Sub-seismic scale characterization of turbidite sandstone architecture and facies in base-of-slope onlaps: A case study in the Battfjellet Formation, Svalbard

Abstract

The boundaries of deep-marine fan deposits have been targets of study over the past few years due to their potential of stratigraphic traps in the oil and gas exploration. The onlap of these deposits on the base-of-slope is one of these boundaries. Although base-of-slope onlaps are considered one of the most common stratigraphic traps mechanism, the internal architecture of these configurations is still unraveled due to the shortage of outcrops and the low-quality of seismic image unable to reveal key internal sedimentary features. This study aims to define the architecture based on facies associations using recollected data which includes sedimentary field logs, photo-panels and field notes from the well-preserved outcrop of the Clinoform 14 part of the Batfjellet formation. This analysis shows a classification of 11 lithofacies comprising 5 facies associations. The facies association analysis suggests that these up-dip pinch-out traps belong to a scour field in the channel-lobe transition zone environment (CLTZ), a zone where erosion, by-pass and infilling are continuously taking place. A division of proximal, middle and distal for this CLTZ is proposed to differentiate the transition of the facies. Base-of-slope architecture comprises several incision surfaces, given by different episodes of scouring. The facies that are infilling the scours in the proximal CLTZ, close to the source, are mainly comprised by thin bedded amalgamated sandstones with grain sizes from very fine to coarse interbedding with thin horizons of siltstones and mudstones. The facies infilling the scours in the medial CLTZ, where major density of scouring is observed, encompasses facies associations from thin to thick amalgamated sandstones, thin bedded sandstones, grain sizes from very fine to coarse and interbedding thin horizons of siltstones and mudstones and several episodes of slumping at the top of the infill. Finally, the facies next to the distal CLTZ at the proximal lobe are dominated by thick bedded amalgamated sandstones and the occurrence of the scours in this area is slighter in comparison with the middle zone. The understanding of the internal characteristics of the base-of-slope onlaps is helpful to approach the geological risks, mainly in trap closure and sealing, encountered during the exploration of this potential reservoirs.