Multi-proxy sedimentary provenance approaches to unravelling complex sand dispersal histories; examples from the Black Sea and Barents Shelf

A vital part of source-to-sink analysis comes from sedimentary provenance that can constrain the size of sedimentary systems, their routing system and help provide an understanding of sand compositional variations within the receiving basin. In this contribution, case studies from two regions with very different exploration histories, the Black Sea and Barents Shelf, explore how an improved understanding of sand dispersal and resultant reservoir properties can be gained using a diverse portfolio of provenance methods.

Black Sea Case Study

The Black Sea is an underexplored basin, where sparse well data makes the projection of outcrop-constrained sediment pathways into the offshore essential for reservoir quality prediction. Sediments derived from the north, from the East European Craton (EEC), have the potential to form high quality, quartz-rich reservoirs, like they do in the adjacent South Caspian Basin. As well as being directly routed into the basin via the Kerch-Taman Strait, EEC-derived sandstones were also reworked during the uplift of the Greater Caucasus as an earlier sediment trap, the Greater Caucasus Basin, was inverted. Sediments derived from this basin were mixed with sediment derived from local basement sources during the Oligocene and younger fill of the Eastern Black Sea basin. Conventional heavy mineral and detrital zircon U-Pb age analysis has enabled these source regions to be distinguished and a complex mixing model between them to be elucidated.Confidence for this mixing model is enhanced because it is independently verified by the Pb isotopic composition of framework K-feldspar grains. This is because the isotopic compositions of K-feldspar grains from the local Greater Caucasus crystalline basement and sources within the EEC are distinct. The former form a tight compositional cluster reflecting their upper crustal Variscan origin, whilst the latter fall along a compositional trendline reflective of multiple crystallisation / metamorphic events over several Ga. Sediments derived from the Greater Caucasus contain a mixture of these two end-member types with a progressive increase in local basement contributions through time.

Barents Shelf case study

On the more intensively explored Barents Shelf, the primary and most widely distributed reservoir interval comprises compositionally mature Lower and Middle Jurassic sandstones. These units are thin and have variable thickness that makes the application of well correlation and seismic interpretation difficult in source-to-sink analysis. Their deposition followed that of a thick Triassic compositionally immature clastic succession often associated with less favourable reservoir properties. The transition from the immature Triassic to mature Jurassic clastic units is attributed to tectonically-driven drainage re-organisation, which initiated widespread sedimentary reworking and rejuvenated extant sedimentary source regions.

Additional challenges to those presented in the Black Sea example are presented by the fact that mature Jurassic successions on the Barents Shelf are petrographically similar and yield detrital zircon patterns that are ineffective provenance indicators. Hence, provenance interpretations require additional heavy mineral and framework grain analysis. In this case, the former comes through combined trace element and age data on detrital rutile and trace element geochemistry on garnet, and the latter through, QEMSCAN compositions and K-feldspar Pb isotopic signatures. With these additional analyses, small sedimentary systems are delineated and indicate least buried sands with greatest direct sediment contribution from the Fennoscandian Shield yield best reservoir properties. In addition, remarkable fertility variations identified in the source rocks can miscarry provenance interpretations without a multi-proxy approach.