Characterization of Upper Jurassic Organic-Rich Caprock Shales in the Norwegian Continental Shelf

Abstract

Characterizing the top seal integrity of organic-rich caprock shale is critical in hydrocarbon exploration and fluid storage sites assessment because the caprock acts as a barrier to the low-density upward migrating fluids. This study investigated the geomechanical properties of the Upper Jurassic caprock shales of various basins from the Norwegian Continental Shelf. Usually, paleo-deposition and diagenesis vary from basin to basin, which influences the geomechanical properties of caprock shale; hence, the seal integrity. Fourteen (14) wells from four (4) different basins within the Norwegian Continental Shelf were analyzed to evaluate the effects of various processes acting on caprock properties. Comparative mineralogy-based caprock properties were also investigated. We include a thorough review of the distribution of organic and inorganic components utilizing SEM and 3D microtomography as they relate to the development and propagation of microfractures. Five (5) wells from three (3) basins contain measured shear sonic logs. These wells were used for petrophysics and rock physics analysis. Three elastic properties-based brittleness indices were estimated and compared. The percentage of different mineral fractions of the studied wells varied significantly between the studied basins, which is also reflected in the mineralogical brittleness indices evaluation. Irrespective of the studied basins, relative changes in caprock properties between wells have been observed. The Young’s Modulus–Poisson’s ratio-based empirical equation underestimated the brittleness indices compared with mineralogy- and acoustic properties-based brittleness estimation. A better match has been observed between the mineralogy- and acoustic properties-based brittleness indices. However, as both methods have limitations, an integrated approach is recommended to evaluate the brittleness indices. Brittleness indices are a qualitative assessment of the top seal; hence, further investigation is required to quantify sealing integrity.

Potential Secondary Seals within Overburden: Observation from the Co2 Storage Sites Aurora and Smeaheia, Northern North Sea

“…The deposition of the shale took place in an open marine environment characterized by restricted bottom circulation and frequent anaerobic conditions (NPD, 2023). The shale itself is composed of a dark gray-brown to black claystone, typically non-calcareous, carbonaceous, and occasionally fissile in nature (Johnson et al, 2022b;NPD, 2023;Rahman et al, 2022e). The reservoir sandstones (i.e., Sognefjord, Fensfjord, Krossfjord) exhibit good to moderate reservoir quality (Dreyer et al, 2005;Mondol et al, 2018;Fawad et al, 2021;Holgate et al, 2015).…”