bluuebunny/crossref_metadata_2025_split huggingface dataset:
The influence of platform morphology and sea level on the development of a carbonate sequence: the Harash Formation, Eastern Sirt Basin, Libya
Authors: Spring D. , Hansen O. P. 
Published: April 2008

Abstract
The Paleocene section of the Eastern Sirt Basin consists of a carbonate succession which developed in response to subsidence and widespread marine transgression following Cenomanian rifting. Within the study area the principal hydrocarbon reservoir is the Upper Sabil Formation, which is sealed by Harash Formation lime mudstones. Facies analysis and seismic mapping indicate that the Upper Sabil Formation consists of a prograding carbonate shelf with a rimmed platform morphology and palaeo-slopes in excess of 5° along the eastern and western flanks of the Agedabia Trough. To the south, the absence of a lagoonal facies and seismically defined shelf edge indicate a ramp morphology with a palaeo-slope of less than 0.5°. The overlying Harash Formation is characterized by its cyclic nature, relatively high terrigenous mud content and a thickness distribution which shows it to be infilling the pre-existing Upper Sabil shelf topography. The morphology of the Upper Sabil shelf coupled with sea-level fluctuations controlled the distribution of the depositional environments within the Harash Formation. A depositional model for the Harash Formation is proposed which subdivides the formation into five carbonate parasequences (named cycles A-E from base to top). In an exploration context the study facilitated the identification of the distribution of Harash Formation sealing and possible reservoir facies within cycles A and B. Furthermore, the study led to the understanding of the cause of anomalous reflections observed on conventional seismic data within the Paleocene section. Work involved the integration of regional well control with conventional seismic and stratigraphic inversion data. The stratigraphic framework and interpretations derived from this study were ultimately tested and confirmed by an exploration well which was cored through the sections of interest.
Critical factors in the exploration of an Atlas intramontane basin; the Western Hodna Basin of northern Algeria
Authors: Mekireche Karim , Sabaou Nordine , Zazoun Reda-Samy 
Published: April 2008

Abstract
The acquisition and evaluation of a substantial integrated surface outcrop and well database over the western Hodna Basin allows the identification of three critical factors (reservoir quality, source maturity and timing and hydrodynamic regime) to which well failures to date can largely be attributed. Reservoir quality is variable in Cretaceous sandstones, because of a high degree of quartz cementation, but improves to the south. Reservoir quality in Eocene limestones is related to the development of vuggy porosity and can now be mapped out from well control. Potential Cretaceous source rocks lie at moderate to high degrees of maturity, with explusion probably preceding the Miocene phases of structure development. Tertiary palaeostress reconstructions from field studies indicate four main structural phases: (1) Late Cretaceous N-S to NW-SE oriented compression; (2) Oligo-Miocene distension; (3) N-S oriented Miocene (pre-Langhian) pull-apart; (4) Langhian compression related to the emplacement of the Tellian nappes. Seismic analysis illustrates some classic late-stage inversion structures. Organic matter seen in outcrops and cores within the Thanetian-Ypresian and lower Lutetian sections is composed mainly of type II kerogen and varies from undermature to marginally mature, leading to the development of shows in many wells that helped in the past to stimulate continued exploration. Study of the hydrology of the formation waters in drilled wells allows large areas of likely reservoir flushing to be delineated. Study of the three critical factors allows a high grading in terms of remaining exploration potential. Based on the interpretations derived from structural, palaeogeography, diagenetic and hydrodynamic studies, the basin can be divided into three areas, which are ranked on the basis of their prospectivity. Each of the factors also needs to be critically assessed in analogous regions of the Atlas, in which such good geological control may not exist.
Introduction to the development, evolution and petroleum geology of the Wessex Basin
Authors: Underhill John R. , Stoneley Robert 
Published: April 2008

Abstract
Despite containing the largest known onshore oilfield in western Europe, the Wessex Basin hydrocarbon province appears to be extremely limited spatially and it currently only consists of three producing oilfields: Wytch Farm, Wareham and Kimmeridge. The main factor which controls hydrocarbon prospectivity in the area appears to be preservation of oil accumulations originally sited in Mesozoic tilted faultblocks. The extensional palaeostructures of Wytch Farm and Wareham are interpreted to have been charged by upwards migration of oil from mature Liassic source rocks situated across the Purbeck-Isle of Wight fault system in the Channel (Portland-Wight) sub-basin prior to, and unaffected by, either significant effects of intra-Cretaceous (Albian-Aptian) easterly tilting or by Tertiary tectonic inversion. To date, only the small Kimmeridge oilfield, which is situated in the core of a periclinal fold created in response to structural inversion, suggests that any hydrocarbon remigration into younger structural inversion structures has taken place.
The geological history of the southern Wessex Basin — a review of new information from oil exploration
Authors: Butler Malcolm 
Published: April 2008

Abstract
Mapping of intervals between the base of the Aylesbeare Mudstone (Permian or early Triassic) and the top of the Penarth Group (early Jurassic) in the Dorset-Isle of Wight area indicates that the deposition of these formations was influenced strongly by northwest-southeast-trending faults, probably related to underlying Variscan elements. There are significant differences between the distribution of the Aylesbeare Mudstone and that of later Triassic formations. An unconformity is recognized within the Sherwood Sandstone Group, separating the syn-rift sequences of Aylesbeare Mudstone, and overlying Budleigh Salterton Pebble Beds and anhydrites from the post-rift Otter Sandstone and Mercia Mudstone (middle-late Triassic). It is postulated that the top of the Sherwood Sandstone Group becomes progressively younger from the Mid Dorset Platform towards the Isle of Wight, steadily replacing the lower Mercia Mudstone. Halite beds are present within the middle Mercia Mudstone of Dorset, although they are absent over much of the Mid Dorset Platform and areas to the east of Wytch Farm. East-west-trending faults became dominant in the Jurassic, and there is strong seismic evidence for syn-depositional movement on these faults during the Lias across the entire Dorset-Isle of Wight area. In Dorset, Many of these east-west faults are listric, hading out into the Mercia Mudstone halite and creating very different structural styles above and below the salt. Major syn-depositional movement is thought to have also taken place along these faults during Kimmeridgian and Portlandian times, creating a series of rollover anticlines in the hanging walls. Seismic evidence indicates that the early Cretaceous sequences of the Portland-South Wight Trough thinned rapidly onto the footwalls of these east-west faults and that very little deposition took place north of the Purbeck Disturbance, either in Dorset or on the Isle of Wight. Several of these east-west faults reversed their throw during compression associated with the mid-Tertiary inversion. Reverse displacement estimates range from 50 m on the Litton Cheney Fault to 1320 m on the Purbeck Disturbance in the Isle of Wight. Offsets of the inversion trends may be related to underlying northwest-southeast-trending Variscan faults which influenced the margins of the Jurassic and early Cretaceous basins, but strike-slip movements may have accentuated some of these offsets during the Tertiary. These offsets appear to be dextral to the west of Wytch Farm oilfield and sinistral in the area to the east. Structural styles suggest that there may also have been strike-slip movements along east-west-trending faults.
Regional uplift in the English Channel: quantification using sonic velocity
Authors: Law Adam 
Published: April 2008

Abstract
Analysis of the substantial well database held by British Gas Exploration and Production Ltd enables regional estimates of uplift for the English Channel area to be derived from anomalous measurements of downhole sonic velocity. Estimates of section uplift, or apparent erosion, can be derived from variations in sonic velocity/travel-time between wells in a basin relative to a function defining the velocity/travel-time and depth relationship for the basin; the normal compaction relationship. These estimates are, however, extremely sensitive to errors in the normal compaction relationship, and to variations in sedimentary facies and/or diagenesis between wells. In this study, these problems have been minimized using regionally persistent, homogeneous thick facies such as shales and by carefully constraining the normal compaction relationship by ‘backstripping’ apparent erosion values to determine ‘residual apparent erosion’.
Apparent erosion has been determined for three intervals from the Mid- and Upper Jurassic, broadly corresponding to the Fuller’s Earth, Oxford Clay and Kimmeridge Clay Formations. Averaging these values and constraining them as above results in robust estimates of total uplift across the English Channel Basin, with uncertainties of around 20%. The results of this study suggest kilometre-scale uplift within the area, with maxima over the Purbeck-Isle of Wight Fault Zone and the Mid Channel High inversion axes. To the north of the Purbeck-Isle of Wight Fault Zone the apparent erosion calculations estimate uplift of 500 m or less, suggesting that this area may have remained relatively stable during basin inversion. When compared to equivalent estimates from AFTA/VR (apatite fission track/vitrinite reflectance) palaeotemperature data, the sonic-derived uplift is similar within errors in inverted areas. However, in more stable areas such as to the north of the Purbeck-Isle of Wight Fault Zone, calculated apparent erosion indicates that little uplift has occurred, whereas uplift estimates derived from AFTA/VR models are still significant.
Inversion tectonics in the Lyme Bay-West Dorset area of the Wessex Basin, UK
Authors: Smith C. , Hatton I. R. 
Published: April 2008

Abstract
Inversion in Lyme Bay and West Dorset was characterized by the reversal of motion on major Jurassic faults, and regional uplift which resulted in the widespread loss of the mid-Cretaceous to Tertiary post-rift cover throughout the area. North-south contraction during the Tertiary resulted in the development of a major anticline in the hanging wall of the Lyme-Portland faults, which was accompanied by widespread folding within the footwall of these faults. Northeast-southwest sinistral and northwest-southeast dextral strike-slip faults were also developed throughout the Lyme Bay-West Dorset area and accommodated much of the north-south contraction. These features are considered to be a consequence of the mechanical effects of an extensive salt interval within the Triassic Mercia Mudstone Group of this area. Major Early Jurassic faults appear to have been directly linked with basement through the salt interval, although smaller faults within the footwall apparently detached on this horizon. The major faults were inverted in a complex manner during the Tertiary, commencing with simple reversal of motion on these faults, followed by detachment of the post-salt carapace as the footwall and hanging wall salt layers were brought into close proximity, thereby resulting in the widespread deformation observed in the footwall of these faults.
The structural development of the Central English Channel High — constraints from section restoration
Authors: Beeley H. S. , Norton M. G. 
Published: April 2008

Abstract
The Central English Channel High is an east-west-trending asymmetric, southfacing, anticlinal structure which forms the southern boundary of the Central Channel Basin. Upper Jurassic rocks are exposed at seabed in the core of the structure, while Tertiary strata are preserved immediately to the south. Elsewhere in the Wessex-Channel basins structures of this overall form are interpreted as linear inversion structures formed during the mid-Tertiary by the reactivation of Mesozoic extensional faults, themselves formed by the partial reactivation of Variscan thrust ramps. Projection of the eroded sequence over the high using the geometry of the preserved section indicates the original presence of an approximately symmetrical Upper Jurassic-Lower Cretaceous sub-basin formed in the hanging wall of a north-dipping Mesozoic extensional structure, the Central Channel Fault. Seismic velocities from wells over the high are consistent with the implied post-Cretaceous relative uplift of up to 1500 m. The deeper geometry of the Central Channel Fault has been determined using the restored hanging wall geometry and the main aspects of the inversion structure have been recreated by forward modelling. The internal structure of the Central Channel High is consistent with a moderately steep fault near surface which passes relatively abruptly into a lower-angle fault. This is interpreted to indicate the presence of a Variscan backthrust at depth which has been partially reactivated during both Mesozoic extension and mid-Tertiary inversion.
Fault size distribution analysis — an example from Kimmeridge Bay, Dorset, UK
Authors: Hunsdale R. , Sanderson D. J. , Hunsdale R. 
Published: April 2008

Abstract
Fault displacement data were measured over four orders of magnitude for a fault set cross-cutting Upper Jurassic rocks exposed along the Dorset coast. Fault data were subdivided into three data-sets, based on data source and field character. Distribution analysis showed that these faults conform to a power law. The scaling relationship is, however, not constant over the entire displacement range of the faults. Faults with displacement >2m are characterized by a negative power law having an exponent of ≈0.96, while faults with displacement <1m are related by a negative exponent of ≈0.7. The change at the small scale is interpreted as being a product of the influence of lithological heterogeneity on fracture initiation and growth.
Towards an integrated study of the depositional environment of the Bencliff Grit (Upper Jurassic) of Dorset
Authors: Goldring R. , Astin T. R. , Marshall J. E. A. , Gabbott S. , Jenkins C. D. 
Published: April 2008

Abstract

An attempt to establish the depositional environment of the Bencliff Grit and its stratigraphic relationships is made using sedimentology, micropalaeontology, palynology and ichnology. Integration of these techniques demonstrates that the main sand units that characterize the Bencliff Grit represent periodic incursions by unidirectional west- and WNW-directed high-density flows, into water depths of at least 5 m from which biological recovery was relatively slow. An essentially autogenic process with dune migration over a colonized interdune area can probably be rejected. The ‘event’ sand units were not associated with colonized sediment-water interfaces. There are at least eight thin units of heterolithic muds and sands closely associated with two suites of marine trace fossils representing sand (
Diplocraterion habichi
) and mud (
D. parallelum
) associations. All mud samples are characterized by a signature of marine palynomorphs and microfossils. The palynofacies show significant differences which correlate with sedimentary facies. These include a palynofacies indicative of rapid sedimentation (in muds immediately following sands). In muds associated with the marine trace fossils a diagenetically modified assemblage is present. A bay or lagoonal setting with back-barrier washover sands is tentatively favoured rather than the intertidal, shoreface or estuary model previously used. The base of the Bencliff Grit does not
a priori
represent a sequence boundary and there are no parasequences within the formation. A sequence boundary may be present within the top 1 m of the formation, associated with a complex transgressive surface.
The Kimmeridge Bay oilfield: an enigma demystified
Authors: Evans Jonathan , Jenkins David , Gluyas Jon 
Published: April 2008

Abstract
The Kimmeridge oilfield was the first commercial discovery in the Wessex Basin. It was discovered in 1959 and is still producing from a single well. To date over 3 million barrels of oil have been extracted. Production is likely to continue into the next century. Oil is trapped in the fractured Cornbrash Limestones in an anticline close to the main inversion axis. The Kimmeridge accumulation remains the only producing field in the hanging wall to the Purbeck Disturbance, making it an intriguing enigma within the basin. Many myths have been perpetuated regarding the size of the field and the results from drilling elsewhere within Kimmeridge Bay. By presenting data on the field here, we hope to dispel some of these myths.
Oblique simple shear in transpression zones
Authors: Jones Richard R. , Holdsworth Robert E. 
Published: April 2008

Abstract
A mathematical and conceptual framework is presented for extending existing models of transpression and transtension to consider vertical and oblique displacements of the deformation zone bounding blocks. This allows transpression and transtension to be considered in terms of the interplay between four biaxial ‘end-member’ strain components, which are shown graphically as the four vertices of a ‘strain tetrahedron’. Transpressional and transtensional strains represented by the strain tetrahedron generally display triclinic symmetry, although the end-member biaxial strains are orthorhombic or monoclinic. Deformation in which vertical simple shear dominates over horizontal simple shear gives rise to finite strains that tend towards plane strain. Finite strains caused by deformation involving oblique simple shear are generally orientated non-parallel to all three Cartesian reference axes.
Analogue modelling of faulting in zones of continental transpression and transtension
Authors: Schreurs Guido , Colletta Bernard 
Published: April 2008

Abstract
Experiments were performed to simulate deformation in zones of continental transpression and transtension. Stratified models consisted of brittle analogue materials overlying a thin layer of viscous material. Oblique deformation was obtained by combining a basal, distributed strike-slip shear component with either transverse shortening (transpression) or transverse extension (transtension). In transpression experiments the imposed ratio of shear strain rate and shortening strain rate exerts an important control on initial fault evolution in the brittle layers of the model. In those experiments with a relatively high strain rate ratio (≥3.6), subvertical, en echelon strike-slip faults develop first, striking at angles of 25–37° to the shear direction. With increasing strain several convergent strike-slip fault zones form displaying positive flower structures. In low strain rate ratio experiments (≤2.7), gently dipping (30–45°), downward converging thrust faults accommodate initial failure. They bound pop-up structures that strike parallel to the shear direction. Increasing strain results in a fault pattern dominated by oblique-slip reverse faults. Partial partitioning of fault motion occurs at late stages of strain when strike-slip faults form within popup structures. The strike-slip faults merge at depth with confining oblique-slip reverse faults, have a curved shape in plan view and a dip direction which changes along strike. Fault patterns can be used as kinematic indicators. En echelon strike-slip faults initially accommodate deformation in a transtension experiment and strike at low angles (6–10°) to the shear direction. With increasing strain, normal faults form parallel to older strike-slip faults. They develop as a result of partitioning of fault motion and gravity failure. There is good agreement between experimental results and natural examples of continental transpressional and transtensional tectonics.
The structural response to evolving plate kinematics during transpression: evolution of the Lebanese restraining bend of the Dead Sea Transform
Authors: Butler R. W. H. , Spencer S. , Griffiths H. M. 
Published: April 2008

Abstract
Structural evolution along continental transform faults may be related to fault zone geometry and to regional variations in plate kinematics. Using a case study of the Lebanese sector of the Dead Sea Transform, the finite geometry of transpression at a restraining bend can be shown to have evolved in time. Relative structural chronologies, calibrated against dated landscape features such as lava-covered palaeosurfaces, coastal erosion surfaces and their incised drainage basins, are used to establish the timing of displacement activity on the major transcurrent faults. The early part of the region’s structural history, up to late Miocene times, was controlled by the geometry of the through-going Yammouneh Fault. Transpression on this right-trending left-lateral structure was accommodated by strike-slip and distributed crustal shortening represented by the initial uplift of Mount Lebanon. For the past 6 Ma the principal active strand of the transform has been the Roum Fault. For much of this period it is presumed to have been a through-going fault which accommodated about 30 km left-lateral displacement. During the Quaternary, the fault zone has become strongly segmented. Although the location of active transcurrent faulting has migrated during the history of the transform, the major site of crustal shortening, the Mount Lebanon-Jabel Barouk structure, has remained broadly fixed. However, the rates of amplification of this structure and the coastal flexure appear to have varied. Continuing uplift of Mount Lebanon and local Plio-Quaternary folding suggest that the offshore continuation of the Roum Fault contains a rightwards, transpressive bend. We relate this multistage history for the Lebanese sector of the transform to an evolving plate tectonic setting: the rotation pole for relative plate motion between Africa and Arabia has migrated through time and the triple junction between the transform and the Tethyan destructive plate margin to the north has moved from onshore SE Turkey to now lie in the NE Mediterranean. Our case study illustrates the transient and evolving nature of deformation in continental restraining bends.