Quartz cement volumes across oil-water contacts in oil fields from petrography and wireline logs: preliminary results from the Magnus Field, northern North Sea

Quartz cement is a significant porosity-reducing mineral cement in many sandstones and thus affects economically significant reserves calculations and flow-rate (through its effect on permeability). The presence of oil in a reservoir is commonly assumed to retard quartz cement precipitation and thus early oil emplacement is often thought to preserve porosity and permeability. A combined petrographic and wireline log approach was utilized to investigate whether quartz cement volumes and the total quantity of quartz do indeed vary across the oil-water contact in a sandstone reservoir. Thin-section point-count data and bulk density, neutron porosity and sonic transit time wireline log data were obtained across the oil-water contact from three wells in the Magnus field, an Upper Jurassic turbidite sandstone reservoir in the Northern North Sea. Reported oil filled inclusions in quartz overgrowths in this reservoir show that quartz cementation occurred either during or after oil emplacement. Point count data were used to determine quartz cement and total quartz volumes across the oil-water contact, whilst wireline data were transformed to reveal the total quantity of quartz across the oil-water contact. Preliminary results seem to show that the volume of quartz cement and the total volume of quartz show little or no variation across the oil-water contact. These data seem to imply that the presence of oil in the reservoir had no appreciable effect on the component processes involved in quartz cementation in this field: a paradox that will be further investigated.