No abstract

The impact of a pre‐existing rift fabric on normal fault array evolution during a subsequent phase of lithospheric extension is investigated using 2‐D and 3‐D seismic reflection, and borehole data from the northern Horda Platform, Norwegian North Sea. Two fault populations are developed: (i) a population comprising relatively tall (>2 km), N‐S‐striking faults, which have >1.5 km of throw. These faults are up to 60 km long, penetrate down into crystalline basement and bound the eastern margins of 6–15 km wide half‐graben, which contain >3 km of pre‐Jurassic, likely Permo–Triassic, but possibly Devonian syn‐rift strata; and (ii) a population comprising vertically restricted (<1 km), NW‐SE‐striking faults, which are more closely spaced (0.5–5 km), have lower displacements (30–100 m) and not as long (2–10 km) as those in the N–S‐striking population. The NW‐SE‐striking population typically occurs between the N‐S‐striking population, and may terminate against or cross‐cut the larger structures. NW–SE‐striking faults do not bound pre‐Jurassic half‐graben and are largely restricted to the Jurassic‐to‐Cretaceous succession. Seismic‐stratigraphic observations, and the stratigraphic position of the fault tips in both fault populations, allow us to reconstruct the Late Jurassic‐to‐Early Cretaceous growth history of the northern Horda Platform fault array. We suggest the large, N‐S‐striking population was active during the Permo–Triassic and possibly earlier (Devonian?), before becoming inactive and buried during the Early and Middle Jurassic. After a period of relative tectonic quiescence, the N‐S‐striking, pre‐Jurassic fault population propagated through the Early‐Middle Jurassic cover and individual fault systems rapidly (within <10 Ma) established their maximum length in response to Late Jurassic extension. These fault systems became the dominant structures in the newly formed fault array and defined the locations of the main, Late Jurassic‐to‐Early Cretaceous, syn‐rift depocentres. Late Jurassic extension was also accommodated by broadly synchronous growth of the NW‐SE‐striking fault population; the eventual death of this population occurred in response to the localization of strain onto the N–S‐striking fault population. Our study demonstrates that the inheritance of a pre‐existing rift fabric can influence the geometry and growth of individual fault systems and the fault array as a whole. On the basis of observations made in this study, we present a conceptual model that highlights the influence of a pre‐existing rift fabric on fault array evolution in polyphase rifts.

The Troll Field in the Norwegian North Sea is one of the largest offshore gas fields in the world. Its western part contains a thin but exploitable oil leg (11–26 m) below the thick gas column, with the majority of the oil located in the late Jurassic Sognefjord Formation. The reservoir geology of the Troll Field was discussed in a few papers prior to production start-up in 1995, but a comprehensive account of the geological model of the Late Callovian – Late Oxfordian Sognefjord Formation has not as yet been published. The present paper reviews the depositional setting and architecture of this reservoir unit based on integration of results from disciplines such as sedimentology, seismic data analysis and stratigraphy. The Troll West reservoir formed on the edge of the Horda Platform during the Late Jurassic rift event; it consists of numerous stacked and generally offlapping sandstone units with intervening finer-grained deposits. The reservoir succession contains three composite sequences, of which the lower two belong to the Sognefjord Formation and the upper one is part of the Draupne Formation. Within this framework, five basic sequences and fifteen reservoir zones occur at the systems tract scale. The lower to middle parts of the studied succession reflect southwestwards growth and decay of a coastal spit system flanked to the east by a tidal backbasin. Brackish water facies have been identified in the eastern parts of the field through the use of detailed palynology. The extensive well database (including 15 cored production wells) and the high quality of the seismic data in the Troll area provide a unique opportunity to gain insight into the evolution of such a major spit-backbasin to tide-dominated deltaic system. A typical spit/strandplain progradation episode results in a clinoform succession comprising bioturbated sands of offshore transition origin overlain by lower shoreface sandstones which pass upward into clean, generally coarser-grained sands of upper shoreface and foreshore origin. The bases of such regressive sandbodies are often sharp, due to rapid facies translations during forced regression, but they may also be characterized by up to 40 m thick coarsening-upwards successions containing alternating siltstones and sandy event-beds generated by storm winnowing of the spit platform. Eastwards, these shallow-marine sandstone units finger into heterogeneous coastal plain deposits. On seismic data, this lateral transition is portrayed by the change from clinoform units in the west to undulatory seismic patterns in the east, as seen on maps based on classification of trace shape. A change in coastal morphology from wave- to tide-dominated took place in the Troll West area in the Late Oxfordian, perhaps related to an increase in tidal range. In the tidal facies tract, heterolithic inshore deposits formed in tidal channels and flats are replaced, in a seaward direction, by more sand-dominated high-energy tidal deposits such as mouth-bars and sand-ridge complexes. Further seawards, muddy shelf or prodelta deposits similar to those found in the wave-dominated facies tract are seen. In the Kimmeridgian, relative sea-level rise and eastward tilting of the Horda Platform caused reworking of the upper part of the Sognefjord Formation and localized hanging-wall shoreline sands formed as the uppermost reservoir intervals on Troll West.

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.