The Effects of Oil Emplacement on Diagenetic Processes: Examples from the Fulmar Reservoir Sandstones, Central North Sea: Geologic Note (1)

Abstract

Sedimentary petrography and petroleum geochemistry studies have been done on Fulmar Formation (Upper Jurassic) cores from Fulmar field, central North Sea. The extent of quartz overgrowths and albitization of K-feldspars is lower in oil-saturated zones compared with water-saturated zones. However, microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil- and water-saturated zones (85-125 degrees C), with maximum recorded temperatures close to the current reservoir temperature (about 130 degrees C). This suggests that quartz overgrowth development in the oil-saturated zones continued after oil emplacement, but was retarded compared with development in the water-saturated zones. The quartz cement, therefore, mus have been essentially locally derived by pressure solution. Albitization of K-feldspars is significantly less (about 50%) in the oil-saturated zones compared with albitization in the water-saturated zones. Because albitization of K-feldspars depends on a supply of sodium and removal of potassium, this process is more likely to be terminated by oil emplacement. The observed difference in the albitization has been used to suggest that the Fulmar reservoir probably was filled during the Neogene. This age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins. The Fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates. This is probably because the Fulmar sandstones, being partly turbiditic, offshore sand deposits, were not exposed to meteoric water. Their close proximity to the marginally mature to mature Kimmeridge Clay Formation seems to have little effect in terms of developing secondary porosity, suggesting that the role of organic acids or CO[2] generated in these source rocks is of minor importance.