Resumen

NET, Laura Inés  y  LIMARINO, Carlos Oscar. Caracterización y origen de la porosidad en areniscas de la sección inferior del Grupo Paganzo (Carbonífero superior), Cuenca Paganzo, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2000, vol.7, n.1-2. ISSN 1853-6360.

The characteristic distribution and origin of porosity in the sandstones of the lower section of Paganzo Group (Upper Carboniferous) in the eastern region of the Paganzo Basin (La Rioja province, Argentina; Figs. 1, 2) are described in this paper. Sandstones of the Lagares Formation in mina Las Mellizas locality are feldsarenites and subfeldsarenites (Q81±7 F19±7 L1±1), while those exposed in Olta and Malanzán area exhibit a larger compositional variety, with plutonic-metamorphic lithoarenites making up the base of the Malanzán Formation (Q41±3 F33±2 L26±5), and feldspathic lithoarenites, feldsarenites and lithic feldsarenites for the rest of this unit and the overlying Loma Larga Formation (Fig. 4). Although in both localities mean total optical porosity values are only moderate (9.7% in Lagares Formation and 7.4% in Malanzán and Loma Larga Formations), porosity distribution is highly irregular and strongly dependent on facies distribution. Maximum values of porosity are registered in high-sinuosity fluvial intervals, in close association with coals and carbonaceous mudstones (Figs. 3, 5). Generation of carboxylic acids caused by decomposition of organic matter is probably the main responsible for the intense feldspar dissolution observed (Surdam et al., 1984), which in this case, and based on petrographic observations, is more pervasive in plagioclase than in K-feldspar grains. Petrographic image analysis (PIA) techniques (Ehrlich et al., 1984; Ehrlich et al., 1991; Bloch, 1994) allowed to recognize four porosity types, named TP1, TP2, TP3 and TP4 (Figs. 6, 7). Porosity type 1 (TP1, up to 15-20%) is mainly related to secondary intergranular macro and mesoporosity in coarse and medium-grained plutonicmetamorphic lithoarenites due to calcite cement partial dissolution; intragranular microporosity is minimal and can be attributed to incipient dissolved feldspars being part of lithic fragments. Porosity type 2 (TP2) is related to massive dissolution of cements, matrix and feldspar clasts in medium and coarse-grained feldsarenites; total optical porosity values in TP2 can reach up to 28%. Porosity type 3 (TP3, maximum value 12%) was only identified in fine sandstones, and it is composed of crypto and micropores generated by the incipient dissolution of matrix and cement. Porosity type 4 (TP4, up to 22%) is an hybrid porosity type that combines intragranular microporosity caused by feldspar dissolution with abundant intercrystalline cryptoporosity in authigenic kaolinite pore-filling patches. Complexity and tortuosity values of individual porosity elements ("porels") increase from TP1 to TP4 (Fig. 9). Taking into account that all the mechanisms generally invoked for feldspar dissolution (inorganic Al³⁺ complexation related to carbonic acid generation, organic Al³⁺ complexes derived from carboxylic acids, or gibbsite hydrolysis) need a considerable pore fluid circulation, secondary porosity generation in these sandstones is supposed to have occurred before significant compaction. Moreover, the presence of abundant authigenic kaolinite patches and common quartz overgrowths strongly suggests that the Al³⁺ mobilized was not exported out of the system, but it combined with poral siliceous-enriched solutions to generate kaolinite. Finally, a model of secondary porosity generation for these sandstones is proposed (Fig. 13). In this model, porosity depends on feldspar abundance and liberation degree (F/L ratio), mean grain size and matrix content. Sandstones with abundant monocrystalline feldspars grains (F/L>>1), medium to coarse grain size and low matrix content show optimal conditions for porosity development, because pore fluid circulation is favored and consequently feldspar dissolution can take place, so secondary porosity of type 2 is generated (A1 trend in Fig. 13). If aluminum was not eliminated out of the system, it would form kaolinite pore fillings typical of porosity type 4. Feldspars as subgrains in polymineralic rock fragments or an episode of early cementation that strongly inhibit fluid circulation would difficult feldspar dissolution, so, in that case, the secondary porosity generation is mostly intergranular, attributed to matrix and cement dissolution (A2 trend in Fig. 13). In fine-grained arenites, the combination of a relatively more quartzose composition and the severe porosity and permeability reduction during diagenesis resulted in very scarce matrix and cement dissolution typical of porosity type 3 (B trend in Fig. 13). Lastly, muddy arenites can not develop appreciable porosity in any case due to their high matrix content (or early cementation processes) that strongly prevents pore fluid circulation (C trend in Fig. 13).

Palabras clave : Porosity; Sandstones; Diagenesis; Petrographic image analysis; Feldspar dissolution.

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