Diagenetic destruction of reservoir potential in shallow marine sandstones of the Broadford Beds (Lower Jurassic), north-west Scotland: depositional versus burial and thermal history controls on porosity destruction

Abstract

The Broadford Beds are a shallow marine mixed siliciclastic-carbonate unit up to 140 m thick that crops out in the northern Inner Hebrides adjacent to a major Tertiary igneous centre. The formation can be divided into three lithological units: a lower limestone-dominated unit, a middle unit of muddy, calcareous sandstones and an upper unit of cleaner sandstones. The limestones underwent early meteoric diagenesis and early diagenetic processes in the overlying sandstones included the precipitation of berthierine, corrensite, phosphate and a small amount of Mg calcite. The most abundant burial cement is ferroan calcite, most of which precipitated during Mesozoic burial at depths of about 1 km. These cements were sourced through the chemical compaction of depositional carbonate within interbedded muddier horizons. Later calcite cements were precipitated in fractures from meteoric fluids during Late Jurassic — Early Cretaceous inversion and Early Tertiary uplift and a final stage of calcite cementation was associated with Tertiary hydrothermal activity. Other burial diagenetic phases include quartz overgrowths and chlorite. Quartz cementation was initiated before the main phase of calcite cementation, but the most significant quartz cementation accompanied the formation of chlorite during Tertiary hydrothermal activity. Overall the most significant factor leading to porosity loss in the Broadford Beds was the abundance of detrital clays and depositional carbonate in the original sediment. This led to extensive calcite cementation that both occuluded primary pores and inhibited the formation of secondary porosity.