X-ray micro-computed tomography of burrow-related porosity and permeability in shallow-marine equatorial carbonates: A case study from the Miocene Pirabas Formation, Brazil

The Miocene Pirabas Formation is of strategic importance for the hydrocarbon industry since it represents the onshore portion of larger offshore basins of Northern Brazil, including the Foz de Amazonas, the Pará-Maranhão, Barreirinhas, Ceará, and the Potiguar basins of the equatorial margin of South America. The Pirabas Formation from the Aricuru outcrop (Pará State) is a shallow-water mixed siliciclastic-carbonate deposit, comprising highly bioturbated wackestone and packstone. However, the impact of these biogenic structures on the reservoir quality of this formation has been overlooked. Here, our results show that pervasive Thalassinoides penetrating the rock matrix, result in remarkably improved porosity and permeability, thus, turning the Pirabas Formation in extremely favorable strata for fluid flow and storage. Since these bioturbated layers are likely to be common in the sub-surface counterparts of this formation, the hydrocarbon potential of these deposits is probably higher than previously expected. Also, considering that the offshore basins of Northern Brazil are directly related to the breakup of the African and American plates during Aptian-Albian, the exploratory interest in the correlated Guinea Gulf (Jubilee play) and French Guiana-Suriname Basin (Zaedyus play) need to be reconsidered. Overall, the results of this research highlight the importance of integrating ichnological datasets with reservoir characterization studies for more accurate reservoir quality assessments.