The influence of salt walls on deformation band formation and implications for reservoir quality

Abstract

A significant number of hydrocarbon reservoirs globally are found in sedimentary sequences affected by salt diapirs and salt walls. As a result, the hydrocarbon industry routinely drills near salt walls. Significant stress perturbations arise in this environment due to the rising salt and associated withdrawal imposing an additional load on the adjacent sediments. The strength of these sediments and their response to external loading is a function of their loading history since deposition. It is therefore critical to account for the stress history in those environments close to salt walls where significant additional loading has been imposed by halokinesis. This study investigates the Triassic fluvial sandstones of the Skagerrak Formation, Central Graben, North Sea where deformation bands are frequently observed in wells drilled adjacent to salt walls, focussing on wells 22/29-2S1 and 22/29-3 in the Seagull Field. Well logs, optical microscopy and statistical analysis are utilised to fully appreciate the salt-sediment interaction and improve our understanding of stress changes near salt walls in mini-basins and how these stresses influence the sediments through their early lithification (<500 m) and later physical and chemical diagenesis. Preferential infiltration of meteoric water and clay material along the complex network of dilational shear (disaggregation) bands has resulted in widespread diagenesis in the sediment adjacent to the salt wall, reducing porosity in sandstones at the margin of the mini-basin. Further analysis of the preserved deformation band orientations reveal that the principal stresses changed over time, reflecting irregular lateral and vertical salt wall movements and minor far-field influence (approximately 1000 m away). This study demonstrates the need to consider the implications of early sandstone deformation coupled with diagenetic processes on the long-term reservoir quality of complex HPHT mini-basin sandstones proximal to salt structures.