Composition of the Lower Cretaceous source rock from the Austral Basin (Río Mayer Formation, Patagonia, Argentina): Regional implication for unconventional reservoirs in the Southern Andes

•We model compositional changes in the black shales from inner to outer shelf.•Increasing information will help in the search of shale oil/gas resources.•The Río Mayer Formation constitutes a highly potential unconventional reservoir.

The importance of the Río Mayer Formation (Lower Cretaceous, prodelta to marine outer shelf) in the hydrocarbon development of the Austral Basin (Patagonia, Argentina) has been recognized by several authors. This unit plays a key-role as source rock and caprock in the “Inoceramus inferior-Springhill” petroleum system, the most important in the basin. In this work we demonstrate that the composition of the Río Mayer Formation highly matches with the requirements to be considered for shale-oil/gas exploitation. The unit studied mainly consists of laminated black siliciclastic mudtones and shales with interbedded marlstones and sandstone levels, and in subsurface shows TOC values up to 3% and kerogen types between II and III. Compositional studies from outcrops of this unit are almost absent and constitute the main focus of this contribution. A total of ten detailed sedimentological logs were measured from the field and X-ray diffraction analysis (XRD), standard petrography, scanning electron microscopy (SEM) and determination of major elements in clay minerals by EDAX were carried out in order to characterize the composition of the Río Mayer Formation. XRD analyses (in fine-grained sediments) indicate a composition dominated by quartz and calcite, with minor proportions of feldspars and clays. Marine sediments are dominated by illite and chlorite, and commonly accompanied by mixed layered illite-smectite (IS), while the prodelta is smectite-dominated with more than 90% of this clay. Both, authigenic and detrital clays were identified using SEM-EDAX. Sandstones of the Río Mayer Formation are generally lithic and feldspar graywackes. Provenance analyses in sandstones make possible the discrimination of four groups of samples differentiated by their ages and palaeoenvironments. In comparison with other important shale-reservoirs from North America (Barnett, Marcellus, Woodford, Haynesville and Doig), the composition of the unit studied have more quartzitic composition, reflecting in a high brittleness index (BI) that result in a clear brittle pattern.