bluuebunny/crossref_metadata_2025_split huggingface dataset:
Integration of petroleum engineering studies of producing Brent Group fields to predict reservoir properties in the Pelican Field, UK North Sea
Authors: Kantorowicz J. D. , Eigner M. R. P. , Livera S. E. , Van Schijndel-Goester F. S. , Hamilton P. J. 
Published: September 2007

Abstract
Diagenetic models and the results of laboratory experiments on cores from producing Brent Group fields (Cormorant and Tern) have been combined to aid prediction of reservoir properties in a deeply buried prospect (Pelican).

In Cormorant and Tern illite particles are thin (20–40 Å), lath-shaped and liable to damage during drying before air permeability measurement, leading to erroneously high permeabilities. The magnitude of this drying effect (
K
air
conventional drying/
K
air
critical point drying) varies with illite morphology and the timing of hydrocarbon emplacement. In Tern pore-filling illites occur above and below the oil—water contact, and there is no significant difference in drying effect. In Cormorant, the drying effect is higher below the oil—water contact where a diversity of late diagenetic pore-filling illite morphologies occur.

Injectivity tests on cores from Cormorant indicate that sea water injection will not lead to impairment of reservoir quality as a result of clay dispersal or other rock-related phenomena. Impairment occurred when flooding cores with produced water and was avoided by introducing divalent cations, saline brines or approximating in situ equilibrium conditions (pH 6.4). This equilibrium point is strongly influenced by the surface chemistry of feldspar and illite, rather than the more abundant kaolinite and quartz. Burial diagenetic replacement of kaolinite by illite will increase this equilibrium towards pH 7 but should not cause impairment during sea water injection into deeper reservoirs.
Pelican contains abundant pore-filling illite. K-Ar age dates (44–25 Ma) suggest illite growth started after burial to over 8000 ft and palaeoburial temperatures of over 80–90°C. Compared with nearby reservoirs, variations in the nature and abundance of illite result from illitisation being arrested by hydrocarbon charge at different times. Optimal reservoir and aquifer properties occur in shallower structures charged from more deeply buried source rocks. Good reservoir properties occur with charge after short-lived illite growth, although later burial may reduce aquifer properties. The more prolonged illite growth experienced by deeply buried reservoirs such as Pelican, charged late in the Tertiary, has resulted in poor reservoir properties and poor aquifer permeabilities may be expected.
Coalbed methane potential of some Variscan foredeep basins
Authors: Fails Thomas G. 
Published: September 2007

Abstract
Five Variscan foredeep coal basins in Germany and Great Britain are compared for coalbed methane (CBM) potential with the productive Black Warrior coal basin in Alabama. Although generally geologically similar, compared with the Black Warrior the European foredeep basins usually have much thicker coal measures, with more numerous coalbeds and thicker net coal thicknesses. Also compared are coal ranks, the present methane contents of the coals, potential depths of production and post-coalification histories. Despite basin to basin similarities, the CBM productive potential depends on characteristics unique to each coal basin. Only the unmined portions of the eastern Ruhr and western South Wales basins appear to be prospective for CBM; both the characterized by high exploration risks for potentially high economic returns. The Oxford and Kent basins appear to be non-prospective for CBM. The CBM potential of the Bristol-Somerset basin is speculative, given the lack of methane content data and severe tectonic compression. Subaerial exposure and erosion during Stephanian-Permian-early Triassic times may have destroyed the CBM potential of the Oxford, Kent and Bristol-Somerset basins. The Belgian and French foredeep basins are not evaluated here.
Coal thickness distributions on the UK continental shelf
Authors: Knight J. L. , Shevlin B. J. , Edgar D. C. , Dolan P. 
Published: September 2007

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.
Assessment of West German hardcoal resources and its relation to coalbed methane
Authors: Juch Dierk 
Published: September 2007

Abstract

To assess West German hardcoal deposits, a 2.5-dimensional computer model was developed. Based on a comprehensive geological evaluation it rendered the automatic construction of approximately 240 000 sectional coalbeds. A combination of this database with data describing coal properties, gas contents and other parameters relevant to the recovery of coalbed methane may provide reliable and detailed estimations of the gas inplace and recoverable gas volume. At the present state of knowledge only rather generalized estimates can be made. Thus a total of 454 × 10
9
m
3
of coal in the West German occurrences may be multiplied by various factors between 1 and 10m
3
/t to obtain a range of more or less realistic volumes of gas in-place.
Main factors controlling coalbed methane distribution in the Ruhr District, Germany
Authors: Freudenberg U. , Lou S. , Schlüter R. , Schütz K. , Thomas K. 
Published: September 2007

Abstract
The Carboniferous coal deposits in the Ruhr District of Germany have been evaluated for their coalbed methane (CBM) potential utilizing data from more than 600 wells collected during the last 20 years of coal exploration. It can be shown that the distribution of methane (and other gases) and therefore the commercial value of the deposit are controlled by several factors that are a result of a combination of processes related to burial during the Carboniferous, subsequent phases of erosion/exposure and, finally, re-burial. Therefore, the distribution and accumulation are not only a function of coal petrology and geochemistry, but also of basin configuration (e.g. tectonics, hydrology, stratigraphy). Thus the assessment of the CBM potential of the basin requires a multidisciplinary approach.
Coal clasts in the upper Westphalian sequence of the South Wales coal basin: implications for the timing of maturation and fracture permeability
Authors: Gayer R. A. , Pešek J. , Sýkorová I. , Valterová P. 
Published: September 2007

Abstract

Coal clasts, common within channel lag deposits within the upper Westphalian C and Westphalian D Upper (Pennant) Coal Measures of the South Wales coal basin, represent reworked previously deposited coal-forming material. Analysis of clast shape, coal petrology and palynology at 16 localities within the eastern part of the South Wales coalfield suggests that three types of clast are present: (1) large elongate rafts of coal, with axial ratios up to 90:1, with irregular, often ‘fish-tail’ terminations and showing post-depositional compaction relative to the enclosing sandstone; (2) rafts of similar dimensions to (1), but showing no evidence for differential compaction and terminated by cleat fractures; (3) small near-equidimensional pebbles that either show no evidence for differential compaction or in which the surrounding sandstone shows greater compaction than the coal pebble. Both types 1 and 2 rafts have miospore assemblage ages that are indistinguishable from coal seams in adjacent sediments and show a similar range of vitrinite-dominated, maceral group composition. The pebbles have miospore assemblages suggesting derivation from coal seams ranging from a similar age to those of the rafts to Westphalian A (Lower Coal Measures) and with a wider range of maceral group composition than the rafts. Type 1 rafts were derived by erosion of partially consolidated and lithified peat deposits from the contemporary alluival plain, and cleat fractures were developed
in situ
during compaction. They formed as extensional cracks parallel to the regional NNW-SSE Variscan compressive stress. In type 2 rafts and pebbles cleat partly controlled the shape of the clasts and was developed before erosion of the clasts from their source coal seams. Vitrinite reflectance (
R
m
%) indicates that maturation of the rafts to bituminous coal rank developed
in situ
within the Pennant Measures, but in the pebbles maturation to bituminous rank may have been developed in the source coal seam before erosion. It is suggested that this maturation, and the concomitant compaction and cleat development, occurred within ≈ 1 Ma of deposition after rapid burial to ≈ 1 km in a high heat flow regime, associated with hot thrust-guided fluids in a foreland basin setting. Rapid maturation of type III kerogen into the oil and gas generation windows, together with the development of cleat, forming effective migration pathways, has important implications for coalbed methane exploration.
Thrust-related permeability in the South Wales Coalfield
Authors: Hathaway T. M. , Gayer R. A. 
Published: September 2007

Abstract

The South Wales Coalfield is a Variscan foreland basin extensively deformed by both linked and isolated thrusts in response to regional NW-SE compressive stress. Thrusts normally strike NE-SW, but transpression has caused a variable dextral rotation of the thrusts to strike E-W and even NW-SE. The range in orientation has allowed thrust-related fractures locally to be opened within the neotectonic stress field in which σ
1
is oriented NW-SE. It is argued that similar thrust-related permeability should be developed in other coal-bearing foreland basins both associated with the Late Carboniferous Variscan/Appalachian orogeny and with younger compressional tectonic systems.

The dominant meso- to major-scale structures formed during the compression of coal-bearing sequences are thrusts and folds. Strains at leading and trailing tip-lines of isolated thrusts and in the immediate hanging wall and footwall generate tension cracks which may act as methane conduits. Unsealed, these allow permeability parallel to thrust strike. Décollements form as bed-parallel detachments within coals, developing as pervasive shear zones, characterized by cleavage duplexes and C-S fabrics in which a penetrative new hinterland-dipping fabric is formed. This fabric, under changing regional stress conditions, may be opened to form a highly effective gas migration pathway. Coal-bearing strata develop chevron folds with flexural slip in fold limbs and tension gashes in competent strata, generating porosity and permeability parallel to the strike of fold limbs. Incompetent coal seams are strongly sheared, producing cleavage duplexes with contrasting vergence in opposing limbs.
Coal microstructure and secondary mineralization: their effect on methane recovery
Authors: Gamson Paul , Beamish Basil , Johnson David 
Published: September 2007

Abstract
Methane production from coal seams rather than porous sandstone reservoirs is now recognized as a valuable and recoverable energy source in Australia. The Bowen Basin of Australia possesses well defined coal seams that contain major methane resources. However, commercial gas production to date has been hampered by the low permeabilities of the coal seams. Recovery of this valuable resource will be assisted by a fundamental understanding of coal microstructures and presence of mineralization, and their influence on the gas flow behaviour through coal.
This paper examines the relationships between coal type, microstructure, secondary mineralization and gas flow behaviour. The study demonstrates that Bowen Basin coals should not be viewed as simply a dual porosity system of micropores which are surrounded by cleats. Instead, studies using scanning electron microscopy show the Bowen Basin coals have a third porosity system comprising a hierarchy of micron-sized fractures and micronsized cavities at a level between the micropores and the cleat/macropore system, which vary according to coal type.
To determine the influence such microstructures have on the flow of gas through the coal matrix, sorption experiments were carried out on small solid blocks of coal, using a new gravimetric technique. The results demonstrate that a clear distinction exists between diffusivity of dull and bright coals in response to coal microstructure. The gas sorption data suggests that both dull and bright coals can be divided into two categories: coal which have a rapid sorption behaviour and coals which have a slow sorption behaviour.
The results of the sorption experiments indicate that size, continuity, connectivity of the microstructures, and the extent of minerals infilling the fractures and cavities, play a significant contribution to overall permeability, and are likely to play a major rate-limiting factor in the flow of methane through coal at a level between diffusion at the micropore level and laminar flow at the cleat level. The studies indicate that the flow behaviour of gas through coal seams in the Bowen Basin is unlikely to be solely dependent on the cleat system but rather a combination of the cleat, microstructure and secondary mineralization in coal.
Enhanced methane desorption characteristics from South Wales anthracites affected by tectonically induced fracture sets
Authors: Harris Ian H. , Davies Gareth A. , Gayer Rodney A. , Williams Keith 
Published: September 2007

Abstract
It has generally been assumed that anthracite, despite containing potentially high levels of sorbed methane, is unsuitable for coalbed methane (CBM) production because of low levels of fracture permeability. South Wales anthracites are commonly strongly affected by tectonically developed fracture systems formed during Late Carboniferous Variscan compressional deformation. The fracture systems occur as thrust-related slip, slickenside and feather fractures, and are superimposed on previously formed extensional fracture systems that include cleat, conchoidal and bed-parallel fractures. When these fracture systems are intensely developed, the anthracite becomes incompetent and friable as a result of extremely close fracture spacing. According to current models for gas desorption and migration, methane initially diffuses through the micropore system of the coal matrix before flowing freely through larger pores and fracture systems. The models suggest that the more closely spaced the fractures, the more rapidly will the coal desorb methane.

The tectonically fractured South Wales anthracites were investigated to determine the relationship between the rate of desorption of methane and tectonically induced fracture spacing. The former was established using gravimetric desorption isotherm experiments and
in situ
quantification of the latter was achieved by using a hand-drill penetrometer. The results demonstrate a clear relationship in anthracite between both the rate of methane desorbed and the amount of structural deformation. It is concluded that regions of deformed anthracite, previously considered unsuitable for CMB production, should be re-investigated.
Model study of the influence of matrix shrinkage on absolute permeability of coal bed reservoirs
Authors: Levine Jeffrey R. 
Published: September 2007

Abstract
The matrix volume of coal shrinks when occluded gases desorb from its structure. In coalbed gas reservoirs, matrix shrinkage could cause the fracture aperture width to increase, causing an increase in permeability. A computer model was developed, based on elastic rock mechanics principles, to evaluate the potential effect of matrix shrinkage on the absolute permeability of coalbed reservoirs as fluid pressure is drawn down during gas production. The model predicts that the fracture width can potentially increase, depending on the combined influence of a number of parameters, particularly Young’s modulus of elasticity, Poisson’s ratio, fracture spacing and matrix shrinkage parameters. Each of these parameters vary depending on coal composition, so each individual coal will behave differently.

A sensitivity study was conducted to evaluate the influence of each model parameter using a geologically reasonable range of input values. ‘Base case’, upper and lower limits were selected, based on published data. Gas production was simulated by reducing fluid pressure from 1290 PSI (8.89 MPa) to 100 PSI (0.70 MPa). The matrix shrinkage parameter ε
max
was found to produce the largest effect on permeability. Permeability changes as large as +250 mD are predicted for the upper case value of ε
max
. If ε
max
is small, however, the predicted permeability change will be negligible. An increase in Young’s modulus, Poisson’s ratio and fracture spacing each cause a predicted increase in permeability. Results of this model study should be verified through additional modelling, laboratory and field-based studies.
Measurement of gas permeability of coal and clastic sedimentary rocks under triaxial stress conditions
Authors: Konečhný Pavel , Kožušníková Alena 
Published: September 2007

Abstract
A method of gas permeability for coal and sedimentary rocks under triaxial stress conditions is outlined. The transmissibility of compressed air through these rocks is shown to vary markedly, depending on both the rock lithology and containment pressure.
Relationship between the hydrogen content of coal and the lithological characteristics of rocks overlying the coal seam
Authors: Kožušníková A. 
Published: September 2007

Abstract

The Prokop and 38a seams were analysed during exploration of the Frenštát-East and Frenštát-West coal fields (the southern part of the Ostrava-Karviná Coalfield in the Czech Republic). The analyses were designed to show possible relationships between the hydrogen content
H
(daf) of coal and other parameters, especially the character of the strata overlying the coal seams. Where originally very porous sediments, such as those of fluvial channel facies, form the roof of seams, a pronounced influence on the elementary composition of coal may be demonstrated. In particular, a definite reduction in hydrogen content may occur. In contrast, where strata of low porosity, such as the siltstones and mudstones of swamp or oxbow lake facies, form the roof, the decrease in hydrogen in the coal seams is not so pronounced. It is argued that the retention of coalbed methane in a coal seam will be influenced by the porosity of the roof rock.
Analysis of the problems associated with the use of image analysis for microlithotype analysis on solid coal mounts
Authors: Lester Edward , Allen Martin , Cloke Michael , Atkin Brian 
Published: September 2007

Abstract
An automated system has been developed by the Coal Technology Research Group at Nottingham University which is capable of producing microlithotype and maceral data on solid coal mounts, compliant with International Standard requirements for particulate coal analysis. The maceral composition of a 50 × 50 µm area (or ‘point’) analysed can be used to classify the microlithotype. However, some problems in classification can occur due to the increased precision of the automated analysis over manual analysis and, because of this, up to 2% of the points cannot be classified when using the ICCP classification scheme. Experimental investigation has revealed that the repeatability of maceral and microlithotype results depend strongly on the scanning pattern used. Using a grid sampling pattern, 100 points produce microlithotype results with barely acceptable levels of repeatability, but the use of 500 points produces results with a level of repeatability compliant with the International Standard.
Petrological coal seam accumulation model for the Zacler Formation of the Lower Silesian coal basin, southwestern Poland
Authors: Nowak Grzegorz J. 
Published: September 2007

Abstract

Coal-bearing strata of Westphalian A–C age occur in the Zacler Formation of the Lower Silesian coal basin of the Central Sudetes. Numerous coal seams were studied petrographically to reconstruct their depositional environments. Coal seams were sampled from several localities, predominantly from mines and boreholes in the Wałbrzych (northwestern), Nowa Ruda (eastern) and Słupiec (southeastern) regions of the Lower Silesian coal basin. The coals studied range from high- to medium-volatile bituminous coals (
R
m
0.72–1.35%). Petrographically, the coals from the northwestern region are characterized by low to high vitrinite contents, low to medium inertinite contents and a constant amount of liptinite. Coals in the eastern part of the basin are vitrinite dominated, with medium to high inertinite and negligible liptinite contents. All of the coals studied in the southeastern area of the basin have a moderate vitrinite content with high percentages of inertinite (semifusinite, fusinite and inertodetrinite) and with constant amounts of liptinite.

Depositional environments were determined from the petrographic maceral analysis indices. Environments in peat swamps were interpreted from triangular facies diagrams and the tissue-preservation index versus gelification index relationships. Forest moors of telmatic and limno-telmatic zones predominate in the northwestern and eastern areas. In the southeastern region coals of reed-moor facies are also present. On the basis of the maceral composition, two main petrological coal facies were identified: (1) vitrinite-fusinite and (2) densosporinite facies. The variation in spore assemblage and petrographic composition of the seams indicates that environmental changes were accompanied by changes in vegetation. The vitrinite-fusinite facies, high in vitrinite, is characteristic of the forest moor/swamp environment (the Wałbrzych and Nowa Ruda regions), where arborescent vegetation predominated. The densosporinite facies, high in inertinite and intermediate in vitrinite, is typical of reed moor/marsh environments where herbaceous plants prevail.