Compaction, rock property evolution and rock physics diagnostics of Askeladd discovery, Norwegian Barents Sea

Abstract

Geologically, Barents Sea consists of platform areas and basins, covered by significant amounts of sedimentary rocks ranging from Paleozoic to Cenozoic age. Several phases of uplift have been occurred, highly influenced the petroleum systems in the region. Askeladd discovery is located in the Hammerfest Basin, South Western part of the Norwegian Barents Sea. In this study, reservoir characteristics of Stø Formation is evaluated according to compaction trends and rock physics diagnostics of five available exploration wells drilled in the area. Compaction processes change the physical properties of rocks such as velocity, density and porosity. Although investigating the compaction trends (velocity/density/porosity versus depth)in the Askeladd discovery reveals the fact that velocity and density increase with depth and porosity reduce as expected as response to mechanical and chemical compaction. Several other parameters such as overpressure, clay mineralogy and organic rich source rock cause variation in compaction trends compared to general compaction curve in the study area. Transition from mechanical to chemical compaction has taken place in the Knurr Formation and its depth increases slightly toward North and reaches 1770 m (BSF) in well 7120/8-3. In order to correct the burial depth after the basin uplift, exhumation estimation is performed in different wells across the Askeladd discovery by applying different published depth trends. Exhumation estimate increase toward North and reaches its maximum at well 7120/8-3 (900 m burial depth). Rock physics make a link between geophysical observable to geological parameters and nowadays becomes an important part of reservoir characterization. Various rock physics models have their own benefits and limitations. Fluid and lithology discrimination are carried out for Stø reservoir by applying different rock physics templates(RPTs). By plotting acoustic impedance (AI) versus Vp/Vs ratio, data points concentrate within a narrow zone indicating high AI and Vp/Vs ratio suggest that application of rock physics template in the study area needs significant modification compared to generalized RPTs. Overconsolidation of the reservoir due to quartz cementation results in high values of AI which causes a great deal of ambiguity for lithology and/or fluid discrimination. Therefore, rock physics diagnostic and its application in the Askeladd area is highly dependent on quality of input data as well as model assumptions.