Imaging Reservoir Properties of the Permian- Triassic- Jurassic and Cretaceous successions at Loppa High, Norwegian Barents Sea. Examples from Gohta, Salina and Skalle Discoveries

• Main Author: Viga, Odd Fossaa
• Format: Master Thesis
• Language: English
• Published: 2019
• Subjects: Permian; Reservoir Characterization; Knurr; Fruholmen; Petrophysical Analysis; Hammerfest Basin; Tempelfjorden; Reservoir; Kolmule; Compaction; Rock Physics; Net-to-Gross; Gohta; Triassic; Shale Volume; Net Pay; Stø; Water Saturation; Ørn; Røye; Fluid Effect; Erosion; Tromsø Basin; Loppa High; Discovery; Carboniferous; Reservoir Properties; Permeability; Cretaceous; Uplift; Skalle; Jurassic; Barents Sea; Norway; Salina; Kobbe; Petrophysics; Petroleum Geology; Cement Volume; Porosity; Net Reservoir; NCS; Tromsø; Loppa; Hammerfest; Tempelfjord*
• Online Access: http://hdl.handle.net/10852/70005; http://urn.nb.no/URN:NBN:no-73134

Exploration in the Barents Sea has shown to be difficult, due to the regional tectonic history to the area. Multiple uplift events, faulting and erosion make the well log data behave differently than expected in other areas on the NCS. This study focuses on the reservoirs in PermianTriassic- Jurassic and Cretaceous succession at the Loppa High area SW Barents Sea, with a, primary focus on Gohta, Salina and Skalle Discoveries. Mainly well log data from six wells has been used in the reservoir characterization: 7120/1-1, 7120/1-2, 7120/1-3 (Gohta), 7120/1- 4 S (Gohta Appraisal), 7120/2-3 S (Skalle) and 7220/10-1 (Salina). The Stø and Kolmule Formation represent the two main target reservoirs for both the Skalle and Salina Discovery, where both of these discoveries mainly consist of gas. The Gohta Discovery stands out from the two others by being located in Røye Formation, consisting of larger amount of carbonates. These formations (Kolmule, Stø, and Røye) are the main focus in the reservoir characterization, where petrophysical analysis and rock physics diagnostics are the two methods used. Petrophysical analysis techniques have been used to calculate the reservoir properties (e.g. shale volume, net-to-gross, porosity, permeability, and water saturation) in the reservoir zone, to provide an insight of the reservoir quality in the study area. The Cretaceous Kolmule Formation shows poorer reservoir quality compared with the Middle Jurassic Stø Formation, which is clearly shown in the amount of shale volume (17-23 % in Kolmule Formation and 7-13 % in Stø Formation) and the net-to-gross values (0.31-0.43 in Kolmule Formation and 0.97-0.99 in Stø Formation). The main contributor to the shale volume in the Stø Formation in the upper part, which is much shalier than the clean lower part of the formation. Both of these formations show high effective porosity values (17-23 % in Kolmule Formation and 14-22 % in Stø Formation). The Permian Røye Formation in well 7120/1-3 (Gohta Discovery) is the only formation containing a higher amount of oil. This formation differs from the two others by being located at a greater burial depth and the content of carbonate rocks. It is a result of relatively shallow burial depth of the central part of the southern Loppa High during Permian time resulting in carbonate buildups. However, the Røye Formation shows good reservoir quality in the discovery well, with relatively high effective porosity (11.5 %) and high net-to-gross value (0.551). Vp-depth trends have been utilized to estimate the uplift in the six wells in this study, based on the transition zone between the mechanical and chemical compaction. The uplift has been estimated to be between 950 m and 1150 m, which correlate well with the published data. In rock physics diagnostics different templates from literature, together with trends in the plots have been used to provide information about the lithology, cementation and fluid effects within the reservoir zones. The target reservoirs show particularly good results in the LMR plots, where the decreasing Vp in hydrocarbon zones is clear. In Røye Formation the main difference in these plots are the significantly higher values of Vp and Vs. Despite this, the formation due shows similar trends (decreasing Vp compared with Vs) in the fluid sensitivity plots (Vp/Vs versus AI, Vp versus Vs and LMR). Kolmule Formation in Skalle Discovery shows some poorer results in the rock physics diagnostics, which probably is due to high shale volume and that the hydrocarbon zone is located in the middle of the formation.