Fault scarp fan deltas, slope apron submarine fans and hanging-wall shallow marine sequences in the synrift Upper Jurassic Viking Group - Implications for reservoir characterization and geological modelling in the southern Halten Terrace, offshore Mid-Norway

Abstract

Thick synrift Upper Jurassic successions occurring in the vicinity of the recent Pil and Bue discovery wells in the southern Halten Terrace adjacent to the Vingleia Fault Complex represent submarine fans and fan-delta slope channel-lobe complexes deposited in a deep nearshore environment resulting from persistent large-scale faulting events, repeated source-area uplift and basin floor subsidence. Stratigraphic architectural elements, sand-body geometries and depositional facies occur in an organized and partly predictable manner. Detailed sedimentological facies analyses integrated with a 3-D seismic reflection dataset allows division of Oxfordian-Tithonian aged Melke and Rogn formations, and definition of two coexisting depositional systems: (1) laterally continuous slope apron and localized fan-delta systems derived from fault scarp degradation, footwall erosion and antecedent drainage networks up-dip on the Frøya High, and (2) shallow marine shoreface successions developed on the Sklinna Ridge hanging-wall dipslope. The distribution of facies is strongly related to the tectonic movements along basin bounding normal faults and resulting changes in accommodation space. The Intra-Melke Sandstone Member records fan delta initiation, growth and abandonment related to development of fault-propagation folding and subsequent monocline breaching along the Vingleia Fault during initial-, peak- and post-rift times. These geological complexities influence the seismic expression and wireline log characteristics of the data analyzed in the Greater Pil and Bue Area. An integrated reservoir characterization of the syn-rift Melke Formation has revealed large-scale reservoir heterogeneities which indicate only limited reservoir compartmentalization. Stratigraphic pinch-outs of sandstones are expected to represent the most significant type of compartments. Defining the reservoir envelope (i.e. the spatial extent of reservoir quality facies) is regarded as the main initial challenge. Integration from a well test, facies analysis and petrophysical measurements suggest that there may be a considerable upside to reserves found in the Pil reservoir based on the observation that the drill stem test interval occurs below the most oil saturated and clean sand interval in the 6406-12/3S Intra-Melke succession. A large uncertainty in reservoir quality still exists in the Rogn Formation along the footwall fault scarp attributed to sediment provenance and diagenetic alteration of immature igneous and metamorphic rock fragments. Analogue outcrops (Wollaston Forland) and more densely drilled reservoirs (Brae Field) are used to help infer spatial statistics that are unobtainable in the present subsurface reservoir dataset and to constrain several implications for future modelling on the southern Halten Terrace.