A catalogue of calcite vein types in porous sandstone and their formation conditions (Huab Basin, Namibia)

Abstract

We present a catalogue of calcite vein types in porous sandstone that have the potential to affect fluid flow properties in a reservoir. Host rock is the Lower Cretaceous aeolian Twyfelfontein Formation, covered by voluminous flood volcanic rock of the Paraná-Etendeka Large Igneous Province in the Huab Basin of NW Namibia. Since the host sandstone lacks carbonate components, these veins document advective fluid flow, where solutes were transported through the fractures from an external source. Crystal growth in veins vary from blocky, to radial, and micritic. Some veins exhibit an intergrowth with kaolinite, or show an initial dolomite and iron oxide precipitation. Deformation stages are evident in a number of veins from breccia to cataclasis and fault-slip slickenfibre generations. Some calcite generations underwent partial dissolution and replacement by chalcedony. U-Pb dating shows a predominant vein formation from 31 Ma to recent, with one outlier at ∼65 Ma. Pore-filling calcite cements of sandstone along veins and in contact with the covering volcanic rock show similar ages. Formation temperatures (16–47 °C) derived from clumped isotope analysis indicate precipitation at shallow burial depths (<1 km). An outlier of 59 °C stems from the vein dated to ∼65 Ma. Carbonate δ¹³C_VPDB shows a trend from −8.3 ‰ to −1.3 ‰ between Oligocene to present and we argue this to be a reflection of the region's climatic shift from warm humid to (semi-)arid conditions in this time. We further discuss, that this shift is responsible for many of the observed vein characteristics.