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This study demonstrates that petroleum and source rocks are enriched in Pt and Pd to the ppb level, and that the 187Os/188Os composition coupled with the Pt/Pd value permits the fingerprinting of petroleum to its source. Oils from the United Kingdom Atlantic Margin (sourced from the Upper Jurassic Kimmeridge Clay Fm.) as well as source rock samples have been analysed for Pt and Pd. When the Pt/Pd value is compared with 187Os/188Os (calculated at the time of oil generation; Osg) the values from both the known source and the oils are similar, demonstrating that they can be used as an oil to source fingerprinting tool. This inorganic petroleum fingerprinting tool is particularly important in heavily biodegraded petroleum systems where traditional fingerprinting techniques (e.g. biomarkers) are severely hampered, e.g. the world's largest oil sand deposit, the West Canadian Oil Sands (WCOS). This has caused the source of the WCOS to be hotly debated, with no present day consensus between inputs from potential source units e.g. Exshaw and Gordondale Fms. 187Os/188Os and Pt/Pd fingerprinting of the oil sands shows that the majority of the petroleum have similar 187Os/188Os and Pt/Pd values, supporting the hypothesis of one principal source. Analysis of the potential source rocks establishes that the principal source of the oil sands to be from the Jurassic Gordondale Fm., with a minor Exshaw Fm. input. Thus, the combination of previously pioneered Re–Os petroleum geochronology with 187Os/188Os and Pt/Pd values of petroleum permits both a temporal and spatial understanding of petroleum systems.

We demonstrate that the Re-Os system can be used to understand the temporal evolution of a petroleum system as well as aid in the identification of source units. Traditional geochemical methods (high-performance liquid chromatography, carbon isotope, gas chromatography, and biomarker analysis) indicate that the main source of 18 oils analyzed for Re-Os geochronology from the United Kingdom Atlantic margin (Clair, Schiehallion, Cuillin, and Foinaven fields) is Late Jurassic marine shales. This is supported by the Os isotopic composition ( 187 Os/ 188 Os) of the Late Jurassic source (∼0.9–2.4) at the timing of oil generation being similar to that of the oil (68 Ma; 0.92–1.12), indicating that the Re-Os system can potentially be used to fingerprint the source of an oil. The Re-Os data for the 18 oils yield an age (68 ± 13 Ma) that agrees with both the relative (basin models) and absolute (Ar-Ar geochronology) timing of oil generation, indicating that that Re-Os oil geochronology records oil generation events.

187 Os/ 188 Os, as expected from a Kimmer- idge Clay Formation source. However, oils from the Viking Graben, with which the Brent fi eld is associated, and main East Shetland Basin display unradiogenic 187 Os/ 188 Os. In this study we seek to demonstrate that these anomalous results are due to interaction with a mantle-like fl uid that propagated through the main fault zone of the Viking Graben and East Shetland Basin. This study enhances our understanding of the Re-Os oil systematics, and supports cur- rent ideas regarding fault architecture, crustal thinning, and associated fl uid fl ow and interac- tion in the North Sea oil system. ABSTRACT United Kingdom North Sea oils sourced from the Kimmeridge Clay Formation contain unradiogenic (~0.17 to ~0.48) and radiogenic (~1.04 to ~3.34) 187 Os/ 188 Os values. The unra- diogenic 187 Os/ 188 Os values are spatially associated with the main basin-bounding faults of the Viking Graben and East Shetland Basin. In contrast, the radiogenic 187 Os/ 188 Os values are associated with North Sea basins located farther away from the basin boundary faults. We suggest that crustal thinning and strain localization within the Viking Graben and East Shetland Basin are suffi cient to have allowed basin-bounding faults to propagate to suffi cient depth to act as conduits for mantle-derived fl uids to interact with oil. This hypothesis is sup- ported by previous geochemical data for North Sea oil fi elds. Furthermore, we demonstrate the use of Os isotopes as an important tracker of crustal-scale fl uid dynamics and petroleum migration pathways in extensional basins.

The offshore Indus Basin is a rift and passive margin basin offshore Pakistan and northwest India. Rifting associated with break up of the India/Madagascar/Seychelles plates began during the Late Cretaceous and was accompanied by a major period of volcanism associated with the Deccan volcanic event. A major volcanic centre was located along the south of the basin adjacent to the Saurashtra Arch transform fault and resulted in the deposition of up to 7 km of extrusive basalts interbedded with Late Cretaceous–Paleocene marine sediments. The basalts show stacked prograding reflection patterns on seismic data. A chain of northeast–southwest-trending volcanic seamounts in the central part of the deep-water basin formed topographic highs for the development of shallow-water carbonates during the Eocene post-rift phase. The passive margin stratigraphy includes up to 9 km of Oligocene–Recent age clastic sediments from the Indus River system. The proximal part of the Indus Fan contains spectacular large-scale channel-levees in the Miocene and Plio-Pleistocene intervals. Fourteen major channel-levee systems have been identified in the Plio-Pleistocene and represent potential reservoir targets. Trap types include extensional rollover anticlines at the shelf edge, drape structures over the Eocene carbonate highs, stratigraphic traps along the Murray Ridge and folds associated with strike-slip faults along the Murray Ridge. A key challenge for future exploration is to determine whether source rocks are present in sufficient quality for commercial discoveries.

Molecular simulations and an inherent structure analysis are used to investigate how the response of a glass to stress depends on its thermal history. Although the stress response at low and high strains is independent of the thermal history of the glass, the stress response at intermediate strains depends strongly on the thermal history. Previous experiments and simulations have shown that the peak stress, defined as the maximum stress the system can withstand before flowing, increases as a glass is annealed. The present inherent structure analysis addresses this phenomenon. The present results show that as glasses are strained, more highly annealed glasses remain in their initial metabasin up to larger strains (and thus larger stresses). This greater stress-absorbing capacity of the metabasins that characterize more highly annealed systems allows the more annealed systems to withstand more stress before flowing.

The roots of a polynomial can be represented as points in the complex plane. The time value of money (TVM) equation that is commonly used in finance is a polynomial equation. (See the appendix for a short description of the TVM equation and an example of its use in finance.) In [1] and [2] it is shown that concepts from financial mathematics can be obtained from the pattern of the roots of the TVM equation. The concepts are given in terms of distances between the roots and other salient points in the plane. This note shows that this particular polynomial, and the technique, can be applied more generally. When a series of data is fed into the coefficients of the polynomial, the mean and standard deviation of the data are seen in the complex plane as combinations of distances between the roots and other salient points. The results are aesthetically pleasing as well as mathematically interesting.

The Triassic reservoirs of the Judy Field, an overpressured petroleum accumulation in the Central North Sea, have been studied to determine their pressure and petroleum filling history. The magnitude of overpressure in the Pre-Cretaceous aquifer is similar across the field at about 24 MPa (3500 psi), but with some higher pressure laterally in areas closest to the major depocentres. Pressure modelling shows that the magnitude of the overpressure can be attributed almost entirely to disequilibrium compaction, and largely due to late rapid Tertiary burial, although modelling does require nanoDarcy (i.e. shale-like) permeability in the Cretaceous chalk section. The contributions from other mechanisms, which may be relevant in this setting, i.e. gas generation and lateral transfer, appear to be small. The existence of both aqueous and petroleum phases in secondary fluid inclusions in microfractures in quartz and feldspar grains allows estimation of palaeopressure. The data show fracture healing from 115 to 136°C during a time when reservoir palaeopressures were above hydrostatic. Reconstruction of the temperature history of the reservoir places the timing of the fluid inclusion formation during fracture healing at between 3 and 1 My ago, coincident with a period of rapid burial. The palaeopressure estimation using fluid inclusions validates the pressure modelling studies, which are derived independently from commercial basin modelling software. The fluid inclusions also revealed a low-GOR oil over the whole field, prior to late stage flooding by gas leading to the variable (low to high) GOR fluids plus gas condensates within the field today.

A three-fold zonation can be imaged within authigenic siderite from sandstones of the Brent Group using back-scatter SEM techniques. We interpret this zonation in terms of the biogeochemical zonation of shallow buried sediment. The innermost siderite crystal zone is very Fe rich (95.0±0.5 mol.% FeCO3), with high Mn levels relative to Ca and Mg. This is interpreted as forming within the Fe reduction zone, with Mn from the closely associated Mn reduction zone. The second siderite crystal zone is frequently represented either by an episode of dissolution, or is impure (80±1 mol.% FeCO3), and this corresponds to the sulphate reduction zone. The outer crystal zone is intermediate in composition, and is equated with the zone of methanogenesis (88±1 mol.% FeCO3). Isotopic values cannot be assigned to individual crystal zones. Bulk δ18O values (−2.7 to −13.0‰ V-PDB) are not consistent with precipitation from seawater at low temperatures, but suggest meteoric pore-waters. δ13C data (−4.3 to −15.7‰ V-PDB) are consistent with microbially-mediated precipitation.Pyrite and siderite are usually mutually exclusive within a single sample. Sedimentary conditions which favour the development of a strong sulphate reduction zone, and hence the formation of pyrite, do not favour the formation of a strong sub-oxic zone, where siderite is preferentially precipitated, and vice versa. There is a strong facies control upon siderite formation, with ripple cross-laminated sands being most strongly siderite cemented.

Pressure transition zones occur where the rate of pressure increase or decrease exceeds a fluid gradient. Transition zones are found between intervals of permeable rock and, in each case, fluid movement is impeded by a 9seal9, which acts as a temporary barrier over geological time. The composite profile of pressure vs. depth is a function of three phenomena: (1) the mechanism responsible for abnormal pressure, (2) redistribution of pressure due to fluid movement during and after the mechanism is occurring, and (3) the lithological profile of the rock succession. Pressure profiles in some overpressured systems can reveal which of the mechanisms is causing the overpressure. Once overpressure has been created, pressure decay and transference away from the interval of generation can strongly influence the nature of transition zones. Transition zones are controlled in a fundamental way by the permeability of the rocks in which the abnormal pressure is found.

Diagenetic kaolinite in reservoir sandstones of the Brent Group precipitated following the dissolution of detrial feldspar. Two distinct morphologies of kaolinite occur: (1) early diagenetic vermiform kaolinite which is often associated with expanded detrital micas; (2) later diagenetic ‘blocky’ kaolinite. Combined hydrogen and oxygen isotopic studies suggest that vermiform kaolinite precipitated at 25–50°C, and blocky kaolinite at 50–80°C, from pore-waters of a similar isotopic composition (δ18O = −6.5 to −3.5‰). These pore-waters are interpreted to be either a mixture of meteoric and compactional waters, or alternatively a meteoric water that had evolved isotopically due to water-rock interaction. Kaolinite precipitation occurred predominantly during the late Cretaceous to early Eocene. Influx of meteoric water into the Brent Group, probably occurred during the Palaeocene. Fluid flow across the entire basin was driven by a hydrostatic head on the East Shetland Platform palaeo-landmass to the west. The development of the two kaolinite morphologies is possibly related to the degree of supersaturation at the time of precipitation. At low degrees of supersaturation, vermiform kaolinite precipitated slowly upon detrital mica surfaces. Blocky kaolinite precipitated more rapidly into open pore-space at higher degrees of supersaturation. Precipitation of blocky kaolinite was perhaps triggered by the decay of oxalate.

Quartz is a major pore-occluding cement in North Sea oilfield reservoir sandstones. Fluid inclusions yield homogenization temperatures ( T h ) which have been thought to represent growth temperatures for the host quartz. However, early diagenetic inclusions from the detrital grain-overgrowth boundary yield excessively high palaeotemperatures which do not match the growth temperatures calculated from the depth of cementation. The temperatures of these inclusions increase with present day depth of burial, and approach present day reservoir temperatures. This suggests that the inclusions have begun to reset towards present day pressure-temperature conditions. The T h of the inclusions hence represent burial temperatures rather than growth temperatures for the quartz. Experimental studies indicate that the amount of resetting undergone by an inclusion varies depending on its size, shape and fluid composition. A relationship between inclusion T h and these variables has been observed in inclusions from deeply buried quartz cements in the North Sea.

Molecular simulations and an energy landscape analysis are used to investigate the effects of shear on aging in a glass. Shear beyond the yield point is shown to change the state of a glass such that it resembles (but is not identical to) a different stage in the aging process. A cycle of large strain rejuvenates the glass by relocating the system to shallower energy minima, while a cycle of small strain overages the glass by relocating the system to deeper energy minima. The balance between overaging and rejuvenation is controlled by how well the glass was initially annealed.

Simulations are carried out to investigate surface segregation in liquid mixtures with strong interspecies attraction. The simulations show that the majority species of particles segregates to the surface, even if the other species has the lower pure-component surface tension. This behavior is expected in metal-metalloid mixtures, such as Ni-P, and mixtures in which hydrogen bonds can form only between unlike species, such as acetone-chloroform.

Overpressure can be produced by the following processes: (1) increase of compressive stress, (2) changes in the volume of the pore fluid or rock matrix, and (3) fluid movement or buoyancy. Loading during burial can generate considerable overpressure due to disequilibrium compaction, particularly during the rapid subsidence of low- permeability sediments. Horizontal stress changes can rapidly generate and dissipate large amounts of overpressure in tectonically active areas. Overpressure mechanisms involving change in volume must be well sealed to be effective. Fluid volume increases associated with aquathermal expansion and clay dehydration are too small to generate significant overpressure unless perfect sealing occurs. Hydrocarbon generation and cracking to gas could possibly produce overpressure, depending upon the kerogen type, abundance of organic matter, temperature history, and rock permeability; however, these processes may be self-limiting in a sealed system because buildup of pressure could inhibit further organic metamorphism. The potential for generating overpressure by hydrocarbon generation and cracking must be regarded as unproven at present. Fluid movement due to a hydraulic head can generate significant overpressure in shallowly buried, "well-plumbed" basins. Calculations indicate that hydrocarbon buoyancy and osmosis can generate only small amounts of localized overpressure. The upward movement of gas in an incompressible fluid also could generate ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received October 17, 1995; revised manuscript received September 4, 1996; final acceptance January 20, 1997. 2Department of Geological Sciences, Durham University, South Road, Durham DH1 3LE, United Kingdom. Osborne e-mail: M.J.Osborne@ durham.ac.uk; GeoPOP web site http://www.dur.ac.uk/~dgl0zz7/ We wish to thank the companies that support the Geosciences Project on Overpressure (GeoPOP) at the universities of Durham, Newcastle, and Heriot-Watt: Agip, Amerada Hess, Amoco, ARCO, Chevron, Conoco, Elf Exploration, Mobil, Norsk Hydro, Phillips Petroleum UK Company Limited, Statoil, and Total. We also thank Neil Goulty (Durham) for commenting on an earlier draft of this paper. Osborne thanks Gordon Macleod (Newcastle) for help with geochemical modeling.