Semi-conventional play: definition, exploration strategy and the example of the Chalk Group in Denmark
Play analysis has been widely used in hydrocarbon exploration for decades with great success. In recent years, progress has also been made to describe reservoir properties of very low permeability reservoirs. However, comparatively little research has been done into play analysis for such reservoirs, which may lead to misleading estimates of their hydrocarbon potential. Here, the concept of a semi-conventional play is defined and characterised as having a reservoir of such low permeability that a hydrocarbon column can form down-dip of an effective dry trap. A new exploration approach is proposed for such plays, using the Chalk Group Play in the Danish North Sea as an example. It is suggested that together with the usual risk elements, a more detailed analysis of ‘charge’ is necessary, paying particular attention to identifying possible hydrocarbon entry points, palaeostructures and the maximum distance from these entry points that the hydrocarbons may have reached since they first entered the reservoir. The application of this novel approach for semi-conventional plays in mature basins can help unlock further resources in proximity of existing fields, and reduce the risk of failure in frontier exploration.
Albrechtsen, T. et al. 2001: Halfdan: developing non-structurally trapped oil in North Sea chalk. SPE Annual Technical Conference and Exhibition. New Orleans, LA, USA, 30 September–3 October, 2001. https://doi.org/10.2118/71322-MS
Blasingame, T.A. 2008: The characteristic flow behavior of low-permeability reservoir systems. SPE Unconventional Reservoirs Conference. Keystone, CO, USA, 10–12 February, 2008. https://doi.org/10.2118/114168-MS
Chadwick, R.A. et al. 2004: Geological reservoir characterization of a CO2 storage site: the Utsira Sand, Sleipner, northern North Sea. Energy 29, 1371–1381. https://doi.org/10.1016/j.energy.2004.03.071
Dennis, H. et al. 2000: Hydrodynamic activity and tilted oil-water contacts in the North Sea. Norwegian Petroleum Society Special Publications 9, 171–185. https://doi.org/10.1016/S0928-8937(00)80016-8
Dennis, H., Bergmo, P. & Holt, T. 2005: Tilted oil–water contacts: modelling the effects of aquifer heterogeneity. Geological Society, London, Petroleum Geology Conference Series 6, 145–158. https://doi.org/10.1144/0060145
Fabricius, I.L. 2007: Chalk: composition, diagenesis and physical properties. Bulletin of the Geological Society of Denmark 55, 97–128.
Fabricius, I.L. et al. 2007: Estimating permeability of carbonate rocks from porosity and vp/vs. Geophysics 72, E185–E191. https://doi.org/10.1190/1.2756081
Fraser, A.J. 2011: A regional overview of the exploration potential of the Middle East: a case study in the application of play fairway risk mapping techniques. Geological Society, London, Petroleum Geology Conference Series 7, 791–800. https://doi.org/10.1144/0070791
Frykman, P. et al. 2004: The history of hydrocarbon filling of Danish chalk fields. Geological Survey of Denmark and Greenland Bulletin 4, 9–12. https://doi.org/10.34194/geusb.v4.4768
Goff, J.C. 1983: Hydrocarbon generation and migration from Jurassic source rocks in the E Shetland Basin and Viking Graben of the northern North Sea. Journal of the Geological Society 140, 445–474. https://doi.org/10.1144/gsjgs.140.3.0445
Grant, S., Milton, N. & Thompson, M. 1996: Play fairway analysis and risk mapping: an example using the Middle Jurassic Brent Group in the northern North Sea. Norwegian Petroleum Society Special Publications 6, 167–181. https://doi.org/10.1016/S0928-8937(07)80017-8
Hancock, J.M. 1975: The petrology of the Chalk. Proceedings of the Geologists’ Association 86, 499–535. https://doi.org/10.1016/S0016-7878(75)80061-7
Hardman, R.F.P. 1982: Chalk reservoirs of the North Sea. Bulletin of the Geological Society of Denmark 30, 119–137.
Harris, R.G. & Goldsmith, P.J. 2001: Water saturation analysis and interpretation of a tilted free-water level in the Joanne/Judy Chalk Field, U.K. North Sea. SPWLA 42nd Annual Logging Symposium. Houston, TX, USA, 17–20 June, 2001.
Hjuler, M.L. et al. 2016: Detailed assessment of geothermal potential by integration of a wide range of geological data: preliminary results of a case study from a lower Triassic Lowenthalpy reservoir in the Tønder area in southern Denmark. European Geothermal Congress 2016. Strasbourg, France, 19–24 September, 2016.
Huuse, M. 1999: Detailed morphology of the top chalk surface in the eastern Danish North Sea. Petroleum Geoscience 5, 303–314. https://doi.org/10.1144/petgeo.5.3.303
Jones, D.W. et al. 2015: Reservoir geology of the Paleocene Forties Sandstone member in the Framdiscovery, UK Central North Sea. Geological Society, London, Special Publications 403, 219–246. https://doi.org/10.1144/SP403.13
Kok, A. & Arnhild, M. 2012: Oil migration and dynamic traps in chalk, Danish North Sea. Search and Discovery, article #120070.
Megson, J.B. 1992: The North Sea chalk play: examples from the Danish Central Graben. Geological Society, London, Special Publications 67, 247–282. https://doi.org/10.1144/GSL.SP.1992.067.01.10
Megson, J.B. & Tygesen, T. 2005: The North Sea chalk: an underexplored and underdeveloped play. Geological Society, London, Petroleum Geology Conference Series 6, 159–168. https://doi.org/10.1144/0060159
Milkov, A.V. 2015: Risk tables for less biased and more consistent estimation of probability of geological success (PoS) for segments with conventional oil and gas prospective resources. Earth-Science Reviews 150, 453–476. https://doi.org/10.1016/j.earscirev.2015.08.006
Mortensen, J., Engstrøm, F. & Lind, I. 1998: The relation among porosity, permeability, and specific surface of chalk from the Gorm Field, Danish North Sea. SPE Reservoir Evaluation & Engineering 1, 245–251. https://doi.org/10.2118/31062-PA
Nelskamp, S. 2017: Geological resource analysis of shale gas and shale oil in Europe. Report T4b of the EUOGA Study (EU Unconventional Oil and Gas Assessment) commissioned by JRC-IET.
O’Connor, S.A., Swarbrick, R.E. & Jones, D. 2008: Where has all the pressure gone? Evidence from pressure reversals and hydrodynamic flow. First Break 26, 55–61. https://doi.org/10.3997/1365-2397.2008013
Rasmussen, S.L. & Surlyk, F. 2012: Facies and ichnology of an upper cretaceous chalk contourite drift complex, eastern Denmark, and the validity of contourite facies models. Journal of the Geological Society 169, 435–447. https://doi.org/10.1144/0016-76492011-136
Van Buchem, F.S.P. et al. 2017: Tectonostratigraphic framework and depositional history of the Cretaceous–Danian succession of the Danish Central Graben (North Sea) – new light on a mature area. Geological Society, London, Petroleum Geology Conference Series 8, 9–46. https://doi.org/10.1144/PGC8.24
Vejbæk, O.V. et al. 2005: The history of hydrocarbon filling of Danish chalk fields. Geological Society, London, Petroleum Geology Conference Series 6, 1331–1345. https://doi.org/10.1144/0061331
White, D.E. 1988: Oil and gas play maps in exploration and assessment: geologic note 1. AAPG Bulletin 72, 944–949. https://doi.org/10.1306/703C911D-1707-11D7-8645000102C1865D
Winefield, P., Gilham, R. & Elsinger, R. 2005: Plumbing the depths of the Central Graben: towards an integrated pressure, fluid and charge model for the Central North Sea HPHT play. Geological Society, London, Petroleum Geology Conference Series 6, 1301–1315. https://doi.org/10.1144/0061301
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