bluuebunny/crossref_metadata_2025_split huggingface dataset:
New oil discoveries in the Ceuta area, SE Lake Maracaibo, Venezuela
Authors: Roberto M. , Mompart L. , Puche E. , Scherer F. 
Published: April 2008

Abstract
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

Abstract
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

Abstract

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

Abstract
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

Abstract
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

Abstract
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

Abstract

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

Abstract
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

AbstractPhosphate 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

Abstract
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

Abstract
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

Abstract

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

Abstract

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

Abstract

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.