Coal thickness distributions on the UK continental shelf

Abstract

A review of the coals on the UK continental shelf as conventional dry gas source rocks and coalbed methane reservoirs has been undertaken using wireline log data from 1747 oil/gas wells. Coals ranging from Late Devonian to Pleistocene age were recorded, with important coal-bearing sequences in the Lower and Upper Carboniferous, Middle Jurassic and Palaeogene. The coals range from lignite to anthracite rank. Coal thicknesses were interpreted from a combination of gamma, sonic and density wireline logs. However, it is apparent that relatively large source/detector spacings and high logging speeds used in deep oil/gas wells fail to completely resolve thin coals. Frequency distributions of coal thickness for the offshore data set cover about 4720 observations in subsets based on geological age and indicate a high positive skewness with evidence of truncation at thicknesses less than approximately 2 ft (0.6 m). A sample of coal thicknesses based on 1227 observations from 66 cored boreholes in the onshore Westphalian displays similar frequency characteristics, but with the truncation confined to coals less than 1 in (0.03 m) thick. Data truncation seen in the offshore wells is attributed to incomplete wireline log resolution. At the maximum end of the distributions, the data generally display an asymptotic shape for larger thicknesses, normally somewhere in excess of 10 ft (3.0 m). To estimate the unresolved thin coals as well as test the statistical validity of the distributions, comparisons were made of the observed frequency distributions with a number of theoretical distributions of the exponential family, including the generalized Pareto distribution. In addition to this frequential approach, a series of thickness-weighted plots, indicative of coal volume, are introduced. Subsequently, conclusions are drawn regarding the estimation of the total coal resource base from this data. In particular, the effects of the truncation can be quantified and are of the order of 20–30%. These are important adjustments to the estimates of overall coal volumes which are used to assess gas-generating capacity.

Documented international enquiry on solid sedimentary fossil fuels; coal: definitions, classifications, reserves-resources, and energy potential

“…In fact, most mature coals are able to produce both. The results clearly demonstrate (Knight et al, 1996) that in UK, for example, most coal beds are thinner than 1 m with high proportion ( F 60%) thinner than 50 cm (exponential distribution, Fig. 9).…”

“…UK Westphalian onshore coal seams frequency distribution. Note that 60% are < 50 cm (after Knight et al, 1996). (Table 15) (1) What should be the modern definition for reserves and resources, and the corresponding appropriate vocabulary?…”

Distribution of Upper Paleozoic Coal Seams in the Southeastern Ordos Basin

“…It should be noted that when the thickness of the coal seam is less than 0.5 m, the logging response might include the contribution of the upper and lower surrounding rocks of the thin coal seam, blurring the boundary between the coal seam logging anomaly and the lithology of the upper and lower surrounding rocks. Therefore, logging's ability to identify thin coal seams is limited [20,21].…”

“…It should be noted that when the thickness of the coal seam is less than 0.5 m, the logging response might include the contribution of the upper and lower surrounding rocks of the thin coal seam, blurring the boundary between the coal seam logging anomaly and the lithology of the upper and lower surrounding rocks. Therefore, logging's ability to identify thin coal seams is limited [20,21]. According to the above identification threshold of logs with different lithologies, the thicknesses of various lithologies in different layers of the study area can be calculated by Eexplorer software.…”