Measurement of Grain Bulk Modulus on Sandstone Samples From the Norwegian Continental Shelf

The grain bulk modulus is a crucial property of a rock frame, determining poroelastic responses such as seismic velocity. Conventionally, the grain bulk modulus is estimated with bulk moduli of compositional minerals. A systematic measurement is still scarce in answering what factors control the grain bulk modulus. We test 38 sandstone samples with porosity ranging from 4% to 35% from three fields on the Norwegian continental shelf through unjacketed tests. The measured grain bulk moduli range from 18 to 61 GPa. The deposition and composition from thin sections and X-ray diffraction data can interpret abnormally low or high grain bulk moduli of samples compared to quartz. Carbonate cement is a factor correlating the low porosity with the high grain bulk modulus in a range of 46-61 GPa for tight sandstones. Illite/chlorite coating on the quartz grains can enhance the result to 48 GPa; nevertheless, bearing clay at the grain-to-grain contact correlates with the low grain bulk modulus. The measured grain bulk modulus overall increases with decreasing porosity. Specifically, cementation and composition, such as calcite and clays, account for the correlation. Therefore, when people model the elastic properties of sandstones, direct usage of the quartz bulk modulus may cause significant errors. We suggest the unjacketed test as the most reliable way to determine grain bulk moduli of sandstones with different geological features.

Automated summary

"The grain bulk modulus is a crucial property of a rock frame, which can be explained by factors such as carbonate cement and clay. It has been found that using the bulk modulus of quartz directly can lead to errors. Therefore, the unjacketed test is the most reliable method for determining the grain bulk modulus of sandstones with different geological features."