Image: G. Parker. Presenters: Henry Chan, Kayla Ireland, Mara Morgenstern, Jessica Palmer, Megan Scott

Transcription

1 Image: G. Parker Presenters: Henry Chan, Kayla Ireland, Mara Morgenstern, Jessica Palmer, Megan Scott

2 Is the Ross Formation a suitable analog for sand-rich turbidite plays in passive margin basins? Play: Potential Oil production from a Reservoir (i.e. high porosity & permeability sand)

3 Turbulent mixture of fluid and suspended sediment creates density difference between fluid and ambient flow above Induced by gravity Hyperpycnal flow Hypopycnal flow Mesopycnal flow Discrete events

4 Paleo-Tectonic Setting Basins are extensional features Earlier lineaments above the Iapetus Suture were reactivated. Basins found on the Northern passive margin of a foreland basin

5 i. Structureless beds ii. Rippled & hummocky beds iii. Sandstone packets iv. Mud-draped scours and megaflutes v. Channels course1.winona.edu/.../images/sedstrux/smhcs.jpg

6 Ross Formation Namurian, Ireland Higgs, 2004

7 Bude Formation Westphalian, England

8 ii. Fill of a basin formed through crustal extension Basin trough running along Iapetus Suture River-fed turbidites prograde along the basin axis from source in the: i. North-west, north, northeast (Collinson, et al., 1991) South-west (Wignall and Best, 2000) Wignall and Best, 2000

9 ii. Fill of a basin hundreds of meters deep formed through crustal extension Basin trough running along Iapetus Suture River-fed turbidites prograde along the basin axis from source in the: i. North-east, north, northwest (Collinson, et al., 1991) South-west (Wignall and Best, 2000) Wignall and Best, 2000

10 Deep Sea Model Continued Mudstones interbedded with turbidite sandstones form from fall-out in a marine environment during quiet conditions and low clastic input. Ripples on turbidite upper surfaces form from waning flow following deposition of sediment load or reworking by later turbidity currents Sandstone filled channels created by migrating delta feeder channels Wignall and Best, 2000

11 i. Western-Ireland Namurian Basin interpreted as part of the Dinantian forelandbasin carbonate shelf ii. Ross and Bude formations interpreted to be lake-shelf sequences iii. Paleo-lake Bude surrounded iv. by lake sill Deposition controlled by glacioeustatic fluctuations v. Tropical paleoenvironment keeps lake full and fresh

12 Fossiliferous bands: maximum flooding surfaces marine setting Turn lake marine or strongly brackish Sandstone packets: combined falling stage systems tract/lowstand systems tract Small packets: Milankovitch cycles Sill keeps the lake full during lower sea-level intervals Accounts for lack of sub-aerial exposure River-fed underflows create turbidity currents carrying coarser sediment to deeper portions of the lake Responsible for carving channels which are later filled by weaker flows

13 Lake model: i. The Ross and Bude formations in Western Ireland and Southern England represent a lake-shelf depositional environment ii. iii. iv. Accounts for evidence of shallow-water setting and lack of emergence Accounts for lack of biological traces for most of the Ross and Bude formations Problems: i. No current evidence for lake sill ii. Proximity of anoxic conditiodns? Deep sea model: i. The Ross and Bude formations represent deposition of turbidity currents on a submarine fan ii. iii. iv. Accounts for the marine fossil bands Accounts for proximity of Clare shale (anoxic) Problems: models do not address ripples or hummocky cross-stratification

14 Conclusions: Hydrocarbon Petroleum Reservoir Potential Great thickness (460m & 1,290m) High sandstone content (70%) Thick individual sand bodies (up to 10m) connected vertically by channels At outcrop Low intergranular porosity in the quartzose, fine to very fine grained SS of the Ross and Bude Formations (<5%) At subsurface Porosity around 5 10 % Carbonate cement applications Potential for secondary porosity by dissolution

15 Hydrocarbon potential (continued) A sand fairway predicted Corresponding to Lake Bude shelf Deep-sea model: No interconnected sandstone packets Lake-shelf model: interconnected sandstone packets PROBLEMS: if the lake-shelf model is believable, this is not a good place to study deep submarine fans as these outcrops are neither marine nor deep-water

16 Conclusions: continued Outcrops show: Anoxic black shales underlying turbidite packtes, shallow water hummocky crossstratification, marine fossil bands, erosional scour marks Deep-sea models: Deposition of turbidity currents on submarine fan Explains evidence for thick layers of black shales, marine fossil bands, and presence of turbidites Lake-shelf model: Deposition of hyperpicnal river flows into a freshwater lake Explains evidence for shallow-water setting, lack of abundant fossils, sandstone turbidite packets

17 References Collinson, J. D, Martinsen, O., Bakken, B., and Kloster, A., 1991, Early fill of the Western Irish Namurian Basin: A complex relationship between turbidites and deltas: Basin Research, v. 3, p Elliott, T., 2000, Megaflute erosion surfaces and the initiation of turbidite channels: Geology, v. 28, n. 2, p Elliott, T., 2000(b), Depositional architecture of a sand-rich, channelized turbidite system: The Upper Carboniferous Ross Sandstone Formation, Western Ireland: GCSSEPM Foundation 20 th Annual Research Conference Deep-Water Reservoirs of the World, p Higgs, R., 2004, Ross and Bude formations (Carboniferous, Ireland and England): Reinterpreted as lake-shelf turbidites: Journal of Petroleum Geology, v. 27, n. I, p Lucchi, F.R., 1995, Sedimentographica; A photographic Atlas of Sedimentary Structures, New York; Columbia University Press, Second Ed. Martinsen, O. J., and Collinson, J. D., 2002, The Western Irish Namurian Basin reassessed a discussion: Basin Research, v. 14, p Wignall, P. B., and Best, J. L., 2000, The Western Irish Namurian Basin reassessed: Basin Research, v. 12, p