bluuebunny/crossref_metadata_2025 huggingface dataset:
Transpressional tectonics along the Karakoram fault zone, northern Ladakh: constraints on Tibetan extrusion
Authors: Searle M. P. , Weinberg R. F. , Dunlap W. J.
Published: April 2008

The Tibetan plateau north of the Himalaya has approximately double normal crustal thickness (60–75 km) and has been homogeneously shortened since the India-Asia collision at 60–50 Ma ago, yet, with minimal erosion rates, has almost no middle or deep crustal rocks exposed at the surface. In the Karakoram range, west of Tibet, early Tertiary crustal thickening and regional metamorphism resulted in Miocene crustal melting producing the Baltoro monzogranite-leucogranite batholith. Late Tertiary transpression along the western margin of the Tibetan plateau, caused by the continued northward penetration of India into Asia, led to exhumation of migmatites in the Pangong range and Baltoro-type granites along the Karakoram fault. Detailed studies of the Karakoram fault zone in northern Ladakh, India, show that Baltoro-type two-mica ± garnet leucogranites, intruded 21–18.0 Ma ago, have been offset a maximum of 150 km right-laterally. Average slip rates since 18.0 ± 0.6 Ma (2σ) are 8.3 mm/a.
40
Ar/
39
Ar mica cooling ages are 11.3 Ma on both sides of the main (southwestern) strand of the fault, suggesting that most of the exhumation of the Pangong migmatites and leucogranites must have occurred between 18.0 and 11.3 Ma. During this time, at slip rates of 8.3 mm/a the rocks would have moved horizontally right-laterally for
c.
56 km and been exhumed by
c.
20 km vertically during transpression, using the measured 20° plunge of lineations.

The high exhumation rate (3.0 mm/a) and amount of erosion (20 km) inferred between 18.0 and 11.3 Ma may also reflect the partitioning between an early transpressional strain associated with crustal thickening and exhumation of the Pangong deep crustal migmatites and leucogranites, and a later dominantly dextral strike-slip phase of fault motion along the central part of the Karakoram fault from
c.
11 to 0 Ma. This timing may also coincide with the initiation of the N-S aligned normal faults and E-W extension in southern Tibet. We suggest that the relatively minor dextral offset (150 km) and the young age of initiation on this bounding fault do not support the model of large-scale extrusion of Tibetan crust, but they suggest instead that deformation of Tibet was taken up predominantly by crustal thickening.
Spectral gamma ray logs: core to log calibration, facies analysis and correlation problems in the Southern North Sea
Authors: Bristow C. S. , Williamson B. J.
Published: April 2008

The aim of this study is to test the usefulness of spectral gamma ray logs in subsurface correlation, lithofacies description and the interpretation of depositional environments of Namurian and Dinantian sandstones in the southern North Sea. Lithofacies and depositional environments were identified from core descriptions and compared with spectral gamma ray logs from thirteen boreholes. The results show that lithofacies and sedimentary environments can be discriminated within single wells. However, there is too much variation between wells to make an unequivocal assessment of depositional environment on the basis of spectral gamma ray logs alone. Comparison of stratigraphically correlated sandstones shows that variations between wells are often greater than variations between lithofacies. The differences between correlated sandstones using spectral gamma ray logs are largely attributed to changes in the logging environment, mainly mud characteristics, borehole quality and contractor. In addition, the occurrence of negative numbers for uranium and potassium in some wells indicates that the algorithm used to calculate elemental concentrations may be in error. For sandstones with a low total gamma ray response, small errors associated with tool calibration and data processing make a comparatively large difference to results, which has made detailed correlation of sandstones untenable. The most significant problem is the correction factor for potassium in KCl drilling mud.
Quantitative density measurements from X-ray radiometry
Authors: Duncan A. R. , Dean G. , Collie D. A. L.
Published: April 2008

Qualitative linear X-ray scanning has an established role in the non-destructive imaging of both slabbed and whole core and has been routinely used in visual assessment and quality control of material being subjected to other physical measurements. Since core may be observed in real time, whole core can be oriented to maximum dip prior to slabbing, especially useful where core has been resin-stabilized within an outer liner. Linear scanning is also useful in the observation of heterogeneous lithologies; the features observed are distinguished by their penetrabilities to X-rays. As a result, the linear scanner produces an image which reflects the density variation in the section analysed. A joint project carried out by Robertson Research International Limited and Amerada Hess Limited on 108ft of heterogeneous sediments has shown that the digital X-ray penetrability values (‘luminance’) can be extracted in order to produce a surface density variation log. X-ray luminance values show a linear relationship with the downhole Formation Density Log and may, therefore, provide an accurate tool for the correlation of core density with log density.
Interpretation of core and log data—integration or calibration?
Authors: Lovell M. A. , Harvey P. K. , Jackson P. D. , Brewer T. S. , Williamson G. , Williams C. G.
Published: April 2008

Core-log interpretation requires the reconciliation of datasets from different measurements. Measurement process, resolution, scale and quality must be appreciated for each dataset. Calibration of measurements involves the use of standards to enable quantitative comparisons locally or globally; this may involve inter-dataset comparison and the process of equalization with the modification of one dataset in preference for another. Calibration should not be confused with integration which aims to maximize the information in an optimal manner and may require the selective choice of data. The clear recognition of the aims of the study at the earliest opportunity enables the best choice of strategy from measurement acquisition through to integration. The final interpretation should realize the original aims but must be compatible with all observations.
Methods for simulating natural gamma ray and density wireline logs from measurements on outcrop exposures and samples: examples from the Upper Jurassic, England
Authors: Ahmadi Z. M. , Coe A. L.
Published: April 2008

Methods for simulating natural gamma ray and density wireline logs from measurements on outcrop exposures and rock samples have been implemented. The signals have comparable amplitudes and resolution to wireline log signals, although the absolute values do not match precisely. The field gamma ray logs were measured on the outcrops at intervals of 30–45 cm using hand-held gamma ray spectrometers. The field density logs were produced by measuring the volume and grain density of selected rock samples, followed by interpolation and filtering of the data. Both techniques are illustrated for the Upper Jurassic of the Wessex Basin, Southern England, with field data from the exposures on the Dorset coast and wireline log data from 11 boreholes between 0.5 km and 170 km away. The Upper Jurassic comprises a range of rock types, giving a wide range of values on which to test the techniques: wireline gamma ray and density values of these strata cover the ranges 15–140 API and 1.8–2.9 g cm
−3
, respectively. Thus these techniques should be widely applicable for the purpose of correlating outcrops with borehole data.
Lithological classification within ODP holes using neural networks trained from integrated core-log data
Authors: Wadge G. , Benaouda D. , Ferrier G. , Whitmarsh R. B. , Rothwell R. G. , Macleod C.
Published: April 2008

Neural networks offer an attractive way of using downhole logging data to infer the lithologies of those sections of ODP holes from which there is no core recovery. This is best done within a computer program that enables the user to explore the dimensionality of the log data, design the structure for the neural network appropriate to the particular problem and select and prepare the log- and core-derived data for training, testing and using the neural network as a lithological classifier. Data quality control and the ability to modify lithological classification schemes to particular circumstances are particularly important. We illustrate these issues with reference to a 250 m section of ODP Hole 792E drilled through a sequence of island arc turbidites of early Oligocene age. Applying a threshold of > 90% recovery per 9.7 m core section, we have available about 50% of the cored interval that is sufficiently well depth-matched for use as training data for the neural network classifier. The most useful logs available are from resistivity, natural gamma, sonic and geochemistry tools, a total of 15. In general, the more logs available to the neural network the better its performance, but the optimum number of nodes on a single ‘hidden’ layer in the network has to be determined by experimentation. A classification scheme, with 3 classes (claystone, sandstone and conglomerate) derived from shipboard observation of core, gives a success rate of about 76% when tested with independent data. This improves to about 90% when the conglomerate class is split into two, based on the relative abundance of claystone versus volcanic clasts.
Core-derived acoustic, porosity & permeability correlations for computation pseudo-logs
Authors: Bastos A. C. , Dillon L. D. , Vasquez G. F. , Soares J. A.
Published: April 2008

In order to improve hydrocarbon production, it is often necessary to obtain more accurate rock, fluid and petrophysical information. For example, to obtain a reservoir porosity map using seismic data as reference, it is necessary to generate reliable correlations between seismic attributes and petrophysical properties like porosity and permeability. Again, to optimize drilling and/or hydraulic fracturing programs, it is also necessary to estimate better formation static mechanical behaviour from geophysical data. The main goal of this work is to establish for an offshore Brazilian field, relationships between compressional and shear wave velocities and petrophysical properties such as porosity and permeability. The large number of limestone samples (120) gave us a precise empirical relationship between
V
s and
V
p for limestone. In order to obtain a calibration reference, we also made, with the same samples, simultaneous measurements of dynamic and static elastic constants. Using all these laboratory relationships, it was possible to generate unmeasured pseudo-logs of
in situ
parameters, which include: shear wave velocity, static and dynamic elastic constants and permeability. The good experimental relationships obtained between
k
-φ and
V
p-φ in this work together with available logs give us an additional method to estimate permeability which is impossible to obtain from
in situ
measurements.
Dolomite cement distribution in a sandstone from core and wireline data: the Triassic fluvial Chaunoy Formation, Paris Basin
Authors: Worden R. H.
Published: April 2008

The distribution of mineral cements in oil fields is critical to the spatial variation of porosity and permeability. The distribution of dolomite cement within fluvial Triassic Chaunoy sandstones in the Paris Basin was studied using core description, petrography, core analysis (porosity and permeability), wireline data interpreted to give mineralogy, porosity and permeability data and geochemical data. Petrographic analysis revealed that dolomite and quartz cements are the main diagenetic minerals. Using sonic transit time, density and neutron porosity log, the overall proportions of quartz, dolomite and shale as well as porosity for each depth interval could be resolved. Petrographic and core analysis data showed that permeability could be calculated from wireline-derived porosity and mineralogy data. There is excellent correlation between core analysis porosity and permeability and their wireline-derived equivalents. There is also excellent correlation between wireline-derived mineralogy data and quantitative petrographic mineralogy data. The wireline-derived mineralogy data show that dolomite is preferentially concentrated at the tops of most sand bodies. Porosity and permeability are consequently lowest at the tops of individual sand bodies due to the localized dolomite cement. There are a number of potential causes for this distribution pattern although geochemical and petrographic data showed that a combination of early pedogenetic dolomite cementation and later recrystallization, possibly due to an influx of organically-derived CO
2
, are most likely.
The integration of electrical image logs with core data for improved sedimentological interpretation
Authors: Goodall T. M. , Møller N. K. , Rønningsland T. M.
Published: April 2008

Electrical borehole images allow the direct integration of sub-surface well-log data with core data on a detailed visual level. For sedimentary interpretation, electrical borehole images are primarily used to obtain bedding orientations and to confirm core-derived sedimentology.

The aims of this paper are two-fold: firstly, to discuss how the sedimentological information provided by electrical borehole image logs is integrated with that obtained from other wireline logs and from cores; and secondly, to show that despite the need to integrate these data electrical borehole images can provide the geologist with unique sedimentological information which can not be obtained from either the cores or the other wireline logs. In a case study from the Oseberg Syd oil and gas field, a Fullbore Formation MicroImager (FMI Mark of Schlumberger) log through a complex interval of shallow marine sediments has been investigated. The interpretation of the FMI log led to the acquisition of very detailed orientation data related to the attitudes of sedimentary bedding surfaces. When these data were integrated with the sedimentary facies, identified from core description, they proved fundamental for understanding the activity of different shallow marine palaeocurrents during sediment deposition.
How to characterize fractures in reservoirs using borehole and core images: case studies
Authors: Haller D. , Porturas F.
Published: April 2008

Microconductivity array and acoustic imaging of the borehole wall provide valuable multidatasets which are used to characterize the geological strata and especially the reservoirs in exploration activities and during development of producing zones. This paper presents a tutorial of the main applications and a methodology to follow when performing fracture interpretation. Borehole image interpretation should not be a routine work referring to ‘a cook book’. It must rely on our geological and structural knowledge and experience, on basic notions about tool principles and image processing, and on geometrical calibrations to cores. Vatious examples will be given, showing:

(a)

natural fractures; how to distinguish them from drilling-induced fractures;

(b)

typical drilling-induced fractures and borehole breakouts; how to identify them;

(c)

a tricky case where cemented fractures might be confused with open ones.

The match between interpretation of borehole images and production data appears now to be the most efficient way to manage fractures in reservoirs.
Quartz cement volumes across oil-water contacts in oil fields from petrography and wireline logs: preliminary results from the Magnus Field, Northern North Sea
Authors: Barclay S. A. , Worden R. H.
Published: April 2008

Quartz cement is a significant porosity-reducing mineral cement in many sandstones and thus affects economically significant reserves calculations and flow-rate (through its effect on permeability). The presence of oil in a reservoir is commonly assumed to retard quartz cement precipitation and thus early oil emplacement is often thought to preserve porosity and permeability. A combined petrographic and wireline log approach was utilized to investigate whether quartz cement volumes and the total quantity of quartz do indeed vary across the oil-water contact in a sandstone reservoir. Thin-section point-count data and bulk density, neutron porosity and sonic transit time wireline log data were obtained across the oil-water contact from three wells in the Magnus field, an Upper Jurassic turbidite sandstone reservoir in the Northern North Sea. Reported oil filled inclusions in quartz overgrowths in this reservoir show that quartz cementation occurred either during or after oil emplacement. Point count data were used to determine quartz cement and total quartz volumes across the oil-water contact, whilst wireline data were transformed to reveal the total quantity of quartz across the oil-water contact. Preliminary results seem to show that the volume of quartz cement and the total volume of quartz show little or no variation across the oil-water contact. These data seem to imply that the presence of oil in the reservoir had no appreciable effect on the component processes involved in quartz cementation in this field: a paradox that will be further investigated.
Physical signature of basaltic volcanics drilled on the northeast Atlantic volcanic rifted margins
Authors: Bücker C. J. , Delius H. , Wohlenberg J.
Published: April 2008

During several DSDP and ODP Legs in the NE Atlantic, basaltic lava flows of the early rifting and break-up phase in the Tertiary have been drilled and logged. The lava flows were deposited subaerially with characteristic variations in their physical and magnetic properties and it is possible to distinguish different intraflow zones (I–IV) by evaluating downhole and core measurements. The physical and magnetic properties are mainly influenced by the degree of vesicularity and alteration, especially in the top and bottom parts of the flows. The pattern of the magnetic properties susceptibility and remanence seems to be helpful in distinguishing different flow types (aa and pahoehoe) as well as intraflow structures.

These zonal characterizations of subaerial basaltic lava flows can be seen frequently in core as well as in downhole measurements. This holds true not only for one hole but also for holes at different locations in the NE Atlantic with an exceptional high correlation down to fine scale variations, pointing to comparable genetic mechanisms during the initial phase of rifting.
The comparison of core and geophysical log measurements obtained in the Nirex investigation of the Sellafield region
Authors: Kingdon A. , Rogers S. F. , Evans C. J. , Brereton N. R.
Published: April 2008

The Sellafield region, west Cumbria, is the focus of one of the most thorough geological investigations in the United Kingdom. The Sellafield Site is defined as an area immediately around the potential repository, extending 6.5 km north-south by 8 km eastwest. Twenty six deep boreholes were drilled within the area up to the end of 1995, with a total depth of approximately 28 km. Most of these boreholes have been continuously cored, a total of over 17 kilometres of core, with average core recovery well in excess of 90%. All boreholes were logged with a comprehensive suite of geophysical logs, including many state of the art tools. Laboratory physical property analysis of hundreds of sample cores has been carried out.

Pilot studies were carried out to compare and contrast datasets and to investigate the relationships between the different data scales. Various techniques, including fractal analysis and Artificial Neural Networks, were tried in order to explore the relationships of these data at a variety of measurement scales.

The pilot study was conducted in two stages:

(1)

evaluation of the primary controlling factors of the physical properties;

(2)

testing the validity of ‘Up-scaling’.

The rocks of the Borrowdale Volcanic Group provided the most challenging problems due to the physical properties being dominated by fracturing and associated alteration zones.

Relationships between data types at different scales were established suggesting that the extrapolation of properties derived from core and wireline logs across three-dimensional seismic grids would allow an understanding of the properties throughout a three-dimensional volume.
In situ
characterization of suspended particles using focused-beam, laser reflectance particle sizing
Authors: Law D. J. , Bale A. J.
Published: April 2008

Measurements of the size distribution of suspended particles were made as part of the LISP (UK) experiment using a novel,
in situ
instrument which employs focused-beam, laser reflectance. Data were acquired over three complete, ebb-flood tidal cycles at two stations possessing different elevations on the intertidal mudflat. The reflectance method, which employs a physical arrangement similar to optical back-scatter sensors for measuring the concentration of suspended particulate matter (SPM), measures sediment particles in the size range 2–1000 µm with negligible disruption to fragile aggregates and can operate over the wide range of SPM concentrations found in estuaries. The results indicated that the median sizes (
D
50
) of particle populations over the intertidal mud flats were relatively constant at 60–100 µm even during mild erosion and resuspension events caused by ebb and flood tides. However, populations with
D
50
values of up to 300 μm were seen at one station within the ridge-runnel systems during resuspension. These large aggregates were attributed to the particularly ‘mobile’ sediment in that environment and to increased biodeposition. In general, variations in the size characteristics of particles measured
in situ
were consistent with the properties of the underlying substrate. Measured sizes compared well with values determined by direct video-microscope observation.
Use of
in situ
flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna
Authors: Widdows John , Brinsley Mary , Elliott Mike
Published: April 2008

A portable
in situ
annular flume was deployed on eight occasions at five stations along a shore normal transect from immediately below the saltmarsh to the mid-tide level of the Skeffling mudflat in the Humber estuary. Both large scale (km) and small scale (m) spatial differences (i.e. between ridges, gullies and pools on ridges) were examined. Biodeposition rates were measured at low current velocities (5 cm s
−1
), while sediment resuspension and erosion rates were determined in response to step-wise increases in current velocities between 10 and 50 cm s
−1
(equivalent to a range in shear stress from 0.02 to 1.6 Pa). Maximum biodeposition rates (6.6 g m
−2
h
−1
) were more than an order of magnitude higher than the mean sedimentation rate in the absence of sediment and biota. There was a significant correlation between biodeposition rate and the density of the suspension feeder
Cerastoderma edule
(
r
= +0.90;
P
< 0.05). The intertidal sites could be separated into three distinct groups on the basis of sediment stability/erodibility. Upper shore sites had the highest stability with critical erosion velocities (
u
crit
) of 30 cm s
−1
, whereas the pools and gullies were the most easily eroded with
u
crit
of 15 cm s
−1
. Site differences in sediment erodibility were not significantly correlated with measured physical properties of the sediment (bulk density, moisture content, % POM, % sand). However, there was a significant correlation between sediment erodibility and the density of
Cerastoderma
(
r
= +0.89;
P
< 0.01) and an index of the bioturbation activity of
Macoma balthica
(
r
= +0.96;
P
< 0.001).
Mapping intertidal sediment distributions using the RoxAnn System, Dornoch Firth, NE Scotland
Authors: Hull J. , Nunny R.
Published: April 2008

The RoxAnn system (manufactured by Marine Micro Systems Ltd) is a remote sensing technique available for mapping sediment distributions. The acoustic returns from a standard boat-mounted echo sounder are electronically processed and the output stream logged together with accurate position data from a Differential Global Positioning System (or similar high accuracy positioning systems). The RoxAnn output comprises depth and indices of bed roughness (E1) and bed hardness (E2). The combination of these indices require to be translated into traditionally used parameters (such as visual appearance and sediment particle size) to provide useful mapping data. The most reliable means of achieving this is through a ground-truthing exercise, where grab samples and video footage are collected at sites of known RoxAnn values. Two problems are encountered in this process: firstly RoxAnn acoustic signatures often do not uniquely represent a specific bed type as described in traditional terms, secondly positioning errors can lead to poor calibration of the RoxAnn signatures in the ground truthing process. The first problem is difficult to correct, and can only be mitigated against through good survey design and execution. The second problem can be addressed in the data processing, and procedures for this are discussed. The RoxAnn data collected from the Dornoch Firth, used in conjunction with qualitative analysis of colour aerial photographs of the upper intertidal areas, provided a detailed classification of substrate types, including a classification of sands which allowed detailed insight into active processes of transport.
Mussels and mussel beds (
Mytilus edulis
) as stabilizers of sedimentary environments in the intertidal zone
Authors: Meadows Peter S. , Meadows Azra , West Fraser J. C. , Shand Peter S. , Shaikh Masroor A.
Published: April 2008

Coastal zone protection of the intertidal zone is of great importance in view of the threats of sea level rise around the globe. Environmentally-friendly methods involving biological control may offer a solution. This paper studies mussels (
Mytilus edulis
) as stabilizers of the intertidal zone because they form beds that armour some intertidal sediments, hence reducing erosion. In field experiments we seeded high energy lower intertidal and low energy upper intertidal environments with isolated mussels and preformed clumps of mussels. More animals were lost from the higher energy site. Those that remained were transported landward for significant distances. Clumping protected animal loss at the low energy but not at the high energy site. Gravel placed under mussels improved their stability because mussels attached threads to them. Laboratory experiments show that mussels form clumps by attaching to other mussels, and when available to gravel which has to be at the sediment surface. Flume studies show single mussels decrease the critical erosion velocity of sediment near them. Similar effects are seen in the field at the edge of small mussel beds.
Compaction — the great unknown in basin modelling
Authors: Giles M. R. , Indrelid S. L. , James D. M. D.
Published: April 2008

Decompaction routines are used in basin modelling packages to calculate sediment thickness and material properties such as thermal conductivity. However, compaction in nature is dependent on initial porosity, composition, and effective stress, and a considerable range of porosity-depth trends exists. Simple thermal modelling demonstrates that significant uncertainties (up to VRE ± 0.5) arise in predicted maturities due to this variation. Furthermore, the validity of using porosity loss as a measure of compaction is questionable because changes in solid volume can occur. Chemical reaction may increase or decrease porosity without changing sediment thickness, although an apparently smooth transition occurs from dominantly mechanical processes of porosity loss (e.g. grain rearrangement) at shallow levels to dominantly chemical processes (e.g. grain dissolution/cementation) at depth. That compaction and porosity loss are processes dependent upon effective stress, time and temperature is illustrated by the observation of overpressuring in the subsurface, comparison of experimental and natural compaction rates and analysis of porosity-depth trends for sediments of different ages. Mechanistic models of the processes involved in compaction (e.g. pressure solution) also indicate time dependency. Time-dependent models of compaction can be constructed, but these are difficult to incorporate into basin models as they cannot be run in a simple backstripping mode.
Analysis of erosion events and palaeogeothermal gradients in the North Alpine Foreland Basin of Switzerland
Authors: Schegg R. , Leu W.
Published: April 2008

For basin modelling purposes, uplift and erosion analysis is as important as subsidence analysis. Within the North Alpine Foreland Basin of Switzerland (NAFB) three major interregional unconformities separate the sedimentary succession: the late Palaeozoic, the base Tertiary and the base Quaternary unconformity. The amount of missing section depends on the palaeogeographic basin position and on different inversion mechanisms (thermal uplift, intraplate stress, tectonic uplift or isostatic rebound movements). Several techniques exist to determine the magnitude and timing of maximum palaeotemperatures and palaeogradients, facilitating direct estimation of missing section. In our study coalification and fission track data are analysed and compared with recently published models concerning erosion in the NAFB. The results indicate that each inversion episode is accompanied by significant erosion (up to several km). The following range of removed section values were reconstructed:

(i)

late Palaeozoic unconformity: 1000–1200 m;

(ii)

base Tertiary unconformity: 800–1800 m;

(iii)

base Quaternary unconformity: 1500–3000 m in the SW, up to 700 m in the NE and 4100–4400 m in parts close to the Alpine front.

The reconstruction techniques provide only rough estimates as each approach is based on certain assumptions and calibration uncertainties. Also, the obtained values are higher than those expected from simple evaluation of the preserved stratigraphic record. Calculated palaeogeothermal gradients are very high (80–90 °C km
−1
) during the late Palaeozoic, slightly higher than normal during the late Cretaceous (30–50 °C km
−1
) and lower than normal (15–30 °C km
−1
) during the Cenozoic.
A multidisciplinary approach to modelling secondary migration: a Central North Sea example
Authors: Symington W. A. , Green K. E. , Huang J. , Pottorf R. J. , Summa L. L.
Published: April 2008

Numerical simulations assist in understanding secondary migration, and can help estimate which traps in a basin may have received a hydrocarbon charge. This is illustrated by a suite of numerical models used to track the secondary migration of hydrocarbons into an Upper Jurassic Fulmar discovery in the Central North Sea. The models include areal flow calculations using Exxon’s proprietary reservoir simulator, and cross-section calculations using TEMISPACK, a commercial basin modelling package. While the application of conventional reservoir simulation techniques to geologic time-scale problems is itself a novel approach, the most important development has been in the integration of fluid flow modelling with techniques for dating the timing of hydrocarbon charge. These techniques have been used to constrain and validate the model results. In addition, sensitivity studies permit evaluation of the required permeability for effective lateral migration.

The areal calculations provide information about migration timing and the fill history of several traps. These fields are charged from mature Kimmeridge source rocks, mapped within the local drainage area. Several model runs were used to evaluate the sensitivity of migration timing to carrier bed permeability and capillary pressure characteristics. Calculations indicated that for reservoir quality carrier beds (permeability ≥ 1 mD) secondary migration can be geologically instantaneous, and that even low permeability silts can serve as effective migration pathways.

A cross-sectional model was used to integrate cross-stratal and strata-parallel migration. The model included hydrocarbon expulsion from Kimmeridge source rocks into the Fulmar sandstone and lateral migration up the flank of the structure. Drilling results indicate that leakage into younger Tertiary strata has occurred. The calculation sugests a portion of the top seal breached by 10–20 Ma. Evidence of partial seal loss includes a seismic chimney interpreted above the predicted breached seal, and an appraisal well high on the Jurassic structure which encountered hydrocarbon-stained reservoir rocks with an apparent palaeohydrocarbon contact. Furthermore, hydrocarbons in the overlying Tertiary reservoirs can be geochemically tied to the remaining Jurassic accumulation. Finally, analysis of authigenic cements, particularly the K-Ar ages of fibrous illite, suggests that hydrocarbons occupied the trap until approximately 10 Ma. Analysis of Tertiary hydrocarbon fluid inclusions further indicates that hydrocarbons entered these Tertiary reservoirs approximately 10 Ma. These latter two techniques now enable the modeller to apply temporal constraints to hydrocarbon charge simulation. Such constraints allow ‘history matching’ analogous to field scale reservoir simulations.
Early Palaeozoic brachiopods and associated shelly faunas from western Gondwana: their bearing on the geodynamic history of the pre-Andean margin
Authors: Benedetto Juan L.
Published: April 2008

The strong provincialism exhibited by early Ordovician to Devonian brachiopod faunas provides an independent tool for testing palaeogeographical hypotheses. Patterns of biogeographical affinities of early Palaeozoic brachiopods from the Argentine Precordillera, the Sierra de Famatina and the Central Andean basin (NW Argentina, Bolivia, southern Peru) suggest that the pre-Andean margin was linked to northern Iapetus Ocean history. The low-grade Vendian-Early Cambrian Puncoviscana Formation and the broadly coeval rocks from the Carolina and Gander terranes may represent sedimentation in a narrow basin developed between Laurentia and Gondwana during rifting. Palaeontological evidence suggests that the Precordillera developed on the passive margin of Laurentia but moved away as an independent plate during the Ordovician; the hypothesis of the Precordillera as a Laurentian plateau does not explain faunal differences between their Late Arenig-Llanvirn brachiopods. Ordovician subduction beneath Gondwana resulted in formation of the Famatina-Puna-Avalonia volcanic-arc system. Affinities between Celtic brachiopods of this age in volcaniclastic rocks from South America (Famatina) and eastern North America (Maine, Gander, Central Newfoundland) suggest some geographical continuity between them, consistent with palaeomagnetic evidence. It is proposed that accretion of allochthonous terranes to eastern Laurentia was related to collision with the northwestern corner of South America in late Ordovician time, an idea supported by the affinities of Silurian and Devonian brachiopods from Venezuela and Colombia.
Ordovician K-bentonites in the Argentine Precordillera: relations to Gondwana margin evolution
Authors: Huff Warren D. , Bergström Stig M. , Kolata Dennis R. , Cingolani Carlos A. , Astini Ricardo A.
Published: April 2008

Ordovician K-bentonites have now been recorded from >20 localities in the vicinity of the Argentine Precordillera. Most occur in the eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle ArenigI. victoriae lunatusgraptolite (Oe. evaeconodont) Zone, and the youngest in the middle LlanvirnP. elegans(P. suecicus) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, titanite and Fe-Ti oxides. The proportions of the mineral phases and variations in their crystal chemistry are commonly unique to individual (or small groups of) K-bentonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition. The sequence is unique in its abundance of K-bentonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established. The ash distribution is most consistent with palaeogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famatinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bentonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.
Time constraints on the Early Palaeozoic docking of the Precordillera, central Argentina
Authors: Ramos Victor A. , Dallmeyer R. D. , Vujovich Graciela
Published: April 2008

40
Ar/
39
Ar incremental-release ages have been determined for muscovite and hornblende concentrates prepared from basement rocks of the Sierra de Pie de Palo, in the western Sierras Pampeanas, east of the Early Palaeozoic Precordillera. The basement is mainly represented by variably metamorphosed units, including ophiolites, orthogneisses and schists. Previous U-Pb, Rb-Sr, K-Ar, and
40
Ar/
39
Ar dating of magmatic and metamorphic zircons from the basement has indicated a Middle Proterozoic age. The
40
Ar/
39
Ar plateau ages constrain a series of ductile deformational events that are correlated with development of structural discontinuities in adjacent foreland basins. The following deformational events are postulated: (1) initiation of collision and drowning of the Precordillera platform against Gondwana (470–460 Ma); (2) flexural extension associated with normal faulting due to tectonic loading of the Gondwana margin; and (3) development of a foreland basin (450–430 Ma). These Mid-Ordovician-Silurian events were related to collision of several exotic Early Palaeozoic terranes along the Gondwana margin. The Punta Negra foreland basin developed further west during Early to Mid-Devonian (410–380 Ma) times, and was linked to the beginning of the docking of the Chilenia terrane along the western margin of the Precordillera.
A Laurentian? Grenville-age oceanic arc/back-arc terrane in the Sierra de Pie de Palo, Western Sierras Pampeanas, Argentina
Authors: Vujovich Graciela I. , Kay Suzanne Mahlburg
Published: April 2008

Whole-rock trace element, field, and petrographical studies indicate that the protoliths of highly deformed metamorphic rocks of the Pie de Palo Complex, Argentina, formed in an oceanic setting like that of SW Pacific supra-subduction complexes (e.g., Lau Basin). Age constraints suggest that this Mesoproterozoic complex, which includes some of the most primitive Grenville-age magmas known on Earth, formed in < 60 million years. Amphibolites (Las Pirquitas/Quemado region) with N-MORB like light-depleted REE patterns and Hf/Ta/Th relations, associated with Fe-rich schists interpreted as hydrothermally altered lavas and gabbros, formed in a back-arc spreading centre. Other ultramafic/mafic units (Quebrada del Gato, Cerros Valdivia/Barboza) formed as arc cumulates, and (central region) as ridge or arc/back-arc lavas (including shoshonite). Intermediate and silicic schists/gneisses are interpreted as arc region greywackes and silicic lavas, and marbles as reef deposits. Quebrada del Gato gneisses with trace elements indicating high pressure residual mineral assemblages are interpreted as younger subduction-related magmas emplaced in a thickened crust. The Pie de Palo Complex probably formed offshore of eastern Laurentia, and subsequently collided with Laurentia east of the Llano uplift, Texas, in Grenvillian times. This block, along with the Precordillera, was rifted from Laurentia in the latest Precambrian, arriving in Gondwana by Ordovician time.
The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Córdoba
Authors: Rapela C. W. , Pankhurst R. J. , Casquet C. , Baldo E. , Saavedra J. , Galindo C. , Fanning C. M.
Published: April 2008

A detailed study of the pre-Silurian geology of the Sierras de Córdoba, Eastern Sierras Pampeanas, is used to define the sequence of magmatic and metamorphic events during the Pampean orogeny. This primarily involved Early to Mid-Cambrian subduction and terrane collision at the western margin of Gondwana during the amalgamation of the super-continent. Evidence for this is based principally on new information concerning (a) regional mapping and field relations, (b) analysis of the structures, deformational history and meta-morphic evolution and (c) geochronology and geochemistry of the igneous and metamorphic rocks. The main events recognized are (1) Late Proterozoic break-up of Rodinia (Nd model ages of 1500 ± 200 Ma, inherited zircons 800–1400 Ma), (2) development of an Early Cam-brian passive margin sequence (Puncoviscana Formation and equivalents), (3) emplacement of metaluminous calc-alkaline granitoids (G1a, dated at 530 ± 3 Ma) as a result of NE-directed subduction, (4) crustal thickening, ophiolite obduction, compression and high-grade metamorphism (M2: 8.6±0.8 kbar, 810 ± 50°C,
c.
525 Ma) related to collision, and culmina-ting in (5) isothermal uplift and widespread low-
P
anatexis (M3, 4.0±0.5 kbar, 715 ± 15°C,
c.
520 Ma). The last event is responsible for the linked generation of highly peraluminous granites (G1b) and cordieritites. Subsequent emplacement into the accreted terrane of Ordovician trondhjemite-tonalites (500-470 Ma) and dextral wrench shear are interpreted as inner cordilleran counterparts of the Famatinian arc, which developed to the west along the newly-formed proto-Andean margin.
U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin
Authors: Sims J. P. , Ireland T. R. , Camacho A. , Lyons P. , Pieters P. E. , Skirrow R. G. , Stuart-Smith P. G. , Miró R.
Published: April 2008

New SHRIMP zircon and monazite
206
Pb/
238
U and
208
Pb/
232
Th ages on structurally controlled units and
40
Ar-
39
Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by
c.
530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at
c.
480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.
The Early Palaeozoic magmatic record of the Famatina System: a review
Authors: Saavedra J. , Toselli A. , Rossi J. , Pellitero E. , Durand F.
Published: April 2008

The Famatina System is an elongated geomorphological zone between the Eastern and Western Sierras Pampeanas, characterized by important Ordovician volcanism. The basement consists mainly of a sedimentary protolith, metamorphosed to low grade during the Pampean orogeny (Early-Mid-Cambrian), together with sparse outcrops of high-grade metamorphic rocks. Overlying this basement, the Volcancito Formation consists of thin sediments of uppermost Cambrian and Tremadocian age, deposited in restricted basins. A much more voluminous series of volcanic and sedimentary deposits, initiated in the Arenig and culminating in Early-Mid-Llanvirn times, indicates a significant increase in the rate of tectonic subsidence. The form of these deposits shows that they were laid down in an ensialic marginal basin that developed contemporaneously with active volcanic activity. The products of the latter were predominantly acidic, generated by crustal anatexis, with much rarer occurrence of basaltic and trachyandesitic compositions. The volcanic rocks are cut by granitoids of unknown absolute age, although the few radiometric ages obtained on them do not exceed 460 Ma. The volcano-sedimentary basin is interpreted as related to subduction, and the granitic and volcanic rocks together represent a continental magmatic arc developed regionally during the Early Ordovician.
Timing of fluid flow in a sandstone reservoir of the north German Rotliegend (Permian) by K-Ar dating of related hydrothermal illite
Authors: Zwingmann Horst , Clauer Norbert , Gaupp Reinhard
Published: April 2008

Authigenic illite is the most abundant clay mineral in Carboniferous and Rotliegend rocks of northern Germany. In spite of extreme present-day burial depths of 4580 to 5280 m, the Rotliegend sandstones are characterized by moderate permeabilities which are controlled, together with their reservoir qualities, by omnipresent illite. It is admitted that deep-crustal hydrothermal fluids, migrating along major fault systems, interacted with the Carboniferous Coal Measures and generated acidic fluids inducing primary leaching processes, mass transfers and concomitant pervasive clay authigenesis.

The investigated samples belong to wells located in a horst-graben structure covering an area of 20 by 20 km. This restricted area allowed investigation of a three-dimensional distribution of fluid flows and related illitization in distinct sandstone reservoirs. The K-Ar ages of the < 2 μm fractions decrease from 210-180 Ma (11 analyses with a mean at 198 Ma) for illite in the horst position to 190-155 Ma (24 analyses with a mean at 177 Ma) for illite in the graben area up to about 4 km away from faults, suggesting a relationship between timing of illite formation and structural position of the host rocks. The decrease in the illite K-Ar ages from the horst into the graben allowed calculation of a migration rate of 0.2 μm per year for the illitization front in the Rotliegend sandstones towards the centre of the graben, which might be of use for prediction of the illite distribution in these rocks.