bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and metamorphic constraints on the tectonic evolution of the upper Pennine Alps
Authors: Ellis A. C. , Barnicoat A. C. , Fry N.
Published: April 2008

A series of thrust and shear zone structures in the upper Pennine Alps dissect the area into a number of horsts and duplexes. The diversity of the mineral stretching lineation directions when combined with shear-sense indicators reveals substantial variation in the movement direction both between and within different units, and in some cases at different times within the same unit. The present-day geometry, illustrated in a new block diagram, is controlled largely by backthrusts. These backthrusts have overprinted, and in some cases reactivated, earlier, more ductile forethrusts and cut out all but one of the regional megascopic folds. While this backthrusting renders structural analysis of the early history of the area intractable, study of the petrogenesis of early, high-pressure assemblages yields some data. Deformation during subduction, leading to the subcretion of rock packets onto the over-riding plate is recorded by a ‘hairpin’ in the P-T paths of several units, and by a few relic structures within ophiolite units. The forms of P-T paths for units which have undergone high pressure metamorphism indicates that they underwent similar histories. Geochronological data for this area are as yet of insufficient resolution to decide if the tectonometamorphic events were continuous or episodic.
West-directed décollement of Austro-Alpine cover nappes in the eastern Alps: geometrical and rheological considerations
Authors: Ratschbacher L. , Neubauer F.
Published: April 2008

The Austro-Alpine unit of the eastern Alps constitutes a thrust belt characterized by multiple deformations under changing external stress regimes during the Alpine orogeny. Problems arise in recognizing direction and geometry of initial detachment tectonics and flow behaviour of mylonites accompanying detachment thrusts.

In the first part of this article, we analyse the actual distribution of Austro-Alpine cover sediments as acquired during the Alpine orogeny and studied detachment thrusting on three sections through thrust systems occupying different vertical positions in the Austro-Alpine stack. In the second part, we study ductile rock flow in mylonites along individual thrusts in the basement-cover décollement zone.

The ramp-flat geometry, the dip of frontal ramps, and the imbrication of youngest sediments at the leading edge of thrust systems show that detachment is directed towards the W or NW and is, thus, in agreement with displacement directions derived from the analysis of microstructures in ductile mylonites of the Austro-Alpine unit. This direction deviates from the current N-ward hypothesis of Alpine décollement.

Cover imbrication occurred along brittle thrust faults, whereas broad belts of mylonites accompany the basement-cover detachment thrusts. Mylonite generation may have started under differential stress conditions as high as 100 MPa. Stress relaxation was achieved in monomineralic rocks mainly by geometric softening, producing highly ordered phyllosilicate, quartz and calcite textures, and, in polymineralic rocks, by pressure solution and grain boundary sliding mainly along quartz-phyllosilicate and phyllosilicate-phyllosilicate contacts. Imposed strain rates range from 10
−11
sec
−1
to 10
−15
sec
−1
, and ductile flow lasted up to 8 Ma.
Plate-scale kinematics and compatibility of crustal shear zones in the Alps
Authors: Lacassin R.
Published: April 2008

Crustal-scale thrusts of the western Alps are characterized by W to WNW directed movements of several hundreds of kilometres. In the internal zones, prominent lineations mark these slip directions. In the central Alps, coeval right-lateral strike slip (Insubric fault) and N-NNW vergent thrusting (Helvetic and more external thrusts) occurred during the upper Tertiary, on crustal-scale faults. It is proposed that WNW directed convergence between NW Apulia and Europe was accommodated mainly by the combination of movements on three first-order faults: in the central Alps, this oblique convergence is partitioned in two components (thrust and strike slip) at a deep fault bifurcation; in the western Alps, it accounts for the observed thrusts. This direction of convergence in the western and central Alps is markedly different from the coeval relative motion of Africa and Europe (N to NNW directed convergence). It implies either specific movements of Apulia with respect to Africa, or the lateral extrusion of NW Apulia during the indentation of the African promontory during the collision.
A review of palaeomagnetic and magnetic anisotropy results from the Alps
Authors: Heller F. , Lowrie W. , Hirt A. M.
Published: April 2008

Numerous palaeomagnetic studies in the Alpine region have documented the effects of tectonic displacement and rotation. Regions in the southern Alps are characterized by palaeomagnetic directions which are more similar to directions predicted by the African path of apparent polar wander than by the Eurasian path. Attempts to define the extent of the region with this African affinity are found in the internal zone of the western Alps, and also in the Lepontine of southern Switzerland, yet directions with European affinity have been reported from S of the Insubric Line. Although the northern calcareous Alps are of southern Tethyan origin, palaeomagnetic directions reveal that these allochthonous units have undergone substantial rotations, in some places clockwise and in others anticlockwise, during emplacement. Several palaeomagnetic studies in the eastern and central Alps show rotated directions due to strong local tectonic effects. In the western Alps, European style directions are found in the undeformed rocks of the Dôme de Barrot and the external Dauphinois zone, whereas directions from the internal zone tend to have African affinity.

Deformation produces not only a deflection of the stable palaeomagnetic vector in a rock. If the deformation is penetrative, grain rotations falsify the palaeomagnetic vector. Preferential grain alignment caused by deformation produces a magnetic anisotropy which can be measured easily and quickly in many rocks. Analogously to the strain fabric, the magnetic fabric can be described by an anisotropy ellipsoid. Several studies in the Alps have confirmed that the principal axes of the magnetic anisotropy are aligned with the corresponding axes of the strain ellipsoid. In deformed Permian red beds in the Alpes Maritimes the anisotropy ellipsoid changes shape and orientation like the strain ellipsoid during progressive deformation. A good qualitative agreement between magnetic fabric and petrofabric is also found in Jurassic shales and slates from the Dauphinois zone of the western Alps, where magnetic lineations often correspond to the direction of nappe transport. In strongly deformed Permo—Triassic red beds from the Lower Glarus nappe complex in central Switzerland the quantitative relationship between strain and magnetic fabric was used to derive equivalent strain ellipsoids in rocks that had no conventional strain markers. In the eastern Alps, the sense of motion on the Insubric Line has been postulated from magnetic fabric observations. Magnetic anisotropy results have been used in attempts to correct palaeomagnetic vectors for the effect of deformation. This technique may be viable in weakly deformed rocks, but it fails in strongly deformed samples; the limits of its applicability have not yet been determined satisfactorily.
Mineral genesis in ironstones: a model based upon laboratory experiments and petrographic observations
Authors: Harder Hermann
Published: April 2008

Although mineral assemblages of ironstones vary widely, with hematite and goethite being the main components of red deposits and berthierine and siderite predominating in green deposits, the bulk major and minor elemental chemical compositions for both types of deposits are very similar. This indicates that the source material and physico — chemical conditions at the site of deposition must have been almost identical and that the different mineral associations probably developed during early diagenesis.

Data from experimental studies including iron clay mineral synthesis throw light upon the post-depositional processes. It is demonstrated that berthierine can be synthesized from iron hydroxides in solution with initial SiO
2
concentrations of 5–9ppm, low concentrations of Mg and K, and with ferrous iron dominant.

Decay of organic matter causes changes of Eh towards reducing conditions, which destabilizes the iron-rich particles and leads to both increased concentration of ferrous ions in the pore solution and the release of silicic acid ions. pH and concentration of silicic and carbonic acid controls whether berthierine or siderite is formed. In red deposits the concentration of organic material is low, suggesting that the presence and amount of organic material is a controlling influence.
The formation of goethitic ooids in condensed Jurassic deposits in northern Switzerland
Authors: Gehring A. U.
Published: April 2008

Goethitic ooids from condensed sequences close to the Middle/Upper Jurassic boundary in the Swiss Jura mountains were analysed for texture and chemistry. The sequences are characterized by discontinuities of sedimentation and reworking, which indicate a marine aerobic depositional environment. The ooids consist mainly of isometric goethite particles with apatite and clay mineral flakes. The coexistence of these mineralogical phases can be explained by different adsorption processes due to electrostatic forces or ligand exchange reactions. Both types of processes are discussed and illustrated schematically.
Minette-type ironstones
Authors: Siehl A. , Thein J.
Published: April 2008

Oolitic ironstones occur in various sedimentary environments: shallow marine to deltaic, lacustrine, fluviatile and pedogenic. Distinction between formational and depositional environment is not always possible. Most of the marine and fluvial minette-type ironstones consist of reworked ferruginous coated grains deposited in agitated water, but there exist also indicative structural features of
in situ
formation in the supporting medium of lateritic and hydromorphic environments. In the zone of oscillating groundwater repeated leaching and subsequent concretionary precipitation of hydrated ferric oxides take place, according to the prevailing Eh/pH-conditions and microbial activity. The moderate Al substitution of goethite from hydromorphic environments corresponds to the observed range in oolitic ironstones. The authors therefore assume erosion, reworking and subsequent fluviomarine redeposition of soil derived ooids to be the major processes of generating minette-type ironstones. Postdepositional diagenetic changes may convert the aluminous, silica-rich ferric oxides into berthierine in reducing environments if the chemical bulk composition of the primary goethite is similar. Since any aquatic milieu with appropriate fluctuations of Eh and pH can produce ferruginous coated grains, marine iron ooids associated with hardgrounds and areas of low sediment input can also occur. But there, release of ferrous iron, transport in saline interstitial waters and fixation of ferric hydroxides — usually with very low Al-substitution — take place in a much smaller scale, unable of generating the huge iron accumulations of minette-type ore deposits.
Cambrian Antarctic archaeocyaths
Authors: Debrenne Françoise , Kruse Peter D.
Published: April 2008

Most Antarctic archaeocyaths have been collected as allochthonous blocks, but in situ collections from the Shackleton Limestone in the vicinity of Nimrod and Byrd Glaciers (Transantarctic Mountains) provide stratigraphic control. Collections from King George Island, the Weddell Sea, Whichaway Nunataks, Argentina Range, Beardmore Glacier, Nimrod Glacier and Byrd Glacier (Cracking Cwm) are all of Botomian age according to the Siberian stage scale for the Early Cambrian. One collection from the Byrd Glacier (Mt Egerton) may be of slightly younger, Toyonian age. A unique species from the Minaret Formation in the Heritage Range has a Late Cambrian (Idamean) age, and is the youngest archaeocyath known. All existing collections are taxonomically revised here. Strong faunal affinities are evident at the species level with in situ Early Cambrian faunas in South Australia and allochthonous faunas in the Permo-Carboniferous Dwyka Tillite of South Africa. This allows the recognition of an Early Cambrian Gondwana province and confirms the existence of a Gondwana supercontinent throughout the Palaeozoic. Archaeocyatha are the most abundant and characteristic early Palaeozoic macrofossils of the Antarctic continent (Fig. 1). Most Antarctic collections have been recovered as allochthonous blocks in moraine or in dredgings at sea, but some in situ faunas are now known. To date, the only in situ collections studied in detail are from the Shackleton Limestone in the environs of the Nimrod and Byrd Glaciers, Transantarctic Mountains (Debrenne & Kruse 1986). On the basis of this latter work, we here make critical comparisons (and synonymies where necessary) of the faunal lists of previous authors to tentatively establish the composition of the Antarctic archaeocyathan fauna (Figs 2–4).
The phenomenon of forest growth in Antarctica: a review
Authors: Chaloner W. G. , Creber G. T.
Published: April 2008

The existence of a temperate Antarctic flora in the Permian, Mesozoic and early Tertiary poses a number of problems that are not soluble by reference to any environmental situation obtaining at the present day (i.e. in which plants live in a regime where a warm summer is followed by a winter without sunlight).

Certain characteristics of the polar climate during a number of geological periods can be deduced from the remains of forests which grew at very high latitudes where tree growth cannot take place at the present day. The demands of tree growth of the order of 3 to 4 mm increase in trunk radius each year are such that an input of light energy of at least 3000 MJ m
−2
a
−1
would be necessary.

Light energy inputs of this magnitude have been measured at very high latitudes in both North and South polar regions and it would appear that polar forest growth would be possible today if the ambient temperatures were sufficiently raised above their present levels. It is also significant that the forest productivity of about 10 t ha
−1
as estimated for the Lower Cretaceous forest on Alexander Island (Palaeolat. 70°S), is a terrestrial productivity that is only matched at the present day in a variety of temperate and subtropical regions of the world.

The evolution of photoperiodic ecotypes in tree species must have facilitated the colonization of polar regions by tree varieties adapted to make all of their annual growth during the long summer light period. There is evidence also that these photoperiodic ecotypes have a high rate of cambial activity concentrating the more rapid growth of wood into the polar summer.
Early Cretaceous biota from the northern side of the Australo-Antarctic rift valley
Authors: Rich T. H. , Rich P. V. , Wagstaff B. , McEwen-Mason J. , Douthitt C. B. , Gregory R. T.
Published: April 2008

A diverse biota including vertebrates, invertebrates, and plants is known from the Early Cretaceous of southeastern Australia. It is preserved in sediments that accumulated in the rift valley formed as Australia began to separate from Antarctica. As there was no significant terrestrial barrier between the two continents at the time, it is likely that the Early Cretaceous biota of the nearest region of East Antarctica may have been quite similar to that of southeastern Australia.

Dominant among the vertebrates are turtles and at least four hypsilophodontid dinosaur species. Three species of theropods, including
Allosaurus
sp., are the only other dinosaurs represented. Other tetrapods include a few scant traces of plesiosaurs, pterosaurs, a lizard, and a labyrinthodont amphibian.

Southeastern Australia was located well inside the Antarctic Circle of the day. Oxygen isotope studies suggest mean annual temperatures for the area between −5 and 8°C. These low temperatures are concordant with the character of both the flora and invertebrate fauna. The presence of juvenile hypsilophodontids implies that these animals were breeding in the area. The large eyes and optic lobes of the brain of these dinosaurs relative to other ornithopods suggests they may have been adapted for active behaviour under low light conditions.
Terrestrial tetrapods in Cretaceous Antarctica
Authors: Molnar R. E.
Published: April 2008

The fossil record of continental vertebrates from Antarctica is practically nonexistent, hence the composition of the supposed Mesozoic terrestrial vertebrate fauna must be inferred indirectly. This may be done using the reconstructed positions of the continents during the Mesozoic. These positions suggest the presence in Antarctica of primitive sauropods, large theropods, hypsilophodontian and iguanodontid ornithopods, ankylosaurs, pterosaurs, crocodilians, lungfish and possibly birds. In the absence of evidence to the contrary, monotremes are considered endemic Australian forms during the Cretaceous. The discovery of hypsilophodontian material in New Zealand corroborates the inference of Antarctic hypsilophodontians from their occurrence in Australia. The existence in the Australian Early Cretaceous of relict taxa and taxa that seem unusual compared to their overseas relatives suggests that the south polar tetrapod fauna was characterized by singular forms.
The nature and timing of biotic links between New Zealand and Antarctica in Mesozoic and early Cenozoic times
Authors: Stevens G. R.
Published: April 2008

Until the Early Cretaceous, when the first phases of rifting that preceded the eventual breakup of the Australasian sector of Gondwana occurred, New Zealand and Antarctica had a close geographic relationship on the eastern margin of Gondwana.

After having been influenced by proximity to the Permian ice sheet, during which New Zealand and Antarctica shared elements of the
Glossopteris
flora, the eastern margin of Gondwana entered into a phase of predominantly cool-temperate climates that lasted throughout the Triassic.

In the Jurassic, rotation of Gondwana had moved New Zealand and Antarctica into mid-latitudes and they experienced warm-temperate climates. These climatic conditions facilitated the dispersion of Tethyan marine faunas into the SW Pacific sector of eastern Gondwana. New Zealand and Antarctica shared many elements of the Tethyan marine faunas.

Starting in Middle Jurassic times, earth movements of the Rangitata Orogeny began to create considerable areas of land in the New Zealand region. Eventually, by Late Jurassic and Early Cretaceous times, a large landmass had been developed, extending northwards towards New Caledonia, eastwards to the Chatham Islands, westwards to the Lord Howe Rise and southwards to the edge of the Campbell Plateau. Creation of this landmass, and its associated land and marine links, together with the equable warm-temperate conditions then apparent over large areas of Gondwana, provided favourable conditions for the dispersal of Gondwana elements into New Zealand. Ancestral stocks of at least some of New Zealand’s archaic endemic terrestrial biota may have dispersed from eastern Gondwana into New Zealand at this time.

In the Early Cretaceous, land connections to eastern Gondwana became weakened by the onset of rifting along the future sites of the Tasman Sea and Southern Ocean. The eastern edge of Gondwana was rotated into high latitudes, and cool-temperate climates returned to New Zealand and Antarctica. Cool-temperate Austral marine faunas developed and were shared between New Zealand, Antarctica and southern South America. Land links between New Zealand and West Antarctica provided access for early angiosperms. However, all land links between New Zealand and Australia/Antarctica were broken after 85 Ma. From this time onwards the ancestral Tasman Sea and Southern Ocean became effective barriers to overland dispersal between eastern Gondwana and New Zealand. Subsequently, all terrestrial colonists had to arrive by either flying, swimming or floating. Many birds did so, but no terrestrial snakes and no mammals, except bats.

Although land connections had been broken in the Late Cretaceous, shallow-water marine links continued to exist between New Zealand and West Antarctica until the late Paleocene (the Weddellian Province). However, as New Zealand moved progressively northwards and the Southern Ocean widened, such southern links gave way in latest Paleocene and early Eocene times to northern (Australian and Malayo-Pacific) links.
Origins and evolution of Antarctic marine mammals
Authors: Fordyce R. Ewan
Published: April 2008

Austral marine mammals evolved in a Southern Ocean influenced by changing post-Gondwana geography and consequent new water-masses, progressive cooling, increased thermal gradients and physical and biological seasonality. The biology of extant species and temperate-to high-latitude fossils suggests, but does not confirm, that such factors have governed evolution, and that details of marine mammal evolution therefore differed from those of the north. Cladistic analyses may eventually allow better interpretation of the record.

Cetacea were present in high southern latitudes by the late Eocene. Mysticetes appeared in the south, where they may have originated, about the early Oligocene. They were diverse by the late Oligocene. Early records of Odontoceti are more ambiguous, but they were diverse by the latest Oligocene, when some taxa had circum-Antarctic distributions. The Neogene records of Austral Odontoceti and Mysticeti broadly complement the taxonomic and ecological diversification seen in the north. Amongst largely southern seals, lobodontine Phocidae radiated in the south about the late Miocene, and perhaps ecologically excluded the later arctocephaline Otariidae from high-latitude pagophilic lifestyles. There are no reports of the important northern groups Odobenidae, Sirenia or Desmostylia. Overall, southern marine mammal faunas were ecologically and/or taxonomically similar to extant faunas by the later Miocene to Pliocene.
Sedimentary structures of clastic rocks identified from electrical borehole images
Authors: Luthi Stefan M.
Published: April 2008

High-resolution electrical borehole images are obtained from the Formation Micro Scanner
*
(FMS) by scanning the borehole wall with arrays of small electrodes pressed against the borehole surface. The images provide detailed sedimentary structural information from clastic rocks which is of great value for geological and reservoir modelling as illustrated by two case studies. Analysis of electrical borehole images through a Pennsylvanian deltaic cycle from Texas shows different types of bedding surfaces, including channel base scours, cross-bedding, slump folds and growth-faulting. The ensemble of the data constrains the subsurface geometry of the reservoir and helps to establish a depositional model. Thresholding may produce a segmentation of sand/shale images. In the aeolian Rotliegendes of the North Sea, electrical borehole images provide a vertical succession of facies, dip and azimuth measurements of interdune layers, bounding surfaces and foresets, as well as thickness measurements of the cross-bedded sets. Statistical modelling using the data indicates scale-invariant crescentic bedforms, and describes sizes, shapes and layering of the cross-bedded sandstones. Computer-aided design tools are used for forward-modelling of structures observed on borehole images in order to test possible interpretation hypotheses. The use of graphic workstations greatly facilitates the interpretation process.
Methodology of Formation MicroScanner image interpretation in Claymore and Scapa Fields (North Sea)
Authors: Harker S. D. , McGann G. J. , Bourke L. T. , Adams J. T.
Published: April 2008

The Formation MicroScanner (FMS)
*
tool is an advanced dipmeter tool capable of producing high-resolution resistivity images of a borehole wall. When integrated with core and open hole logs, FMS images are of great utility in evaluating the thin bedded reservoir sequences of the Claymore and Scapa North Sea oil fields. Sedimentary features as small as 0.5 inches (1.2 cm) can be resolved on FMS images. Such detail is employed in the distinction of massive, parallel laminated, ripple cross-laminated, bioturbated and conglomeratic bedding types. The application of the FMS tool to reservoir description is not limited to bedding type identification. Bed boundaries, as well as small and large scale structural features, can be distinguished, including erosional contacts, slumps, fractures and faults. In the hydrocarbon-bearing zone permeability distribution outwith cored intervals can inferred. This can be confirmed by relating resistivity changes to core measured permeability values. The proposed approach to FMS interpretation involves the integration of all available open hole logs with detailed local geological knowledge and core data. Aspect and image quality of the FMS log are important considerations. By imaging the borehole wall, the aspect of the FMS tool provides a direct method of relating features detected using open hole logs to those observed in cores. Image quality is largely dependent on resistivity contrast in the formation and correlation with cores is imperative to establish confidence limits for extrapolation into uncored intervals.
The language of rocks: an example of the use of syntactic analysis in the interpretation of sedimentary environments from wireline logs
Authors: Griffiths C. M.
Published: April 2008

The sedimentary sequence as recorded in vertical section, i.e. by boreholes and logged sections, is a very uneven representation of the passage of time. Thick sand sequences may be deposited instantaneously with respect to geological time, whereas thin shales may represent either millions or hundreds of years, depending on their environment of deposition. This uncertainty concerning the relative time values of sedimentary units and the breaks between them adds a certain spice to attempts to correlate rather than match well sequences. Consideration of Gressly’s original facies concept, and the formal application of ‘Walthers Law’ suggests the basis of a sequence analysis tool that can be used to correlate adjacent wells. The syntactic pattern recognition technique demonstrated here is based on the development of a grammar of unit associations, and the parsing of logderived lithology units. The development of stratigraphic syntax is shown for both wireline log and grain-size data, with example taken from the North Sea.
Geophysical logging of water exploration boreholes in the Deccan Traps, Central India
Authors: Buckley D. K. , Oliver D.
Published: April 2008

Drilling and geophysical logging of Deccan basalt during groundwater exploration in Central India demonstrated that the Deccan lavas comprise a series of simple lava flows, 15–20 m thick, having a porous upper cooling surface with thin clay development and a layered structure. Geophysical logging has proved capable of resolving the layering and produced consistent log responses. Gamma-ray logs were especially useful to identify the weathered flow surfaces but further revealed certain lavas have distinctly higher radioactivity. The higher radioactivity flows were found to be of regional significance. The resolution of the layering and positive identification of flows by geophysical logging permitted individual lavas to be recognized and traced over distances of at least 100 km with no apparent displacement. The logs provided a framework for a detailed stratigraphic sub-division of the lava sequence, and also a key to investigate lateral and vertical variation in aquifer properties.
Geochemical well logs: calibration and lithostratigraphy in basaltic, granitic and metamorphic rocks
Authors: Anderson R. N. , Dove R. E. , Pratson E.
Published: April 2008

Elemental analyses of rock compositions have been measured
in situ
with a geochemical logging tool (GLT) in drillholes through igneous and metamorphic terrains. Comparisons of dry weight percent oxide analyses from laboratory measurements on core samples and the log-derived results demonstrate that accuracies are fair in Ocean Drilling Program Hole 504B basalts, good in Cajon Pass, California granites, and very good in Moodus, Connecticut metamorphic rocks and Palisades, New York diabases. Improved accuracies of GLT measurements are attainable through the use of a boron sleeve to isolate the borehole fluid from wellbore effects, and in smaller diameter, land-based boreholes. Precisions within each well are very good in all cases. That is, only a linear calibration shift was required to bring core and laboratory analyses into general agreement in all cases. Variations in mineralogy, derived from inversion models of the log-derived elemental abundances at each well, produce a lithostratigraphic interpretation that is useful for determining the structure of the oceanic crust (Hole 504B), the fractionation sequence of basaltic intrusions in a new rift basin (Palisades Sill), the deformation history near the San Andreas Fault (Cajon Pass), and the variations in degree of metamorphism across a shallow decollement surface caused by collision tectonics (Moodus).
Preliminary geochemical results from DSDP/ODP Hole 504B: a comparison of core and log data
Authors: Brewer T. S. , Lovell M. A. , Harvey P. K. , Pelling R. , Atkin B. P. , Adamson A.
Published: April 2008

DSDP/ODP Hole 504B, located in the Eastern Equatorial Pacific in 5.9 Ma old crust, penetrates both basaltic pillows and dykes down to a depth of 1562.1 m below the sea floor. Core recovery is poor, averaging 230ut falling to a 12% average in the deepest sections. Geochemical logs give a continuous measure of elemental (Si, Ca, Fe, S, Ti, Gd, H, Cl, K, Th, U) abundances. Correlation between laboratory (XRF) geochemical data and log data is difficult because of the lack of accurate depth information for the cores. Pattern recognition techniques are of little help for locating sample depths in the uniform basaltic lithologies. Non-hierarchical clustering techniques prove the existence of relationships between core descriptions and gross geochemical variations; major lithostratigraphic zones are readily identified from the log-derived data.
Fracture detection from logs of North Sea chalk
Authors: Lehne Karl Audun
Published: April 2008

Depending on the rock type, logs are variously affected by fractures. Only field experience permits us to define logging methods which give the best results locally. This article presents the results of different fracture detection methods based on logs from two wells in the Tommeliten Gamma Field in the North Sea. Cores, dipmeter, formation microscanner, circumferential acoustic, digital sonic (amplitude, spike and criss cross analysis and Stoneley reflection) and echolog (Stoneley reflection) have been used, which allows a comparison of almost all the tools that are available on the market for fracture detection. The Borehole Televiewer was not used because of the high mud weight. The two wells are both deviated, which affects some of the fracture logs. Conventional logs (caliper, resistivity, density-neutron etc.) were of no significant value in fracture detection. Well tests and production logs will be run later to improve this fracture evaluation.
A correlation of hydraulic conductivity from pulse tests with sonic log amplitudes
Authors: Goldberg D. , Speed D. , Wilkinson C. , Scholz E.
Published: April 2008

A 100 m water resource well penetrating shaly dolostone in East Fishkill, NY was tested to identify fracturing and to relate in situ hydraulic conductivity to acoustic properties derived from sonic waveform logs. The vertical fracture distribution was determined by borehole televiewer imaging. Sonic full waveforms were also recorded in the well and were analysed for compressional and Stoneley amplitudes, and differential energy logs were calculated. Hydraulic pulse tests were conducted in 1.5 m intervals isolated using a re-inflatable borehole packer system, and the measured conductivity in unfractured zones ranged from 10
−9
to 10
−5
cm s
−1
. Our preliminary comparison of results show that hydraulic conductivity is not related to Stoneley amplitude and weakly related to compressional amplitude and Stoneley differential energy. However, hydraulic conductivity may be reduced by partially closed or clay-filled fractures and volumetric differences in the packed interval which simultaneously increase attenuation. Further analysis by hydraulic flow tests in highly permeable zones are expected to improve the correlation. In principle, independent estimation of hydraulic conductivity from sonic attenuation measurements alone could be extended to untested intervals in nearby wells.
Investigating stress regimes in sedimentary basins using information from oil industry wireline logs and drilling records
Authors: Bell J. S.
Published: April 2008

Estimates of the in situ stress tensor in sedimentary basins can be inferred from information gathered during the drilling and logging of hydrocarbon exploration wells. Principal stress magnitudes can be obtained from information provided by density logs, drilling histories, leak-off test results and pore pressure measurements. Horizontal principal stress orientations are indicated by borehole breakouts recorded on four-arm dipmeter logs. The stress magnitude estimates are comparable with values obtained from hydraulic fracturing and the orientation data provide a level of detail hitherto unobtainable. The information obtained assists in designing hydraulic fractures, in secondary recovery programmes and in obtaining optimum drill bit performance. It also contributes to understanding lithospheric plate movements, crustal subsidence and tectonic deformation.
Geological evolution and hydrocarbon habitat of the ‘Arctic Alaska microplate’
Authors: Hubbard Richard J. , Edrich Steven P. , Rattey R. Peter
Published: April 2008

Depositional sequence mapping has been used to analyse the late Devonian to Recent geologic evolution and hydrocarbon habitat of north Alaska and northwest Canada. Eight depositional megasequences have been identified, each of which records a discrete, major phase of basin evolution. The three oldest megasequences are named the Ellesmerian and reflect deposition on a subsiding fold belt terrane. We name the subsequent two megasequences of early Jurassic to Aptian age, the Beaufortian. They record a 100 Ma period of extension during which a Jurassic failed rift episode was followed by onset of the successful rift episode in the Hauterivian. This extension led to the opening of the oceanic Canada Basin. The final three megasequences record geographically distinct pulses of Brookian orogenesis.

The major proven hydrocarbon habitat occurs on the Barrow Arch of north Alaska. This is a volumetrically large, but greatly restricted, hydrocarbon province, which developed as a result of constructive interference between Beaufortian rift and Brookian orogenic tectonics. Two other, relatively minor, hydrocarbon provinces have also been discovered. They are the Mackenzie Delta and Kugmallit Trough provinces of northwest Canada, which developed in passively subsided basins, located just beyond the influence of Brookian orogenic uplift.
The Northern Gulf Coast Basin: a classic petroleum province
Authors: Fails T. G.
Published: April 2008

The US portion of the Gulf Coast Basin is that country’s greatest petroleum province, having produced about 31% of the oil and about 48% of the gas. Jurassic evaporites, deposited in basins opened by Mesozoic drift, underlie much of an area which received marine, mainly clastic, upper Mesozoic and Cenozoic deposits. Three small tensional basins were filled by late Eocene time. A large paralic basin remains an area of active deposition. Petroleum is produced from Mesozoic carbonates and clastics in the tensional basins, often from salt-related features, and from Cenozoic clastics in the paralic basin, where salt domes are common. Eocene fault-trapped production dominates in salt-free south Texas. Oligocene production, often from diapiric structures, is most important in coastal Texas and southwestern Louisiana. Miocene and younger production, usually salt-related, dominates in onshore and offshore south Louisiana. Favourable factors include large volume deposition and rapid burial, early and continuing structural development, lack of sand cementation and excellent source characteristics.
Geochemistry of carbonate source rocks and crude oils in Jurassic salt basins of the Gulf Coast
Authors: Sassen R.
Published: April 2008

The lower member of the Jurassic Smackover Formation is a source rock for crude oil across the northern Gulf Rim from east Texas to Florida. Type-I crude oils occur in Smackover reservoirs of east Texas, north Louisiana, south Arkansas, and Mississippi, and were generated by algal-derived kerogen preserved in an anoxic environment. Type-II crude oils in Smackover reservoirs of Alabama and Florida reflect an origin from kerogen with a terrestrial component preserved in a less anoxic environment. Thermal maturity differences across the Gulf Rim caused generation and destruction of crude oil to occur at much shallower depths in east Texas, north Louisiana, and south Arkansas than in Mississippi, Alabama and Florida. Jurassic through Cretaceous timing of migration and faulting permitted vertical migration of Type-I Smackover crude oils to many Upper Jurassic, Lower Cretaceous and Upper Cretaceous reservoirs along the Gulf Rim. Vertical migration of Type-II crude oils was less significant, especially in Alabama, because of evaporite barriers to migration. The high contents of hydrogen sulphide and carbon dioxide in deep, hot Smackover reservoirs are related to thermochemical sulphate reduction that has driven early destruction of methane. Compositionally distinct crude oils from Upper Cretaceous and Lower Tertiary siliciclastic source rocks are found in reservoirs south of the Gulf Rim.
Origin of the Infracambrian Salt Basins of the Middle East
Authors: Husseini Moujahed I. , Husseini Sadad I.
Published: April 2008

During the Infra and Lower Cambrain, the Arabian Gulf and Zagros Mountains (Hormuz Formation), South Oman (Ara Formation), Kerman in Central Iran (Ravar Formation), and the Salt Range Province in Pakistan (Salt Range Formation) were all sites of salt precipitation in subsiding rift basins along the Middle Eastern edge of Gondwanaland. Also from 600 to 540 Ma, a rift system extended westwards from the Sinai Peninsula and North Egypt across NE Africa, while a second rift system (about 600 to perhaps 510 Ma) extended northwards from the Sinai Peninsula across the Jordan Valley and Dead Sea into SE Turkey.

The development of these rifts was contemporaneous with the left-lateral Najd strike-slip fault system which dislocated the Arabian Peninsula by 300 km. The Najd system may be interpreted as a transform fault system which accommodated rifting between NE Egypt, the Jordan Valley, North Pakistan, Kerman and South Oman. The Najd system is parallel to the Zagros Line, and the latter is also the eastern depositional limit of the Infracambrian Hormuz Formation. The Infracambrian Zagros Line may be interpreted as a right-lateral, strike-slip fault system which caused Infracambrian rifting in the Arabian Gulf and Zagros Mountains.

Palaeontological and radiometric age constraints, together with stratigraphic considerations, suggest that the syn-rift evaporites were first precipitated during a transgression which commenced in the Latest Precambrian (600 Ma) and again during a Lower Cambrian regression. The regression terminated a period dominated by marine carbonates and evaporites and initiated the deposition of progradational alluvial to marginal marine sediments. Finally, in the late Lower Cambrian and Middle Cambrian, transgressive marine deposits swept across the Middle East.
Geology and reservoir characteristics of Lower Cretaceous Kharaib Formation in Zakum Field, Abu Dhabi, United Arab Emirates
Authors: Alsharhan A. S.
Published: April 2008

The Zakum oil field is an east-west plunging anticline, located in offshore Abu Dhabi, UAE. The field was discovered in 1963 by seismic survey, and came on production in 1967 at an initial rate of 50 000 bopd from the Lower Cretaceous carbonate sediments. The Zakum structure is a broad asymmetric anticlinal feature characterized by gentle dips throughout; the steepest dips occur in the north and northeast where the average dip is about 2.5°. The structure is about 45 km long and 28 km wide, and has a vertical closure in excess of 1200 ft.

At Zakum field the Lower Cretaceous Thamama Group is a thick shallow-marine carbonate sequence with an average thickness of about 2300 ft, and is divided into four formations: Shuaiba, Kharaib, Lekhwair and Habshan (in descending order). An informal reservoir-oriented classification was set up for this group. It was subdivided into six zones given a numerical notation from top to bottom (Zones I to VI).

The Kharaib Formation represent shallowing upward cycles formed in very shallow epicontinental seas. The energy level grades from low-energy subtidal to high-energy setting at or above wave base. It is characterized by dense-argillaceous pyritic bioclastic lime mudstones/wackestones, peloidal-ooidal packstones/grainstones, dolomitic limestones and thin black shale laminae.

The reservoir quality of the formation is controlled by vertical and lateral distribution of porosity, permeability and stylolites. The porosities ranges from 5 to 28%, and permeabilities from less than 1 to 100 Md. The influence of initial environment of deposition, diagenesis and petrophysical properties on the reservoir characteristics is evaluated.
People’s Democratic Republic of Yemen: a future oil province
Authors: Paul S. K.
Published: April 2008

The People’s Democratic Republic of Yemen has recently joined the ranks of oil producing countries and it is hoped that in the near future there will be every likelihood that it will become a major oil producing country and ‘a Classic Petroleum Province’, similar to the surrounding Arabian Gulf region.
Sub-salt imaging in the Gulf of Suez
Authors: Zaki R. , McDowell H. , Threadgold I. , Oldfield O.
Published: April 2008

Sub-salt imaging on seismic data is a serious geophysical problem affecting exploration in the Gulf of Suez. Here, thick Miocene evaporite, shale and salt sequences with salt tectonics overlie rift structures involving pre-Miocene and Miocene oil-producing formations. This paper discusses the seismic imaging problems associated with salt swells in the Gulf of Suez and presents experimental work by GUPCO that has provided a better understanding and possible solution to the imaging problem. A realistic model for the Gulf was used to generate synthetic shot records using finite difference modelling. Processing of the simulated data in conjunction with actual field data led to improved understanding of the wave propagation behaviour in the subsurface, and to a realization of the validity of the common midpoint stacking technique and post-stack depth migration.
Petroleum geology of the Niger Delta
Authors: Doust H.
Published: April 2008

The Niger Delta is one of the World’s largest Tertiary delta systems and an extremely prolific hydrocarbon province. It is situated on the West African continental margin at the apex of the Gulf of Guinea, which formed the site of a triple junction during continental break-up in the Cretaceous. Throughout its history, the delta has been fed by the Niger, Benue and Cross rivers, which between them drain more than 10
6
km
2
of continental lowland savannah. Its present morphology is that of a wave-dominated delta, with a smoothly seaward-convex coastline traversed by distributary channels. From apex to coast the subaerial portion stretches more than 300 km, covering an area of 75 000 km
2
. Below the Gulf of Guinea, two enormous lobes protrude a further 250 km into deeper waters.

The delta sequence comprises an upward-coarsening regressive association of Tertiary clastics up to 12 km thick. It is informally divided into three gross lithofacies: (i) marine claystones and shales of unknown thickness, at the base; (ii) alternations of sandstones, silstones and claystones, in which the sand percentage increases upwards; (iii) alluvial sands, at the top.

Delta structure and stratigraphy are intimately related, the development of each being dependant on the interplay between sediment supply and subsidence rates. The dominant subsurface structures are syn- and post- sedimentary listric normal faults which affect the main delta sequence. They die out upwards into the alluvial sands and sole out at depth near the top of the marine claystones. Major growth-fault trends cross the delta from northwest to southeast, dividing it into a number of structural and stratigraphic belts, called depobelts, which become younger towards the south. The deltaic sequence in each of these depobelts is distinct in age, so that they actually represent successive phases in the delta’s history.

Hydrocarbons have been located in all of the depobelts of the Niger Delta, in good quality sandstone reservoirs belonging to the main deltaic sequence (the ‘paralic sequence’ of common usage). Most of the larger accumulations occur in roll-over anticlines in the hanging-walls of growth faults, where they may be trapped in either dip or fault closures. In the 30 or 50 years since the first discoveries were made, approximately 4 × 10
9
m
3
of oil and an unestablished, but very substantial quantity of associated and unassociated gas have been discovered. Most fields are small, ranging up to 50 × 10
6
m
3
, though several larger fields contain recoverable reserves in excess of 80 × 10
6
m
3
. The hydrocarbons are found in multiple pay sands with relatively short columns, and adjacent fault blocks usually have independent accumulations.
Petroleum geology of the ‘Fossa Bradanica’ (foredeep of the Southern Apennine thrust belt)
Authors: Sella M. , Turci C. , Riva A.
Published: April 2008

The ‘Fossa Bradanica’ is located in Southern Italy, East of the Apennine mountain chain. It has undergone a complex geotectonic evolution: from platform (Mesozoic carbonates) to unstable foreland (Eocene-Miocene) and finally to foredeep of the Apennine thrust belt (Plio-Pleistocene). Petroleum exploration targets in this province are represented by: (a) Gas and oil in carbonates of the pre-Pliocene substratum. In this domain traps are only structural; (b) Gas and subordinately oil in Plio-Pleistocene sands, where structural, stratigraphic and combination traps can be found. The oils produced range in gravity from 3° to 45° API, with prevailing values of 15–20° API. Two families of oils have been recognized: oils generated by a shaly source deposited in oxygenated waters and oils derived from a carbonate source deposited in a euxinic environment. Most of the oils are classified as non-mature or marginally mature oils. Gas is mostly dry: 79% of the gas discovered in Fossa Bradanica being of biogenic, 6% of thermogenic and 15% of mixed origin. Many fields have high CO
2
concentrations (50 to 90% in the carbonates). The CO
2
content decreases, moving upwards in the stratigraphic column.