results presented in this paper demonstrate that the volcanic rocks in the Serra Geral Group have a heterogeneous pore system, similar to that in carbonate rocks.
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