Role of forced regression in controlling Brent Group reservoir architecture and prospectivity in the northern North Sea

Abstract

The Middle Jurassic Brent Group of the northern North Sea presents a mature and highly productive reservoir play fairway where a combination of effective facies analysis and depositional sequence stratigraphy offers real potential to optimize exploitation. The north of the Brent province differs from classically studied southern areas in being dominated by marginal-marine delta-front facies. A core- and log-based study of 37 wells around the Don fields was performed to establish a sequence stratigraphic framework, map facies and thereby describe facies architecture. The results demonstrate that reservoir quality and productivity are regionally and locally controlled by facies. Of particular interest are intervals of fluvio-estuarine channel to sharp-based shoreface sandstone that formed during sea-level lowstands, since it is these packages that boost well productivity but, conversely, also increase the risk of early water breakthrough on production. Analogy with the Saloum Delta of Senegal highlights the importance of rapid and continuous barrier migration and destruction in controlling the deflection and switching of fluvio-estuarine channels, explaining also the preferential preservation of channel-floor deposits over continuously eroded barrier and delta-top facies. Interpretations suggest that deposition in the study area was dominated by punctuated progradation of the Brent Delta, as periods of delta plain incision alternated with episodes of base-level rise and delta aggradation/progradation. A model of regression for the Brent Delta is presented, where the Rannoch, Etive and Ness formations are an amalgam of highstand, falling stage and lowstand systems tract deposits, and the Tarbert Formation is a transgressive systems tract deposit, with the delta responding to regional relative sea-level changes driven by uplift and deflation of the mid North Sea dome. The prograding Brent Delta is characterized as a succession of attached shorefaces formed by alternating periods of normal and forced regression. Significantly, this explains the long distance (>200 km) build out of the Brent Delta and the continued presence of coarse-sandstone packages, as well as the potential for high-quality reservoirs even in the distal reaches of the system. It also suggests that there is limited potential for lowstand fan plays beyond the northernmost tip of the delta.

Episodic river flooding events revealed by palynological assemblages in Jurassic deposits of the Brent Group, North Sea

“…1) occupies the northern limit of the Middle Jurassic Brent Delta of the North Sea Viking Graben; a location which has previously been identified as turbidite-prone . forced regression (Olsen and Steel, 2000;Mjøs, 2009;Went et al, 2013). In addition, Wei et al (2016) identified a tidal signature within the Rannoch Formation c. 50 km to the south-east of the Penguin Cluster in the axial part of the basin, suggesting that the Brent delta was more tidally influenced than previously thought.…”

“…The sharp boundary with the underlying Broom Formation, and presence of reworked sandstone and mudstone clasts, indicate that the contact likely represents an erosional ravinement surface. This mud-prone interval as a whole records maximum flooding prior to the rapid reversion to normal and subsequent forced regression (Went et al, 2013).…”

“…6C-E), interpreted hyperpycnites and shoreface deposits separate slightly more strongly. The northwards progradation direction of the Brent Delta (Richards, 1992;Went et al, 2013) would imply that well 211/13A-8 was located closer to a palaeoshoreline/ river mouth compared to other cores and the abundant Botryococcus spp. throughout this well, which originates from a non-marine source, supports this.…”

Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

“…This succession is capped by (5) low-angle to planar parallel stratified sandstone beds, often rooted, and interpreted as beach or "foreshore" deposits, which may be overlain by coals. While this model is robust as a general description and has predictive power in subsurface settings (Hodgetts et al 2001;Bullimore & Helland-Hansen 2009;Went et al 2013), significant complexity and variability exists on the intra-parasequence scale and within the individual facies belts. Intraparasequence complexities n such deposits include the presence of bedsets, which are upwardcoarsening discontinuities related to minor sea-level fluctuations or variations in climate or sediment supply (Storms & Hampson 2005;Sømme et al 2008), the presence of river-dominated deltaic intervals within an otherwise wave-dominated shoreface (Hampson & Storms 2003;Charvin et al 2010;Ainsworth et al 2011;Eide et al 2014) or the presence of discontinuous mudstone beds within shoreface sandstones (Eide et al 2014).…”

Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

“…This succession is capped by (5) low-angle to planar parallel stratified sandstone beds, often rooted, and interpreted as beach or 'foreshore' deposits, which may be overlain by coals. While this model is robust as a general description and has predictive power in subsurface settings (Hodgetts et al 2001;Bullimore & Helland-Hansen 2009;Went et al 2013), significant complexity and variability exists on the intra-parasequence scale and within the individual facies belts. Intraparasequence complexities in such deposits include the presence of bedsets, which are upwards-coarsening discontinuities related to minor sea-level fluctuations or variations in climate or sediment supply (Storms & Hampson 2005;Sømme et al 2008), the presence of river-dominated deltaic intervals within an otherwise wave-dominated shoreface (Hampson & Storms 2003;Charvin et al 2010;Ainsworth et al 2011;Eide et al 2014) or the presence of discontinuous mudstone beds within shoreface sandstones (Eide et al 2014).…”