bluuebunny/crossref_metadata_2025 huggingface dataset:
Regional dolomitization of subtidal shelf carbonates: Burlington and Keokuk Formations (Mississippian), Iowa and Illinois
Authors: Harris David C. , Meyers William J.
Published: April 2008

Cathodoluminescent petrography of crinoidal limestones and dolomites from the Mississippian (Osagean) Burlington and Keokuk Formations in Iowa and Illinois has revealed a complex diagenetic history of calcite cementation, dolomitization, chertification and compaction. Dolomite occurs abundantly in subtidal, open-marine facies throughout the study area. Three luminescently and chemically distinct generations of dolomite can be recognized regionally. Dolomite I, the oldest generation, is luminescent, thinly zoned, and occurs mainly as a replacement of lime mud. Dolomite II has dull red unzoned luminescence, and occurs mainly as a replacement of dolomite I rhombs. Dolomite III is non-luminescent, and occurs as a syntaxial cement on, and replacement of, older dolomite I and II rhombs.

Petrography of these dolomite generations, integrating calcite cement stratigraphy, chertification and compaction histories has established the diagenetic sequence. Dolomites I and II pre-date all calcite cements, most chert, intergranular compaction and styloites. Dolomite III precipitation occurred within the calcite cement sequence, after all chert, and after at least some stylolitization. The stratigraphic limit of these dolomites to rocks older than the St Louis Limestone (Meramecian) suggests that dolomitization took place before or during a regional mid-Meramecian subaerial unconformity.

A single dolomitization model cannot reasonably explain all three generations of dolomite in the Burlington and Keokuk limestones. Petrographic and geochemical characteristics coupled with timing constraints suggest that dolomite I formed in a sea water-fresh water mixing zone associated with a meteoric groundwater system established beneath the pre-St Louis unconformity. Dolomite II and III may have formed from externally sourced warm brines that replaced precursor dolomite at shallow burial depths. These models therefore suggest that the required Mg for dolomite I was derived mainly from sea water, whereas that for dolomites II and III was derived mainly from precursor Burlington—Keokuk dolomites through replacement or pressure solution.
Carbonate cements in clastic reservoir rocks from offshore Norway—relationships between isotopic composition, textural development and burial depth
Authors: Saigal G. C. , Bjørlykke K.
Published: April 2008

Many Jurassic and Lower Cretaceous sandstone reservoir rocks from offshore Norway contain carbonate-cemented intervals which typically make up 1–10% of the total sequence. Being nearly impermeable to fluid flow, their distribution may strongly influence reservoir properties. It is therefore important to understand the genesis of these carbonate cements.

Studies of the Jurassic and Lower Cretaceous reservoir rocks from the Haltenbanken and Viking Graben area show that the sandstones contain dominantly two types of calcite cements: (i) microsparry and sparry calcite (Type I) and (ii) poikilotopic calcite (Type II). Type I calcite with δ
18
O values between −4.5 and −8.4‰ PDB precipitated relatively rapidly at shallow burial and at temperatures between 15 and 40°C, possibly from porewater of meteoric origin. Type II calcite with δ
18
O values between −10 and −12‰ PDB have precipitated later at temperatures corresponding to 50–60°C (at ∼ 1.5 km burial depth). Fluid inclusion data also indicate that Type II calcites have crystallized at temperatures between 56 and 68°C.

Silicate grains are often corroded by Type I calcite while grains contained in Type II calcite are uncorroded, suggesting that the porewater was in near equilibrium with the main silicate phases (quartz and feldspar) during the precipitation of Type II calcite.

The degree of supersaturation in the porewater with respect to low Mg calcite probably decreased with increasing burial depth as sources of more soluble carbonate (aragonite, high Mg calcite) were exhausted, causing slower precipitation rates and increased crystal size. The Type II (poikilotopic) calcite is thought to have formed at the expense of only slightly more soluble low Mg calcite crystals.

The fact that the Type II calcite shows nearly the same δ
18
O value (−10 to −12‰ PDB) in sandstones ranging in present burial depth from 1.6 to 5 km suggests that this type of carbonate cementation probably took place early (at temperatures less than 60–70°C), and that very little dissolution and reprecipitation occurred at higher temperatures (greater burial depth).
The early Tertiary stress regime in NW Britain: evidence from the patterns of volcanic activity
Authors: England R. W.
Published: April 2008

Major dyke swarms emplaced throughout the Palaeocene volcanic episode in the British Tertiary Volcanic Province indicate NE-SW extension occurred perpendicular to the evolving NE Atlantic continental margin. Sigmoidal N-S orientated secondary swarms cutting Lewisian basement structures indicate a possible dextral shear component within the regional NE-SW extensional stress field, rather than a structural control on dyke emplacement. Both groups of swarms are considered to be the surface expression of linear intrusions emplaced in the lower to middle crust under the influence of the regional extension. Patterns of faulting within the province are consistent with the stresses recorded by the dyke swarms, indicating the stress field persisted into the Eocene during which a change to NW-SE extension occurred.
Palaeogene volcanism: the sedimentary record in Denmark
Authors: Nielsen O. B. , Heilmann-Clausen C.
Published: April 2008

Approximately 200 volcanic ash layers are known from Palaeocene and Eocene marine clays in Denmark. The oldest layers are present in the upper part of the Upper Palaeocene and are sporadic, thin and acidic in composition. The main volcanic phase took place at the Palaeocene-Eocene transition. During the main phase, a supply from the N to NE is indicated, while a few layers probably had a source to the NW or W. A later volcanic phase is represented by thin and more sporadic ashes in the Lower to Middle Eocene. These are almost totally transformed to smectite.

In the Upper Palaeocene, and partly in the Lower Eocene, smectite is a highly dominant clay mineral in the interbedded clays. Zeolites of the heulandite-clinoptilolite type are also present, while cristobalite is sporadically present in the lower part. A major supply of smectite from the N Atlantic may be indicated. Below the ash-bearing sequence the only evidence for volcanic activity is the presence of very smectite-rich, zeolitic clays with scattered opal-CT and slightly silicified horizons. Calculated chemical compositions of smectites from both ash layers and clay layers between ash layers and below the ash-bearing sequence are almost identical, indicating a volcanic source for the smectite.
Clastic and carbonate lacustrine systems: an organic geochemical comparison (Green River Formation and East African lake sediments)
Authors: Katz B. J.
Published: April 2008

Source rock properties were determined on samples obtained from three modern African lakes—Tanganyika, Kivu and Edward—and on thermally immature material from Eocene Lake Uinta of Utah and Colorado (Green River Formation). The modern rift lakes represent largely clastic lacustrine systems, whereas Lake Uinta is a carbonate-dominated system. The sediments from both systems exhibit a wide range of organic enrichment: up to ∼ 13 wt.% TOC in the clastic lake samples and sometimes greater than 30 wt.% TOC in the carbonate-dominated system. Total hydrocarbon generation potentials, as determined by ‘Rock-Eval’ pyrolyses, also indicate higher yields in the carbonate system (up to ∼200 mg HC g
−1
rock) as compared to the clastic systems (up to ∼ 90 mg HC g
−1
rock). Differences in hydrocarbon yield are greater than can be explained by the organic content. They appear to be the combined result of mineral matrix effects and kerogen character. Elemental and visual analyses of isolated kerogen also support the idea of differences in organic character. The kerogen from the Green River Formation was used to define the Type I reference curve. The kerogen isolated from the clastic lakes appears intermediate between the Type I and Type II reference curves and contains varying and, in some cases, very significant proportions of Type III and/or residual (inert) organic material. In both cases, the principal product will be a high-wax crude oil. The primary differences in organic enrichment and character appear to be related to the relative quantity of allochthonous sedimentary material, both organic and inorganic.
Interactions of iron, carbon and sulphur in marine and lacustrine sediments
Authors: Davison W.
Published: April 2008

Sulphur occurs in marine sediments chiefly as pyrite, which can form both directly or diagenetically within the sediment, or directly at the surface of the sediment. Direct formation occurs when reducing and oxidizing conditions co-exist, whereas diagenesis of monosulphides to pyrite involves oxidative transfer of electrons by, for example, decomposition of organic matter. The limited data available indicate that pyrite occurs in low concentrations in freshwater sediments and is formed directly at a redox boundary. Although most of the sulphur is bound within organic compounds, there is relatively little compared with that present in marine sediments. The absolute concentrations of pyrite, and the ratios of carbon to total sulphur, carbon to pyrite sulphur and organic sulphur to pyrite sulphur all have their uses as palaeosalinity indicators. Although their use is soundly based due to the elevated concentrations of sulphate in sea compared with fresh waters, their validity is not proven. There are sufficient gaps in our knowledge, particularly about pyrite concentrations and formation processes in freshwater environments, to suggest that at present they should be used cautiously.
The lacustrine Burdiehouse Limestone Formation—a key to the deposition of the Dinantian Oil Shales of Scotland
Authors: Loftus G. W. F. , Greensmith J. T.
Published: April 2008

Oil shales, and the less common limestones associated with them, represent freshwater lacustrine phases within the Dinantian Oil Shale Group at the eastern end of the Midland Valley of Scotland. The understanding of the areally extensive Burdiehouse Limestone Formation is fundamental to an appreciation of the palaeolimnology of the lake in which algal oozes (consisting mainly of planktonic algae analogous to the modern form
Botryococcus braunii
), later to become oil shales, accumulated. The lake, referred to as Lake Cadell, was shallow (< 100 m) with gently sloping margins. It was meromictic and formed a closed system during deposition of the limestones, draining eastwards to the sea during oil shale deposition. The climate was wet tropical ensuring ideal conditions for a prolific flora of algae and vascular plants both around and within the lake, and providing a regular freshwater input. The lacustrine phase forms part of the cyclical Oil Shale Group succession in which each sedimentary cycle can be resolved into four phases indicating long-term lake expansion and contraction.
Characteristics of Mesozoic and Cenozoic non-marine source rocks in north-west China
Authors: Binjie Luo , Xinghua Yang , Hejie Lin , Guodong Zheng
Published: April 2008

The formation and distribution of Mesozoic and Cenozoic lacustrine source rocks in north-west China is determined by climatic factors and the tectonics of terrestrial sedimentary basins. The most suitable condition for the formation of non-marine source rocks was a humid to semi-humid climate and a deep to intermediate lacustrine basin during the expansion stage of the lakes. Dominantly fresh to brackish water bodies with sluggish hydrodynamic bottom environments were conducive to the accumulation and preservation of organic matter. The non-marine source rocks show low to high organic matter contents, mainly from a mixture of terrestrial and aquatic sources. Thus, they are characterized by high wax, low sulphur, high nickel and low vanadium. They also have a series of corresponding organic-geochemical signatures. All of these are important indicators in studies of terrestrial source rocks, crude oil and oil-source correlations. A chart with the δ
13
C value of the crude oil against the log plot of pristane/phytane values can be used to discriminate the environment of deposition of the source rock of the oil.
Delta or coastal plain? With an example of the controversy from the Middle Jurassic of Yorkshire
Authors: Alexander J.
Published: April 2008

Deltaic and coastal plain deposits can be distinguished from each other in the rock record where enough data are available to interpret the three-dimensional distribution of facies and unconformities over a wide area. This distinction is based on the distribution of facies rather than purely on a marine-to-alluvial transition as has been the case in the past. The distinction between deposits of the two environments is often very difficult or impossible, and if based on limited data may lead to inaccurate and even incorrect models that can cause problems with prediction of reservoir rocks or coal location in the subsurface. Models of the gross facies distribution for tectonically, eustatically and autocyclically controlled coastal plains are seen as the end-members of a continuum between coastline depositional styles.

The Middle Jurassic sedimentary rocks of the Yorkshire Basin are used to illustrate the controversy that can arise from imprecise definitions and the application of incorrect facies models. The Middle Jurassic Ravenscar Group is interpreted here as the deposits of a tectonically and eustatically influenced coastal plain. Repeated fluctuations between shallow-marine and non-marine conditions are interpreted as mainly caused by vertical tectonic movements and sea-level changes rather than by autocyclic processes.
Deltaic reservoir prediction from rotational dipmeter patterns
Authors: Selley R. C.
Published: April 2008

The dipmeter is a useful logging tool for predicting the distribution of petroleum reservoirs in deltaic sediments, particularly in the Tertiary deltas of the Gulf Coast petroleum province of the USA. Conventionally upward-coarsening genetic increments are defined on gamma or sonic potential logs. Then progradational dip motifs are back-plotted to define the apex of the delta lobe and hence the highest sand-to-shale ratio of the reservoir. This method can also be applied to lobate deposits of alluvial and submarine fan origin.

The dipmeter technique can be extended by noting how depositional dips rotate as a fan lobe progrades. When viewed down-fan the depositional dip slope rotates clockwise on the right flank and anticlockwise on the left flank. These rotational dips may be detectable on the dipmeter log. Rotating dips are poorly displayed on the conventional tadpole plot but are readily apparent on graphic vector plots. Thus it may be possible to determine which side of a fan lobe is well penetrated. Dip data from two wells may define the origin and termination of the fan axis and hence locate the parent distributary channel that may contain the best reservoir sand development.
The influence of basin subsidence and depositional environment on regional patterns of coal thickness within the Namurian fluvio-deltaic sedimentary fill of the Kincardine Basin, Scotland
Authors: Read W. A.
Published: April 2008

Quantitative studies of a varied spectrum of Namurian fluvio-deltaic sediments deposited in Scotland’s most rapidly subsiding Namurian basin, the Kincardine Basin, have revealed the following three simple regional geometrical patterns of coal thickness: (i) basin patterns, (ii) inverse basin patterns and (iii) wedge patterns. These characterize both the overall thickness of coal within a given stratigraphical interval and the thickness of some individual coal seams. The patterns become clearer once local sedimentary and tectonic effects have been filtered out by trend-surface analysis.

In a basin pattern coal thickness is closely related to the net subsidence of the basin. In contrast, in an inverse basin pattern coal-forming peat was selectively preserved on the flanks of the basin and vegetation growth was inhibited in the centre by repeated flooding and channel erosion. A wedge pattern shows coal thickness increasing fairly regularly right across the basin, so that it is directly linked to net subsidence on only one flank. Such a pattern can be linked to saline-water incursions from one direction or to optimal peat growth in a locality sheltered from repeated flooding. Similar coal-thickness patterns seem to be present in some other basins, of various ages, scattered over the world.
Genesis of mid-ocean ridge basalts
Authors: Fujii Toshitsugu
Published: April 2008

Analyses of the compositions of liquids from various melting experiments indicate that the liquids coexisting with magnesian olivine, orthopyroxene, clinopyroxene and spinel at 10 kbar occupy a large volume in the normative projection OL-CPX-PL-SIL and that the application of the invariant melting concept analogous to the simple system CMAS is not warranted in the natural peridotite system. The compositions of magnesian mid-ocean ridge basalts (MORBs) are similar to those of liquids coexisting with olivine, orthopyroxene, clinopyroxene and spinel at 10 kbar. The relationships between Cr/(Cr+Al) in spinel and Mg/(Mg+Fe) in olivine or orthopyroxene in both abyssal peridotites and the solid phases of the melting experiments of peridotites indicate that abyssal peridotites may represent the residual materials after the extraction of liquids formed at pressures around 10 kbar. Reconstructed modal compositions of abyssal peridotites indicate that most of them included calcium-rich clinopyroxene at their solidus temperatures. It is suggested that many magnesian MORBs represent primary magmas formed at pressures around 10 kbar, leaving residual materials composed of olivine, orthopyroxene, calcium-rich clinopyroxene and spinel.
Origin of hybrid lavas from Agua de Pau volcano, Sao Miguel, Azores
Authors: Storey M. , Wolff J. A. , Norry M. J. , Marriner G. F.
Published: April 2008

The island of Sao Miguel in the Azores exhibits bimodal volcanism: eruptives from Agua de Pau volcano and the ‘Waist’ consist almost entirely of basalt or trachyte with rare intermediate products. Although some intermediate lavas are fractionates from basalt, the majority are mixed-magma hybrids anomalously enriched in europium, barium and potassium. These features clearly indicate that they have also accumulated alkali feldspar. The source of the alkali feldspar contaminant must be the least evolved trachyte in the Agua de Pau system, because barium, europium and strontium are rapidly depleted in liquids and crystals with differentiation to more evolved trachytes. Many less evolved trachytes are also accumulative in alkali feldspar, suggesting a genetic association with the hybrid lavas. Geochemical modelling indicates that the hybrid lavas are the result of contamination of basalt by accumulative trachyte containing up to 75% alkali feldspar.

Field and petrological evidence points to the episodic injection and ponding of basalt beneath trachytic magma at Agua de Pau volcano. One trachytic lava, with alkali feldspar-contaminated basaltic inclusions, represents a sample of this interface. Density calculations indicate that highly accumulative trachyte can have a neutral or negative buoyancy with respect to basalt and could descend through the interface. Resorption of alkali feldspar will lower the bulk density of the contaminated basalt, with the effect of favouring even greater contamination by overlying accumulative trachyte. The most attractive mechanism for achieving the accumulative trachyte compositions is the slumping of side-wall cumulates and the formation of magmatic density currents which flowed along the compositional interface between trachyte and basalt. Flowage differentiation and crystal settling would result in local ‘parcels’ of denser highly accumulative trachytic magma. The process is illustrated with results from a simple laboratory experiment.
The influence of pre-existing basin structure on thrust system evolution in the Western Alps
Authors: Butler R. W. H.
Published: April 2008

Collision mountain belts have recently been interpreted as being large scale stacked sedimentary basins, where the wholesale reactivation of pre-existing normal faults exerts the primary control on orogenic style. This model is explored here using examples from the Western Alps. The use of stratigraphic separation across faults, in the absence of corroborative sedimentological data, provides an ambiguous criterion to determinate fault reactivation. Variations between ‘compressional’ and ‘extensional’ stratigraphic separations across the Frontal Pennine thrust are more likely to be due to regional unconformity patterns developed in lower Tertiary times rather than normal fault reactivation along the eastern margin of the external zones. Mesozoic-Tertiary basin structure has been important in controlling the sites of major thrust ramps where stratigraphic horizons suitable for activation as thrust detachment zones have been offset. Examples include the thrust front in the Jura (Triassic detachment dropped to W by faults bounding the Bresse basin) and Vercors (basement uplifted to W at Faille de l’Isere, bounding the Mesozoic sub-Alpine basin to W). Buttressing structures, where thrust displacement is converted into vertical pure shear, may indicate buried normal faults but can occur at either hinterland or foreland-directed basin faults. The overall structural style in the sub-Alpine thrust belt indicates detachment between basement and cover with basement involvement along the Sole thrust which carries the external basement massifs. There is no indication from surface geology that this thrust originates from reactivation of a Mesozoic basin fault although this process may have occurred at depth. Similar styles of detachment-dominated thrusting with large horizontal relative to vertical displacements also characterize internal Alpine structure. Preliminary results from the ECORS-CROP deep seismic reflection profile across the Western Alps support the detachment thrusting model although they do not rule out minor normal fault reactivation.
The Lewis Thrust/Rocky Mountain trench fault system in Northwest Montana, USA: an example of negative inversion tectonics?
Authors: Powell C. M. , Williams G. D.
Published: April 2008

In the Rocky Mountains of Northwest Montana, USA, Oligocene basins occur within the Laramide thrust system. During emplacement of the Laramide Lewis thrust sheet in the Glacier National Park area, a penetrative cleavage was developed with an asymmetry suggestive of NE directed movement. This cleavage can be used as a pre-extensional marker to reinterpret the geometry of the Rocky Mountain Trench extensional fault system. The cleavage is confined to a zone of constant thickness above the thrust. Thus its upper limit can be considered a mappable surface horizon from which the depth to the Lewis Thrust can be interpolated. Sequentially restored sections from the Rocky Mountain Trench to the leading edge of the Lewis Thrust illustrate the interpreted geometries and the interaction of the extensional and contractional fault systems.
The tectonics of the southern Alps and the Austro-Alpine nappes: a comparison
Authors: Laubscher H. P.
Published: April 2008

The southern Alps as well as the Austro-Alpine nappes originate from the southern margin of Tethys with close analogies both in basement petrography and stratigraphic sequence. Although they played vastly different roles in Alpine orogeny they developed similar tectonic features. In both domains there is a ramp-flat geometry with associated folds, analogous to those found in thrust belts worldwide, and the basement consists of thrust plates not more than 5 km thick with a conspicuous frontal ramp fold, complicated by imbrications. Regional décollement levels are at the base of the middle Triassic, the Carnian, and the Rhaetian. Both the southern Alps generally and at least the western end of the Upper Austro-Alpine Lechtal nappe are affected by transverse segmentation which in the southern Alps is demonstrably due to reactivation of Jurassic normal fault zones. Tectonic wedging into the regional décollement layers was a frequent phenomenon. In contrast to the southern Alps, the Silvretta-Lechtal nappe forms part of the Alpine nappe system with total translations of many hundreds of kilometres. It now overlies nappes deriving from different palaeogeographic domains (Lower Austro-Alpine, south Pennine, mid-Pennine, and north Pennine nappes in the Rätikon). Reactivation of the equivalent of the original Lechtal sole thrust, although at a somewhat deeper level within the south Pennine Arosa zone, led to the introduction of exotic Arosa masses into the Carnian interval of the Lechtal nappe, and reactivation of the transverse faults and their propagation to deeper levels resulted in non-cylindrical kinematics of Penninic nappes along the Rhine line.
Concentrated and lean oolites: examples from the Nubia Formation at Aswan, Egypt, and significance of the oolite types in ironstone genesis
Authors: Bhattacharyya D. P.
Published: April 2008

Two textural types of oolitic ironstones occur in the Late Cretaceous Nubia Formation at Aswan, Egypt. These are: (1) relatively thin bedded (5–20 cm) muddy or ‘lean’ oolites containing up to 300oids, proto-ooids and peloids scattered in a matrix of ferruginous mud, and (2) thicker beds (as much as 2.5 m thick) of ‘rich’ or ‘concentrated’ oolites comprising 60–800oids.

Sedimentologic and textural attributes of the ‘concentrated’ oolites suggest that these represent ooids transported and redeposited away from their site of formation. In fact, in ironstone-bearing successions elsewhere the concentrated oolites are either associated with winnowed sandstones or, in sand-poor deposits, they substitute for sandstone. Together the ooids and the detrital sand form bar and sand wave complexes in upward shoaling marginal marine sediments. In unbioturbated or less bioturbated occurrences these sandbodies of ferruginous ooids are replete with primary sedimentary structures indicative of strong current transport. Thin beds of similar ferruginous ooids in sequences of off-shore carbonate or argillaceous basinal mudstone, such as those in the Silurian Clinton Group of eastern United States, are storm deposits characterized by basal scours and hummocky cross-stratification. The muddy or ‘lean’ oolites, on the other hand, are commonly associated with prograding subtidal mudflat deposits. Textural characteristics of these ironstones suggest that they probably represent the primary sites of ooid formation. Different stages of ooid formation preserved in these ‘lean’ ironstones and their relationship with the concentrated oolites provide information which is significant for a more realistic and comprehensive genetic modelling for the Aswan ironstones.

In the model proposed here the deposition of muddy, ferruginous sediments on prograding, near-shore, subtidal mudflats begins during the waning stages of detrital sediment supply. The ooids are formed on these shallow-water mudflats by mechanical accretion of detrital kaolinitic clay and precipitated iron hydroxide minerals, mostly around faecal pellets or flocculated peloids, under the influence of gentle wave or current agitation. Periodic high-energy events, such as storm-generated or tidal currents, transport the ooids to off-shore areas to be reworked there into oolitic sand bar and shoal complexes. Following deposition, the kaolinitic ooids, peloids and matrix are transformed into berthierine under a chemically reducing diagenetic environment.
Oolitization processes in Palaeozoic ironstones of France, Algeria and Libya
Authors: Chauvel J-J. , Guerrak S.
Published: April 2008

On the basis of the study of Palaeozoic ironstones from France, Algeria and Libya, some oolitization processes are documented, particularly intrasedimentary accretion and ‘snow-ball’ type accretion. In a single ooid, successive layers of the cortex can have been formed by different processes. Chemical analysis of an ooid cortex can help to reconstruct its history. Chamositic layers are always weathered in their outer part, with the leaching of aluminium and magnesium. Haematitic layers result from iron, and often titanium concentration in the weathered outer parts of primary chamositic layers. Crystallization of siderite appears to be late diagenetic, and often took place at the same time as, or after, the development of quartz overgrowths.
Time and space distribution of Palaeozoic oolitic ironstones in the Tindouf Basin, Algerian Sahara
Authors: Guerrak S.
Published: April 2008

The Tindouf Basin is located, mainly in Algeria, in the SW part of the Saharan Platform. It comprises Cambro-Ordovician to Carboniferous marine formations overlain by the continental Cretaceous and Pliocene Hamada cover. On the southern limb of the Tindouf Basin several oolitic ironstone deposits occur within Silurian and Devonian sediments. They occupy an area of 500 × 40 km and have an east — west trend. These oolitic ironstone deposits are part of the major North African oolitic ironstone belt, extending more than 3000 km, from Zemmour to Libya. These occurrences are of the LOID type (Local Ironstone Deposition), and contain more than 10000 million tonnes of ironstone reserves (1500 million tonnes in the Silurian and 9200 million tonnes in the Devonian).

The various ironstones were formed in shallow marine environments such as barriers and deltas. The major features of these deposits are: (i) a palaeogeographical control of the sediment which was mainly formed on the flanks of uplifts; (ii) the occurrence of the ironstones at the top of coarsening-upward sequences, mostly located towards the end of major regressive cycles, particularly in the Pragian and Famennian; (iii) ooids developed in quiet environments such as lagoons or embayments, within an iron-rich mud; (iv) a southern, relatively close, iron source area and; (v) a palaeolatitudinal distribution giving successively cold and temperate climates.

The Palaeozoic history of the basin involved pericratonic sedimentation on the borders of a large epicontinental sea.
The Aztec fish fauna (Devonian) of Southern Victoria Land: Evolutionary and biogeographic significance
Authors: Young G. C.
Published: April 2008

The Aztec fauna is one of the most diverse fossil vertebrate faunas from Antarctica, with over 30 taxa belonging to the four major groups of jawed fishes (acanthodians, placoderms, osteichthyans, chondrichthyans), and one group of agnathans (thelodontids). Biogeographically the strongest affinity is with eastern Australia, as part of the East Gondwana Province. Biostratigraphically the Aztec succession includes probably the youngest known non-marine thelodont agnathans at the base, and the oldest known phyllolepids at the top. This transition, and an association of species of
Turinia
and
Bothriolepis
, is not seen in any other sequence. Most or all of the Aztec sequence is regarded as Givetian (late Middle Devonian) in age. The Aztec fauna contains older and/or more primitive representatives of several groups typical of younger (late Palaeozoic) strata in Euramerica, suggesting a biotic dispersal episode between Gondwana and Euramerica in the Late Devonian. Centres of origin and dispersal hypotheses in Palaeozoic biogeography can be related to disappearance of faunal barriers as manifested in different stratigraphic ranges in different areas. The change for Devonian vertebrates from the endemic patterns seen in the Early Devonian to the close similarity of Famennian faunas between East Gondwana and Euramerica suggests disappearance of a major faunal barrier in the Late Devonian, presumably the ocean between Gondwana and Euramerica.
Patterns of evolution in the Dicynodontia, with special reference to austral taxa
Authors: DeFauw Sherri L.
Published: April 2008

Dicynodonts were the dominant primary consumers of the latest Palaeozoic and early Mesozoic ecosystems. Of the 66 currently accepted genera of dicynodonts, three (possibly four) of these occur in the Fremouw Formation of Antarctica (i.e.
Kingoria, Lystrosaurus, Myosaurus
and ?
Kannemeyeria
). Modifications to the basic shearing mechanism of the jaws as well as postcranial morphology presumably reflect differences in the physical nature of the food consumed and foraging techniques used by dicynodonts. To date, five dicynodont feeding types have been recognized: invertebrate specialists, grubbers, browsers, forest litter foragers and flexible foragers. Three out of the five ‘central’ dicynodont feeding types are represented among the members of the early Mesozoic Antarctic palaeofauna. In order to more accurately assess systematic and ecomorphic relationships among dicynodonts, comprehensive generic diagnoses (that incorporate postcranial characters) are warranted; suggested revisions to the generic diagnoses of Antarctic dicynodonts are provided. Dicynodont distributions are evaluated in light of ecomorphic considerations, tectonic-orogenic events, sea-level fluctuations, palaeoclimates and floral changes. Available evidence, pertaining to austral taxa, suggests that: (1) Antarctica served as a refugium for kingoriamorphs in the Early Triassic, and (2) the informal members of the Fremouw Formation are correlated with more than one African biozone.
The relevance of dip profiles from outcrops as reference for the interpretation of SHDT dips
Authors: Herweijer J. C. , Höcker C. F. W. , Williams H. , Eastwood K. M.
Published: April 2008

The ability of SHDT processing techniques to produce detailed sedimentological information has been evaluated by comparing the data with 3000 ft of cores covering shallow marine, deltaic, lacustrine, fluvial and aeolian sedimentary environments. It has been shown previously that, by using side-by-side processing, realistic sedimentary dip sequences through sedimentary bodies can be obtained from SHDT data under normal conditions. However, optimum results require processing parameters to be chosen compatible with the scale of the sedimentary structures present. Furthermore, post-processing selection procedures are required to remove noise and to identify dips representing foreset inclination which can be interpreted in terms of flow direction and sand body orientation.

The sedimentological interpretation of downhole dips is often difficult. It may be improved by comparison with vertical dip sequences from outcrops. Sedimentary dip profiles have been measured as part of field work carried out in the Pocahontas Basin of Kentucky. Extensive roadcuts not only enable an unambiguous sedimentological interpretation, but also exhibit numerous relict blast-boreholes, in which vertical dip profiles can be measured under conditions similar to downhole logging. The data now available show that even contradictory dip sequences may be a proper representation of the reality. Systematic collection of sedimentary dip sequences will provide guidelines for a realistic interpretation of downhole records of sedimentary dips.
Geological applications of geochemical well logging
Authors: Herron M. M. , Herron S. L.
Published: April 2008

Recent advances in geochemical logging and interpretation have made it possible to obtain in situ concentration logs for at least ten of the chemical elements present in sedimentary formations: Al, Si, Ca, Fe, S, Ti, K, Th, U, Gd and possibly Mg. Each of these elements is concentrated in the solid portion of the formation as opposed to the pore fluids, and together these elements provide an array of measurements with a large dynamic range and tremendous diagnostic strength for geological interpretation.

The combination of a few diagnostic elements, such as silicon, aluminum, and calcium, provides sufficient information for a rapid but accurate lithological description. In the case of siliciclastic reservoir rocks, it is possible to discriminate between sand and shales and to determine types of sandstones using the ratios of SiO
2
/Al
2
O
3
and Fe
2
O
3
/K
2
O. Calcium is used in conjunction with these ratios to differentiate between non-calcareous, calcareous, and carbonate rocks. On a more sophisticated level, a set of chemical abundances can be incorporated into a sedimentary normative analysis to determine quantitatively both the framework and clay mineralogy of siliciclastic formations. Derived mineral assemblages can provide valuable information for the interpretation of depositional environments and diagenesis. In shales, elemental data can be used alone or in conjunction with derived mineralogy to derive total organic carbon and thereby begin to evaluate source rock potential. By using individual elemental concentration logs or any of the interpreted formation units it is possible to enhance the characterization of vertical sequences and the recognition of well-to-well correlations.
Circumferential acoustic logs detect natural fractures and determine their orientation
Authors: Fertl W. H.
Published: April 2008

As the direction of fractures shifts from horizontal/near-horizontal toward the vertical plane with respect to the intersecting wellbore, conventional acoustic logging devices become unreliable for fracture detection purposes. Under these circumstances, circumferential acoustic logging technology has already proven to be an excellent technique to determine vertical and/or near-vertical open fracture systems that intersect a wellbore and to measure the directional subsurface trends of these detected fractures. In addition to visual ‘quick-look’ interpretation of the recorded acoustic waveform data, novel digital interpretive techniques enhance the fracture evaluation.
The Venezuelan hydrocarbon habitat
Authors: James K. H.
Published: April 2008

Venezuela has produced some 40 billion barrels of oil. Remaining recoverable reserves are currently in the order of 60 billion barrels of oil and 92 trillion cubic feet of gas. The Orinoco Heavy Oil Belt, the world’s largest known oil accumulation, has some 1.2 trillion barrels of oil-in-place. Only part of the 267 billion barrel, ultimate recoverable of this belt is included in the current reserves.

Most of Venezuela’s hydrocarbon reserves are located in the Maracaibo, Eastern Venezuela, and Barinas—Apure Basins, and most are derived from Upper Cretaceous, marine source rocks. Tertiary, terrestrial source rocks have also contributed hydrocarbons in the Maracaibo and Eastern Venezuela basins, and Tertiary marine and terrestrial source rocks have generated hydrocarbons in the Falcon and offshore Venezuelan basins.

The hydrocarbon habitat has been profoundly influenced by Mesozoic—Recent platetectonics. Many of the structural traps are the consequence of Carribbean—South America dextral relative movements. Some of these structures, mainly in the northwest, were enhanced by stress generated in the Pacific. The northern part of the country has suffered considerable shortening and inversion. The resultant mountain chains limit the modern hydrocarbon basins. Reconstruction of late Cretaceous source rock palaeogeography, of Late Cretaceous to Palaeocene kitchens (now metamorphosed), and of Eocene to Recent kitchens, shows that the latter are remnants of a much larger petroliferous province and provides an explanation for the great abundance of hydrocarbons in the area.

Significant volumes of hydrocarbons remain to be discovered in extrapolations of traditional and developing plays.
Structural styles of growth faults in the U.S. Gulf Coast Basin
Authors: Lopez J. A.
Published: April 2008

Structure and sediments related to shelf margin growth faults are an important exploration objective in the Cenozoic in the U.S. Gulf Coast basin. The syndepositional nature of these fault systems has been recognized for decades with well control and seismic. Recent seismic of higher quality and deeper penetration has begun to reveal more clearly the variety of structural styles of the ‘typical’ Gulf Coast shelf margin growth fault.

Generally five different shelf margin growth fault types are recognized: (1) listric faults which detach above undeformed older section, (2) listric faults which detach on mobile, overpressured shales above older undeformed sediments, (3) faults which sole into autochthonous salt withdrawal basins, (4) faults which detach along allochthonous salt sill intrusions, and (5) non-detached faults caused by differential compaction of thick shale intervals or basement faulting. The first four are the dominant structural types and their different structural styles reflect various modes of accommodation of fault offset within the sedimentary section. The fifth structural type, although not as commonly recognized, may be obscured by other structural complexities.

These five fault types have different structural and stratigraphic characteristics which create different trap-source-reservoir relationships for each type. By understanding and identifying the distribution of these structural types one may better understand the complex production trends established in the Gulf Coast basin and more accurately assess potential production on prospective traps.
The Middle East Basin: a summary overview
Authors: Stoneley R.
Published: April 2008

In a review of classic petroleum provinces, it is instructive to consider the most prolific of them all and once again to ask ourselves why it is that the Middle East is so rich in oil and gas. The question has been addressed many times (e.g. see the discussion by Beydoun) but, as new data become available, it will continue to be asked and new insights may be gained by fresh attempts to provide the answer.

The part of the region considered in this brief overview surrounds the Arabian/Persian Gulf, and does not extend either northwest far into Iraq or southeast into Oman, where much of the oil is Palaeozoic in age. Neither is the abundant gas in the Permian Khuff Formation included, since little information about it is available — and even less on its source. We are therefore concerned with the Mesozoic and Tertiary accumulations in a limited part of the basin.
Niger Delta reservoir geology: historical growth of the sedimentological model and its application to field development
Authors: Weber K. J.
Published: April 2008

In 1953 when the exploration in Nigeria moved towards the Niger Delta, it was realised that the recent delta could be used as a guide to the past. Consequently many studies were carried out between 1953 and 1964, ranging from surveys of the shallow offshore, the coastal zone and the rivers to an extensive shallow coring campaign (1960–62).

The studies of the recent delta were soon augmented by the analysis of cores from several newly discovered fields (1963–69) and also by the description and grain size analysis of numerous side-wall samples. Around 1970 the basic facies types comprising the individual deltaic cycles of the hc-bearing paralic sequence were well established. The facies types occur in a very repetitive pattern of combinations and successions which facilitates recognition and correlation. A fairly reliable system of diagnostic criteria was worked out based on petrophysical logs and side-wall samples and calibrated by the cores and the recent delta.

With the sequencial introduction of new logging tools the diagnostic methods are being refined while additional cored wells continuously enlarge the data base. Detailed seismic analysis at reservoir level is showing considerable promise for sand body delineation. For each facies a representative data base of sand body geometry could be compiled based on detailed reservoir correlations and recent examples. There is a good understanding of typical permeability and grain-size distributions. Knowledge on shale intercalation continuity is particularly useful in the planning of well completions to retard gas or water breakthrough to the well bore. It was shown that permeability can be computed from log derived porosity and grain-size distribution measured on side-wall samples.

The practical value of the reservoir geological research lies in the possibility to correlate and model reservoirs in a reliable and consistent manner. Reservoir and well behaviour can be predicted and where necessary reservoir simulation models can be constructed. Continuous updating of reservoir studies has demonstrated the validity of this approach by checking past predictions and analysis of infill wells. In this way the production planning and reserve estimation is given a solid base.
Hydrocarbon generation and migration in the Western Canada sedimentary basin
Authors: Creaney S. , Allan J.
Published: April 2008

A total of ten marine, petroleum source rocks ranging in age from Middle Devonian to Late Cretaceous exist within the Western Canada Basin. Principal hydrocarbon reserves occur in the Lower Cretaceous Mannville Group and subcropping Paleozoic carbonates, where 270 × 10
9
m
3
(1.7 trillion barrels) of heavy oil form one of the largest deposits of oil on Earth. Biomarker analyses of these heavy oils, many conventional oils and most source rocks in the basin have been combined with basic geology to establish the source and migration history of the Western Canada Basin. Several pre-Cretaceous source rocks (principally the Lower Jurassic Nordegg Formation, the Mississippian Exshaw/Bakken Formation, the Triassic Doig/Phosphate Formation and the Upper Devonian Duvernay Formation) have contributed to the Mannville accumulations. The Joli Fou shale, overlying the Mannville, is a regional seal for the Mannville and older section. Oils present in the overlying Viking, Cardium, and Belly River sands are all of a single family, sourced from the Colorado Shale section and are quite distinct from the oils in the Mannville and older section.

Meteoric water incursion at the eastern limb of the basin permitted biodegradation, which converted the conventional Mannville crude oil to heavy oil. The Western Canada Basin also contains an estimated 5 × 10
12
m
3
(180 TCF) of gas. Gas sources are the marine, oil-prone source rocks where overmature, as well as Lower Cretaceous Mannville coals and coaly shales. Hydrogen sulphide is contributed from overmature Paleozoic evaporites/carbonates with an estimated 36% of the total in-place gas reserves being classified as sour.