Changes

The geology of the western [[Barents Sea]] is well known (Smelror et al., 2009<ref name=Smelroretal2009>Smelror, M., O. V. Petrov, G. B. Larsen, and S. Werner, eds., 2009, Atlas—Geological history of the Barents Sea: Geological Survey of Norway, Trondheim, 135 p.</ref>; Henriksen et al., 2011<ref name=Henriksenetal2001 />, and references therein). The geological description of the study area given here is largely constrained to the elements that are significant for the understanding of leakage from structural traps of the [[Jurassic]] play. The location of the study area and the main structural elements (from Gabrielsen et al., 1984<ref name=Gabrielsenetal1984>Gabrielsen, R. H., R. B. Faerseth, G. Hamar, and H. C. Rønnevik, 1984, Nomenclature of the main structural features on the Norwegian Continental Shelf north of 62nd parallel, in A. M. Spencer, S. O. Johnsen, A. Mørk, E. Nyséther, P. Songstad and Å. Spinnangr, Petroleum geology of the North European margin: Norwegian Petroleum Society, Graham & Trotman, London, p. 40–60.</ref>) are shown in [[:file:M114CH10FG01.jpg|Figure 1]]. Ostanin et al. (2012<ref name=Ostaninetal2012a />) later separated the [[fault]]s in the area in four classes: first-order faults off-setting the Jurassic reservoir units and extending to the top of the [[Cretaceous]] and sometimes to the Upper Regional Unconformity (URU), second-order faults that offset [[reservoir]] rocks but do not extend to the top Cretaceous, the polygonal faults, and the [[Paleocene]] to [[Eocene]] faults that do not connect to the deeper faults.

The geology of the western [[Barents Sea]] is well known (Smelror et al., 2009<ref name=Smelroretal2009>Smelror, M., O. V. Petrov, G. B. Larsen, and S. Werner, eds., 2009, Atlas—Geological history of the Barents Sea: Geological Survey of Norway, Trondheim, 135 p.</ref>; Henriksen et al., 2011<ref name=Henriksenetal2001 />, and references therein). The geological description of the study area given here is largely constrained to the elements that are significant for the understanding of leakage from structural traps of the [[Jurassic]] play. The location of the study area and the main structural elements (from Gabrielsen et al., 1984<ref name=Gabrielsenetal1984>Gabrielsen, R. H., R. B. Faerseth, G. Hamar, and H. C. Rønnevik, 1984, Nomenclature of the main structural features on the Norwegian Continental Shelf north of 62nd parallel, in A. M. Spencer, S. O. Johnsen, A. Mørk, E. Nyséther, P. Songstad and Å. Spinnangr, Petroleum geology of the North European margin: Norwegian Petroleum Society, Graham & Trotman, London, p. 40–60.</ref>) are shown in [[:file:M114CH10FG01.jpg|Figure 1]]. Ostanin et al. (2012<ref name=Ostaninetal2012a />) later separated the [[fault]]s in the area in four classes: first-order faults off-setting the Jurassic reservoir units and extending to the top of the [[Cretaceous]] and sometimes to the Upper Regional Unconformity (URU), second-order faults that offset [[reservoir]] rocks but do not extend to the top Cretaceous, the polygonal faults, and the [[Paleocene]] to [[Eocene]] faults that do not connect to the deeper faults.

The western Hammerfest Basin was formed as a response to a Late Jurassic to Early Cretaceous rifting episode with a largely east–west extension in the western part of the study area. This rifting had an oblique stress component that resulted in local north–south extension in the eastern part of the basin (Berglund et al., 1986; Faleide et al., 2008). A Late Cretaceous–Early Tertiary megashear system developed along the margins of the Norwegian–Greenland Sea, which resulted in local transpression and transtension along restraining and releasing bends of this shear system. Some of the Jurassic to Early Cretaceous normal faults were rejuvenated at this time period in the Hammerfest Basin (Gabrielsen, 1984; Berglund et al., 1986).

The western Hammerfest Basin was formed as a response to a Late [[Jurassic]] to Early [[Cretaceous]] rifting episode with a largely east–west extension in the western part of the study area. This [[rift]]ing had an oblique stress component that resulted in local north–south extension in the eastern part of the basin (Berglund et al., 1986<ref name=Berglundetal1986>Berglund, L. T., J. Augustson, R. Férseth, I. Gjelberg, and H. Ramberg-Moe, 1986, The evolution of the Hammerfest Basin, in A. M. Spencer, E. Holter, C. J. Cambell, S. H. Hanslien, P. H. H. Nelson, E. Nyséther, et al., eds., Habitat of hydrocarbons on the Norwegian continental shelf: Norwegian Petroleum Society (NPF): London, Graham & Trotman, p. 319–338.</ref>; Faleide et al., 2008<ref name=Faleideetal2008>Faleide, J. I., F. Tsikalas, A. J. Breivik, R. Mjelde, O. Ritzman, Ø. Engen, et al., 2008, Structure and evolution of the continental margin off Norway and the Barents Sea: Episodes, v. 31, no. 1, p. 82–91.</ref>). A Late Cretaceous–Early [[Tertiary]] [[megashear]] system developed along the margins of the Norwegian–Greenland Sea, which resulted in local [[transpression]] and [[transtension]] along restraining and releasing bends of this [[shear]] system. Some of the Jurassic to Early Cretaceous normal [[fault]]s were rejuvenated at this time period in the Hammerfest Basin (Gabrielsen, 1984<ref name=Gabrielsen1984>Gabrielsen, R. H., 1984, Long-lived fault zones and their influence on the tectonic development of the southwestern Barents Sea: Journal of the Geological Society of London, v. 141, p. 651–662.</ref>; Berglund et al., 1986<ref name=Berglundetal1986 />).

[[file:M114CH10FG03.jpg|300px|thumb|{{figure number|3}}Stratigraphy and main tectonic events of the Hammerfest Basin. TWT = two-way travel time. Modified from Ostanin et al. (2013<ref name=Ostaninetal2013>Ostanin, I., Z. Anka, R. di Primio, and A. Bernal, 2013, Hydrocarbon plumbing systems above the Snøhvit gas field: Structural control and implications for thermogenic methane leakage in the Hammerfest Basin, SW Barents Sea: Marine and Petroleum Geology, v. 43, p. 127–146.</ref>), courtesy of Elsevier Ltd.]]

[[file:M114CH10FG03.jpg|300px|thumb|{{figure number|3}}[[Stratigraphy]] and main [[tectonic]] events of the Hammerfest Basin. TWT = [[two-way travel time]]. Modified from Ostanin et al. (2013<ref name=Ostaninetal2013>Ostanin, I., Z. Anka, R. di Primio, and A. Bernal, 2013, Hydrocarbon plumbing systems above the Snøhvit gas field: Structural control and implications for thermogenic methane leakage in the Hammerfest Basin, SW Barents Sea: Marine and Petroleum Geology, v. 43, p. 127–146.</ref>), courtesy of Elsevier Ltd.]]

The litho- and chronostratigraphy of the Mesozoic and Cenozoic sequences of the Hammerfest Basin, as well as the main tectonic events and main components of the petroleum system in the area, are shown in Figure 3. The main reservoir rocks in the area, and the only ones considered in this study, are the Early Jurassic sandstones deposited in coastal plain (Nordmela Formation) and shallow marine (Stø Formation) environments. These rocks were overlain by the organic poor shales of the Fuglen Formation, which separates the Jurassic reservoirs from the main source rock in the area (Late Jurassic Hekkingen Formation). The Lower Cretaceous rocks mainly consist of shales of the Knurr and Kolje formations and are overlain by the Kolmule Formation, which has a somewhat higher silt content (Mark et al., 1999). This formation is again overlain by a Cenomanian to Campanian rock sequence, which consists of condensed calcareous units of the Kviting Formation in the central Hammerfest Basin and of claystones of the Kveite Formation elsewhere in the study area. The Kveite, Kviting, and upper part of the Kolmule formations are intersected by polygonal faults in parts of the study area, which may have served as fluid flow pathways and connected gas from underlying reservoir to the Paleocene strata of the Torsk Formation (Ostanin et al., 2012a). The Paleocene to Eocene transition is characterized by an angular unconformity, on top of which clinoforms reflect prograding sediments with shaly and some coarser material intermixed (Knutsen and Vorren, 1991). The overburden rocks thus mainly consist of shales but with occasional coarser (permeable) layers in distinct sequences. About 1 km (0.6 mi) of Cenozoic rocks have been removed from the Hammerfest Basin (Nyland et al., 1992) after maximum burial in Oligocene to Miocene times (Doré and Jensen, 1996). The amount of erosion increases eastward, with differing suggestions of the timing and amount of individual erosional episodes (Cavanagh et al., 2006). The top of the eroded rocks, termed the URU, also marks the base of the 100–300-m (328–984-ft)-thick Quaternary glaciogenic sediments. The water depth in the area is about 300 m (984 ft).

The [[lithostratigraphy|litho-] and [[chronostratigraphy]] of the [[Mesozoic]] and [[Cenozoic]] sequences of the Hammerfest Basin, as well as the main [[tectonic]] events and main components of the [[petroleum system]] in the area, are shown in [[:file:M114CH10FG03.jpg|Figure 3]]. The main [[reservoir]] rocks in the area, and the only ones considered in this study, are the Early [[Jurassic]] [[sandstone]]s deposited in coastal plain ([[Nordmela Formation]]) and shallow marine ([[Stø Formation]]) environments. These rocks were overlain by the organic poor [[shale]]s of the [[Fuglen Formation]], which separates the Jurassic reservoirs from the main [[source rock]] in the area (Late Jurassic [[Hekkingen Formation]]). The Lower [[Cretaceous]] rocks mainly consist of shales of the [[Knurr formation|Knurr]] and [[Kolje formation]]s and are overlain by the [[Kolmule Formation]], which has a somewhat higher silt content (Mørk et al., 1999<ref name=Morketal1999>Mørk, A., W. K. Dallmann, H. Dypvik, E. P. Johannesen, G. B. Larsen, J. Nagy, et al., 1999, Mesozoic lithostratigraphy, in W. K. Dallmann, ed., Lithostratigraphic lexicon of Svalbard: Review and Recommendations for Nomenclature Use. Upper Paleozoic to Quaternary Bedrock, Norsk Polarinstitutt, Tromsø, p. 127–214.</ref>). This formation is again overlain by a [[Cenomanian]] to [[Campanian]] rock sequence, which consists of condensed calcareous units of the [[Kviting Formation]] in the central Hammerfest Basin and of claystones of the [[Kveite Formation]] elsewhere in the study area. The Kveite, Kviting, and upper part of the Kolmule formations are intersected by polygonal [[fault]]s in parts of the study area, which may have served as [[fluid flow]] pathways and connected [[gas]] from underlying reservoir to the [[Paleocene]] strata of the [[Torsk Formatio]]n (Ostanin et al., 2012<ref name=Ostaninetal2012a />). The Paleocene to [[Eocene]] transition is characterized by an angular [[unconformity]], on top of which [[clinoform]]s reflect prograding [[sediment]]s with shaly and some coarser material intermixed (Knutsen and Vorren, 1991<ref name=Knutsenandvorren1991>Knutsen, S. M., and T. O. Vorren, 1991, Early Cenozoic sedimentation in the Hammerfest Basin: Marine Geology, v. 101, no. 1–4, p. 31–48.</ref>). The [[overburden]] rocks thus mainly consist of shales but with occasional coarser (permeable) layers in distinct [[sequence]]s. About 1 km (0.6 mi) of Cenozoic rocks have been removed from the Hammerfest Basin (Nyland et al., 1992<ref name=Nylandetal1992>Nyland, B., L. N. Jensen, J. Skagen, O. Skarpnes, and T. Vorren, 1992, Tertiary uplift and erosion in the Barents Sea: Magnitude, timing and consequences, in R. M. Larsen, H. Brekke, B. T. Larsen, and E. Talleraas, eds., Structural and tectonic modelling and its application to petroleum geology: Norwegian Petroleum Society Special Publications 1, p. 153–162.</ref>) after maximum burial in [[Oligocene]] to [[Miocene]] times (Doré and Jensen, 1996<ref name=Doreandjensen1996 />). The amount of [[erosion]] increases eastward, with differing suggestions of the timing and amount of individual erosional episodes (Cavanagh et al., 2006<ref name=Cavanaghetal2006>Cavanagh, A. J., R. Di Primio, M. Schenck-Wenderoth, and B. Horsfield, 2006, Severity and timing of Cenozoic exhumation in the southwestern Barents Sea: Journal of the Geological Society of London, v. 163, p. 761–774.</ref>). The top of the eroded rocks, termed the URU, also marks the base of the 100–300-m (328–984-ft)-thick [[Quaternary]] [[glaciogenic]] sediments. The water depth in the area is about 300 m (984 ft).

The hydrocarbons were largely sourced from the deep Cretaceous Tromsø Basin to the west, although minor contributions from locally mature source rocks above the deeper structures may also have occurred. The trap filling mainly occurred from Middle Cretaceous times to the time of maximum burial. The leakage that resulted in the present underfilling of traps occurred after this time. Gas exsolution from oil and pore water and gas expansion due to fluid pressure decrease during erosion also resulted in increased gas volumes in the traps and contributed to overpressure generation here (Hermanrud et al., 2013b). Hermanrud et al. (2014) used the observation that the gas–water contacts coincide with the depth of the top reservoir surface at fault intersections or relay ramps as a main argument for these positions as being leakage locations. This suggestion implies that the leakage took place late in the erosional history or after it.

The [[hydrocarbon]]s were largely sourced from the deep [[Cretaceous]] [[Tromsø Basin]] to the west, although minor contributions from locally mature [[source rock]]s above the deeper structures may also have occurred. The [[trap]] filling mainly occurred from Middle Cretaceous times to the time of maximum burial. The leakage that resulted in the present [[underfill]]ing of traps occurred after this time. [[Gas]] exsolution from [[oil]] and pore water and gas expansion due to fluid pressure decrease during erosion also resulted in increased gas volumes in the traps and contributed to [[overpressure]] generation here (Hermanrud et al., 2013<ref name=Hermanrudetal2013bHermanrud, C., J. M. Venstad, J. Cartwright, L. Rennan, K. Hermanrud, and H. M. Nordgård Bolås, 2013b, Consequences of water level drops for soft sediment deformation and vertical fluid leakage: Mathematical Geosciences, v. 45, no. 1, p. 1–30.</ref>). Hermanrud et al. (2014<ref name=Hermanrudetal2014 />) used the observation that the [[gas-water]] contacts coincide with the depth of the top [[reservoir]] surface at [[fault]] intersections or [[relay ramp]]s as a main argument for these positions as being leakage locations. This suggestion implies that the leakage took place late in the erosional history or after it.

==See also==

==See also==