A new look at the provenance of sandstones deposited on the Barents Shelf using lead isotopes in detrital K-feldspar

The analysis of sandstone provenance has been a useful methodology for oil and gas exploration for many years. As well as providing information on sediment dispersal patterns, basin tectonic evolution and improving stratigraphic correlation, its use as a means of discriminating potential areas of higher reservoir quality has been suggested (e.g. Vincent et al., 2013). A range of increasingly sophisticated techniques are now available for provenance analysis. Of these techniques, the geochemistry and geochronology of detrital grains has revolutionised the study of sandstone provenance and have vastly improved the predictive capabilities of provenance studies.

Lead isotopes in K-feldspar as a provenance tool

One such geochemical technique involves the analysis of lead isotopes in detrital K-feldspar grains. Pb isotopes in K-feldspar vary according to the age and tectonothermal history of the crust. Measured in crystalline rocks, K-feldspar Pb isotopic compositions have successfully been used to identify and define terranes which differ in their crustal evolution at a regional scale (e.g. Stacey & Stoeser 1983). Recent technical advancements mean that in situ Pb isotopic compositions of individual detrital K-feldspar grains contained within sandstones can now be measured accurately, expediently and at a precision which is useful for sedimentary provenance studies (Tyrrell et al. 2006, 2012a). Several studies have successfully applied the Pb in K-feldspar tool to confirm, complement, refine and define palaeodrainage models (Tyrrell et al. 2012b and references therein).

The main advantages of the Pb in K-feldspar technique come from the relatively labile nature of K-feldspar and the fact that it forms a framework grain in both sandstones and the eroded rocks in the sedimentary source region. K-feldspar is susceptible to mechanical and chemical breakdown during a single sedimentary cycle of weathering, erosion, transport and diagenesis. This contrasts with widely employed zircon sedimentary provenance techniques. Zircon is robust and may survive multiple cycles of mountain building, erosion, transportation and incorporation into new sedimentary rocks. Moreover, it is unevenly distributed in rocks where K-feldspar is more uniform and so introduces bias to the interpretation of zircon data. The techniques are most powerful when used in combination because the component of detritus that has only been through a single sedimentary cycle and the proportion of zircon that has been recycled can be identified (Fig. 1; Flowerdew et al. 2012).

Targeting arkosic reservoirs and tight gas reservoirs where some K-feldspar has survived secondary porosity formation with this technique may be most fruitful.

Sedimentary provenance of the Barents Shelf sandstones

The reservoir properties of sandstones on the Barents Shelf have been shown to be linked to provenance (Scott et al., 2012). The interpretation of seismic (Glørstad-Clark et al. 2011), petrographic and heavy mineral data from outcrop and well materials (Mørk 1999; Bue Pozer & Andresen 2013) has suggested that the sandstones were derived from several competing sedimentary source regions (Fig. 2). The most important of these are interpreted to be from the Baltic Shield (Caledonides and Fennoscandian Shield) and the Uralides. Although the two source regions are best distinguished from each other by some heavy mineral indicators, most notably detrital zircon ages and high abundances of some heavy mineral species, considerable uncertainty persists in linking sedimentary provenance variations with reservoir properties. Ambiguity is primarily introduced by sedimentary recycling, which has occurred within the sedimentary source region, and also by mixing and recycling more proximally to the basin.

Potential of the Pb in K-feldspar technique on the Barents Shelf

A compilation of published K-feldspar Pb isotopic data from crystalline rocks within candidate source regions for sandstones deposited on the Barents Shelf form a series of non-overlapping / poorly overlapping fields (Fig. 3). The clear Pb isotopic distinction between the Caledonian and Fennoscandian terranes potentially make the Pb in K-feldspar technique ideal for separating sedimentary provenance from these regions which may otherwise be blurred by significant sedimentary recycling. In assessing the contributions from these source regions, more accurate predictions of reservoir quality in sandstones on the Barents Shelf may be possible.