bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mesozoic palaeoceanography of the North Atlantic and Tethys Oceans
Authors: Roth Peter H. 
Published: April 2008

Summary

The biogeography and preservation of calcareous nannofossil assemblages allow mapping of surface water fertility patterns and put constraints on carbonate dissolution in the global oceans of the middle Cretaceous. Cyclic changes in carbonate dissolution and surface water biologic productivity in the Cretaceous oceans were linked to oceanographic and climatic changes that had frequencies close to the Milankovitch cycles (tens of thousands to hundreds of thousand years). Evolution of calcareous nannoplankton during the Jurassic and Cretaceous periods occurred in two primary cycles of about 75 Ma duration that were punctuated by major diversification events, occurring every 15–30 Ma, that marked the beginning of periods of increased marine organic matter production and preservation (stagnation events). Calcareous nannoplankton evolved largely in shallow seas during the Rhaetian to Kimmeridgian. In the latest Jurassic, calcareous nannoplankton conquered the oceanic realm, became major producers of pelagic carbonates, and are thus responsible for the shift of carbonate deposition from shallow seas to the deep ocean. In the late Cretaceous chalk seas, nannofossils replaced shallow water benthos as major carbonate producers. Warm climates, due to high atmospheric CO
2
concentrations and increased albedo during high sea-level stands caused by global tectonic processes, made these tiny creatures flourish during the late Mesozoic.
Changes in the organic carbon burial during the Early Cretaceous
Authors: de Boer P. L. 
Published: April 2008

Summary
The early and middle Cretaceous exhibit a paradoxical coincidence between low primary production in the oceans and entrapment of large amounts of organic matter in pelagic sediments. The high average temperatures and the decreased intensity of the (haline) circulation led to a strong decrease of the supply of oxygen to deep water, encouraging the accumulation of organic matter in black shales.
The nutritious elements, involved in the excess burial of organic matter, were available because nutrients of the terrestrial cycle of sedimentation and erosion were removed into the marine domain during the global rise of sea-level.

With respect to the warm climate, the sluggish circulation, and the widespread deposition of black shales, it is suggested that an initial rise of CO
2
, related to increased spreading and emissions of juvenile carbon, and to increased carbonate production, led to a first ‘greenhouse warming’. This was reinforced by a transfer of CO
2
from the warming oceans into the atmosphere. During subsequent cooling, CO
2
re-dissolved within the oceans, and conditions returned to ‘normal’.

Model calculations, based on the effect of changes of burial rate of organic matter upon δ
13
C of carbon in the ocean, show that the excess storage involves an increase of burial of organic carbon of the order of 20% only.
Geolipids of black shales and claystones in Cretaceous and Jurassic sediment sequences from the North American Basin
Authors: Farrimond P. , Eglinton G. , Brassell S. C. 
Published: April 2008

Summary
Aliphatic hydrocarbon distributions in DSDP black shales and green claystones from the North American Basin have been examined by gas chromatography—mass spectrometry (GC-MS). The sediment sequences studied were: (i) black shale and claystone horizons (Callovian-Cenomanian) from the Blake-Bahama Basin (Leg 76, Site 534), and (ii) black shales and associated green claystones (Cenomanian-Turonian) from the US East Coast Continental Rise (Leg 93, Site 603). In both series of black shales, specific biological marker compounds provide evidence for contributions of organic matter from terrestrial, marine and bacterial sources. In addition, several of these geolipid distributions (e.g. hopanoids) are markedly similar for both sequences, indicating some common inputs of organic matter, and comparable diagenetic histories. For the Leg 93 samples, the relatively organic-lean green claystones are found to contain mainly terrigenous marker compounds, in constrast to their interbedded black shales which show a larger contribution from marine sources.
All the sediments are immature, with a trend of increasing diagenesis evident from changes in the geolipid distributions down the Leg 76 sequence. One sample, however, is anomalous; its enhanced maturity may be due to an input of reworked, older organic material at the time of deposition.
Turbidites, the principal mechanism yielding black shales in the early deep Atlantic Ocean
Authors: Degens E. T. , Emeis K.-C. , Mycke B. , Wiesner M. G. 
Published: April 2008

Summary
A series of organic and inorganic geochemical parameters and facies indicators were studied in Late Jurassic and Cretaceous black shales and organic rich sediment sections from basins offshore Angola, Ghana and North Carolina, and from the western Wealden Basin in NW-Germany. Based on these data the sediments can be grouped into: (i) intracratonic (Bueckeberg Formation, NW Germany), (ii) shelf (Cretaceous basins offshore Ghana), (iii) continental rise turbidite (Hatteras Formation of Leg 93, Hole 603B, Section IV), and (iv) deep basin (Leg 75, Hole 530B, Section VIII) black shales. Trends of geochemical evolution are interpreted as the effect of multiple reworking and equilibration, when organicrich sediments from shelfbound basins are transferred into oceanic depocentres as turbidites. It is suggested: (i) that during turbidite phases, organic-rich suspensates originating from these primary basins on continental margins are advected along density boundaries within the water column overlying oceanic basins, and (ii) that these substrates are introduced into zones where responding microbial populations create expanded zones of oxygen depletion. Microbial degradation affects predominantly marine organic compounds contained in these recycled sediments, leaving a residual kerogen composed of structureless organic matter with pronounced terrigenous signals.
During slow sinking of these suspensions, chemical equilibration, possibly with the ambient water, leads to an association of certain meals, i.e. vanadium, zinc, and nickel, with the organic fraction. Pyrite placers form as the result of gravitational separation from oxygen depleted suspensates at the bottom of black shale beds. It appears that oceanic anoxia is not a prerequisite for black shale sedimentation in the Cretaceous basins of the Atlantic Ocean.
Comparison of Mesozoic carbonaceous claystones in the western and eastern North Atlantic (DSDP Legs 76, 79 and 93)
Authors: Rullkötter Jürgen , Mukhopadhyay Prasanta K. 
Published: April 2008

Summary
Mesozoic carbonaceous claystones were recovered on both margins of the North Atlantic Ocean in a number of holes drilled by the Deep Sea Drilling Project (DSDP). Geochemical investigation revealed that the organic matter in these sediments belongs to different organofacies types, as indicated by varying degrees of preservation and mixing of marine and terrigenous material. The intensity of terrigenous organic matter supply, coastal upwelling, downslope transport events, and local oxygen depletion are major factors controlling the composition and preservation of the organic matter in most of the Mesozoic carbonaceous claystones, whereas an ocean-wide anoxic event, if at all, can only be invoked for the organic-matter-rich sediments deposited at the Cenomanian-Turonian boundary. Examples discussed are from the Mazagan Escarpment (Site 547, DSDP Leg 79), the Blake Bahama Basin (Site 534, DSDP Leg 76), and the continental rise off Cape Hatteras (Site 603, DSDP Leg 93).
Organic-rich sedimentation at the Cenomanian-Turonian boundary in oceanic and coastal basins in the North Atlantic and Tethys
Authors: Herbin J. P. , Montadert L. , Müller C. , Gomez R. , Thurow J. , Wiedmann J. 
Published: April 2008

Summary
One of the most striking results of the Deep Sea Drilling Project is the proof that organic-rich sediments have a widespread geographical distribution during the period from Upper Cenomanian to Middle Turonian. Such sediments were drilled at North Atlantic DSDP Sites: 105, 135, 137, 138, 367, 398, 551, 603. They are also present (from outcrops or oil wells) on the shelf of the African continental margin (Senegal, Tarfaya and Agadir Basins), in the Tethys (former Alboran Block, Southern Spain, Algeria, Tunisia and Umbrian Apennines/Italy), and in the North Sea. Although these sediments have different lithologies and depositional environments (from shelf areas to the deep sea), their study, mainly based on organic geochemistry with additional data on sedimentology, biostratigraphy and palaeo-bathymetry, suggests that a unique ‘pulse’ of organogenic accumulation characterizes the Cenomanian/Turonian Boundary Event (CTBE).
The content and type of organic matter are related to the depositional environment and organic preservation. The organogenic accumulation is distributed according to various trends. Off the African continental margin the organic content increases from onshore areas to the shelf (Casamance area), and, moreover, increases also in deep sea areas, with a gradual transition from terrestrial type III to marine type II (the best preservation of the organic matter being in the deepest areas, i.e. Site 367). Off the American continental margin Site 603 shows the same TOC and type of organic matter as at Site 105. The CTBE is also well recorded in the northern part of the Atlantic (Celtic margin, North Sea) by a drastic lithological change (black shales within chalks), but the type of organic matter is mainly terrestrial. In the Tethyan area the organic matter is of marine origin and well preserved. Results are compared with those of Pratt (1984) from the Western Interior Basin of the USA. Different hypotheses to explain this synchronous widespread accumulation of organic matter are discussed.
Mid-Cretaceous of the Gibraltar Arch Area
Authors: Thurow J. , Kuhnt W. 
Published: April 2008

Summary
Mid-Cretaceous depositional environments of the Gibraltar Arch and adjacent areas exhibit at the Cenomanian-Turonian boundary a special type of organic carbon-rich siliceous sedimentation which is intercalated in all the different environments—from the shelf down to the deep—of this area. This will be called the Cenomanian-Turonian Boundary Event (CTBE). Summarizing the most important features of the CTBE it can be pointed out: (1) the initiation of the CTBE is nearly coeval; (2) very high TOCs, especially at the base of Turonian; (3) kerogen is exclusively of type (I-) II in the undiluted strata; (4) independent of given palaeogeographic setting (e.g. E. and W. margin of the Atlantic ocean); (5) bituminous sedimentation is also very pronounced in environments with high detrital input and strong dilution (e.g. deep sea fans). Here too, most strata comprise kerogen of type II; (6) as far as we know, there is a striking co-occurrence of these sediments with strong enrichment in biogenic silica from diagenetically altered radiolaria, but there is no link between good preservation of radiolaria and bituminous sediments (e.g. Scaglia-facies); (7) rich radiolarian faunas are common in the carbon-rich and carbon-free sediments of shallow marine and deep sea environments; (8) the important change in the evolution of planktonic organisms (foraminifera/radiolaria) which marks the Cenomanian-Turonian boundary coincides with the peak of anoxic conditions and biogenic silica-rich sedimentation.
All these features of the CTBE observed in the Gibraltar Arch area are comparable with coeval DSDP-facies in the North Atlantic and allow on-shore-off-shore tracing of the CTBE.
Development of N.W. Europe’s Southern Permian Gas Basin
Authors: Glennie K. W. 
Published: April 2008

Summary

The Southern Permian Basin contains over 4.1 × 10
12
m
3
of recoverable non-associated gas, most of which is found in Early Permian Rotliegend sandstones. The most important source of this gas is Carboniferous coal deposited in a proto-Tethys foredeep basin. Where the coals were overlain by Rotliegend desert sands and by Late Permian Zechstein halite, the scene was set for creating important reservoirs for gas.

Deformation of the reservoir and seal to create a trap, and the generation and migration of Carboniferous gas, resulted from the structural history of the area, which was shaped in part by underlying crustal blocks and zones of crustal weakness. Much of this history was related to events most clearly expressed beyond the limits of N.W. Europe in the North Atlantic and Tethys:

(1)
E.-W. tension, which gave rise to the oblique-slip-induced subsidence of many sub-basins within the Southern Permian Basin, caused the consecutive creation of the Viking-Central graben system and Rockall Trough, and ended with the crustal spreading of the Atlantic Ocean; and

(2)
the early Mesozoic opening of Tethys, its Cretaceous closure and ensuing Alpine orogeny, which induced inversion in many areas of the Southern Permian Basin that are now gas bearing.
Strike-slip Basins and Inversion: a possible model for the Southern North Sea Gas Areas
Authors: Gibbs A. D. 
Published: April 2008

SummaryThis paper examines the structural style of strike-slip zones and of the associated inverted basins and presents a tectonic model for their evolution as part of a linked system. The structure of strike-slip zones may be modelled geometrically by turning an extensional or compressional dip-slip system on its side. The duplex geometries (Dahlstrom 1969a) become braided wrench zones and the transfer components (Gibbs 1984) become linking ‘flats’ with dip-slip displacements between steep strike-slip belts. The stratigraphic implications of this model are not discussed in detail.The model developed in this paper suggests that in an area such as the Southern North Sea Gas Area, the pattern of inversions at a regional scale can be related to major crustally linked systems of steep faults which ride at different levels on flat detachments. Where the detachments step up, or down the over-lying block or leaf will invert. At a more local scale individual structures may invert by progressive rotation of individual faults or fault bounded blocks. Simple shear geometries of both extension and shortening above a relatively simple basement shear zone can be modelled by reference to the strain ellipse. The rotation of strained elements gives rise to the classic ‘flower structure’ seen in cross-section. The Gas Area Basin is a particularly good area to study the effects of rotational simple shear because of the low strength salt decoupling cover from basement and for studying the long range regional inversions across crustal shear zones and the postulated detachments on long regional seismic lines. The data presented in this paper are intended to illustrate the variety of structural styles produced by progressive evolution of structure on a linked system of strike-slip and detachment faults.
An Interpretation of Vitrinite Reflectance Data From the Southern North Sea Basin
Authors: Cope Michael J. 
Published: April 2008

Summary
Vitrinite reflectance data for the Carboniferous in 68 Southern North Sea Basin wells have been used to determine a maturity-depth relationship for the basin and to provide an estimate of basement inversion. The derived maturity-depth relationship is comparable with maturity gradients in other Paleozoic coal and gas basins known to have had similar geothermal histories. The earliest onset of gas generation in the Southern North Sea was in the late Jurassic following the accumulation of ± 11700 ft (3500 m) of sediment above the Carboniferous source beds. Estimates of inversion of the Sole Pit area using vitrinite data are similar to those based on shale velocity methods (± 1500 m maximum). In some areas the amount of inversion of the Carboniferous basement apparently exceeds that of the post-Zechstein section which may be attributed to removal of significant amounts of late Carboniferous strata during late Hercynian times; these areas are also associated with the principal gas discoveries in the basin.
Evaluation Procedures in the Carboniferous of Northern Europe
Authors: Draxler J. K. , Edwards D. P. 
Published: April 2008

Summary
Formation evaluation in the Carboniferous has always been an extremely difficult task due to complex formation properties coupled with adverse logging conditions. A study was undertaken in co-operation with oil companies to propose optimum data acquisition techniques and formation evaluation procedures.
The study covers a 300 kilometre-wide area of North Continental Europe. It includes data from 12 wells with different logging conditions and depths ranging from 2200 to 6700 meters.
Cores were analysed for porosity, permeability and formation factor under reservoir pressure conditions. Thin sections, SEM photographs and X-ray diffraction results, have been utilised for clay typing and to establish the lithological model best suited to Carboniferous evaluation. This model includes: quartz, ferroan dolomite, haematite and mica. Illite and kaolinite are the two main clays.
Formation water salinity was found to decrease from 230 Kppm in the uppermost formations, close to the Zechstein cap rock, to 26 Kppm in the oldest Westphalian horizons. Determination of the true formation resistivity takes account of the influence of the so-called Groningen effect.
Core and log observations pointed to the presence of a dual porosity system with variations of cementation exponent with overburden pressure. Rock mechanical property computations, estimation of in-situ stress directions and fracture detection completed the study.
Application of Image Ray Tracing in the Southern North Sea Gas Fields
Authors: Thorn S. A. , Jones T. P. 
Published: April 2008

Summary
The application of Time Migration to seismic data has won a great deal of popularity due to its ability to better locate seismic reflection events. Commercially available Time Migration algorithms all involve different approximations to the Wave Equation. In practise these simplifying assumptions work effectively in areas such as the Northern North Sea, where the shallowest reflection with a significant structural dip is often close to the target horizon. In the Southern North Sea Gas Basin, however, the section which overlies the Top Rotliegendes target horizon commonly shows a large degree of structural dip. Due to the resulting high lateral velocity variation the wave equation approximations of Time Migration are no longer valid, and this results in the incomplete imaging of the seismic data. Depth Migration potentially offers a complete solution to full imaging of seismic data beneath a section with laterally varying velocities. However, in either its pre-stack or post-stack mode, Depth Migration is an expensive iterative process. In certain circumstances incomplete imaging of the Time Migrated data can be accommodated by application of Image Ray Tracing techniques. The repositioning of subsurface structures demonstrated by both techniques could potentially influence reserves, development plans, and equity determinations.
This paper contains Image Ray Tracing examples from the Southern North Sea Gas Area. It discusses a practical procedure to incorporate the modelling results, and highlights situations where this technique has proven to be successful, and others where problems with interpreting the results of modelling still exist.
Reservoir Geology of the Morecambe Field
Authors: Bushell T.P. 
Published: April 2008

Summary
The Morecambe Gas Field, located in the Irish Sea Basin, came on stream on 1 January 1985 and is to be used as a seasonal storage facility. Structurally the field is a northwest-southeast trending faulted anticline divided into two parts by a narrow southwest-northeast graben system. Gas is produced from the upper part of the Lower Triassic Sherwood Sandstone Group. The reservoir sandstones were deposited predominantly by the action of low sinuosity braided streams during periods of high stage flow. Facies associations recognised from core and wireline log data include: Channel Sandstones, Secondary and Ephemeral Channel Sandstones, Sheetflood Sandstones, Floodplain fine member and Channel abandonment deposits. Reservoir quality is dependent on grain size, shape, sorting, facies control and the distribution and abundance of authigenic cements. Quartz and dolomite cements are a major factor in causing porosity reduction, whilst authigenic illite controls effective permeability. Platy illite only occurs below an identifiable datum believed to represent a ‘paleo’ gas-water contact. Below the illite datum permeability is impaired by up to one hundred fold. A change in illite crystal morphology in the lower parts of the reservoir results in a basal zone which exhibits improved quality. The main producing horizons are stacked sandbodies within the Keuper Waterstones and Keuper Sandstone Frodsham Member. On the basis of facies modelling a three dimensional reservoir model has been developed for simulation purposes. A simplified three layer model which incorporates the influence of authigenic illite has also been erected and will control future field development. A phased development programme is planned for the field over the next ten years. The first stage is currently underway, utilising slant drilling technology to economically drain the reservoir. Maximum gas production from the field will be 1200 million cubic feet per day.
Esmond Gas Complex
Authors: Bifani Ron 
Published: April 2008

Summary
Esmond Gas Complex consists of three gas fields (Esmond, Forbes and Gordon Fields), located in Quadrant 43 of the Southern North Sea. These fields are all simple anticlines caused by deep sale piercement and are relatively small in areal extent. The reservoir, composed of a 400 ft to 500 ft layer of Lower Triassic, Bunter Sandstone, consists of predominantly very fine to fine grained sandstone deposited by fluvial processes in an arid to semi-arid environment.
Porosity and permeability are controlled by original texture and by subsequent diagenesis. The presence of considerable quantities of halite, as a cementing agent, resulted in a very large apparent ‘gas effect’ on the FDC-CNL log in certain low porosity intervals. Variation in halite content throughout the reservoir required customised log analysis and special consideration in core preparation in order not to dissolve halite or alter the delicate pore geometry. This paper deals with techniques used in identifying this problem, and in procedures used to convert the data into useable information from which definitive reserve and deliverability determinations were made.
Field Development was approved by the Department of Energy and production commenced in June 1985.
The Leman Sandstone Formation in U.K. Block 49/28 Sedimentation, Diagenesis and Burial History
Authors: Arthur T. J. , Pilling D. , Bush D. , Macchi L. 
Published: April 2008

Summary
Block 49/28 lies on the northeastern side of the Sole Pit Inversion. The Leman Sandstone Formation increases in thickness from 280 ft to 702 ft southeast wards across the block. The section can be subdivided by facies and by the recognition of major bounding surfaces between aeolian sequences. Transverse dunes up to 100 ft in height have been identified. Early secondary porosity development accompanied by growth of mixed layer clays, feldspar, dolomite and silica cements with minor anhydrite was followed by burial related illitisation of clay minerals and late kaolinite authigenesis.
The most important control on reservoir quality in the block is the development of illite, which reduces permeability. The extent of fibrous illite growth was controlled by pre-Cretaceous burial. Late kaolinite, related to the migration of pore waters accompanying gas, is restricted to gas bearing structures. The relative timing of authigenic mineral growth and the structural history of the area can be combined to pinpoint the timing of gas accumulation as Middle to Late Jurassic.