Cherts, spiculites, and collapse breccias – Porosity generation in upper Permian reservoir rocks, Gohta discovery, Loppa High, south-western Barents Sea

Abstract

Spiculitic cherts are uncommon reservoir rocks and their porosity evolution is poorly understood compared to sandstones and carbonates. In the Gohta oil and gas discovery on the Loppa High in the south-western Barents Sea, the reservoir is in brecciated, silicified, and dolomitized Permian spiculites below the Permian/Triassic unconformity. It represents the infill of several collapsed cave systems with spiculite clasts in a micritic matrix, separated by intact cave roofs of shale and spiculite facies. The cave collapse was related to dissolution of less resistant (?limestone and ?spiculitic) beds, due to percolation of freshwater during latest Permian – earliest Triassic emersion. During later transformation of opal-A to opal-CT, associated growth of silica concretions left the margins of spiculite clasts depleted in SiO2 and thus highly porous. Subsequent transformation of opal-CT to quartz resulted in precipitation of texture-preserving quartz and chalcedony cements. The latter show a systematic decrease of δ18O from the first to last precipitated phase, implying crystallization under increasing temperatures during renewed Middle–Late Triassic burial. Later diagenesis includes in situ brecciation and fracturing, dolomitization affecting mainly the micritic matrix of cave-collapse facies, chemical compaction, and calcite cementation. The best reservoir properties are in cave-collapse facies (commonly 10–25% and 0.03–19 mD) where the pore system is dominated by (1) uncemented interspicule pores and central parts of spicule molds within clast margins, and (2) intercrystalline pores between dolomite crystals in the breccia matrix. The primary depositional facies have much lower porosity and permeability (rarely exceeding 10% and 1 mD, respectively). This study shows that porosity in the cave fill most likely formed by local redistribution of silica to form concretions and dissolution of the carbonate matrix to source the growth of dolomite crystals, while prolonged subaerial exposure only played an indirect role by isolating spiculite clasts and preventing their complete silicification during burial.