Complex stratigraphic fill of a small, confined syn-rift basins: an Upper Jurassic example from offshore mid-Norway
Authors: Jones George E. D. , Welbon Alastair I. F. , Mohammadlou Hossein , Sakharov Alexander , Ford John , Needham Tim , Ottesen Christian 
Published: September 2020

Extensional rift basin systems have been a focus of study for hydrocarbon exploration and have provided significant economic reserves in recent decades, which has led to advances in our understanding of their tectono-sedimentary evolution. However, with the increasing maturity of such settings, focus is shifting from pre-rift structures to the underexplored syn- and late-rift plays. This change in focus brings with it a significant increase in complexity when trying to develop an integrated understanding of the sedimentological, sequence stratigraphic and structural conditions that control the distribution of syn-rift reservoirs. Here we present a subsurface example from offshore Norway of a small, confined, syn-tectonic basin sourced from a local basement high. 3D seismic data, sedimentary facies analysis and stratigraphic correlations based on well and core data from six key wells constrain the local and regional tectonic controls. The PL586 licence partners drilled several wells between 2014 and 2015 targeting syn-rift Upper Jurassic stratigraphy on the southern Halten Terrace. The wells penetrated a complete syn-rift stratigraphic sequence, which provided an excellent subsurface record of the inherent complexities encountered in small, confined (3 by 5 km) syn-rift basins. Seismic data reveals that the Late Jurassic hanging-wall basin architecture and the footwall hinterland of the Frøya High were controlled by activity on the basin-bounding Vingleia Fault Complex. Facies analysis from well and core data reflects a complete syn-rift succession. The early-rift succession comprises mainly mud and siltstone with minor fine-grained sandstones interpreted to be deposited by sediment gravity flows in isolated mini-basins. The peak-rift deposits comprise coarse-grained sediment gravity flows that were sourced locally from the uplifting footwall and deposited as coarse-grained submarine fan systems in the immediate hanging wall. Significant footwall uplift during the peak-rift phase resulted in fault scarp instability and the emplacement of several large landslide complexes into the basin. The late-rift phase is dominated by a mud-rich sequence with minor coarse-grained apron fans shedding sediment off the basin bounding fault and shoreface deposits flanking the hanging-wall dip-slope and subtle intra-basinal highs. This subsurface study from the southern Halten Terrace is significant in the number of wells that have specifically targeted syn-rift stratigraphy and the level of detail at which it captures the stratigraphic variability of these deposits within a small, confined basin system. Integration of seismic geomorphology and well data with robust provenance, dating and stratigraphic correlation has enabled the development of a tectono-stratigraphic model specific to the scale and local tectonic setting. The model reveals that structural evolution ultimately controlled the development of sediment supply routes, their age and distribution, and allows several conclusions to be drawn:

The stratigraphy of small, confined syn-rift basins is broadly comparable with more typically documented larger syn-rift settings, although lateral facies changes occur more rapidly.

The cannibalistic nature of syn-rift systems, where a large volume of pre- and syn-rift sediments are uplifted and eroded then rapidly re-deposited makes determining the relative age of laterally variable depositional settings difficult.

In order to adequately constrain the timing and depositional extent of syn-rift stratigraphy, it is essential to understand the structural evolution of the system and integrate this information with other disciplines and datasets. This integrated understanding will ultimately improve predictions of reservoir presence and quality.