Imaging Reservoir Quality of the Triassic-Jurassic succession of Bjarmeland Platform, Norwegian Barents Sea. Examples from Wisting, Norvarg and Ververis discoveries.

The Barents Sea is a prospective but challenging area in terms of hydrocarbon exploration. High technical success rate and projected future resources, with around half of Norway’s estimated undiscovered hydrocarbons expected to lie here, are in contrast to the region’s complex geological history. Some of the main concerns in terms of exploration and development are associated with complicated depositional environments, with high lateral facies variations, significant Cenozoic uplift and erosion, and its associated effects on the petroleum systems, dominance of gas discoveries, with a relatively large distance from potential markets, and environmental concerns, among others. The Triassic (Kobbe and Snadd) and Jurassic (Stø) Formations located on the southern part of the Bjarmeland Platform, in SW Barents Sea are the focus of this study. The Jurassic Stø Formation is known to have good reservoir quality in the SW Barents Sea, but is thin (∼28 m) and not laterally extensive in the study area. The Triassic Kobbe and Snadd Formations are several hundred meters thick, but their reservoir quality is significantly poorer. The main objective was to characterize reservoir properties within these formations. Geophysical well logs from 8 exploration wells constitute the main database for the study, which is supplemented by published literature. Petrophysical analysis shows only thin reservoir intervals are present in the Kobbe Formation (up to 18 m), with poor reservoir properties (shale volume up to 37%, effective porosity up to 16%). However, most of the reservoir intervals within the formation have some amount of gas. The Snadd Formation is found to have numerous reservoir intervals, with intra-Snadd channel sandstone bodies having the largest thicknesses (up to 58 m) and good reservoir properties, albeit with relatively high shale volume (shale volume up to 36%, effective porosity up to 24%). Unfortunately, most of the Snadd Formation reservoirs are brine saturated, and only a few show traces of hydrocarbons. The Stø Formation sandstones are found to have the best reservoir qualities (shale volume up to 26%, effective porosity up to 26%), compared to the other formations. A significant oil accumulation and a significant gas accumulation exist in the Stø Formation in the study area. Rock physics diagnostics was used to estimate the degree of consolidation. Reservoirs of the Kobbe Formation are found to have 1-5% quartz cement, the Snadd Formation reservoirs are found to have 0-5%, and the Stø Formation reservoirs are found to have 0-2% quartz cement. Correlation with thin section analyses should be done to better constrain the interpretation. VP/VS versus AI and Lambda-Mu-Rho plots work well in discriminating lithology and fluid effects, although cementation, relatively high shale volumes and variance in mineralogy are thought to obscure the fluid effects, especially in the deeper reservoirs. Forward AVO modelling, combined with fluid substitution, showed that hydrocarbon saturation has the effect of producing significant AVO responses, well outside the background trend for most of the reservoirs. The sandstones in most of the reservoirs are found to have lower impedances than the overlying shales. Some of the deeper buried Kobbe reservoirs, however, are probably too consolidated to yield meaningful AVO results.