Abstract: Westphalian coal seams in the Kinetica/Taurus exploration licence EXL 282, which covers part of the North Staffordshire Coalfield, are estimated to ...
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Geological controls on coalbed prospectivity in part of the North Staffordshire Coalfield, UK F. J. M A C C A R T H Y 1, R. M. T I S D A L E 2 & W. B. A Y E R S , J R 2
1Kinetica Limited, 20 Bedfordbury, Covent Garden, London W C 2 N 4BL, UK 2 Taurus Exploration Inc., 2101 Sixth Avenue North, Birmingham, Alabama 35203, USA
Abstract: Westphalian coal seams in the Kinetica/Taurus exploration licence EXL 282, which covers part of the North Staffordshire Coalfield, are estimated to contain over 5.6 x 10l° m 3 of initial coalbed gas in place. In the evaluation of coalbed gas prospectivity in the licence area, data from the coal and the oil and gas industries were used to map coal occurrence, coal rank, the gas content of coal seams, gas in-place, structure, face cleat and in situ stress. The North Staffordshire Coalfield is structurally and stratigraphically complex. Westphalian strata occur in a SSW plunging synclinorium that is highly segmented by faults with displacements as great as 500 m. These faults may either be barriers or conduits for fluid migration. The maximum in situ horizontal compressive stress is generally oriented NNW, subparallel to face cleats and some faults, which may favour openness and transmissivity of those fractures. In the southern part of the licence area, intraformational unconformities occur in the Westphalian. Total coal thickness is greatest (more than 40 m) and coal rank is highest (medium-volatile bituminous) in the northern part of the licence area, coincidentwith the greatest thickness of Westphalian strata. Predictably then, gas content and initial gas in-place per unit area are greatest in the northern part of the licence area, coincident with the occurrence of thick, high rank coals. In this area, initial gas in-place is estimated to exceed 4.0 x 108 m3 per km2 in some areas. Although gas resource trends have been identified, production rates of coalbed gas can only be established through well testing. Exploration Licence EXL 282 was awarded to Kinetica Limited and Taurus Exploration U K Limited in 1993. This licence area is situated in the west-central U K (Fig. 1) and covers 600 km 2 in the counties of Staffordshire, Shropshire and Cheshire. The Westphalian coal-bearing sequence in the area was part of the Late Carboniferous Pennine Basin (Guion & Fielding 1988) within which the coalfields of central and northern England were initially contiguous. The Pennine Basin has been interpreted on a regional scale (Leeder 1982) as a thermal sag basin, infilling an earlier (Early Carboniferous-Namurian) rifting stage, with a basin depocentre in the south Lancashire region. Within the Pennine Basin, coal-bearing strata are of Namurian to Westphaiian D age, with most of the coal resource occurring in the Westphalian A to C interval. Coals crop out in the North Staffordshire area and in the Coalbrookdale Coalfield in the southwest of the licence area (Fig. 2); coalbearing strata are overlain by Permo-Triassic sediments in the Stafford Basin between these two exposed coalfields. The Westphalian Coals in the licence area were deposited in fluvio-deltaic environments with occasional marine influence and were subject to a multi-phase deformation during the Variscan Orogeny. At that time some coal and gas resource were lost due to the erosion of coals and the removal of overburden required
for a gas seal. In addition the Westphalian, strata comprising the coalbed reservoir interval were segmented by faulting. The subsequent Permian and Mesozoic history of the area is poorly understood due to the incomplete stratigraphic record of these ages. This paper focuses on the northern part of the licence area (North Staffordshire Coalfield Area; Fig. 3), where there is a large volume of data in the public domain, in contrast with the relative scarcity of data elsewhere in EXL 282.
Stratigraphy An outline of Westphalian and Stephanian stratigraphy in north EXL 282 is given in Table 1. The main coal-bearing sequence is of Westphalian A to Westphalian C age and is divided into the stratigraphic units of Lower, Middle and Upper Coal Measures (British Geological Survey 1993), but minor coals also occur in the overlying Newcastle Formation (Westphalian D). The Coal Measures strata were deposited in fluvio-deltaic environments within which two broad facies associations were recognized in a study of the Westphalian A and B (Guion & Fielding 1988); an upper delta plain facies (no marine influence) and a lower delta plain facies (some marine influence). There are considerable variations in the thickness of the
From Gayer, R. & Harris, I. (eds) 1996, Coalbed Methane and Coal Geology, Geological Society Special Publication No 109, pp 2742.
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