The Chubut Group (Cretaceous, Golfo San Jorge Basin): Allogenic controls, hydrocarbon habitat, and analogs

PAREDES, J.M; FOIX, N; ALLARD, J.O; OLAZÁBAL, S.X; VALLE, M

La Plata

Congreso; XVIII Reunión Argentina de Sedimentología - IX Congreso Latinoamericano de Sedimentología; 2023

Asociación Argentina de Sedimentología

The Chubut Group (¿Barremian?-Maastrichtian) in the Golfo San Jorge Basin (Argentina) currently provides 45 % of the liquid hydrocarbons of the country. It is a continental succession preserved in an endorheic basin characterized by a high content of volcaniclastic components reworked in fluvial and lacustrine environments, with minor ash-fall deposits. It comprises six continental formations (Pozo D-129, Matasiete, Castillo, Bajo Barreal, Laguna Palacios, and Lago Colhué Huapi) totalizing up to 6,000 m thick in the deepest depocenters, and it is coeval with the early stages of the uplifting of the Andes Cordillera. For over a century, subsurface studies of productive units and the analysis of outcrop analogs along the San Bernardo Fold Belt have provided an extensive database of sedimentological and stratigraphic studies that guided the hydrocarbon exploration and production. With an emphasis on assessing the scientific and resource-based research of the Chubut Group during the last five decades, this contribution is divided into four parts. Part A provides an introduction and geological background. Part B focuses on the Chubut Group´s stratigraphy, analyzing typical subsurface features and the outcrop analogs along the San Bernardo Fold Belt. Part C discusses the importance of allogenic controls on sedimentation, integrating outcrop and subsurface research. Part D presents the main conclusions and perspectives. The contribution focuses on the main features of fossil analogs of hydrocarbon reservoirs in terms of their overall (paleo)geomorphic scenario and planform evolution, evaluating alluvial styles, stacking density, and geometry through the analysis of multiple sedimentological sections in the Matasiete, Castillo, and Bajo Barreal formations. From this, we evaluate the relations among tectonic and climatic processes during the deposition of the Chubut Group, highlighting the role of pyroclastic-volcaniclastic participation in the construction of the overall succession. Current analogs of endorheic basins in a seasonal climate occur in the south-central of Africa (Okavango Macrobasin) and central-northwest of Africa (Lake Chad basin). Both endorheic basins contain several drainage catchments of variable orientation with multiple source areas, variable development of fluvial processes (ephemeral, strongly seasonal, perennial) regulated by the overall patterns of precipitation and temperature distribution within the drainage catchments, and multiple base levels (lacustrine, ephemeral pans). The hydrological basins change between fully integrated, partially connected, or disintegrating according to water availability. Both current analogs record significant changes in the size of lakes and ephemeral pans through their evolution, regulated by the overall re-organization of the drainage catchments by river capture and climatic cycles; comparable features have not been recognized within the Chubut Group in the subsurface of the Golfo San Jorge basin until now. We conclude that the sedimentary evolution of the Chubut Group has been regulated by the development of several extensional phases and climatic cycles in a basin that changed its shape and boundaries, which give characteristics patterns to the stratigraphic architecture, also influenced by long-lasting periods of incorporation of volcaniclastic particles in variably-integrated drainage catchments in a distal, peri-volcanic environment.