Does Early Oil Emplacement Stop Quartz Cementation in Deeply Buried Reservoirs? Evidence from Ula Field, Norwegian N. Sea

Quartz cement is an important porosity-occluding cement in sandstone reservoirs that are exposed to elevated temperature and effective stress for a significant period of time. The effect of oil emplacement is controversial with some concluding that early oil emplacement inhibits quartz cementation and preserve porosity in deeply buried reservoirs while others claim that quartz cementation continues unabated whether oil is present or not. In this work, we have studied shallow marine, Upper Jurassic sandstones from the Ula field in the Norwegian North Sea in an attempt to determine the effect of oil emplacement on diagenesis with particular attention to the distribution of quartz and other cements relative to the oil water contact. Instead of approaching this problem with a desired outcome, we have taken an objective approach and looked at all controls on porosity and permeability and not simply focussed on one control (i.e. the effect of oil emplacement).

Core samples collected from samples from the oil and water legs and the transition zone were studied using a range of techniques: light optics, scanning electron microscope (SEM), cathodoluminescence (CL), X-ray diffraction (XRD) and fluid inclusion UV-petrology and thermometry. The distributions of all controls on porosity and permeability have been quantified so that we have been able to eliminate the effects controls on quartz cement other than fluid type. Thus we have defined the distribution of grain size and sorting, chlorite coats, microcrystalline quartz coat and general facies variations. Using published burial and thermal histories, fluid inclusion studies have revealed that oil emplacement into the Ula reservoir commenced prior to the onset of quartz cementation. The results show that there is a consistent difference in the amount of quartz cement and porosity between the oil and water legs that cannot be explained by difference in facies, chlorite or microquartz coats etc. We must therefore conclude that oil emplacement has significantly inhibited the rate and amount of quartz cementation. Early emplacement of oil into sandstones seems to be an important control on reservoir quality.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California