Silicate and associated cements in an Oxfordian marine—freshwater transition, Inner Moray Firth, UK North Sea

Abstract

Wells 13/28-2, 13/28-3 and 13/29-1, located in the Inner Moray Firth, penetrated a predominantly arenaceous marine to non-marine sedimentary complex of Oxfordian age banked up against an argillized Caledonian granite. These sediments exhibit a close relationship between the pattern of mineral diagenesis and the physico-chemical conditions of the original depositional environment. Both marine and non-marine lithofacies have undergone a complex alternation of mineral cementation and grain dissolution. The marine lithofacies is dominated by silicate cements. The earliest cements consist of chert, phosphate and pyrite; these were followed, sequentially, by fine-grained quartz and minor mica, quartz overgrowth, dolomite (non-ferroan to ferroan), and calcite (non-ferroan to ferroan). The non-marine lithofacies is dominated by clay mineral cements. The earliest cement is kaolinite, followed by quartz overgrowth cement, then by mica cement and, finally, by a late phase of kaolinite cement. The porewaters responsible for the non-marine mica cement have caused extensive argillization of the granite occurring adjacent to the non-marine sediments in Well 13/29-1. K-Ar radioisotope dating suggests that the precipitation of the mica cement and the granite argillization both took place sometime during the late Early Cretaceous (95–115 Ma, late Aptian to Albian). The close correlation between style of mineral diagenesis and depositional environment in this sedimentary complex developed during burial within an essentially sealed and overpressured reservoir in which the different facies stewed in their own porewaters. At between 4000 and 6000 ft of burial, minor tectonic perturbations caused the limited invasion of the marine sediments by non-marine porewaters and of non-marine sediments by marine porewaters. These were responsible for the presence locally of (i) allochthonous non-marine cements (mica, late kaolinite) within the marine lithofacies, and (ii) allochthonous marine cement (ferroan calcite) within the non-marine lithofacies. Prior to the entry of hydrocarbons during the Early Tertiary at a burial depth of between 6000 and 9000 ft, the sandstones experienced a major loss of pore-fluid pressure, localized collapse, and the precipitation of allochthonous baryte cement in both the marine and non-marine lithofacies.