Print this pageCryptobioturbation and Reservoir Quality: The Role of Mucus-Supported Microburrows in Rock Porosity Reduction
Cryptobioturbation is an important feature in reservoir characterization. Its presence in well-sorted, fine-grained, apparently unburrowed Mesozoic and Cenozoic sandstones is well documented, usually enhancing permeability and porosity of these rocks. However, cryptobioturbation present in apparently clean, fine- to very fine-grained quartzose sandstones of the Rio Bonito/Palermo sedimentary succession (Artinskian-Kungurian, Paraná Basin, southern Brazil) tells us a different story. Low-angle planar cross-stratification and horizontal lamination are the main sedimentary structures in these sandstones, which show the very peculiar “fuzzy” aspect normally associated with cryptobioturbation. These features suggest deposition in shallow-marine settings, ranging from the foreshore to the upper/middle shoreface, where biogenic activity of meiofaunal organisms has a major potential of preservation. These sandstones have been noted as one of the potential reservoir-quality deposits in the succession. However, few or no traces of hydrocarbons or groundwater have been reported, in contrast to those beds deposited in estuarine channels and tidal bars. 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. Mucus is commonly excreted by wormlike animals, because it prevents skin dehydration and favors metabolic exchanges with the surrounding substrate. However, mucus easily dries up, requiring permanent stock up by the burrowers and forming a thick organic mud lining which is probably pre-cemented even before early diagenesis. Thus, the presence of thick mud-lined microburrows in fine-grained sandstones impacts in reservoir quality, reducing, instead of enhancing, their porosity.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil