Snøhvit field is located in the SW Barents Sea and comprises a reservoir in Lower to Middle Jurassic sandstones of Stø Formation containing gas with thin oil lag. The sandstones of Upper Triassic to Lower Jurassic Nordmela and Tubåen Formations have also shown gas in few wells. Several stages of uplift and erosion of the entire region resulted in dramatic changes in reservoir rock properties and petroleum system. Hence, analysis of this sedimentary basin as a normally subsiding basin would mislead the exploration results. An integrated approach that incorporates compaction analysis, rock physics diagnostics, AVO modeling and post-stack seismic inversion has been carried out to predict reservoir properties of the Snøhvit field. Six exploration wells drilled in the study area and three 2D seismic lines have been considered for detail compaction, AVO modeling inversion and detail rock physics analyses.
Naturally compacted rocks in the well 7121/5-1 indicate transformation in compaction domain (MC to CC) at present depth 1922 m (BSF) corresponding to present day temperature of 66⁰C. An abrupt velocity increase at this depth is inferred as a result of grain framework stiffening related to precipitation of micro-quartz cement. The higher velocity-depth gradient of this sediment than laboratory experimental curves is related to the burial history and subsequent uplift. However, the estimated exhumation in the studied area varies between 300 and 760 m. The present day temperature of well 7121/5-1 at transition depth after exhumation is 93⁰C which is quite enough to change the rock stiffness. But, the present day temperature (50⁰C) at transition depth in well 7120/5-1 indicates the paleo-temperature history in this basin was different.
The velocity of the Stø Formation in the eastern well is lower than the western well. The eastern well is located more close to the shore line which may control deposition of coarse and well sorted sediments. Hence, the influence of compaction (both mechanical and chemical) is lower in the east than west. However, the reservoir quality of the Stø Formation is decreasing from east to west. Moreover, the reservoir quality is changing because of vertical lithological alterations. The rock physics template is not good for lithology identification, but good enough for fluid separation. The stø reservoir in the studied area has been overconsolidated. Hence, the fluid separation using the rock physics template is very difficult and risky (in exploration phase).
The sensitivity analysis of different fluid saturations indicates substantial change of effective rock properties when added only 10% gas into a gas-water system, mainly because of changing the effective fluid modulus. Higher gas saturation (50 and 90%) has slightly changed the fluid modulus as resulted from synthetic seismic traces. The lateral variation of the cap rock (Fuglen Formation) elastic properties greatly controls the AVO characteristics of the reservoir rock (Stø Formation). Lithological heterogeneity and lateral thickness variation changed the AVO response. The diagenetic changes (different depth level reservoirs) within reservoirs also affect AVO responses. Moreover, the AVO gas sand classes based on reflectivity and intercept-gradient cross-plots have given a quick preliminary AVO evaluation of the Stø reservoirs.
The post-stack seismic inversion shows the low impedance within the reservoir (Stø Formation) zone compared to the upper and lower units. Moreover, inversion is useful to track laterally the lower impedance hydrocarbon filled reservoirs. Thus, it can be concluded that the geophysical techniques help to understand the reservoir, leading to its proper management and better evaluation of new structural and stratigraphic prospects for exploration and development planning.