The Claromecó frontier Basin: Hydrocarbon source rock potential of the Tunas Formation, southwestern Gondwana margin, Argentina

Abstract

Coal-bearing deposits of the Tunas Formation suggest the existence of potential gas-prone source rocks in the Claromecó Basin, south of Buenos Aires province, Argentina. Two wells, PANG 0001 and PANG 0003, containing Permian rocks assigned to the Tunas Formation were studied through core observation, X-Ray diffraction, petrographic description, geochemical analysis, and organic petrography to investigate the hydrocarbon potential of the Claromecó Basin and define horizons with source rock potential. The analyzed sedimentary successions are composed of medium- to fine-grained sandstones interbedded with tuffs, mudrocks, carbonaceous mudrocks, and coal. Two main horizons with potential source rocks were determined: an upper horizon, at the top of the succession (300–400 m) and a lower one, at the base (740–850 m). These layers are mainly composed of mudrocks, carbonaceous mudrocks, and coal beds up to 4.50 m in cumulative thickness. Total organic carbon (TOC%) content is high, with most samples ranging from 0.5 to 2% in organic-rich mudrocks, and from 26 to 53.9% in carbonaceous mudrocks and coals, characterizing the coal-bearing layers as good-quality source rocks. The organic matter is of terrigenous origin as indicated by the abundance of woody plant debris. Predominant organic matter constituents belong to vitrinite group (type III kerogen), with a low abundance of the inertinite group macerals. Vitrinite reflectance (Ro) values in carbonaceous mudrocks and coals ranged from 1.3 to 1.6% for the upper horizon to 1.9–2.4% at the lower one, reflecting a catagenesis to late catagenesis stage within the wet to dry gas window. The organic matter quantity, type, and maturity in Tunas Formation coal-bearing deposits suggest that this unit has a good potential as gas-prone source rocks.