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[[file:M114CH10FG01.jpg|300px|thumb|{{figure number|1}}Location of the western Hammerfest Basin with main [[structure|structural]] elements, [[gas]] discoveries, and [[seismic]] data coverage.]]

[[file:M114CH10FG01.jpg|300px|thumb|{{figure number|1}}Location of the western Hammerfest Basin with main [[structure|structural]] elements, [[gas]] discoveries, and [[seismic]] data coverage.]]

[[file:M114CH10FG02.jpg|300px|thumb|{{figure number|2}}[[Hydrocarbon]] filling and shows occurrence in the Hammerfest Basin, (A) underfilling (shown as vertical distance between the structural spillpoint and the hydrocarbon–water contact) and (B) shows to or deeper than the structural spillpoint (presented as the vertical distance between the base of shows and the spillpoint depth). GWC = [[gas–water]] contact. Modified from Hermanrud et al. (2014<ref name=Hermanrudetal2014>Hermanrud, C., M. E. Hakjelsvik, K. Kristiansen, A. Bernal, and A. C. Strömbäck, 2014, Column height controls in the western Hammerfest Basin: Petroleum Geoscience, v. 20, p. 227–240.</ref>).]]

[[file:M114CH10FG02.jpg|300px|thumb|{{figure number|2}}[[Hydrocarbon]] filling and shows occurrence in the Hammerfest Basin, (A) underfilling (shown as vertical distance between the structural spillpoint and the hydrocarbon–water contact) and (B) shows to or deeper than the structural spillpoint (presented as the vertical distance between the base of shows and the spillpoint depth). GWC = [[gas-water]] contact. Modified from Hermanrud et al. (2014<ref name=Hermanrudetal2014>Hermanrud, C., M. E. Hakjelsvik, K. Kristiansen, A. Bernal, and A. C. Strömbäck, 2014, Column height controls in the western Hammerfest Basin: Petroleum Geoscience, v. 20, p. 227–240.</ref>).]]

==Introduction==

==Introduction==

The western Hammerfest Basin is situated in the [[Barents Sea]], north of [[Norway]] ([[:file:M114CH10FG01.jpg|Figure 1]]). It contains several [[gas]] discoveries, most (eight) of which are being produced and are collectively referred to as the [[Snøhvit Field]]. Ten of the eleven discoveries in this area were apparently not filled to their structural capacity at production start-up, and [[hydrocarbon]] shows have been noted between the [[gas–water]] contact and the depth of the structural spillpoint in all but one of the [[structure]]s ([[:file:M114CH10FG02.jpg|Figure 2]]). These observations testify to leakage as a major controlling factor for the hydrocarbon column heights in the area.

The western Hammerfest Basin is situated in the [[Barents Sea]], north of [[Norway]] ([[:file:M114CH10FG01.jpg|Figure 1]]). It contains several [[gas]] discoveries, most (eight) of which are being produced and are collectively referred to as the [[Snøhvit Field]]. Ten of the eleven discoveries in this area were apparently not filled to their structural capacity at production start-up, and [[hydrocarbon]] shows have been noted between the [[gas–water]] contact and the depth of the structural spillpoint in all but one of the [[structure]]s ([[:file:M114CH10FG02.jpg|Figure 2]]). These observations testify to leakage as a major controlling factor for the hydrocarbon column heights in the area.

While leakage has been recognized as a main contributor to the reduced column heights, there has not been a consensus on how or where the leakage actually took place. Explanations that have been put forward include overpressure-related seal failure (Sales, 1992), stress release between the eastern and western parts of the study area related to the uplift (Linjordet and Grung Olsen, 1992), fault reactivation (Larsen et al., 1993), tilting of traps (Doré and Jensen, 1996), increased seal brittleness at shallow depths (Doré, 1995), and semipermeable seals and fill–spill balancing (Ohm et al., 2008; Henriksen et al., 2011).

While leakage has been recognized as a main contributor to the reduced column heights, there has not been a consensus on how or where the leakage actually took place. Explanations that have been put forward include [[overpressure]]-related [[seal]] failure (Sales, 1992<ref name=Sales1992>Sales, J. K., 1992, Uplift and subsidence of northwestern Europe: Possible causes and influence on hydrocarbon prospectivity: Norsk Geologisk Tidsskrift, v. 72, no. 3, p. 253–258.</ref>), stress release between the eastern and western parts of the study area related to the [[uplift]] (Linjordet and Grung Olsen, 1992<ref name=Linjordetandgrungolsen1992>Linjordet, A., and R. Grung Olsen, 1992, [http://archives.datapages.com/data/specpubs/fieldst2/data/a014/a014/0001/0300/0349.htm The Jurassic Snøhvit Gas Field, Hammerfest Basin, Offshore Northern Norway], in Michel T. Halbouty, ed., Giant oil and gas fields of the decade 1978-1988: AAPG Memoir 54, p. 349–370.</ref>), [[fault]] reactivation (Larsen et al., 1993<ref name=Larsenetal1993>Larsen, R. M., T. Fjéran, and O. Skarpnes, 1993, Hydrocarbon potential of the Norwegian Barents Sea based on recent well results, in T. O. Vorren, E. Bergsager, Ø. A. Dahl- Stamnes, E. Holter, B. Johansen, E. Lie, et al., eds., Arctic geology and petroleum potential: Norwegian Petroleum Society (NPF) Special publication 2, Elsevier, Amsterdam, p. 321–331.</ref>), tilting of [[trap]]s (Doré and Jensen, 1996<ref name=Doreandjensen1996>Doré, A. G., and L. N. Jensen, 1996, The impact of late Cenozoic uplift and erosion on hydrocarbon exploration: Offshore Norway and some other uplifted basins: Global and Planetary Change, v. 12, p. 415–436.</ref>), increased seal brittleness at shallow depths (Doré, 1995<ref name=Dore1995>Doré, A. G. 1995, Barents Sea geology, petroleum resources and commercial potential: Arctic, v. 48, p. 207–221.</ref>), and semipermeable seals and fill–spill balancing (Ohm et al., 2008<ref name=Ohmetal2008>Ohm, S. E., D. A. Karlsen, and T. J. F. Austin, 2008, Geochemically driven exploration models in uplifted areas: Examples from the Norwegian Barents Sea: [http://archives.datapages.com/data/bulletns/2008/09sep/BLTN08028/BLTN08028.HTM AAPG Bulletin], v. 92, no. 9, p. 1191–1223.</ref>; Henriksen et al., 2011<ref name=Henriksenetal2011>Henriksen, E., A. E. Ryseth, G. B. Larssen, T. Heide, K. Rønning, K. Sollid, et al., 2011, Tectonostratigraphy of the greater Barents Sea: Implications for petroleum systems, in A. M. Spencer, A. F. Embry, D. L. Gautier, A. V. Stoupakova, and K. Sørensen, eds., Arctic petroleum geology: Geological Society (London) Memoir 35, p. 163–195.</ref>).

Gartrell et al. (2004) demonstrated by numerical modeling that stress concentrations take place at fault intersections during fault movement, and that vertical leakage is a natural response to such stress concentrations. Rotevatn et al. (2009) used lidar imaging to demonstrate the high intensity of reservoir faults at relay ramps, and we suspect that the flexuring that resulted in such faults could also have influenced the overlying caprocks. Field evidence of fluid transport along fault intersections is present along the Moab fault, where they are concentrated along a hard-linked (fault surfaces are joined) relay ramp (Urquhart, 2011). As distinctions between hard- and soft-linked relay ramps often cannot be made from seismic data, and since significant stress variations occur at relay ramps, relay ramps should also be considered as likely positions for vertical fluid flow even if hard links cannot be confirmed in the seismic data.

Gartrell et al. (2004<ref name=Gartrelletal2004>Gartrell, A., Y. Zhang, M. Lisk, and D. Dewhurst, 2004, Fault intersections as critical hydrocarbon leakage zones: Integrated field study and numerical modeling of an example from the Timor Sea, Australia: Marine and Petroleum Geology, v. 21, p. 1165–1179.</ref>) demonstrated by numerical modeling that stress concentrations take place at [[fault]] intersections during fault movement, and that vertical leakage is a natural response to such stress concentrations. Rotevatn et al. (2009<ref name=Rotevatnetal2009>Rotevatn, A., S. J. Buckley, J. A. Howell, and H. Fossen, 2009, Overlapping faults and their effect on fluid flow in different reservoir types: A LIDAR-based outcrop modeling and flow simulation study: [http://archives.datapages.com/data/bulletns/2009/03mar/BLTN07092/BLTN07092.HTM AAPG Bulletin], v. 95, p. 407–427.</ref>) used [[LIDAR]] imaging to demonstrate the high intensity of reservoir faults at relay [[ramp]]s, and we suspect that the flexuring that resulted in such faults could also have influenced the overlying [[cap rock]]s. Field evidence of [[fluid]] transport along fault intersections is present along the [[Moab fault]], where they are concentrated along a hard-linked (fault surfaces are joined) relay ramp (Urquhart, 2011<ref name=Urquhart2011>Urquhart, A. S. M., 2011, Structural controls on CO2 leakage and diagenesis in a natural long-term carbon sequestration analogue: Little grand wash fault, Utah: M. S. thesis, University of Texas, Austin, TX.</ref>). As distinctions between hard- and soft-linked relay ramps often cannot be made from [[seismic data]], and since significant stress variations occur at relay ramps, relay ramps should also be considered as likely positions for vertical [[fluid flow]] even if hard links cannot be confirmed in the seismic data.

Hermanrud et al. (2014) investigated the position of the gas–water contact in ten discoveries and four dry structures in the western Hammerfest Basin. They demonstrated that the fluid contact of all the underfilled structures coincided with the position of intersecting faults or relay ramps at the top reservoir surface, within the uncertainty of the definition of this surface in the depth domain. They also demonstrated that the only structure that was apparently filled to its structural capacity had no intersecting faults above the fluid contacts, and that all of the four dry structures had major fault intersections updip of the well position. As a consequence, the authors suggested that vertical leakage at fault intersections or relay ramps controlled the positions of the gas–water contacts in the western Hammerfest Basin.

Hermanrud et al. (2014<ref name=Hermanrudetal2014 />) investigated the position of the [[gas-water]] contact in ten discoveries and four dry structures in the western Hammerfest Basin. They demonstrated that the fluid contact of all the underfilled structures coincided with the position of intersecting [[fault]]s or [[relay ramp]]s at the top [[reservoir]] surface, within the uncertainty of the definition of this surface in the depth domain. They also demonstrated that the only structure that was apparently filled to its structural capacity had no intersecting faults above the [[fluid contact]]s, and that all of the four dry structures had major fault intersections [[updip]] of the well position. As a consequence, the authors suggested that vertical leakage at fault intersections or relay ramps controlled the positions of the gas-water contacts in the western Hammerfest Basin.

Several of the underfilled structures that were addressed by Hermanrud et al. (2014) also have fault intersections updip of the fluid contact (see below table). The authors inferred that significant vertical leakage did not take place along these intersections. They could however not distinguish between leaky and nonleaky fault intersections, which limited the practical significance of their findings.

Several of the underfilled structures that were addressed by Hermanrud et al. (2014<ref name=Hermanrudetal2014 />) also have fault intersections [[updip]] of the [[fluid contact]] (see below table). The authors inferred that significant vertical leakage did not take place along these intersections. They could however not distinguish between leaky and nonleaky fault intersections, which limited the practical significance of their findings.

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Studies of seismic signatures in overburden rocks have been reported frequently in the last few years (Chand et al., 2008, 2009, 2011; Perez-Garcia et al., 2009; Nickel et al., 2012; Ostanin et al., 2012a, 2012b, 2013; Rajan et al., 2013). Most of these studies relate main classes of observations (typically pockmarks and methane hydrates and remnants of such) to leaking gas. A common approach for all of these investigators is that they base their work on shallow observations. Ostanin et al. (2013) suggested that recent periods of gas leakage, as inferred from shallow amplitude variations in overburden rocks, resulted from leakage from the underlying Snøhvit and Albatross gas discoveries. They did, however, not provide specific information on how or where they thought these structures had leaked.

Studies of [[seismic]] signatures in overburden rocks have been reported frequently in the last few years (Chand et al., 2008<ref name=Chandetal2008>Chand, S., J. Mienert, K. Andreassen, J. Knies, L. Plassen, B. Fotland, 2008, Gas hydrate stability zone modelling in areas of salt tectonics and pockmarks of the Barents Sea suggests an active hydrocarbon venting system: Marine and Petroleum Geology, v. 25, p. 625–636.</ref>, 2009<ref name=Chandetal2009>Chand, S., L. Rise, D. Ottesen, M. F. J. Dolan, V. Bellec, and R. Bøe, 2009, Pockmark-like depressions near the Goliat hydrocarbon field, Barents Sea: Morphology and genesis: Marine and Petroleum Geology, v. 26, p. 1035–1042.</ref>, 2011<ref name=Chandetal2011>Chand, S., L. Rise, J. Knies, H. Haflidason, B. O. Hjelstuen, and R. Bøe, 2011, Stratigraphic development of the south Vøring margin (Mid-Norway) since early Cenozoic time and its influence on subsurface fluid flow: Marine and Petroleum Geology, v. 28, p. 1350–1363.</ref>; Perez-Garcia et al., 2009<ref name=Perezgarciaetal2009>Perez-Garcia, C., T. Feseker, J. Mienert, and C. Berndt, 2009, The Håkon Mosby mud volcano: 330 000 years of focused fluid flow activity at the SW Barents Sea slope: Marine Geology, v. 262, p. 105–115.</ref>; Nickel et al., 2012<ref name=Nickeletal2012>Nickel, J. C., R. di Primio, K. Mangelsdorf, D. Stoddard, and J. Kallmeyer, 2012, Characterization of microbial activity in pockmark fields of the NW Barents Sea: Marine Geology, v. 332-334, p. 152–162.</ref>; Ostanin et al., 2012<ref name=Ostaninetal2012a>Ostanin, I., Z. Anka, R. di Primio, and A. Bernal, 2012a, Identification of a large Upper Cretaceous polygonal fault network in the Hammerfest basin: Implications on the reactivation of regional faulting and gas leakage dynamics, SW Barents Sea: Marine Geology, v. 332–334, p. 109–125.</ref>, 2012<ref name=Ostaninetal2012b>Ostanin, I., Z. Anka, R. di Primio, and A. Bernal, 2012b, Hydrocarbon leakage above the Snøhvit gas field, Hammerfest Basin SW Barents Sea: First Break, v. 20, p. 55–60.</ref>, 2013<ref name=Ostaninetal2013 />; Rajan et al., 2013<ref name=Rajanetal2013>Rajan, A., S. Büntz, J. Mienert, A. J. Schmidt, 2013, Gas hydrate systems in petroleum provinces of the SW Barents Sea: Marine and Petroleum Geology, v. 46, p. 92–106.</ref>). Most of these studies relate main classes of observations (typically pockmarks and [[methane]] hydrates and remnants of such) to leaking [[gas]]. A common approach for all of these investigators is that they base their work on shallow observations. Ostanin et al. (2013<ref name=Ostaninetal2013 />) suggested that recent periods of gas leakage, as inferred from shallow [[amplitude]] variations in overburden rocks, resulted from leakage from the underlying Snøhvit and Albatross gas discoveries. They did, however, not provide specific information on how or where they thought these structures had leaked.

Thus, while ample evidences of gas-related amplitude variations in overburden rocks have been documented, this information has not yet been fully utilized to understand leakage from hydrocarbon reservoirs. Leaking hydrocarbons may leave amplitude signatures in overburden rocks that could potentially reveal where individual traps leaked. Both seismic chimneys and bright amplitudes have previously been used for this purpose (Teige and Hermanrud, 2004; Heggland, 2005; Løseth et al., 2009). We are however not aware of studies that have attempted to identify the position of vertical leakage for fluid contact identification in a suite of underfilled structures by combining knowledge of the structural setting, the position of the fluid contact, and overburden amplitude variations.

Thus, while ample evidences of gas-related [[amplitude]] variations in overburden rocks have been documented, this information has not yet been fully utilized to understand leakage from [[hydrocarbon reservoir]]s. Leaking [[hydrocarbon]]s may leave amplitude signatures in overburden rocks that could potentially reveal where individual traps leaked. Both [[seismic]] chimneys and bright amplitudes have previously been used for this purpose (Teige and Hermanrud, 2004<ref name=Teigeandhermanrud2004>Teige, G. M. G., and C. Hermanrud, 2004, Seismic characteristics of fluid leakage from an underfilled and overpressured Jurassic fault trap in the Norwegian North Sea: Petroleum Geoscience, v. 10, no. 1, p. 35–42.</ref>; Heggland, 2005<ref name=Jegg;amd2--5>Heggland, R., 2005, [http://archives.datapages.com/data/specpubs/hedberg2/chapter16/CHAPTER16.HTM Using gas chimneys in seal integrity analysis: A discussion based on case histories], in P. Boult and J. Kaldi, eds., Evaluating fault and cap rock seals: AAPG Hedberg Series 2, p. 237–245.</ref>; Løseth et al., 2009<ref name=Losethetal2009>Løseth, H., M. Gading, and L. Wensaas, 2009, Hydrocarbon leakage interpreted on seismic data: Marine and Petroleum Geology, v. 26, p. 1304–1319.</ref>). We are however not aware of studies that have attempted to identify the position of vertical leakage for fluid contact identification in a suite of underfilled structures by combining knowledge of the structural setting, the position of the fluid contact, and overburden amplitude variations.

The purpose of this study is to document the spatial relationships between positions of vertical reservoir leakage and amplitude variations in the overburden rocks. This documentation was motivated by the hopes of obtaining (1) a further verification of the leakage model of Hermanrud et al. (2014), (2) an improved understanding of the leakage processes, (3) a better understanding of how gas leakage influences amplitudes in overburden rocks, and (4) a set of guidelines that can be helpful in sealing analyses in the area. We addressed these issues by attempting to identify the fluid flow pathways that resulted in the presumably gas-related bright amplitudes in overburden rocks and by analyzing the occurrences, shapes, and intensities of seismic chimneys above and outside the positions of presumed vertical reservoir leakage.

The purpose of the study contained in the [http://archives.datapages.com/data/specpubs/memoir114/data/239_aapg-sp2030239.htm full paper] associated with this Wiki article is to document the spatial relationships between positions of vertical reservoir leakage and amplitude variations in the overburden rocks. This documentation was motivated by the hopes of obtaining (1) a further verification of the leakage model of Hermanrud et al. (2014<ref name=Hermanrudetal2014 />), (2) an improved understanding of the leakage processes, (3) a better understanding of how [[gas]] leakage influences [[amplitude]]s in overburden rocks, and (4) a set of guidelines that can be helpful in sealing analyses in the area. We addressed these issues by attempting to identify the fluid flow pathways that resulted in the presumably gas-related bright amplitudes in overburden rocks and by analyzing the occurrences, shapes, and intensities of seismic chimneys above and outside the positions of presumed vertical reservoir leakage.

==Geological Setting==

==Geological Setting==

The geology of the western Barents Sea is well known (Smelror et al., 2009; Henriksen et al., 2011, 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, 1990) are shown in Figure 1. Ostanin et al. (2012a) later separated the faults 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 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).