bluuebunny/crossref_metadata_2025 huggingface dataset:
New techniques in lithofacies determination and permeability prediction in carbonates using well logs
Authors: Dorfman M. H. , Newey J.-J. , Coates G. R.
Published: April 2008

Carbonate reservoirs pose many challenges to geologists and engineers. Lithofacies variations are often the key to the determination of commercial value and efficient reservoir exploitation. Thus, techniques which permit the identification of the involved lithofacies would be useful. This paper illustrates a successful application of multidimension techniques in two-dimensional space, by using cross plots of a variety of well logs that have a principle component sensitivity to the lithofacies. This paper also examines the appropriateness of techniques which relate log data to core-derived permeability. These approaches have previously been associated with the intergranular shaly sand rocks and not carbonates. The technique shown approaches this from a Kozeny-core correlation which is used to predict the surface area parameter. This is then followed by development of a link to log-derived bulk water values. The log derived variables can be made to work providing irreducible conditions are present or can be established.
Sediment cyclicity from well logs
Authors: Worthington P. F.
Published: April 2008

The application of wireline well logs to the recognition and interwell correlation of sediment cyclicity is governed by the resolving powers of the different logging tools relative to measured sedimentation rates and cyclic periodicities. These variables, which are not always well defined, give rise to diverse capabilities of well logs in recording and correlating cyclic events. It is therefore important that criteria be established for the successful application of wireline logs in cyclostratigraphy, especially when a succession reflects several lithoperiodicities due to climate forcing. Quantification of these criteria leads to the introduction of a log association parameter (LAP) to compare the effective cyclic resolutions of different logging tools in diverse geological environments. This parameter relates sedimentation rates and tool vertical resolutions to the targeted time-periods of cyclicity. It is tool- and site-specific but is independent of formation lithology and chemistry. High LAP values, above the critical level of unity, suggest a strong cyclic resolution. Similar supracritical LAP values for (different) logs in different boreholes imply a high degree of potential interwell correlation. The benefits of a LAP-based approach are evident from illustrative examples, even at this preliminary stage. Field application ideally requires the conjunctive use of log and core information, a strategy which reflects the current trend in earth science towards a more meaningful integration of data measured at different scales.
Interwell matching using a combination of petrophysically derived numerical lithologies and gene-typing techniques
Authors: Griffiths C. M. , Bakke S.
Published: April 2008

The use of gene-typing techniques in stratigraphic matching problems has been steadily developing since Smith & Waterman published an example in 1980. The regular use of these techniques to assist in stratigraphic correlations using wireline log data has, however, been hindered by the lack of suitable precisely defined lithological units. In an oil-field production or exploration environment, the porosity logs and a natural gamma log can be used to derive various types of numerical lithologies. These lithologies provide a convenient starting point for the use of gene-typing algorithms since they comprise a relatively small set of precisely defined reproducible units. The advantage of using a gene-typing technique is that it enables multiple matching hypotheses to be produced from sequences which contain gaps, stretched zones, and duplicated sections. These matches can assist the production of stratigraphic correlations. Examples of matching are presented from reservoir sections in the Statfjord and Murchison fields.
Feasibility of in situ elemental analysis in mudrock evaluation
Authors: Primmer T. J. , Kerr S. A. , Myers K. J.
Published: April 2008

Geochemical logging now offers the possibility of performing quantitative in situ elemental analysis. Laboratory measurements of element abundances on selected sections of mudrocks were made to simulate the potential use of data available from geochemical logging. The analyses centre on mudrocks from the Magnus Field (North Sea) and Dorset Kimmeridge Clay Formation sequences (Southern England). Subsequent multivariate statistical processing of these analyses showed that pre-defined units within the sections studied can be distinguished geochemically without recourse to mineralogical description. This is demonstrated firstly by cross plotting elemental abundances to pick out significant sympathetic trends and thereafter by clustering and factor analysis techniques which utilize a more extensive dataset. This study shows that the direct application of statistical tests to raw element abundances determined downhole is an appropriate methodology by which conventionally impractical topics, such as the subsurface discrimination of mudrock facies, may be tackled. This conclusion will, no doubt, be tested further as geochemical logging develops over the next few years.
An investigation into nuclear log responses of North Sea Jurassic sandstones using mineralogical analysis
Authors: Humphreys B. , Lott G. K.
Published: April 2008

The results of laboratory mineralogical analyses have been compared with downhole log responses to determine the possibility of detecting individual minerals using the gamma-ray, natural gamma-ray spectroscopy, formation density and litho-density logs. Cored sequences from the Murchison, Thistle, Kittiwake, Gannet and Tartan Fields, North Sea, are used as case examples. K-feldspars, micas, detrital clays (illite, and possibly kaolinite) and phosphatic cements are shown to produce a measurable response on the gamma-ray and natural gamma-ray logs. Authigenic kaolinite did not impart radioactivity to the sandstones in the examples studied. The photoelectric absorption index (
Pe
) was found to be particularly sensitive to Fe-bearing minerals and baryte. A number of problems constrain full characterization of the mineralogy of a sandstone: specific detection of quartz and plagioclase feldspars is not possible by nuclear logs, distinction between minerals is problematical when a log responds to several minerals simultaneously, and calibration for minerals with a variable composition such as ferroan dolomite and mixed-layer illite-smectites is difficult.
Identification of source rocks from wireline logs
Authors: Stocks A. E. , Lawrence S. R.
Published: April 2008

This paper reviews the properties of organic-rich rocks which constitute petroleum source rocks and investigates petrophysical methods which can be used for their assessment. A review is made of the common log data and their response to an appreciable organic content in shale formations. Associations and effects which might lead to an erroneous assessment are also discussed. Previous researchers have highlighted relationships between organic content and various log parameters or values. These rely on geochemical control and tend to be stratigraphically and regionally specific. In this paper we show how standard petrophysical processing can be applied for qualitative and semi-quantitative evaluation of organic content and therefore source rock potential. Recommendations are made for a reform in geochemical sampling which will enable better quantitation methods for more widespread application.
In situ crustal stress in the United Kingdom from borehole breakouts
Authors: Evans C. J. , Brereton N. R.
Published: April 2008

Until recently very little in situ stress data have been available from the UK crust. Literature reviews list only a limited number of measurements which include information on the direction of maximum horizontal compression. Unequal horizontal crustal stresses imposed on vertical boreholes will often cause localized rock spalling along the direction of least compressive stress. The resultant elongations of the borehole cross section, called breakouts, can be identified from four arm caliper records collected routinely with conventional geophysical dipmetre logs. Eighty-one boreholes in the United Kingdom have been analysed for breakouts using criteria for identification of breakouts unique to this paper. The majority of borehole breakouts in southern Britain are aligned NE-SW, indicating a maximum horizontal stress in a NW-SE direction. There are insufficient data to confidently assess a dominant borehole breakout direction in Northern Britain, although the four boreholes analysed indicate possible variations in the crustal stress field.
Electrical properties of basalts from DSDP Hole 504B: a key to the evaluation of pore space morphology
Authors: Lovell M. A. , Pezard P. A.
Published: April 2008

Archie’s Law is a well known empirical relationship between electrical formation factor and porosity:

FF =
a
φ

−
m

.

As such it is widely applied in the analysis and interpretation of downhole measurements. Despite its frequent application, however, the mechanics underlying the relationship that it embodies remain largely a mystery. A brief review of some previously published, but often neglected, data is used here to describe recent results for laboratory and downhole electrical measurements in basaltic ocean crust. The results disagree with many previously published studies of crystalline rocks in that
a
≠ 1.0 and
m
is not always close to 2.0. Instead
a
increases with increasing fracturing, while
m
remains low and constant, though it increases slightly in the altered rock, possibly implying increased ‘tortuosity’. The value of
a
allows discrimination between fractured pillows and massive flows and may be inversely related to the degree of crack connectivity. Cation exchange capacity measurements on core samples show there is potential for surface conduction to occur, but probably only at higher temperatures. The precise nature of this temperature dependence is unclear, as is the interplay of the surface conduction mechanism with that of electrolytic (pore fluid) conduction.
An assessment of the use of Christensen’s equation for the prediction of shear wave velocity and engineering parameters
Authors: Entwisle D. C. , McCann D. M.
Published: April 2008

The full wave train sonic log is widely used in civil engineering for the determination of formation dynamic elastic moduli. In some instances, the shear wave amplitude is too small to time its arrival accurately. In these situations Christensen’s equation can be used to predict the shear wave velocity from the compressional wave velocity and the formation density. In this paper the origins of Christensen’s equation are investigated in relation to both field and laboratory studies on a wide range of natural materials. It is suggested that differences between the measured and predicted shear wave velocities are lithologically based. The equation is not valid for uncemented sediments. The dynamic elastic moduli reflect these differences. Bulk modulus is least affected by large differences in shear wave velocity. There is no relationship between the predicted and the measured values for Poisson’s ratio. The use of Christensen’s equation will be improved if corrections are made locally for lithology.
Geological integration and evaluation of Northern Monagas, Eastern Venezuelan Basin
Authors: Aymard R. , Pimentel L. , Eitz P. , Lopez P. , Chaouch A. , Navarro J. , Mijares J. , Pereira J. G.
Published: April 2008

Over the last 60 years, the Eastern Venezuelan Basin has been heavily explored with giant oil accumulations being developed in such areas as Oficina, Anaco, Quiriquire and the Orinoco Oil Belt in the states of Anzoategui and Monagas. Since 1986, interdisciplinary studies and seismic interpretations have led to discovery of a trend of deep structures on the northern flank of the basin, some of them beneath old early Tertiary producing fields. The first exploratory well, FUL-1, tested 12 000 BOPD of 28° API crude. The new reservoirs consist of massive sandstones of the Eocene Caratas Formation and Oligocene Naricual and Los Jabillos Formations between depths of 13 000 and 17 000 feet. The main structures placed in a thrust fold belt as a series of anticlines, extending over 100 km across Northern Monagas; currently four wells are being drilled and six more exploratory wells are planned to be drilled in the new future. To date over 1900 MMBBLS of reserves have been discovered, and possible reserves for the area are close to 8.6 billion BBLS.
New oil discoveries in the Ceuta area, SE Lake Maracaibo, Venezuela
Authors: Roberto M. , Mompart L. , Puche E. , Scherer F.
Published: April 2008

The Ceuta area forms the southeast extension of a major oil accumulation in the Maracaibo Basin, at present Venezuela’s main oil producing area. The Ceuta Field, with current reserves of 600 MMBBls of light to medium gravity oil, is a major wrench fault structure, producing from Miocene and Eocene sandstones. Several major transcurrent faults are recognized in the Maracaibo Basin, each defining major producing oil fields like the Ceuta Field. These structures, roughly parallel to each other, are oriented in a North-South direction.

The principal plays in the Maracaibo Basin consist of Miocene and Eocene sandstones and, at greater depths, fractured Cretaceous carbonate reservoirs and fractured basement, all charged from the rich marine La Luna source rocks of Upper Cretaceous age.

The discovery of the Ceuta Field was the result of an intensive drilling campaign executed in the 1970s, based on conventional seismic techniques. At that time the exploration of the area was oriented towards the main North-South trending structures. The new exploration, undertaken during the last four years, was based on the structural modelling of the area in terms of the known transcurrent faulting. This allowed the recognition, in areas between the major trends, of three new types of traps related to different styles of faulting, such as normal, compressive and antithetic transcurrent.

The successful drilling of these structures has led to the discovery of an additional 1100 MMBBls of oil outside the existing Ceuta Field, confirmed by an intensive drilling appraisal campaign.
Aspects of the petroleum geology of the Oriente Basin, Ecuador
Authors: Dashwood M. F. , Abbotts I. L.
Published: April 2008

The Oriente Basin of Ecuador is one of the most productive of the South American Sub-Andean Basins. Cumulative production of oil to the end of 1986 was over one billion barrels, and current production stands at approximately 300 000 barrels per day. At least 20 oil fields have been discovered to date, including five giant fields, and exploration is currently in the early stages of a new and already successful phase in which ten new operators are scheduled to drill over 40 wells between 1985 and 1992.

The Oriente Basin contains a sedimentary fill of Palaeozoic to Recent age. Major commercial interest is confined to the Cretaceous depositional cycle and all of the significant production comes from fluvio-deltaic and marine sandstones of the Hollin and Napo Formations. Most of the productive structures are low-relief, north-south orientated, anticlines of two distinct types: foot wall anticlines associated with normal faults, (Type 1) or hanging wall anticlines associated with reverse faults, (Type 2). Evidence points to the importance of pre-Miocene structural growth, comprising either early-mid Cretaceous rejuvenation of pre-Cretaceous ‘basement’ faults in the case of Type-1 structures and ‘Early Andean’ (latest Cretaceous to Oligocene) compression in the case of Type-2 structures. Few commercial discoveries have been made associated with ‘Late Andean’ (Miocene-Pliocene) compressional structures, essentially because these structures appear to post-date the main phase of primary oil generation and migration.

The origin of the oils is problematical because the potential source rocks, the marine claystones and limestones of the Cretaceous Napo Formation, are generally immature or marginally mature within the confines of the present-day Oriente Basin. Oil analyses indicate a single family of oils. Available evidence, combined from structural and geochemical data, supports a major phase of Early Andean age oil generation and migration.

There is a considerable variation of oil type from 37° API paraffinic oils with a GOR of 250–300 to altered 10° API oils. Marked variations exist not only between oilfields but also between reservoirs in the same well. The observed trends can in most cases be accounted for systematically in terms of: early primary oil generation, migration and entrapment; and subsequent structural evolution locally involving the processes of biodegration, water washing and/or flushing, and oil remigration.
Yates and other Guadalupian (Kazanian) oil fields, U. S. Permian Basin
Authors: Craig D. H.
Published: April 2008

More than 150 oil and gas fields in west Texas and southeast New Mexico produce from dolomites of Late Permian (Guadalupian [Kazanian]) age. A majority of these fields are situated on platforms or shelves and produce from gentle anticlines or stratigraphic traps sealed beneath a thick sequence of Late Permian evaporites. Many of the productive anticlinal structures are elongate parallel to the strike of depositional facies, are asymmetrical normal to facies strike, and have flank dips of no more than 6°. They appear to be related primarily to differential compaction over and around bars of skeletal grainstone and packstone. Where the trapping is stratigraphic, it is due to the presence of tight mudstones and wackestones and to secondary cementation by anhydrite and gypsum. The larger of the fields produce from San Andres-Grayburg shelf and shelf margin dolomites. Cumulative production from these fields amounts to more than 12 billion bbl (1.9 × 10
9
m
3
) of oil, which is approximately two-thirds of the oil produced from Palaeozoic rocks in the Permian Basin. Eighteen of the fields have produced in the range from 100 million to 1.7 billion bbl (16−271 × 10
6
m
3
). Among these large fields is Yates which, since its discovery in October 1926, has produced almost 1.2 billion bbl (192 × 10
6
m
3
) out of an estimated original oil-in-place of 4 billion bbl (638 × 10
6
m
3
). Flow potentials of 5000 to 20 000 bbl (800 to 3200 m
3
) per day were not unusual for early Yates wells.

The exceptional storage and flow characteristics of the Yates reservoir can be explained in terms of the combined effects of several geologic factors: (1) a vast system of well interconnected pores, including a network of fractures and small caves; (2) oil storage lithologies dominated by porous and permeable bioclastic dolograinstones and dolopackstones; (3) a thick, upper seal of anhydrite and compact dolomite; (4) virtual freedom from the anhydrite cements that occlude much porosity in other fields which are stratigraphic analogues of Yates; (5) unusual structural prominence, which favourably affected diagenetic development of the reservoir and made the field a focus for large volumes of migrating primary and secondary oil; (6) early reservoir pressures considerably above the minimum required to cause wells to flow to the surface, probably related to pressures in a tributary regional aquifer.
Southern Gulf of Suez, Egypt: structural geology of the B-trend oil fields
Authors: Helmy H. M.
Published: April 2008

The Nubia sandstone, the primary reservoir in B-trend oil fields (GS 365, Sidki, East Zeit, and Hilal) was mapped by projecting from the deepest reliable seismic event (base of Miocene evaporites), using the intermediate isopach maps. This was based on Gebel El Zeit stratigraphy and structure. The Gebel El Zeit outcrop lying 10 km to the west was used to describe a geological model analogous to the fields; since a close correlation was found between structural style and Miocene stratigraphy in both the surface and the subsurface data sets. The implementation of this subsurface model results in the enhancing of the oil potential of the B-trend.
Aspects of the petroleum geology of the Junggar Basin, Northwest China
Authors: Lawrence S. R.
Published: April 2008

The Junggar Basin is one of a family of productive interior basins in Northwest China. These are generally referred to as ‘flexural’ or ‘orogenic’ basins nucleated by Variscan orogenic episodes. Their uniqueness is enhanced by later rejuvenation and reactivation in the successive orogenic phases affecting Central Asia.

The northwest margin of the Junggar Basin has been productive since the discovery of the Karamay-Wuerhe group of fields in the 1950s. The source rocks are generally considered to be Permian lacustrine sediments which provide mostly liquid hydrocarbons to basin margin alluvial fan reservoirs of Permian, Triassic and Jurassic age. The Karamay Thrust Belt, which forms the northwestern margin of the Junggar Basin, was initially formed during Variscan movements and re-activated during several subsequent structural episodes. According to KOC work there are many trap permutations associated with the Thrust Belt in ‘hanging-wall’ and ‘footwall’ situations.

Recent studies have focused on the southwestern part of the Junggar Basin. Here marginal production from Himalayan periclinal structures adjacent to the Tien Shan, dates back to the beginning of the century. More recently Carboniferous ‘basement’ rocks on the southward extension of the Karamay Thrust Belt, the Hong Che Fault Belt, have proved productive.

The stratigraphy shows the fairly continuous existence of lacustrine conditions in the mid basin since the Late Palaeozoic. Other studies have shown that ‘black mudstone’ conditions are prevalent in the Permian, Triassic, Jurassic and Early Tertiary and that oil-prone algal-rich intervals, indicating permanent or semi-permanent anoxia, are most likely in the Permian. The stratigraphic development also demonstrates the occurrence of suitable reservoir units in marginal alluvial fan-fluviatile clastics at most stratigraphic levels. The source-reservoir spatial relationships demonstrate favourable conditions for primary and secondary migration. The maturation history shows ‘deep’ Permian source rocks becoming mature for oil generation in the Triassic and for gas-generation from Cretaceous times. More gas-prone higher level source rocks generated hydrocarbons progressively from the Cretaceous through the Tertiary.

The structural history shows migration routes controlled by regional tilting into the Tien Shan foredeep, and local migration and entrapment by re-activation of the Karamay Thrust Belt along the northwestern basin margin, and of intra-basinal late Variscan transpressional trends. Late Himalayan structuring along the southern margin and basin-wide tilt has effected a phase of re-migration and further funnelling of hydrocarbons to the productive zones of the northwest.
The Cadomian orogeny and its influence on the Variscan evolution of western Europe
Authors: Cogné J.
Published: April 2008

The south-north zonation of the Variscan orogen in Western Europe, from Ligerian internal mobile zones in the south, to Hercynian external ensialic zones in the north, appears to be controlled by the north-south zonation in the older Cadomian orogenic belt which was developed in the northern margin of Gondwana. A juvenile crust (mostly younger than 650–700 Ma) was built up from Brioverian times onward in the Proto-Tethys domain between two Precambrian supercontinents. There are a few basement relicts (2000 Ma) preserved within this zone which have been derived from the Eburneantype massifs of Gondwana. The mobility and progressive consolidation of the Proto-Tethys zone are both directly linked to successive distensive and convergent tectonic episodes which characterize the geodynamic history of this zone and which results in the superposition of the Cadomian, Variscan and finally Alpine orogenies.
Early Ordovician shelly phosphorites of the Baltic Phosphate Basin
Authors: Ilyin A. V. , Heinsalu H. N.
Published: April 2008

The Baltic Phosphate Basin contains a condensed sequence of late Upper Cambrian-early Ordovician sediments. These consist mainly of mature quartz sandstones with debris of phosphatic shells of inarticulate brachiopods, notably
Schmidtites
and
Ungula
. The sequence also contains
Dictyonema
shales and glauconitic sandstones. A large-scale exploration project completed recently has delineated several new promising areas, including the Rakvere deposit in northeastern Estonia where resources amount to as much as 700 Mt of P
2
O
5
contained in easily beneficiated ores grading 10% P
2
O
5
. Palaeogeographically, the Baltic phosphate basin opened westwards into a large euxinic basin that occupied Sweden, southern and central Norway, and Denmark. Alum shales accumulated in the latter basin from early Middle Cambrian times. Further west, the basin joined the Iapetus Ocean.
The distribution of phosphatic facies in the Georgina, Wiso and Daly River Basins, Northern Australia
Authors: Howard Peter F.
Published: April 2008

The lithological logging and chemical testing of cuttings from some 600 water bores and Bureau of Mineral Resources stratigraphic holes, together with the modelling of aeromagnetic, gravity and elevation data, have defined a carbonate-siltstone-chert phosphatic lithofacies of Middle Cambrian age. This in turn has led to the recognition of two new phosphorite deposits, the deposition of which was related to the basement configuration and its depth. The phosphatic lithofacies, which have been followed entirely in the subsurface, occur as belts peripheral to and within the Georgina, Wiso and Daly River Basins. These phosphatic belts have an average width of 32 km, a thickness of 10 to 190 m, and have been traced over a distance of 2,100 km. Of the newly discovered phosphorite deposits, the Lady Judith in the Wiso Basin rests on volcanic rocks and interdigitates with carbonates of the Montejinni Limestone, while the Ammaroo belt in the southwestern portion of the Georgina Basin is contained within a depression bounded by limestones of the Arthur Creek Formation. The phosphatic sediments are believed to have been deposited primarily as an Ordian Middle Cambrian event in the west with a ‘younging’ transition through Ordian and/or Early Templetonian to Late Templetonian in the southeast. The basins are extensional, exhibiting a series of broad downwarps crossed by peripheral aulacogens, grabens, half grabens formed in the Late Proterozoic and modified subsequently by the development of plateaux, narrow horst blocks and adjacent deeps during the Middle Cambrian along basin-dividing arches. The basement to the shallow-water phosphatic lithofacies consists of Proterozoic sediment or Early Cambrian volcanic plateaux or peripheral sloping platforms which in the Brunette Sub-basin have present elevations of 0–300 m ASL.
Two decades of phosphorite investigations in India
Authors: Choudhuri R.
Published: April 2008

Phosphate rock has been known for many years in some of the sedimentary basins of India. Several discoveries in the 1960s and 1970s have proved of economic importance although the country continues to remain dependent upon imports for most of its phosphate requirements. Scope for further finds of phosphate rock have probably not been exhausted and a much more co-ordinated exploration programme is recommended. Proterozoic basins continue to offer the best exploration potential and currently account for the bulk of the known domestic phosphate rock resources. Nevertheless, geologically younger basins should not be discounted in terms of their phosphate potential.

Geological ages for the main phosphorite sequences in India are not well established and there is no precise stratigraphic correlation with Proterozoic-Cambrian phosphorite provinces in other parts of the world. Since the discovery of phosphorite beds of Palaeozoic age near Dehra Dun, Uttar Pradesh, and Jaisalmer, in southeastern Rajasthan, considerable progress has been made in both the exploration for and exploitation of phosphate deposits in India. One notable, promising development in recent years has been that associated with research, in India, on the beneficiation of low-grade and highly dolomitic phosphate rock.
Glacially-influenced sedimentation on the Hebridean slope, northwestern United Kingdom continental margin
Authors: Stoker M. S.
Published: April 2008

Pleistocene sediments on the northern Hebrides Slope form a localized, mounded slope-front deposit up to 200 m thick of limited downslope and alongslope extent. In seismic profiles, this slope-front deposit is characterized by a prograding, oblique-parallel reflection configuration. The bulk of the succession is characterized by three distinct depositional packages, each comprising an amalgamated sequence of mass flow deposits consisting primarily of debris flow diamicts with turbidite sands and muds. They represent re-sedimented glacimarine deposits. The mass flow packages are bounded by discrete, slope-wide, moderate-to high-amplitude reflectors. These reflections may be due to different degrees of consolidation of the sediments and/or the presence of thin, interbedded units of glacimarine dropstone mud. They indicate periods of reduced sediment supply to the slope. The slope facies interdigitate with a submarine end-moraine at the shelf-edge, and deposition probably occurred during a single glacial cycle. The restricted occurrence of the debris flows is thus attributed to a local source reflecting the shelf-edge position of an ice-sheet in this area at the time of sedimentation. Fluctuations in the sedimentation rate, independent of major climatic variation, suggest that the ice-sheet may have experienced alternate periods of basal melting and freezing. Ice-marginal processes have contributed significantly to the construction of the slope apron on the northern Hebrides Slope. During this period of sedimentation, the shelf-edge has prograded seaward, to the NW, by up to 5 km.
Brecciation and fracturing within neotectonic normal fault zones in the Aegean region
Authors: Stewart Iain S. , Hancock Paul L.
Published: April 2008

Neotectonic normal fault zones cutting bedrock carbonates within the Aegean extensional province contain contrasting breccia types that record different stages of fault zone evolution at shallow (< 500 m) crustal levels. Initial blind faulting at depth produces a near-surface shatter zone. Locally, blocks within the shatter zone become increasingly disorganized as slip planes advancing through the zone, create incohesive breccia belts ahead of propagating tips. Mineralization and frictional wear accompanying fault movement form narrow compact breccia sheets adjacent to slip planes. In some major fault zones, fault movement is concentrated along metre-wide zones of intensely deformed stylobreccia. When slip planes reach the free surface, post-seismic stress release initiates a new fracture network that is superimposed on and restricted to breccia belts and sheets.
The role of second phase in localizing deformation
Authors: Olgaard David L.
Published: April 2008

Small fractions of second phases may play a major role in localizing deformation in rocks. Second phases inhibit the migration of grain boundaries and hence may prevent grain growth. Since flow laws for many deformation mechanisms show a grain size dependence, second phases may indirectly control deformation rate. To illustrate this, a hypothetical sequence of micritic limestones, two impure layers sandwiched between two pure calcite layers, are metamorphosed and then deformed. The initial grain size is assumed to be less than 5 μm. The thermo-mechanical behavior of this sequence is predicted based on three types of experimental data: (1) calcite grain growth kinetics; (2) calcite grain size as a function of second phase; (3) plastic flow laws on natural and synthetic calcite rocks under conditions for grain-size insensitive (GSIC) and grain-size sensitive creep (GSSC). Extrapolations of the results predict that metamorphism at 400°C will increase the grain size in the pure calcite layers to 780 μm in 10 000 years. In a layer containing 5% of 0.3 μm particles, however, the grain size will increase to only 6 μm. Under a differential stress of 100 MPa, the coarser-grained layers deform by GSIC at a strain rate of 10
−14
s
−1
while the 6 μm layer deforms by GSSC at 10
−9
s
−1
. This large difference in deformation rate insures that virtually all of the strain will be localized in the fine-grained layer.
Ductile deformation mechanisms in micritic limestones naturally deformed at low temperatures (150–350°C)
Authors: Burkhard Martin
Published: April 2008

Deformation microstructures have been examined in a series of micritic limestone samples from the Helvetic nappes of western Switzerland, deformed at temperatures ranging from 150–350°C. Finite strain has been determined with
R
f
/φ-ϑ and centre—centre techniques applied to pellets and varies between
R
s
= 2 to 10 in samples in fold nappe interiors; much higher strains occur in samples from thrust planes. Optical micrographs from three ultrathin sections per sample and 200 to 500 grains per section were quantified by image analysis. Grain size is fairly constant from diagenesis through to anchizone (3–6 µm) and increases only within the epizone (6–10µm). Grain shape preferred orientations are weak and invariably much smaller than strain ratios
R
s
. Long axis orientations reflect the finite strain orientation. Roundness measurements are consistent with grains having relatively simple boundaries, frequently meeting in 120° triple junctions independently of temperature, finite strain or grain size. No crystallographic preferred orientation could be detected in the low temperature (up to 300°C) micrites despite large finite strains. Strong
c
-axis preferred orientation is found in one extremely finegrained (< 1 µm) low temperature (
T
< 180°C) faultrock. Some degree of preferred orientation is widespread in the coarser grained epizonal (
T
> 300°C) limestones which all show twinning. Grain Boundary Sliding (GBS) is inferred to be the dominant ductile deformation mechanism in moderately deformed micrites (
R
s
< 10) between 200–300°C. Coarsening of grains with increasing temperature by dynamic grain boundary migration recrystallization leads to a net increase in grain size above 300°C. This favours twinning instead of GBS, which in turn leads to pronounced
c
-axis fabrics.
Experimental and observational constraints on the mechanical behaviour in the toes of accretionary prisms
Authors: Karig Daniel E.
Published: April 2008

The mechanical behaviour of porous sediments in the toes of accretionary prisms can be analyzed by combining the principles of soil mechanics and the results of experimental sediment deformation with the in-situ physical properties observed from deep sea drilling. Porosity is the only physical property that has been extensively collected and even these data have generally been inadequately analysed. After correction for porosity rebound, variations due to lithology, and lack of representative sampling, porosity data must be converted from a spatial or Eulerian description to a Lagrangian description, which follows the behaviour of a given sediment element as it moves through the prism toe. Such an approach shows that sediment elements compact during deformation in prism toes; strongly in the Nankai prism and much less so in the Lesser Antilles. Compactive stress paths are ductile, and there is evidence that much of the deformation, at least in the Nankai toe, is of this type. Nevertheless, there is also a very strong component of brittle deformation, both on diffuse structures and along the major faults. This brittle overprint is explained in the light of experiments as showing that sediments already at a state of ductile failure will shear brittlely if post-failure deformation is associated with a decrease in σ
m
′. Such reduction of σ
m
′ probably occurs in prism toes during fluctuations of pore pressure. Persistent brittle faults are modelled as zones in which porosity and pore pressure are higher than in the surrounding sediments, a condition maintained by fluid channelled up the faults from deeper in the prism. Outward diffusion of fluid from the faults may be responsible for broad zones of brittle failure, characterized by scaly clay.
Structural implications of compactional strain caused by fault block rotation: evidence from two-dimensional numerical analogues
Authors: Iliffe J. E. , Lerche I. , Nakayama K.
Published: April 2008

The effects of compaction on tilted fault blocks are assessed using a two dimensional mathematical model which assumes homogeneous non-rigid blocks. Lithology is described by a depth-dependent porosity function.

Compaction is restricted to the upper parts of the hangingwalls of blocks and is greater in blocks bounded by sub-vertical faults rotated by 30° or more. The fault planes are unaffected.

Significant sub-vertical lines of maximum compaction are noted directly below block crests. Since secondary faults occur in similar orientations in rigid blocks, rotation in the Earth’s gravitational field is proposed as a mechanism for secondary faulting.

Fluid migration from the compressed hangingwall to the undeformed footwall has implications for shale diapir formation.
Alpine deformation on Naxos (Greece)
Authors: Urai J. L. , Schuiling R. D. , Jansen J. B. H.
Published: April 2008

A detailed structural study of Naxos (Attic-Cycladic massif, Greece) reveals two major deformation events. The first one is associated with large scale thrusting and high-pressure-low-temperature metamorphism during an early Alpine subduction episode. The second event occurred during continental extension and the associated development of localized thermal domes, where lower crustal rocks were brought into contact with upper crustal units along a major shallow dipping shear zone. We agree with a model of Naxos as a Cordilleran type Metamorphic Core Complex. However, our observations show that the sense of shear was ‘upper plate moving North’ during the second event, calling for a reinterpretation of existing tectonic models of the Cyclades.
Timing, style and sedimentary evolution of Late Palaeozoic-Mesozoic extensional basins of East Greenland
Authors: Surlyk Finn
Published: April 2008

The late Palaeozoic-Mesozoic sedimentary basins of East Greenland record a complex series of events which eventually led to the successful opening of the Norwegian-Greenland Sea in the late Palaeocene. Major tectonic events and regional sea-level changes caused by plate movements and reorganizations, overprinted by the effects of local tectonics and associated sea-level changes are reflected in thick, unconformity bounded sequences. Caledonian crustal shortening and thickening culminated in the Silurian, and was succeeded by extensional collapse, vulcanism and intrusion of post-tectonic granites in the Devonian. Large, transtensional pull-apart basins were formed and became filled with continental red beds. Basin margins underwent repeated episodes of thrusting. A change to rifting took place in the latest Devonian. Rapid subsidence and continental deposition punctuated by episodes of igneous activity and contractional deformation continued from the Devonian until early Permian times for about 120 Ma leaving a record of alluvial fan and flood plain siliciclastics alternating with rhyolitic and alkaline basaltic intrusions and extrusions. Later basin subsidence reflected thermal cooling and contraction following the prolonged late Palaeozoic period of crustal thinning. Important phases of block faulting occurred at several intervals during Mesozoic times climaxing in the Volgian-Valanginian interval. Cenozoic basin inversions resulted in dramatic decoupled uplift and tilting of large blocks. The degree of Devonian extensional collapse, the position and orientation of Caledonian thrust planes, and the localization of mid- and late Palaeozoic transtensional strike-slip basins exerted a profound control on localization and style of the compartmentalized Mesozoic rift. The importance of pre-Mesozoic history, timing of Mesozoic tectonic events and the interplay of tectonism, and rates of subsidence, sea-level change and sediment influx have close parallels in the North Sea area. Examples from the exposed Mesozoic succession of East Greenland may thus serve as excellent predictive guides for subsurface geology around the British Isles.
The significance of normal faults in the exploration and production of North Sea hydrocarbons
Authors: Hardman R. F. P. , Booth J. E.
Published: April 2008

The Mesozoic geological history of the northern North Sea was dominated by extensional faulting. In addition to forming the most important hydrocarbon traps, such faults can be shown to exert a significant degree of control over the distribution of mature source rocks, and the deposition and erosion of reservoir rocks. As exploration and production have progressed the importance of small normal faults has been increasingly recognized. Although they do not totally offset reservoir formations, such small faults may provide lateral seals, thus disrupting hydrocarbon migration routes, forming subtle traps and causing technical problems during production by unanticipated compartmentalization of fields. Consequently, as the commercial size of fields is reduced by improving production technology and the North Sea matures as a hydrocarbon province, a thorough understanding of the origins and characteristics of normal faults as well as a command of the increasingly sophisticated techniques for analysing the behaviour of these faults will be vital to successful exploration programmes.
Mechanisms generating normal fault curvature: a review illustrated by physical models
Authors: Vendeville Bruno
Published: April 2008

This paper briefly reviews the mechanisms generating fault curvature during extension, illustrates them using physical models, and discusses their applicability at different scales and for specific extensional styles. These mechanisms are in two groups. In the first, initially curved fault planes are essentially of mechanical origin. They commonly result from marked vertical changes in stress orientation and rheology within the faulted layer. Experiments suggest that such mechanisms are more likely to influence secondary faults that form where deformation and displacements are severely constrained. The second group includes faults that become curved after they form, either by volume loss due to compaction, by internal deformation of fault blocks, or by interacting sedimentation and faulting. Fault curvature due to compaction is mainly restricted to growth-faulted areas. Internal deformation of blocks affects fault shapes at the transition zone between continuous and discontinuous deformation. Block rotation during rapid deposition can lead to strongly curved fault profiles. Blocks or faults are distorted during block rotation to maintain geometric compatibility within and between the blocks. This review suggests that normal growth faults in deltaic areas are more likely to be listric than crustal-scale normal faults.
Provenance research: Torridonian and Wealden
Authors: Allen P.
Published: April 2008

Torridonian regional flow-directions are indicated unequivocally by sediment geometries and palaeocurrent structures. In the Wealden more reliance has to be placed on textural and compositional gradients. Flow directions in both guide the search for sourcemassif ‘stumps’: uncovered, covered or inferred. ‘Putting back’ the detritus on to the stumps leads to broad palaeogeographies; but erosional histories, recycling, basin budgets and changes in basinal configuration are major uncertainties. Diagenetic interference with the evidence is minimal. Overlapping opposing flows are consistent with extensional tectonic regimes.

Torridonian waste provides evidence on the timing of mantle contributions to the Laurentian crust. Refreshingly new Hercynian petrographies in the Wealden may bear on a similar problem in European Gondwana.

Single-grain dating (perhaps to include quartz) and rapid and ultra-precise geochemical and isotopic analysis (including whole-rock) make the prospects for provenance research exciting. Tourmaline, zircon and glauconite have considerable potential for dating and justify fuller investigation of their isotopic and mineral chemistries.
On the use of turbidite arenites in provenance studies: critical remarks
Authors: Zuffa Gian Gaspare
Published: April 2008

When we determine framework petrography of deep-sea arenites for provenance studies we should concentrate not only on what particles make up sedimentary beds but also when, where and how they form. Taking such information into account, grains can be classified according to their temporal, spatial and compositional characteristics. This will allow a better understanding of palaeogeography and palaeotectonics of the basin/source system. Two examples from the literature that support this approach are reported.
Organic carbon flux and development of oxygen deficiency on the modern Benguela continential shelf south of 22°S: spatial and temporal variability
Authors: Bailey Geoffrey W.
Published: April 2008

Anoxia in continental shelf waters has previously been ascribed to lack of circulation, advection from a remote source and in situ formation. It appears that in situ formation resulting from degradation of organic-rich material derived from phytoplankton blooms is generally the controlling factor in coastal upwelling ecosystems such as the Benguela system. Here, spatial variation in accumulation of organic-rich shelf sediments and associated anoxia is closely related to the degree to which primary production is enhanced at sites ‘downstream’ of coastal upwelling centres. Consequently, at sites where there is strong seasonal variation in the input of recently upwelled water, there is temporal variability in the incidence of oxygen-deficient waters overlying the shelf and anoxic sediments.

The extent of phytoplankton blooms is also dependent on coupling between the benthic and pelagic zones and physical characteristics of the water column. This is supported by observations of phytoplankton abundance, sedimentation of particulate organic matter and benthic oxygen deficiency in the Benguela upwelling system, where the continental shelf is marked by oxygen-deficient bottom waters and anoxic sediments equatorward of three of the major upwelling centres where such coupling occurs.

This work reaffirms the view that primary production rather than anoxia exerts direct control on the accumulation of organic-rich sediments.
The dysaerobic zone revisited: a magnetic facies?
Authors: Rhoads Donald C. , Mulsow Sandor G. , Gutschick Raymond , Baldwin Christopher T. , Stolz John F.
Published: April 2008

The original dysaerobic zone concept defined a unique assemblage of benthic organisms and sedimentary fabrics associated with dissolved oxygen concentrations ranging between 0.3 and 1.0 ml/l. Subsequent studies of the Santa Barbara Basin and oxygen minimum zone (OMZ) off the California coast suggest that this facies is bounded by dissolved oxygen in the range 0.1 to 0.5 ml/l.

This range of dissolved oxygen is optimal for the growth of magnetotactic bacteria. We present a hypothesis predicting that the dysaerobic zone should be associated with a high inventory of biogenic magnetite and that this magnetite can be used as a palaeo-oxygen indicator.

We have tested this hypothesis with a magnetic analysis of samples from aerobic, anaerobic, and dysaerobic sediments taken off the Peruvian coast. The mean magnetic susceptibility of the dysaerobic zone is 3 to 5 times higher than the aerobic and anaerobic parts of the Peruvian oxygen minimum zone (OMZ). Micrographs (TEM) of these sediment samples indicate that only the dysaerobic zone contains appreciable amounts of fine-grained magnetite.