Controls on Calcite Cemented Horizons by Sequence Stratigraphy and Hydrocarbon Infilling of the Reservoir, Troll Field, Norwegian Continental Shelf

Abstract

On average 10% of the 200-m-thick main reservoir on Troll consists of calcite cemented horizons that vary from small nodules to larger laterally extensive layers. Filling of Troll field with hydrocarbons began in the west in early Tertiary and progressed to the eastern part of the field in late Tertiary. As a result, early marine calcite cements are preserved in the hydrocarbon zones of the early filled western portion of the field. Calcite cemented horizons in the eastern hydrocarbon zones and in the water zones across the field are extensively modified by influx of meteoric waters during the later burial history. Early calcite cemented horizons are associated particularly with maximum flooding surfaces and sequence boundaries. This early cement formed near the sediment-water interface mainly from seawater at 20-40C (based on {delta}{sup 18}O values of 0 to {minus}5{per thousand} PDB). {delta}{sup 13}C values of {minus}20 to {minus}40{per thousand} PDB indicate that the carbonate was derived from oxidation of methane. Texturally, early cementation is characterized by rim cements, displacive prismatic cements within biotites, and microfracturing of the clastic components. Such textures have previously been reported from caliches, but not from marine cements. Later cementation related to freshwater influx and biodegradation of hydrocarbonsmore » is probably associated with the Intra-Oigocene uplift in the eastern part of the field. Isotopic signatures indicate temperatures of 20-40C ({delta}{sup 18}O {minus}5 to {minus}12{per thousand} PDB) and carbonate sourced by methanogenesis ({delta}{sup 13}C 0 to +10{per thousand} PDB). Late cementation is characterized by blocky to poikiolotopic ferroan calcite which encloses authigenic K-feldspar overgrowths on detrital feldspar and absence of microfractures.« less