Comportamento do sistema conduto-selo de rochas carbonáticas-siliciclásticas mistas em cavernas da formação salitre, Bacia de Una-Utinga (BA)

Abstract

Kartisification processes in carbonate reservoirs are widely discussed in the literature due to their great relevance for oil and gas exploration. However, the development of mixed-carbonate-siliciclastic reservoirs can sometimes be difficult to understand. The studied caves presented in this work comprise two main approaches: sedimentary and structural. The main objective is to understand the behavior of a conduitseal system in a mixed-carbonate-siliciclastic plataform in the Lapinha and Paixão caves, located at Una-Utinga Basin, Bahia, Northeast Brazil. The study is based on micro to macro scale analyses, through Light Detection and Ranging (LiDAR) surveys, Unmanned Aerial Vehicle (UAV) images, lithological profiles, gamma spectrometry, Uniaxial Compressive Strength (UCS) using Schmidt hammer, structural field mapping and laboratory data analysis, petrographic and petrophysical analyses. The sedimentary results comprise the definition of three sedimentary facies described in a vertical columnar section of 22 m in Lapinha and 10 m in Paixão, as well as in thin sections. At the bottom of sections, the Lime mudstone facies (Lm) comprises 10% of siliciclastic sediments, which characterizes Lm as the most soluble facies, responsible for the development of most cave passages. Heterogeneous calcarenite facies with carbonate and siliciclastic intercalations may present a conduit or seal behavior, depending on siliciclastic sediments variation (ca. 30-50%). The mixed limestone facies with siliciclastic sediments (LmTs), about 50% of siliciclastic content, can be characterized as a sealing facies due intense chemical compaction. In addition to the sedimentary control, it was possible to identify in these caves a structural control, based on quantitative analysis, associated with stratigraphic data. The conduit facies (Lm and/or CaLm) present great density and intensity of fractures indexes, while the sealing facies (LmTs), the opposite. In conclusion, siliciclastic sediments along the described sedimantary facies controls karstification processes at different scales in a mixed carbonate-siliclastic platforms. Thus, Lapinha and Paixão caves can potentially be analogues of mixed carbonate-siliciclastic reservoirs, using the sedimentary and structural patterns described in the present work. The conduitseal system in a mixed carbonate-siliciclastic platform may be interesting for characterization of these mixed reservoirs and for the prediction of storage and conduction of oil and gas.