bluuebunny/crossref_metadata_2025 huggingface dataset:
Palaeomagnetic dating of ancient fluid-flow events in the Arbuckle Mountains, southern Oklahoma
Authors: Elmore R. Douglas , Campbell T. , Banerjee S. , Bixler W. G.
Published: April 2008

Palaeomagnetic and geochemical data, and petrographic observations from Palaeozoic rocks in the Arbuckle Mountains provide data on the timing and pathways of fluid flow events. The lower part of the Upper Cambrian Reagan Sandstone, the basal palaeoaquifer in the Palaeozoic section, and the upper part of underlying 525 Ma Colbert Rhyolite are geochemically altered and contain two late Palaeozoic secondary magnetizations. The rhyolite as well as the overlying strata contain apparent primary Cambrian magnetizations. One of the secondary magnetizations (
D
= 150°,
I
= 5°, α95 = 5°, 45% tilt corrected) resides in hematite and is interpreted to be a chemical remanent magnetization (CRM) that formed due to hematite authigenesis as a result of fluid migration. The other secondary magnetization (
D
= 146°,
I
= 17°, α95 = 4°, 80% tilt corrected) resides in magnetite and is older than the CRM in hematite on the basis of the pole positions. The origin of this magnetization is more ambiguous and could have been caused by hot fluids resetting previously formed magnetite. Evidence for alteration by fluids includes secondary mineralization of calcite, hematite and quartz in veins, depletion/enrichment of elements in the rhyolite, and authigenic hematite cement in the lower Reagan Sandstone. Fluids also migrated along fractures during multiple episodes in the late Palaeozoic, locally remagnetizing and altering younger Palaeozoic strata in the Arbuckle Mountains.
The origin of helium in deep sedimentary aquifers and the problem of dating very old groundwaters
Authors: Pinti Daniele L. , Marty Bernard
Published: April 2008

Noble gases, inert elements having isotopes produced by the decay of long half-life radionuclides, offer a powerful approach for tracing fluid circulation and dating groundwaters. The (U + Th)-
4
He water ages — calculated from the accumulation rate in water of radiogenic
4
He produced by decay of U and Th contained in the aquifer rocks — is frequently higher than the hydrological ages. This discrepancy is generally interpreted by two contrasting models: (i) heterogeneities of the aquifers, which allow water stagnation and accumulation of large amounts of radiogenic
4
He, or (ii) addition of
4
He produced in deeper regions of the continental crust. In this contribution, we propose that the apparent contrast between (U + Th)-
4
He ages and hydrological ages in the Paris Basin reflects the mixing of different types of water, having different residence times. We show, using the helium isotopic signatures of waters, that this mixing occurs between three aquifers, the Middle Jurassic, the Triassic and the Palaeozoic basement, which have contrasting helium contents and heterogeneous chemical compositions and permeabilities. The difference of radiogenic
4
He/
40
Ar* ratios between the aquifers of Triassic and Middle Jurassic strongly suggests that a significant fraction of helium is produced in the aquifer rocks. This implies residence times for groundwaters circulating in the Middle Jurassic carbonate aquifer much longer than those obtained from hydrological studies. Independent fluid age estimates, based on the ground palaeotemperatures recorded in the same groundwaters by the atmosphere-derived noble gases, seem to confirm the presence of very old groundwaters in the Paris Basin.
The relationship between faults and pressure solution seams in carbonate rocks and the implications for fluid flow
Authors: Peacock D. C. P. , Fisher Q. J. , Willemse E. J. M. , Aydin A.
Published: April 2008

Pressure solution plays a major role in fault initiation and development in carbonate rocks. Strike-slip faults in the Triassic and Jurassic limestones of Somerset, UK, initiated as en echelon extension fractures, which became linked by pressure solution seams. Shear occurred along the pressure solution seams as the bridges between the veins rotated. The linked vein-pressure solution seam systems developed into pull-aparts and eventually into through-going faults. Normal fault planes in the Cretaceous Chalk at Flamborough Head, Yorkshire, UK, commonly have the pitted appearance of slickolites. Phyllosilicates are often concentrated along faults in Chalk, the thickness of the phyllosilicate gouge being proportional to fault displacement. The enrichment of phyllosilicates along the faults is due to pressure solution rather than simply to phyllosilicate smear. Pressure solution can be concentrated at the contractional quadrants of faults, particularly where there is a contractional overstep onto an adjacent fault. Metre-scale oversteps between strike-slip faults in Somerset often have pressure solution seams, while contractional oversteps and bends at Flamborough Head are accommodated by compaction of beds, with pressure solution apparently being important. The concentration of phyllosilicates by pressure solution can hinder fluid flow along and across faults in carbonate rocks. A high density of pressure solution seams between overstepping faults can effectively link these faults, increasing both the effective length of the barrier to fluid flow and the fault seal potential.
A seismic inversion study of the axial magma chamber reflector beneath the East Pacific Rise near 10°N
Authors: Collier J. S. , Singh S. C.
Published: April 2008

We have applied a full waveform inversion method to seismic reflection data from two adjacent fourth-order morphological segments of the East Pacific Rise near 10°N. Our inversion results show the ridge to be underlain by a thin (30 m) low P-wave velocity layer which we interpret as magma with less than 20±10% crystals. At one of the locations, the base of the mostly molten layer is underlain by a velocity gradient. Here the ridge is at an intermediate stage in its volcanic cycle and we interpret the gradient zone to be a result of an increase in crystals with depth, reaching 40–90% crystallinity over a vertical distance of 50 m. In contrast, beneath the second segment, which is at the onset of a vigorous volcanic stage, the mostly molten layer is underlain by a mostly solid floor. We infer that the melt layer has been newly emplaced. Our results provide support for the idea that the crustal magma system beneath fast spreading ridge axes is segmented on a similar scale to the fourth-order bathymetric segmentation.
Rare earth element mobility in a mineralized alteration pipe within the Troodos ophiolite, Cyprus
Authors: Wells D. M. , Mills R. A. , Roberts S.
Published: April 2008

Rare earth element (REE) mobility has been investigated in stockwork mineralized lavas and dykes that lie in a zone of former black smoker fluid upflow within Troodos oceanic lithosphere. Most lavas from the Pitharokhoma alteration pipe have REE compositions similar to Troodos volcanic glasses, indicating that on- and off-axis hydrothermal alteration processes have not induced significant net REE mobilization. A degree of REE mobility is inferred for some lavas which are depleted in the light REE ± Eu. Within the lavas, high- and low-temperature hydrothermal alteration phases analysed by laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) display contrasting REE patterns (variably light REE enriched and depleted respectively). Thus during high- and low-temperature hydrothermal alteration of the lavas there may have been some relative loss or gain of light and heavy REEs, that did not necessarily cause any substantial net REE mobilization. Low-temperature hydrothermal phases have been identified as the major repository for the REEs in many of the altered lavas. These data suggest that much of the REE signature of the alteration pipe is not a relict of on-axis greenschist facies alteration, but a lower-temperature post-mineralisation overprint acquired in the waning stages of hydrothermal activity and during subsequent ageing of the oceanic basement.
The fossil record of hydrothermal vent communities
Authors: Little C. T. S. , Herrington R. J. , Maslennikov V. V. , Zaykov V. V.
Published: April 2008

There are 19 known fossiliferous volcanogenic massive sulphide (VMS) deposits which range in age from the Silurian to the Eocene. Most of these are in the Ural Mountains, Russia. The deposits contain assemblages of inarticulate and rhynchonellid brachiopods; gastropod, bivalve and monoplacophoran molluscs; and a small diversity of worm tube morphologies, some of which may be attributable to alvinellid polychaetes and vestimentiferans. The fossils are preserved mainly as external moulds of pyrite, which is consistent with biomineralization processes occurring at modern vent sites. Most of the fossil taxa are new to science, but the lack of original shells and organic tubes makes placement in existing phylogenetic schemes difficult. A comparison between modern vent communities and fossil vent assemblages shows that vestimentiferans, alvinellid polychaetes, bivalves, gastropods, monoplacophorans and perhaps brachiopods are shared at higher taxonomic levels, but that arthropods are found only in the modern communities. There are no direct ancestor-descendant relationships between the fossil and modern vent molluscs and brachiopods. This demonstrates that the modern vent environment is not a refuge for the known Palaeozoic and Mesozoic shelly vent taxa. Hence, taxonomic groups have moved in and out of the vent ecosystem through time. These findings are discussed in relation to alternative hypotheses for the origins of modern vent communities.
Global glacial isostatic adjustment and coastal tectonics
Authors: Peltier W. R.
Published: April 2008

A global and gravitationally self-consistent model of the process of glacial isostatic adjustment (GIA) has been developed that extremely well reconciles the vast majority of available records of Holocene relative sea-level history, not only from sites that were ice covered at last glacial maximum (LGM) but also from sites that are well removed from such locations. There do exist, however, data that have been construed to constitute a significant challenge to this theory, namely, the long records of relative sea-level history derived on the basis of U/Th-dated coral sequences from the Huon Peninsula of Papua New Guinea and from Tahiti in the central equatorial Pacific Ocean. Following a review of the theoretical model and a discussion of the extent to which it is able to successfully reconcile a very wide range of Holocene shoreline observations, the discussion focuses upon the interpretation of these very important and interesting records, which are subject to different levels and types of tectonic contamination. These analyses suggest that existing estimates of the levels of Holocene tectonic contamination at both locations may require revision. In this context, it is suggested that the global model of the GIA process is sufficiently accurate that the magnitude and form of local tectonic effects during the Holocene period might be sensibly estimated by simply subtracting the GIA prediction for a given site from the observed variation of relative sea level.
Carbonate ramps and the Pleistocene-Recent depositional systems of the Arabian Gulf
Authors: Walkden Gordon , Williams Alun
Published: April 2008

The term ramp was devised to be helpful. It was to provide an interpretative model with genetic significance for a specific category of carbonate body and one with predictive value in oil exploration. The present-day southern Arabian Gulf is commonly cited as a ramp but it fails to provide a good example. In contrast with most of its pre-Miocene history, the Arabian Gulf is a foreland basin with a tectonically, climatically and glacioeustatically controlled mixed carbonate-clastic fill. Because of glacioeustasy and its relatively shallow depth, the Gulf has spent much of the last 2.5 Ma emergent. Three separate and equally important sedimentary systems currently contribute to its development: the Arabian Gulf marine carbonate system, the Mesopotamian fluviodeltaic system and the Arabian continental aeolian system. The Arabian Gulf carbonate system is a ‘high stress’ regime that is in tectonic, eustatic and depositional disequilibrium. Its structural, morphological and sedimentological diversity discount it as a Recent analogue for the thick carbonate bodies in the geological record that show persistent accumulation of depth-related facies developed across gentle slopes lacking a marked shelf-slope break. The Arabian Gulf fails some fundamental tests for whether it should be regarded as a stable carbonate ramp depositional system and its use as an example of a ramp is not helpful.
Carbonate production and offshore transport on a Late Jurassic carbonate ramp (Kimmeridgian, Iberian basin, NE Spain): evidence from outcrops and computer modelling
Authors: Aurell M. , Bádenas B. , Bosence D. W. J. , Waltham D. A.
Published: April 2008

The complex interplay between shallow-water carbonate production, pelagic sedimentation, and sediment erosion and redeposition on a Kimmeridgian carbonate ramp is analysed from field data and computer modelling. Field data come from reconstructed cross-sections near Zaragoza and near Teruel, NE Spain. Inner ramp areas are dominated by coral patch reefs, ooid shoals and bioclastic packstones and grainstones, whereas middle ramp areas are typified by tempestites and pinnacle reefs. Outer ramp areas are dominated by carbonate muds and marls. The large-scale stratigraphic features of the ramp are simulated using the forward modelling computer program
carbonate
6. A good match is found between real and simulated stratigraphic thicknesses along the two sections, large-scale internal stratigraphic geometries, amounts of aggradation and progradation, and the location of sequence boundaries, flooding surfaces and systems tracts in the two modelled cross-sections. To investigate the origin of the offshore carbonate mud, two synthetic stratigraphies, which both closely replicate the overall ramp geometries, were generated by computer simulation. These two simulations are based on two different hypotheses that had not been resolved from field investigation: (1) that most of the outer ramp mud was derived from erosion and redeposition from inshore areas; (2) that most of this mud originated from pelagic sedimentation. Detailed comparison between the simulated occurrence of sedimentary facies (characterized in the program by depth and process of deposition) and the real facies occurrence indicates that the closest match is given by hypothesis (1) where most of the offshore mud is produced by resedimentation from inshore areas. Offshore transport and resedimentation is thought to have been favoured by the windward orientation of the ramp, and offshore transport was highest during periods of sea-level highstands.
Lower Triassic storm-dominated ramp sequence in northern Hungary: an example of evolution from homoclinal through distally steepened ramp to Middle Triassic flat-topped platform
Authors: Hips Kinga
Published: April 2008

Lower Triassic formations were studied in sections of the overthrust Silicicum, in the Aggtelek-Rudabánya Mountains, in northern Hungary. The sequence is represented by evaporitic, siliciclastic, mixed siliciclastic-carbonate, and carbonate rocks
c.
800 m in thickness. Sixteen facies, from tidal flat to outer ramp, were documented in three succeeding depositional systems. They are as follows: wave- and storm-dominated siliciclastic ramp, mixed high-energy storm-dominated ramp, and low-energy carbonate ramp. Based on the vertical stacking patterns of the metre-scale cycles, five depositional units were defined within the Scythian sequence. Each depositional unit is composed of a deepening- and a shallowing-upward part. Occasionally, a lowermost transitional part is also recognized from the top of the previous depositional unit. There is a good correlation between these units and the Scythian depositional sequences of western Tethys. In Early Triassic time, the following main ramp stages were recorded: (1) homoclinal ramp with siliciclastic sedimentation; (2) homoclinal ramp with siliciclastic sedimentation and incipient oolite shoals on the inner ramp; (3) homoclinal to distally steepened ramp with mixed sedimentation; (4) distally steepened ramp with mixed and subsequent carbonate sedimentation; (5) drowning and gradual restriction of carbonate ramp. The following Anisian development is briefly summarized. In Early Anisian time, the restricted outer ramp filled up, which allowed shallow-water carbonates to advance laterally. In Mid-Anisian time, a tectonically preformed flat-topped platform began to develop. As a consequence of Mid-Triassic rifting, a deep pelagic basin was formed. Reef-building organisms, which first appeared in Late Anisian time after the Permian-Triassic boundary extinction, colonized the faulted margins.

The sedimentation on the Scythian ramp was controlled by several factors. The gently sloping ramp was formed by slow thermal subsidence. Absence of frame-builders because of the Permian-Triassic boundary crisis played an important role in maintenance of the ramp morphology. The climate significantly influenced the terrigenous influx and carbonate production. Owing to the windward palaeogeographical position of the Aggtelek ramp, storms had a particularly important role in the sediment transport.
Diagenesis of magnetic mineral assemblages in multiply redeposited siliciclastic marine sediments, Wanganui basin, New Zealand
Authors: Wilson Gary S. , Roberts Andrew P.
Published: April 2008

In many sedimentary environments, early diagenetic sulphidization reactions cause extensive dissolution of detrital magnetic minerals which can alter or destroy the palaeomagnetic signal. Pliocene sediments from the Wanganui basin, North Island, New Zealand, have been subjected to multiple cycles of erosion and redeposition in sulphatereducing environments before reaching their present setting. Despite this, the sediments retain a measurable and stable remanent magnetization. The only Fe-Ti oxide recognized from detailed sedimentary petrographic characterization of magnetic extracts and bulk sediments is ilmenite, with compositions that lie within the range from which paramagnetic behaviour is expected. There are no Fe-Ti oxide grains with compositions from which ferrimagnetic behaviour is expected (except for rare chromite). A direct link was made between magnetic behaviour and mineralogy by conducting electron probe microanalysis and back-scattered electron imaging of the same grains that were subjected to magnetic analysis. These studies demonstrate that stable magnetic behaviour is displayed by grains that appear to be homogeneously composed of ilmenite. This magnetic behaviour is attributed to the presence of ferrimagnetic Fe-enriched (hemo-ilmenite) microstructural domains that have been previously reported from ilmenite grains. These hemo-ilmenite domains appear to be dominantly responsible for the remanence of the Wanganui basin Pliocene sediments. Calculations of the reactivity of detrital iron-bearing minerals to sulphidization indicate that ilmenite is much less reactive than magnetite and other detrital magnetic minerals. Given the resistance of hemo-ilmenite, to dissolution, it is suggested that microstructural ferrimagnetic domains within ilmenite grains may be a source of palaeomagnetic information in sediments that have undergone diagenetic magnetic mineral dissolution during multiple episodes of erosion and redeposition.
Magnetic anisotropy indications of deformations associated with diagenesis
Authors: Hrouda F. , Ježek J.
Published: April 2008

Anisotropy of magnetic susceptibility (AMS), as revealed by mathematical modelling and empirical studies, sensitively reflects initial ductile deformation associated with diagenesis and early tectonism. In rocks shortened vertically because of the gravitational loading by the weight of overlying strata, the degree of AMS increases and the magnetic fabric becomes more oblate. The magnetic foliation remains parallel to the bedding and the magnetic lineation remains near the water current directions. In rocks shortened laterally, usually by tectonic forces, the degree of AMS and the magnetic fabric oblateness initially decrease and only later increase. The magnetic lineation deflects from the water current direction and the magnetic foliation from the bedding, with its poles creating a girdle pattern. The AMS changes are due to the physical rotation of magnetic grains, which has serious implications for palaeomagnetism, but are major indicators of the change from a diagenetic to a tectonically controlled regime.
The Andrew Formation and ‘biosteering’ — different reservoirs, different approaches
Authors: Holmes N. A.
Published: April 2008

Biosteering, the provision of real-time biostratigraphic data for the drilling of horizontal wells, has developed and evolved a wide range of approaches and applications in recent years. Its value to the success of a development project has grown beyond its core role as an effective and rapid method by which to define the relative stratigraphic position of the drill bit for steering purposes. High-resolution biostratigraphy and biosteering are now being used as part of an integrated service which is being incorprated in the planning of a horizontal drilling programme and individual well paths. This paper illustrates some of this diversity of applications by describing two very different styles of biosteering from age-equivalent turbidite sands of the Andrew Formation, on the United Kingdom continental shelf (UKCS).

The first example describes biosteering through thin (10–15 ft) turbidite sands from the Joanne Field. The reservoir is one of several thin allochthonous sand and limestone subunits, requiring a very detailed biostratigraphic framework to provide sufficient resolution to identify each of the subunits. The framework utilizes both
in situ
and derived/reworked assemblages and associations (microfacies) to identify up to 10 discrete and correlative lithostratigraphic subunits. Local dip variations, faults of subseismic resolution and thinness of sand hampered efforts to continuously remain within the reservoir during horizontal drilling operations. Well-site micropalaeontological interpretation correctly identified the position of the drill bit in relation to the reservoir sand, allowing the correct well-path deviation to be enacted. In the absence of this technique, hundreds of feet of additional non-reservoir section would have been drilled.

The second example describes the Andrew Field, again comprising deep-marine turbidite sandstones of the Andrew Formation. Development of the Andrew Field has incorporated both the biosteering function of well-site biostratigraphy as well as pre-drill, high-resolution facies analysis of the microfaunas to assist in depositional modelling of the field. Microfacies analysis provides data relating to the potential sealing qualities and lateral continuity of adjacent silt and claystones, and how these may act as barriers or baffles during production. This information is utilized in the planning of the well path to optimize production through well placement and stand-off from oil-water and gas-oil contacts, and at the well site during horizontal drilling to confirm, through micropalaeontological analysis, key horizons at which to geosteer.
Microfossil assemblages as proxies for precise palaeoenvironmental determination — an example from Miocene sediments of northwest Borneo
Authors: Simmons M. D. , Bidgood M. D. , Brenac P. , Crevello P. D. , Lambiase J. J. , Morley C. K.
Published: April 2008

Hydrocarbon reservoirs in northwest Borneo are often developed in ‘paralic’ depositional settings, although current exploration is evaluating relatively deep-water turbiditic plays. In the absence of conventional core, and with only ambiguous wireline log and seismic signatures being available, the use of microfossil data is considered to determine precise depositional setting. This is important because different depositional settings imply different reservoir qualities in terms of architecture, connectivity, heterogeneity and poroperm characteristics.

Equivalents of the reservoir succession are well exposed in northwest Brunei, and contain well-preserved sedimentary features and ichnofossils to determine precise depositional setting. Microfossil assemblages (both palynomorphs and foraminifera) have been sampled from each depositional environment identified at outcrop and by using an iterative approach, and incorporating data on modern distributions, diagnostic microfossil assemblages and taxa have been identified which can be used as precise palaeoenvironmental proxies. By using this approach distal turbidite, proximal turbidite, open shelf with slumping, open shelf, lower shoreface, upper shoreface, tidal flat with tidal channels, lower distributary channel, lagoon-distributary channel margin and upper distributary channel depositional environments can be recognized.
Applied palaeontology: a critical stratigraphic tool in Gulf of Mexico exploration and exploitation
Authors: O’Neill B. J. , DuVernay A. E. , George R. A.
Published: April 2008

Recent exploration has relied heavily on detailed biostratigraphic correlations using foraminifera and calcareous nannoplankton, particularly in salt flank positions poorly imaged by seismic. Shell’s Bonnie, a deep near-salt well, was drilled at Eugene Island Block 95 to the depth of projected amplitude anomalies without encountering significant sand bodies. Detailed palaeontological correlation to nearby wells verified that the objective section had been penetrated. Objective sands were interpreted to be missing because of stratigraphic thinning on to the dome. A sidetrack well encountered significant hydrocarbon reserves. Appraisal drilling in the Mars basin (Mississippi Canyon blocks 763, 806 and 807) was aided by the recognition of regional condensed sections between gravity-flow units providing correlation of individual reservoir units within the field. Biostratigraphy showed stratigraphic pinch-out of reservoir sands rather than absence from erosional or structural truncation near vertical salt faces. The interpretation improved volume estimates and the planning of future wells.
Deep seismic structure of the continental margin in the Gulf of Guinea: a summary report
Authors: Rosendahl Bruce R. , Groschel-Becker Henrike
Published: April 2008

Post-stack reprocessing of the PROBE seismic data set has resulted in a refined tectonic model of the continental margin between Cameroon and southern Gabon. The margin is divided into the North Douala and Gabon Basins by the Kribi Fracture Zone, a 75 km wide transform fault zone that trends northeast-southwest and intersects the coastline between 2 and 3°N. North of the Kribi Fracture Zone, oceanic crust extends essentially to the coastline and floors the narrow margin off Cameroon. Little or no offshore rift margin exists in the northern Douala Basin. Reflection Moho beneath oceanic crust is relatively weak but continuous throughout the northern Douala Basin. Oceanic crustal thickness is extremely uniform in the North Douala Basin, averaging
c.
1.75 s in two-way travel time. A distinct northeast-southwest trending structural grain at both basement and reflection Moho levels occurs within the Kribi Fracture Zone. Much of the margin off Equatorial Guinea is influenced or controlled by fracture zone tectonics. South of the Kribi Fracture Zone, oceanic crust is offset
c.
350 km to the southwest, resulting in a broad rift margin off Gabon that is unaffected by major-offset fracture zones. This margin seems to have evolved by a form of imperfect pure shear, involving brittle, upper continental crustal sheets overlying a series of ductile lower sheets that generally dip easterly to southeasterly. The ductile sheets may unroof at outer highs, which mark the boundary between oceanic and continental lithosphere. Dominant reflection events are often associated with the ductile shear zones that define the lower sheets, but a regionally coherent event representing reflection Moho cannot be delineated. The concept of reflection Moho beneath rifted continental crust may be invalid. The tectonic and rheological models presented here can be used to frame hydrocarbon exploration tactics and strategies along this passive margin. In a broader context, elucidation of margin architecture may be a cost-effective way to approach frontier passive margins in the next century.
Petroleum habitats of the Atlantic margin of South Africa
Authors: Jungslager Eric H. A.
Published: April 2008

Petroleum system logic is applied along South Africa’s western margin to improve understanding of the observed petroleum habitats and the evaluation of the area’s potential for hosting profitable hydrocarbons. Three major groups of source rocks are identified and, consequently, the source-centred petroleum systems and complementary exploration plays form three major groups: (1) those sourced from syn-rift, Upper Jurassic-Neocomian, generally lacustrine source rock pods; (2) those sourced from Barremian-lower Aptian, generally marine, transitional succession source pods; (3) those possibly sourced from the Cenomanian-Turonian boundary, generally marine, drift succession source pods. These groups are further subdivided into individual petroleum systems in terms of specific individual source rocks, level of certainty, type of hydrocarbon and extent; and into individual exploration plays in terms of stratigraphic level of the reservoir and trap type. The geographic location of the known and actively generating oil and gas pods, whether in the syn-rift or transitional successions, is the major control on the observed distribution of petroleum habitats. The predominantly gas-bearing areas coincide with the main depocentre of the Orange Basin where the lower Aptian source rocks are in the gas window. More oil-bearing areas are predicted outside the depocentre, in deeper water, where the lower Aptian source rocks may become more oil prone and are possibly located in the oil window. These possibly more oil-prone areas coincide with major syn-sedimentary structures and hypothetical deepwater turbidite sandstones. They represent a new frontier for future exploration.
Non-marine ostracod assemblages of the Pre-Salt rift basins of West Africa and their role in sequence stratigraphy
Authors: Bate Raymond H.
Published: April 2008

Early Cretaceous ostracods living within the rift lakes of Central Gondwana reflect, through their morphology, abundance and species diversity, depositional environments and changes in water chemistry that occurred within the lakes at that time. By studying the indigenous ostracod assemblages it is possible to interpret past environmental changes with respect to both water depth and water chemistry, and to utilize this information to estimate the probability of source and reservoir sediment deposition. The alternation of siliclastic and carbonate sequences throughout rift lacustrine sediments can be explained by changes in the depositional environments and related water-level movements. Such changes being brought about as a direct result of tectonism leading to increases or decreases in precipitation as a consequence of river capture or regional changes in climate. Two cycles of sedimentary infill are recognized, each initiated by an onlapping, fining-upwards transgressive systems tract within which a maximum flooding surface is developed. This is followed by a coarsening-upwards highstand systems tract in which a prograding sequence retains the diverse ostracod fauna of the transgressive systems tract below. A coarsening-upwards lowstand systems tract, having a low diversity ostracod fauna, brings each cycle to a close. An understanding of the cyclic sequence stratigraphy of these basins suggests that the break-up of Gondwana commenced as early as the late Barremian. Commercially, these rift basin lakes contain excellent hydrocarbon source rocks, but it is the interpretation of the lacustrine faunas that provides the essential information as to lake size and hydrocarbon potential.
Walvis Ridge barrier: its influence on palaeoenvironments and source rock generation deduced from ostracod distributions in the early South Atlantic Ocean
Authors: Dingle R. V.
Published: April 2008

The early South Atlantic was subdivided latitudinally by the proto-Walvis Ridge, which separated early Cretaceous open marine conditions in the south from non-marine environments to the north. Changes in the palaeogeography, which can be monitored by the distribution of ostracod assemblages, determined the establishment of a wide variety of depositional environments, including evaporitic and total organic carbon-rich facies. Limited northward marine influxes across the barrier are postulated for mid-Aptian time, while largescale southward migrations of marine ostracods from the northern sector occurred in early Cenomanian and/or Turonian times. The latter established cosmopolitan faunas in the southern Atlantic and effectively marked the breakdown of the Walvis Ridge as a physical barrier to north-south migrations, although possible oceanographic factors militated against longitudinal faunal movements. Variations in the composition of ostracod assemblages during so called ‘oceanic anoxic events’ and the production of potential source rocks is briefly reviewed.
Detachment faulting and petroleum prospectivity in the Rio Muni Basin, Equatorial Guinea, West Africa
Authors: Turner Jonathan P.
Published: April 2008

The change from rifting to drifting is generally signified by a characteristic end-rift, or break-up, unconformity. In the Rio Muni Basin, two such unconformities bound an Aptian-Cenomanian transitional sequence of allochthonous carbonates and clastics considered unique among the South Atlantic passive margins. The transitional sequence comprises allochthonous thin-skin ‘rafts’, up to 2500 m thick, of shallow water carbonate and clastic sediment emplaced in a relatively deepwater setting along shallow detachments. A new model for the rift to drift development of the Rio Muni Basin interprets the raft tectonics as a response to a combination of end-rift-early post-rift uplift of the basin margins and deep off-shore bathymetry
c.
1800 m. Seismic interpretations reveal spectacular hanging wall geometry in the rafts as a consequence of their movement across a geometrically complex, ramp-flat basal detachment. In places, the hanging walls of the rafts display an ‘eggbox’ topography of closely spaced anticlines, often with four-way dip closure. Information from wells and analogy with adjacent basins indicates potential source rocks in the Aptian and Turonian. Several migration paths are identified, including the juxtaposition of hanging wall reservoirs with the source-bearing stratigraphy across transverse fault zones. Raft tectonics has considerable isostatic and thermal consequences, with attendant implications for predicting the timing of potential source rock maturity. Modelling maturity at variously located wells and pseudowells indicates the principal effect of the raft tectonics to have been a very rapid episode of burial in the basin centre and isostatic rebound of the unloaded margins.
Tectono-stratigraphic evolution of the North Falkland region
Authors: Lawrence S. R. , Johnson M. , Tubb S. R. , Marshallsea S. J.
Published: April 2008

An interpretation of regional seismic data acquired between 1993 and 1995 has been used to establish a regionally consistent tectono-stratigraphic framework for the North Falkland Shelf. This incorporates three rift (extensional) and two post-rift sequences separated by important tectono-stratigraphic breaks representing unconformities at near-Base Upper Jurassic, near-Base Cretaceous, Mid-Cretaceous and Early Tertiary levels. In the structural framework derived from seismic interpretation two major zones are recognized. The northeast Graben Zone constitutes three tectono-stratigraphic provinces, the north-south orientated North Falkland Graben, and the Western Rift Complex and Southern Half-graben which are dominated by northwest-southeast rift trends. The Falkland Islands form part of the southwest Uplift Zone which has a tectono-stratigraphic history, shown by the interpretation of apatite fission track data, to be compatible with that of the North Falkland Shelf and southern South America. A regional northeast-southwest seismic line shows an extensional northern edge to the Falkland Plateau and no apparent westerly extension of the Agulhas-Falkland Fracture Zone. A ‘rigid’ or ‘non-rotational’ reconfiguration of southwest Gondwanaland (i.e. with no independent rotation of a Falkland Islands microplate) shows a regional consistency of Late Triassic-Mid-Jurassic back-arc and Late Jurassic-Early Cretaceous South Atlantic rift trends. This is presented as circumstantial evidence for a ‘non-rotational’ model for the break-up of southwest Gondwanaland.
Scaling systematics of vein size: an example from the Guanajuato mining district (Central Mexico)
Authors: Loriga Maria Antonietta
Published: April 2008

Fractals have been applied to vein systems to assess whether they have an organized and systematic structure. Understanding vein systematics allows realistic interpolation and extrapolation from available data by accurate prediction of the numbers and sizes of veins within a target volume. Vein geometries were measured from the Guanajuato epithermal Ag-Au field (Mexico) with vein attributes ranging from the millimetre to metre scale. Ores occur as mineralized quartz-calcite veins and stockworks in Tertiary sedimentary, volcanic and acid intrusive rocks. Mineralization is controlled by north-northeast-south-southwest regional extension. 1D sampling was found to be the most objective and effective way to compare the different data sets. Vein thickness distributions indicate an underlying power-law, with the scaling exponent varying between −0.66 and −0.88 over two orders of magnitude. When combined, the data maintain the same relationship over four orders of magnitude and are characterized by a slope of −0.75. Outcrop data, however, display higher densities within the stockwork. The agreement in scaling exponents between the various data sets suggests that they belong to the same overall structure and are most likely related to the same growth process.
Discrete fracture network modelling applied to groundwater resource exploitation in southwest Ireland
Authors: Jones Mark A. , Pringle Alec B. , Fulton Iain M. , O’Neill Shane
Published: April 2008

A discrete fracture network model has been applied to increase the probability of sinking a successful groundwater production borehole at a well-field in Carboniferous rocks in southwest Ireland. The model employs the FracMan software to explicitly represent fractures. Model construction is based on a synthesis of data from a variety of geological and hydrogeological sources. The model is verified and calibrated against borehole measurements and multiborehole pumping tests. The process of model construction indicates that the intensity of flowing fractures is controlled primarily by fracture zones running through the well-field, and that there is also a relationship between flowing fractures and zones of weathered dolomite. Simulations allow the optimal orientation of any production borehole to be determined, and suggest that the most favourable orientation increases the mean number of flowing fractures intersected by up to 30%. The likely yield for different borehole locations can be assessed by conducting flow simulations. Within fracture zones, the probability of a successful production well is high. Flow simulations are also used to quantify the degree of hydraulic interference between boreholes. The results of the modelling can be used to assess the most significant uncertainties and develop a programme of future site characterization that addresses them.
Prediction of static and dynamic fluid pathways within and around dilational jogs
Authors: Connolly Peter , Cosgrove John
Published: April 2008

The distribution of stresses around a fault controls the development of second order fractures and influences fluid migration pathways. This paper demonstrates how the regions of enhanced fracture-induced permeability and fluid migration pathways, in both static and dynamic situations, can be predicted for dilational jogs subjected to simple shear. Mean stress is one of several key factors influencing the migration of fluids through a system. By combining mean stress data with second order fracture predictions obtained from experimentally determined stress trajectory and differential stress data around fault geometries, it is possible to derive predictions for the fluid pathways associated with a static (homogenous) system. Superimposing the mean stress states obtained for the same jog at different applied loads enables the changes in mean stress (and hence fluid migration) associated with the cyclical loading of a fault during stick-slip faulting to be studied. In addition, the mean stress patterns associated with successive stages in the jog’s evolution have been studied and the variations in these patterns, and therefore of the associated fluid migration pathways, mapped in order to track the change in these pathways that occurs as the overlap of the jog-defining faults increases.
Carboniferous basin evolution of central Ireland — simulation of structural controls on mineralization
Authors: Lewis Helen , Couples Gary D.
Published: April 2008

Much is known about the Carboniferous-hosted, hydrothermal, Zn-Pb deposits of central Ireland, including temperatures, geochemistries and fluid properties at the mineralized sites. However, this knowledge is insufficient to force a unique genetic interpretation of the palaeohydrogeological system responsible for mineralization. Two hypothetical flow systems survive previous mass-balance testing: a deep, topographically driven, regional flow, and deep, convective flow systems local to each deposit. In order to develop models of these flow systems a 4D, Early Carboniferous restoration of basin development in central Ireland has been created. The structural style is one of tilted fault blocks of the pre-Carboniferous ‘basement’; the Carboniferous sedimentary rocks are deposited in the accommodation space created by movement of the basement. Fluid and heat flow simulations based on the geological restorations show that most of the rising, hot flows are focused on faults. The 4D restorations suggest that such faults are at active, syn-depositional, fault-block corners. Buoyancy driven convective flow systems, operating within both the fractured basement and its sedimentary cover, are a viable mineralization system that agrees with the geological interpretation. Topographically driven flow systems developed from the geological interpretation do not reproduce the important characteristics of hydrothermal flow as deduced from evidence at the deposit sites.
Gravity lineaments and Carboniferous-hosted base metal deposits of the Irish Midlands
Authors: O’Reilly B. M. , Readman P. W. , Murphy T.
Published: April 2008

Central Ireland contains an extensive carbonate succession of Courceyan-Brigantian age, deposited in fault-bounded sub-basins across the Irish Midlands. Carbonate sediments host base metal Pb-Zn deposits of mostly pre-Arundian age, with mineralization episodes controlled by syn-rift crustal extension. As the region is highly glaciated, geological structure is not well mapped. Here, it is shown that the mineralization is spatially correlated with a network of ‘Caledonian’ northeast-southwest trending and interconnecting weaker north-northeast-south-southwest and subtle northwest-southeast trending gravity lineaments. Gravity derivative maps are used to demonstrate these relationships and it is shown that the occurrence of the base metal deposits is related to the local magnitude and direction of the derivatives. The continuity of lineaments across the Carboniferous regions into older Palaeozoic terranes, where they usually correlate with mapped faults, indicates strong Caledonian basement control on Variscan tectonic development and related episodes of base metal mineralization. The reactivation of older basement fractures controlled the migration pathways of mineralizing fluids and the spatial distribution of deposits. These methods applied to densely spaced gravity data are a useful tool in understanding basement and basin-fill properties which influence ore genesis.
Tertiary-Quaternary magmatism within the Mediterranean and surrounding regions
Authors: Wilson Marjorie , Bianchini Gianluca
Published: April 2008

Tertiary-Quaternary magmatism within the Mediterranean and surrounding regions, including the European foreland of the Alps, the North African margin and the Eastern Mediterranean, occurs in three distinct associations:

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anorogenic, extension-related intra-plate magmas, typically Na-rich alkali basalts, basanites and their differentiates, but locally including subalkaline basalts and potassic partial melts (e.g. leucitites) of the mantle lithosphere;

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orogenic, subduction-related/post-collisional magma series related to plate convergence; typically with potassium-rich geochemical characteristics which may be related to subduction of continental crustal materials;

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subalkaline basalts, similar to mid-ocean ridge basalts, formed at localized oceanic spreading centres.

The magmatism is spatially and temporally associated with the Late Cretaceous-Cenozoic convergence of Africa-Arabia with Eurasia which resulted in the progressive closure (by subduction) of oceanic basins in the Mediterranean domain and ultimately the collision of the Alpine orogen with the southern passive continental margin of Europe. Break-off of subducted lithospheric slabs may have provided an important trigger for magmatism in several localities.

The timing and geographical distribution of magmatism of orogenic and anorogenic affinity is summarized. Detailed discussion of the major and trace element and Sr-Nd-Pb isotope characteristics of the magmatism in the Central Mediterranean region provides important insights into the petrogenesis of the magmas and the nature of the main mantle source components.
Crustal structure of the North Tyrrhenian Sea: first result of the multichannel seismic LISA cruise
Authors: Mauffret A. , Contrucci I.
Published: April 2008

During a deep-penetration multichannel seismic cruise, the North Tyrrhenian basin was investigated. This basin has a shallow Moho along the Italian coast. Two kinds of dipping reflections were identified in the crust. The westward-dipping reflections are interpreted as Apennines or/and Alpine thrusts whereas the eastward-dipping reflections may be the seismic expression of extensional detachments. The deep part of the 10 km thick Corsica Basin is well imaged on the continental shelf. This basin was formed before or during the opening of the Balearic-Ligurian basin and is related to the collapse of the Alpine Corsica belt. We emphasize the active role of plutonic bodies that rise from a 3 km depth during the mid-Pliocene. The thinning of the North Tyrrhenian crust could be explained by a delamination of the Adriatic lithosphere that retreats towards the east.