Origin and Significance of Fracture-Related Dolomite in Porous Sandstones: An Example from the Carboniferous of County Antrim, Northern Ireland

  1. Sadoon Morad
  1. R. Evans1,
  2. J. P. Hendry1,
  3. J. Parnell1 and
  4. R. M. Kalin2

Published Online: 17 APR 2009

DOI: 10.1002/9781444304893.ch18

Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution

Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution

How to Cite

Evans, R., Hendry, J. P., Parnell, J. and Kalin, R. M. (1998) Origin and Significance of Fracture-Related Dolomite in Porous Sandstones: An Example from the Carboniferous of County Antrim, Northern Ireland, in Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution (ed S. Morad), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304893.ch18

Author Information

  1. 1

    School of Geosciences, The Queen's University of Belfast, Belfast BT7 1NN, UK

  2. 2

    Department of Civil Engineering, The Queen's University of Belfast, Belfast BT7 1NN, UK

Publication History

  1. Published Online: 17 APR 2009
  2. Published Print: 29 MAY 1998

ISBN Information

Print ISBN: 9780632047772

Online ISBN: 9781444304893

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Keywords:

  • origin and significance of fracture-related dolomite in porous sandstones;
  • carboniferous of County Antrim, Northern Ireland;
  • Dinantian fluvio-deltaic sandstones at Ballycastle County, Antrim, Northern Ireland;
  • Rathlin basin – Carboniferous-Cretaceous transtensional sedimentary basin;
  • dolomite-cemented fractures

Summary

Dinantian fluvio-deltaic sandstones at Ballycastle, County Antrim, Northern Ireland, provide a record of the palaeofluid hydraulics in operation during the structural and diagenetic evolution of the area. Dolomitized fractures cause a reduction in reservoir quality by structural compartmentalization, which may be significant for subsurface analogues in the prospective Rathlin basin. An integrated field, petrographic and stable isotopic study has elucidated the physical and diagenetic origin of the cemented fractures and provides clues to their timing in the context of the regional tectonic evolution of northeast Antrim. The dolomite is highly ferroan but with near-stoichiometric Ca2+ content (50–53 mol% CaCO3; 32–42 mol% MgCO3; 5–18 mol% FeCO3), and has oxygen and carbon isotopic compositions of −3.8 to −0.9‰ PDB, and −9.5 to −4.2‰ PDB, respectively. Planar crystal fabrics and a preponderance of monophase aqueous fluid inclusions indicate a relatively low cementation temperature and negate the involvement of hydrothermal fluids.

The results of the study demonstrate that dolomite was precipitated during multiple episodes of dilatational reactivation of cataclastic slip bands, in response to elevated pore fluid pressures associated with tectonic enhancement of subsurface fluid flow. Such hydraulic fracturing has rarely been recognized in porous sandstones, and a minor dextral shear component recorded in the fractures concurs with a late Carboniferous origin. Dolomite was sourced either from compactional dewatering of basinal pro-delta shales augmented by strain cycling, or from local mudrocks via transfer across active faults. An alternative interpretation is of Tertiary fracturing and fluid input from venting of overpressured mudrocks during catagenesis in the deep Rathlin basin. This would also fit the data, but is considered less realistic from structural considerations. The tightly constrained orientation of cemented fractures suggests that a reservoir sand body of similar nature would be strongly heterogeneous rather than completely ineffective, and further integration of field and petrological data is required to assess the regional importance of structural–diagenetic compartmentalization in potential reservoirs.