The Barents Sea is an active area for frontier petroleum exploration .This study focuses on an area across the Tornerose prospect in the Hammerfest Basin, South West of the Barents Sea. Cenozoic exhumation of the entire region resulted in dramatic changes in rock properties and eventual petroleum systems therein. Analysis of this sedimentary basin as a normally subsiding basin would yield misleading results. The focus of this thesis comprise of two phases which are compaction analysis and evolution of rock properties as a function of depth coupled with AVO/AVA (amplitude versus offset/angle) evaluation and modeling of the Stø and Snadd reservoirs.
Petrophysical logs from 5 wells, published natural compaction curves and experimentally compacted mudstone and sandstone curves have been used to investigate compaction and evolution of rock properties of the area. Mechanical compaction dominates as a monotonic function of vertical effective stress from the overburden to depths of about 1530m BSF (well 7123/4–1A). An abrupt velocity increase at this depth corresponding to present day temperatures of 45.9⁰C is inferred to as resulting from grain framework stiffening related to precipitated micro-quartz released from the transformation of the clay mineral smectite to illite via mixed layer smectite – illite (SI). This marks the transition zone to chemical compaction. The high velocity/depth ratio of these sediments compared to experimentally compacted synthetic mudstones is related to the burial history and subsequent uplift. Correcting for exhumation yields estimates in the range of 1200 to 1600m.Velocity and density inversion in the organic rich Hekkingen Formation due to high pore pressures, among other factors, could possibly have contributed to the good reservoir quality in the underlying Stø reservoir sandstones.
Lateral variation of the cap rock elastic properties greatly control the AVO character of the Stø reservoir. Lithological heterogeneity within the Snadd reservoir reduces the impedance contrast with the overlying Fruholmen Formation giving low AVO response. A systematic increase in pore fluid compressibility in the Stø reservoir results in a corresponding decrease in reflection coefficients. Substituting brine with an initial 10% gas as reservoir pore fluid results in remarkable changes on seismic. However these changes are not evident for higher gas saturations of 50 to 90%. AVO modeling effectively quantifies these fluid effects at all gas saturations. The result presented herein establishes possible AVO variation trends for increasing gas saturation within the Stø reservoir. The various models within the framework of this thesis give a quick preliminary AVO evaluation of the Stø and Snadd reservoirs. With an expanding data base, more constrains could be incorporated into these elementary models.