Calcite veins as local fluid flow barriers in reservoir rock? The odd occurrence of veins in highly porous aeolian sandstone in Namibia

In porous sandstone, fluids are guided by major features such as faults or lithologic discontinuities. At the local scale, deformation bands are common structures to baffle fluid flow in such rock. Potential flow-hindering structures less frequently reported of are veins in porous sandstone (Skurtveit et al., 2015, as a rare case), which may root in the circumstance that they do not appear very often and/or have simply been overlooked. 

We here present a case where calcite veins formed in the highly porous (up to 25 % porosity) and partially poorly lithified eaolian Lower Cretaceous Twyfelfontein Formation in NW Namibia. This sandstone was buried by the extrusion of voluminous Paraná-Etendeka flood basalts at around 130 Ma and was since then subject to exhumation. Calcite veins occur in roughly half of the visited outcrops of the Twyfelfontein Formation and their dominant parallel trend to the continental margin suggests a tectonic origin. As the host rock is void of carbonate framework material or cement, the veins must have formed through advective fluid circulation. An external source of the calcium may possibly be the alteration of overlying and intercalated basalt. The veins exhibit a remarkable multitude of textures ranging from blocky, colloform, to microcrystalline calcite generations, that have partially experienced brecciation. This argues for highly variable formation conditions, potentially spanning from normal fluid advection to boiling and injection (c.f., Moncada et al., 2012; Salomon et al., 2021). 

Preliminary clumped isotope data of the veins indicate a low temperature formation in the range of 19-61°C, which suggests overall shallow burial conditions. This is in agreement with the diagenetic paragenesis of the rock arguing for late stage vein formation, i.e. during exhumation of the rock. Upcoming U/Pb calcite dating is expected to bring greater clarity on this regard. A halo in the host rock surrounding the veins became calcite cemented due to the growth of calcite from the fractures into the sandstone body. This appearance demonstrates the following evolution: (1) fracturing of the sandstone, which enhances advective fluid flow in the rock body; (2) vein precipitation and near-vein host-rock cementation; and consequently (3) reduction of permeability in the fracture and adjacent wall rock. 

Due to their potential of forming effective barriers to fluid flow, we stress that their formation needs to be understood in greater detail. The variable vein textures indicate differing formation conditions, which sets the base for a more common occurrence of calcite veins in porous uncemented sandstone. 

References:

Moncada, D., et al. (2012). Journal of Geochemical Exploration 114, 20-35, doi:10.1016/j.gexplo.2011.12.001.

Salomon, E., et al. (2021). Journal of Structural Geology 153, 104463. doi:10.1016/j.jsg.2021.104463.

Skurtveit, E., et al., (2015). Petroleum Geoscience 21, 3-16, doi:10.1144/petgeo2014-031.