Clinoform geometry, geomorphology, facies character and stratigraphic architecture of a sand‐rich subaqueous delta: Jurassic Sognefjord Formation, offshore Norway

Abstract

The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta-scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud-prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta-scale subaqueous clinoforms are common in recent tide-influenced, wave-influenced and current-influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient deltascale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super-giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening-upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave-dominated or current-dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic 'envelope' of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform 'envelope' rolls over to define a topset-foreset-toeset geometry. All clinoforms are consistently oriented sub-parallel to the edge of the Horda Platform (N005-N030). In the eastern half of the field, individual foresets are relatively gently dipping (1°to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine-grained, hummocky crossstratified sandstones. Towards the west, clinoforms gradually become steeper (5°to 14°) and bound thicker (15 to 60 m) clinothems that comprise mediumgrained, cross-bedded sandstones. Topsets are consistently well-developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near-linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north-east and sculpted by the action of currents sub-parallel to the clinoform strike.

Three-dimensional modeling of clinoforms in shallow-marine reservoirs: Part 1. Concepts and application

“…This ratio is predicted to be influenced by the calcite-cemented concretionary beds that are abundant in the Sognefjord Formation (Kantorowicz et al, 1987;Lien et al, 1992;Evensen et al, 1993). These are present within delta-front parasequences, which are seismically imaged as clinoform sets, and along their bounding flooding surfaces (Gibbons et al, 1993;Bakke et al, 1996;Dreyer et al, 2005;Holgate et al, 2014;Patruno et al, 2015). The Jurassic Bridport Sand Formation, a close sedimentologic analog present onshore United Kingdom, contains similarly abundant calcite-cemented concretionary beds.…”

“…Each couplet of fine-grained, micaceous sandstone and overlying coarse-grained sandstones corresponds to the lower and upper part of a single delta-front parasequence (Dreyer et al, 2005). The 3-D seismic data image laterally extensive (up to 30 km [19 mi] along depositional strike), nearlinear, north-northeast-south-southwest-trending clinoforms that dip west-northwestward at 1.5°-4° ( Dreyer et al, 2005;Patruno et al, 2015). The structure of the Troll West reservoir is defined by two rotated fault blocks that formed after reservoir deposition, and the reservoir is further segmented by smaller postdepositional faults that trend west-northwesteast-southeast to north-northwest-south-southeast (Dreyer et al, 2005) (Figure 12B).…”

“…The Sognefjord Formation is interpreted to record deposition in a mixed fluvial-, tide-, and wave-influenced delta system (Dreyer et al, 2005;Patruno et al, 2015). The formation is up to 170 m (558 ft) thick in the Troll Field and consists of five, vertically stacked regressive-transgressive successions bounded by major flooding surfaces (informally referred to as the 2-, 3-, 4-, 5-and 6-series in the reservoir; Figure 12C) (Dreyer et al, 2005).…”

Three-dimensional modeling of clinoforms in shallow-marine reservoirs: Part 2. Impact on fluid flow and hydrocarbon recovery in fluvial-dominated deltaic reservoirs

“…Seismic data may, in some cases, identify a change in lithology at, or across clinoforms (e.g., Dreyer et al, 2005;Holgate et al, 2014;Patruno et al, 2015), but such changes are not consistently imaged. Similarly, core and wireline-log data may allow the identification of clinoforms at a limited number of locations within the reservoir, but the extent of barrier coverage away from well penetrations is difficult to evaluate.…”

“…Such effects cannot be captured simply by modifying effective (single-phase) permeability. Although seismic reflection data may allow the position of clinoforms to be mapped in some shallow-marine reservoirs (e.g., Dreyer et al, 2005;Patruno et al, 2015), clinoform distribution is at best only partially resolved (e.g., Holgate et al, 2014). To gain further understanding of the impact of varying clinoform distribution, oil recovery was simulated in models with spacing between clinoforms of either 25, 50, or 100 m (82, 164, or 328 ft) in models of a single delta-lobe deposit (parasequence 1.6, Figure 3C).…”