Role of depositional environment on clay coat distribution in deeply buried turbidite sandstones: Insights from the Agat field, Norwegian North Sea

The preservation of reservoir quality in deeply buried sandstones largely depends on the presence of grain-coating clays that can inhibit quartz overgrowth and cementation. These grain-coating clays are the focus of today's hydrocarbon exploration as they enable new oil and gas discoveries in sedimentary basins. However, the origin and distribution of these clays in turbidites are poorly documented in scientific literature. Our study addresses this knowledge gap by investigating the mechanisms and parameters governing the clay coat development in turbidite sandstones, with a focus on the Agat Formation (Fm) in the Norwegian North Sea. Petrographic observations, scanning electron microscopy, and X-ray diffraction analyses were used to identify the origin of clay minerals and their distribution, as well as to understand the diagenetic evolution of the sandstone and its impact on reservoir quality. Based on the sedimentary facies description, the Agat Fm comprises four principal members deposited in various depositional environments, ranging from distal lobe fringe to amalgamated proximal lobes and weakly confined channels. Chlorite and kaolinite are the main authigenic clay minerals in these sandstones, associated with a variable amount of inherited detrital clays consisting in Fe-bearing kaolinite and mixed-layer minerals composed of illite and hydroxyl-interlayered clay minerals formed in acidic soil environments. Well-developed chlorite coats are associated with the highest reservoir quality (φ > 10%), while discontinuous chlorite coats are linked to extensive quartz cementation and poor reservoir quality (φ < 5%). Samples with abundant pore-filling chlorite also exhibit poor reservoir quality (φ < 7%). Proximal lobe deposits and weakly confined channels show well-developed chlorite coats, whereas discontinuous chlorite coats are mainly found in proximal lobe deposits associated with high velocity escaping fluids during dewatering. Pore-filling chlorites are typically present in distal lobe fringes and levee deposits. Chlorite in the Agat Fm is mainly derived from the replacement of inherited detrital clays via an intermediate berthierine precursor forming during early diagenesis. The dissolution of Fe-rich grains such as biotite and ferric detrital clays under reducing conditions governed the kinetics of berthierine/chlorite growth. The late diagenesis is characterized by extensive feldspar dissolution in a closed diagenetic system, leading to the precipitation of kaolinite and quartz cement. The presence of chlorite coatings around detrital grains preserved the porosity from extensive quartz cementation during deep burial, highlighting the role of chlorite in maintaining the quality of deeply buried turbidite reservoirs.