bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen and carbon isotopes in planktonic foraminifera as indicators of upwelling intensity and upwelling-induced high productivity in sediments from the northwestern Arabian Sea
Authors: Steens Tineke N. F. , Ganssen Gerald , Kroon Dick
Published: April 2008

Oxygen and carbon isotopes in the tests of
Globigerina bulloides
and
Neogloboquadrina dutertrei
from the Indian Ocean provide indicators for the intensity of upwelling and upwelling-related high productivity. The difference in δ
18
O between the surface dwelling
G. bulloides
and the thermocline dwelling
N. dutertrei
reflects the mixing of the water column during upwelling situations, and hence the intensity of upwelling. The difference in δ
13
C isotope distribution between the two species is smallest within upwelling areas, indicating that upwelling induced the high productivity. The two parameters are applied to Hole 728A (ODP 117), in the Arabian Sea on the continental margin of Oman. Both upwelling indicators show that from isotopic stage 2 till stage 11 upwelling was stronger during interglacial periods, and that upwelling intensified down core for the same period. In older sediments, stages 12 and 13, the signal from the upwelling indicators is reversed, showing stronger upwelling during glacials.
Physical upwelling processes, upper ocean environment and the sediment record of the southwest monsoon
Authors: Anderson David M. , Brock John C. , Prell Warren L.
Published: April 2008

We used a one-dimensional model of upwelling driven by the surface windfield to examine the relationship between wind-driven physical upwelling processes, changes in the plankton environment and modern (coretop) sediments in the northwest Arabian Sea. The model resolves the vertical upwelling velocity into a coastal component related to transport away from a solid boundary and a second component related to the curl in the wind stress. The coastal component is an order of magnitude larger than the curl component but restricted to <100 km offshore. In offshore waters (>100 km), upwelling is directly related to the curl in the wind stress. Model-derived upwelling is in reasonable agreement with estimates of the vertical velocity off Oman, and correlated with the cool waters and thin mixed layer found along the coast of Oman. The response of the biota to these environmental changes is observed in remotely sensed images of phytoplankton biomass during the Southwest Monsoon, and in distinct foraminifer species distributions in modern (coretop) sediments.
A lamina-scale geochemical and sedimentological study of sediments from the Peru Margin (Site 680, ODP Leg 112)
Authors: Aplin A. C. , Bishop A. N. , Clayton C. J. , Kearsley A. T. , Mossmann J.-R. , Patience R. L. , Rees A. W. G. , Rowland S. J.
Published: April 2008

Individual laminae (mm-cm) were isolated from late Pliocene-Holocene sediments cored at Site 680 during ODP Leg 112 (Peru upwelling region). At all depths, individual laminae alternate between (1) pale, high porosity, clay-poor, diatom ‘framework’ oozes containing a high proportion of complete, well preserved diatom frustules and (2) dark, low porosity diatomaceous muds containing abundant diatom debris and foraminifer tests. In some instances, detrital-rich laminae with low amounts of diatom debris were also observed.

Hydrogen-rich organic matter is finely disseminated throughout the sediments, perhaps occurring as coatings on detrital and biogenic grains. Both carbon isotopic and pyrolysis-GCMS analyses suggest that the organic matter is derived mainly from a marine algal and/or bacterial input. Py-GCMS further shows that, even in the sediments from 4.91 mbsf, the organic matter is significantly altered from the composition of living organisms. Only in two organic-lean Pliocene samples was there any sign of a significant input of terrestrial organic matter. Relative to laminae deposited during the Pleistocene-Holocene, late Pliocene laminae are depleted in TOC. This may reflect variations in primary productivity. TOC contents also vary between adjacent diatomaceous mud laminae in Pleistocene-Holocene sediments. In these laminae, TOC is inversely proportional to diatom abundance (which is believed to be related to productivity) but correlates positively with detrital content. Variations in the TOC content of closely spaced laminae is apparently not controlled by variations in productivity or preservation, but probably by variations in the TOC content of particulate matter reaching the sediment-water interface.

Some parameters (organic S/C; δ
34
S) vary on a lamina scale — no larger scale trends are apparent. These variations are presumably the result of short-term, local changes in diagenetic conditions and are hard to quantify. In these H
2
S-rich sediments, differential uptake of sulphur into organic matter may be caused by variations in the availability of specific, reactive inorganic sulphur species, perhaps polysulphides.

This study shows that conventional geochemical analysis of bulk, or of widely spaced samples can hide significant variations in sediment composition and inferred palaeoceanographical conditions. Although bulk samples may be used to determine the gross characteristics of a sedimentary sequence, fine-scale sampling is needed to define the processes which determined those characteristics.
The spatial distribution and seasonal succession of planktonic foraminifera in the California Current off Oregon, September 1987 – September 1988
Authors: Ortiz Joseph D. , Mix Alan C.
Published: April 2008

Preliminary results are presented from the September 1987 to September 1988 deployment of the Multitracers Project, a moored sediment trap experiment in the California Current. Three sediment trap moorings were deployed on a transect off Cape Blanco at 130, 280, and 650 km offshore. The seasonal distribution of foraminifers in the California Current at 42°N reflect variations in the temperatures and water masses present at each site. The winter assemblage of foraminifers at the sites 130 and 280 km offshore comprise mainly the subarctic/transitional species:
N. pachyderma
(right),
N. dutertrei, G. bulloides, G. quinqueloba
and
G. glutinata
. From late June to September 1988 at the site 130 km offshore,
G. bulloides, G. falconensis, N. pachyderma
(left) and the P-D intergrade category record high fluxes, apparently in response to upwelling conditions. The subtropical species
O. universa, G. ruber
and
T. humilis
are most important at the site 650 km offshore. The foraminiferal communities present from September 1987 to September 1988 off Cape Blanco seem to be more diverse but less productive than assemblages from the Alaskan Gyre at Station P (45°N, 150°W) as described in the literature.
The palynological expressions of post-Palaeogene upwelling: a review
Authors: Powell A. James , Lewis Jane , Dodge John D.
Published: April 2008

This paper reviews the known palynological expressions of post-Palaeogene upwelling, and changes in upwelling. In Plio-Pleistocene upwelling regimes, levels of amorphogen tend to swamp other palynoclasts present; the application of transmission electron microscopy has great potential for elucidating the origins of amorphogen. The recovery of foraminiferal test linings may be controlled by their progressive microbial destruction; the more juvenile stages in the ontogenetic record stand the greatest chance of preservation. The study of dinoflagellate cyst biofacies offers the greatest potential for palynological characterization and interpretation of post-Palaeogene upwelling systems. Cysts of heterotrophic peridiniacean dinoflagellates (P-cysts) tend to dominate cysts of autotrophic gonyaulacacean forms (G-cysts). The ratio of P-cysts to G-cysts (P/G ratio) is therefore considered to be a useful indicator of variable upwelling strength in these settings, reflecting the degree of water column turbulence. It is uncertain to what degree alynological patterns recognizable in one system are representative of other contemporaneous upwelling regimes. Nevertheless, palynological studies of upwelling systems should be used to interpret geochemical data and thereby provide a tangible link between cause (palaeobiological) and effect (geochemical).
Variations in the content and composition of organic matter in sediments underlying active upwelling regimes: a study from ODP Legs 108, 112, and 117
Authors: Ten Haven H. L. , Eglinton G. , Farrimond P. , Kohnen M. E. L. , Poynter J. G. , Rullkötter J. , Welte D. H.
Published: April 2008

An overview is presented of organic geochemical studies on sediments underlying the active upwelling cells off Northwest Africa (ODP Leg 108; Site 658), off Peru (Leg 112; Sites 679, 681, 684, 686) and off Oman (Leg 117; Sites 723, 725, 728). The investigated sediments are all characterized by high organic carbon contents (0.5–9% C
org
), being the sedimentary expression of an increased primary bioproductivity induced by upwelling currents of deep nutrient-rich waters. The organic matter is predominantly of marine origin with variable admixtures of a terrigenous component; the highest relative contribution is found at Site 658. The extractable lipids are composed of a wide variety of compounds, of which the most characteristic and abundant compounds were quantified. Long-chain unsaturated ketones usually dominate. Steroids, alkanediols and their corresponding keto-ols are also present in high concentrations, although strong variations in their abundances were noted. In addition, organic sulphur compounds are omnipresent and reveal strong variations in their distribution, but do not make a major contribution to the total amount of extractable lipids.
Carbon/sulphur/iron relationships in upwelling sediments
Authors: Morse John W. , Emeis Kay C.
Published: April 2008

Carbon/sulphur/iron relationships in sediments from beneath three upwelling areas (Benguela, Peru and Oman) were investigated using DSDP and ODP cores. The three areas displayed markedly different C/S/Fe characteristics. Metabolizable organic matter appears to limit production of reduced sulphur in the Benguela upwelling area sediments, as evidenced by incomplete reduction of dissolved sulphate. On the Peru shelf infusion of a sulphate-containing brine at depth has a major influence on diagenesis.

Weight ratios of organic carbon to reduced inorganic sulphur are generally higher in sediments from upwelling areas than in normal marine sediments. C/S ratios generally increase with increasing organic carbon concentration in upwelling sediments. This does not appear to be the result of limited availability of reactive iron. Both carbon and reduced sulphur accumulation rates are well correlated with sediment accumulation rates, but are higher than in non-upwelling sediments having similar accumulation rates.
Strong deglacial minimum in the δ
13
C record from planktonic foraminifera in the Benguela upwelling region: palaeoceanographic signal or early diagenetic imprint?
Authors: Schneider R. , Dahmke A. , Kölling A. , Müller P. J. , Schulz H. D. , Wefer G.
Published: April 2008

A strong deglacial decrease in the δ
13
C values of the planktonic foraminifera
G. inflata
and
G. bulloides
, spanning a 1000-year period, was measured in a core from the Benguela upwelling region. The deglacial δ
13
C minimum exceeds background values by more than −1‰, and is between −1.6 to −2.0‰ below Holocene values. The unusual low δ
13
C values could reflect a change in the isotopic composition of the upwelled South Atlantic Central Water (SACW) and/or changes in the stratification of the surface waters in the Benguela coastal upwelling region. The observed δ
13
C decrease correlates with a proposed zone of authigenic CaCO
3
precipitation at 6 m depth in the anaerobic sediments. This raises the question whether the δ
13
C minimum was caused by diagenetic alteration of the δ
13
C record or not. Calculations based on estimated interstitial water δ
13
C
TCO2
values and flux rates of Ca predict a 5% authigenic CaCO
3
precipitation additional to the primary CaCO
3
content in the sediment, which could account for an average diagenetic imprint of −1.0‰ δ
13
C
CaCO3
if precipitated on foraminiferal tests. However, missing visual evidence (SEM) for authigenic CaCO
3
phases greatly weakens the case for diagenetic causes for the negative δ
13
C spike. The deglacial negative δ
13
C spike is therefore assumed to be a primary palaeoceanographic signal.
Calcareous nannofossils of Plio-Pleistocene sediments from the northwestern margin of tropical Africa
Authors: Pujos Annick
Published: April 2008

Coccoliths are skeletal remains of organisms living in the uppermost part of the oceans, so they reflect the climatological and hydrological variations of the uppermost water layers. The main goal of the present study is to use their fossil remains (calcareous nannofossils) to capture information about the environment where they lived for the last 700 000 years. The work is based on a study of sediment from Site 658, ODP Leg 108 located in a major upwelling area in the northeastern tropical Atlantic, close to the African continent. The hydrological and atmospherical conditions of the area are complex: seasonally windy and alternatively humid and arid. Downcore the effects of these changes are complicated by the presence of climatic stages including various glacial/interglacial alternations.

The interpretation of Plio-Pleistocene climatic history at Site 658 is based on analysis of abundance matrices (calcareous nannofossils as well as other <25 μm organic and inorganic elements) and of two nannofossil-based transfer functions (giving estimates for summer-winter temperatures, summer-winter salinities and productivity). The nannofossils most useful for paleoecological interpretation proved to be
Rhabdosphaera
(related to increased temperature and decreased productivity),
Coccolithus pelagicus
(related to sharp decreases in temperature and inversely to salinity),
Syracosphaera
(related to increased salinity) and
Helicopontosphaera
(related to sharp decreases in salinity and increased productivity).

A tentative paleoenvironmental and paleoecological model is given for a typical glacial/interglacial alternation. During
glacial
periods the African continent was arid, with seasonal winds blowing towards the ocean, and the uppermost oceanic waters showing increased salinity. At first oceanic temperatures were low, and organic matter linked with fecal pellets was abundant due to wind-induced upwelling; temperature then increased slightly, before decreasing at the end of the glacial period under the influence of northern ice sheets. At the
glacial/interglacial boundary
, rainy monsoons caused strong and sudden increased runoff, which lowered salinity;
interglacial
time was characterized by low salinity surface waters. As interglacials began, fluvial outflows supplied abundant inorganic and organic matter, while the Canary Current brought cooler waters south from northern latitudes, and there was a short record of upwelling. Then, temperature increased slowly, as the area became dominated by tropical waters.
Miocene intensification of upwelling along the California margin as recorded in siliceous facies of the Monterey Formation and offshore DSDP sites
Authors: White Lisa D. , Garrison Robert E. , Barron John A.
Published: April 2008

Diatomaceous sediments and their diagenetic equivalents in the Monterey Formation record a variable history of upwelling along the California margin. Distrinctive dark opal-CT and quartz chertz found in distal basins of the Monterey Formation are the result of burial diagenesis of pure biosiliceous oozes (biosiliceous oozes without significant admixtures of clay) and are therefore evidence of intensified coastal upwelling during the early middle Miocene. Dating of six sections of the Monterey Formation, largely by diatom biostratigraphy, suggests that at the Point Reyes and Point Año Nuevo sections in north-central California, the age of the earliest chert intervals is between 13.8 and 15.0 Ma, and 14.3 and 14.8 Ma, respectively. In south-central California, ages from the Shell Beach, Mussel Rock, and Lions Head sections imply that the age of the base of the chert intervals is between 12.7 and 13.3 Ma. Both ages correlate to an early middle Miocene high latitude cooling step that resulted in more vigorous surface water circulation, upwelling of nutrient-rich waters, and increased biosiliceous sedimentation in the North Pacific. The north-south difference in age of the base of the chert interval probably reflects a progressive intensification of the California Current from 15.0 to 12.7 Ma.

The age of the onset of biosiliceous sedimentation at DSDP sites of the northeastern Pacific is also generally younger at the more southern sites; however, these particular DSDP sites were located some distance from the centres of coastal upwelling and are not as reliable indicators of the intensification of upwelling along the California margin.
Planktonic foraminifera of the coastal upwelling area off Peru during the Pleistocene
Authors: Ibaraki Masako
Published: April 2008

Planktonic foraminifera of the coastal upwelling area off Peru during the Pleistocene have been quantitatively examined in ODP Leg 112 cores obtained from five sites beneath the upwelling area. Laminated diatomaceous facies in some cores suggest they had been in upwelling centres during the Pleistocene. The Pleistocene fauna dominantly consists of cold water and eurythermal elements, but also contains diverse tropical-subtropical species. The fauna is similar to the living one in the modern upwelling area, although the former seems to have much more cold-water elements. The most important species in the coastal upwelling area off Peru during the Pleistocene were
Globigerina quinqueloba
, concentrated in upwelling centres,
Neogloboquadrina dutertrei
, dominantly found in surrounding areas, and many kinds of tropical-subtropical species. Typical temperate water dwellers are rare.
Palaeoproductivity oscillations during the last 130 ka along the Portuguese and NW African margins
Authors: Abrantes Fatima
Published: April 2008

Diatom abundance and taxonomic changes were quantitatively studied in the latest Quaternary (∼ 130 ka to present day) sequences of the upwelling zones associated with the continental margins of Portugal and northwest Africa. Marked oscillations in diatom abundances accompanied by changes in the assemblage composition are evident during the late Quaternary and reflect changes in primary productivity related to upwelling intensity. Upwelling increased during glacial episodes (Isotopic Stages 2 and 6) relative to interglacial intervals (Isotopic Stages 1, 3 and 5). During the late Holocene, primary productivity levels are similar to those for Stage 5, and, lower than today in the early Holocene.

The palaeoproductivity reconstruction based on the diatom record is supported by independent estimates of palaeoproductivity based on the organic carbon content of the sediments.
Outcrop gamma-ray logging to improve understanding of subsurface well log correlations
Authors: Slatt Roger M. , Jordan Douglas W. , D’Agostino Anthony E. , Gillespie Robert H.
Published: April 2008

Reliable, reproducible gamma-ray logs of outcrops have been generated by two techniques which aim to improve the visualization of interwell-scale lateral continuity (and discontinuity) of strata and to demonstrate the reliability and potential pitfalls of subsurface wireline log correlations. One innovative technique was developed which uses a standard gamma-ray sonde run from a logging truck in order to log vertical cliff or quarry faces. A second technique employs a hand-held gamma-ray scintillometer to log more easily accessible outcrops.

Examples are presented from the Jackfork Group (Pennsylvanian), Arkansas, USA, of outcrop gamma-ray logging of both laterally continuous and discontinuous turbidites in structurally simple and complex settings. Because the strata from which the logs were measured can be visualized and discussed at the outcrop, these examples can clearly illustrate the following aspects of wireline log correlation: (1) reliability can be greatly improved by understanding expected subsurface depositional geometries and lateral facies changes; (2) wireline log correlations in stratigraphically and structurally complex settings, such as many oil or gas fields, may not be reliable without sufficient coring and special logging in addition to well testing; (3) erroneous correlations can result from the common practice of inferring three-dimensional rock geometries from two-dimensional well log data. Additional examples demonstrate that correlations and interpretations of subsurface wireline logs in both oil/gas fields and in exploration areas can be improved by comparing the subsurface logs with outcrop gamma-ray logs of nearby analogous strata.
In situ stress orientations in the Witch Ground Graben, North Sea, revealed by borehole breakouts: preliminary results
Authors: Cowgill S. M. , Meredith P. G. , Murrell S. A. F. , Brereton N. R.
Published: April 2008

Directions of minimum horizontal stress (σ
h
) derived from borehole breakouts are presented for seven boreholes in the Witch Ground Graben, North Sea. We have found a clear tendency for a dominant N-S (more accurately a NNW-SSE) direction of σ
h
, approximately perpendicular to the graben axis. However, significant variations from this general trend were encountered even within this relatively small area, especially within the boreholes from the Tartan and Highlander fields. Contrasts in local structure and lithology within the area may explain these differences although such contrasts have yet to be quantified.

Broad regional trends of σ
h
are apparent throughout Northwest Europe, although previously published data show a certain amount of variability. It has been shown that the minimum compressive stress direction changes significantly across many of the graben structures within the North Sea basin, and this also appears to be the case within the Witch Ground Graben.
Fracture permeability and alteration in gabbro from the Atlantis II Fracture Zone
Authors: Goldberg D. , Broglia C. , Becker K.
Published: April 2008

Geophysical logs recorded over 500 m in Ocean Drilling Program Hole 735B have been analysed to identify the in situ properties and the fracture permeability of basement rocks in the Atlantis II Fracture Zone. Hydrous alteration minerals were most abundant in the upper section of the hole. Packer tests also indicate the highest permeabilities in this zone. This interval is characterized by the presence of several open fractures imaged by the acoustic televiewer and by temperature log indications of fluid flow through transmissive fractures. Negative temperature gradient and large semblance anomalies were found to correlate well with the largest of the observed fracture zones. Most of the 70 identified fractures strike subparallel to the north-south fracture zone and dip steeply WSW. These fractures probably formed as a result of normal faulting on the transform side of the uplifted ridge in the Atlantis II Fracture Zone, where 735B was sited, rather than as a direct result of horizontal plate tectonic motion.
Thermal conductivity prediction from petrophysical data: a case study
Authors: Griffiths C. M. , Brereton N. R. , Beausillon R. , Castillo D.
Published: April 2008

Thermal conductivity measurements were carried out on over 400 samples from Tertiary and Cretaceous sediments and Jurassic basalts during Ocean Drilling Program Leg 123 in the Argo Abyssal Plain, North West Australia. Ship-board physical property measurements were obtained from cores over the same intervals, and petrophysical wireline logs were run. New algorithms are demonstrated relating thermal conductivity to matrix grain type and porosity, derived only from wireline log data. Comparison with heat flows calculated using high-resolution temperature measurements at Site 765 suggest that thermal conductivities predicted from wireline logs can be used to estimate heat flow. This implies that existing exploration and production wells in which high resolution temperature measurements and thermal conductivities have not been measured could still be used to prepare regional maps of heat flow variation.
Determination of Young’s modulus of the rock mass from geophysical well logs
Authors: McCann D. M. , Entwisle D. C.
Published: April 2008

Geophysical logging of site investigation boreholes can provide a wide range of useful geotechnical information ranging from the formation density (gamma/gamma log) to the dynamic elastic moduli (full wave train sonic log). In this paper, the use of Young’s modulus as a measure of the deformation characteristics of the rock mass is examined. A comparison is made between the dynamic value obtained from geophysical borehole logging and the static value obtained directly from the laboratory testing of samples collected from the borehole. The data are obtained from a number of BGS boreholes. It is shown that although the prediction of the static value of Young’s modulus from the dynamic value is dependent on a knowledge of the borehole lithology, the use of the dynamic elastic moduli can yield considerable useful engineering information in the assessment of the deformation properties of the rock mass.
The Temperature Decay Log: a different approach to presenting a temperature survey
Authors: Alm Per-Gunnar
Published: April 2008

In connection with ‘shut-in’ temperature surveys numerous temperature logs are run over a period of time. When it comes to presenting the results, several of the recorded temperature logs are often shown in the same figure. In order to increase the readability of the figure, some of the recorded logs might have to be excluded. This can cause vital information concerning the formation surrounding the borehole to be excluded. The paper gives an example of a different way to present the recorded temperature logs. Normally the presentation shows temperature versus depth. A different approach is to study the change of temperature versus time, after shut-in. In this case, the temperature behaviour at every depth is studied throughout the survey. Thereafter all the temperature logs are included in one final temperature decay log. This means that all the temperature logs run during the survey are used in the final presentation. The method gives as a result one single curve showing the temperature recovery after shut-in. The curve shows the time of recovery versus depth. In the figure, the different zones of the borehole are easily discerned. By use of several of the above mentioned temperature decay logs it is possible to distinguish parts of the borehole with uniform temperature behaviour. The paper gives an example from a fractured formation where the two methods of presenting temperature logs are compared. (The conventional one and the one suggested in the paper.)
Determining total organic carbon contents from well logs: an intercomparison of GST data and a new density log method
Authors: Myers K. J. , Jenkyns K. F.
Published: April 2008

A detailed case study shows that more accurate in situ organic carbon (TOC) estimates can be made by the interpretation of conventional density logs than by the carbon/oxygen log method, and at a fraction of the cost. The performance of the two methods has been compared in a cored section of the Schiste Carton source-rock in the Paris Basin. The density log method described in this paper method is relatively quick and simple to use, whereas the C/O method involves the interpretation of a specially run log.

Results show that with the density log method 80% of the TOC estimates fall within ±20% of the core measured values in a TOC range 2–6.5 wt%. The C/O log method, which involves 30 ten minute stationary measurements or three passes at 600 ft/hour, achieves comparable accuracies at higher TOCs (> 5wt%), but is considerably less accurate at lower TOCs with only 50% of values within ±20% of the core measured values.
The hydrology of basement complex regions of Africa with particular reference to southern Africa
Authors: Farquharson F. A. K. , Bullock A.
Published: April 2008

Basement complex rocks occur extensively throughout Africa and experience a wide range of climatic conditions. Over much of the continent, potential evaporation exceeds rainfall and runoff is consequently limited. Much of the continent is short of reliable water supplies, and hence understanding the hydrological behaviour of river basins throughout this important region of Africa is of vital importance to the development of the area.

There is a considerable body of rainfall and river flow data throughout the continent of Africa, but whilst climatic data has been collated and analysed on the regional or continental scale, river flow data has been analysed only at the national or sub-regional scale.

An attempt is made here to summarize the hydrological characteristics of those regions of Africa having outcrops of basement complex rocks, illustrating general observations with the results of detailed analyses of available data from Malawi and Zimbabwe. It is suggested that the results of studies in these two countries may be of wider benefit to workers in southern and eastern Africa in particular.
An exploration strategy for higher-yield boreholes in the West African crystalline basement
Authors: Boeckh Eduard
Published: April 2008

In Burkina Faso investigations have been carried out to test satellite imagery and electromagnetic (EM) methods as a means of improving the siting of water supply boreholes in fissured rocks of the crystalline basement. The results show that these methods may be used to locate boreholes with above average groundwater yields, and to reduce the failure rates.

Geometrical analysis of Landsat-MSS lineaments reveals the relevant regional fracture pattern, the present-day principal stress field and the resulting system of oriented hydraulic conductivities. Structural analysis of the lineament pattern and identification of dry-season-active vegetation on Landsat-TM imagery permit promising locations to be pinpointed for groundwater abstraction. EM-measurements on a two-dimensional grid using the ‘very low frequency’ (VLF) and ‘horizontal loop EM’ (HLEM) methods are used to locate potential fractures on a local scale as a basis for rational borehole siting.

The methods, applied separately or in combination, offer an economical approach for use in rural and community water supply, small-scale irrigation and water resources planning. The methods appear to be generally transferable to fissured hard rock aquifers in other regions. The best suited settings are believed to be on cratons and platforms in a semi-arid climate.
Thermal history reconstruction using apatite fission track analysis and vitrinite reflectance: a case study from the UK East Midlands and Southern North Sea
Authors: Bray R. J. , Green P. F. , Duddy I. R.
Published: April 2008

Apatite fission track analysis (AFTA) is a method of obtaining thermal history information in sediments. In sections that have been hotter in the past, AFTA provides estimates of maximum palaeotemperatures, and the time at which a sedimentary section began cooling from maximum palaeotemperatures. It is useful for studying the thermal history of sequences containing source rocks, and can provide information critical to the understanding of the timing of oil generation. Integration of AFTA and vitrinite reflectance (VR) data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining AFTA and VR data. This approach allows definition of the major facets of thermal history based on directly measurable parameters, rather than on assumed values of palaeo-heatflow, and provides rigorous estimates of the amount of uplift and erosion. The case study demonstrates the application of this approach to the East Midlands Shelf region. Here AFTA data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 Ma. Combined AFTA and VR data show that this cooling was due to kilometre-scale early Tertiary uplift and erosion under near normal geothermal gradients. Our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion, but was regional in extent. This has implications for the evolution of source rock maturity in this region and in other basins in around the UK where AFTA has indicated early Tertiary heating and where the extent and amount of Tertiary uplift and erosion may have also been underestimated by previous workers.
Estimation of Cenozoic water depths in the Western Central Graben, UK North Sea, by subsidence modelling
Authors: Joy Aidan M.
Published: April 2008

Variations in water depth with time at a point within a marine sedimentary basin can in theory be evaluated by fitting a tectonic subsidence curve for the point to a subsidence model. By combining results from numerous points, palaeobathymetric maps can be constructed for specified time horizons. This has been attempted for the Cenozoic interval in part of the Western Central Graben in the United Kingdom sector of the North Sea Basin. A modified version of the instantaneous extension model has been used as the basis for the method. The initial results suggest that water depths over most of the study area increased dramatically from the end of the Danian to the end of the Eocene. Thereafter sedimentation outpaced subsidence, reducing water depths at first slowly (Oligocene-Pliocene) and then rapidly (Pleistocene). These results are reviewed with reference to observational data: palaeontological and sedimentological data, stratigraphical data from seismic reflection profiles and data on the uplift and denudation histories of the adjacent land masses. The review procedure both illustrates the strengths of the method and highlights areas in which refinements are necessary. The iterative nature of this study, in which the observational data are used progressively to constrain the tectonic subsidence model, is emphasized.
The Angus Field, a subtle trap
Authors: Hall Stephen A.
Published: April 2008

The Angus Field is situated near the centre of the North Sea in UKCS Blocks 31/21 and 31/26a. High-quality shallow marine sands of late Kimmeridgian to early Ryazanian age (the Angus Sands) form the reservoir. Although the field is a simple four-way-dip closure at Base Chalk level, the distribution of the reservoir sands beneath this surface is complex and reflects the impact of basement faulting on both sediment deposition and preservation. Five facies have been recognized within the Angus Sands and a depositional model has been developed that relates their distribution to the structural history of the area. Four of these facies are of good to excellent reservoir quality, the fifth is only poorly developed but may act as a local permeability barrier on the southern flank of the field. Over much of the field area the reservoir sands are too thin to be seismically definable. In such areas a sand isopach has been created using well data points, the basement fault pattern and the results of reservoir modelling. Based on this isopach the Angus Field is calculated to contain 18.5 MMB STOIIP. By adopting an innovative approach to development that makes use of the high sustainable productivity of the reservoir Amerada Hess believes that it can develop the field economically on a standalone basis.
Facies analysis of the Kopervik sand interval, Kilda Field, Block 16/26, UK North Sea
Authors: Guy Martin
Published: April 2008

The Kilda Field is a large gas condensate accumulation located in UK Block 16/26. The reservoir sands, informally termed Kopervik, are Lower Cretaceous in age and are developed on the southern dip slope of the Fladen Ground Spur. They represent a localized sandy development within the lower Sola and upper Valhall formations, associated with the Austrian orogeny. Sands were carried out into the basin by a variety of sediment gravity flow processes, and were deposited as an apron near the foot of the slope. Their distribution is controlled largely by the nature of the flow and by pre-existing topography. Six main sand facies have been recognized, three of which are high-density turbidite (HDT) types and contain virtually all reservoir quality rock. HDT S
3
sands resulted from suspension collapse. They exhibit porosities around 15%, and permeabilities around 40 md. HDT Laminated sands probably formed from traction sedimentation. They have similar average porosities but reduced permeabilities between 10 and 15 md. Sands of the Marginal facies are of an inferior reservoir quality. The remaining three facies consist of non-reservoir quality sands deposited by liquefaction, debris or low-density turbidite flow. Post-depositional disturbance suggests that deposition took place on an unstable palaeoslope. Subsequent porosity loss within clean sand intervals has been dominantly through compaction.
The Nelson Field: a successful application of a development geoseismic model in North Sea exploration
Authors: Whyatt M. , Bowen J. M. , Rhodes D. N.
Published: April 2008

UK North Sea Block 22/11 has had a long history of exploration involving six companies drilling four wells over two decades. The latent exploration potential of the block was recognized by Enterprise in 1985 following geoseismic studies carried out in the neighbouring Montrose and Arbroath Fields. The modest reserves attributed to the oil and gas shows encountered in the poor quality Palaeocene reservoir of the first well, 22/11-1, drilled in 1967 were considered uneconomic at the time. The true reservoir potential of the Palaeocene appeared to have been overlooked. Enterprise pursued the new play concept, firstly by farming in and thereafter by acquiring the sole exploration rights of the block in a series of complex commercial asset and acreage deals.

Applying modern and ancient submarine fan analogues with a detailed appreciation of local geology, Enterprise recognized sand-prone ‘channels’ on reflection seismic data and established that the 22/11-1 well had penetrated an ‘interchannel’ facies. Subsequent drilling confirmed this model and led to the discovery of the Nelson Field, in the centre of what is considered to be a mature hydrocarbon province.

The geoseismic model, exploration concepts and reservoir geology of this late Palaeocene turbidite clastic play are presented to illustrate how a detailed production geological/geophysical model can significantly benefit exploration.
Reservoir heterogeneity in the Vanguard Field, UKCS
Authors: Lahann Richard W. , Ferrier Jon A. , Corrigan Susan
Published: April 2008

The Rotliegendes Group in the Vanguard Field, UKCS, consists of a series of aeolian, fluvial, and reworked aeolian sediments, organized into four broadly correlatable lithostratigraphical units. Within these units, reservoir properties are strongly linked to lithofacies, with aeolian dune sandstones possessing the best porosity and permeability.

A comparison of production log spinner profiles (PLT) with detailed sedimenttological logs from three development wells shows that gas production is intimately associated with specific lithofacies and their spatial organization. Although dune sandstones in the upper section of the Rotliegendes have good petrophysical characteristics, they do not contribute significantly to production because they are contained within laterally extensive and relatively impermeable interdune sandstones. Most production comes from 15–30 feet thick and highly permeable (100–1000 mD) dune foreset intervals deposited near the bases of large aeolian dunes.

Significant interwell variation in production rates and in porosity exists for all lithofacies due to the variable extent of diagensis. Petrographical studies, combined with whole-rock geochemical analysis, have outlined a diagenetic sequence for the field. Two end-member mineralogical assemblages are identified which are closely linked to reservoir quality. High porosity wells contain a chlorite-K-feldspar-quartz-dolomite assemblage while low porosity wells contain an illite-quartz-ankerite assemblage. Empirical evidence shows that porosity loss is associated with conversion of K-feldspar, chlorite and dolomite to illite and ankerite, and is due to compaction rather than cementation. Post-depositional faults acted as conduits for fluid which altered mineralogy, removed soluble components and, in one case, introduced anhydrite cement.
Effects of heterogeneities in a braided stream channel sandbody on the simulation of oil recovery: a case study from the Lower Jurassic Statfjord Formation, Snorre Field, North Sea
Authors: Høimyr Øistein , Kleppe Amund , Nystuen Johan P.
Published: April 2008

Three heterogeneous models of a channel sandbody of braided stream origin were constructed by combining reservoir data from the middle reservoir unit of the Lower Jurassic Statfjord Formation in the Snorre Field in the Norwegian Sector of the North Sea, with facies and geometry data from an exposed analogue channel sandbody in the Pennsylvanian-Permian Maroon Formation, Colorado. Permeability classes corresponding to those in the Statfjord Formation were distributed according to facies variation in the analogue sandbody and to the recorded vertical distribution in the Middle Statfjord sandbodies. Relative permeability and capillary pressure curves were made for individual permeability classes. Two-dimensional (
yz
-plane) reservoir simulation of waterflooding was carried out with injection at the down-dip end and production at the up-dip end of the models, which measured 104 m × 8.75 m and had a structural dip of 8°.

The results were compared with simulations on two reference models, one of a homogeneous sandbody and one with the permeability zonation organized in parallel layers with decreasing permeability from base to top. All models had the same volume-weighted arithmetic mean permeability. The 104 m long basic models were also compared with scaled-up versions measuring 312 m and 936 m, respectively.

The simulations show that variation in internal heterogeneity has considerable impact on recovery efficiency. Compared with the homogeneous reference model, the heterogeneous models had reduced recovery efficiencies at water breakthrough of 11% to 33%, depending on the type and degree of heterogeneity, and a corresponding reduction in mean effective permeability of 9% to 42%. This study also demonstrated the importance of using capillary pressure data in simulations of this type.

Capillary pressure allows imbibition into the low permeability rocks during waterflooding and explains the higher recovery efficiency on simulation runs on the scaled-up models compared to the short basic models. The effect of capillary pressure also results in higher recovery efficiency in runs with low flow rate than in runs with high rate. Similar simulation studies, also in 3-D, should be performed on various types of internal heterogeneities in complex fluvial reservoirs.
Geometry and facies of large-scale flow units in fluvial-dominated fan-delta-front sequences
Authors: Dreyer Tom
Published: April 2008

In heterogeneous reservoirs, an optimal number of large-scale flowunits (here termed building blocks) must be defined prior to detailed geological modelling. For the fluvial-dominated fan-deltaic parts of the Tilje Formation, Mid-Norwegian shelf, four fan-delta-front building blocks have been defined. Quantifications with regard to the geometry and spatial arrangement of these building blocks have been made based on outcrop studies of similar sediments in the Ridge Basin, California. The four building blocks are composite delta-front units (CDF), mouth-bar sandstones (MS), mouth-bar-related mudstones and carbonate-cemented zones (MMC), and distal delta-front mudstones (D). The main reservoir units in the Tilje Formation are the composite delta-front units. These have large lateral extents (tens of kilometres), and occur in three intervals separated by impermeable zones of building block D. Within the CDF units, the main heterogeneity is the alternation between permeable mouth-bar sandstones of building block MS and impermeable zones of building block MMC. The mouthbar sandbodies have width (W)/thickness (T) ratios of 220–525, and constitute a considerably larger percentage of the reservoir volume than the less widespread MMC flow-barriers. Hence, the internal communication within the CDF units is believed to be generally good.
Methods for improving the chronometric time-scale
Authors: Smith Alan G.
Published: April 2008

The problems of improving the chronometric time-scale are discussed with reference to the chronogram method for the estimation of the Cretaceous-Tertiary boundary. The need for better assessment of stratigraphic errors is emphasized. Suggestions are made on how the position of dated horizons relative to stratigraphic boundaries may be more precisely described. The assumption that stratigraphic thickness is linearly related to time is considered in relation to the likely gaps in the stratigraphic record from a variety of environments.
Magnetic polarity timescales: a new test
Authors: Mussett Alan E. , McCormack Alan G.
Published: April 2008

The magnetic polarity timescale, for times older than a few million years, is based upon sea-floor spreading magnetic anomalies, but the relative ages deduced from the anomalies must be calibrated to allow for non-uniform spreading rates. Calibration is carried out by using well-dated samples of known position in the magnetic timescale, but such samples are rare. This paper presents a method for testing the accuracy of a (portion of a) proposed magnetic timescale, using a body of polarity-plus-age data unconnected with sea-floor spreading anomalies. The method also suggests how a timescale could be improved by displacing or offsetting parts of it by a constant amount.

When this method is used with data for the British Tertiary Igneous Province, the timescale of Harland
et al.
(1989) is found to be superior to those of Harland
et al.
(1982) and Berggren
et al.
(1985) for part of the early Tertiary period (
c.
62–52 Ma). Over this interval the Harland
et al.
(1989) timescale is close to optimum for this data set, but it might be improved by subtracting about 0.25 Ma from the ages of all polarity boundaries.
Quantitative marine sediment core matching using a modified sequence-slotting algorithm
Authors: Thompson Roy , Clark R. M.
Published: April 2008

An automated core correlation procedure has been developed in order to match sequences, such as core logs, based on a mathematical sequence-slotting approach. The programme can be applied to univariate, multivariate or compositional data. In addition to finding the optimal slotting, the programme also quantifies the quality of match. It can also be used to detect sections of the sequences which are tightly constrained, by the match, or only fit together more loosely. An important aspect of any multivariate analysis is scaling. This topic is addressed through a simplex optimization method.

We have used the sequence-slotting method to analyse high quality oxygen-isotope data from ODP Site 658 on the continental slope, west of Cap Blanc, on the northwest African coast. Excellent agreement is found with the global oxygen-18 reference curve and this allows the sediments to be dated. By and large the resulting depth-age profile is very similar to that derived in previous studies. However, small changes in the age estimates between the original stratigraphy and our sequence-slotting lead to major differences in the pattern of sediment accumulation rate.

An hiatus spanning 24 500 SPECMAP years some 50 000 years ago has been proposed in the Site 658 sediments. However, as we find no compelling mathematical evidence for the hiatus, Site 658 may contain an even better stratigraphic record of palaeoclimatic change than previously thought.