bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrocarbon generation, migration, alteration, entrapment and mixing in the Central and Northern North Sea
Authors: Barnard P. C. , Bastow M. A.
Published: April 2008

There is little doubt that sediments of Upper Jurassic to lowermost Lower Cretaceous age, particularly those of the Kimmeridge Clay Formation and its equivalents, are the source of the vast bulk of the oil in the Central and Northern North Sea. This is true both of oil in conventional Mesozoic plays and in Tertiary clastic reservoirs. The timing of oil generation and migration ranges from the Late Cretaceous through to the present.

The oil productivity of the Kimmeridge Clay Formation in the Central Graben, Moray Firth and South Viking, Graben areas of the North Sea is estimated to be in excess of 250 billion barrels, of which about 25% is accounted for in accumulations discovered to date. There is strong areal differentiation between oil trapped in Mesozoic reservoirs as opposed to Upper Cretaceous Chalk and Tertiary reservoirs.

This paper is particularly concerned with hydrocarbons in Tertiary reservoirs, which have a wide range of compositions and appear to have complex accumulation and degradation histories. Migration to Chalk and younger Tertiary reservoirs occurs often through thick shale sequences. The evidence for the timing and mechanism of this process is considered and a preferred model for the vertical migration and subsequent lateral migration, mixing and degradation is proposed.
Contrasting characteristics attributed to migration observed in petroleums reservoired in clastic and carbonate sequences in the Gulf of Mexico region
Authors: Thompson K. F. M.
Published: April 2008

Systematic differences among two subsets of petroleums selected geographically and by reservoir type were attributed to lithologically-characteristic migration phenomena. Most oils in offshore Louisiana Tertiary clastic reservoirs are deficient in light ends, evidently because of evaporative fractionation. Residual oils are enriched in cycloalkanes and aromatic hydrocarbons. Spuriously low evident maturities based upon concepts of paraffinicity are observed. Numerous oils are both fractionated and biodegraded. Mesozoic carbonate-sealed reservoirs at the margins of the basin generally contain pristine oils with intact light ends. However, many oils appear to be depleted in preferentially water soluble compounds: benzene, toluene, light cycloalkanes and phenanthrene, evidently due to aqueous fractionation during intraformational migration.
Petroleum seepage and post-accumulation migration
Authors: Clarke R. H. , Cleverly R. W.
Published: April 2008

The processes governing the loss of petroleum from traps and its migration to the surface are poorly understood. Leakage from traps does not necessarily result in surface seeps, nor are all occurrences of surface petroleum the result of present-day trap leakage. Three types of seep are recognized: unaltered gassy seeps, seeps affected by surface dispersal processes, and seeps altered by subsurface processes. Unaltered seeps form about 30% of the recorded population and are characterized by the presence of a free gas component. The buoyancy of the gas phase appears to drive most seeps to the surface. Non-associated oil seeps, often sharing their point of efflux with spring water, are altered by near-surface processes, especially the segregation of the oil and gas phases at the water table. Seeps altered by subsurface processes are frequently sour, containing free hydrogen sulphide, and are often associated with leakage pathway mineralisation. The geological context and characteristics of seepage deserve attention, leading to more realistic models for oilfield growth and decay, and to a better understanding of the environmental and mineralising effects of seepage.
A review of platform sedimentation in the Early and Late Permian of Oman, with particular reference to the Oman Mountains
Authors: Lee C. W.
Published: April 2008

The Permian of Oman consists of two distinct portions. An early Permian sequence of clastics and carbonates, that are conformable with the Carboniferous, and a late Permian sequence of carbonates that may rest with strong unconformity on the underlying sequence. Lower Permian clastic rocks crop out in S and E Central Oman and they also occur extensively in the subcrop of South and Central Oman. They show a glacial affinity and are correlated with the Dwyka glaciogenic deposits of Gondwanaland. Lower Permian marine carbonates are also found in the Haushi region of E Central Oman and from the Batain coast where they are known as the Lusaba Limestone and Qarari Limestone, respectively. The Upper Permian carbonates are found extensively throughout the Oman Mountains where they constitute the Bih, Hagil and Saiq Formations, the Khuff Formation in the subcrop and as blocks in the Batain Melange of NE Oman. Permian carbonates are also found as ‘exotics’ masses in the higher thrust slices (Haybi Complex) beneath the Semail Ophiolite. These carbonates were deposited on block faulted crust by a major transgression (enhanced by a post-glacial rise in sea level) caused by the onset of continental rifting. The rocks which characterize the autochthonous (parautochthonous) portion of Oman show affinities with Gondwanaland or the southern side of Tethys, while the faunas collected from the Batain coast area of NE Oman have affinities with Timor and possibly the northern side of Tethys in Early Permian times.
Chronostratigraphy of the Cretaceous carbonate shelf, southeastern Arabia
Authors: Scott R. W.
Published: April 2008

Carbonate deposits dominated the southeastern part of the Arabian Shelf from Berriasian to Santonian time, approximately 60 Ma. Deposition was terminated by the development of the Oman Foredeep in Coniacian-Santonian following the initial emplacement of the Hawasina and Semail nappes in the Turonian. The Lower and middle Cretaceous units in Arabia correlate with European reference sections and the current ages of the stage boundaries can be used to form a quantitative scale.

The Thamama-Kahmah Group to the Ilam-Halul Limestone consists of four major unconformity-bounded, shoaling-up depositional sequences that range in duration from 3 to 22 Ma. These sequences represent significant transgressive events on the shelf. The Rayda-Salil-Habshan sequence represents drowning and extensive incursion of the deep shelf landward; it lasted about 13 Ma. Small-scale, possibly climatic cycles comprise the Salil and Rayda formations. The contact with the overlying Lekhwair-Kharaib-Shuaiba sequence is normally sharp with no evidence of erosion. This sequence lasted about 18 Ma and consists of numerous shoaling-up cycles and local disconformities. The unconformity between the Shuaiba and Nahr Umr is a regional disconformity with evidence of local exposure. The hiatus spans most of the Late Aptian in the Oman Mountains, and in Abu Dhabi it spans the youngest part of the Late Aptian.

The Nahr Umr-Natih sequence in Oman spanned about 22 Ma and consists of four shoaling-up cycles. An important drowning event in the Oman Mountains occurred during the earliest Middle Cenomanian and is represented by the submarine hardground at the top of the ‘E’ member. The youngest sequence consists of the Laffan-Ilam formations in the Khatiyah Basin and lasted about 3 Ma. These depositional sequences correspond to transgressive events on other plates and represent eustatic sea level changes. The onlapping unconformities in the Oman Mountains represent times when the peripheral swell was active.

Restricted, intraplatform basins, in which organic-rich muds were deposited, developed four times. These units are best developed in the Bab-Khatiyah Basin and locally extended eastward into Oman. The Bab Member of the Shuaiba Formation is mainly Late Aptian. The Khatiyah Formation is coeval with the Early Cenomanian, lower ‘E’ member of the Natih in Oman. The ‘B’ member of the Natih is Late Cenomanian in age. The Laffan Formation is mainly Coniacian in age and represents the final restricted basin upon the southeastern Arabian Shelf.
Geochemical stratigraphy and porosity controls in Cretaceous carbonates near the Oman Mountains
Authors: Wagner P. D.
Published: April 2008

Geochemical stratigraphy is an investigative tool useful (a) for discerning the mode and spatial extent of diagenetic processes in carbonates, (b) for making diagenetic correlations between localities, and (c) for inferring palaeotopography. δ
13
C and δ
18
O stratigraphies of a five-well transect across central Oman indicate that most of the Cretaceous U. Kahmah-Natih interval stabilized under semiclosed, shallow burial, marine-water-like conditions. A ‘depositional’ δ
13
C isotopic signal is preserved. Local anoxic-suboxic events are also recognized, as well as depth-limited effects of early meteoric diagenesis. Isotopic stratigraphic relationships clearly indicate the presence of a local palaeotopographic high in central Oman during deposition of the Natih E member. Good diagenetic-stratigraphic correlations are shown between locations in central Oman and Sharjah, a distance of about 300 miles.

Porosity type and distribution within Cretaceous carbonates is related both to early diagenesis and to later structural events. Early cementation associated with marine hardgrounds and subaerial exposure surfaces occluded primary porosity in a limited depth-thickness sense. Later, all existing porosity types were regionally destroyed below the Hawasina thrust sheet in central Oman by thrusting-induced burial stylolitization and associated cementation. Foreland basin carbonates not involved in nappe emplacement retained excellent reservoir properties. Porosity in foreland basin wells consists of primary macro- and micro-porosity, and secondary selective-leach microporosity; selective leaching fluids moved along the subsurface ‘plumbing system’ of the primary porosity-permeability network. Small volumes of pore space were formed in low porosity subthrust carbonates by late aggressive (meteoric?) leaching along fractures and stylolites. Rock texture (facies) did not play a major role in defining the distribution of porosity in the immediate study area.
Mesozoic carbonate slope facies marking the Arabian platform margin in Oman: depositional history, morphology and palaeogeography
Authors: Watts K. F.
Published: April 2008

Carbonate slope sediments of the Sumeini Group mark the northeastern margin of the Arabian carbonate platform. Most slope sediments exposed in the Oman Mountains are of Jurassic or Cretaceous age. Older slope deposits at Jebel Sumeini indicate establishment of the Oman continental margin and subsidence accompanied by faulting in the Early Triassic (Dienerian), Middle Triassic (Ladinian) and possibly earlier (Permian).

Widespread Jurassic slope deposits indicate significant variations in declivity and facing direction of the continental margin slope in Oman. At Jebel Sumeini, a gullied bypass slope formed an apron along the NNW-trending slope, previously established as a Triassic block-faulted escarpment. In the Dibba Zone, a steep bypass slope formed along an inactive, NE-trending, transform lineament. Farther south near Wadi Qumayrah, a Middle to Late Jurassic base-of-slope apron containing megabreccias formed along a possible NNW-trending fault scarp. East of Wadi Qumayrah, an open-end embayment in the margin separated this area from coarse-grained slope apron deposits at Jebel Sham. Carbonate slope deposits at Saih Hatat indicate a complex history of subsidence culminating in Jurassic platform drowning and development of a long-lived submerged plateau bounded by gently inclined ramp slopes. This general geometry of the Oman passive continental margin was maintained through Jurassic and Early Cretaceous time.

Major changes in platform margin sedimentation occurred in the Late Cretaceous (Cenomanian to Coniacian), as recorded by the upper Mayhah Formation and overlying siliceous Qumayrah Formation. Thick Cenomanian(?) to Coniacian syn-orogenic megabreccias associated with pelagic sediments formed due to collapse of oversteepened slopes, possibly resulting from tectonic downbowing of the Oman margin toward a northward-dipping oceanic subduction zone. Later, the geometry of the passive continental margin may have had a significant effect on the emplacement of nappes and formation of the Sumeini culminations during the structural development of the Oman mountain belt.
Sedimentary evolution and palaeogeographical reconstruction of the Mesozoic continental rise in Oman: evidence from the Hamrat Duru Group
Authors: Cooper D. J. W.
Published: April 2008

The mainly Mesozoic Hamrat Duru Group represents deep-water, primarily continental rise Neo-Tethyan sediments derived from the Oman continental margin and later emplaced onto the Arabian carbonate platform during the Late Cretaceous obduction of the Semail ophiolite.

Sediment patterns reflect fluctuations in sea level relative to the platform, with carbonate input dominant during periods of platform stability. High and low sea-level stands, controlled by eustacy or tectonism, were accompanied by significant reductions of carbonate production and input into the basins. Raised calcite compensation depths associated with high sea levels resulted in the deposition of cherts whereas, during low stands, terrigenous clastics prograded across or by-passed the platform into the Hawasina basin.

The Hawasina basin was divided into two sedimentary basins separated by a submarine high. The shale-rich ‘Al Ayn sub-basin’ contained coarse-grained sediments that formed either small fans, or laterally coalescing sediment bodies that were fed by multi-source input of carbonate and terrigenous clastics. This is preserved as the Hamrat Duru Group of the northern and central Oman Mountains. The carbonate-rich ‘Duru sub-basin’ was fed exclusively from the south and large sediment bodies with sheet-like morphologies extended northwards parallel to the margin, outboard from the Al Ayn sub-basin, with thinning and fining trends to the NW and NE. Duru basin units are found in the central and southern Oman Mountains. Contrasting sedimentation patterns in the Al Ayn subbasin and the Duru sub-basin may reflect the influence of prevailing SE trade winds on the margin.
Permian to Cretaceous radiolarian biostratigraphic data from the Hawasina Complex, Oman Mountains
Authors: De Weaver P. , Bourdillon-de Grissac Ch. , Bechennec F.
Published: April 2008

The analysis of foraminifera and radiolaria in 3000 carbonate and 150 siliceous rock samples from the Oman Mountains results in revision of the existing stratigraphy and supports the definition of new units. In the Hawasina allochthonous unit the main results are as follows: (i) Permian bedded chert exists near the base of the sequence and is thus the first Permian bedded chert occurrence reported from the Tethyan region); (ii) an important volcanic event is dated as Triassic; (iii) bedded chert horizons are dated as Liassic; (iv) the thick turbiditic sequence has been divided into several units of Middle and Late Jurassic age. In the Samail ophiolite: (i) the ages of four tectonic episodes were established from the beginning of oceanic spreading (Albian—Early Cenomanian), to the obduction of the ophiolitic nappe (Campanian).
Subduction and obduction: two stages in the Eo-Alpine tectonometamorphic evolution of the Oman Mountains
Authors: Le Métour J. , Rabu D. , Tegyey M. , Béchennec F. , Beurrier M. , Villey M.
Published: April 2008

The platform unit of the Oman Mountains is composed of shelf sedimentary and volcanic rocks, ranging in age from late Proterozoic to late Turonian-Coniacian, that were deformed mainly during the polyphase Eo-Alpine tectogenesis.

In the late Turonian-Coniacian tangential shearing deformation with NE (oceanward) vergence occurred under HP/LT conditions of regional metamorphism. This tectonic phase was largely responsible for the structural and metamorphic zonation of the platform unit. A SW-NE gradient from external to internal zones has been recognized from the south flank of Jabal Akhdar, where fracture cleavage appears, to the eastern part of Saih Hatat, where schistose glaucophane ecologites are preserved. This tectonometamorphic event was related to partial subduction of the northeast corner of the Arabian platform beneath the neo-Tethyan oceanic lithosphere.

A later phase of higher-level shearing deformation with SW (continentward) vergence, during the Campanian, accompanied the thrusting of the Hawasina Nappes and the obduction of the Semail Ophiolite. These tectonic units were transported SSW respectively from the Permian-Cretaceous cover of the Arabian continental margin and from the oceanic floor of the Semail marginal basin that opened during the Albian-Cenomanian. Tectonic slicing and strike-slip faulting within the platform unit, on the northern margins of Jabal Akhdar and Saih Hatat, formed frontal and lateral ramps during the advance of the nappes onto the Arabian Platform.

During a third phase of deformation, probably during the Campanian-Maastrichtian, large-amplitude, long-wavelength, open, near-upright folding accompanied by poorly developed, subvertical, SSW-NNE fracture cleavage indicates moderate shortening during uplift perpendicular to the earlier structures.

The resulting Eo-Alpine foreland belt of the Oman Mountains was partly covered by marine carbonate deposits during the middle Maastrichtian to late Maastrichtian or early Palaeogene before being affected by Alpine tectonics during the Miocene.
Structure of the Jebel Sumeini-Jebel Ghawil area, Northern Oman
Authors: Searle M. P. , Cooper D. J. W. , Watts K. F.
Published: April 2008

Detailed structural mapping of the Sumeini half-window, Jebel Sumeini and Jebel Ghawil areas in northern Oman, shows that the dominant Late Cretaceous thrusting was a WSW-propagating piggy-back sequence modified by late-stage, deeper level thrusts in the Sumeini slope facies carbonates which breached up into the overlying Hamrat Duru thrust sheets. The uppermost Semail thrust carrying the complete ophiolite sequence overlaps lower Haybi thrust sheets and inverted metamorphic isograds are displayed in the footwall. Amphibolites and greenschists of the metamorphic sole have been subsequently imbricated with Haybi alkaline and tholeiitic volcanics, Triassic Oman exotic limestones and melanges of the Haybi duplex. A major E-W trending lateral ramp is inferred along the southern edge of Jebel Ghawil downthrowing the Sharm peridotite block up to 1000 m to the south. The basal Hamrat Duru thrust overlies Cretaceous Mayhah Formation slope facies carbonates and has been folded around the lower, later culminations primarily of Late Cretaceous age, enhanced by further structural growth and short-distance post-Maastrichtian (? Eocene) thrusting. Thrusts at the base of the Sumeini Group have breached through the overlying Hamrat Duru sheets and locally overthrust Maastrichtian Faiyah Formation limetones along the SW margin of Jebel Wasa. Culmination of Jebel Sumeini promoted hanging-wall drop fault development along the trailing (NE) edge of the Sumeini duplex, down-faulting Hamrat Duru rocks to the east. The amount of displacement of the Jebel Sumeini culmination increases towards the south, toward the axis of the Sumeini half-window.
Tectonics, geochronology and geochemistry of the Precambrian rocks of Oman
Authors: Gass I. G. , Ries A. C. , Shackleton R. M. , Smewing J. D.
Published: April 2008

precambrian rocks crop out in NE Oman (Qalhat and Jebel Ja’alan), in SE Oman (near Mirbat) and offshore on the Kuria Muria Islands. Similar rocks occur on Socotra and Abd el Kuri. In all these localities the Precambrian rocks include metasediments of greenschist or amphibolite facies, intruded by dolerites, granodiorites and granites and cut by doleritic and felsitic dykes which in the Mirbat area, form a dense swarm. Foliation trends vary but are predominantly ENE-WSW. Geochronological data show that the rocks are late Proterozoic (800-600 Ma) in age and are therefore chronologically and compositionally similar to the Pan-African rocks of the Afro-Arabian Shield. Unmetamorphosed, weakly deformed Infracambrian sediments in the Huqf-Haushi Uplift and in the Saih Hatat and Jebel Akhdar windows are not cut by the dyke swarms, which are therefore probably late Proterozoic in age although radiometric dating suggests a lower Palaeozoic age. These widely distributed late Proterozoic metamorphic rocks indicate that the middle Proterozoic (
c.
1700 Ma) gneisses in the Afif Terrain in eastern Saudi Arabia do not mark the eastern limit of the Pan-African belt. Before the breakup of Gondwanaland that limit must have lain between the Oman-Somalia region and the Archaean and middle Proterozoic terranes of the Bundelkhand and Aravalli areas of India.
Intraplatformal basin-fill deposits from the Infracambrian Huqf Group, east Central Oman
Authors: Wright V. P. , Ries A. C. , Munn S. G.
Published: April 2008

The Infracambrian Huqf Group, well exposed in the Huqf-Haushi area of Central Oman, is an alternating sequence of clastic (Abu Mahara and Shuram Formations) and carbonate (Khufai and Buah Formations) sequences overlain, in the subsurface, by the evaporitic Ara Formation. The Khufai and Buah Formations are dolomites whose lowest members have previously been interpreted as lagoonal deposits; they are here reinterpreted as sub-wave base deposits which were probably formed in an intraplatformal basin. The upper parts of both formations consist of shoal and back shoal (peritidal) deposits with evaporites. The intervening Shuram Formation is a mixed siliclastic-carbonate unit deposited in an extensive shallow wave and tidally-influenced setting.
Carbonate/evaporite deposition in the Late Precambrian — Early Cambrian Ara Formation of Southern Oman
Authors: Mattes B. W. , Conway Morris S.
Published: April 2008

The Ara Formation of southern Oman is a geological rarity — a late Proterozoic sedimentary sequence which contains rich hydrocarbon source rocks and large accumulations of oil. It consists of a cyclic sequence of carbonates and evaporites deposited in a restricted basin. The source rocks were deposited in relatively deep, anoxic parts of the basin; reservoir rocks, most of them dolomitic, were deposited in shallow water on adjacent shelf or ramp areas. Since the sedimentary sequence overlying the Ara Formation has been truncated by numerous unconformities, the Ara source rocks have never been buried very deeply and have had a long history of oil generation in many areas, they are still within the oil window. Thick evaporites provide a regional seal for the intraformational trapping of hydrocarbons. In addition, post-depositional basin inversion led to structural elevation of what had been the deeper parts of the basin along a broad NW dipping flank. Episodic down-dip salt removal along this flank allowed Ara oil to bypass the salt seal and to charge overlying reservoirs ranging in age from Cambrian to Tertiary.

The Ara Formation represents at least five third-order tectono-eustatic cycles of carbonate to evaporite sedimentation. Carbonates formed during periods of relatively high sea level; evaporites during sea level low stands, compounded by evaporative drawdown. The carbonates were deposited in a variety of marine to mesohaline environments; in addition to an abundance of cyanobacterial remains and framestone fabrics, they contain one of the earliest shelly faunas in the geologic record.
Chemical transects across intra-oceanic arcs: implications for the tectonic setting of ophiolites
Authors: Taylor Rex N. , Murton Bramley J. , Nesbitt Robert W.
Published: April 2008

Systematic spatial variations in Ti, Y, and Zr are found in volcanic sequences sampled as transects from back-arc, through arc, to trench within the western Pacific intra-oceanic arc systems. These correlations occur regardless of eruption age, reflecting the retention of relative positions between individual tectonic units. Concentrations of these incompatible elements are taken to reflect the nature of the underlying mantle at a particular trench-volcano distance during the subduction process. Volcanics become more depleted trenchwards, implying that progressively shallower levels melts were extracted from more refractory or harzburgitic mantle. A model is proposed whereby a subduction system is superimposed on a compositionally stratified mantle, which is a product of an earlier spreading event. On the basis of this evidence, the trench-distance variation model is applied to the Troodos ophiolite complex. Similar spatial changes in lava chemistry are found across the ophiolite, providing evidence for the relative position of the Troodos trench and direction of subduction beneath the Troodos basin. These findings have implications for studies in other orogenic terrains where structure and exposure permit their application.
Petrology and geochemistry of the plutonic rocks of the Halmahera ophiolite, eastern Indonesia, an analogue of modern oceanic forearcs
Authors: Ballantyne Paul
Published: April 2008

Ophiolitic rocks form the basement of the eastern part of the island of Halmahera in eastern Indonesia. The ophiolite is tectonically dismembered but all the elements of a complete ophiolite are present with the uncertain exception of a sheeted dyke complex. The ophiolite was formed in a supra-subduction zone setting before the Late Cretaceous and is interpreted to represent the forearc of a Mesozoic arc whose remnants are now found within and close to the margins of the Philippine Sea Plate. The plutonic rocks of the ophiolite include periotites interpreted as a ‘mantle sequence’ and cumulates interpreted as the deeper parts of an ophiolitic crustal sequence. The mantle sequence includes subordinate lherzolites of relatively enriched chemistry which represent upper mantle previously depleted by the extraction of MORB. A high degree of melting of the lherzolites under hydrous conditions left a mantle residue of depleted harzburgites, which are more refractory than the harzburgites of Troodos and Oman. Abundant cumulates which crystallized from the magma include peridotites and gabbroic rocks rich in orthopyroxene. The chemistry and mineralogy of the cumulates indicate open-system crystallization from a high Si, Mg, low Ti, LREE-depleted magma. Highly calcic plagioclase characteristic of island-arc plutons and found in the cumulate sequence of the Troodos ophiolite is not found in the Halmahera cumulates. The Halmahera ophiolite is interpreted as the onland analogue of the present-day Mariana forearc. Rocks dredged from the Mariana Trench, which like Halmahera is also situated at the margin of the Philippine Sea Plate, are remarkably similar to those of the Halmahera ophiolite.
Mobility of rare earth elements in zones of intense hydrothermal alteration in the Pindos ophiolite, Greece
Authors: Valsami E. , Cann J. R.
Published: April 2008

During the formation of the Jurassic Pindos ophiolite of northwest Greece, a convective seawater-fed hydrothermal system was initiated at or near the spreading centre. As a result, typical ophiolitic background metamorphic assemblages developed during the downward circulation of seawater, and epidosites and stockworks formed in the upwelling part of the system. The epidosites were analysed for major and trace elements including the REE. Substantial chemical changes occur during the evolution of the epidosites from their protoliths. While some elements (Ti, Zr, Y, Nb) appear immobile during this transformation, when a bulk quartz addition is taken into account, the REEs are mobile under these alteration conditions. Analyses show: (a) either enrichment of all the LREEs; or (b) ‘anomalous’ selective enrichment of certain elements (La, Pr, Eu) and depletion of others (Ce, Sm). These patterns can best be explained by: (a) greater mobility in the bulk of the alteration zone of the LREEs compared to the HREEs; (b) variable properties of Ce and Eu due to changes in their valence state; (c) variable water/rock ratio in epidosite zones; (d) heterogeneous permeability of reaction zones; (e) control of the rock alteration mineralogy on the REE distribution. Evidence for the mobility of the REE is also provided by the analyses of hydrothermal solutions from black smokers at 13°N and 21°N on the East Pacific Rise. REE can be very good indicators of the degree of water/rock interaction at high water/rock ratios.
Burial histories and hydrocarbon source rocks on the North West Seaboard
Authors: Parnell J.
Published: April 2008

The basins on the North West Seaboard of the British Isles had widely differing burial histories and consequently contain different successions. Basins active in the Permo-Triassic saw early maturation of Carboniferous source rocks, but Jurassic sections remain immature. Basins active in the Cretaceous — Tertiary saw later maturation of the Carboniferous, followed by maturation of the Jurassic. Potential source rocks range from Ordovician to Tertiary age, but Carboniferous source rocks are probably the most widely distributed. Samples from onshore Carboniferous outcrops in Northern Ireland and western Scotland give good pyrolysis yields, with a potential for gas and limited quantities of oil. The Jurassic rocks of the Hebrides include bitumen-impregnated sandstones where Jurassic shales have achieved maturation in the vicinity of Tertiary intrusions. The North West Irish Basin shows evidence for oil migration in Carboniferous rocks and the burial history suggests that gas may have been generated from the Carboniferous in eastern Ulster where the succession is analogous to that in the Morecambe Gas Field.
Late Triassic to Middle Jurassic stratigraphy, palaeogeography and tectonics west of the British Isles
Authors: Morton Nicol
Published: April 2008

The Upper Triassic to Middle Jurassic stratigraphy of the Atlantic margin basins west of the British Isles (including Celtic Sea, Porcupine, Slyne, Erris, Hebrides, and west of Shetland) is summarized in terms of six major genetic sequences, defined as packages of conformable strata separated by surfaces of abrupt major facies change. They indicate common tectonic histories and are interpreted as representing episodes of evolution comprising extensional phases with laterally variable sequences (1A Late Triassic to Early Sinemurian; 2A latest Toarcian to Late Bajocian), thermal and loading sag phases (1B mid-Sinemurian to earliest Toarcian; 2B Late Bajocian to Bathonian) and stabilization phases with sequences of reduced thicknesses and hiatuses (1C Toarcian; 2C Late Bathonian to Callovian).

The basins west of Shetlands, north of the Wyville-Thomson Ridge, have a different history, especially in the Early Jurassic. Similarly, basins south of an extension of the Clare Lineament (south of Celtic Sea and Porcupine Basins) differ in having limited Middle Jurassic subsidence. These transverse elements divide the Atlantic basin system into local subsystems, each of which has a different tectonic history.
Petrography and diagenesis of the Permo-Triassic of Scotland
Authors: McKeever Patrick J.
Published: April 2008

Sediments considered to be of Permian or Permo-Triassic age are exposed in many areas of Scotland and within several offshore basins. These sediments were deposited within fault-bounded, intermontane basins and represent a variety of continental facies typical of arid or semi-arid areas. A major study of the diagenetic evolution of the Scottish Permo-Triassic reveals that the basins were isochemical and experienced similar patterns of diagenesis, largely controlled by the climatic aridity of the depositional environment. Depositional pore-waters in the Permo-Triassic sediments of Scotland were well oxygenated and alkaline in nature, allowing the rapid alteration of the unstable mineral assemblage. Ionic complexes released as a result of this alteration were precipitated as a suite of eodiagenetic minerals with oxygen isotope analysis on the authigenic K-feldspar phase confirming an origin from meteoric waters in an arid climate. These eodiagenetic modifications resulted in a major loss of primary porosity. Further burial resulted in minor pressure solution while further depth-related changes have been identified in the authigenic clay mineral suite. Telodiagenetic precipitation of kaolinite and minor quartz was associated with a moderate development of secondary porosity. The hydrocarbon reservoir potential of the Scottish Permo-Triassic is low, primarily due to the early destruction of porosity.
Mid-Jurassic lagoonal delta systems in the Hebridean basins: thickness and facies distribution patterns of potential reservoir sandbodies
Authors: Harris J. P.
Published: April 2008

In the Inner Hebrides the facies distribution patterns and mineralogy of outcropping Great Estuarine Group (Bathonian) sandbodies indicate SSW progradation of copiously supplied lagoonal deltas in two sub-basins (Sea of the Hebrides Basin and Inner Hebrides Basin). These were both westerly tilted half graben and were probably separated by a slowly subsiding basement ridge known as the mid-Skye palacohigh. In the Bathonian these basins had an intermittent connection with the open sea and were supplied with clastic sediment from mineralogically distinct source areas in the Scottish Landmass to the east and an Outer Hebrides Landmass to the west. Fluvial-dominated and fluvial-wave interaction deltas supplied by low sinuosity rivers form successive sandbodies intercalated with lagoonal mudstones. The facies distribution patterns and mappable depositional limits of these sandbodies are dependent on the style and rate of fluvial supply, the configuration of the basins, variations in wave energy and salinity fluctuations. This demonstrates concepts which are probably applicable to the prediction of reservoir thickness and facies patterns in related sedimentary basins.
Palaeoenvironment of a Lower Carboniferous sandstone succession northwest Ireland: ichnological and sedimentological studies
Authors: Buckman J. O.
Published: April 2008

The shallow-marine deltaic sandstones and shales of the Lower Carboniferous Mullaghmore Sandstone, northwest Ireland, contain abundant trace fossils. A twenty-five metre thick measured section of Mullaghmore Sandstone from the northwest corner of Mullaghmore Head contains a minimum of twenty-eight ichnogenera and four informally named trace fossils. At least four coarsening-upward sequences occur within the measured section, with a possible total of six. Sedimentary structures and trace fossils indicate that each sequence represents deposition in a shallow-marine deltaic environment; prodelta-interdistributary bay passing upwards into a subaqueous channel environment. These represent a
Cruziana
-type ichnofacies, with an interbedded tempestite-associated
Arenicolites
ichnofacies, and a
Skolithos
-type ichnofacies respectively. The ichnogenus
Neonereites
, more typical of the
Nereites
ichnofacies, also occurs within the
Cruziana
ichnofacies, but only in limited numbers, and is atypical of the ichnofauna. Local lateral variation in ichnogenera and their sporadic occurrence along bedding planes indicates the need for extensive investigation, if trace fossils are to be used for detailed environmental reconstruction.
Origin of reddening and secondary porosity in Carboniferous sandstones, Northern Ireland
Authors: Wang W. H.
Published: April 2008

Carboniferous sandstones in the Ballycastle district of Northern Ireland were deposited in fluvio-deltaic environments. Significant secondary porosity was generated in the sandstones, particularly in red-coloured sandstones, due to the dissolution of carbonate cement. Minor secondary porosity was also created by the leaching of feldspar grains. Virtually all the observed porosity in the Carboniferous sandstones has a secondary origin. Subsequently, the precipitation of kaolinite reduced the porosity. The red-coloured sandstones are important in showing the depth of invasion of oxidising meteoric fluids following late Carboniferous-Permian uplift. The red colour is due to iron oxides liberated during the dedolomitization of a ferroan dolomite cement. Several instances of reddened Carboniferous strata across the UK probably have a similar origin and are potential oil and gas reservoirs as reddened sandstones may have a high secondary porosity.
Petroleum geology of the Slyne Trough and adjacent basins
Authors: Trueblood Stephen
Published: April 2008

The Slyne Trough lies to the west of Ireland, northeast of the Porcupine Basin and southeast of the margin of the Rockall Trough. The Slyne Trough can be demonstrated to have adequate source, reservoir and seal rocks, but the timing of hydrocarbon migration is more problematical. Structures exist on both large and small scales within the Central and Northern Slyne Troughs, with the possibility of hydrocarbon entrapment at a number of horizons. An excellent source rock has been identified in the Toarcian (Portree Shale Formation) with up to 7% TOC. A secondary source exists within the Sinemurian/Pleinsbachian (Pabba Shale Formation) with up to 4% TOC. Reservoir quality sands have been encountered in the Bathonian (Elgol Sandstone Formation) and Hettangian/Sinemurian (Broadford Beds Formation). A possibility for further reservoir development can be demonstrated in the Bajocian and Pleinsbachian. Sealing intervals have been found in the Bathonian (various levels in the Great Estuarine Group), the Toarcian/Aalenian (Portree Shale Formation/Dun Caan Shale Member) and the Sinemurian/Pleinsbachian (Pabba Shale Formation). The Liassic source rocks are marginally mature in the 27/13-1 well. In the deeper parts of the basin they are probably mature for generation and migration. The Lower and Middle Jurassic evolution of the basin can be estimated with fair accuracy from the 27/13-1 well. The Post-Middle Jurassic history can be interpreted from analogous basins in the region. The North Porcupine sub-basin section shows the complete erosion of the Liassic source horizons. The oil found within the Porcupine basin is believed to be sourced from the Upper Jurassic, which are mature here due to a post-Jurassic history different to that of the Slyne Trough. The Erris Trough is believed to have had a similar Jurassic evolution to the Slyne Trough, but with locally much greater erosion during the Lower Cretaceous. The Jurassic petroleum potential within this basin relies on preservation of the full Liassic sequence in downthrown fault blocks.
Early Tertiary submarine fan deposits in the Porcupine Basin, offshore Ireland
Authors: Shannon Patrick M.
Published: April 2008

The Porcupine Basin, offshore western Ireland, contains up to 10 km of Mesozoic and Cenozoic sediments. Seismic sequence analysis of the Palaeocene-Eocene strata on the southwestern and southeastern margins of the basin reveals a mounded and draped geometry. Four seismic sequences are defined and are coeval with a series of deltaic deposits in the northern and central parts of the basin. In the southwestern area these are interpreted as stacked submarine fan deposits. In the southeastern area the lower two sequences are interpreted as shelf-slope deposits which are overlain by two submarine fan complexes. Funnelling of the sediment was controlled by erosive channelling through interfan topographic lows. The seismic response indicates that the fans are dominated by lower fan sheet sand deposits with minor amounts of upper fan channellized sandstones. They probably developed in response to an interplay of eustatic sea level changes, tectonic uplift on the basin margins, and thermal subsidence towards the basin centre.
Evolution of Mesozoic sedimentary basins around the North Central Atlantic: a preliminary plate kinematic solution
Authors: Srivastava S. P. , Verhoef J.
Published: April 2008

There is significant correlation between the tectono-stratigraphic record in the sedimentary basins round the Central North Atlantic and the seafloor spreading history of the North Atlantic. This has been taken as an indication that the formation of these basins can be related to the relative motion between lithospheric plates. Based on many criteria, plate kinematics of the North Atlantic have been derived for the rifting periods of these basins. Reconstructions of the North Atlantic based on these plate kinematic solutions show a progressive overlap between the reconstituted plate boundaries. These overlaps exist because the margins of the original plates stretched during rifting: present-day plate boundaries are therefore extended beyond their pre-rift locations. From the overlaps, it is possible to calculate the amount of extension during rifting and to restore the plates and their enclosed basins to their pre-stretched configurations. The paper discusses techniques for the de-stretching of plate margins and describes a set of reconstructions showing the configuration of these basins as they evolved. For the first time, the reconstructions give a pictorial presentation of the formation of these basins.
A critical appraisal of the Whittle 3-D open-pit mine optimization software package
Authors: Costelloe D. J.
Published: April 2008

In 1965 Lerch & Grossman produced a paper describing a 3-D algorithm based on graph theory which could be specifically applied to the optimization of open-pit mine designs. At the time, most computers were incapable of performing the large quantities of iterative calculations required by the method. A 2-D algorithm was also described which, though effective on sections, lost its optimized quality when sections were combined. The 3-D algorithm therefore remained the preferred option.

In 1986 the Whittle 3-D open-pit optimization package was launched by Whittle Programming Ltd. This package utilized the Lerch-Grossman algorithm for the first time in a commercial software application. Since then the package has undergone a number of changes and improvements.

The purpose of an optimization package is to produce the most cost effective and most profitable open-pit design from a block model of an orebody. It must be capable of rapidly analysing alternatives accurately; it must be able to carry out sensitivity analyses of different components to assess financial risk; it must allow frequent updates to take into account changes in costs etc. and it must allow the production of interim designs. In today’s mining world, a hastily produced or non-rigorous design is financially unacceptable as the design determines the potential for profit or loss. To develop this further it is essential not just to make a profit but to maximize the return on investment.

This paper appraises the ability of the Whittle 3-D package to satisfy this objective.
Grade control for Australian open pit gold mines
Authors: Davis J. G.
Published: April 2008

The deposits exploited in Australian open pit gold mines are often small and of low grade with highly variable gold distributions. Consequently, they require detailed grade control. The nature of these grade control programmes is outlined together with a description of the various computational methods used. Particular attention is given to geological input into the various stages of the grade control program so as to ensure the reliability of sampling; to aid the production of bench extraction outlines; and to predict and minimize dilution and ore loss. The application of geostatistical techniques in this respect is also critically examined.
Ore reserve estimation in Los Pelambres, A Chilean porphyry copper
Authors: Faunes A. , Forkes J. , O’Leary J.
Published: April 2008

Los Pelambres is a world-class Chilean porphyry copper orebody located in the Andes Range adjacent to the border with Argentina at an elevation of 3100 m. It is characterized by a stock-size Tertiary granodiorite complex intruding a lower Cretaceous volcano-sedimentary sequence. Several multi-episodic stages of hypogene alteration and mineralization related to late porphyries and hydrothermal breccias occur, as well as secondary enrichment, generating a well defined orebody with bornite-chalcopyrite-chalcocite as the main copper minerals with minor molybdenite. Additionally, some gold and silver values are present. Grade zoning and kriging methods have been used to evaluate the richest east portion of Los Pelambres.
Faunal and floral indicators of ocean coastal upwelling (NW African and Peruvian Continental Margins)
Authors: Thiede J. , Jünger B.
Published: April 2008

Sediments under coastal upwelling regimes contain pelagic fossil assemblages that differ from those of non-upwelling coastal areas and of the open ocean. In this paper we describe the composition and biogeographical distribution of skeletal material of living planktic and benthic organisms in two of the most important coastal upwelling regions. Particular attention is paid to planktic foraminifers and pelagic gastropods off NW Africa and off NW South America. In both regions, near-surface water masses covering coastal waters of the upwelling region
per se
, of the coastal regions to the North and South of the upwelling centres as well as of the adjacent open ocean, were sampled during 1971 and 1977. The upwelled water masses were characterized by very high concentrations of radiolarians, planktic foraminifers, pteropods and small fish bones (the latter off NW South America only).

The biogeographical distribution of planktic foraminifers and pelagic gastropods can be related particularly well to upwelling. Off NW South America,
Globigerinoides ruber, Globigerina bulloides
and
Globoquadrina dutertrei
are found in high abundances near the coast. A region of high concentrations of these species extends from the areas of maximum occurrence towards the west and south into the open Pacific Ocean. Analogous biogeographical patterns can be observed off NW Africa. Concentrations of pelagic gastropods also are high in both coastal upwelling regions. Typical upwelling species are
Limacina inflata, L. trochiformis
and
Creseis acicula a.
. The biogeography of pelagic fossil groups is therefore particularly well suited to delimit the spatial extent of coastal upwelling regions.
Planktonic foraminiferal faunal and stable isotopic indices of upwelling: a sediment trap study in the San Pedro Basin, Southern California Bight
Authors: Thunell Robert , Sautter Leslie Reynolds
Published: April 2008

A planktonic foraminiferal faunal and stable isotopic study has been carried out on sediment trap samples collected in 1988 from the San Pedro Basin (Southern California Bight) in order to examine the response of this group of plankton to coastal upwelling. Hydrographic monitoring indicates that a period of upwelling occurred in the basin from late April to early June, with a brief spring bloom of planktonic foraminifera occurring prior to upwelling. The onset of upwelling resulted in a significant increase in total foraminiferal shell flux; the upwelling assemblage was initially dominated by
Globigerina quinqueloba
, with
Globigerina bulloides
dominating the latter half of upwelling. Following upwelling, the upper water column became thermally stratified and
Neogloboquadrina dutertrei
dominated the fauna. The observed pattern of faunal succession may be partly related to changes in food availability.

Stable isotope analyses (δ
18
O and δ
13
C) of
Neogloboquadrina pachyderma, N. dutertrei
and
G. bulloides
indicate that all three species adjust their depth habitats in response to upwelling.
Globigerina bulloides
migrates from below the thermocline to the surface at the onset of upwelling. Both
N. pachyderma
and
N. dutertrei
appear to adjust their depth habitats in order to remain within specific temperature ranges.