bluuebunny/crossref_metadata_2025_split huggingface dataset:
Carbonate ramps and the Pleistocene-Recent depositional systems of the Arabian Gulf
Authors: Walkden Gordon , Williams Alun 
Published: April 2008

Abstract
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

Abstract

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

Abstract

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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract

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

Abstract
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

Abstract
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

Abstract
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.