Imaging reservoir quality of Alka discovery, Norwegian Barents Sea : Petrophysical, rock physical and AVO modeling approach

The main focus of this study is to image reservoir quality of the ‘Alka discovery located in the southern part of the Hammerfest Basin, Norwegian Barents Sea. A total of five exploration wells (7120/12-1, 7120/12-2, 7120/12-3, 7120/12-4 and 7120/12-5) drilled in the study area of which four of them (7120/12-1, 7120/12-2, 7120/12-3, 7120/12-5) are located in the Hammerfest Basin, while the other one (7120/12-4) is located in the Finnmark Platform. The main reservoir unit is the Stø Formation of Middle Jurassic sandstones while U. Jurassic Hekkingen Formation acts as the main source rock for hydrocarbons. Four other potential reservoirs (Nordmela, Tubåen, Fruholmen and Snadd) are also present in the study area of Middle-Upper Triassic to Middle Jurassic age. An integrated approach is used to investigate the reservoir quality of the five reservoirs of Kapp Toscana Group though the main emphasis is given to the Middle Jurassic Stø Formation; the prime reservoir in the Norwegian Barents Sea. Petrophysical analyses, rock physical diagnostics and AVO modeling are utilized to investigate the reservoir quality using data from the five exploration wells. The Stø Formation shows good reservoir quality because of high net-to-gross ratio and less amount of shale present in the formation. However, the quality of the reservoir varies with depth and location of the wells. The thin interbedded shales present within the Stø Formation reduce the quality of the reservoir sandstone at certain places. Petrophysical analyses indicate that the existence of gas saturated zones in the Stø Formation are present in the wells 7120/12-2 and 7120/12-3. The reservoir intervals consist of clean, fine to medium grained and well sorted sandstones. Nordmela, Tubåen and Fruholmen Formations have moderate reservoir quality due to a higher amount of shale content and intermediate value of net-to-gross ratio. The Snadd Formation has poor reservoir quality as depicted by low net-to-gross ratio, high percentage of shale and poor sorting. A series of rock physics diagnostics have been employed to study the porosity, clay content, cementation and fluid saturation of the five reservoir horizons of Kapp Toscana Group. The crossplot of porosity versus Vp shows that the Stø Formation in the wells 7120/12-5 and 7120/12-1 is more cemented as compared to the other two wells 7120/12-2 and 7120/12-3. In the wells 7120/12-2 and 7120/12-3 the gas saturated zones can be identified by low Vp in the porosity versus Vp cross plot. Although the cross plot of AI versus Vp/Vs is an excellent indicator to discriminate fluid types but the gas saturated zones identified earlier fall on the water saturated line in AI versus Vp/Vs cross plot. However, the cross plot of Lamda-Rho and Mu-Rho clearly shows that these gas saturated zones have lower values of Lambda-Rho (incompressibility) and Mu-Rho (rigidity). AVO modeling indicates that change in rock properties are more pronounced by introducing 10% gas saturation than at higher percentages of gas. The top of the Stø Formation exhibits the Class IV gas sand and the zero offset reflection coefficient increases with increase in gas saturation. The outcomes of the study is to enhanced understanding of quality of the reservoir rocks by integrating cross disciplinary techniques including petrophysical analyses, rock physics diagnostics and AVO modeling which may not be that developed at the time when the Alka discovery was discovered. As the study area has experienced several phases of uplift and erosion which may have consequence on reservoir quality, maturity of source rock and reservoir pressure, thus compaction study and exhumation estimation are necessary to understand better the associated uncertainties and to reduce the risk for further exploration and development in the study area.