Compaction and evolution of rock properties and rock physics diagnostics of Albatross discovery, SW Barents Sea

• Main Author: Butt, Arif
• Other Authors: Nazmul Haque Mondol
• Format: Master Thesis
• Language: English
• Published: 2012
• Subjects: VDP::450; Barents Sea; Nordmela; Norway; Stø; Tubåen; Hammerfest; Hammerfest Basin; Snøhvit; midnight sun
• Online Access: http://hdl.handle.net/10852/34250; http://urn.nb.no/URN:NBN:no-33167

The Albatross discovery is located approximately 140 km northwest of Hammerfest (city of midnight sun), Norway in the central part of Hammerfest Basin, SW Barents Sea. The Albatross discovery included within Snøhvit field development project (the first gas development project in the Barents Sea) with two other discoveries, Snøhvit and Askeladd, in the area. The reservoirs contain gas and condensate in the Lower and Middle Jurassic sandstones of the Stø Formation. The study focuses compaction and rock perperties evolution of the whole sedimentary package penetrated by six exploration wells and to investigate physical and acoustic behavior of the reservoir sandstones applying rock physics diagnostics tool. In addition, an uplift estimation of the greater Snøhvit area (Snøhvit, Askeladd and Albatross) has also been performed. An integrated approach using well log data, published compaction trends and rock physics diagnostics methodology has been utilized in order to understand the evolution of rock properties with increasing burial and to investigate physical and acoustic behavior of reservoir sandstones of the Stø Formation. Bottom hole temperature has been used to infer the transition zone temperature from mechanical to chemical compaction. On the basis of transition from mechanical to chemical compaction, an estimation of eshumation is investigated. The sandstones of Nordmela, Tubåen and Fruholmen Formations, other possible reservoir rocks in the area, have also been investigated by rock physics diagnostics techniques. Results from this study clearly show that due to the combine effect of mechanical and chemical compaction, the rock properties such as velocity, density and porosity altered significantly as a function of depth. On the basis of abrupt velocity increase within a narrow depth interval the transition from mechanical to chemical compaction has established. The transition from mechanical to chemical compaction occurred within the Knurr Formation at varying depth and temperature depending upon geothermal gradient and structural configuration. The abrupt velocity change reflects stiffening of grain framework due to quartz cementation. The quartz cementation increases with depth as long as the surface area is available for precipitation of quartz and temperature is higher than 70ºC. The detail investigation suggested that the present day transition zone temperature is far bellow the temperature usually for the transition zone from mechanical to chemical compaction reflecting the study area as an exhumed basin. It can be stated that the transition zone temperature, before exhumation, was sufficient enough to initiate the chemical compaction. When observed Vp versus depth trends of all wells have been compared with published compaction trends, there was a clear mismatch observed. On the basis of difference between compaction published trends for normally compacted basin and trends observed in studied wells a rough exhumation was estimated. The exhumation estimates differ for Snøhvit, Albatross and Askeladd discoveries depending upon the structural configuration. The exhumation of Snøhvit field is in between 300 to 800 m increasing from west to east whereas in the Albatross discovery it increases in opposite direction ranging from 700 to 1000 m. In the Askeladd discovery it ranges from 300 to 1000 m and decreasing from south to north. This exhumation estimation is in accordance with the published literature The reservoir quality of Stø sandstones, investigated by rock physics diagnostics, decrease from eastern wells to western wells with different depth levels. This change in the quality of sandstones is due to the lithological variations within the Stø Formation. The depositional history suggested that the eastern wells are close to the shoreline (proximal zone) than the western wells (distal zone), controlling the deposition of coarser and well sorted sediments causing decrease in velocity. Hence, the impact of compaction (mechanical and chemical compaction) of Stø sandstones is lower in the east compared to west. Due to different deposition history resulted the grain sorting and variable geothermal gradient, the composition may different that also reflects different degree of cementation and, hence, different rock physical properties. Over-consolidation of reservoir rocks due to quartz cementation resulted in high impedance sandstones difficult to discriminate the effect of pore fluids. The study demonstrates that the complex burial history of Hammerfest basin uplift, erosion and renewed burial during Cenozoic time has influenced the distribution of hydrocarbons in the reservoirs and the position of fluids contacts. Exhumation suggested in this study can be used to calibrate the porosity/density/velocity versus depth relationships used in reservoir characterization work flows and also in assessing the degree of tertiary migration from traps due to exsolution of gas.