bluuebunny/crossref_metadata_2025 huggingface dataset:
Recharge components in a semi-arid area: the Sana’a Basin, Yemen
Authors: Alderwish A. M. , Dottridge J.
Published: April 2008

Credible estimates of recharge are essential for assessment of sustainable water resources in semi-arid areas, which rely on groundwater for irrigation and domestic supply. Investigation of the mechanisms and rates of recharge to the aquifers of the Sana’a Basin, Yemen, showed that the main components of recharge are infiltration of surface flows, irrigation return and urban leakage. Limited data and inaccessible, mountainous terrain required the use of robust, low-cost methods of quantifying the recharge components. Infiltration of surface flows from 12 wadis averages 39 Mm
3
a
−1
, with high interannual variation. The recharge was calculated using a combination of a rainfall-run-off model and computation of the water balance for the wadi. Irrigation return flows of 30 Mm
3
a
−1
, 20 to 40% of the water applied, were derived from measurements of soil moisture and matric potential for the three most extensive crops. Urban recharge from mains leakage and seepage through septic tanks was estimated from a well inventory, water supply records and usage rates, as 59% of the water supplied. The total recharge to the aquifers of the Sana’a Basin in 1993 was estimated to be 102 Mm
3
, significantly less than the total groundwater abstraction, of about 184 Mm
3
.
Palaeozoic lithofacies correlatives and sequence stratigraphy of the Saharan Platform, Algeria
Authors: Fekirine B. , Abdallah H.
Published: April 2008

This paper attempts to unravel the interacting effects of eustacy, subsidence and sediment supply in controlling facies distributions in the Palaeozoic of the Algerian craton. These strata, particularly the Cambro-Ordovician section, contain no diagnostic index fossils, and the identification of sequence boundaries is therefore of paramount importance in defining a stratigraphic framework for the region. The wealth of outcrop and subsurface data in the Algerian Palaeozoic enables us to recognize six major sequences, the boundaries of which correspond to major events such as Taconic and Hercynian tectonism, the Ordovician glaciation and global-scale sea-level falls. Formations within these sequences exhibit thickness variations and lateral facies changes that are traceable and predictable in the subsurface. Unconformities of regional extent generally tie to significant facies changes and to inflection points on the global sea-level curve. These trends stem from the high sensitivity of this area, which was structurally stable over most of its Palaeozoic history, to eustatic sea-level fluctuations. The Cambro-Ordovician strata feature a high proportion of coarse clastic rocks, with subtle lateral and vertical facies variations, which were deposited within continental fluvial and shallow marine environments. The Silurian, Devonian and Early Carboniferous sequences show a typical pattern of marginal shallow marine sand belts, which migrate with changing sea-level and pass rapidly into deep marine shales, many of which are organic rich. The Upper Carboniferous strata consist of sediments deposited in continental and restricted basin settings, created as a result of tectonic movements heralding the Hercynian event. With respect to the occurrence of petroleum, the most important reservoirs are found in lowstand tracts in the Cambro-Ordovician and Siluro-Devonian sequences. Thick radioactive marine Silurian and Devonian black shales deposited during major transgressions provide both source and cap-rocks.
Sedimentological and diagenetic controls on Cambro-Ordovician reservoir quality in the southern Hassi Messaoud area (Saharan Platform, Algeria)
Authors: Djarnia Mohamed Robert , Fekirine Berrached
Published: April 2008

The Cambro-Ordovician reservoirs of the Hassi Messaoud area comprise quartzitic sandstones, which rest unconformably on granitic basement and are capped by the Hercynian unconformity. Two sequence stratigraphic cycles are identified: a lower cycle of lowstand, transgressive and highstand deposits, and an upper cycle in which only lowstand deposits are preserved below the Hercynian unconformity. Sandstone of the greates thickness and highest porosity and permeability occur within the lowstand deposits of the two sequences. Petrographic and scanning electron microscope studies were conducted in two wells in the southern Hassi Messaoud area on five sandstone units, to which the following chronostratigraphic terms are applied, from base to top: Ra + Ri lithozones (which contain the bulk of the reserves), Alternance Zone, El Atchane Sandstones and Hamra Quartzite. Reservoir quality is found to bear a strong relationship to clay content and mineralogy. All reservoirs are characterized by an extensive quartz cementation, which reduces porosities to generally below 10%. Permeability is related to clay content and type, with effective permeabilities generally limited to the quartz- and kaolinite-rich Ra lithozone. Permeability is lowest in reservoirs rich in fibrous illite and/or chlorite, such as the Hamra Quartzite. As porosity shows a less direct relationship to matrix mineralogy, there is a poor statistical relationship observed between porosity and permeability. Comparative diagenetic studies carried out within both the oil-bearing and the water-bearing parts of the reservoirs have determined that all the secondary processes occurred under freely operating diagenesis, pre-dating oil emplacement in the structure.
Tectonic controls on the petroleum geology of NE Africa
Authors: Keeley M. L. , Massoud M. S.
Published: April 2008

During the Palaeozoic and early Mesozoic, the NE sector of the African craton remained an entire, integral part of Gondwanaland. It experienced two episodes of intracratonic sagging and marginal cratonic rifting, linked to episodes of strike-slip affecting both the craton interior and its northern margins. Source kerogens accumulated during Palaeotethyan highstands within these sags, across which reservoir clastics prograded during the corresponding lowstands. Such source-reservoir associations are known from the Silurian, Devonian and Carboniferous. The fragmentation of the NE African margin with Mesotethys took place over the period Mid Triassic-Bathonian. From Palestine to Cyrenaica, a rift propagated westwards, detaching crustal terranes along shears that later formed Asia Minor. Deep marine shales within this rift have some proven oil-generating potential. However, the principal Jurassic oil kerogen is found associated with coal swamp-delta clastics, deposited during the Bathonian lowstand, in basins formed between the shears along the new cratonic edge. There is also now evidence for a Mid-Triassic rift penetrating the Sirt Basin from the northwest. Sinistral strike-slip associated with these Mesotethyan shears penetrated deeper into NE Africa during the Late Cretaceous, with subsidence reaching a maximum in the Jebel el Akhdar, Abu Gharadiq and proto-Clysmic Basins, in which rich oil-prone marine source rocks accumulated during the Turonian-Coniacian lowstand. Inversion followed, as the sense of strike-slip reversed with the onset of ‘Syrian Arc’ movements, starting in the latest Cretaceous, and proceeding as a series of pulses into the Late Oligocene. The final phase of ‘Syrian Arc’ tectonics was linked to the opening of the Red Sea and Gulf of Suez Basins, and transtension along the Gulf of Aqaba in the Early Miocene. Falling Miocene sea levels then permitted the accumulation of the restricted ‘Globigerina Marls’ facies within the active graben system of the Gulf of Suez.
Trap architecture of the Early Cretaceous Sarir Sandstone in the eastern Sirt Basin, Libya
Authors: Gras Rutger , Thusu Bindra
Published: April 2008

The Sarir Sandstone is the principal reservoir for oil accumulations in the eastern Sirt Basin in Libya. The main phase of the rifting in this area took place in the Late Jurassic-Early Cretaceous, during which time the Sarir Sandstone was deposited as a non-marine, intra-continental clastic syn-rift sequence. Although successfully explored from 1959 onwards, the prolific eastern Sirt Basin is in a relatively immature stage of exploration regarding wildcat drilling and 3D seismic data acquisition. The most recent phase of exploration, utilizing 3D seismic techniques, revealed a complex structural development. The trap geometries are often related to E-W trending, basement-controlled fault systems, oblique to the NNW-SSE Sirt Basin trend. The fault systems were active during the Sarir Sandstone deposition, giving rise to structural as well as well as combined structural-stratigraphic traps. An increased understanding of trap architecture has led to both re-evaluation of older fields and new discoveries. Complex structural traps exhibiting four-way dip or fault closures, and combined structural-stratigraphic traps, have been successfully explored in recent years, and will continue to provide exploration opportunities. The prospective areas comprise the faulted basin margins and the Sarir Sandstone depositional and erosional edges. Modern geophysical techniques including high-resolution 3D seismic data acquisition are critical in discovering and developing the remaining potential.
The exploration history and controls on hydrocarbon prospectivity in the Wessex basins, southern England, UK
Authors: Buchanan James G.
Published: April 2008

The Wessex basins were formed during Mesozoic extension and were subsequently modified by Cretaceous uplift and Alpine inversion events. The basin geometry and evolution of the area is strongly controlled by the long-lived fault systems which cross the area.

Two primary plays are recognized within the Wessex basins; the Triassic Sherwood Sandstone and the Jurassic Bridport Sandstone. These plays contribute the vast majority of the recoverable reserves in the area. The Triassic play consists of a Sherwood Sandstone Group reservoir, a Liassic mudstone source rock and a Mercia Mudstone Group regional seal. Hydrocarbons were generated from the Late Jurassic to Tertiary in the main kitchen area and migrated into a range of fault-related traps. The main risks on the play are reservoir quality together with the timing and the route of hydrocarbon migration into valid traps.

The Jurassic play consists of the Bridport Sandstone Formation which is also sourced from the Liassic mudstones and the top seal is provided by the Fuller’s Earth Formation. The dominant risk on this play is the extent and quality of reservoir facies. In the onshore domain trap definition, using two-dimensional seismic data, is an additional risk on both plays.

The basins of southern England have been explored for hydrocarbons for over 50 years. The initial phase of exploration focused on the onshore and tested anticlinal structures which had been mapped at surface. Onshore activity reached a peak in the successful drilling by British Gas of the Jurassic and Triassic reservoirs in the Wytch Farm Field in 1973 and 1977, respectively.

The first offshore well in the United Kingdom Continental Shelf (UKCS) was drilled off Lulworth Cove by BP in 1963. More recently, offshore exploration concentrated on fault-related traps within or on the edges of the Portland-Wight Basin. Although offshore activity has been reduced in the last 5 years, the 14th UKCS Licensing Round rekindled interest in the area as companies were awarded previously unlicensed acreage for exploration.

Successful future exploration in the Wessex basins will require a more complete understanding of reservoir development and three-dimensional basin evolution. Future drilling will focus on untested fault bounded prospects and stratigraphic traps with suitable reservoir quality.
The tectono-stratigraphic development and exploration history of the Weald and Wessex basins, Southern England, UK
Authors: Hawkes P. W. , Fraser A. J. , Einchcomb C. C. G.
Published: April 2008

Exploration drilling for hydrocarbons in southern England commenced over 50 years ago prompted by numerous seepages along the Dorset coast and gas shows in water boreholes. The two main depocentres, the Weald and Wessex basins, exhibit many similarities in both tectonic and stratigraphic evolution through Early Triassic-Tertiary times, reflecting the regional influences of Atlantic Margin rift-subsidence processes and subsequent Tertiary (Alpine) inversion tectonics. Given these similarities, the development of the hydrocarbon play system in both these areas can be compared using a common tectono-stratigraphic framework which has been identified and calibrated using well and seismic data. In detail, facies variability recognized within individual depositional units highlights differences in both source and reservoir distribution between the Weald and Wessex areas, which have in turn exerted fundamental controls on the hydrocarbon prospectivity of the two basins. These differences are influenced by the position of major basin-bounding fault systems and by the changing importance of the Cornubian-Armorican and the London-Brabant Massifs as clastic provenance areas through time. Sequence isopachs illustrate the same discrete episodes of rift-related subsidence in both areas. These rifting episodes led to the development of extensional trapping geometries particularly during Late Jurassic-Early Cretaceous times. Subsequent Late Cretaceous thermal subsidence led to the formation of mature source kitchen areas focused within the hanging walls of the earlier extensional faults. Hydrocarbon expulsion from the source kitchen areas ceased during Tertiary uplift and subsequent cooling. Inversion movements on the old extensional faults led to the development of hanging wall anticlines which rely upon remigration mechanisms, including exsolution of gas from uplifted Liassic source rocks, to receive hydrocarbon charge. To date over 300 million barrels of oil and 100 bcf of gas have been discovered.
Use of palynofacies analysis to define Lower Jurassic (Sinemurian to Pliensbachian) genetic stratigraphic sequences in the Wessex Basin, England
Authors: Cole David C. , Harding Ian C.
Published: April 2008

Quantitative palynological and palynofacies analyses allow the Lower Jurassic succession of the Dorset coast (Wessex Basin, southern England, UK) to be placed within a genetic sequence stratigraphic framework. The multifaceted approach employed utilizes various palynological criteria adopted from previous publications (e.g. both relative and absolute particle abundance data, allochthonous versus autochthonous palynomorphs ratios, phytoclast type, etc.), and suggests new parameters which may be studied in this context (e.g. ratios of acritarch spine length). Interpretation of plynofacies trends within the studied succession enables the identification of eight genetic sequences informally described as Genetic Stratigraphic Sequences I–VIII. Genetic stratigraphic sequence boundaries are placed at the following horizons: Mongrel, Grey Ledge, Pavior, the Coinstone, Hummocky, the Belemnite Stone, Day’s Shell Bed and the
margaritatus
Stone. These results compare well with sequence stratigraphic frameworks for published for the British Lower Jurassic interval both in Dorset and elsewhere, suggesting that the maximum flooding events recognized are likely to have been of inter-regional extent.
The documentation of a latest Jurassic-earliest Cretaceous uplift throughout southern England and adjacent offshore areas
Authors: McMahon Neil A. , Turner Jonathan
Published: April 2008

The development of the Wessex Basin, Celtic Sea Basins and the Western Approaches Trough was initiated during the break-up of Pangea with active subsidence continuing throughout the Mesozoic until the basins were inverted in the Tertiary. The Mesozoic stratigraphy in these basins is characterized by a number of unconformities. In the Early Cretaceous two major erosional events can be identified, one in the Berriasian and the second in the Aptian. These unconformities have a significant bearing on both the Cretaceous stratigraphy and the hydrocarbon potential in the region.

Stratigraphic data from onshore exposures in the Wessex Basin, from over 100 boreholes, and seismic data document the evolution and erosion pattern of both unconformities. The Aptian unconformity has been well documented by previous authors, and may be related to uplift associated with commencement of sea-floor spreading in the Bay of Biscay. However, an older unconformity can be identified as Berriasian in age on the basis of tracing its correlative conformity using vertical and lateral facies changes and a rapid lateral increase in the thickness of the Wealden Group. This older unconformity is herein interpreted to have been formed by a latest Jurassic-earliest Cretaceous uplift event and can be recognized in all basins surrounding the Cornubian Platform. The areal extent of the unconformity implies that the interpreted uplift was of regional significance, centred on the Cornubian Platform, but unlike the younger Aptian event was unrelated to any specific intra-basinal extensional event or the Apto-Albian sea-floor spreading in the nearby Bay of Biscay. Its effects dominated Early Cretaceous sedimentation, the results of which are demonstrated in subsidence patterns within these basins.

The thermal maturity of the Lias (Early Jurassic), the main hydrocarbon source rock in the area, has also been affected by the latest Jurassic-earliest Cretaceous uplift, as well as Tertiary uplift and inversion. The latest Jurassic-earliest Cretaceous event resulted in the erosion of much of the Jurassic succession in the basins closest to the Cornubian Platform, and in many areas uplifted the Lias from the top of the oil window. Only on the flanks of the uplift, where high sedimentation rates occurred during the deposition of the Purbeck and Wealden Beds, were source rocks buried to depths sufficient to generate hydrocarbons. Only by understanding the tectonic evolution of the latest Jurassic-earliest Cretaceous uplift can accurate play risk modelling and prediction of the hydrocarbon fairway take place.
Biodegradation of seep oils in the Wessex Basin — a complication for correlation
Authors: Bigge M. Ashley , Farrimond Paul
Published: April 2008

A detailed organic geochemical investigation of seep oils from the Dorset coast has revealed notable variation in both the extent and pathways of biodegradation. All the seep samples analysed from Mupe Bay, Stair Hole (near Lulworth Cove) and Osmington Mills have had their
n
-alkanes and acyclic isoprenoid alkanes removed, but some samples are more extensively degraded, with partial loss of steranes and/or hopanes. At Mupe Bay (conglomerate matrix samples; see Parfitt & Farrimond 1977 this volume) the hopanes have been preferentially attacked, whilst at Stair Hole the steranes appear heavily degraded although there has been no alteration to the hopane distribution. 25-Norhopanes were not detected in any of the samples.

Biological marker distributions of seep oils which have suffered no hopane or sterane biodegradation are compared with those of three reservoired oils from the area (Wytch Farm and Kimmeridge fields). Molecular parameters indicate significant variation in source rock facies and maturity within the oils of the Wessex Basin.
Reservoir architecture of the upper Sherwood Sandstone, Wytch Farm field, southern England
Authors: McKie T. , Aggett J. , Hogg A. J. C.
Published: April 2008

The Sherwood Sandstone Group reservoir in the Wytch Farm field comprises a
c.
150 m thick succession of arkosic sandstones deposited in a variety of fluvial, lacustrine and aeolian depositional systems. These systems show at least three orders of facies variability, which are interpreted to be the depositional response to climatic changes. These comprise a first-order evolutionary trend over the entire Sherwood Sandstone Group from perennial braidplain to ephemeral sheetflood systems to ephemeral lacustrine conditions. This trend culminated in deposition of the Mercia Mudstone Group, and reflects a long-term waning of sand supply and increasing ‘flashiness’ of the fluvial system. This trend is further subdivided into second-order cycles defined by five areally widespread floodplain and lacustrine deposits containing minimal development of fluvial sandstones. These represent widespread, episodic reductions in fluvial sediment supply and rising base level during more ‘humid’ climatic conditions. These horizons form the basis for the reservoir layering scheme. Each floodplain episode is increasingly more mud-rich upwards through the Sherwood section, and the sand-rich fluvial packages between become systematically more ephemeral in character. Third-order cycles are defined by thin (<2 m), but alreally widespread floodplain and lacustrine horizons which are most readily identifiable in the upper half of the Sherwood section. The sandstones between these cycles are composed of aeolian and sheetflood deposits, but are incised by coarse-grained multistorey-multilateral channel deposits. The incisions are interpreted to be the result of fluvial erosion during dry climatic conditions when lake levels fell and the alluvial plain was devegetated. These incised fluvial deposits form the principal producing intervals in the upper part of the reservoir, particularly in the eastern part of the field. Higher frequency stratigraphic cycles are locally expressed by variations in ephemeral lake levels, palaeosol development and episodic development of wind-blown sand patches. At outcrop, the stratigraphically equivalent Otter Sandstone Formation (
c.
100 km to the west) shows comparable evolutionary patterns, albeit with a subtly different facies make-up. The recognition of a hierarchy of climatically driven cycles within the reservoir permits high-resolution correlation and the recognition of subtle, but important, changes in sandbody geometry and connectivity within successive cycles.
Exhumation of UHP rocks by transtension in the Western Gneiss Region, Scandinavian Caledonides
Authors: Krabbendam Maarten , Dewey John F.
Published: April 2008

In the Western Gneiss Region (WGR) in the Scandinavian Caledonides, Scandian ecologites (
P
= 16 to >28 kbar) occur in a large area of reworked Proterozoic gneisses, structurally below a series of Scandian nappes. The top-to-the-west, extensional Nordfjord-Sogn Detachment (NSD) separates the WGR from allochthonous units, which include several late-orogenic Devonian basins. The allochthon has not experienced Scandian high-pressure (HP) metamorphism. Below the NSD, the WGR is intensely deformed under late-orogenic amphibolite-facies conditions. This deformation is bulk-constrictional, as indicated by a linear feldspar fabric within augen gneisses and tight to isoclinal, lineation-parallel folds within layered gneisses. In a later stage, the NSD, the WGR and the Devonian basins were folded by east-west trending folds, coeval with continuing movement along detachments.

To explain these features we propose a transtensional model for the late-orogenic evolution of the WGR. Transtension in West Norway had a sinistral sense and was partially partitioned with increasing transtensional angle towards the NE-SW trending Møre-Trøndelag Fault Zone in the NW. During transtension, there is a strong tendency for rejuvenation of detachments, because detachments fold and may lock as they move. In the WGR, the younger Hornelen Detachment developed above the older NSD. Transtension was the principal exhumation mechanism of the HP and ultra-high-pressure (UHP) rocks in the WGR and involved oblique plate divergence of Laurentia and Baltica during the Early Devonian.
The estimation of modal mineralogy: a problem of accuracy in core-log calibration
Authors: Harvey P. K. , Brewer T. S. , Lovell M. A. , Kerr S. A.
Published: April 2008

In the case study described here the quantitative modal mineralogy of a number of core samples was determined with the objective of using these modes to calibrate geochemical logs. Modal estimates were obtained for the core samples by quantitative X-ray diffraction, infrared spectroscopy, point counting of thin sections, and indirectly by calculation from a complete chemical analysis of the samples. In the case of calculated modes, three different algorithms were applied. A by-product of this particularly complete dataset is the possibility of evaluating the most accurate method of modal analysis, and although no certain conclusion is reached on this point the analysis of these data does demonstrate the difficulty of obtaining accurate modal estimates. The core samples, taken at regular intervals through a sand, sandy-shale sequence, capped by a carbonate unit, have a mineralogy which, although dominated by quartz, includes feldspars, carbonates, and clays (illite, kaolinite) together with minor phases. There was generally good agreement between methods in the estimation of quartz, total carbonate, albite, kaolinite, total clay and pyrite. The results for illite and K-feldspar were poor, a reflection of their relatively low concentrations (<10%), and problems of compositional co-linearity in the calculated modes.
Quantitative lithology: open and cased hole application derived from integrated core chemistry and mineralogy database
Authors: Herron M. M. , Herron S. L.
Published: April 2008

A new quantitative lithology interpretation is based on elemental concentrations of silicon, iron, calcium and sulfur available from logs. The lithology interpretation is founded on an integrated chemistry-mineralogy core database comprising over 400 samples from many wells of predominantly sand and shaly sand composition located on four continents. The lithological components include ‘clay’, which is the sum of all clay minerals; ‘carbonate’, which is the sum of calcite and dolomite; ‘anhydrite’, which is the sum of anhydrite plus gypsum; and ‘sand’ or ‘quartz-feldspar-mica’, which is the remainder of the formation essentially constituting the sand fraction. The new interpretation demonstrates that the elements aluminium alone or a combination of silicon, calcium, and iron provide a much more accurate estimation of clay than either gamma ray or its individual components potassium, thorium and uranium. Calcium alone or calcium and magnesium are used to determine carbonate concentrations. Calcium and sulfur can be used to estimate the anhydrite fraction. Having estimated the total clay, carbonate, and anhydrite fractions, the remainder of the formation is assumed to be composed quartz, feldspar, and mica minerals. Examples of the new lithology interpretation are provided for core data and also for geochemical log data from both open and cased hole environments.
Effects of water salinity, saturation and clay content on the complex resistivity of sandstone samples
Authors: Denicol P. S. , Jing X. D.
Published: April 2008

Complex resistivity measurements were made on sandstone samples in the frequency range from 10 Hz to 2 MHz. The main objective was to investigate the frequency response of complex resistivity and phase angle as a function of salinity, water saturation and clay content. The results showed the classical frequency dependence behaviour where the complex resistivity decreases with increasing frequency. The complex impedance behaviour in the intermediate frequency range (10–100 kHz) was used to relate the effect of frequency dispersion with interface polarization and, hence, pore geometry, specific surface area and permeability.

Both water saturation and salinity were found to influence the gradient and the relaxation frequency of the complex resistivity versus frequency relationship. A variation in water saturation from full to partial saturation resulted in a dramatic increase in the gradient and a clear shift of the relaxation frequency. Both the saturation and salinity dependence can be attributed to the polarization of both the rock-fluid and fluid-fluid interfaces within the pore space, which depend on the geometry and physical characteristics of the interfacial layers. The results presented in this paper can have important applications in identifying low resistivity and low contrast pay zones.
Sedimentation on a Humber saltmarsh
Authors: Brown Sue L.
Published: April 2008

Sediment deposition was measured on two transects down the profile of Skeffling marsh in four short term studies, ranging from two tidal covers to a spring tide cycle, and in longer term studies of four intervals over 18 months. Sedimentation in
Spartina
and bare areas at the same elevation at the marsh front was measured to determine the role of
Spartina
in accretion, and the effect of two levels of stem density on short term deposition was compared.

In terms of sediment deposition per number of tidal inundations at the different elevations, accretion was greatest on the upper parts of the marsh. At lower elevations, subjected to more frequent inundation, deposition generally increased with the number of tidal covers in three of the four short-term studies, although there was evidence of resuspension and/or bedload movement. During an autumn high spring tidal cycle, large amounts of sediment were deposited on the upper marsh (up to 4–5 mm), but very little sediment was retained on the lower marsh and erosion was observed on the bare mud at the marsh edge.

The longer term studies revealed accretion at all sites on the marsh compared with the initial baseline measurements, ranging between 3.2 and 6.3 cm after 18 months, depending upon the transect and elevation. At both transects, accretion was generally continuous at the upper levels, but deposition was more variable in the lower zones, particularly the bare mud at the marsh edge. This area accreted the most during the summer months, but lost material during the winter of 1996–1997 while the upper zones continued to build up.

There was no evidence to suggest that
Spartina
enhances deposition in the pioneer zone at Skeffling. The tide advances rapidly across the expanse of mudflats and scours channels around the vegetation at the marsh edge.
Spartina
is probably more effective as a sediment stabilizer during periods of erosion and any resulting net accretion is likely to be apparent only when measured over longer time scales.
Temporal variation in sediment erodibility and suspended sediment dynamics in the Dollard estuary
Authors: Kornman Bart A. , Deckere Eric M. G. T. De
Published: April 2008

The influence of biological activity on sediment erodibility was studied in the Dollard estuary, The Netherlands. The sediment erodibility was measured with an
in situ
flume on an intertidal flat. The hydrodynamics and suspended sediment concentrations above the flat were monitored from a measuring-bridge and in the tidal channel from a platform, using conventional instrumentation. The data from the flume, bridge and platform show that erosion resistance varied considerably from March until August 1996. A diatom bloom at the beginning of April appears to have resulted in an increase in resistance. Currents and waves were no longer able to resuspend sediment from the flat. This resulted in very low mean suspended sediment concentrations. The increase in erosion resistance did not only occur at the research site but on all flats in the Dollard. At the beginning of June, sediment erodibility increased and consequently the suspended sediment concentrations increased. This coincided with the disappearance of diatoms in May and with an increasing density of the amphipod
Corophium volutator
on the flat in June.
100 years of environmental change in a coastal wetland, Augusta Bay, southeast Sicily: evidence from geochemical and palaeoecological studies
Authors: Cundy Andrew B. , Collins Philip E. F. , Turner Simon D. , Croudace Ian W. , Horne David
Published: April 2008

Recent (post-1950
ad
), industrial development in southeast Sicily has resulted in extensive modification of the coastline (port construction) and a potential for persistent, chronic pollution (oil refinery and petrochemical developments). High resolution radio-metric dating (
210
Pb and
137
Cs) and palaeoenvironmental analysis of a short saltmarsh core from the Mulinello estuary (Augusta Bay) provides a record of change over the period 1880–1995
ad
. Palynological data indicate expansion of halophytic vegetation across the study area between
c
. 1880 and 1945
ad
. An increase in sediment accumulation occurred between 1945–1965
ad
, with an associated rapid change in saltmarsh vegetation occurring at the beginning of this period. Palynological and geochemical data indicate that this increase in sediment accretion is probably due to an enhanced input of catchment-area derived material. This ante-dates the main period of industrial activity in the area, and probably reflects fluvial/catchment processes rather than local anthropogenic changes. Port and industrial developments, in comparison, have had a less significant impact on marsh vegetation and sediment supply, despite large-scale disturbance at the rear of the marsh. Trace element analyses show little input of industrial effluent to the Mulinello despite its location in a heavily-industrialized area. This, combined with preliminary diatom studies, indicates that the Mulinello receives most of its sediment from inland and local sources. The rapidity of local environmental change revealed in this sequence illustrates the dynamic nature of wetlands in microtidal settings and emphasizes their potential sensitivity to both local and regional disturbance. This study also demonstrates the use of combining palaeoecological with geochemical analyses to reconstruct environmental change in fluvial and estuarine wetland settings.
Microscale biogeotechnical differences in intertidal sedimentary ecosystems
Authors: Meadows Peter S. , Murray John M. H. , Meadows Azra , Wood David Muir , West Fraser J. C.
Published: April 2008

Intertidal sediments are inhabited by organisms that can modify the geotechnical and sedimentological properties of the sediment. We have analysed small scale differences in these properties at four closely adjacent sites on Ardmore Bay, Clyde Estuary, Scotland. The sites were an
Enteromorpha
algal mat site (EAM), a
Corophium volutator
site (CV), the head shafts of
Arenicola marina
(AMHS) and the tail shafts of
Arenicola marina
(AMTS) burrows. Three replicate cores were taken from each site. We measured load resistance, particle size parameters of mean particle size, sorting, skewness and kurtosis, and total organic matter (TOM) and carbonate. Load resistance was measured with a newly developed microscale load resistance penetrometer which measured load resistance of the sediment at 1 mm intervals through the sediment core from the surface to 100 mm. Sediment cores were then sectioned every 10 mm and particle size, total organic matter and carbonate content measured.
In situ
shear strength measurements were also taken at the four sites. The data were analysed by bivariate correlation analysis and multivariate cluster analysis. There was a number of significant positive and negative correlations between the parameters at the four sites. Significant down-core changes in the sediment parameters and their correlations were noted. These differed between the four sites. The CV site had the largest number of significant correlations, the EAM site had the least. Cluster analyses of the sites showed that in general the sites clustered separately, although there were a number of overlaps. The EAM site showed distinct clusters for its three separate replicate cores, while the CV site clustered into a top part and a bottom part of the core. Cluster analyses of the depths across all the sites identified a break in the data at depths of between 40 and 70 mm. There was a linear relationship between field shear strength and laboratory penetration resistance. The results are discussed in relation to the fine-scale geotechnical and sedimentological heterogeneity of intertidal sediments, and the effects of biological activity.
Simulation of oil expulsion by 1-D and 2-D basin modelling — saturation threshold and relative permeabilities of source rocks
Authors: Okui A. , Siebert R. M. , Matsubayashi H.
Published: April 2008

In order for oil to accumulate in economic quantities, it first has to be generated and expelled from source rocks in sufficient quantities. In spite of long term efforts, the mechanism of oil expulsion from the source rocks is not completely understood. For modelling expulsion, the adoption of pressure-driven multiple-phase fluid flow governed by Darcy’s law is widely accepted. However, relative permeabilities for fine-grained source rocks, which is an essential parameter for this model, are very difficult to specify. The conventional reservoir-rock curve is obliged to be used for the modelling. Simplification of the relative permeability model is generally used for one-dimensional basin modelling. The 1-D model also requires substantial optimization of the key parameter, saturation threshold.

Laboratory measurement of relative permeabilities for fine-grained rocks is very difficult, therefore, we carefully interpreted the analysed laboratory data on various sandstones to establish a relationship between relative permeability curves and pore geometry parameters, with a view to extrapolate the relationships developed in sandstones to fine-grained source rocks. It was found that the relative permeability curves, or irreducible water saturation are controlled by two factors ; grain size and clay content. Since the total surface area of the pore system becomes greater as the grain size decreases, we consider surface water adsorbed on the grain surface as a part of irreducible water. Since clay contains micro-porosity which cannot be displaced by oil due to high capillary pressure, we regard this to also play a role. Prediction of relative permeability curves for fine-grained rocks by both processes results in the curve with high irreducible water saturation.

The new relative permeability curves were tested by both one-dimensional and two-dimensional basin modelling. Test results indicate that the new curves can reproduce expulsion efficiency, locations of accumulations and leaking through cap rock, which is consistent with actual observations.
Some thoughts on porosity reduction — rock mechanics, overpressure and fluid flow
Authors: Waples D. W. , Couples G. D.
Published: April 2008

A currently popular paradigm, that porosity reduction occurs as a direct consequence of the effective stress acting on the rock framework grains, is mechanistically incorrect. The commonly observed covariance between porosity and effective stress does not reflect a cause-and-effect relationship. Instead, it arises because both low effective stress and slow porosity reduction are consequences of the inability of compacting rocks to expel their pore fluids quickly enough to maintain normal fluid pressures. The process of porosity loss is here divided into sequential steps, and we argue that the expulsion of pore fluids is the rate-determining step leading to overpressuring. Thus, Darcy’s law assumes equal importance with the relationships describing the mechanical compaction of sediments. In this paper we describe how compaction can be treated as a Coulomb-plastic response that is functionally dependent on effective mean stress, deviatoric stress, and the state of compaction. In the next generation of basin models, a mechanistically correct approach is needed, combining both rock mechanics and hydrogeology.
Central North Sea overpressures: insights into fluid flow from one- and two-dimensional basin modelling
Authors: Darby D. , Haszeldine R. S. , Couples G. D.
Published: April 2008

Jurassic and Triassic reservoirs in the Central North Sea are highly overpressured (>40 MPa above hydrostatic pressure). Simulation of the interplay between rapid Tertiary subsidence, seal permeability and fluid flow allows insight into the geological controls on the distribution and magnitude of the overpressure. One-dimensional models demonstrate that, unlike other basins, the overpressure developed in the Graben is not determined by the thickness and permeability of the shale pressure seal. A two-dimensional model simulating lateral flow beneath the pressure seal provides an accurate simulation of the overpressure distribution. Disequilibrium compaction of shale-dominated off-structure regions forms the principal overpressuring mechanism. Lateral flow in the permeable Fulmar sandstones leads to high overpressure and focused vertical escape on an axial high, where the seal is thinner above a subcropping Fulmar Fm. A layered hydrogeological regime is suggested, with shallow Tertiary pressure cells separated from deep Cretaceous-Triassic pressure cells by normally pressured, permeable Palaeocene sandstones. The pre-Cretaceous rift-associated configuration of the Graben, in combination with 3 km of Tertiary subsidence, controls the distribution of overpressure.
A review of techniques used to determine geological and thermal history in the Southern North Sea
Authors: Archard G. , Stafford J. , Bardwell K. , Bagge M. , Stafford J. , Bardwell K.
Published: April 2008

The Southern North Sea has been a prolific source of gas to the UK economy over the last 30 years. As many of the producing fields are now becoming depleted, attention has switched to exploration in more marginal areas of the basin. In order to evaluate suitable structures in these areas, an understanding of the complex structural and thermal history of the Southern North Sea Basin is required. Particular emphasis must be placed upon the prediction of source rock maturity, the timing of generation and migration of hydrocarbons, and the thermal controls exerted on potential reservoir sand units.

Over large parts of the basin the Carboniferous source rocks are not at their maximum depth of burial at present day. A variety of techniques utilizing sonic velocity, vitrinite reflectance and apatite fission track analysis have been used to quantify the magnitude of uplift that the Carboniferous section has undergone through geological time. With the exception of apatite fission track analysis, these techniques provide only an estimate of ‘net uplift’, i.e. the amount of uplift relative to maximum depth of burial or maximum temperature. They record neither the timing and effects of individual uplift events nor the timing of maximum burial prior to net uplift. In order to accurately reconstruct the burial and thermal history of the basin these techniques must be integrated with a detailed knowledge of the geological history of the Southern North Sea.

The use of these techniques in isolation, together with a lack of appreciation of the limitations of the data has in the past resulted in spurious results, leading to criticism and mistrust of the techniques. This paper attempts to show that when correctly applied and in appropriate geological circumstances, each analytical technique can provide useful data for burial and thermal history reconstruction. Examples are shown to highlight the effects of incomplete data interpretation and the impact of inaccurate determination of present day temperatures. Particular attention is paid to determining the burial and thermal history of the economically important Carboniferous source and Permian reservoir units, and the interpretation of uplift data is restricted to the post-Zechstein section.
Aspects of applied basin modelling: sensitivity analysis and scientific risk
Authors: Thomsen R. O.
Published: April 2008

Basin modelling is a powerful quantitative tool and in order to be able to use modelling for the correct purpose and to expand the variety of problems where basin modelling with advantage can be applied, it is necessary to view basin modelling in the context of the general exploration process. Results from basin modelling are often directly used for ranking and risking of exploration targets. Due to the integrated nature of basin modelling it is applied more or less routinely throughout the exploration process for quantitative assessments. Awareness of some fundamental limitations of the models used is therefore vital in order to assess the overall uncertainty associated with the results. It will be shown how, through sensitivity analysis and risking of the modelling results, uncertainty ranges and confidence levels can be attached to the results. Examples of how sensitivity analysis helps in the process of understanding the system behaviour, reveals the critical factors, and provides the resolution limits on specified parameters will be given. Finally, a probabilistic procedure for assessing the uncertainty ranges and the level of confidence associated with modelling results will be given. A case history shows how the procedure can be used for providing risked entry parameters for the general risk assessment.
The Argentine Precordillera: its odyssey from the Laurentian Ouachita margin towards the Sierras Pampeanas of Gondwana
Authors: Dickerson Patricia wood , Keller Martin
Published: April 2008

The Argentine Precordillera (Cuyania terrane) and the Marathon/Solitario basin (west Texas), both underpinned by Grenvillian Laurentian basement, evolved together during Early Cambrian to Mid-Ordovician times. Ages, Pb-isotope and geochemical data for Precordilleran and for west-central Texas basement (including the Llano uplift) are strikingly similar. Carbonate platform sequences developed on both sides (e.g., El Paso and Chica de Zonda) of the Marathon/Solitario outer-shelf to slope basin and hosted homologous reef organisms which were unique to the Laurentian Ouachita margin. The Marathon/Solitario basin received sediments from both north and south; much detritus came from the northern shelf, including olistoliths bearing shelf fauna. Erosional vacuities on the platforms correlate with coarse detrital basin deposits. Cuyania constituted the long-sought southern source. Stratigraphy and structures of the Precordillera/Marathon/Solitario basin are consonant with plate reconstructions that place southern Laurentia near western Gondwana from Late Cambrian into mid-Ordovician time. During Caradoc time, Cuyania moved beyond range of faunal exchange with Laurentia, and tholeiitic basalts were intruded into off-shelf turbidites in the western Precordillera. The attenuated, thermally weakened, Laurentian slab broke apart with continued extension and right-oblique separation of Laurentia and Gondwana. Severance of Cuyania from Laurentia was complete before the onset of Taconic or Ocloyic orogenesis.
The Famatinian magmatic arc in the central Sierras Pampeanas: an Early to Mid-Ordovician continental arc on the Gondwana margin
Authors: Pankhurst R. J. , Rapela C. W. , Saavedra J. , Baldo E. , Dahlquist J. , Pascua I. , Fanning C. M.
Published: April 2008

A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Córdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 ± 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470–450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial
87
Sr/
86
Sr (0.7075–0.7105) and low ɛNd
t
(−4.6 to −6.3), inherited from lower crust. Sm-Nd model ages of 1600–1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.