Patterns and origins of the porosity in the productive reservoirs of the upper part of the Chubut Group, southern flank of the Golfo de San Jorge Basin, Patagonia Argentina

The Mina del Carmen, Bajo Barreal and Cailaclon Seco Formations bear the most important Cretaceous reservoirs in the southern flank of the Golfo de San Jorge Basin, in which the porosity is mainly secondary in origin. Primary porosity is about 4% in average and can be divided in intergranular and intercrystalline porosity, the former is better preserved where chlorite rim cement is abundant, and the second occurs among crystals of the clay cement (main kaolinite). Secondary porosity comprises dissolution of feldspar, ductile fragments, clay cements, lithic grains, and carbonate-zeolite cements. Dissolution of feldspar produces the formation of authigenic kaolinite and secondary porosity; this mechanism dominates in the western area of the southern flank due to sandstones show higher amounts of plagioclase grains, which were very reactive to diagenetic dissolution. Ductile fragments, composed of tuffaceous mudstones (probably of intrabasinal origin), were strongly deformed during the compaction reducing the primary porosity and closing the pore system. Nevertheless, in sandstones containing similar proportions of intrabasinal and extrabasinal clasts, some ductile grains suffered massive dissolution increasing the secondary porosity. Another source of secondary porosity comprises the dissolution of tuff fragments, and locally, carbonate and zeolite cements. The secondary porosity includes the types redistributional (RDS) and pervasive (PVS). This division can be related to previous diagenetic model for the Bajo Barreal Formation. In this way, redistributional secondary porosity is linked to the first and the second events of diagenetic dissolution, while the pervasive secondary porosity to the third dissolution event, which likely had a telogenetic origin.