bluuebunny/crossref_metadata_2025 huggingface dataset:
MECHANICALLY INDUCED STYLOLITES AND LOSS OF POROSITY IN DOLOMITES
Authors: Braithwaite C. J. R.
Published: December 2007

Devonian dolomites are described, in which broken grains and internal crystal silts are associated with stylolites and pressure‐solution seams. These features are interpreted as resulting from a progressive mechanical collapse, following dissolution of anhydrite. The transfer of loading by failure within the rock unit was responsible for stress concentrations, which led to the initiation of the stylolites with fluids driven out by compaction. The evidence suggests occlusion of a formerly extensive, vuggy, porosity.
A REVIEW OF THE CRETACEOUS FORMATIONS IN THE ARABIAN PENINSULA AND GULF: PART I. LOWER CRETACEOUS (THAMAMA GROUP) STRATIGRAPHY AND PALEOGEOGRAPHY
Authors: Alsharhan A. S. , Nairn A. E. M.
Published: December 2007

This paper is a review of the standard stratigraphic column accepted by many workers for the Lower Cretaceous (Berriasian‐Aptian) of the Arabian Gulf. Type sections, synonymy and summary paleontological details are included. A historical introduction provides an explanation of the profusion of formational and member names. The observed fades variation in the region, both vertical and lateral, can best be understood in terms of the migration of predominantly shallow water carbonate facies‐belts across a shelf. Two, predominantly carbonate, cycles can be recognised, with the maximum development of the marginal clastic fades occurring within the upper cycle.
OIL AND GAS ACCUMULATIONS AND THEIR POSSIBLE SOURCE ROCKS IN SOUTHERN ENGLAND
Authors: Ebukanson E. J. , Kinghorn R. R. F.
Published: December 2007

A number of commercial hydrocarbon accumulations, which include oil and gas, are now known in southern England. Investigations into the organic maturity and hydrocarbon potentials of sedimentary rocks in the area suggest that the Jurassic Lias, Oxford Clay and Kimmeridge Clay units are the most probable sources for these hydrocarbon accumulations. The Lias on the southern side of the Purbeck‐Isle of Wight Disturbance appears to be the main source of the Dorset oils (Wytch Farm and Kimmeridge Bay) on the bases of adequate maturity, n‐alkane distribution, alkane ratios and stable carbon and hydrogen isotope ratios. On the basis of organic maturity, the oil accumulations in the Weald and the adjacent Hampshire areas are considered to be composed of varying contributions of bitumen generated from the three Jurassic source formations.

Application of the modified Lopatin method of basin modelling suggests the Lias in the Weald as the probable source for the natural gas deposit at Godley Bridge in Surrey and predicts a possible Liassic sourced wet‐gas field in the Dorset‐Isle of Wight area.
PETROGRAPHICAL AND GEOCHEMICAL STUDIES OF CRETACEOUS CARBONATE ROCKS, AIN ZALAH OILFIELD, NORTH IRAQ
Authors: Ahmed Walid A. , Elzarka M. , AI‐Dabbas Moutaz
Published: December 2007

Petrographical and geochemical studies of 135 core rock samples from Cretaceous formations at the Ain Zalah oilfield suggests that the carbonate rocks contain different percentages of dolomite and calcite.

Dolomitisation, stylolisation and neomorphic processes were observed in these formations. This study suggests that the Upper Cretaceous formations were deposited in a deep‐sea environment under low‐energy conditions, while the Middle and Lower Cretaceous formations were deposited in a shallow‐marine environment associated with tidal conditions far from wave attack.
SUPERPOSITION AND THE LAW OF REGULARITY IN STRATAL ORDER—KEYS TO THE PRACTICE AND THEORY OF GLOBAL STRATIGMPHY
Authors: Patterson J.R.
Published: December 2007

In petroleum geology, the widespread practice of mapping lateral variations of lithology within the confines of general formations instead of time‐zones inevitably provokes differences of opinion over the scope and definition of stratigraphy. These differences can be traced back to a blending together of the separate concepts of two founders of geology, William Smith and Abraham Gottlob Werner. Werner originated a lithologic definition of rock formations; also, he taught the concept that the relative ages of formations could be determined by fossils. Smith contributed to the development of stratigraphy with his invention of the term ‘stratigraphical’, which referred to the occurrence of specific fossils and rock types within units of strata.

The outcome of reconciling these different concepts has been that geologists have used Smith's basic terminology, the principle of superposition, and the formational units of stratigraphy, but have redefined them to accord with Werner's theory of Earth history. These changes in definition have confused the concept of stratigraphy, because they misrepresent its spatial form evident from ‘regularity in stratal order’. This general law was accepted by William Smith as the basis for his stratigraphical arrangement of rocks and fossils, which in the practice of correlating rocks by superposition led to a stratigraphic system of strata and the economy of a single stratigraphic scale.
THE BEJUCO PALEOCANYON (CRETACEOUS‐PALEOCENE) IN THE TAMPICO DISTRICT, MEXICO
Authors: Cantu‐chapa A.
Published: December 2007

The Bejuco area (4,200 sq. km) is in Eastern Mexico, south of the Tampico District. Ammonites, gumma‐neutron logs, and lithologic samples of 65 exploratory wells were studied in order to construct structural maps at several levels within the Neocomian and Upper Jurassic. The purpose of this work was to study the stratigraphy, paleogeography, erosional unconformities, and also the occurrences of hydrocarbons of Late Jurassic‐Neocomian age.

In most of the area, the Huasteca series of Middle‐Late Jurassic age lies discordantly on rocks of continental facies. In a few places in the NW, south, SE and centre, the Huasteca overlies intrusive rocks, and in the NW, the Huasteca also overlies metamorphic rocks in a few localities.

Marine sedimentation commenced at the beginning of middle Callovian time in the eastern part of the region. Several successive periods of transgression took place: (1) during the early Kimmeridgian (central and western area); (2) the late Tithonian (north); and (3) the late Hauterivian (southern part).

An important geological phenomenon is the submarine erosional unconformity which represents the Neocomian‐Eocene time‐interval. The area underlain by this unconformity has a delta shape and extends from the SW to the east. This is the precise area of the Bejuco paleocanyon. The unconformity today lies at a depth of between 1,200 and 2,900 m.

The presence of hydrocarbons in either Tithonian or Neocomian rocks is controlled by structural traps, and in some places by combination traps.
THE “ALBIAN TRANSGRESSION” IN THE SOUTHERN NORTH SEA BASIN
Authors: Crittenden Stephen
Published: December 2007

The “Albian transgression” may be recognised lithostmtigraphically in a number of boreholes in the UK sector of the southern North Sea Basin. This event is also recognised onshore the United Kingdom. The importance of this event for hydrocarbon exploration is discussed, and comparisons are made with sections in West Germany. The “Albian transgression” is a regional event in NW Europe which is important for understanding the pattern of basin evolution during Early and “Mid” Cretaceous times. It is also important because the diachronous coarse clastics associated with the transgression may prove to be of important reservoirpotential in the southern North Sea area (UK sector).
A MODEL TO EXPLAIN THE EARTH'S MAGNETIC FIELD AND OTHER GEODYNAMIC PHENOMENA
Authors: Melton Charles E. , Giardini A.A.
Published: December 2007

Paleomagnetism and other geodynamic processes, such as mantle convection and sea‐floor spreading, are of basic importance to petroleum geology. We present a model that explains the cause of these phenomena.

The model is founded on the fact that the core‐mantle interface is a super‐conducting region, and on the concept that the gravitational interaction between the Moon and the Earth's solid inner core decouples the angular velocities of the solid inner core, liquid outer core and mantle. The decoupling is analogous to the crust‐ocean decoupling that produces ocean tides. The continuous decoupling and resultant frictional heat produce a rotating equatorial “hot spot” on the outer surface of the liquid core. The “hot spot” generates a thermal power of up to 10
13
watts and an equatorial current of about 10
9
amperes. The current is held to a narrow equatorial path by the well‐known “pinch effect”. The total resistance of the equatorial circuit is no greater than 10
‐10
ohms. The 10
9
ampere current creates the “polar” magnetic field. The secondary “non‐polar” fields are induction effects due to minor fluctuations (ripples) in the equatorial current.

Paleomagnetic polarity reversals are initiated by turbulence, and by imbalances in kinetic pressure (nkT) and magnetic pressure (B
2
/2μ0). Westward drift is explained by the common direction of rotation and the differential angular velocities of the Earth's core and the Moon. The heat generated by the model explains the persistent molten state of the Earths core over a long geologic time, the nature of the deeper part of the geothermal gradient, and the non radiogenic heat emitted by the Earth. This heat is sufficient to drive all thermally‐driven geodynamic processes that are of deep origin.

The model is consistent in general with planetary magnetism, in that those planets that are known to have significant magnetic fields also have moons.
DEPOSITIONAL ENVIRONMENTS AND DIAGENESIS OF ALBIAN CARBONATES ON THE CALABAR FLANK, SE NIGERIA
Authors: Reijers T.J.A. , Petters S.W.
Published: December 2007

Lithostratigraphic interpretations are here presented of a carbonate body which rims the exposed basement on the SE hinge zone of the Niger delta, and which thickens to over 450 m on a nearby subsurface basement horst. This faulted margin, which is known as the Calabar Flank, was occupied by carboqate platforms similar to those in the Gabon and Angola Basins, during the pronounced South Atlantic Albian transgression. Stromatolitic, oncolitic/pisolitic, oolitic and fossiliferous coralline algal lithofacies are well displayed both in outcrop and in the subsuface of the carbonate body (the Mfamosing Limestone). These lithofacies suggest a facies progression from tidal flats, through restricted marine bays and lagoons to a possible reef front, with ooid bars and encrusting and ramose coralline algae. In the subsurface, the Mfamosing Limestone, which is gas‐bearing and oil‐impregnated, exhibits fair to good porosities in which calcarenitic bars and chalkiped lagoonal, calcarenitic and extensively dolomitized limestones are the potential reservoirs.
THE SEISMIC EXPRESSION OF REAL AND APPARENT BURIED TOPOGRAPHY
Authors: Jenyon M.K.
Published: December 2007

Subaerial, and in some case submarine, erosional processes sometimes leave a depositional surface which is not plane. During any subsequent transgression and sedimentation, remnant relief on the old surface may be preserved, particularly where depositional rates during the early period of the transgression are high, and the environment energy of deposition is relatively low. As well as in circumstances involving normal clatic sediments, similar situtions can arise in the inundation of a volcanic surface including relief features like vents, fissures and mounds, or of a karstified surface‐carbonate unit.

Apparent, or “false” buried topography can be produced by a wide range of geological processes. The difference between such features and those of true buried topography is that at the time of transgression, the latter formed part of the contempory Earth surface, whilst in the case of the false features, the surface at which they were formed was already buried beneath varying depths of sediment.

Attention has been directed in recent years to the potential importance of buried topographic features in hydrocarbon exploration. From this viewpoint, the difference between true and false features of this type may be trivial in some cases, but important in others, usually because of the details of the sedimentological processes involved.
STRATIGRAPHY AND ROCK UNIT NOMENCLATURE IN THE OIL‐PRODUCING AREA OF INTERIOR OMAN
Authors: Clarke Michael W. Hughes
Published: December 2007

The sedimentary rock sequence in the oilfield areas of Interior Oman ranges from the Late Proterozoic through Paleozoic, Mesozoic and Tertiary. A number of major gaps or changes in sedimentation are recognised, and these punctuate a subdivision into major rock units ranked as groups. Eleven named groups are recognised: in stratigraphic order from oldest, Huaf (Upper Proterozoic), through Haima (Lower Paleozoic), Misfar (Devonian), Haushi and Akhdar (Carboniferous to Triassic), Sahtan (Jurassic), Kahmah (Lower Cretaceous), Wasia (“Middle Cretaceous”), Aruma (Upper Cretaceous), Hadhramaut (Paleogene) to Fars (Neogene). Most of these groups are further subdivided and 35 named formations are described giving lithologic characteristics, boundaries, inferred age span and variability. These descriptions, together with reference well sections indicating wire‐line log characteristics, provide a sufficient basis for consistent future use of the rock units.

Hydrocarbon occurrence within this sedimentary sequence is primarily controlled by the presence of good impermeable sealing units, particularly those of sufficient thickness to be least breached by faulting. Seven important sealing units are recognised, ranging from the evaporites of the Huaf Group to the basal shale of the Hadhramaut Group.

Commercial hydrocarbon accumulations are retained by all these seals. Reservoirs are in both carbonate and clastic units, and in South Oman are commonly in much older units than the seal, which overlines unconformably.
FOLDING OF THE MESOZOIC COVER IN SW SOMALIA: A COMPRESSIONAL EPISODE RELATED TO TEH EARLY STAGES OF INDIAN OCEAN EVOLUTION
Authors: Boccaletti M. , Dainelli P. , Angelucci A. , Arush M. A. , Cabdulqaadir M.M. , Nafissi P. , Nafissi P. , Piccoli G. , Robba E.
Published: December 2007

The tectonic‐sedimentary evolution of SW and central Somalia is characterized by two main depositional cycles. The first cycle (Triassic to Early Cretaceous) is characterized by subsiding basins related to a process of crustal thinning, and is associated with the separation of Madagascar from Africa, between 165 and 121 Ma*. The second cycle, starting in the Late Cretaceous with a regional unconformity, is related to the separation and NEward drift of India, at approximately 80 Ma. The folds that affect the Jurassic and Early Cretaceous formations of the Lugh‐Mandera basin in SW Somalia are the result of a compressive phase connected with dextral transcurrent movements along NE‐S W trending fracture zones. These zones are developed parallel to the oceanic system of transform faults, in connection with the change in the stress regime intervening at the shift of the direction of spreading between the first and the second stages of evolution of the Indian Ocean.
GEOLOGY, GEOMORPHOLOGY, AND PETEROLEUM POSSIBILITES OF THE EL GODO AREA, IQUIQUE, CHILE
Authors: Cecioni G. , Meyerhoff A.A. , Teitz H.H.
Published: December 2007

The presence of solid bitumen in the EI Godo area of Tarapacá province, northern Chile, has been known for centuries but, from a petroleum viewpoint, the region has been little studied. A major deterrent to exploration has been the presence of volcanic accumulations in the objective section, a marine Jurassic sequence more than 3, 000 m thick, which has been intruded by some younger silicic plutons. Objectives older than the Jurassic are unknown but may be present, especially in the deep part of the Jurassic basins and in the offshore.

Two basins are present with a total prospective area of about 20,000 sq. kms. They contain shale and carbonate source rocks, shale and evaporite seals, and probable reservoir rocks (carbonates, terrigenous clastics, breccias). Anticlines, formed during the Nevadan orogeny, are present. The widespread presence of bitumen in outcrops indicates that oil has been generated. The western basin has thick syndepositional volcanics, and is the less attractive of the two basins. The eastern basin is nearly free of volcanic rocks and is a prime prospect.

An ancient. deeply‐weathered, igneous rock mass forms a basement ridge that separates the two basins. This ridge apparently is an extension of the Precambrian Arequipa massif of southern coastal Peru. Possibly this massif was exposed at times during the Jurassic, supplying quartz detritus at least to the coastal and offshore basins.

Karst phenomena, unrecognised before 1957 and almost unnoticed in the literature of this huge desert region, are widespread. The landforms are mature in the west. where poljes dominate, and very youthful in the east, where dolines (sinkholes) are dominant. The landscape east of the Coastal Range at Iquique has a fossil, inactive karst, but north of there, karst development is still active. The karst phenomena are related in only a general way to the structures known to underlie the area.
SOME DEFORMATION EFFECTS IN A CLASTIC OVERBURDEN RESULTING FROM SALT MOBILITY
Authors: Jenyon M. K.
Published: December 2007

In a basin containing an evaporite sequence that includes salt rock, should the salt become mobile and deform plastically, it will produce important structural effects in overlying sediments. These effects may include folding, faulting, slumping, and in some cases where dissolution of the salt also occurs, collapse structures. The movement of the salt/overburden system during plastic deformation of the salt may be syndepositional, in which case rim syncline sedimentation will take place at the surface. The form taken by the rim syncline deposits will be related to the stage of development of the salt structures with which they are associated.

An understanding of the mechanisms involved is important, particularly in a basin containing hydrocarbons. Also, study of the behaviour at depth of sediments which are subject to deformational stress may assist in solving structural problems on the Larger, regional scale. Selected examples of various effects seen in the North Sea Zechstein salt basin are discussed and illustrated with recently‐acquired seismic data.
THRUST CONTROL ON THERMAL MATURITY OF THE FRONTAL OUACHITA MOUNTAINS, CENTRAL ARKANSAS, USA
Authors: Underwood M. B. , Fulton D. A. , McDonald K. W.
Published: December 2007

Three types of anomalies in thermal maturity exist within the frontal thrust‐belt of the Ouachita Mountains, Arkansas, USA. The anomalies were documented by combining structural mapping with measurements of vitrinite reflectance; all samples were obtained from surface exposures. Maturity‐inversion occurs along the Y‐City Fault, where Jackfork and Johns Valley strata (Lower Pennsylvanian) have been thrust over less‐mature sequences of the lower Atoka Formation (Middle Pennsylvanian). An apparent shear‐heating aureole is also associated with this important thrust, although the spatial extent of the aureole is limited. When allowances are made for both structural and stratigraphic position, we find that internal gradients in vitrinite reflectance within individual thrust‐sheets are abnormally low. We attribute these gradients to tectonic burial. Weak remnants of the primary stratigraphic gradients are retained in most instances, so organic metamorphic reactions evidently failed to equilibrate during thrust‐loading; rapid unroofing of the thrust‐belt may have begun even as some faults remained active. Similar patterns of maturation should be expected within other thrust‐belts where peak burial and heating events were synchronous with deformation. Under these circumstances, subsurface projections of maturation trends should be keyed to structural (rather than stratigraphic) position.
SOME PROBLEMS IN PETROLEUM GEOCHEMISTRY
Authors: Stannage W.
Published: December 2007

Limited geological events are occasionally observable, but petroleum geochemistry is inevitably a wholly inferential science. It can only be simulated in the laboratory, with occasional help from industrial‐scale work, by using the Arrhenius equation to convert observations in human time to geotime by means of temperature change.

There are difficulties here. First, the Arrhenius equation is purely empirical. The analogy with the theoretically rigorous van't Hoff isochore cannot give the Arrhenius equation any more than empirical status. A departure from linearity unobservable in human time could well make the extrapolation to geotime subject to significant errors.

Secondly, the inability to define petroleum generation in terms of specific molecules prevents the assignment of specific activation energies to specific reactions. Energies of activation can vary significantly between even similar reactions.

Thirdly, catalysis can accelerate a reaction by ten or twenty powers of ten, and even variations in natural clays can alter reaction rates almost a thousandfold.

Each of these three factors could easily introduce errors of two powers of ten in translating laboratory simulation to geotime.

From equilibrium considerations, petroleum generation and degeneration belong to a continuous spectrum of reactions in which very large molecules are broken down into smaller ones, and ultimately methane. Pressure inhibits the equilibrium of such reactions to such an extent, especially as the molecules become smaller, that there can be no validity to simulation experiments which are not carried out at geopressures.

Altogether. it seems unlikely that the age of oil can be limited by thermodynamic considerations.

Finally, an inorganic origin for some petroleum ought not to be completely ignored.
PETROLEUM GEOLOGY OF THE UNITED ARAB EMIRATES
Authors: Alsharhan A. S.
Published: December 2007

The Permain to Holocene sediments of the United Arab Emirates consist mainly of epeiric shelf carbonates, associated with minor clastics and evaporites, reflecting major cycles of transgression and regression. These were deposited on the eastern margin of the Arabian Sheild, which lay along the southern margin of the Tethys Ocean during the Mesozoic‐Cenozoic eras. Sedimentation patterns were controlled by prominent regional structural features, epeirogenic movements and/or sea‐level fluctuations.

The tectonic history of the UAE in the Mesozoic‐Cenozoic is connected with the opening (Triassic) and closure (Upper Cretaceous‐Paleogene) of the southern Neo‐Tethys Ocean.

The distinctive structural style, together with the tripartite development of source‐reservoir‐seal, has produced in the UAE one of the world's richest Jurassic ‐ Cretaceous oil habitats. Significant oil discoveries have also been made in the Permian; Middle and Upper Jurassic; Lower‐Middle‐Upper Cretaceous and Oligo‐Miocene carbonates.

Two main source rocks have been identified. One is the Upper Jurassic Diyab/Dukhan Formation, which supplies the most prolific reservoirs in the Upper Jurassic (Arab Formation) and Lower Cretaceous (Thamama Group). The other is the Middle Cretaceous Shilaif/Khatiyah Formation, which feeds both Mishrif and Simsima reservoirs. Other minor potential source rocks have also been identified in the study area.

There are two principal sealing formations ‐ the Hith Anhydrite and the Nahr Umr shale; these are the main seals for the oil and gas accumulations in the underlying Arab Formation and Thamama Group, respectively. Secondary seals and barriers also exist in the stratigraphic sequence.
LA PENA FORMATION (APTIAN): A CONDENSED LIMESTONE‐SHALE SEQUENCE FROM THE SUBSURFACE OF NE MEXICO
Authors: Cantü‐Chapa A.
Published: December 2007

An integrated stratigraphic study was made of the La Peña Formation, which is penetrated by a number of wells in NE Mexico. the study was conducted with the aid of ammonites (Cheloniceratidae, Deshayestidae, Acanthohoplitidae), microfossils (Colomiella, Nannoconus), and radioactivity logs to investigate the age of the unit ‐ late Aptian.

A series of wells extending 400 Km NW ‐ SE were studied. The results show that the stratigraphic ranges of Colomiella recta, C. mexicana, and certain Nannoconus species are greater than currently believed. In contrast, the ammonite groups mentioned above are present only in the La Peña Formation.

The formation is composed of a condensed section of limestone and shale in this area. The La peña and the units below it are cut by normal faults, in particular the Paras Fault System, which is described here for the first time.
SOME COMMENTS ON PALEOTEMPERATURE ESTIMATION FROM APATITE FISSION TRACK ANALYSIS
Authors: Green P.F , Duddy I.R.
Published: December 2007

Huntsberger ans Lerche (1987) recently presented a method of obtaining on information on the varaition paleo heat‐flux with time in sedimentary basins by use of fission tracks on apatie. While this paper has the merit of drawing attention to the advantages of Apatite Fission Track Analyis over more convential paleotemperature indicators, we suggest that it is flawed in various important respects
PETROLEUM EXPLORATION IN THE ETHIOPIAN RED SEA
Authors: Savoyat E. , Shiferaw A. , Balcha T.
Published: December 2007

The Red Sea is a pericratonic Upper Tertiary basin which developed on the continental margin of Africa during the separation of Arabia from Africa, and is characterized by halokinetic tectonics in the upper stuctual stage and block‐rifting in the lower strctual stage. The margin is overprinted on a pre‐rift Mesozoic platform.

Hydrocarbon exploration interest in the Ethiopian Red Sea area is based on three objectives, which correspond to stages in geotectonic evolution: Mesozic pre‐rift targets; Miocene rift presalt targets; and upper Miocene‐Pliocene post‐rift drift sequences.

Reservoirs with appropriate sealing units (salt or shale) have been recorded. Models of sourcerock maturation using observed geothermal gradients are consistent with anaytical resuits, and prove that oil generation and preservation is to be xpected in the Ethiopian Red Sea.
FAULT‐BLOCK TECTONISM IN THE DEVONIAN SUBSURFACE, WESTERN CANADA BASIN
Authors: Greggs R. G. , Greggs D. H.
Published: December 2007

An alternative model for the formation of Devonian carbonate reefs in the Western Canada Basin is proposed. This model is based on the epeiric clear‐water sedimentation model of Irwin (1965), with structural modfication throughout the Phanerozoic by repeated PreCambrian basement tectonism.

The cratonic structural pattern is dominantly controlled by the global regmatic fracture‐system, which comprises a series of NW ‐ SE and NE ‐ SW vertical faults. These faults delineate a mosaic of fault blocks across the entire Western Canada Basin. Wrench faulting and vertical epeirogenic movements throughout the Paleozoic. Mesozoic, and Cenozoic subjected these blocks to further tectonic effects ‐ structural deformation and hydrothermal diagenesis. Preservation of down‐faulted carbonate blocks, and their subsequent isostatic recovery into overlying anhydrite or shale sequences, has led to the formation of carbonate complexes generally referred to as “organic reefs”.

In addition, repeated emergence and submergence of the cratonic platform, particularly throughout the Middle and Upper Paleozoic, produced numerous regional and inter‐regional unconformities. These periods of erosion contributed to the leaching, and partial dolomitization, of the Devonian carbonates formed during periods of cratonic submergence. Repeated faulting is considered to have been a major factor in the diagenetic alteration of Devonian carbonates through associated hydrothermal activity, and a dominant control in the migration of hydrocarbons.
ALUM SHALE BITUMEN MATURATION AND MIGRATION: IMPLICATIONS FOR GOTLAND'S OIL
Authors: Dahl J. , Chen R. T. , Kaplan I. R.
Published: December 2007

Thermal maturation of Swedish Alum Shale kerogen and bitumen has been determined from core samples from Eastern and Central Sweden. In samples from Eastern Sweden (Öland and Gotland), the kerogen and bitumen are thermally immature with respect to petroleum generation. In some areas of Central Sweden (Närke, ÖstergÖtland, and Kinnekulle in VästergÖtland), the kerogen is immature, whereas the bitumen is marginally mature to mature. In other areas of Central Sweden (Halleberg‐Hunneberg in VästergÖtland), the kerogen is supermature and the bitumen mature. This suggests that bitumen from a mature source‐rock has migrated into the Alum Shales of Central Sweden. Migration in Central Sweden is further evidenced by the occurrence of obviously‐migrated bituments in vugs and voids in the organicpoor Ordovician limestone overlying the Alum Shale in Central Sweden, and in concretions within the Alum Shale itself.

Based on biomarker distributions of extracted bitumen, Alum Shale kerogen pyrolysate and obviously‐migrated oils, and the fact that the Alum Shale in most of the areas studied is the only petroleum source‐rock extant, it is suggested that the migrated bitumen in Central Sweden is from the Alum Shale itself. Bitumen has migrated from areas where the Alum Shale is in close proxmity to Permo‐Carboniferous intrusions, such as Halleberg‐Hunneberg, into nearby areas such as Närke and ÖstergÖtland, where there is no evidence of intrusion and the indigenous organic matter is thermally immature. Other areas, where Alum Shales were associated with intrusions and consequently sourced oil, may have been eroded away.

There are producing wells on the island of Gotland, where the Alum Shale is also thermally immature. It is therefore assumed that heating which was responsible for generating Gotland's oil was very localized (such as by an intrusion) or that the oil has migrated from a thermally moremature, distant area. On the basis of reservoir rock porosity, and the fact that the Alum Shale of Gotland contains no migrated component, localized heating is favored.
PLASTIC FLOW AND CONTRAFLOW IN SUPERPOSED ZECHSTEIN SALT SEQUENCES
Authors: Jenyon M. K.
Published: December 2007

A competent band within a salt‐rock sequence can be a useful indicator of the form taken by any plastic deformation movement that occurs in the salt. The acoustic impedance contrasts provided by such a band often result in strong seismic reflection events that exhibit the effects of all stages of deformtion accompanying salt movement, from gentle undulations through complete disintegration the competent unit.

Examples are shown in seismic data from the North Sea Zechstein of deformation of the Plattendolomit band that separates the underlying Z2 ‐ cycle salt from the salt of the overlying Z3 cycle. Cases are discussed in which the evidence suggests contraflow in the two salts, where the Z2 salt has flowed bi‐directionally away from the axial trough of a salt‐withdrawal basin. This has led to flow of the overlying Z3 salt bi‐directionally towards the same axis. The observations tell us something about the rheological behaviour of superposed salts, and about the deformational behaviour of competent carbonate/anhydrite bands within a salt sequence. They also highlight potential problems of well correlation in the absence of good seismic data, or due to misunderstanding of salt‐flow phenomena.
A NUMERICAL KEROGEN TYPE INDEX
Authors: Taiming Li , Shanfan Lu
Published: December 2007

With the increasing importance of quantitative studies of hydrocarbon generation, an accurate classification of kerogen types is needed. On the basis of the widley‐used trichotomic classification, which depends on elemental composition, a numerical kerogen type index (NKTI) has been depveloped; this takes the maturation level as a basic parameter, and provides a numerical value to describe the composition or type of a kerogen.
THE GULF OF ADEN RIFT: HYDROCARBON POTENTIAL OF THE ARABIAN SCETOR†
Authors: Haitham F.M.S. , Nani A.S.O.
Published: December 2007

Stratigraphic results from eight offshore exploration wells drilled between 1979 and 1983 are described, and the successions penetrated are compared with the adjacent land outcrops and facies; these are also depicted in lithostratigraphic correlation charts/sections for the area. The Oligo‐Miocene Shihr Group is divided into formations which are described and correlated between the wells, and a new post‐Shihr formation is likewise described and correlated.

Mesozoic succession thickness variations or erosion cut‐outs and syn‐ and post‐rift sediment thickness increases in some of the wells reflect the extension of structural episodes of uplift and erosion (or of basin subsidence and thick sedimentation) seen on land, into the offshore domain; they highlight the major role these have had on source, reservoir/seal and trap development, within the context of regional and local basin evolution.

A number of source rocks, including Upper Jurassic, Upper Cretaceous and Paleocene‐Eocene argillaceous sediments, are identified, and the results of oil/oil and oil/source‐rock correlations on fluids recovered from Paleogene carbonate reserviors in the Sharmah and Ras Ghashwah wells, and an Upper Cretaceous sandstone reservoir in the Sarar well, are, respectively, correlated with sources in the Paleocene‐Eocene formations, and the Upper Cretaceous Mukalla Formation. The geothermal gradient, calculated from differing results in each well, is high, and averages 3.5°C/100m, which indicates an oil “window” ranging between depths of 1,750 m and 3,500 m; this is broadly supported by vitrinite reflectance and Thermal Alteration Index (TAI) measurements carried out on the samples. It is concluded that favourable conditions exist for commercial hydrocarbon accumulations in the offshore area
PETROLEUM AT THE ROOF OF THE WORLD: The geological evolution of the Tibet (Qinghai‐Xizang) Plateau Part II
Authors: Taner I. , Meyerhoff A. A.
Published: December 2007

Abundant field data collected within the past twelve years or so show that all published explanations for the origin of this 2.4MM sq. km region violates some of these data. Among them, one fact is of paramount importance. This is the discovery that none of the great east‐west fracture zones is a boundary between different, but coeval, lithofacies. Instead, several major facies boundaries (in rocks of various ages) form a zigzag pattern across the various blocks of the region. This fact, in turn, means that most published tectonic interpretations, including those of plate tectonics, are incorrect.

In Part 2 of this paper, the Authors now point out that the recently‐proposed surge‐tectonics hypothesis explains all field data. If the surge‐tectonics hypothesis is correct, most of the Tibetan Plateau, including its Cambrian‐through‐Tertiary section, is prospective. They conclude, therefore, that Tibet could eventually become a major petroleum‐producing province of the world.
THE NEOGENE‐QUATERNARY SECTION IN THE NILE DELTA, EGYPT: GEOLOGY AND HYDROCARBON POTENTIAL
Authors: El‐Ella R. Abu
Published: December 2007

The Nile Delta area covers nearly 60,000 sq. kms in the northern part of Egypt. The Nile Delta basin contains a thick sequence of Neogene‐Quaternary clastics that are considered to be prospective for oil and gas. The information available for investigating the basin sediments, and evaluating their hydrocarbon potential, has been gathered from some 24 wells. As most of these wells have not penetrated below the Miocene, deeper sediments could not be investigated. The studied section is uniform across the northern Delta, consisting of at least 15,000 ft. of shales and sandstones: it becomes thinner southwards. The section is composed of three sedimentay cycles, including eight formations. Thick, organic‐rich sediments were deposited under favourable conditions for oil and gas genesis in several parts of the studied basin. The northern part of the onshore area, and the eastern and western parts of the offshore area, are favourable sites for hydrocarbon generation and accumulation for the following important reasons:(1)mature source rocks,(2)structural relief capable of trapping hydrocarbons in the eastern and western parts of the offshore area (Abu Qir and El‐Temsah localities),(3)stratigraphic traps in the northern part of the onshore area, and (4)the depositional features of the Abu Madi Formation.
THE SEDIMENTOLOGY OF THE BRENT GROUP IN THE CORMORANT BLOCK IV OILFIELD
Authors: Livera S. E. , Caline B.
Published: December 2007

Cormorant Block IV is one of four, large‐scale fault‐bounded accumulations which together comprise the Cormorant oilfield. The field lies some 150 kms NE of the Shetland Islands, in the East Shetland Basin. Oil is trapped in the Middle Jurassic (Aalenian‐Bathonian) Brent Group. Block IV is a separate accumulation in a downthrown area formed by a large synsedimentary listric fault, which produced a dome‐shaped structure with further internal fault‐block compartmentalisation. A concerted data‐gathering effort on the accumulation included the coring of 43% of the reservoir section penetrated by wells.

The Brent Group in Block IV fits into the “classical” overall regressive/transgressive, wavedominated delta model widely accepted for these sediments. However, the synsedimentary boundary fault has had some influence on the shallow‐marine Broom and Tarbert Formations and the coastal‐plain Upper Ness sediments, in each case preserving a unique and thicker sequence on the downthrown eastern side. The lowermost Broom Formation includes a shallow‐marine, Gilbert‐delta type deposit that infilled a topographic feature formed by the fault, and is genetically distinct from the rest of the Group. The Rannoch and Etive delta‐shoreface sediments, the main resrvoir interval, appear to have prograded through the area towards the NNE so rapidly that no fault influence was felt. Lateral variability at this level can be explained by the location of distributaries in the upper shoreface, controlling the grade of sediment supply and progradation rates of the entire system. The barrier‐attached coastal‐plain sequence of the lower Ness is chronostratigraphically equivalent to the Etive. and thins rapidly towards the north. The upper Ness is strongly layered by repeated periods of emergent coastal‐plain development separated by at least block‐wide lagoonal drowning events. Erosion at the base of the shallow‐marine Tarbert Formation, and the marked variability of sediment type and thickness in this horizon, suggests that it formed over a considerable time‐period at the end of Brent Group deposition, as the structure of the block was still developing.
THE TECTONIC MOSAIC OF THE SOUTHERN LEVANT: IMPLICATIONS FOR HYDROCARBON PROSPECTS
Authors: Cohen Z. , Kaptsan V. , Flexer A.
Published: December 2007

A comprehensive study of the Levant region, using geophysical, well and surface data, has elucidated the tectonic regime which preceded the Late Cretaceous tectonic inversion in the Levant region. A tectonic mosaic existed in the study area during the Paleozoic and Mesozoic, and was made up of NE‐trending grabens (e.g. the Pleshet, Judea and Galilee grabens) and their neighbouring horsts, and a number of NW‐trending blocks. These tectonic elements were bounded by regional faults.

The grabens were formed in three known phases: pre‐Upper Permian: Early to Middle Triassic; and Early to Middle Jurassic. Formation of each graben resulted in a depositional sag basin centered above the graben and extending beyond its boundaries.

Two types of inversion were active—inversion of regional tilts, and inversion of vertical tectonic movements. Examples of regional tilts are the SE landward Middle Triassic Anisian sedimentary wedge. and the NW basinward wedge of the Early Cretaceous (Berriasian to middle Aptian) sequence. A vertical inversion occurred in the Late Cretaceous. The Paleozoic‐Mesozoic grabens became raised blocks, whereas the horsts turned into depressed areas, in which synclinal sediments, rich in organic matter, accumulated in asymmetrical fault‐fold sag basins. These events occurred as a result of intraplate stresses, and are related to major global orogenies.

The grabens, the sag basins, and also the argillaceous parts of the basinward sedimentary wedges, are considered to be sites of hydrocarbon generation. Commercial and sub‐commercial production, as well as abundant oil and gas shows, confirm the presence of source rocks. Traps of various forms are widely available; in the horsts, in the grabens, in the sedimentary wedges, and in the sedimentary sag basins.
COMPUTER SIMULATION OF PRIMARY OIL MIGRATION IN KUWAIT
Authors: Özkaya I.
Published: December 2007

Primary oil migration from a source rock has been simulated using a computer. The simulation model is based on a finite difference solution of one‐dimensional fluid‐flow differential equations. Oil generation is simulated by the time‐temperature index. Shale porosity is reduced by compaction. Oil saturation rises as organic matter is conversed to oil, and the source‐rock porosity decreases during burial. When oil saturation reaches an irreducible level, surplus oil is expelled by capillary and excess pore fluid pressure.

The model is applied to an actual case. Oil generation and migration from a possible source rock in Kuwait is simulated. A reasonable estimate of the time of oil expulsion is obtained. even though values of several parameters had to be assumed. The results confirm the validity of the model and point to the possible use of such computer simulation models in oil exploration.
OIL AND GAS EXPLORATION IN CUBA
Authors: Echevarria‐Rodriguez G. , Hernandez‐Perez G. , Lopez‐Quintero J. O. , Lopez‐Rivera J. G. , Rodriguez‐Hernandez R. , Sanchez‐Arango J. R. , Socorro‐Trujillo R. , Tenreyro‐Perez R. , Yparraguirre‐Pena J. L.
Published: December 2007

Oil and gas exploration in Cuba began with the discovery of the Motembo oilfield in 1881. and some 25 oilfields have subsequently been discovered. the largest of which are Boca de Jaruco and Varadero.

Two oil‐bearing provinces are known: the Northern Cuban Province, which includes the overthrust belt; and the Southem Cuban Province, to the south of the overthrust belt, which includes a number of Cretaceous‐Tertiary basins.

Rocks in the overthrust belt are intensely faulted and folded. with folds trending NE in western Cuba and NW elsewhere on the island. The Northern Cuban Province is about 1,000–kms long and 80– to 100–kms wide: sediments here are generally 10– to 12–kms thick. Seven overthrust surfaces have been identified by drilling, and correspond to different oil zones: thus, oilfields are characteristically located within different tectonic‐stratigraphic units.

The Southern Cuban Province covers an area of about 60.000 sq. kms, and is composed of several basins. whose development was different from those to the north. Oil has been located in the Central Basin, where oilfields are present in positive structures, but not so far in any other basins here, although surface and subsurface oil seeps are common in this area.
GEOLOGIC AND HYDROCARBON EVOLUTION OF BARBADOS
Authors: Speed R. C. , Barker L. H. , Payne P. L. B.
Published: December 2007

Barbados island has risen above sea‐level in the last one million years as a pinnacle on the structural high of the extensive Barbados accretionary prism. The island provides by outcrop and many wells a glimpse of the structural history of the oldest and thickest realm of the prism, and a model for evolution of oil and gas in an accretionary terrane rich in quartz sanaktones and muddy temgenous and hemipelagic rocks that are moderately rich in organic substances. The island contains four major tectonostratigraphic units: basal complex of Eocene accretionary origin: prism cover: the Oceanic allochthon, of marly forearc basin strata, that was thrust over the basal complex and prism cover: and mud‐matrix mélange diapirs. Oil and gas are sourced chiefly from the basal complex at depths below 7 km, the minimum maturation depth. Oil reservoirs are in sandstones of the basal complex in fault‐fold structures. Seals are provided by mudstones of the basal complex and, more regionally, by thick, impermeable successions of prism cover and the Oceanic allochthon. Current oil reservoirs probably evolved mainly in Late Neogene time during a late stage of deformations. Oil reservoirs are best known in southern Barbados, in the Woodboume Trough, where sealing conditions were optimized.

Barbados' marine territory also includes an extensive fold‐thrust belt for some 50 km west of the island. This belt includes rocks of both the Barbados accretionary prism and strata of the Tobago Trough forearc basin. The thrusting is due to the westward wedging of the prism into the forearc basin between roof and floor detachments.
OIL SOURCE ROCK POTENTIAL OF THE LACUSTRINE JURASSIC SIM UUJU FORMATION, WEST KOREA BAY BASIN: Part I: Oil source rock correlation and environment of deposition
Authors: Massoud M. S. , Killops S. D. , Scott A. C. , Mattey D.
Published: December 2007

The offshore West Korea Bay (WKB) Basin is one of several NE‐SW rift basins which formed over thin continental crust in Korea and eastern China during the Late Mesozoic and Early Cenozoic, and is characterised by a two‐layered structure, with a Cenozoic basin superposed on a pre‐Tertiary basin. Paraffinic oils recovered from Mesozoic and Tertiary sandstone intervals are thought to have been generated from different source beak. The Sim Uuju Formation consists mainly of fluvially‐derived shales and sandstones deposited in a large, open, fresh‐water, deep, tectonic lake. which occupied wide sub‐basins formed by normal faults under a semi‐humid climate, as a result of initial rifting during the Upper Jurassic. A probable source bed (400– to 500–m thick) has been geochemically located in the basinal offshore area of the Sim Uuju sequence penetrated by Well 606. The Jurassic bed could be considered as a potential source for the Mesozoic oil, provided that it contains the right type and amount of organic matter which has reached the stage of oil generation.
ALPINE GEOWARPINGS IN THE ZAGROS‐TAURUS RANGE: INFLUENCE ON HYDROCARBON GENERATION, MIGRATION AND ACCUMULATION
Authors: Ameen M. S.
Published: December 2007

Megascale monoclinal flexures (‘geowarpings“) with maximum angles of dip ranging from 5° to 13° occur in the Zagros‐Taurus range of Iraq and Iran. They face south and SW. with amplitudes of 1.4 to 14.44 kms and axial lengths of 43 to 230 kms. meir axes are parallel to the host mountain range and lie in an en Pchelon configuration, and many are located in the high mountain zone, at or close to the mountain front (i.e. the border with the foothills zone).

Neotectonic and stratigraphic evidence suggests the contemporaneous development of these flexures with their host fold‐belt during the collision of the Eurasian and Afro‐Arabian Plates, which culminated during Upper Miocene‐Pliocene time. Differential vertical movements caused greater shortening and thickening in the upwarded regions (10–33%). which lie closer to the plate suture zones, compared to the downwarped regions (3–17%).

Development of the geowarpings resulted in circumstances favourable for hydrocarbon generation in downwarped regions. where source rocks were buried sufficiently deeply to produce oil and gas: this then migrated upwards to be trapped in the growing folds, and was sealed by Miocene evaporites. On the other hand, upwarped regions were elevated, and suffered extensive erosion, leading to a reduction in the depth of burial of potential source rocks, and consequently limited possibilities of hydrocarbon generation. Furthermore. the non‐deposition and/or erosion of Miocene evaporites in upwarped regions led to the dissipation of considerable volumes of the hydrocarbons which might have been generated. The effect of geowarpings on oil and gas reserves is evident from the distribution of oil‐ and gasfields. which are almost exclusively located in the downwarped. foothills zone.

Geowarpings may be located in the Taurus Range of E Turkey and NE Syria. but a lack of accurate data makes this difficult to investigate. Further studies are needed to develop an efficient exploration plan for oil and gas in that area.