ABSTRACT

Jurassic sandstones from the oil zone in a well from the Haltenbanken area offshore mid-Norway contain both aqueous and hydrocarbon fluid inclusions within quartz overgrowths. Homogenization temperatures for the inclusions range from 92°C up to the present formation temperature of approximately 118°C. Since the inclusions mostly are located at the boundaries between quartz clasts and quartz overgrowths, the homogenization temperatures indicate that quartz cementation started at temperatures slightly below 92°C.

Poikilotopic ferroan calcite cement usually predates formation of quartz overgrowths, and most calcite cement must be precipitated at temperatures below 90°C, although in one calcite cemented zone ferroan calcite encloses quartz overgrowths with inclusions trapped at temperatures of up to 105°C.

None of the examined hydrocarbon inclusions homogenized below 102°C, and this suggests that oil started entering the reservoir at temperatures of around 100°C. The burial history of the examined sandstones indicates that temperatures of 100°C were not reached before the late Pliocene and that filling of the reservoir has taken place during the last few million years.

Homogenization temperatures approximately equal to present formation temperature suggest that quartz cementation may still be in progress, and that emplacement of oil did not necessarily stop precipitation of quartz cement. Possible continued quartz cementation after oil filled the reservoir could be explained by dissolution of quartz at stylolites and grain contacts as transport of dissolved quartz from these sources might possibly take place by diffusion through irreducible water.