Diagenesis and burial history of the Tunas Formation (Permian) at the Claromecó Basin (Buenos Aires province, Argentina)

FEBBO B.; CESARETTI, N.N.; TOMEZZOLI R.N.

Simposio; Twenty-Ninth Meeting of Swiss Sedimentologists ? Fribourg, February 25, 202; 2023

Swiss Sedimentology

The Claromecó foreland Basin (Carboniferous–Permian; Ramos, 2008) extends from the Ventania orogenic belt (southern Buenos Aires province, Argentina) to the Argentinean continental shelf, covering about 65000 km2, with a maximum sediment thickness of 10.5 km. Due to its paleogeographic position, understanding the geological history of this area is key to reconstruct the geodynamic evolution of southwestern Gondwana margin, as it records the tectonic events resulting from the assembly of the continents to form the Pangea in the Permian-Triassic. Also, this basin has attracted industry interest focusing on energy resource exploration due to the presence of subsurface coal beds contained within the sandstone Tunas Formation (Early Permian; Harrington, 1947) which might represent a target for conventional and/or unconventional (CBM) gas resources (Lesta and Sylwan, 2005; Febbo et al., 2022). However, despite its scientific and economic relevance, given the lack of outcrops in the foreland area and few available subsurface data, its geological knowledge is poor.This study aims at determining the diagenetic evolution of the Tunas Formation and constraining the factors and processes that acted during its burial history. To achieve this goal, different analytical methods were carried out on cores samples of the Tunas Formation (PANG 0001 and 0003 wells), such as microscopy determinations (petrographic studies), fluid inclusions analysis and reflectance vitrinite measurements. The analysed sedimentary sequences are composed of sandstones interbedded with mudrocks, carbonaceous mudrocks, tuffs and coals. Petrographic and fluid inclusion studies (petrography, fluorescence and microthermometry) were performed in sandstones while vitrinite reflectance (%Ro) were carried out in shales, carbonaceous shales and coals. Sandstones are medium- to fine-grained, framework-supported and moderate-to-well sorting, with 5-20% of matrix. Grain contacts are predominantly concave-convex, with subordinate long and sutured contacts. Quartz, feldspar and lithic fragments dominate framework components. Authigenic minerals are calcite and laumontite and minor proportions of quartz and feldspar (mostly albite) overgrowths and clay minerals (micas and illite). They constitute between 5 and 25% of the bulk rock volume and could occur as cements, with poikilotopic and intergranular textures, or replacement of unstable grains (frequently feldspars and lithic fragments). The porosity measured by optical microscopy varies from 0.1 to 4% of the total rock volume, prevailing values less than 1%. Porosity is secondary type, predominantly by fracturing and dissolution of grains (feldspar and lithic fragments) and carbonate cement. Twenty-Nineth Meeting of Swiss Sedimentologists – Fribourg, February 25, 2023_______________________________________________________________________________________14Fluid inclusions (FIs) studies were performed in sandstones cements and quartz veins. Primary, pseudo-secondary and secondary aqueous and organic inclusions were recognized. Organic FIs show yellow and light blue fluorescence, which could indicate the presence of organics components (liquid or gas hydrocarbons; Roedder, 1984) within the inclusion. Homogenization temperatures (Th) values obtained from microthermometry studies range from 130 to 168°C in carbonate cements and 160 to 230°C in quartz veins. Some measurements do not reach homogenization even above 230°, behaviour that may be due to the presence of hydrocarbons within the inclusion (Roedder, 1984). Carbonaceous levels analysed are mainly composed of macerals from the inertinite and vitrinite group and minor proportions of solid bitumen and liptinite. Vitrinite reflectance values range from 1.63 to 2.8 %, with an average of 1.9-2.3%. This large variation is related to the great difficulty to distinguish vitrinite from semifusinite (intertinite) particles. The minimum values obtained (1.65 to 1.90%) were measured on very small and scarce particles, which could correspond to vitrinite while the largest population measured (2-2.2%) could be semifusinite. The results obtained allowed to determine the different diagenetic stages experienced by the Tunas Formation and to estimate the maximum paleotemperatures reached by this unit. The main diagenetic processes that acted during burial are physical and mechanical compaction and cement precipitation, which contributed to the loss of primary porosity. However, dissolution of grains and carbonate cements and fracturing of the rock and grains led to the generation of secondary porosity. The diagenetic minerals recognized as laumontite-albite and illite, which act as geothermometers, indicate a formation temperature between 140 and 200°C (Aoyagi and Kazama, 1980). In addition, the Th obtained in fluid inclusions signal that they were formed between 130 and 230°C. These paleotemperatures confirm that the Tunas Formation reached a mesogenic stage (Choquete and Pray, 1970) within the inorganic diagenesis. Regarding the organic diagenesis, vitrinite reflectance values (VR: 1.9-2.3%) reflect that the unit is in a late catagenesis stage within the wet-to-dry gas window (Suárez-Ruiz et al., 2012).