Geochemical Characterization vs fracture frequency of the Tunas Formation – Claromecó Basin

The Claromecó foreland Basin (Carboniferous-Permian) is located in the southern of Buenos Aires province, Argentina. The sedimentary infill of the basin was recorded by the PANG0001 and PANG0003 reporting Tunas Formation presence (Pillahuincó Group, Permian). The study of their synorogenic deposits is of significant importance, as key data to understand the evolution of the SW sector of the Gondwana margin. Also, Claromecó Basin is considered a frontier basin due to its potential for unconventional reservoir, particularly CBM (coal bed methane).  

This work presents published data related to organic geochemistry, thermal maturity, and mineralogy, aiming to assess the source rock potential of the Tunas Formation and its relevance as an unconventional reservoir. Geochemical characterization of carbonaceous mudrocks in the Claromecó Basin is also critical, as these organic-rich intervals are chronostratigraphically correlated with other formations in southwestern Gondwana. The integration of geochemical data from correlative unit, such as the Irati Formation in the Paraná Basin, highlights the basin's broader regional significance.

Moreover, organic geochemical parameters, such as total organic carbon (TOC) content in these mudrocks, may serve as proxies for elevated fracture density, as their disorganized and abundant organic matter increases rock brittleness, potentially enhancing reservoir quality.

A comprehensive review and reinterpretation of data from exploratory wells PANG0001 and PANG0003 was performed. Techniques considered include petrographic analysis, X-ray diffraction, geochemical assays, and organic petrography. Key parameters reviewed were total organic carbon (TOC), vitrinite reflectance (Ro%), hydrogen index (HI), maceral composition, and mineralogical features of the carbonaceous deposits in the Tunas Formation.

Two main horizons with significant hydrocarbon generation potential were identified: an upper horizon at 300–400 m depth, and a lower one between 740–850 m. These intervals are dominated by shales and carbonaceous mudrocks, interbedded with coal layers that reach a cumulative thickness of up to 4.5 m. TOC values range from 0.5–2% in organic-rich shales and from 26–53.9% in coal and carbonaceous levels, classifying them as high-quality source rocks. The organic matter is mainly of terrigenous origin (type III kerogen), with Ro values ranging from 1.3–1.6% in the upper horizon (wet gas window) and 1.9–2.4% in the lower (dry gas window). HI values between 30–90 mg HC/g TOC support a gas-prone nature, and Tmax values from 455–470 °C indicate a mature to overmature thermal stage. 

This study plus the relationship between the geochemical characteristics of carbonaceous mudrocks, particularly TOC content, and natural fracture frequency. The presence of high TOC is associated with structurally disorganized organic matter that increases rock brittleness, favoring fracture development. By integrating TOC data with fracture attributes measured using Marrett's scanline method, this work explores how organic matter enrichment may act as a proxy for fracture density in the Tunas Formation.

The Tunas Formation exhibits excellent geochemical and thermal properties, confirming its potential for unconventional gas generation, particularly methane. Additionally, the characteristics of carbonaceous mudrocks, such as high TOC and poorly structured organic matter, suggest increased brittleness, potentially favoring fracture development