--> --> Fractured Carboniferous Limestone Sealed by a Volcanic Ash - A New Play in the East Midlands UK, by Peter Boult, Brett Freeman, and Graham Yielding; #10212 (2009)

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 PSFractured Carboniferous Limestone Sealed by a Volcanic Ash - A New Play in the East Midlands UK*

 

Peter Boult1, Brett Freeman2, and Graham Yielding2

 

Search and Discovery Article #10212 (2009)

Posted October 30, 2009

 

*Adapted from poster presentation at AAPG Convention, Denver, Colorado, June 7-10, 2009

 

1Low New Biggin Oil, London, United Kingdom  ([email protected])

2Badley Geoscience, Spilsby, United Kingdom

 

Abstract

 

The Strelley-1 encountered a minimum 27 m hydrocarbon column in significant karstic porosity and permeability that occurs within the brittle Carboniferous/Dinantian Limestone beneath a thick, low velocity, relatively ductile volcanic ash seal. This volcanic ash is a widespread, previously unrecognised sealing unit that forms a significant new play where it overlies fractured limestone along the northern flank of the Widmerpool Gulf, which also contains proven significant source rocks.

 

The influx of meteoric water and dolomitisation, occurring along the northern edge of the Widmerpool Gulf, probably had a structural control and has created a fracture set capable of producing over 2000 BOPD per vertical well. A horizontal well is planned to intersect fractures within the oil column. Understanding the structural history and fracture development of the area is thus very important.

 

Fault population analysis of previous detailed fault mapping in the Nottinghamshire coal field and the application of 3D fault plane modelling in the Strelley area have revealed: (1) N-S Dinantian rifting, which caused syn-depositional normal faults related to previous Caledonian structural grain; (2) Late Westphalian / Stephanian NNE-SSW extension, which caused conjugate normal faults and fracturing; (3) Late Westphalian / Stephanian NW compression, which caused dextral strike slip on the basement related Caledonian / Dinantian weaknesses and local extension and reactivation of NNW-SSE faults; (4) Late Westphalian / Stephanian compression, which caused a series of NW-SE trending interference related anticlinal domes that may be widespread along the northern edge of the Widmerpool Gulf.

 

Forward modelling to predict fracture orientation needs to be confirmed within an exploration well before a horizontal well is planned. We will present a first pass fracture model based on elastic dislocation theory and fault interpretation derived from the predicted structural evolution of the area.

 

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fig01

Figure 1. Location map, depth to basement, and Devonian subcrop map.

fig02

Figure 2. SW-NE cross section.

fig03

Figure 3. Fault map example for the Barnsley/top Hard Coal level, showing three sets of faults (NW-SE, NE-SW and NNW-SSE). Coloured faults are underlying Dinantian structures.

fig04

Figure 4. Base volcanic ash (top reservoir) depth (mss) map. Contour interval = 10 m. Polygons are used for estimates of gross rock volume, Green polygon = minimun AND mode area. Red = maximun area. The mapped up-dip potential for Strelley is > 120 m. Note Ilkeson 1 was a shallow well.

fig05

Figure 5. 3D model of the Strelley structure at top reservoir horizon looking northward. Shows major panelled faults in the model and transparent blue time slice equivalent to the FWL.

 

References

 

Dee, S., B. Freeman, G. Yielding, A. Roberts, and P. Bretan, 2005, Best practice in structural geological analysis, First break, v. 23, p. 49-54.

 

Dee, S.J., G. Yielding, B. Freeman, D. Healy, N.J. Kusznir, N. Grant, and P. Ellis, 2007, Elastic dislocation modelling for prediction of small-scale fault and fracture network characteristics, L. Lonergan, R.J.H. Jolly,  K. Rawnsley, and D.J. Sanderson (eds), Fractured Reservoirs, Geological Society, London, Special Publications 270, p. 139–155.

 

Edinburgh Oil and Gas PLC, 1987, Geology and potential of the East Midlands, DTI2D57.

 

Healy, D., G. Yielding, and Kusznir, 2004, Fracture prediction for the 1980 El Asnam, Algeria earthquake via elastic dislocation modeling, Tectonics, v. 23, TC6005.

 

Holliday, D.W., J.M. Allsop, M.R. Clarke, R.C. Lamb, G.A. Kirby, W.J. Rowley, N.J.P. Smith, and P.W. Swallow, 1984, Hydrocarbon prospectivity of the Carboniferous rocks of eastern England, British Geological Survey report 84/4.

 

Swann, G. and J. Munns, 2003, The hydrocarbon prospectivity of Britain’s onshore basins, DTI.

 

RTZ, 1987, UK Onshore round 2 application for exploration licenses – DTID76a.

 

World Stress Data, 2008, Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences.

 

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