bluuebunny/crossref_metadata_2025 huggingface dataset:
Provenance of braided alluvial deposits of the Thari Formation, Rhodes, SE Aegean: evidence for major erosion of an ophiolite-bearing thrust sheet
Authors: Edgington Lisa , Harbury Neil
Published: April 2008

The Middle to Upper Oligocene Thari Formation sediments in central and southern Rhodes, Greece were deposited adjacent to an active thrust front. They consist of clast-supported massive to horizontally stratified conglomerates, massive matrix-supported conglomerates interbedded with ophiolitic sandstone, and silty mudstone forming lenticular, discontinuous beds with common erosional contacts. Debris flow, stream channel, sheet flood deposits and the high gravel to sand ratios (90: 10), suggest deposition by processes within a gravel-dominated braided alluvial system. Facies relationships demonstrate a clear proximal (NW) to distal (SE) variation across an alluvial fan. Provenance relationships based on 2400 clast counts show that the main constituents of the conglomerates in the north west area are rounded to well rounded pebbles and cobbles derived from an ophiolite sequence (80%) and include harzburgite, dunite, gabbro, serpentinite, and dolerite whilst the other major components, pelagic limestone and chert (20%), have an increasing importance to the south west of the area. These relationships suggest that the Thari Formation formed part of a flexural foreland basin-fill that developed adjacent to a deformed ophiolite stack eroded after early Paleocene to mid-Oligocene convergent tectonics.
Tectonic significance of the Hellenic-Dinaric ophiolites
Authors: Smith Alan G.
Published: April 2008

Isotopic and biostratigraphic data show the Hellenic ophiolites (those SE of the Scutari-Pec transform zone in Albania) are of Middle to Late Jurassic age. They are of two contrasting types. The first type of ophiolite is complete, usually harzburgitic, forms allochthonous sheets, commonly develops metamorphic soles, generally includes significant chrome deposits and shows supra-subduction zone (SSZ) trace-element characteristics with an evolution from MORB to IAT and occasionally boninites. The second type is characterized by incomplete ophiolites which lack chrome and mantle tectonite rocks. In contrast to the first type, these are slightly younger, are exposed in situ, show contact metamorphism against the country rock, are similar to MORB and show little influence of a subduction zone.

The harzburgitic ophiolites from a western and an eastern outcrop belt. The relationship between the two is unclear, principally because of large, probably strike-slip, offsets in the Vardar zone, best documented in former Yugoslavia.

The western Hellenic ophiolites (Pindos, Vourinos, Othris, Evvia ( = Euboea) and Argolis) all formed above a SW-dipping subduction zone in early Middle Jurassic time. They are continuous with a similar belt in the Dinarides of Yugoslavia whose SW margin gradually becomes lherzolitic as it is traced northwards. It is totally lherzolitic at its termination in NW Yugoslavia. This gradual change in ultramafic character is speculatively linked to the changes in spreading rates in a wedge-shaped marginal basin.

Emplacement took place in later Middle to Late Jurassic time probably along a low-angle detachment zone that was close to the lithosphere/asthenosphere boundary shortly after spreading ceased. The SW half of the basin was pushed over the NE half, which was subducted to the SW but has left very little trace of its former existence. In the Hellenides the ophiolites were emplaced onto the Pelagonian continent and in the Dinarides onto the Drina-Ivanjica continent.

Precise analogues of the complete ophiolites appear to be absent in the present-day major ocean basins — because of their SSZ character — or any island arcs that border the present-day oceans — because their marginal stratigraphy shows no trace of arc sediments or volcanics.

The incomplete ophiolites, which appear to be restricted to the Axios zone in Greece, seem to be the expression of transient motion on short extensional segments of a plate margin whose geometry resembled the early stages of the opening of the Gulf of California.
Tectonics of the Mozambique Belt in East Africa
Authors: Shackleton Robert M.
Published: April 2008

The Late Proterozoic Mozambique Belt in East Africa is reviewed, using new geochemical and other data. This review leads to the following tectonic interpretation.

A first collision, between 750 and 800 Ma ago, after the obduction of the Baragoi and associated ophiolites of north-central Kenya, produced intense regional recumbent fabrics and structures, a trans-Mozambique Belt stretching lineation, crustal thickening and metamorphism reaching granulite-facies in the lower crust.

Uplift and erosion were followed by closure of a second oceanic area, obduction of the West Pokot ophiolite and collision along the West Pokot suture, possibly about 580 Ma ago. This second collision produced strong within-plate deformation (Baragoian-Barsaloian) especially in N-S dextral shear zones over 100 km east of the West Pokot suture. The shear zones are associated with late Barsaloian within-plate granites.

It is inferred that eastward subduction led to the successive accretion of plates to an eastern foreland, perhaps represented by Malagasy, culminating in the collision of east and west Gondwana.
The isotope and trace element geochemistry of basalts from the volcanic islands of the southern Red Sea
Authors: Rogers N. W.
Published: April 2008

Alkali basalts from the Hanish-Zukur and Zubair island groups in the southern Red Sea have been analysed for major, trace elements and Sr, Nd and Pb isotopes. They have close to primary compositions with high Mg# (60–70) and Ni and Cr concentrations up to 298 ppm and 597 ppm respectively. Many have only experienced olivine fractionation and this can be readily corrected. Compositional variations in the corrected analyses are interpreted to result from variations in the degree of melting, and a simple inversion of the corrected incompatible element abundances reveals a source composition intermediate between that of N-MORB and primitive mantle. This is suggested to result from the interaction of depleted asthenospheric mantle with less depleted material derived from the nearby Afar plume.

The overall geochemical characteristics of the basalts are comparable with ocean island basalts erupted through thin (young) lithosphere. SiO
2
varies from 44.0–47.9%, Nb/Y from 0.8–1.4 and Ce/Y averages 1.64, close to values from Ascension Island.
143
Nd/
144
Nd ratios show minimal variation between 0.51301–0.51309 whereas
87
Sr/
86
Sr ranges between 0.70323–0.70396, and the analyses define a horizontal trend extending to the right of the mantle array. Pb isotopes also show marked variation,
206
Pb/
204
Pb values varying from 18.675–19.077, but the analyses remain within the ranges defined by MORB and are generally comparable with samples from the Red Sea axial trough and the Gulf of Aden.

Comparison with basalts from Afar and the Main Ethiopian Rift (MER), together with those from other parts of the Red Sea and Gulf of Aden reveal systematic shifts in isotope ratios with degree of extension. Along the constructive plate margins and including the island basalts, isotope results are dominantly asthenospheric, showing variations from N-MORB towards a possible plume end-member. In Afar, where β factors are lower (<2.5), isotope ratios show a greater deviation from oceanic basalts and this is most marked in the MER basalts where β factors are lowest (< 1.2).

These data are tentatively related to a model of passive extension over a mantle plume, driven by plate motions, with the possibility of active, plume-driven extension being restricted to the early stages of Ethiopian flood basalt volcanism and the subsequent development of the MER during the Miocene.
Advances in analytical technology and its influence on the development of modern inorganic geochemistry: a historical perspective
Authors: Potts Philip J. , Hawkesworth Chris J. , van Calsteren Peter , Wright Ian P.
Published: April 2008

Since the 1960s, a range of new instrumental analytical techniques have become widely available in geochemical laboratories, replacing traditional wet chemical procedures that were in common use throughout the first half of the twentieth century. These new techniques have resulted not only in an enormous advance in analytical productivity, but also in the sensitivity with which a wide range of trace elements and isotope ratios may now be determined on a routine basis. It is argued that there is a symbiosis between analytical technology and geochemical application which has resulted in geochemistry currently being regarded as one of the dominant strands of modern geological sciences. The present paper reviews the development of this analytical revolution, evaluates the impact that successive techniques have had on geochemical thinking and looks to the future to suggest areas that may result in advances over the next decade.
Some examples and possible explanations for oil generation from coals and coaly sequences
Authors: Thompson S. , Cooper B. S. , Barnard P. C.
Published: April 2008

Coals and associated shales are important oil source rocks in certain deltaic environments, in which selective processes of degradation or reworking increase the content of hydrogen-rich kerogen. The formation of hydrogen-rich kerogen in a deltaic setting is either by preferential concentration of plant cuticles and spores after reworking of delta top freshwater peats, or, by accumulation of delta margin peats under saline conditions. In each case the source rocks and derived oils may be expected to show distinctive indicators of the depositional environment in the associated alkane content, sulphur content, biomarker and carbon isotope signatures.

Coals in the Oligo-Miocene Talang Akar Formation of South Sumatra and Northwest Java Basins contain perhydrous vitrinite and are the source of isotopically relatively light waxy oils which contain biomarkers derived from tree resins. Waxy oils are also produced in the Sunda Basin, between Sumatra and Java but in this case the upper (coaly) part of the Talang Akar Formation is immature and the oils are derived from algal kerogen in the older, lacustrine Banuwati Formation shales; these oils are distinguished by low contents of land plant biomarkers and by a markedly heavier carbon isotopic signature.

In the Western Desert of Egypt, coals of the mostly marine Jurassic Khatatba Formation have sourced oils with a relatively heavy isotopic signature. It is proposed that the isotopically heavy character of the coals and oils is due to a combination of microbial contribution, and raised salinity (and sulphate) levels due to marine influence.

In northern England, Namurian (Pendleian) marine shales are generally distal turbidites derived from pro-delta sediments but mostly without oil source potential. Rich oil source rocks occur as basinal shales at the extreme margins of or interdigitating with the turbidites or as condensed sequences on structural highs. They contain kerogen derived by reworking of peats of a deltaic-coastal plain environment, and may be correlated with many of the oil shows and produced oils of the area.

The examples demonstrate that the recognition of source rock facies aids exploration by guiding the prediction of the location and potential of oil-prone facies, but that this is only possible if all facets of oil and source rock geochemistry are integrated with a knowledge of sedimentary processes.
A maturity and palaeoenvironmental assessment of condensates and oils from the North Sumatra Basin, Indonesia
Authors: Matchette-Downes C. J. , Fallick A. E. , Karmajaya  , Rowland S.
Published: April 2008

Five light oils and condensates from the wells drilled in Lower to Upper Miocene reservoir rocks of the North Sumatra Basin were analysed by gas chromatography-mass spectrometry and stable isotope ratio mass spectrometry. In conjunction with data from over 1600 well and outcrop samples the results were used to describe the character of the presumed source rocks in terms of organic matter maturity and palaeoenvironment of deposition. The source rock character was further constrained by the maturity of the drilled sections, geothermal gradient observations and re-evaluation of the geology of the region.

Dominant lacustrine with subordinate ombrogenous raised peat bog palaeoenvironments of deposition are indicated. It is envisaged that raised mires fringed large lakes in the area. Tectonic disturbance probably caused periodic flooding of the swamps and ultimately a complete marine incursion occurred.

The oils and condensates are mature to extremely mature. Some oils are mixtures of oils of different maturities and discrete terrestrial sources.
The oil potential of Mid-Jurassic coals in northern Egypt
Authors: Bagge Mark A. , Keeley Martin L.
Published: April 2008

Geochemical analysis of selected coals indicates that some of the Jurassic coals in Egypt have oil-generating potential. Other coals have just gas potential, but may have already generated oils as they are now post-mature for oil generation. Petrographic analysis of the coals reveals a composition dominated by vitrinite, but with variable liptinite contents and the occurrence of fluorescent vitrinite in some coals. It is not clear which components of the coal are responsible for the oil potential; it is probable that liptinite, hydrogen-rich vitrinites and a fine-grained groundmass of algal origin all contribute. A combination of Rock-Eval Hydrogen Indices and pyrolysis-gc P2 fingerprints provides the best method for determination of the oil- or gas-prone nature of the coals.

Attempts at correlating oils to source rock extracts illustrate the problem of oil-source correlations involving a rapidly varying source rock environment. The bitumen extracts from the coals reveal variable characteristics which reflect facies variations in the coals. Several of the coals have unusual saturated hydrocarbon and biological marker distributions that are atypical of humic coals and give a poor correlation to the waxy oils from the Meleiha field. The best oil-source rock correlation is with a carbonaceous shale, not a coal, but it is likely the oils were derived from large volumes of source rock including coals and carbonaceous shales. As the coals have unusual extract characteristics, any oils derived from a more localized environment in the coal swamp may show very unusual geochemical characteristics, and would not be recognized immediately as being derived from terrestrial coals.
Coal and coal-bearing strata as oil-prone source rocks: current problems and future directions
Authors: Scott Andrew C. , Fleet Andrew J.
Published: April 2008

At present, the principal problems concerning coaly sequences as oil-prone source rocks are, still, how can we identify which coals expel oil (liquid-phase petroleum) and how do we predict the presence and distribution of such coals in the sub-surface? To tackle these problems four key areas of study need addressing. Firstly, the representatives of the database, on which our current empirical understanding is based, needs verification. Secondly, the nomenclature used to describe coals and related kerogens must be clearly integrated. Thirdly, the constituents of these coals and kerogens, their botanical precursor and their petroleum products need to be established. Finally, controls on oil migration out of coal-bearing sequences need elucidating. Ways of addressing these problems might include a multi-/interdisciplinary study, ‘bottom-up’ studies from plant constituents, and comparative migration studies based on suites of coals from different settings.
Tectonic control of the sedimentary record and stress-induced fluid flow: constraints from basin modelling
Authors: Van Balen R. , Cloetingh S.
Published: April 2008

Many basins show deviations from the simple subsidence pattern predicted by the stretching model for basin evolution. Incorporating finite strength of the lithosphere during rifting and stresses acting on the lithosphere during the post-rift phase of basin evolution can successfully explain these deviations. Finite strength of the lithosphere during rifting causes flexurally supported rift-shoulders. The erosion of these rift-shoulders induces both additional uplift at the basin margin and sediment loading-induced subsidence in the basin centre, causing tilting of syn-rift sediments. A ‘break-up’ unconformity results from the progressive diminishing of this tilting towards the end of the rifting period, separating the syn- and post-rift sediments. As the amount of erosion is primarily controlled by climatic conditions, basins having the same tectonic history but, for example, located at different lattitudes will not have developed the same pattern of stratigraphic fill. The same effect is expected if another mechanism causes the uplift of the basin margin, as, for example, a change in the level of intraplate stress.

Plate reorganizations are the main cause for changes in the level of intraplate stress. We demonstrate that the in-plane stress variations affect the fluid flow regime in rifted basins, with possible implications for the diagenesis of sediments, primary migration of hydrocarbons, faulting and localization of economic resources. Examples include the North Sea and Pannonian Basin. An increase in the level of compressive stress causes flank uplift and basin centre subsidence, inducing a contemporaneous increase of meteroic water influx and compaction-driven fluid overpressures. An increase in the level of tensile inplane stress induces the opposite effects.
Fluid flow and heat transport in the upper continental crust
Authors: Deming David
Published: April 2008

The upper 10–15km of the continental crust is saturated with aqueous brines and is sufficiently permeable to allow the circulation of these fluids. The most important driving forces for fluid flow in the continental crust are topography gradients, sediment compaction and diagenesis, and buoyancy forces. Topographically-driven flow is generally the most efficient mechanism for mass and heat transport, and therefore has recently been favoured in theories of the origin of Mississippi Valley-type lead-zinc deposits. The magnitude of heat transport by fluid movement in the crust has an exponential dependence upon the depth of circulation. In areas of the continental crust with appreciable topography gradients (c.0.01) and permeabilities greater thanc.10−17m2, the background thermal state will likely be appreciably perturbed by groundwater movement. Fluid circulation in the upper crust is both continuous and pervasive due to topography and fluid-density gradients. The old conception of the continental crust as an unchanging body of solid rock should be replaced by a paradigm that recognizes the continental crust as a two-component system; a solid framework which continuously evolves through thermal, chemical, and mechanical interaction with crustal fluids.
Fluid chemistry and hydrological regimes in geothermal systems: a possible link between gold-depositing and hydrocarbon-bearing aqueous systems
Authors: Nicholson Keith
Published: April 2008

Geothermal fluids form by the infiltration, heating and circulation of meteoric waters in the Earth’s crust. Rock-water reactions, boiling and condensation processes influence the chemistry of the fluids. The composition of geothermal fluids ranges from that of gold-depositing, dilute waters to saline, oil-field brines; as such they form a link between these two aqueous systems. Techniques developed to examine the processes active in geothermal systems may find valuable application in gold exploration and hydrocarbon reservoir modelling. The concept of hydrological regime is introduced and applied to recognising deposits of geothermal activity, the level of preservation and permeable, potentially gold-bearing zones in ancient epithermal systems. Mixing models, such as Cl-enthalpy plots, and geothermometry can aid identification of breakthrough of edge waters and casing damage. Differences in the equilibration time of geothermometers may enable the thermal profile or history of the reservoir to be developed. Statistical models enable major controls or sources of the solutes to be isolated, and aid in the correlation of formation waters between wells.
The role of geopressure zones in the formation of hydrothermal Pb-Zn Mississippi Valley type mineralization in sedimentary basins
Authors: Fowler Anthony D.
Published: April 2008

Geopressure zones immediately subjacent to platform carbonate rocks can be modelled to serve as proximal sources of hydrothermal fluids for epigenetic Pb-Zn deposits in sedimentary basins. In some geopressure zones of the Gulf of Mexico, geothermal gradients can be as high as 10°C 100 m
−1
. This arises because the water-saturated geopressured shale masses act as thermal insulators. Geopressure zones may have sufficient fluid pressure to rupture overlying strata, providing a vertical conduit for hot mineralized brine to migrate directly into carbonate host rocks.
The use of natural He, Ne and Ar isotopes to study hydrocarbon-related fluid provenance, migration and mass balance in sedimentary basins
Authors: Ballentine C.J. , O’nions R.K.
Published: April 2008

Hydrocarbon accumulations contain rare gases derived from the atmosphere, the crust and, in some cases, the mantle. The distinctive isotopic structure of these different rare gas components allows them to be resolved. The relative abundances of the He, Ne and Ar in the crustal, mantle and atmosphere-derived components provides information on the physical processes which have operated in the subsurface. When combined with mass balance considerations, which place constraints on the scale of fluid movements in sedimentary basins, the rare gases provide powerful constraints on fluid provenance and transport.

Results of case studies from two extensional basin systems, the Pannonian and Vienna basin, and the sub-Alpine loading basin of the Po, illustrate how the rare gases provide information about the extent of hydrocarbon-groundwater interaction, constrain the mechanism of hydrocarbon gas transport, and provide an insight into the behaviour of the fluid regimes in different types of sedimentary basin.
Some comments on the classification of resources and reserves
Authors: Armitage M. G. , Potts M. F. A.
Published: April 2008

Resource and reserve classification systems are used by stock markets, investors, mining companies etc. in order to make their decision making apparently more soundly based. In view of the subjectivity of geology and diversity of mining methods, in producing and adhering to such systems those charged with the responsibility of estimating reserves run the risk of inadvertently misleading these people and of risking their own integrity. This is not to say that we should not have classification systems at all, but more that they need to reflect the reality as much as the requirement. There are additional problems at present because of the number of classification systems currently in use and the widely different uses to which they are commonly put. The worldwide acceptance of a single terminology of resource and reserve classification would go some way to reducing these. Different classification systems may be required for different applications. In addition, a move to a more systematic quantitative assessment of confidence is proposed which may reduce some of the problems inherent in such an imprecise science.
INTMOV: a program for the interactive analysis of spatial data
Authors: Hatton W.
Published: April 2008

A set of programs have been developed to compute moving-windows statistics from spatial data, which are fully interactive, allowing the user to compute and display two plan views of the window statistics, the parent population histogram and the two histograms of the windows statistics all on one graphical display. Stationarity of a deposit can be checked instantly at differing window sizes with all the sample data or with partitioned subsets. Data can be read into the routine from a current exploration campaign or from working site-survey data, allowing comparisons between ‘projected’ and ‘ground’ truth. Screen and DXF file output of the local neighbourhood window statistics are plotted in a colour which corresponds to standard confidence bands of the parent or global distribution. Rapid identification of local anomalies is possible. Window statistics for all the neighbourhoods can be written to file at any subset number of the original sample number. Plots of the window means and standard deviations versus numbers of boreholes allow the exploration geologist to check stationarity in a retrospective way. The main aim in the development of these routines was to reduce the amount of user keystrokes needed to compute and display the maximum amount of neighbourhood information. The technique is demonstrated with examples from UK Coal Measures.
Quantifying differences between computer models of orebody shapes
Authors: Sides E. J.
Published: April 2008

Several different methods have been used for the representation of orebody shapes in computer systems including serial-slice, gridded seam, regular block, mathematical functions, solid geometry, and boundary representation models. The accuracy and precison of such models are affected by factors such as errors associated with the original data used, natural variations, errors in data capture and processing, etc.

In order to asses the relative importance of such factors it is necessary to quantify differences in accuracy and precision between different model structures. To meet this objective a new technique which relies on statistical study of a series of sample lines, intersecting two or more different orebody models, is proposed.

Application of this technique on orebody models at the Neves-Corvo copper-tin deposits in southern Portugal suggested that in certain circumstances regular block models can give very poor estimates of the total volume of materials of different geological types. In addition it was shown quantitatively that shape interpretations close to the plane of drill-sections are better, in terms of accuracy and precision, than interpretations for zones falling between the drill-section planes.
Computer modelling of dewatering a major open pit mine: case study from Nevada, USA
Authors: Cameron R. I. , Middlemis H.
Published: April 2008

Operations began in 1985 at Amax Gold Inc.’s Sleeper Mine in Nevada, and the major open pit has been one of the largest North American gold producers. The orebody in Tertiary volcanic rocks is overlain mainly by the Desert Valley basin fill gravels. Natural groundwater levels at the mine are around 10–15 m below surface, and dewatering has been a major activity since initial overburden stripping. With a final projected mine depth of 215 m, a properly designed and cost-effective dewatering scheme, fully integrated with the mine plan, was required. Field investigation and numerical computer modelling proceeded in parallel, the feedback between the two activities being essential to the optimization of the dewatering scheme.

The basis of effective dewatering has been the interception of groundwater in the overburden before it can reach the pit. This paper describes the groundwater model, based on the USGS
modflow
code, which was specifically designed to develop a wellfield design for optimum dewatering. The model enabled the determination of the most cost-effective wellfield configuration, pumping rate and distance from the pit. Subsequent field observations showed that real drawdowns were within 50f those predicted.

The interceptor model has also been used to investigate the dewatering rate and cost implications of expanding the pit in the direction of the wellfield; and the implications of power failure at the wellfield for determination of standby generating capacity.
Volcanism and sedimentation in a rifting island-arc terrain: an example from Tonga, SW Pacific
Authors: Clift Peter D. ,   ODP Leg 135 Scientific Party
Published: April 2008

Scientific drilling of narrow sub-basins within the Lau back-arc basin system of the SW Pacific has recovered uppermost Miocene to Recent volcaniclastic sediment and pelagic nannofossil oozes. Pliocene sediment gravity flow and turbidite sands from the western Lau Basin indicate a local source for the sediment, probably intrabasinal seamount volcanoes active during the initial stages of arc rifting. Derivation of abundant material from either the remnant volcanic arc (Lau Ridge) or the new Tofua arc is ruled out by the rugged topography of the basin and the proximal nature of the facies. Sediments from the Tonga platform, adjacent to the present day Tofua arc, indicate a peak in volcanic activity prior to and during the generation of the first back-arc basin crust at 5.25 Ma. A 2.0 Ma hiatus in arc volcanism on the trench side of the basin after rifting was brought to an end by the foundation of the Tofua arc at 3.0 Ma (Late Pliocene). On the basis of the sedimentary, geochemical and seismic data it is suggested that basin rifting involved an initial stage of extension of the original island arc, accompanied by volcanism in the form of major seamount volcanoes within the basin. These produced volcanic ash by submarine eruption, which was then reworked into adjacent sub-basins by slumping, gravity flow or turbidity current. Basin opening proceeded with a trenchward migration of extension and volcanism with time. This system was disrupted by the southward propagation of the Eastern Lau Spreading Centre into the southern Lau Basin at 1.5–1.0 Ma. This resulted in extension and volcanism being concentrated along the median valley of the spreading centre and a cessation in explosive volcanism of wide compositional range. Sedimentation in the Lau Basin since that time has been principally pelagic with minor ash layers mostly derived from the Tofua arc.
Geochemistry of Lau Basin volcanic rocks: influence of ridge segmentation and arc proximity
Authors: Pearce Julian A. , Ernewein Michelle , Bloomer Sherman H. , Parson Lindsay M. , Murton Bramley J. , Johnson Lynn E.
Published: April 2008

Side-scan sonar imaging of the Central Lau Basin (SW Pacific) has revealed a Central Lau Spreading Centre (CLSC) propagating southwards at the expense of an Eastern Lau Spreading Centre (ELSC) with a small Intermediate Lau Spreading Centre (ILSC) forming a ‘relay’ between the two. Volcanic rocks sampled along these spreading centres, and from two adjacent seamounts, are glassy to fine-grained pillow lavas and sheet flows of basalts, ferrobasalts and andesites. The evolved rocks are mostly confined to the propagating tip of the CLSC and can be explained by a high rate of cooling relative to magma supply, as invoked for magma genesis at propagating ridges elsewhere. Compared with equivalent rocks from the eastern Pacific, the most evolved members of the CLSC suite require similarly high degrees (>90%) of fractional crystallization from their basaltic parents. Fractional crystallisation cannot, however, account for the compositional differences between CLSC and ELSC lavas. Whereas the composition of the CLSC lavas lies just within the compositional spectrum of typical N-MORB, the ELSC lavas are distinctly enriched in alkali and alkaline earth elements, reach oxide and apatite saturation at lower Fe, Ti and P concentrations and generally show greater vesicularity despite slightly greater depths of eruption, all indicative of a water-rich subduction component. They also have lower contents of Ni, Sc, Na and Fe and higher contents of Si at a given MgO concentration that indicate a more depleted and more hydrous mantle beneath the ELSC compared with the CLSC. These results provide further evidence that, beneath the Central Lau Basin, the source composition changes progressively from MORB-type to island arc tholeiite type as the subduction zone is approached, both eastwards from the CLSC to ELSC and southwards along the ELSC to the Valu Fa Ridge. They also indicate that the composition of the subduction component may vary systematically away from the arc, with Th, LREE, Ba, Rb and H (as H
2
O) all present close to the arc, only Ba, Rb and H
2
O present at intermediate distances and just H
2
O perceptible at the furthest distances.
Seismic stratigraphy of an accommodation-zone margin rift-lake delta, Lake Malawi, Africa
Authors: Scholz C. A.
Published: April 2008

The Ruhuhu River enters Lake Malawi adjacent to an accommodation zone, a major linkage region between two adjacent half-graben basins that switch dip-polarity along the axis of the Malawi Rift. A large drainage basin developed because of the drop in elevation of the rift shoulder at the accommodation zone. Single-channel digital seismic reflection data indicate that the Ruhuhu River delta’s offshore depositional slopes are steep (2–7°), and that subsurface reflections show a strongly divergent stratal character. Sets of prograding clinoforms are observed at various stratigraphic levels and are interpreted as sand-prone lowstand delta lobes. These prograding packages are 20–50 m thick, and typically cover an area of
c.
10 km
2
. Such accommodation zone delta margins are considered prospective areas for hydrocarbon exploration because of their enhanced structural closure, high input of coarse clastic material for reservoir facies and close proximity to organic-rich deep-water shales.
Environmental dust analysis in opencast mining areas
Authors: Merefield John , Stone Ian , Jarman Philip , Rees Geraint , Roberts Jo , Jones Jeff , Dean Andrew
Published: April 2008

An atmospheric dust database has been set up for the western end of the South Wales Coalfield. This, the first of its kind in the UK, comprises dust data from British Standard 1747 four-way directional gauges, a window ledge sampler and an experimental frisbee deposit gauge. X-Ray diffraction was used to characterize the dust on the basis of mineralogy. Scanning electron microscopy was used to examine particle size and shape and an energy dispersive X-ray analysis system was used to locate coal particles.

For 18 months, a total of 15 directional and three frisbee gauges were used to collect dust on and around the Ffos Las opencast coal site in South Wales, UK. Additionally, two more directional gauges have been installed in the Brecon Beacons and at Exeter in south-west England to provide data from outside the coal mining area.

Processed data from the X-ray diffractograms, along with the weather data from an onsite weather gauge, are input to a geographical information system (ArcInfo), where they can be manipulated statistically. Rose diagram plots for the area can thus be presented, giving the mineralogical contents of dust from the four directions of each sampling station, set in a spatial context. A series of these plots introduces the temporal element.

This new dust data technology already shows considerable potential for use in nuisance complaints, planning applications, site licence renewals and for the implementation of future European environmental protection legislation.
Controls of coalbed methane prospectivity in Great Britain
Authors: Baily H. E. , Glover B. W. , Holloway S. , Young S. R.
Published: April 2008

Only in the last few years has there been a significant increase in exploration for coalbed methane (CBM) in Great Britain. There are several geological controls on British CBM prospectivity which combine with socio-political constraints in limiting the direct transfer of experience and technology from the USA where, until recently, CBM development has been concentrated. In many parts of the world, the depth of burial and rank of coals may be used as approximate indicators of CBM potential. However, in most British coal-bearing basins, one of the most important factors controlling the amount of preserved adsorbed methane in coals appears to be the degree of syn- and post-depositional basin inversion. Although enough methane to saturate the coals was probably generated during the formation of the Late Carboniferous basins, extensive degasification took place during the end-Carboniferous Variscan orogeny. Subsequent Permo-Triassic and later reburial of coals seems to have been insufficient to replenish adsorbed methane over much of the CBM target areas of Great Britain. Those British coal-bearing basins which can be identified as having been originally deeply buried, only midly inverted during the Variscan orogeny and which have remained relatively deeply buried beneath Mesozoic cover until early Cenozoic times are likely to contain the most preserved adsorbed methane and consequently prove the best prospects. Identification of a consumer and an adequate infrastructure further limit the potential of CBM prospects. In addition, at present, geological factors such as
in situ
stress, Coal Measures sedimentology, coal cleat orientation, hydrogeology and hydrology, and planning and environmental issues are considered in more detail only at the well-siting stage. As a knowledge of British CBM grows, however, these factors will become more important in initial licence acquisition.
Pre-sedimentary palaeo-relief and compaction: controls on peat deposition and clastic sedimentation in the Radnice Member, Kladno Basin, Bohemia
Authors: Opluštil Stanislav , Vízdal Petr
Published: April 2008

Pre-sedimentary palaeo-topography was a major control on facies distribution during the deposition of the basal unit (the Radnice Member) in the Kladno Basin. The undisturbed accumulation of peat took place in calm valleys protected by surrounding ridges from the effect of fluvial dynamics, whereas in open depressions only thin or split coals occurred. Facies distribution and the thickness of deposits depend on the palaeo-relief which influenced compaction rates and peat bog development without a major role of local synsedimentary tectonism within the study area. Different positions of both the maximum thicknesses of the clastic interseam intervals and lines of splitting of the coals have been documented. The thickest coals (up to 7m) originated near the base of the Carboniferous deposits in the axes of depressions, either locked or protected by the adjacent ridges. They are underlain mostly by fine-grained sediments, responsible for an even compaction rate compensated by peat accumulation. Stratigraphically higher coals have maximum thicknesses of only 2.5 m at the depression margins or in shallow depressions. Towards the axes of the main depressions they are either split or thinner due to the rapid compaction rate of a thick sequence of fine-grained sediments and organic matter. Moreover, the drainage pattern changed as a result of the burial of relief. Owing to compaction, however, the river systems occupied routes along the palaeo-valley axes. No mineable coals occur in the area underlain by a thick sequence of coarse-grained sediments as a result of weak compaction.
Organic carbon as a palaeoenvironmental indicator in the marine realm
Authors: De Leeuw J. W. , Frewin N. L. , Van Bergen P. F. , Sinninghe Damsté J. S. , Collinson M. E.
Published: April 2008

Recent developments in biogeochemistry have made accessible all information currently known to be present in sedimentary organic compounds, i.e. the carbon skeleton structure, the positions of functional groups and stable carbon isotope ratios. This combined information has strengthened the applications of sedimentary organic compounds as biomarkers and palaeoenvironmental indicators. This review summarizes the most important biomarkers and their applications in reconstructing marine palaeoenvironments and processes. The marine environment is characterized by the combined presence of specific compounds, such as organic sulphur compounds, dinosteroids, 24-
n
-propylsteroids (even in trace amounts), high amounts of C
25
HBI hydrocarbons and the absence of long chain
n
-alkanes with odd over even predominance, bicadinanes and specific higher-plant triterpenoids (e.g. oleanane). Furthermore, palaeoenvironmental conditions, such as photic zone anoxia, hypersalinity, microbial sulphate-reducing activity, algal blooming, palaeo-upwelling and palaeo-surface seawater temperature are indicated by molecular and/or isotopic characteristics. Biomarkers from terrestrial sources are included because land-derived organic matter is an important contributor to coastal and shallow-marine environments. A short case study, within a Holocene terrestrial/marine setting (Florida Bay, USA), is also presented to illustrate the reconstruction of transgressive-regressive trends using selected organic compounds.
Application and analysis of a new method for calculating tensor permeability
Authors: King Michael J.
Published: April 2008

Geostatistical techniques generate multi-million gridblock reservoir descriptions which must be scaled up to coarser models before being utilized in standard reservoir simulators. When the underlying structure is cross-bedded, and the imposed pressure drop does not follow the layering, the effective permeability must be represented as a full tensor to capture the flow properties of the system. We present a novel method of computation for effective permeability, which preserves tensorial cross-flow effects while remaining compatible with standard (non-tensor) simulation. The resulting mixed finite element formulation, periodic boundary conditions and ‘self-consistent’ inlet and outlet pressures provide an unique non-linear scale-up computation. The method is applied to a synthetic data set drawn from a North Sea reservoir. For a typical value of Kv/Kh — about 5% — the cross-terms dominate over the diagonal in the computation of vertical permeability. In overall response, the use of the standard diagonal tensor and five-point finite difference scheme is in error by about 15%, and in error by a factor of 100% in the evaluation of Kv/Kh. The results are especially important in evaluating the productivity from horizontal wells, where vertical flow dominates.
Towards measurement of capillary pressure and relative permeability at representative wettability
Authors: Kokkedee J. A. , Boutkan V. K.
Published: April 2008

Equipment and experimental methods are presented for simultaneous measurement of capillary pressure and relative permeability, using a modification of the continuous injection method. Initial experiments on Berea core material using laboratory fluids showed that the reproducibility of the measured data is good. Capillary pressure and relative permeability curves measured by our new methods were consistent with previous results. Experiments with a dead reservoir crude demonstrated that wettability changes can be monitored as a function of time. This possibility will be used in future experiments to compare capillary pressure and relative permeability curves under reservoir conditions with those under less severe conditions.
SAGA injection: a new combination IOR process for stratified reservoirs
Authors: Hanssen Jan Erik , Surguchev Leonid M. , Svorstøl Idar , Blaker Tore
Published: April 2008

Specific features of North Sea sandstone reservoirs and reservoir fluids determine quite special conditions for water-alternating-gas (WAG) injection which must be taken into account when tuning WAG parameters for the particular stratification and anisotropy of a heterogeneous reservoir. Combining advantages of waterflooding and gas injection, WAG can be very effective in stratified reservoirs. Even so, gravity segregation and early breakthrough may cause poor sweep, especially in reservoirs with low restriction to the cross-flow within the layers and with low permeable layers underlying high permeable layers. The first part of this paper presents results of WAG prediction and process tuning for a reservoir of this class. The second part introduces a new process for selectively controlling injectant flow, surfactant-alternating-gas-ameliorated (or SAGA) injection, which may yield further improvement. In this process, a foaming surfactant added to part of a water half-cycle preferentially enters the swept region. Foam that is generated by gas/surfactant displacement in a subsequent gas half-cycle will block or reduce further entry of gas and water into the high permeable layer, diverting injectant to unswept layers as the reservoir is maintained on WAG flood. Analytical estimates and numerical simulations indicate positive effects of SAGA injection on oil recovery and GOR over WAG injection. Simulation results quantifying the effect of foam plugging, injection parameters and time factor on WAG performance are discussed. Experiments in a physical stratified reservoir model demonstrate the selective placement of foamer, foam-plug generation, and effectiveness of the combined action of WAG and foam. The visualization of physical model experiments is essential for demonstrating foam process mechanisms and is useful in illustrating segregation and viscous/gravity effects in SAGA injection.
Rapid evaluation of the impact of heterogeneity on miscible gas injection
Authors: King M. J. , Blunt M. J. , Mansfield M. , Christie M. A.
Published: April 2008

To predict and manage petroleum production there are two challenges: first, to generate and utilize tremendous detail in our reservoir description and second, to recognize that the majority of this detail is uncertain. Techniques, therefore, are needed to assess uncertainty while making predictions of oil, water, and gas production. We demonstrate that by using a mixture of old and new techniques (streamlines and fine grid simulation) we obtain the speed of the first while retaining the rigour and accuracy of the second. The method is applied to the rapid evaluation of the impact of reservoir heterogeneity on miscible gas injection.
A WAG injection pilot in the Lower Brent Formation at the Gullfaks Field
Authors: Dalen Vilgeir , Instefjord Rune , Kristensen Reidar
Published: April 2008

The Gullfaks Field is a large oilfield in the Norwegian Sector of the North Sea. The field is compartmentalized by a dense and complex fault pattern, and most of the oil is contained in poorly consolidated but high-quality reservoir sands. Production started in 1986, and waterflooding is the main recovery method. Water-alternating-gas (WAG) injection — or supplementary injection of gas in existing water injection wells — has been identified as a possible method for increasing and accelerating oil recovery from Gullfaks. To verify the potential of this technique, a WAG pilot was initiated in 1991. The target area for the gas was primarily one fault block where major water breakthrough had occurred. In this paper, relevant field observations are presented, and the effects of the WAG pilot injection on flow performance and recoveries are discussed on the basis of the field observations and a detailed, history-matched 3D simulation model for the pilot area. In the present case, the gas migrates rapidly towards the top of the formation and accumulates in secondary gas caps. Improved immiscible displacement of attic oil is the main benefit of gas injection in this case.
Improving oil recovery from Oseberg Gamma North using horizontal wells
Authors: Madsen T. , Lie O. , Velvin M.
Published: April 2008

The Oseberg Gamma North field is a small satellite to the Oseberg field. The reservoir is a tilted heterogeneous sand package with a thin oil zone between a gas cap and an aquifer. The field has been produced from one horizontal subsea well since October 1991. The main reason for the development is to supply gas for injection in the Oseberg field, but also to optimize oil recovery.

In planning the development of a small field like Gamma North, particular attention must be paid to existing uncertainties in geology and reservoir conditions, concept feasibility and cost. Our strategy at Oseberg Gamma North therefore emphasizes flexibility by allowing stepwise decisions to be made as more detailed reservoir information becomes available.