Provenance, depositional setting and diagenesis as keys to reservoir quality of the Lower Cretaceous in the SW Barents Sea

Abstract

This study examines the role of the depositional environment for the final reservoir quality in four Lower Cretaceous sandstone reservoirs in the southwestern Barents Sea by linking facies to the distribution of primary textures, composition, and diagenetic alteration. Facies analysis reveals slope-to-basin-floor, distal shallow-marine, and deltaic depositional environments. The slope-to-basin-floor sandstone has the highest porosity of 3–19% (avg. 13%). It is attributed to good sorting, non-pervasive carbonate cementation that inhibited compaction and allowed for secondary porosity through later dissolution, and moderate clay infiltration that resulted in clay cutanes on grain rims and the precipitation of chlorite (which inhibited quartz growth). For the deltaic sandstone, moderate to fluctuating energy and sediment supply provided good conditions for mechanical clay infiltration and varying porosity of 2–18% (avg. 8%). The distal shallow-marine sandstone reservoir has the lowest porosity of 1–12% (avg. 7%). Based on its fine-grained and bioturbated character, deposition in a low-energy environment with low sediment supply seems likely. The combination of fine-grained lamina, interstitial matrix and bioturbation led to porosity reduction. Abundant mica and feldspar grains in the shallow-marine sandstone, partly a result of the provenance, and deep burial also resulted in extensive illitization. High mineralogical maturity, much monocrystalline quartz in the quartz-grain populations, and similar felsic chemical rock compositions for all facies associations and wells indicate similar source rocks with some variations. Abundant mechanically unstable mica makes the nearby Loppa High a plausible catchment, which is supported by the seismic geometries. This study demonstrates that the porosity evolution of the studied Lower Cretaceous sandstone reservoirs is determined mainly by the depositional environment despite minor provenance and major diagenetic variations