The development and destruction of porosity within Upper Jurassic reservoir sandstones of the Piper and Tartan Fields, Outer Moray Firth, North Sea

Abstract

AbstractUpper Jurassic Piper Formation sandstones of the Outer Moray Firth Basin, UK North Sea, form the main reservoir for the Piper and Tartan Fields, and are sealed by either the Kimmeridge Clay or Cretaceous marls. The main reservoir sandstones can be broadly considered as stacked coarsening-upward units that accumulated in a predominantly high-energy, shallow marine complex with depositional waters of inferred normal marine salinity, Eh and ionic composition. The amount of porosity and its distribution throughout these sandstones differs between the individual structural entities of the Piper and Tartan Fields. On the Piper structure all the sandstones are highly porous. Oil-zone porosities average 25% and are only slightly reduced in the water zone. Sandstones in the upthrown block of the Tartan structure are also generally porous but porosities are typically reduced to below 20%. Porosities are the most variable in the Tartan downthrown block and in the 15/17-9 well, off-structure Piper, where porous sandstones with between 14 and 18% porosity are associated with highly compacted and cemented sandstones with <5% porosity. Thin-section and SEM textural evidence indicate that much of the porosity in these sandstones is secondary, produced through the dissolution of an intergranular cement that provided support for the detrital framework, was peripherally replacive with respect to most detrital grains, and extensively replaced feldspars. Nodular concretions and sporadic, irregular crystals of ferroan calcite are inferred to be the remnants of this intergranular cement. The distribution of porosity zones in the Tartan downthrown block is related to the present structure and oil-water contact, supporting the interpretation of a secondary origin for much of the present porosity. Highly compacted sandstones interbedded with the porous sandstones result from a combination of early compaction, brittle framework collapse following the generation of secondary porosity and late grain-to-grain contact dissolution. Aggressive fluids responsible for the generation of the cement dissolution porosity are inferred to have been expelled from the Kimmeridge Clay in the adjacent Witch Ground Graben. The preservation of secondary porosity in Piper Formation sandstones is due to the relative timing of secondary porosity generation and subsequent hydrocarbon migration.