Evidence for resetting of fluid inclusion temperatures from quartz cements in oilfields

Quartz is a major pore-occluding cement in North Sea oilfield reservoir sandstones. Fluid inclusions yield homogenization temperatures ( T h ) which have been thought to represent growth temperatures for the host quartz. However, early diagenetic inclusions from the detrital grain-overgrowth boundary yield excessively high palaeotemperatures which do not match the growth temperatures calculated from the depth of cementation. The temperatures of these inclusions increase with present day depth of burial, and approach present day reservoir temperatures. This suggests that the inclusions have begun to reset towards present day pressure-temperature conditions. The T h of the inclusions hence represent burial temperatures rather than growth temperatures for the quartz. Experimental studies indicate that the amount of resetting undergone by an inclusion varies depending on its size, shape and fluid composition. A relationship between inclusion T h and these variables has been observed in inclusions from deeply buried quartz cements in the North Sea.