The truth about Triassic salt tectonic models for the Northern North Sea

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1 The truth about Triassic salt tectonic models for the Northern North Sea Christopher A-L. Jackson Basins Research Group (BRG), Department of Earth Science & Engineering, Imperial College, Prince Consort Road, London, SW7 2BP, UK Data: Triassic Park (FORCE) Friday 20 th October 2017 Stavanger, Norway Software:

2 Talk Outline Provide an overview of Triassic salt-tectonic models for the Northern North Sea Outline the implications of model choice for reservoir development, trapping style, and basin-scale structural style and kinematics Test models using observations from published and unpublished datasets

3 In the case of Triassic salt-tectonic models for the Northern North Sea, I call to the stand Penge et al. (1993) and Hodgson et al. (1993)

4 Rift-Raft Salt Tectonics depth (ms TWT) Block 23/11, Jaeren High, NCS post-triassic Triassic salt subsalt Penge et al. (1993) Fault-bound blocks of Triassic above salt Thin salt below blocks; thick salt (reactive diapirs) between blocks Largely 2D profiles; few map-view images showing salt geometry Limited information on detailed Triassic stratigraphic architecture

5 Rift-Raft Salt Tectonics East Central Graben, UKCS salt Triassic (+thin Jurassic) Penge et al. (1993)

6 Rift-Raft Salt Tectonics brittle cover extension increasing extension ductile cover (e.g. boudinage) Penge et al. (1993)

Diagnostic Criteria Triangular-shaped salt diapirs Inward-dipping fan of normal faults younging upwards towards diapir crest Structural attenuation of overburden towards diapir flank Oldest

7 Diagnostic Criteria Triangular-shaped salt diapirs Inward-dipping fan of normal faults younging upwards towards diapir crest Structural attenuation of overburden towards diapir flank Oldest overburden broadly tabular True stratigraphic onlap absent Elongate salt walls and minibasins trend normal to slope Widespread depositional systems later cut by normal faults and diapirs...

8 Passive Diapirism Eastern Trough, Central Graben, UKCS Conceptual model Hodgson et al. (1993)

9 Passive Diapirism Hodgson et al. (1993)

10 Diagnostic Criteria Rounded diapirs Minibasins only gently folded; faulting generally absent Thickness changes in earliest/deepest minibasin fill Irregular minibasin shapes and distributions Stratigraphic onlap occurs near minibasin base and persists though minibasin-fill Depositional systems restricted to minibasin centre...

11 so, does it matter which model we choose?

Structural Implications The Curse of the

12 Structural Implications The Curse of the Kinematic Requirement extensional gap (x) extensional gap (y) Penge et al. (1993) original length of section present length of section (x+y) Prekinematic overburden Syn-/postkinematic salt Vendeville (2005) Jackson et al. (2015)

13 Structural Implications Stage 1 Early extension Jackson et al. (2015) Stage 2 Extrusion Stage 3 Postkinematic erosion

14 Structural Implications Jackson et al. (2010)

Summary Rift-raft model Vertical and horizontal overburden motions; kinematically requires complimentary shortening, which should be of: (i) broadly similar age; (ii)

15 Summary Rift-raft model Vertical and horizontal overburden motions; kinematically requires complimentary shortening, which should be of: (i) broadly similar age; (ii) similar structural trend; and (iii) of broadly similar magnitude Passive diapirism model Predominantly vertical motions; complimentary shortening not kinematically required

16 so, what do the data say? Høiland et al. (1993)

17 Utsira High top Triassic time-structure map Jackson et al. (2010)

18 Utsira High salt Jackson et al. (2010)

19 Utsira High salt Jackson et al. (2010)

20 Jæran High Høiland et al. (1993)

21 Jæran High Høiland et al. (1993)

22 Jæran High overburden salt Høiland et al. (1993)

23 Jæran High Høiland et al. (1993)

24 Jæran High Leong (2015)

25 Jæran High top salt time-structure map salt isochron Leong (2015) Triassic isochron

26 Jæran High Leong (2015)

27 Jæran High Leong (2015)

28 Mandal High Høiland et al. (1993)

29 Mandal High TWT (ms) Base salt N m Aldaif (2016)

30 Mandal High TWT (ms) Top salt N m Aldaif (2016)

31 Mandal High TWT (ms) N 12500m Aldaif (2016)

32 Mandal High Aldaif (2016)

33 Regional Observations Karlo et al. (2014)

34 Triassic Seismic Stratigraphy Southern West Central Graben, UKCS Karlo et al. (2014)

35 Hybrid Salt-Tectonic Models Karlo et al. (2014)

36 Evidence for Triassic shortening Western Norwegian-Danish Basin, NCS Karlo et al. (2014)

37 Evidence for Triassic shortening shortening Lwr.Triassic shortening Mid.Triassic Upr.Triassic Karlo et al. (2014)

38 Data favour the passive diapirism model for Triassic salt tectonics in the Northern North Sea. So, we re all good, yes...? No......because of the Mechanical Gremlin...

39 The Density Paradox Overburden sinks into salt due to excess density Pure salt density = 2.2 kg/m 3 ; sediment densities = <2.2 kg/m 3 until several hundred to >1 km thickness Hudec et al. (2009) How to trigger Early Triassic subsidence below such thin overburden?

40 The Density Paradox Gulf of Mexico Hudec et al. (2009)

41 The Density Paradox Mandal High Aldaif (2016)

42 So, how to trigger passive diapirism...?

43 Minibasin Propagation? A) B) C) D) Peel (2014) Minibasin array propagates from dense seed (i.e. sediment mound) Does not explain plan-view geometries (e.g. polygonal walls or isolated minibasins)... Depositional system type providing initial seed/seeds unclear... Karlo et al. (2014)

44 Syn-Salt Density Overturn? Fernandez et al. (2017)

45 Seismic Evidence for Intrasalt Variability

46 Seismic Evidence for Intrasalt Variability

47 Conclusions Two end-member models for Triassic salttectonics; passive and reactive diapirism Diagnostic criteria (or varying strengths) allow model differentiation; model selection has kinematic and exploration consequences Seismic reflection-based observations (e.g. onlaps, lack of faulting, lack of coeval contraction) largely support passive diapir model (reactive model may be locally applicable on tilted fault blocks) Latest Permian, intrasalt depositional heterogeneity may explain the so-called Density Paradox

Lithology Distribution in the Zechstein Supergroup and Controls on Rift Structure: Greater South Viking Graben, Northern North Sea

Lithology Distribution in the Zechstein Supergroup and Controls on Rift Structure: Greater South Viking Graben, Northern North Sea Christopher Jackson 1, Elisabeth Evrard 1, Gavin Elliott 1, Robert Gawthorpe