bluuebunny/crossref_metadata_2025 huggingface dataset:
The palaeoenvironments and dynamics of the Rannoch—Etive nearshore and coastal succession, Brent Group, northern North Sea
Authors: Scott Elaine S.
Published: September 2007

This paper investigates the palaeoenvironments and dynamics of the Rannoch Formation in the East Shetland Basin and the sedimentology of the Etive Formation in one particular part of the basin, the southern Cormorant area. The study utilizes core and well data from the North Sea as well as comparative outcrop data from Utah, western USA.

Rannoch Formation facies, coupled with a slight coarsening and cleaning upward of sand-dominated profiles above offshore shales and below channelized delta top sediments, are indicative of a prograding shoreface. The middle shoreface is dominated by hummocky cross-stratified fine sand characterised by horizontal to very low angle, planar to undulatory laminations, often cut by steeper scour features.

The Etive Formation is characterized in the southern Cormorant area by development of a microtidal, wave-dominated barrier cut by associated inlet channels.

These associations are consistent with an interpretation of the Rannoch Formation as a high energy, storm-dominated environment, with strong wave oscillatory motion accompanied by weak translatory currents as the most likely depositional setting. In the overlying Etive Formation, shoreline characteristics are those of a dissipative coastal system with a complex three-dimensional inshore topography and at least one nearshore bar system developed during fairweather periods.
Samarium-neodymium isotopic constraints on the provenance of the Brent Group
Authors: Mearns Euan W.
Published: September 2007

Samarium-neodymium data for the Brent Group of Gullfaks oilfield provide a detailed record of provenance. The Broom Formation, yielding relatively high Sm-Nd ages in the range 1700–1800 Ma, is interpreted as being derived from a proximal southwesterly source on the Shetland Platform. The Rannoch and lower Etive Formations, having lower Sm-Nd provenance ages (1550–1650 Ma) than the Broom, are interpreted in terms of easterly provenance involving transport westwards to Gullfaks by marine currents. The upper Etive Formation returns to the proximal southwesterly source. This source area grew southwards during initial deposition of the Ness Formation to encompass the Forties volcanic centre resulting in young Sm-Nd provenance ages in some Ness strata. Provenance ages in the upper Etive-Ness section thus have a large range from 1300 Ma to 1800 Ma. It is therefore suggested that the Brent Group was deposited by at least two major river systems: one draining an easterly source in southern Norway and the other draining a proximal southwesterly and southerly source in the Shetland Platform and the central North Sea.
Provenance of Brent Group sandstones: heavy mineral constraints
Authors: Morton A. C.
Published: September 2007

Heavy mineral suites in Brent Group sandstones have been heavily influenced by hydraulic and diagenetic processes in addition to provenance. Geochemical studies of detrital garnet assemblages have shown a great complexity in provenance that argues against a dominant sediment supply northward from an uplifted central North Sea dome. Much of the sediment was fed laterally, particularly in the Broom and Oseberg Formations. The Rannoch, Etive and Tarbert sequences are dominated by similar suites to those in the Oseberg Formation, implying that most of the shoreface and barrier sands were transported longshore from the east. Ness mineralogy is extremely complex. It has a broadly southerly source (possibilities including the southern Shetland Platform and Horda Platform). Close to the western margin of the basin there are strong northern Shetland Platform influences. The lateral and stratigraphic variation in Ness assemblages argues for deposition by several small-scale river systems rather than one single major river. The strong mineralogical variations provide a basis for reservoir subdivision in most of the studied fields, enable the identification of Ness channels that occur at the Etive level, and identify deposits within the Ness sequence that were derived from the barrier sequence into the Ness lagoonal sequence.
Statistical analysis of garnet compositions and lithostratigraphic correlation: Brent Group sandstones of the Oseberg Field, northern North Sea
Authors: Stattegger K. , Morton A. C.
Published: September 2007

Garnet compositions from sandstones of the Middle Jurassic Brent Group of the Oseberg Field, northern North Sea, have been treated statistically in order to assess objectively their value in stratigraphic classification and grouping and for provenance analysis. The data set consists of the compositions of 1550 garnet grains from three wells, as determined by electron microprobe analysis. The statistical techniques used include summary statistics, exploratory data analysis, analysis of correlation structure and of subcompositions, discriminant analysis and fuzzy clustering. General univariate statistics provide the characteristic properties of the data distribution pattern. Almandine is the prevailing component, followed by pyrope, grossular and spessartine. Variation in the bulk data is generally low except for many high spessartine outliers. Stepwise linear discriminant analysis is the first step in stratigraphic grouping of the samples and in end-member modelling. Using all 1550 garnet grains, group classification and separation is efficient for the Tarbert Formation and the Etive-Oseberg interval, but not for the Ness Formation. End-member analysis by fuzzy c-means clustering generates similar stratigraphic garnet logs to those generated by discriminant analysis, the advantage of the method being that fuzzy clustering allows a dynamic interpretation by assuming that the calculated cluster centres represent the garnet compositions of source rocks or source areas. There is an overall increase in almandine with time: the Etive-Oseberg interval is characterized by high pyrope contents: the Ness Formation has high grossular and spessartine contents and depletion of pyrope: and the Tarbert Formation is depleted in grossular. Therefore multivariate end-member modelling of garnet compositions has provided objective criteria for classifying and differentiating samples. The resulting grain and sample distribution patterns provide a clearer picture of the stratigraphic variations than those obtained by using the geochemically-defined garnet end members.
Diagenetic processes in the Brent Group (Middle Jurassic) reservoirs of the North Sea: an overview
Authors: Bjørlykke Knut , Nedkvitne Tor , Ramm Mogens , Saigal Girish C.
Published: September 2007

The Brent Group is extensively cored in the northern Viking Graben and East Shetland Basin in the range between 1.8 and 4.5 km. It therefore offers the opportunity to study diagenetic processes arrested at different burial depths and temperatures. The sandstones of the Brent Group remain poorly indurated and contain only small amounts of quartz cement down to burial depths of about 2.5–3.5 km. The quartz cementation increases with depth reflecting progressive silica cementation sourced by pressure solution. Dissolved feldspar and authigenic kaolinite occur in the shallowest of the Brent Group reservoirs and secondary porosity shows no significant increase with depth. Petrographic analyses show a strong depletion of K-feldspar with depth. This can be explained by albitization of K-feldspar at intermediate burial depths (2.5–3.5 km) and dissolution of K-feldspar during illitization of kaolinite on deeper burial (below 3.7–4.0 km). Extensive quartz cementation and formation of illite are the main causes of an observed rapid permeability reduction below 4 km burial depth. K-Ar dates of authigenic illite indicate that illite formed mostly in Early Tertiary at rather shallow burial depth. Illite formed by alteration of kaolinite and K-feldspar seems, however, to increase very significantly at present burial depths of 3.5–4.0 km, suggesting a more recent formation. Emplacement of oil probably does not stop all diagenetic reactions in the remaining residual pore water, and precipitation of illite and quartz may continue in a closed system.
The reservoir properties and diagenesis of the Brent Group: a regional perspective
Authors: Giles M. R. , Stevenson S. , Martin S. V. , Cannon S. J. C. , Hamilton P. J. , Marshall J. D. , Samways G. M.
Published: September 2007

A regional study of Brent Group diagenesis and reservoir properties has been undertaken in order to determine the main controls on porosity and permeability in the sandstone reservoirs. Data from 44 wells from block 211/7 in the north to block 3/8 in the south and spanning current depths from 6700 to 13 400 ft include 9000 porosity, permeability and grain density determinations, quantitative petrographic information from 850 thin sections, and stable isotopic and K/Ar analyses of authigenic phases.

The diagenesis of the sediments is similar across the study area and most of the diagenetic phases occur in all formations. The sequence of precipitates and dissolution events reflects early porewater evolution in shallow burial environments and later reactions which were essentially isochemical and controlled largely by increasing burial depth and therefore temperature. Early diagenetic products include siderite, calcite, chlorite and vermicular kaolinite. Only where the calcite cements form concretions or cemented horizons have they a significant effect on reservoir properties. Local dissolution of feldspars and carbonate cements took place on the crest of some fault blocks inferred to have been emergent during the Jurassic and this has caused local enhancement of porosity in some crestal wells.

In general, the porosity of each of the reservoir facies decreases systematically with depth but permeability only starts to decrease significantly at depths greater than 10 200 ft (3109.0 m). The general decrease in porosity can be attributed to compaction, together with burial cementation by quartz and iron-rich carbonates. Secondary porosity resulting from feldspar dissolution is increasingly common at depth but there is no net increase in porosity as much of the dissolution was evidently accompanied by the precipitation of authigenic quartz initially with kaolinite but at greater depths with illite. The systematic changes in porosity and the decrease in permeability are compatible with thermally driven dissolution of feldspar and the local reprecipitation of the authigenic silicates; the decrease in permeability corresponds to the presence of increased quantities of illite at depth.
K-Ar dating of illites in Brent Group reservoirs: a regional perspective
Authors: Hamilton P. J. , Giles M. R. , Ainsworth P.
Published: September 2007

This paper reviews existing K-Ar dates from diagenetic illites from the Brent Group and presents new age data from 11 wells from across the East Shetland Basin.

K-Ar ages from diagenetic illites record illite growth in various parts of the Brent Group of the East Shetland Basin from about 60 Ma until 17 Ma. Diagenetic illites from the Cormorant field yield ages, which when combined with burial history simulation, suggest substantial illite growth from 75°C. This temperature is believed to be close to the temperature for the start of illite growth in the Brent Group. Other illite dates, when combined with burial history simulations, demonstrate continued illite growth to temperatures in excess of 110°C. Etive Formation samples of wells in which the overlying Kimmeridge Clay Formation source rocks are mature for oil generation, show a strong correlation between the K-Ar age of the finest size fraction of diagenetic illite and the calculated time at which the overlying source rocks reached a vitrinite reflectance of 0.62%. Data from other formations do not follow this trend and frequently give ages which are considerably older than those from the Etive Formation. It is considered likely that many of these older dates may arise as a result of contamination by detrital illites which are likely to be less common in the highly reworked sands of the Etive Formation barrier complex.

No recent (< 10 Ma) K-Ar dates have been obtained from diagenetic illites of the water zone of any wells so far studied in the Brent province. It is also apparent from many wells where diagenetic illites from both the hydrocarbon and water zones have been dated, that illite growth apparently stops in the water zone relatively soon after hydrocarbon emplacement. Furthermore, the time span for illitization, suggested by dating different illite size fractions of the same sample, is much shorter than that suggested by comparison of the range of temperatures illite apparently grew at and burial histories of Brent Group wells. Consideration of these points has led the authors to conclude that illite begins to form at a temperature of about 75°C probably at quite a low reaction rate. The process of oil migration at somewhat higher temperature perturbs the static aqueous pore fluid medium and increases effective water-to-rock ratio by fluid transport. This results in increased rates of illite growth. Subsequent to the peak of oil generation, fluid flow rates decrease, with a concomitant decrease in illite-forming reaction rates. Any attempt to isolate illite formed at lower or higher temperatures will inevitably also sample illite from the most rapid period of growth and so yield a false and reduced time span of illite formation and apparently old ages for most recently formed illite in the water zone.
Cathodoluminescence of quartz cements in Brent Group sandstones, Alwyn South, UK North Sea
Authors: Hogg A. J. C. , Sellier E. , Jourdan A. J.
Published: September 2007

Quartz cements can, at best, enhance sandstone reservoir quality through strengthening the clastic framework against compaction or, at worst, degrade poroperm properties by occluding pore spaces and constricting fluid pathways. An understanding of the controls on distribution and timing of quartz cementation can therefore aid exploration and production strategy.

In the Middle Jurassic Brent Group sandstones in Alwyn South authigenic quartz, kaolinite and illite development has resulted in severe deterioration of reservoir properties. Further resolution is obtained using a scanning electron microscope (SEM) adapted for cathodoluminescence (CL) mode observations. CL images show a clear distinction between detrial and authigenic quartz. In detail quartz overgrowths are complex and multiply zoned and a recurrent sequence of three major and two minor CL zones is distinguished. Zones become increasingly euhedral from core to rim and reflect variation in crystal growth rates and silica supply during several pulses of silicification. Heterogeneous zones and dissolution-boundary development shows that quartz precipitation is flux-controlled. Correlation of CL zones with clay mineral inclusions allow cement paragenesis to be refined and correlations with fluid inclusion data allow the conditions of quartz precipitation to be constrained.
Migration of petroleum into Brent Group reservoirs: some observations from the Gullfaks field, Tampen Spur area North Sea
Authors: Larter Stephen , Horstad Idar
Published: September 2007

Whereas the processes of petroleum generation and primary migration are beginning to be understood from a quantitative viewpoint, the process of secondary migration is still incompletely quantitatively understood. We estimate that typically the carrier systems feeding Brent Group reservoirs contain an average of around 3% residual oil saturation, the petroleum flowing in high saturation petroleum rivers through the most permeable carrier bed zones. Most of the carrier porosity contains no petroleum. The filling of the Brent Group reservoir in the Gullfaks field (Tampen Spur) is elucidated by petroleum geochemistry and this study indicates that the field has filled from more than one source basin and has been biodegraded post oil-emplacement. The complex filling and degradation history suggests that geochemical study of reservoir petroleum columns is a useful exercise providing information on the fillpoints of these complex reservoir systems and potentially information on the palacohydrogeology of the area.
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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Coal geology, chemical and petrographical characteristics, and implications for coalbed methane development of subbituminous coals from the Sorgun and Suluova Eocene basins, Turkey
Authors: Karayigit A. I. , Eris E. , Cicioglu E.
Published: September 2007

The Sorgun and Suluova basins contain a thick and laterally extensive Lower Eocene coal seam. The stratigraphy of both basins comprises basement rocks, the Yozgat Granitoid of Pre-Eocene age in the Sorgun basin and Jurassic-Cretaceous limestones in the Suluova basin, overlain unconformably by the coal-bearing Lower Eocene Celtek Formation and by transgressive units of Middle-Upper Eocene age. In the Sorgun basin the Artova Ophiolitic Complex tectonically overlies the transgressive units. Neogene and Quaternary deposits cover all the older units unconformably in both basins. The single coal seam is located at the base of the Celtek Formation and reaches a thickness of 6 m in the open-cast mine at Sorgun and 8 m in boreholes in the Suluova basin. Some coal is produced by opencast mining, but most is produced by underground mining methods.

A total of 75 coal samples was collected from both basins and analysed chemically and petrographically. The coals are characterized by a relatively low moisture content (0.7–9.3 wt% on an air-dried basis) and variable ash yields (2.4–67.8 wt% on an air-dried basis) and huminite contents (28.4–89.6 vol%). Measurements of the mean random reflectance of ulminite (mainly eu-ulminite A) vary between 0.46 and 0.60% (0.53% average). These reflectance values and the chemical analyses show that the rank is subbituminous A or transitional to high-volatile bituminous C coal according to the ASTM classification.

The coalbed methane potential of both basins is at present unknown. No borehole has been drilled to explore for this resource and no gas capacity value has been obtained for the coal seam and bituminous shales. The methane explosions which have occurred during coal production in the underground mines imply that both basins may have been an important potential for coalbed methane.
Abrupt changes in marine conditions, Sunneshine Fiord, eastern Baffin Island, NWT during the last deglacial transition: Younger Dryas and H-0 events
Authors: Andrews J. T. , Osterman L. E. , Jennings A. E. , Syvitski J. P. M. , Miller G. H. , Weiner N.
Published: September 2007

Sunneshine Fiord is a shallow fiord that lies at the easternmost tip of Baffin Island, NWT,
c
. 66°33′ N on the western end of Davis Strait, a 600 m deep sill that links Baffin Bay to the north with the Labrador Sea and the North Atlantic in the south. Offshore surface circulation today consists principally of the southward transport of Polar Water in the Baffinland Current. Raised marine deposits in the outer part of the fiord are radiocarbon dated at >57 ka and can be associated with large lateral moraines on both walls of the fiord; Holocene raised marine sediments appear to be absent. A gravity and piston core together (HU82-031-SU5) sample about 8.5 m of the sediment fill. Nine AMS
14
C dates on marine shells and foraminifera indicate that the coring recovered sediments that span around 14 calibrated (cal) of record. Seismic surveys indicate that the upper sediment is acoustically transparent and overlies acoustically stratified materials. There is a major reflector towards the base of the cored section. Analyses of physical properties, lithofacies and biofacies indicate that there is a very sharp break in the record at around 12.4 cal ka, which is interpreted as a change from relatively ice-distal conditions to one characterized by a dramatic increase in cold, ice-proximal foraminiferal benthic species, notably
Elphidium excavata
forma
clavata
. At the same time, there was an extremely abrupt increase in detrital carbonate, most of which is dolomite. Because there is no known local source of detrital carbonate within the fiord, this abrupt event signals an increase in the transport of detrital carbonate derived from icebergs and/or meltwater from sources further north in Baffin Bay, or immediately to the south from Cumberland Sound. The onset of this event at 12. 4 cal ka is synchronous with evidence for an abrupt increase in detrital calcitic carbonate emanating from an ice re-advance within and probably across Hudson Strait. These events are the probable source of detrital calcitic carbonate in the northwestern North Atlantic associated with the Younger Dryas cold period and iceberg rafting event, H-0.
Shelf erosion and glacial ice proximity in the Labrador Sea during and after Heinrich events (H-3 or 4 to H-0) as shown by foraminifera
Authors: Jennings Anne E. , Tedesco Kathy A. , Andrews John T. , Kirby Matthew E.
Published: September 2007

Collapses of the marine-based Laurentide Ice Sheet during the last glaciation are documented by large, abrupt influxes of detrital carbonate into the northwest Labrador Sea, known as detrital carbonate or Heinrich events. The extent of grounded ice on the continental shelf is poorly constrained for these events, but has been assumed to be at the shelf edge. Possible grounding line positions of ice streams in Hudson Strait and Cumberland Sound over the last
c.
34 ka are delimited using the abundance of shelf-dwelling benthic foraminifera, sediment colour and calcite to dolomite ratios in two Labrador Sea cores (87033-009 LCF and 75-009-IV-55 PC). Heinrich events 1 and 2 contain no evidence of glacial erosion of the continental shelf, suggesting that the Hudson Strait ice stream may have grounded on the Hudson Strait sill or on the inner shelf rather than at the shelf edge during these events. Dark grey, dolomite-rich sediments with abundant shelf-dwelling foraminifera document an advance of Cumberland Sound ice onto the shelf after H-3 or H-4 (
c.
34 ka), but before H-2 (
c.
21 ka). Cumberland Sound ice readvanced to a position at or near the shelf edge at
c.
11.1 ka, during the Younger Dryas chron. This advance is nearly synchronous with an advance of Labradorean ice on the southeastern Baffin Shelf and with a detrital carbonate event in Sunneshine Fjord, to the north, suggesting climatic forcing of separate ice dispersal centres during the Younger Dryas.