Sedimentological controls on the distribution and geometry of chlorite-coated intervals in deeply buried tide-influenced sandstones, Tilje Formation, Norwegian Continental Shelf

Abstract

Chlorite-coated intervals occurring in deeply buried sandstone reservoirs preserve porosity and permeability by inhibiting quartz cementation. However, the geometry of such chlorite-coated intervals and the controls on their distribution remain unclear. The Lower Jurassic Tilje Formation of the Halten Terrace, Norwegian Continental Shelf, shows sparsely distributed chlorite coats preserving high permeability at great depth. Changes between chlorite-coated and quartz-cemented intervals can occur at a centimetre scale and are thus hard to predict. In this study, detailed sedimentological investigations of 12 wells in the Smørbukk Field, integrated with biostratigraphic data and combined with permeability analysis of more than 5000 core plug samples, facilitate the interpretation of depositional elements and the tracing of chlorite-coated intervals across the field. Ten depositional environments are recognised, representing deposition as a large tide-dominated delta that prograded in an embayment for most of the Tilje Formation. A shift in depositional processes occurs towards the top of the formation, with the development of a latest Pliensbachian candidate relative sea-level fall and incision rapidly followed by the onset of a latest Pliensbachian–earliest Toarcian transgressive phase. The transgressive phase is associated with a change to an open-marine seaway and the deposition of marine tidal dunes. Two types of chlorite-coated intervals occur: (i) thick (20 to 30 m), stratigraphically bounded intervals and (ii) thin (<3 m), sparsely distributed intervals. Chlorite-coated intervals occur mostly in fluvial-tidal channels but also occasionally in other depositional environments, suggesting that sedimentological controls do not fully explain their distribution. Instead, regional controls such as the composition of the hinterland, climate, relative sea-level change, tectonics and geochemical conditions in the primary depositional environments are considered important factors. This study illustrates the various geometries of chlorite-coated intervals in the subsurface, having important implications for building predictive models of their distribution and for conventional and CO₂ reservoir characterisation.