Heterogeneous cementation and compaction in a sandstone and the role of temperature – an example from the Etjo Formation, Namibia

Diagenetic processes alter petrophysical properties, such as porosity and permeability, of clastic sediments. An understanding of these processes is therefore pivotal for any reservoir quality assessments. Thermal exposure is among the crucial factors influencing diagenesis and can vary significantly in widespread formations, e.g. due to regional variations of the burial history. Especially quartz overgrowth is controlled by temperature and we here demonstrate the effect of spatial thermal exposure variability on the degree of cementation. Our field example is from the aeolian Jurassic Etjo sandstone that was buried by the Lower Cretaceous Paraná-Etendeka Large Igneous Province in Namibia and we show data from outcrops lying >100 km apart from each other – named Waterberg, Mt. Etjo, and Gamsberg. 

Our petrographic analysis shows significant differences in the degree of compaction and cementation between the localities (Salomon et al., 2024). Waterberg samples have a mean quartz cement volume of 6.5 % and intergranular volume (IGV) of 23.7 %. Mt. Etjo samples have a higher quartz cement volume (15.4 %), but lower IGV (19.7 %). As Waterberg samples had an on average 72 % larger nucleation surface area available for quartz cement growth than samples from Mt. Etjo, we argue that the latter locality has experienced a higher thermal exposure. This is supported by a kaolinite-to-illite transformation that only occurs at Mt. Etjo. We attribute the temperature differences to the localities’ position underneath the volcanic cover with Mt. Etjo being closer to the volcanic center than Waterberg. Gamsberg samples have a very high mean IGV of 30.7 % and very high mean quartz cement volumes of 24.3 %. Here, two quartz growth generations are evident and separated by an exhumation period of the sandstone. The origin of the first generation is yet unclear, whereas the second one is also attributed to the Etendeka burial. 

Our case study serves as a prime example for the variability in diagenetic character within a sandstone formation, depending on its geographic position. It underscores the importance of understanding spatial variabilities in the burial/temperature history when predicting the diagenetic properties of a reservoir rock. 

Reference: Salomon, E., Stollhofen, H., Caracciolo, L., Bonnell, L.M., Lander, R.H., Kittel, M., 2024. Burial of a sand sea: Heterogeneous compaction and cementation of the jurassic Etjo Formation, Namibia and what it tells. Marine and Petroleum Geology, 168, 107044.