bluuebunny/crossref_metadata_2025 huggingface dataset:
An investigation of the thermal history of the Ahnet and Reggane Basins, Central Algeria, and the consequences for hydrocarbon generation and accumulation
Authors: Logan Paul , Duddy Ian
Published: April 2008

In an attempt to better understand the thermal history of the Ahnet and Reggane Basins, the techniques of apatite fission track analysis and zircon fission track analysis were employed on samples from a number of exploration wells previously drilled in the study area. The results indicated clear evidence for a major heating event at
c.
200 Ma, overprinted on the effects of heating caused by simple burial before the Hercynian uplift. It is proposed that at least two major phases of hydrocarbon generation took place within the study area; an early, pre-Hercynian phase, in which chiefly liquid hydrocarbons were expelled and a later phase, associated with a ‘heat spike’ at
c.
200 Ma, in which significant quantities of dry gas were generated and expelled.
Mesozoic rifting and basin inversion along the northern African Tethyan margin: an overview
Authors: Guiraud R.
Published: April 2008

The northern African Tethyan margin registered three major rifting episodes from the latest Palaeozoic-earliest Mesozoic to the earliest Cenozoic. Break-up of Gondwana was initiated in the late Carboniferous. Along the northern African-Arabian plate margin rifting propagated westward from the northeastern Arabian margin to Morocco during the Permian and Triassic, and was accompanied by Mid-Late Triassic-earliest Liassic extensive alkaline flow basalts. Rifting continued during the Liassic, e.g. in the Moghrebian Atlas troughs. A second stage of rifting occurred in the Late Jurassic and continued into, or was rejuvenated during, the Early Cretaceous. Along the east Mediterranean margin, some large E-W trending rifts formed, often with associated volcanism, e.g. southern Sirt and Abu Gharadig. Most researchers believe the oceanization of the eastern Mediterranean basin occurred at this time. Flexural subsidence affected the Moghrebian Atlas, including the Riffian-Tellian domain, where a thick flysch series was deposited. Two short-lived, but widespread and tectonically important, compressional or wrench-dominated events occurred during the Santonian and the latest Maastrichtian. From Morocco to Oman, most sedimentary basins were folded and inverted at these times. The Santonian event resulted from onset of the collision between the African-Arabian and Eurasian plates. New stress-fields favoured NE-SW extension along the African plate margin, generating or rejuvenating some rifts, e.g. in the Euphrates trough and in the Libya-Tunisia region (northwestern Sirt, Pelagian Sea). Rifting and magmatism continued in these regions into the Paleocene. During the Mesozoic, therefore, the northern margin of the African-Arabian plate registered both rifting resulting in the oceanization of the Tethys and rifting resulting from the initiation of the closure of the Tethys. The intraplate domain exhibited echoes of the tectonic events affecting the margin.
An overview of the petroleum systems of Morocco
Authors: Morabet Al Moundir , Bouchta Rabah , Jabour Haddou
Published: April 2008

Magoon & Dow’s petroleum system concept is utilized in this paper to evaluate the origin of the proven hydrocarbon resources of Morocco, which lie chiefly in the Essaouria, Prerif and Rharb Basins. Several systems are interpreted, in which the confidence in the source-petroleum tie varies from ‘known’ to ‘speculative’, involving charge from Silurian (eastern Essaouira), Lias (Prerif), Oxfordian (western Essaouira), Cretaceous (Ain Hamra) and an intra-Miocene biogenic system in the Rharb Basin. The origin of the Cap Juby heavy oil accumulation in the offshore Tarfaya basin is uncertain, with a Jurassic carbonate source currently the favoured interpretation. The petroleum system concept is extended in this paper to predict further, currently undrilled and unproven systems. These include areas where the proven source systems referred to above are likely to extend, plus additional systems predicted on the basis of analogues to surrounding regions. An example of the latter is a series of postulated Triassic systems developed in lacustrine deposits in offshore half-grabens. Palaeozoic source rocks may be regionally developed and may still be in the oil and gas windows over wide, generally unexplored, regions. Lower to Middle Jurassic source rocks are predicted in lagoonal settings in platform sags and in outer shelf environments, whereas Cretaceous source beds occur in continental rift complexes and passive margins, with maturity likely to be reached in depocentres and below thrusts. Biogenic gas potential is probably confined to Neogene deposits in foreland and divergent margin basins. Further application of the petroleum system concept should help to significantly reduce geological risk, particularly in frontier exploration areas.
Application of fault seal analysis techniques in the Western Desert, Egypt
Authors: Richardson S. M. , Vivian N. , Cook R. J. , Wilkes M. , Hussein H.
Published: April 2008

The Western Desert is one of Egypt’s major hydrocarbon provinces. Most discoveries to date have been in the Cretaceous, with the Jurassic being the most underexplored interval, with the highest potential for new discoveries. Although trap styles vary, the majority are structural fault blocks, requiring fault seal on one or more bounding faults. Despite the number of excellent reservoirs present in the Western Desert, large field sizes are rare, with less than 50 MMBOE STOIIP (million barrels oil equivalent, oil initially in place) being predominant. This apparent failure to regularly prove larger field sizes can be considered a direct function of trap failure resulting from absence of fault seal. A number of fault seal analysis techniques including fault-plane profiles, smear-gouge ratios, incremental strain analysis and structural sections are available to directly address trap integrity. Case histories from the Western Desert are used to demonstrate the use of these techniques, and to highlight potential pitfalls. These case histories show that fault seal techniques allow a more accurate assessment of trap integrity in the Western Desert, provided they are applied iteratively, and integrated with other geological and geophysical data.
The thin-skinned style of the South Atlas Front in Central Algeria
Authors: Bracène R. , Bellahcène A. , Bekkouche D. , Mercier E. , de Lamotte D. Frizon
Published: April 2008

Seismic lines cutting through the southern front of the Sahara Atlas show that the front is not superimposed on a major basement fault. Folded Cretaceous rocks can be observed on these lines to overlie a décollement surface climbing from Triassic to Early Cretaceous, below which flat-lying sediments can be recognized. The structural style is thus interpreted to be thin skinned and the folds underlining the front as ramp- related features. The development of duplexes in the core of some anticlines explains the apparent thickening of Cretaceous and/or Jurassic strata revealed by boreholes. This thickening was previously interpreted to be related to now-inverted extensional half-grabens, a model which cannot now be supported. This new interpretation allows a reassessment of other parts of the Sahara Atlas system, the large-scale structural model for which is that of large half-graben system that has undergone inversion because of shortening between the High Plateau massif and Sahara Platform.
A proposed latest Triassic to earliest Cretaceous microfossil biozonation for the English Channel and its adjacent areas
Authors: Ainsworth Nigel R. , Braham William , Gregory F. John , Johnson Ben , King Christopher
Published: April 2008

Three detailed biostratigraphic zonations, based on ostracods, foraminiferids and dinocysts, are proposed for the latest Triassic through to the earliest Cretaceous of the Portland-Wight Basin and its adjacent areas. A total of 112 zones-zonules (34 ostracod zones/subzones, 25 foraminiferid zones, 53 dinoflagellate zones, subzones and zonules) are recognized. All three zonation schemes are based on samples obtained from both outcrop (the Dorset coast) and boreholes, the latter from ditch cuttings, with some conventional and sidewall core samples, in association with published data, and have where possible been calibrated to the standard ammonite zones recognized throughout northwest Europe.
Multiple heating episodes in the Wessex Basin: implications for geological evolution and hydrocarbon generation
Authors: Bray Richard J. , Duddy Ian R. , Green Paul F.
Published: April 2008

The timing of maturity development and hydrocarbon generation with respect to structuring is a primary factor controlling oil and gas accumulation. Hydrocarbon generation is largely a temperature-dependent process and in order to predict accumulation it is necessary to reconstruct thermal history. In this study of the Wessex Basin and adjacent areas, direct thermal history data are obtained from Apatite Fission Track Analysis (AFTA) on samples from wells, shallow boreholes and outcrops, enabling the key facets of a complex thermal and tectonic history to be reconstructed. Several distinct styles of thermal history are recognized, with clear differences across the region in the time of cooling from maximum palaeotemperatures. Areas of east Devon show evidence of an early thermal episode, sometime after the mid-Triassic and before ∼170 Ma. The cause remains uncertain at present. In the area of the ‘Wytch Farm block’ maximum palaeotemperatures were reached prior to cooling in the early Cretaceous, in the range ∼140–100 Ma, due mainly to uplift and erosion. This area remained relatively high through the rest of its history compared to the areas to the north and south. The Lias (early Jurassic) was not heated sufficiently to cause significant generation and any that occurred effectively ceased in the early Cretaceous. South of the Portland-Isle of Wight faults, maximum palaeotemperatures were reached prior to cooling due to uplift and erosion in the mid- and late Tertiary, at ∼40 Ma and ∼20 Ma, respectively. Tertiary uplift and erosion affected a wide region. Maximum erosion occurred immediately to the south of the Purbeck structure where maturation of Lias source rocks continued through peak oil and into the gas generation phase until terminated by the Tertiary cooling. In most wells, AFTA and vitrinite reflectance data define low or non-linear palaeogeothermal gradients indicative of an element of heating due to fluid movement in addition to heating due to burial, making quantitative estimation of erosion difficult, but any reasonable model requires kilometre-scale uplift and erosion over a wide region. West of Portland in Lyme Bay, a key question is whether maturation history is dominated by early Cretaceous or by mid- to late Tertiary events. At present there are not sufficient data to extrapolate confidently from the area of well control east of Portland. No parts of the basin are currently generating hydrocarbons, implying that surface seepages are from previously reservoired accumulations.
Influence of salt on the structural evolution of the Channel Basin
Authors: Harvey Michael J. , Stewart Simon A.
Published: April 2008

The north margin of the Channel Basin is defined in Dorset and the Isle of Wight by Mesozoic extensional faults which were reversed during Tertiary contraction, causing forced folding of the Albian-Oligocene post-rift cover. The resulting contractional structures (the Purbeck and Isle of Wight Monoclines and the reversed Abbotsbury-Ridgeway Fault) have previously been used as type examples for inversion tectonics. The Jurassic-Cretaceous basin margin crosses the eastern edge of a Triassic salt basin (Dorset Halite, Upper Triassic Mercia Mudstone Group) near Swanage. Detachments in Triassic salt around Weymouth resulted in extension and inversion styles which differ from the better known basement-linked structures to the east. The Abbotsbury-Ridgeway Fault is listric and unlike the Tertiary hangingwall folds to the east the Weymouth Anticline formed as an extensional rollover, only tightened during inversion.

Seismic and well data and structure maps are used to illustrate for the first time the increasing effects of salt on structural style as the salt thickens westwards. The onset of detachment on the basin margin is placed at Ringstead, where a north-stepping shift in the margin at pre-salt level defines a relay zone. Here extension was accommodated on many basement faults and the ratio of basement fault displacement to salt thickness was reduced sufficiently to allow the post-salt section to detach throughout extension (Abbotsbury-Ridgeway Fault). Tight Tertiary compressional folds in this area may result from linkage of the pre- and post-salt sections during inversion, a consequence of salt welds developed during the last stages of extension.

In the western part of the Channel Basin (Lyme Bay) the Triassic salt is sufficiently thick to have detached the pre- and post-salt sections over a wide area. North-south shortening during basin inversion may have been accommodated in the Lyme Bay post-salt section by displacements on conjugate, northwest-southeast- and northeast-southwest-oriented, strike-slip faults such as the Mangerton Fault. The Dorset Halite is a complex sequence of halite and mudstone interbeds, different from the ‘pure’ halites seen in the North Sea Basin, and its influence on structural style differs accordingly. Variation in salt tectonics style between the Weymouth area (salt swells and rollers) and Lyme Bay (salt rollers only) may reflect lateral facies variation and hence rheology in the Dorset Halite Formation.
Transpression and transtension zones
Authors: Dewey J. F. , Holdsworth R. E. , Strachan R. A.
Published: April 2008

Transpression and transtension are strike-slip deformations that deviate from simple shear because of a component of, respectively, shortening or extension orthogonal to the deformation zone. These three-dimensional non-coaxial strains develop principally in response to obliquely convergent or divergent relative motions across plate boundary and other crustal deformation zones at various scales. The basic constant-volume strain model with a vertical stretch can be modified to allow for volume change, lateral stretch, an oblique simple shear component, heterogeneous strain and steady-state transpression and transtension. The more sophisticated triclinic models may be more realistic but their mathematical complexity may limit their general application when interpreting geological examples. Most transpression zones generate flattening (
k
< 1) and transtension zones constrictional (
k
> 1) finite strains, although exceptions can occur in certain situations. Relative plate motion vectors, instantaneous strain (or stress) axes and finite strain axes are all oblique to one another in transpression and transtension zones. Kinematic partitioning of non-coaxial strike-slip and coaxial strains appears to be a characteristic feature of many such zones, especially where the far-field (plate) displacement direction is markedly oblique (<20°) to the plate or deformation zone boundary. Complex foliation, lineation and other structural patterns are also expected in such settings, resulting from switching or progressive rotation of finite strain axes. The variation in style and kinematic linkage of transpressional and transtensional structures at different crustal depths is poorly understood at present but may be of central importance to understanding the relationship between deformation in the lithospheric mantle and crust. Existing analyses of obliquely convergent and divergent zones highlight the importance of kinematic boundary conditions and imply that stress may be of secondary importance in controlling the dynamics of deformation in the crust and lithosphere.
Extended models of transpression and transtension, and application to tectonic settings
Authors: Fossen Haakon , Tikoff Basil
Published: April 2008

We introduce a spectrum of transpressional and transtensional deformations that potentially result from oblique plate interaction. Five separate types of deformation are designated, in which a simple shear deformation is combined with an orthogonal coaxial deformation. The types vary in the amount of extension v. contraction, both parallel to the margin and vertically. The interaction between the angle of convergence, kinematic vorticity, infinitesimal strain axes, finite strain, and rotation of material lines and planes is investigated. Quantification of the finite strain indicates that the orientation, magnitude, and geometry (flattening, constriction, etc.) change continually during steady-state transpression.

These results are then applied to the cases of transpression, particularly resulting from oblique plate convergence of terranes. The obliquity of plate motion and the geometry of the plate margin determine which of the types of transpression or transtension is favoured. A component of margin-parallel stretching also potentially causes terrane motion to locally exceed oblique plate motion, or move opposite to the general direction of movement between the converging plate boundaries. The kinematic models also suggest that the boundaries between converging terranes are likely to exhibit vertical foliation, but either vertical or horizontal lineation. Finally, narrow transpressional zones between colliding blocks may have very high uplift rates, resulting in exhumation of high-grade metamorphic fabrics.
Strike-slip partitioned transpression of the San Andreas fault system: a lithospheric-scale approach
Authors: Teyssier Christian , Tikoff Basil
Published: April 2008

Physical and numerical experiments on transpression indicate that strike-slip partitioning is facilitated by low angle of convergence, such as occurs on the modern San Andreas fault system. Cross-sections across the San Andreas fault overestimate the amount of margin-normal contraction, because they do not include the effect of horizontal stretching caused by the wrench component of deformation. When this correction is made assuming a strike-slip partitioned transpression model, contraction estimated from cross-sections is consistent with both the normal and tangential movements imposed by plate motion: there is no San Andreas discrepancy.

Seismic anisotropy (shear-wave splitting) and teleseismic analyses suggest that the upper mantle in the San Andreas region is thick and largely undeformed to the SW of the San Andreas fault, but thin and intensely sheared in a zone 50–100 km wide beneath the NE region of the San Andreas fault system. Seismic anisotropy is best explained by a transpression zone of vertical foliation (and possibly horizontal lineation) which penetrates the asthenospheric mantle. Using a lithospheric-scale approach, the transfer of the displacement field from the penetratively deformed mantle to the strike-slip partitioned upper crust necessitates zones of accommodation in the mid- to lower crust, corresponding to the flatlying detachments that underlie the San Andreas fault system. The mechanics of the San Andreas fault are best considered in connection with the strength of the various lithospheric layers involved in this system.
Transtension in southeastern Africa associated with Gondwana break-up
Authors: Watkeys M. K. , Sokoutis D.
Published: April 2008

The southeastern margin of Africa is marked by a zone of complex faulting associated with crustal extension related to the Mesozoic break-up of Gondwana. The oldest age of faulting (180–175 Ma) displays differences in style in regions with Archaean as opposed to Proterozoic basement. Faulting in the Archaean region shows extension normal to the proto-plate boundary whereas transtension prevailed in the Proterozoic region. The north-south trending normal faulting in the Archaean craton, which cuts across basement trends, displays an asymmetry indicating simple shear extension of the lithosphere. In contrast, transtension occurred in the region of Proterozoic basement by ENE strike-slip movement of blocks along terrane boundaries. As these blocks moved, normal faults developed behind them as well as within them at high angles to the terrane boundaries, to accommodate extension. Analogue modelling reproduced the observed different fault patterns. It is proposed that sinistral movement along the interconnected Gastre Fault System and Falkland-Agulhas Fracture Zone, together with the presence of a releasing bend, are the reasons for extension in the Lebombo region. This movement may have been caused by plate edge events between the Patagonia block and the proto-Pacific Ocean.
Transpressional tectonics and strain partitioning during oblique collision between three plates in the Precambrian of southeast Brazil
Authors: Ebert Hans Dirk , Hasui Yociteru
Published: April 2008

Two orogenic belts have been recognized in south-east Brazil, which are interpreted to have been formed as a product of diachronous collisions between three continental plates. Wide crustal-scale shear belts have developed both between and inboard of the collided and amalgamated plate borders. These shear belts record frontal, oblique or lateral displacements during oblique plate converence and A-type subduction. The overall structural style of each belt depends on the angle subtended between the plate boundary and the convergence vector. The E-W branch between the São Paulo and Brasília plates, the Campo do Meio strike-slip shear belt, has undergone dominantly sinistral wrench dominated transpression along a set of folds and shear zones dipping southwards. The NE-SW branch between the São Paulo and Vitória plates, the Paraíba do Sul strike-slip shear belt, has undergone a partitioned dextral transpression, whereas the north-south branch between the Brasília and Vitória plates, the Paraíba do Sul strike-slip shear belt, has undergone a partitioned dextral transpression, whereas the north-south branch between the Brasília and Vitória plates is essentially a frontal thrust system with only a weak component of dextral strike-slip. These complex structural patterns, formed at deep to mid-crustal levels, reflect temporal and spatial partitioning at all scales between flattening and non-coaxial deformation, and down-dip and strike-slip shearing, in tangential as well as in transcurrent structural domains. Additionally, this area demonstrates that regional flower structures, lateral extrusion and other secondary deformations across the
yz
sections of transpressional belts are important in accommodating shortening in obliquely convergent orogens.
The main phase of the Hercynian orogeny in the Pyrenees is a dextral transpression
Authors: Gleizes G. , Leblanc D. , Bouchez J. L.
Published: April 2008

Synthesis of several recent structural studies in the Pyrenees points to a syntectonic emplacement of granitic plutons, more or less early during the D2 main phase of the Hercynian orogenesis. A dextral E-W component of simple shear is demonstrated for D2, which was previously considered as a pure N-S compression. This component is recorded in the granites by large shear bands and sigmoidal features, and in the country rocks by asymmetrical strain shadows of foliation trajectories in map view and by systematically dextral sense of shear in
xz
sections. The compressive component of D2, well known in the country rocks where it induced a strong flattening related to ubiquitous upright isoclinal tight folds, was also observed in the granites. There, we documented a reverse movement within magmatic or solid-state shear bands. The addition of all these data constrains a new interpretation for the D2 main Hercynian phase in the whole Pyrenees: this phase was a dextral transpression.
Three-dimensional retro-modelling of transpression on a linked fault system: the Upper Cretaceous deformation on the western border of the Bohemian Massif, Germany
Authors: Tanner David C. , Behrmann Jan H. , Oncken Onno , Weber Klaus
Published: April 2008

The Zone of Erbendorf-Vohenstrauß (ZEV) on the western margin of the Bohemian Massif was deformed by an Upper Cretaceous intra-plate deformation event. Dextral transpression was caused by the reactivation of pre-existing structures. Using the extensive geological database available, we have constructed a three-dimensional virtual model of the ZEV. The model was deformed in reverse, to remove the effects of the Upper Cretaceous event. This involved moving the hanging wall (the ZEV) in a sinistral transtensive sense northwards above a composite active fault surface composed of two steep faults, perpendicular to each other in strike, and a detachment intersecting both faults at 9.5 km depth. Hanging-wall deformation was accommodated by antithetic inclined shear. Seven kilometres heave of the hanging wall fulfilled the geological constraints. Calculated uplifts range from 2 to 6 km. Deformation is mostly only contained within the ZEV. The hanging-wall deformation above a linked fault system was highly complex, causing rollover above one fault and drag-folding above the other. The most important control on the vertical movement and deformation of the hanging wall was a 30° change in the strike of one of the coupled faults.
Transpressional kinematics and magmatic arcs
Authors: Blanquat Michel De Saint , Tikoff Basil , Teyssier Christian , Vigneresse Jean Louis
Published: April 2008

Most continental magmatic arcs occur in obliquely convergent settings and display strike-slip movement within, or adjacent to, the magmatic arc, and contractional structures in the forearc and backarc regions. Thus, three-dimensional transpressional kinematics typifies many arc settings, both modern and ancient. Intrusions cause magma-facilitated strike-slip partitioning, even in cases where the relative angle of plate convergence is almost normal to the plate boundary. Transpressional systems are preferentially intruded by magmas because of the steep pressure gradients in vertical strike-slip shear zones and their ability to force magma upward. Both buoyancy and transpressional dynamics cause a component of magma overpressuring, which in turn expels granitic magma upward following the vertical pressure gradient. The tectonic and magmatic processes are linked in a positive feedback loop which facilitates the upward movement of magma.

We propose a lithospheric-scale, three-dimensional model of transpressional arc settings. Strike-slip motion is partitioned into the magmatic arc settings because of the linear and margin-parallel trend of the vertical, lithospheric-scale weakness caused by ascending magma. The parallelism of contraction structures in the forearc and backarc regions is caused by mechanical coupling through the lower crust and upper lithospheric mantle. The displacement field of the basal layer of the arc system provides the boundary condition for the upper-crustal, strike-slip partitioned deformation.
A review of up-scaling and cross-scaling issues in core and log data interpretation and prediction
Authors: Corbett P. W. M. , Jensen J. L. , Sorbie K. S.
Published: April 2008

In a heterogeneous geological formation, each rock petrophysical property (e.g., permeability, porosity, and electrical conductivity) reflects the heterogeneity and varies in a manner related to the underlying changes in fabric (grainsize, mineralogy, lamination, wettability, etc.). However, measurements, both laboratory and downhole, are made at certain volume scales dictated by the size of the core plug used or the wireline log resolution. The comparison of core and log data needs to account for both the
scale
and
physics
of the particular measurements and how these relate to the underlying scale of the geological heterogeneity of the formation.

In this review, these two fundamental issues are addressed as follows:

(a)

measurement scale and how it relates to the ‘true’ or ‘required’ petrophysical properties of the formation is defined as ‘
up-scaling
’;

(b)

measurement physics and how we relate the physics of one measurement (e.g. permeability) to that of another (e.g. density, electrical, or acoustic properties) is termed ‘
cross-scaling
’.

We illustrate how these two issues arise in the comparison and prediction of permeability using several published studies. We also outline an approach to petrophysical measurement reconciliation termed ‘genetic petrophysics’. This combines all three elements—measurement scale, measurement physics, and geology—to provide an integrated and robust model. We illustrate this approach for permeability to provide fit-for-purpose models of anisotropy in the near-well region of a reservoir.
Forward modelling of the physical properties of oceanic sediments: constraints from core and logs, with palaeoclimatic implications
Authors: Lauer-Leredde C. , Pezard P. A. , Touron F. , Dekeyser I.
Published: April 2008

A new methodological approach based on the analysis of core data, logs and high-resolution electrical images of borehole surfaces (FMS) is developed in order to improve the study of oceanic sediments from physical properties. This approach is tested on data obtained in the context of the Ocean Drilling Program (Japan Sea, Leg 128, Hole 798B). The downhole measurements and FMS images exhibit a cyclic pattern reflecting variations in oceanic surface productivity combined with continental aeolian supply due to palaeoclimatic changes. On the basis of m-scale physical measurements, cm-scale FMS images and measurements on core, the objective is to deconvolve the variations in sedimentary supply of oceanic and continental components through time and to compute the intrinsic formation factor versus depth. The latter topic is approached in two ways: first by conventional log analysis, then with a new iterative forward modelling method. In the second case, the low frequency electrical resistivity log (SFL) is modelled using a numerical modelling code (Resmod2D
©
) in order to obtain an accurate formation electrical resistivity model (Rt), where individual beds are derived from FMS images. An analytical routine is also used to model the natural gamma-ray measurement (CGR). While the conventional log analysis allows deconvolution of the sedimentary supplies, the forward modelling leads to a greater resolution and accuracy in more precise sediment characterization, such as that obtained from the derivation of the formation factor.
Complementary functions reveal data hidden in your logs
Authors: Samworth J. R.
Published: April 2008

Many logging tools make multiple measurements of the same type that have more than one depth of penetration. Common examples are Compensated Density, Compensated Neutron and Array Induction logs. The purpose of the compensation is to reduce or remove the effects of a disturbance that distort the true measurement. Examples of this disturbance are the borehole size, mudcake and salinity.

A general technique can be derived based on a theory of Linear Perturbation which requires no prior knowledge of the nature of the perturbation, the only requirement being that it is approximately locally linear. Various interpretations can be made of the general equation depending on the particular circumstances.

The technique also produces a Complementary Parameter associated with the degree of correction. This parameter is usually discarded or paid scant regard, but can often be of some significant value and exposes surprising information. A number of examples can be used to illustrate these techniques, showing that they have wide applicability in situations ranging from difficult logging conditions (e.g. density through casing) to the apparently routine, where unusual and unexpected borehole fluids are revealed from neutron logs.
Acoustic wave anisotropy in sandstones with systems of aligned cracks
Authors: Shakeel A. , King M. S.
Published: April 2008

Seismic anisotropy has been studied on a number of dry cubic sandstone specimens, of 51 mm side, in which a system of aligned cracks has been first introduced progressively by the application of a polyaxial state of stress, and then closed by hydrostatic stress. One P- and two S-wave velocities polarized at right angles, along with the deformation, have been measured at each stress level in each of the three principal stress directions. Thomsen’s (1986) anisotropy parameters (ɛ, γ, δ) have been calculated at each stress level during the cracking and crack closing cycles using Nishizawa’s (1982) theory. Test results indicate that anisotropy in the P-wave velocity is greater and more sensitive to the presence of aligned cracks than that observed for S waves. Modelling studies show that the P-wave anisotropy parameter ɛ is always greater than that of anisotropy parameter δ, for low crack densities and for small aspect ratios. The reverse is true for high crack densities and low aspect ratios. The results of numerical studies indicate that S-wave anisotropy is independent of the nature of the saturating fluid and that it is possible to observe elliptical anisotropy in a medium containing aligned dry ellipsoidal inclusions.
In situ
stress prediction using differential strain analysis and ultrasonic shear-wave splitting
Authors: Widarsono B. , Marsden J. R. , King M. S.
Published: April 2008

Knowledge of the
in situ
states of stress in rock masses is of considerable importance to a number of subsurface engineering activities, including those involved in exploiting petroleum and geothermal energy reserves. In this paper, a comparison is made of two laboratory techniques, based upon stress-relief microcracks, for determining the
in situ
state of stress: differential strain analysis (DSA) and ultrasonic shear-wave splitting (USWS). Measurements on ten sandstone samples recovered from deep boreholes, made using the well-established technique of DSA, have been compared to those made by the comparatively new technique of USWS and to sleeve fracturing measurements of
in situ
stress made in the corresponding boreholes. The results obtained indicate that the USWS technique, with its ability to test a large number of samples quickly, provides a useful adjunct to DSA and sleeve fracturing in determining trends in
in situ
stresses. Used in combination, the two laboratory techniques have also proved useful for examining rock micro-structural features.
Measurement scale and formation heterogeneity: effects on the integration of resistivity data
Authors: Jackson P. D. , Harvey P. K. , Lovell M. A. , Gunn D. A. , Williams C. G. , Flint R. C.
Published: April 2008

Core and downhole logging resistivity data gathered during Leg 133 of the Ocean Drilling Program are used to illustrate the wide range of scales of resistivity data available for reservoir characterization. The differences in scale and sampling interval between quantitative log resistivity data and conventional core plug data is shown to be central to reconciling these two datasets. Resistivity images of fine scale sedimentary structures taken on half-round cores are presented (at the same resolution as the downhole borehole wall imaging tools) and these fine structures are shown to be ‘lost’ if investigated using conventional core plugs and downhole resistivity logging tools. The limitations of conventional measurements on core plugs are presented and contrasted with the benefits of logging all of the core in the laboratory at a resolution comparable to the borehole wall imaging tools. An example of integrating different scales of resistivity data using a modelling approach is presented and is shown to be applicable to both core and log data. Visualizing and comparing the scale content of different resistivity datasets has been achieved in an intuitive way using a spectral method which illustrates the ‘data gap’ in quantitative resistivities which exists between core and log data.
Aspects of core-log integration: an approach using high resolution images
Authors: Lofts J C. , Bristow J. F.
Published: April 2008

Core-log comparisons are often not considered routinely enough within the exploration environment. This may be for a number of reasons, such as problems with depth-matching the core and log datasets, the environment of acquisition of both datasets, the lack of understanding of log and core acquisition or a lack of confidence in laboratory or log measurements. These problems are discussed as a preliminary step to the development of a strategy aimed at improving core-log integration.

Using recent technological advances in the side-by-side presentation of core and high resolution image data from logging tools, a strategy is presented with the aim of making core-log integration more rigid and routine. Features in both core and images are correlated interactively—thus ensuring the best possible integration. This is a two stage process involving core-to-image matching, and then image-to-log matching. This strategy has the potential to make core-log integration more accurate and as a result enable the interpreter to realize the most from sub-surface data.
Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, I: stratigraphy of eruptive products and accretion processes
Authors: Ayadi M. , Pezard P. A. , Laverne C. , Bronner G.
Published: April 2008

Hole 504B is located about 200 km south of the Costa Rica Rift and constitutes the reference section for the structure of the upper oceanic crust. Compared to core, the continuous electrical resistivity (at m scale) and the high-resolution electrical images (at cm scale) recorded in Hole 504B, provide a continuous and detailed lithostratigraphic description of the effusive section at Site 504. Flow thicknesses measured from cm scale FMS images average 0.5 (±0.1) m. The massive units, known to bound fluid circulation at large scale into the crust, are constituted with a series of 20 to 50 individual flows. If Site 504 was created over two volcanic cycles, each volcanic cycle allows the emplacement of [0.60 (±0.30)] × 10
6
m
3
of magma per m along the ridge axis. This computation leads to an estimate of magma volume for a single eruption of [0.003 (± 0.001)] × 10
6
m
3
per m along the ridge axis, and eventually, a gradient in magma pressure within the magma chamber lens of 52 (±26) MPa, appropriate for one eruption.
Development of the Cote D’Ivoire-Ghana transform margin: evidence from the integration of core and wireline log data
Authors: Gonçalves C. A. , Ewert L.
Published: April 2008

The primary objective for drilling the Cote d’Ivoire-Ghana Transform Margin during ODP Leg 159 was to assess the sedimentary and deformation processes resulting from the different stages of continental break-up and related transform tectonism. In view of the structural importance of the leg, integration of logging and core data is important to help understand the main tectonic and deformation events that occurred.

The effect of the transform deformation can be seen in physical properties data, for instance the porosity data derived from index properties measurements. Major breaks in porosity are associated with the tectonized lower Cretaceous and Cenozoic boundary, a trend also reflected in the P-wave velocity measurements. At each site, core and well log data show the presence of a major unconformity between the Cretaceous and Cenozoic, marked by an offset in porosity, density and P-wave data. The physical properties of log data are also heterogeneous, reflecting variations in consolidation, age and lithology.

Another interesting aspect covered by core-log integration was the structural relationship within the sediments. As well as the direct measurements made on cores, in situ structural measurements have been obtained using the Formation MicroScanner (FMS; Mark of Schlumberger) logging tool in two of the holes. The measurements cover the Eocene to Turonian-upper Santonian limestones. Bedding planes dip predominantly towards NWNNW and show an increase with depth which can be interpreted to be the result of steady subsidence of the Deep Ivorian Basin. Break-outs and fracturing were also observed. Breakout occurrences depend on sediment type and their axes are perpendicular to the maximum compressive horizontal stress east-northeast west-southwest. Fracturing occurs as normal and reverse microfaults, with dispersion of dips and azimuth directions in these zones. The presence of fault zones are also correlated with changes in the physical properties of the sediments.
The stability of a mudflat in the Humber estuary, South Yorkshire, UK
Authors: Amos Carl L. , Brylinsky M. , Sutherland T. F. , O’Brien D. , Lee S. , Cramp A.
Published: April 2008

The stability of a mudflat in the Humber estuary, South Yorkshire, was investigated at seven sites along a shore-normal transect during early spring. This was carried out using the benthic flume Sea Carousel, and from the investigation of surface samples in a laboratory equivalent to the Sea Carousel, the Lab Carousel. A clear trend in erosion threshold [τ
c
(0)] was evident, showing two maxima: the greatest on the inner mudflat (0.78 Pa); and a second on the central mudflat (0.75 Pa). We ascribe these maxima to two causes: (1) desiccation of the inner mudflats; and (2) biostabilization by algae of the central flats. Our results suggest that of these factors biostabilization was the dominant one at the time of the study. Most of the variation in erosion rate is explained through the sediment bulk density (ρ
b
) and colloidal carbohydrate (DCHO): [τ
c
(0)] = 0.5[1.93log
10
(DCHO + 2.5 × 10
−4
ρ
b
] + 1.7;
r
2
= 0.63. Thus variations in DCHO had O(10
4
) greater impact on erosion threshold than equivalent variations ρ
b
. The relationship between mean erosion rate (
E
m
) and current speed (
U
y
) was similar for all sites and is defined by the exponential function:
E
m
= 2.47 × 10
−6
· 10

(3.749
U
y
)

kgm
−2
s
−1
.
In situ
mean still-water settling rates (
W
s
) were up to 2.46 × 10
−3
ms
−1
, which is up to an order of magnitude faster than was measured in other estuaries at similar suspended sediment concentrations,
S
. The decay constant,
k
, for still-water settling appeared to be a linear function of
S
, and compared favourably with values derived from five Canadian coastal mudflats. Results from Lab Carousel showed that the erosion thresholds were the same as those measured
in situ
, using Sea Carousel. Furthermore the spatial trends in erosion were the same in both studies with the exception of the innermost sites (A and A/B). Differences are explained by consolidation and desiccation caused by solar radiation which were not simulated in the lab. Mean erosion rates showed a similar exponential relationship with current speed to that determined from Sea Carousel. Lab-derived values of
k
were much higher than the Sea Carousel ones and indicated much faster sediment settling under laboratory conditions.
k
was a weak inverse function of applied stress. Values of τ
d
derived from Krone’s (1962) method yielded threshold values of between 0.03 and 0.32 Pa. These values were 38 ± 16% of the erosion threshold τ
c
(0).
Measurements of the turbid tidal edge over the Skeffling mudflats
Authors: Christie M. C. , Dyer K. R.
Published: April 2008

The effects of wave and current action on the shallow water’s edge are quantified, by describing the relationships between velocity, bed shear stress and suspended sediment concentration. Advective processes dominated the sediment fluxes. Wave effects were negligible and high concentrations were generally limited to the first few minutes of covering by the flood tide. Velocities over 0.5 m s
−1
and concentrations above 10 gl
−1
were measured in the advancing water’s edge at this time. For depths <1 m, there was a linear relationship between concentration and velocity. An apparent erosion threshold occurred at a velocity of 0.2 m s
−1
.
Ecological interaction and sediment transport on an intertidal mudflat I. Evidence for a biologically mediated sediment-water interface
Authors: Ruddy G. , Turley C. M. , Jones T. E. R.
Published: April 2008

Using biological, chemical and physical data, evidence of biologically mediated cohesive sediment dynamics is presented for an intertidal mudflat in the Humber Estuary, UK. The data suggest that in excess of 98% of sediment fluxes to the sediment bed on the mudflat are subject to resuspension, forming a bedload that is important in generating the shear stress required to erode the sediment bed. Benthic micro-algae photosynthesize and are grazed by macro-heterotrophs within this 300–3000 μm deep dynamic surface layer. The data suggest that the algal growth within this layer is limited by nitrogen fluxes, resulting in ‘excess’ carbon fixation by photosynthesis. A significant positive relationship is apparent between ‘excess’ carbon fixation rates and the sediment critical erosion stress. This indicates that nitrogen limitation and sediment stability were directly proportional to each other on the mudflat. Grazing of the algae by
Macoma balthica
(a burrowing bivalve) profoundly affects the carbon and nitrogen cycles in the dynamic surface layer, and hence also ‘excess’ carbon fixation. We propose that a highly interdependent community of benthic algae, bacteria and macro-heterotrophs, acts to regulate sediment dynamics via small-scale nitrogen cycling and the algal exudation of ‘excess’ carbon as sediment-binding polymers.
Ecological interaction and sediment transport on an intertidal mudflat II. An experimental dynamic model of the sediment-water interface
Authors: Ruddy G. , Turley C. M. , Jones T. E. R.
Published: April 2008

A one-dimensional model is presented which combines simplified biological, chemical and physical dynamics across the sediment-water interface of intertidal mudflat sediments. Interactions between ecology, biogeochemistry and sediment dynamics are simulated during iteration through a short-term sequence of light and tides. The model shows that, under ‘normal’ intertidal conditions, a biolayer composed of unicellular algae can develop at the sediment-water interface. Rapid algal growth generates regularly recurring nitrogen limited conditions in the biolayer. In the model, the algal carbohydrate polymers produced and exuded during nitrogen limitation profoundly influence model sediment dynamics by adjusting sediment cohesion. Stable, but dynamic, sediment surfaces result in the model from a combination of carbohydrate exuding algae, bacterial remineralization of exuded carbohydrate and grazing of the algae by bioturbating macroheterotrophs. Model predictions for sites on an intertidal mudflat in the Humber Estuary, UK show good agreement with the field observations for a range of different physical, chemical and biological variables. The results imply that sediment accretion may be more directly coupled to nitrogen supply than to sediment supply. The results are discussed in terms of an algal growth advantage to carbohydrate exudation.
Measurements and preliminary modelling of current velocity over an intertidal mudflat, Humber estuary, UK
Authors: Wood R. G. , Black K. S. , Jago C. F.
Published: April 2008

This paper presents tidal current velocity data from Station D on the LISP transect, Humber estuary, UK, collected during April 1995. Ten impeller-type current meters, aligned along the major flood-ebb tidal axis, were deployed in a vertical array at the site. Five of these measured flood speeds and five measured ebb speeds. First, depth-averaged flow is considered. Observed depth-averaged velocities are compared to results from three sets of modelled velocities: one set in the Hawke channel taken from a 2D model of the Humber estuary, one from a 1D hydrodynamic model of a flooding/drying transect, and one from a uniform elevation, volume-conservation model of a flooding/drying transect. The uniform elevation model is simpler, faster and gives a smoother response than the hydrodynamic model at this scale; and the observed velocities are closest in magnitude and direction to results from this model. The nature of the vertical velocity profile is then considered for both Spring and Neap tidal cycles. It is suggested that the vertical advection of horizontal velocities (due to the large ratio of water level change to water depth) has a significant effect on the velocity profile during the ebb, modifying the expected deviation in profile due to the flow acceleration.