Reduction In Reservoir Potential by Cryptobioturbation. A Case Study In Upper Paleozoic Shallow-Marine Sandstones (Rio Bonito/Palermo sedimentary succession, Paraná Basin, south Brazil)

Meiofaunal organisms generally burrow or swim between the sediment grains, promoting small-scale disruption of the grain fabrics through the sandstone laminae, which reworks totally the sediments but preserves, rather than destroys, the primary physical structures. The result is “fuzzy” laminations in apparently unburrowed sandstones, whose original grain arrangement was changed by cryptobioturbation, enhancing or reducing permeability and porosity. The sandstones of the Rio Bonito/Palermo sedimentary succession (Permian, Artinskian-Kungurian, Paraná Basin, south of Brazil) contain significant amounts of hydrocarbons and, in some areas, represent important aquifers. These apparently clean, fine- to very fine-grained quartzose sandstones with low-angle planar cross-stratification, hummocky cross-stratification and parallel lamination representative of shallow-marine deposits, ranging from the foreshore to the shoreface, have been noted as one of the potential gas and water reservoirs in the succession. However, few or no traces of hydrocarbons or groundwater have been reported in them, in contrast to those beds deposited in estuarine channels and tidal bars. The clean sandstones show a very peculiar “fuzzy” aspect, normally associated with cryptobioturbation. Ten thin sections of this “fuzzy” laminated sandstone, from both bedding plane and cross-section views, were prepared and analyzed by optical microscope, revealing a complete homogenization of the grain fabric, and the presence of mud-lined, unbranched millimetric trace fossils. The disruption of the grain fabric through the laminae seems to be due to the intense reworking by the burrowers, cross-cutting each other and preserving only the last-emplaced trace fossils. The thick mud lining observed in the microburrows of the Rio Bonito/Palermo “fuzzy” sandstones suggests originally mucus-supported burrows, a common strategy developed by meiofaunal nematodes. Thus, cryptobioturbation may be responsible for the absence of hydrocarbons or groundwater in these foreshore-shoreface sandstones, reducing, instead of enhancing, their porosity and permeability.