Early Carboniferous basins in Co. Fermanagh and south Co. Tyrone, oil and gas, mineral resources, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
D M Reay
This area covers the Carboniferous outcrop that extends from Co. Fermanagh eastwards through south Co. Tyrone to Co. Armagh (P947901) . Westwards it extends to Counties Cavan, Leitrim, Sligo, Monaghan and south Co. Donegal in the Republic of Ireland. In Counties Fermanagh and Cavan, the Early Carboniferous section comprises 3500 m of marine sedimentary rocks that include organic-rich mudstone and sandstone which are potential hydrocarbon source and reservoir rocks respectively. Late Carboniferous rocks of the Fintona Block and in the Coalisland area of east Co. Tyrone contribute to a cumulative maximum thickness of c. 7000 m of Carboniferous strata (see Carboniferous article).
Five wells were drilled in 1965–1966, resulting in gas shows at Dowra and Macnean in Co. Cavan and at Big Dog and Owengarr in Co. Fermanagh while the Glennoo No. 1 well, in the Slieve Beagh area of Co. Tyrone, was dry. Interest in the area lapsed when testing of the wells produced non-commercial gas flows. Dowra No. 1 was re-entered in 1981 and the reservoir interval was hydraulically fractured producing a tenfold increase in flow-rates to 250 000 cubic feet of gas per day (250 mscfg/day) from the Dowra Sandstone Member of the Bundoran Shale Formation (P947934). This led to further exploration which combined a seismic reflection survey with the drilling of wells at Slisgarrow and Kilcoo Cross in Co. Fermanagh and Macnean No. 2 and Drumkeeran in Co. Cavan in 1984-85. Gas shows were again encountered but the wells did not flow to surface when tested.
In 1996, licences were awarded for the area between Co. Fermanagh and the west shore of Lough Neagh. Six wells were drilled in 2001, four in Co. Fermanagh and two in the Republic of Ireland. The reservoir intervals were hydraulically fractured and extended well tests performed. Unfortunately, the fractures did not propagate as well as had been hoped and this, combined with low formation pressures, led to low gas flow rates of <100 mscfg/day. The wells were considered non-commercial given the high drilling costs and lack of infrastructure in the area. Horizontal drilling of the reservoir may be the key to the successful future development of this play.
The main reservoir target is the Mullaghmore Sandstone Formation (see Carboniferous article). About 55% of the Formation is composed of sandstone but, even in the best wells, only 20% reaches tight gas sandstone reservoir quality. The Dowra Sandstone Member of the Bundoran Shale Formation is a deeper reservoir target of restricted distribution. The calcareous mudstone of the Bundoran Shale and Benbulben Shale formations are moderate to good source rocks. Vitrinite reflectance (R0) and Conodont Alteration Index (CAI) values indicate that the source rocks are mature for oil and gas at surface and for wet or dry gas at depth . The Benbulben Shale Formation forms an effective and widespread seal to the underlying Mullaghmore Sandstone reservoir. Structural traps may be associated with gentle flexures within the basin or tighter, pop-up or flower structures adjacent to major faults such as the Castle Archdale Fault Zone. The widespread occurrence of gas shows and hydrocarbon residues in the cuttings samples may be an example of ‘basin-centred gas’ and indicate that structural trapping is not an essential factor because of the poor poroperm characteristics. Migration from the source rocks into the adjacent reservoirs is not seen as problematical.
The Mullaghmore Sandstone Formation normally consists of fine- to medium-grained sandstone and siltstone deposited in a delta complex which prograded southwards into the basin. It is about 200 m in Big Dog No. 1 but thins southwards to less than 50 m in Drumkeeran No. 1 where it comprises a silty mudstone (P947902). It also thins and shales out in the Upper Lough Erne area where the Formation is represented by thin, sandy siltstone in Owengarr No. 1 well and could not be recognised in Wind Farm No. 1. Further east, on Slieve Beagh, it is present in outcrop and in Glennoo No. 1 is >200 m thick. In the basinal wells five major lithological cycles have been recognised in the Mullaghmore Sandstone Formation starting with a basal cycle of pro-delta to distal delta slope mudstone and siltstone . This is succeeded by four cycles coarsening-upwards from delta slope to platform facies with channel and inter-channel deposits. Each cycle is terminated by a fining-upward trend to fine-grained sandy oolitic limestones deposited in a shallow shelf setting (P947903). Through the Mullaghmore Sandstone Formation there is evidence of increasing water depths leading to the deposition of the Benbulben Shale Formation. Thus reservoir characteristics reflect the changing sedimentary environments with the quality and thickness decreasing from north to south and the better reservoir quality confined to channel sandstones.
Measured and calculated porosities in the Mullaghmore Sandstone Formation range from 0% to 10% in boreholes and up to 15% in outcrop samples . Petrographic (4,5) and combined petrographic/SEM studies reveal that the major cement components are ferroan carbonate (calcite and dolomite) and/or quartz overgrowths, together with feldspar-replacing illite . It appears that primary porosity was totally occluded by the matrix and pore-filling cements. Secondary porosity created by dissolution of grains and cements by meteoric water, associated with periods of uplift, is usually insignificant. However, locally developed secondary porosity filled with hydrocarbons suggests an important dissolution phase prior to hydrocarbon migration. Later brittle fracturing of the sandstone may also have created secondary porosity although these fractures are often filled with carbonate cement. The higher outcrop values reflect near-surface leaching of carbonate cements and are not relevant to the reservoir quality of the deeper targets. The irregular distribution of secondary porosity translates into low permeabilities, often <0.1 mD.
Gas shows and hydrocarbon fluorescence are recorded from various intervals in the wells with the best in the Dowra Sandstone Member in Dowra No. 1 and the Mullaghmore Sandstone Formation in Big Dog and Slisgarrow No. 1. The best shows were in cycle 1 (brown oil staining and streaming cut fluorescence in Big Dog No. 1) and cycle 2 (moderate gas show and slow streaming white fluorescence in Slisgarrow No. 1).
The best quality and most mature potential source rocks in the area are Viséan organic-rich mudstone and Namurian mudstone and coal , although the latter are too high in the sequence to provide a source for the Mullaghmore Sandstone Formation. Lower Palaeozoic black mudstone in the Pomeroy and Lisbellaw inliers have vitrinite reflectance values in the oil or gas windows but with very low Total Organic Carbon (TOC) content are poor potential source rocks .
Measured TOC content for the Carboniferous lithologies of <1% in exploration wells ranks them as poor quality source rocks (P947904). However, Rock-Eval and maturity indicator analyses show that they have generated most of their potential hydrocarbons . Original TOC content would thus have been at least twice the present values making them originally good to excellent source rocks. The organic content consists of marine lipid and terrigenous humic organic matter which yield mostly wet and dry gas with minor oil or condensate (P947905). At outcrop, a TOC of 6.32% has been recorded in the Meenymore Formation (P947935) which is the source of most of the surface oil shows . This Formation is, however, stratigraphically too high to be a source rock for the Mullaghmore Sandstone Formation.
The vitrinite reflectance values (R0) of potential source rocks in well samples are generally between 1.45 and 2.75 indicating that they are below the oil floor (R0 <1.4) and above the dry gas preservation limit (R0 >3.2)( in Co. Fermanagh. Big Dog has the least mature R0-depth profile and this is consistent with the live oil shows in the well. Rock-Eval analysis indicates that little hydrocarbon-generating capacity remains. Kerogen maturity (R0-depth) profiles indicate that substantial uplift and erosion has taken place. Projecting the maturity profile back to a near-surface value of R0=0.2% gives an estimate of 3500–4500 m of uplift. It appears that hydrocarbon generation has been suspended since soon after major uplift and erosion began. Although these maturity characteristics extend eastwards to the Glennoo No. 1 borehole, around Lough Neagh vitrinite reflectance values at outcrop and in boreholes decrease significantly (P947906).
The Benbulben Shale Formation provides an effective seal for any hydrocarbons trapped in the Mullaghmore Sandstone Formation. Although the seals may have been ruptured as a result of Variscan faulting (see Variscan (Hercynian) Orogenic Cycle article), leakage of hydrocarbons would have been very low due to the diagenetic reduction in the permeability of the reservoir. The Bundoran Shale Formation acts as a seal for the Dowra Sandstone Member reservoir target.
Burial history, hydrocarbon generation and trapping mechanisms
Understanding the burial history of a rock sequence is critical to defining the relative timing of hydrocarbon generation and migration, the diagenetic reduction of porosity and permeability, trap creation and breaching, and subsequent escape of hydrocarbons. These processes are the key to the formation and preservation of oil and gas accumulations.
In Co. Fermanagh the high maturity of Lower Carboniferous rocks indicates a significantly greater depth of burial than at present. In Northern Ireland, major unconformities correspond to episodes of uplift in the late Westphalian to Early Permian (Variscan Orogeny), Early Jurassic to early Late Cretaceous (Cimmerian), latest Cretaceous to early Palaeogene (Hebridean Uplift), Eocene to late Oligocene and mid-Neogene (Alpine) intervals (P947907). Structural trap formation is most likely to have occurred during the Variscan, Cimmerian and Alpine orogenic events. Younger strata, which could have covered the early Carboniferous basin, include up to 3000 m of Namurian to Westphalian rocks, 3000 m of Permian to Lower Jurassic rocks and 1000 m of Upper Cretaceous sedimentary and Palaeogene volcanic rocks. In Co. Fermanagh and south Co. Tyrone, it is calculated that up to 1800 m of late Viséan and perhaps 1500 m of Westphalian strata were removed by erosion chapter. Estimates of palaeotemperature and palaeogeothermal gradients for the Carboniferous strata from vitrinite reflectance data indicate moderate geothermal gradients (35–50ºC km-1, mean 44.7ºC km-1 from 4 wells) at the time of peak palaeotemperatures . The timing of peak maximum burial and palaeotemperature was probably in the pre-late Mesozoic but could have also preceded the Variscan or Cimmerian episodes of uplift as well. Apatite fission track data from an offshore well west of Ireland, predicts c. 1900 m of post-Middle Jurassic to pre-early Palaeogene (late Cimmerian?) uplift. Modelling of onshore apatite fission track data gives estimates of post-early Palaeogene denudation greater than 2500 m . This last scenario would be favourable for the preservation of hydrocarbons, with generation probably persisting until the Alpine event, allowing hydrocarbons to migrate into structures formed during all three orogenic phases (P947957).
It is likely that Carboniferous source rocks were mature for oil, and wet and dry gas, prior to Variscan uplift. Renewed subsidence in the Permian and Triassic returned the source rocks in most of the basin to the wet gas/dry gas windows. Late Cimmerian uplift possibly suspended hydrocarbon generation but Alpine (mid-Neogene) uplift certainly did.
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