bluuebunny/crossref_metadata_2025 huggingface dataset:
Subduction systems and magmatism
Authors: Hamilton Warren B.
Published: April 2008

Most published subduction modelling and much palaeotectonic speculation incorporate the false assumption that subducting oceanic plates slide down fixed slots. In fact, hinges roll back into oceanic plates and slabs sink more steeply than the inclinations of the Benioff zones which define transient positions of the slabs. The lower parts of overlying mantle wedges sink with the slabs, pulling away from partial-melt zones higher in the wedges. The complex behaviour of arc systems can be comprehended in terms of this mechanism of subduction. The common regime in overriding plates is extensional, and leading edges are crumpled only in collisions. Shear coupling between subducting slabs and overriding plates is limited to shallow depths and varies widely, with corresponding variations in tectonic erosion, accretion, and regurgitation of high-
P
subducted materials. Arcs can advance, lengthen, change curvature, festoon around obstacles, rotate while deforming, and fold and pinch shut. Two arcs can collide as an intervening oceanic plate is subducted simultaneously beneath both, or they can migrate apart as new lithosphere is formed between them. Subduction cannot occur simultaneously beneath opposite sides of a rigid plate because impossible retrograde subduction would be required beneath one of them. Histories, including inception ages, collisions, polarity reversals and stage of petrological evolution, vary greatly along continuous arc systems. Long-continuing steady-state systems are uncommon.

Magmatic arcs are properly viewed as features migrating with sinking lower plates, not as fixed features of upper plates. Hot inclined zones within mantle wedges, midway between sinking slabs and overriding crust, are avenues for replenishment of mantle pulled away with subducting plates and also are sites of generation of arc protomelts as volatiles rise into them from dehydrating slabs. Back-arc basins form by spreading behind migrating arcs; strips of arcs may be abandoned in the spreading systems. An arc can migrate so rapidly that it plates out oceanic lithosphere rather than producing a welt.

Exposed sections of the upper mantle and basal crust of arcs show that the Mohorovičić discontinuity is a self-perpetuating density filter and that the already-evolved basaltic and melabasaltic melt that leaves the mantle forms great basal-crust sheets of norite, gabbro and granulite. All more-evolved rock types in these sections are generated in the crust by fractionation, secondary melting and contamination (and this falsifies much petrological modelling).
Cretaceous to Cenozoic volcanism in South Korea and in the Sea of Japan: magmatic constraints on the opening of the back-arc basin
Authors: Pouclet André , Lee Jin-Soo , Vidal Philippe , Cousens Brian , Bellon Hervé
Published: April 2008

The major element, trace element, and radiogenic isotope compositions of volcanic rocks in the back-arc area of the eastern Eurasian continental margin provide insight into the nature of the mantle wedge and constrain the magmatic evolution of the Japan Sea back-arc basin linked to its tectonic history. Different phases of post-Early Cretaceous volcanic activity are identified along the Korean margin and in the Japan Sea. Volcanic rocks from Korea include (1) Cretaceous and early Cenozoic calc-alkaline lavas of a volcanic arc at an active margin, and (2) Pliocene and Quaternary intraplate flood basalts and volcanic islands of alkaline composition. Japan Sea volcanic rocks consist of (1) early Cenozoic andesite flows of a remnant arc in the Yamato Bank, (2) early Miocene basalts of the Japan Sea basin basement, which share compositional characteristics of island arc tholeiites, continental rift tholeiites and back-arc basin basalts, (3) late Miocene seamounts of tholeiitic and mildly alkaline compositions, and (4) Pliocene and Quaternary alkaline volcanic islands.

Geochemically, these rocks belong to three broad magmatic groups: (1) an arc-related, calc-alkaline group of a continental, Andean margin type, which prevailed prior to the opening of the Japan Sea between the Cretaceous and early Miocene, (2) continental rift tholeiites and back-arc basin basalts, formed during the rifting stage in the early Miocene, and (3) an intraplate alkaline group similar to OIB, erupted later during spreading, between late Miocene and Holocene times.

Trace element and Sr, Nd and Pb isotopic compositions of selected samples show that the sources of magma Group 1 calc-alkaline lavas and magma Group 2 tholeiitic lavas included varying contributions of two main mantle components: an Indian Ocean MORB-like depleted mantle source (DMM) and an enriched mantle component similar to EM II. The latter component could represent DMM contaminated by subducted oceanic sediments incorporated into the lower lithosphere during the long-lived subduction of west Pacific crust. During the opening of the Japan Sea back-arc basin, the relative proportion of the DMM component dramatically increased between the rifting and spreading stages. It is also necessary to postulate a third component present in the sources of the Group 3, post-opening alkaline lavas, perhaps enriched mantle of EM I composition, which may also have resided in the subcontinental lithospheric mantle.
Arc and back-arc geochemistry in the southern Kermadec arc-Ngatoro Basin and offshore Taupo Volcanic Zone, SW Pacific
Authors: Gamble J. A. , Wright I. C. , Woodhead J. D. , McCulloch M. T.
Published: April 2008

Back-arc basin basalts from the Ngatoro Basin (the southern end of the Havre Trough) are similar geochemically to, yet subtly distinct from, basalts of the Havre Trough to the north. Whole rock and glass chemistry are consistent with derivation from a fertile mantle source with subsequent evolution by fractionation of olivine (+ Cr-spinel) + plagioclase, and then clinopyroxene. Basalts from the vicinity of Rumble IV seamount at the southern end of the Kermadec island arc, and the eastern Ngatoro rift escarpment, are strongly porphyritic relative to the back-arc basin basalts and show trace element (high LIL abundances and highly depleted HFS abundances) and isotopic signatures of subduction zone basalts.

At its southern end, the Ngatoro Basin penetrates the continental crust of New Zealand creating a major, 3000 m deep bathymetric re-entrant in the slope-break; the slope-break marks the transition from oceanic to continental crust. Basalts from the floor of the Ngatoro Basin re-entrant are isotopically distinct from the basalts of the oceanic sector in that they have higher Sr and correspondingly lower Nd isotope ratios and are comparable to basalts of the Taupo Volcanic Zone (TVZ) to the south.

In contrast to the basalt-dominated oceanic sector, basalts from the offshore TVZ, a 100 km long area extending roughly NNE from White Island to the submarine Whakatane arc volcano at the edge of the continental slope-break, occur in association with andesites, dacites and rhyolites. These basalts are generally strongly porphyritic (olivine + plagioclase + clinopyroxene) and show trace element abundances typical of suprasubduction zone rocks. However, offshore TVZ basalts show subtle distinctions from onshore TVZ basalts to the south; the former have more radiogenic Sr isotopes. Furthermore, their high field strength element and transition element systematics appear to overlap with those of basalts from the Kermadec arc to the north.

The authors attribute these lateral (along arc) and transverse (across arc) variations to source heterogeneity and variable fertility in the sources of the arc and back-arc basin magmas. Sources of the arc-front magmas are more refractory and also more susceptible to contamination by slab-derived fluids than sources for back-arc basin magmatism, reflecting the dynamic nature of flow from the back-arc into the mantle wedge beneath the volcanic front.
Cretaceous-Tertiary volcanism and syn-subduction crustal extension in northern central Peru
Authors: Petford Nicholas , Atherton Michael P.
Published: April 2008

Although subduction of the Nazca plate beneath the Peruvian continental margin has been in progress for nearly 200 Ma, the Andean cycle in western Peru is dominated by periods of contemporaneous rifting and large scale crustal extension. It began with the formation of the west Peruvian trough (WPT), a major depositional structure and one of a series of ‘marginal’ basins that developed along the entire Pacific coast of South America during the Cretaceous. The western element of the WPT north of Lima is the Albian Casma basin, composed of nearly 9000 m of mostly basaltic to intermediate volcanic rocks and minor intrusives. Facies analysis of the basinal fill is consistent with a spreading system in a relatively isolated, deep-sea environment.

The Tertiary is marked by two major episodes of extensional, subaerial volcanism: the
c.
53-15 Ma Calipuy Group, followed in the Late Miocene-Pliocene (
c.
7.6–4.65 Ma) by the more easterly Yungay volcanic rocks. Although both were extruded during periods of crustal extension, the prevailing tectonic regimes differ in detail. Thus the acid-intermediate Calipuy Group was extruded from fissure-type volcanoes over a long period with only minor plutonic activity, whereas the more easterly Yungay volcanic rocks, located furthest from the present trench, are closely associated with major batholith intrusion within a strike-slip transtensional system.

Although the volcanic rocks of the Casma-Calipuy-Yungay groups are dominantly ‘calc-alkaline’ in the classic sense, the role of extension has clearly been fundamental in their genesis. In contrast, their relation to contemporaneous subduction is less clear.
Developments in the structural geology of rifts over the last decade and their impact on hydrocarbon exploration
Authors: Morley C. K.
Published: April 2008

Three different aspects of rift geometry and their impact on hydrocarbon exploration are examined: rift architecture, fault angle and basin inversion. Rift architecture is controlled by fault geometry, which itself is heavily influenced by pre-existing fabrics. At the largest scale, rifts are divided into segments tens to hundreds of kilometres long. They can be joined along offset segments, separated by rift jumps and gaps, or terminated in splays. Rift jumps and gaps provide the entry points for large rivers to enter rifts and create along-axis drainage systems, while rift offsets are areas where anomalous structural patterns develop in response to oblique extension. Within rift segments transfer zones between boundary faults mark important changes in rift geometry and are the preferred sites of coarse clastic sedimentation. In the Gulf of Suez, for example, the structures and the syn-rift reservoir units within two major transfer zones are associated with over four billion barrels of reserves out of a total of six billion barrels. The way individual tilted fault blocks terminate include strike ramps, forced folds, rhomb-blocks, transfer faults, and major and minor cross-strike faults. In areas of relatively poor seismic data quality the choice of fault block termination interpreted on geophysical maps can make a considerable difference to structural interpretation. There has been considerable debate as to whether rifts are composed of high-angle (45–75°) faults or a mixture of high- and low-angle faults. Low-angle faults occur in the sedimentary sequence of rifts, where they occur in a variety of structural settings. Such faults can have a significant economic impact in field development. The presence of low-angled basement faults is more problematic. They do apparently occur in rifts, both due to reactivation of older faults and as completely new faults. Rifts can evolve through a variety of low- and high-angle fault structures with time, which impacts trap creation and destruction. Rifts commonly undergo changes in their stress regime which may lead to basin inversion features. This affects trap development and destruction and can lead to a variable subsidence history for basins, as the basin bounding faults change their senses of motion. The recognition of inversion features in many rifts indicates that this is a very important aspect of rift history.
Strike variation in deformation and diagenesis along segmented normal faults: an example from the eastern Gulf of Corinth, Greece
Authors: Roberts Gerald P. , Gawthorpe Robert L.
Published: April 2008

Structural and diagenetic studies at several localities along the strike of a single fault segment within the eastern Gulf of Corinth show that deformation and diagenetic textures vary along-strike. Uplifted phreatic carbonate cements fill fractures within footwall localities in the centre of the fault segment, whereas, at the termination of the fault segment within the Psatha interbasin transfer zone, fractures filled with vadose calcite cements are associated with an extensive karst system. A small-scale offset between two fault strands along the centre of the fault segment within the Asprokambos intrabasin transfer zone is also dominated by fracture-filling phreatic cements. Fractures filled with phreatic cements exhibit crack-seal textures and low porosities, and formed in fluid-saturated conditions below the water table. In contrast, fractures filled with vadose cements have asymmetric cement textures associated with connected karstic porosity, and formed above the water table. The variations in deformation and diagenesis result from decreasing displacement and footwall uplift towards the ends of the fault segment giving differential uplift across the water table/sea level. Variation in fracture styles and diagenetic textures have been correlated with the position of sample sites along the fault segment. It is therefore possible to use outcrop studies of deformation and diagenetic textures from different portions of fault segments to produce simple predictive models to aid hydrocarbon exploration and production strategies.
Structural controls on sedimentation in continental rifts
Authors: Lambiase J.J. , Bosworth W.
Published: April 2008

Structure and climate have each been proposed as the primary control on sedimentation in continental rifts. Numerous Phanerozoic rifts spanning a wide range of inferred palaeoclimates have consistent relationships between structure and sedimentation, and between structural evolution and stratigraphic sequences. Six principal structural features strongly influence sedimentation patterns in rifts. (1) Topographically high accommodation zones segment rifts into 50–150 km long structural half-graben, each one a discrete depocentre. (2) Reactivation of pre-existing structural elements commonly controls the location and orientation of accommodation zones. Drainage systems outside the rift then may be captured and diverted towards the rift axis, establishing some accommodation zones as input points for sediment. (3) Footwall uplift at main border faults directs most drainage away from young rifts. Along the rift flanks, uplift is minimized at accommodation zones, enhancing their role as sediment input points. (4) Gentle roll-over of topography on flexural margins produces drainage systems that are generally small, and that do not introduce large amounts of sediment into the rift. (5) Breakup of the basin floor into elongate, rift-parallel fault blocks greatly enhances along-axis sediment transport. (6) Development of fault relay zones and transfer faults localizes down-dip sediment movement. These structural controls occur in the present-day low latitude rifts of East Africa, where climate ranges from arid to humid. Similar features have been identified in ancient rifts that are interpreted to span a comparable range in climates. Clearly, climate is important in determining the sedimentary history of continental rifts. The present study indicates that climatic effects, however, are superimposed on the fundamental structural geometries that evolve during rifting.
Characteristics of selected Palaeogene and Cretaceous lacustrine source basins of Southeast Asia
Authors: Williams Harold H. , Fowler Martin , Eubank Roger T.
Published: April 2008

Lacustrine rift systems have sourced a large portion of the known hydrocarbon reserves in Southeast Asia, and their importance as exploration targets is well established. Our exploration experience has shown that the source rock quality, maturation and the presence of a post-rift marine sag sequence containing reservoirs are the most important factors controlling the hydrocarbon potential of these basins. However, variability in rift architecture, depositional processes, sedimentary geochemistry and thermal history of rift systems is poorly documented in the published literature. Often in the early stages of exploration of rift basins, data available to assess their prospectivity is limited. A more extensive data base of analogues to draw on, particularly non-producing analogues, would allow the explorationist to make better interpretations. In this paper the rift architecture, depositional sequences and sedimentary geochemistry of four Palaeogene and one Cretaceous/Palaeogene graben systems in Indonesia (Sumatra), Thailand and China are described. The graben described are in the Bandar Jaya Basin in South Sumatra, Kampar Kanan Basin in Central Sumatra, Ombilin Basin in West Sumatra, Phetchabun Basin in North Thailand and Dongting Basin in China. These graben cover a range of rift architectures related to wrench tectonic regimes and represent depositional sequences deposited in freshwater to saline lacustrine environments in deep to shallow water lakes. Climatic conditions range from arid in Dongting to tropical throughout Indonesia and Thailand. The geochemical characteristics of the source rocks in these graben are described in the context of their depositional systems.
Karoo Rift basins of the Luangwa Valley, Zambia
Authors: Banks N. L. , Bardwell K. A. , Musiwa S.
Published: April 2008

An integrated study of the Luangwa Valley Karoo Rift basins of NE Zambia has been carried out. Approximately 3000 km of seismic data were used in addition to gravity and aeromagnetic surveys and data from two exploration wells. The present day Luangwa Valley is underlain by two opposing Karoo half-graben separated by a central accommodation or transfer zone. The basins contain up to 8000m of Permo-Triassic, mainly clastic, sediments. The Lower Karoo consists of basal glacial deposits overlain by a sequence of interbedded coal, shale and siltstone, and capped by a very thick (up to 5000 m) lacustrine section of interbedded siltstone and mudstone. The Upper Karoo consists of interbedded coarse sandstones and siltstones thought to be of braided stream and flood plain origin. Rifting appears to have been initiated in the early Permian. The timing of subsequent rifting and inversion events has been related to strike-slip movements along the Mwembeshi Shear Zone, a major Precambrian lineament. Study of the seismic data has revealed a number of structural and stratigraphic plays in the basin. The best potential for hydrocarbon accumulation occurs within the lowest part of the Karoo section where source rocks and reservoirs are postulated in the Luwumbu Formation and the lowest part of the Madumabisa Mudstone. Since these intervals were not reached by either of the exploration wells drilled it is felt that the hydrocarbon potential of the Luangwa Graben has not yet been fully tested.
Hydrocarbon habitat in the Songliao Rift Basin, China
Authors: Desheng Li
Published: April 2008

The Songliao Basin covers an area of 260 000 km
2
and is a Jurassic-Cretaceous rift depression basin which overlies basement that is composed of Palaeozoic metamorphic rocks and Hercynian granites. The basin has undergone four evolutionary stages, including: (1) the Triassic to middle Jurassic Thermo-Arch Stage; (2) the Late Jurassic to Early Cretaceous Taphrogenic Stage; (3) the Early to Late Cretaceous Depression Stage; and (4) the Late Cretaceous to Quaternary Shrink-Folding Stage

The Lower Cretaceous non-marine Qingshankou Formation provides very favourable oil and gas source rocks. The supergiant Daqing Oilfield, discovered in 1959, is located in the central anticline belt of the basin. Reservoir formations are Lower Cretaceous S, P and G Reservoir Group sandstones. Since 1976, Daqing Oilfield has yielded more than 50 million tons of crude oil per year (one million barrels/day) and has maintained its peak production for 17 years. Its cumulative oil production by the end of 1992 was 1.18 billion tons (8.2 billion barrels).
Application of sedimentology to the development and extraction of deep-mined coal
Authors: Fulton Iain M. , Guion Paul D. , Jones Neil S.
Published: April 2008

Most deep-mined coal in the UK comes from the Westphalian A and B (Langsettian-Duckmantian) of the coalfields of the Pennine Basin, in which several sedimentary facies have been identified. Deep mining of coal may be divided into four main operations: shaft sinking and driving of access drifts; drivage of underground roadways; excavation of coal faces; and the provision of temporary underground storage. The prediction of the lithological, geometrical and geotechnical properties of the various sedimentary facies likely to be encountered during mining is important for the success of all these operations.

Roadway location may be influenced by the geotechnical attributes of the sedimentary facies, which also govern the type of machinery and methods used to cut and support the roads. The thickness, quality and continuity of the coal, and the properties of the roof and floor, are important in selecting the location of longwall faces and in their productivity. Facies composed of strong sedimentary rocks are likely to be the most suitable for the location of staple shafts and bunkers for coal storage. The geotechnical properties of each of the sedimentary facies is described, with particular reference to their excavation and support characteristics, and the discontinuities of sedimentary origin that are likely to be present. Distal lacustrine delta facies appear to provide the optimum roof conditions for coal faces and roadways.

A simple scheme for the prediction of the sedimentary facies which may influence mining is presented, and a variety of mapping techniques, including facies maps, isopach maps and palaeocurrent analysis, are outlined. An improved knowledge of the properties and distribution of the various sedimentary facies at a mine site should lead to improved productivity and a reduction in risks and costs.
Discrimination of regionally extensive coals in the Upper Carboniferous of the Pennine Basin, UK using high resolution sequence stratigraphic concepts
Authors: Hampson Gary
Published: April 2008

During the late Carboniferous, the Pennine Basin, UK was a thermally subsiding, intracratonic basin with little active tectonism. The sedimentary fill of the basin consists of coarsening-upward coal-bearing deltaic cyclothems bounded by widespread faunalconcentrate condensed horizons. The Yeadonian (Namurian G
1
) Rough Rock Group comprises three such cyclothems that have been studied at outcrop and in core using detailed sedimentological logging and then applying high resolution sequence stratigraphical concepts. Key surfaces and systems tracts are recognized and a sequence stratigraphic framework is constructed using these criteria and incorporating basin biostratigraphy and palaeogeography.

Two regionally extensive coals are present in the Rough Rock Group. They formed under relatively long-lived, basin-wide conditions of (1) clastic sediment starvation and (2) rising water-table, creating accommodation space for peats to accumulate and be preserved in mires. These conditions are characteristic of deposition in transgressive systems tracts and in the context of the Rough Rock group sequence stratigraphic framework these coals are identified as up-dip equivalents of initial flooding surfaces. This example raises wider implications for the discrimination of other regionally extensive coals in the upper Carboniferous strata of the Pennine basin using high resolution sequence stratigraphic concepts.
Overview of the influence of syn-sedimentary tectonics and palaeo-fluvial systems on coal seam and sand body characteristics in the Westphalian C strata, Campine Basin, Belgium
Authors: Dreesen Roland , Bossiroy Dominique , Dusar Michiel , Flores Romeo M. , Verkaeren Paul
Published: April 2008

The Westphalian C strata found in the northeastern part of the former Belgian coal district (Campine Basin), which is part of an extensive northwest European paralic coal basin, are considered. The thickness and lateral continuity of the Westphalian C coal seams vary considerably stratigraphically and areally. Sedimentological facies analysis of borehole cores indicates that the deposition of Westphalian C coal-bearing strata was controlled by fluvial depositional systems whose architectures were ruled by local subsidence rates. The local subsidence rates may be related to major faults, which were intermittently reactivated during deposition. Lateral changes in coal seam groups are also reflected by marked variations of their seismic signatures. Westphalian C fluvial depositional systems include moderate to low sinuosity braided and anastomosed river systems. Stable tectonic conditions on upthrown, fault-bounded platforms favoured deposition by braided rivers and the associated development of relatively thick, laterally continuous coal seams in raised mires. In contrast, rapidly subsiding downthrown fault blocks favoured aggradation, probably by anastomosed rivers and the development of relatively thin, highly discontinuous coal seams in topogenous mires.
Palaeoecological models, non-uniformitarianism, and tracking the changing ecology of the past
Authors: Bottjer David J. , Campbell Kathleen A. , Schubert Jennifer K. , Droser Mary L.
Published: April 2008

Palaeoecological models are commonly used by palaeontologists and sedimentary geologists to reconstruct ancient palaeoenvironments. In order to illustrate the ways in which palaeoecological models develop as new information is discovered, four examples are discussed: (1) reefs and fossil cold seeps; (2) biofacies models for strata deposited in ancient oxygen-deficient environments; (3) palaeoenvironmental distributions of post-Ordovician stromatolites; and (4) onshore-offshore trends of trace fossils. The development of physical sedimentological and geochemical criteria that can independently be used for evaluating ancient depositional environments provides a base line with which to assess palaeoecological change through geological time. Thus, the possibility now exists to free palaeoecological models and the study of ancient ecology from traditional uniformitarianism and Lyell’s dictum that the ‘present is the key to the past’, so that palaeoecological models may be developed which are useful for segments of time not anchored in the present. This approach will also be essential for evaluating the changing ecology of the past, which at present is only poorly understood. Future development and independent testing of such palaeoecological models will allow a more complete appreciation of the changing roles of environment, ecology and evolution in the history of life.
Fossil indicators of nutrient levels. 2: Evolution and extinction in relation to oligotrophy
Authors: Brasier M. D.
Published: April 2008

An overview of hypotheses on the impact of nutrients on biosphere history reveals the need for quantitative fossil indices for oligotrophy. Emphasis is given to indicators for endosymbiosis in relation to oligotrophic regimes, and a middle Eocene case history is selected for closer examination. Quantitative data, relating to changes in cellular integration in foraminifera are consistent with a catastrophic shift from highly oligotrophic towards eutrophic conditions, associated with global cooling near the end of the middle Eocene.
Organisms and the substrate: response and effect
Authors: Goldring Roland
Published: April 2008

Benthic organisms are invaluable in the interpretation of ancient environments. The principles of animal behaviour at and below the substrate surface (e.g. encrustation, burrowing, boring, feeding) are well established. Three areas in particular are currently being developed: (1) Ichnofabrics are replacing ichnofacies in facies interpretation. The use of archetypal ichnofacies is abandoned in favour of an integrated approach (including palynology and geochemical analysis), using autecology followed by synecology and synthesis, and ichnofabric analysis; this provides greater resolution; (2) Comparisons of deep-sea studies with ancient bathyal sediments are leading to a better understanding of turbidite facies: the factors involved in the formation of the complex tiering of the pre-event ichnocoenoses and those involved in the colonization of the turbidite. The morphology and ethology of several shallow and deep-water ichnotaxa (e.g.
Zoophycos, Paleodictyon
) are still imperfectly understood; (3) The
fabric
of shell and skeletal concentrations reflects the dynamics of the final depositional environment, and the
taphonomic signature
, in the main, the environmental conditions under which the shells formed and accumulated. Together, these aspects, and their relative stratigraphic frequency, reflect regional patterns of hydraulics and basin dynamics. Advances in the appreciation of time-averaging should provide renewed interest in research into palaeocommunities.

The stratigraphical distribution of hardgrounds remains an enigma and their potential significance for basin analysis awaits full evaluation. Two groups of hardground are recognized: whether they formed under omission and wave bevelling, or solely under omission.
Which sub-seismic heterogeneities influence waterflood performance? A case study of a low net-to-gross fluvial reservoir
Authors: Jones Alistair , Doyle James , Jacobsen Torgrim , Kjønsvik Dagrun
Published: April 2008

Waterflood performance in a fluvial reservoir is examined to assess which aspects of the reservoir are important to describe for accurate simulation of recovery performance. The study is based on a low net-to-gross fluvial reservoir and we consider the influence of sedimentary heterogeneities which are sub-seismic-scale in a full-field model.

A detailed hierarchical 3D geological model is developed and flow properties are upscaled. Particular attention is given to representing the influence of cross-stratification. A screening study is then conducted to assess which heterogeneities and scales have the greatest influence on recovery predictions. By using experimental design the influence of a large number of factors is examined in an efficient manner.

The results show that parameters describing channels and channel belts have the greatest influence. Somewhat surprisingly, the number of channels in a belt and their width and thickness has at least as much influence on sweep efficiency as net-to-gross and the stacking of belts. In addition to the influence of these large-scale heterogeneities, capillary effects at the laminae scale within cross-stratification have a significant effect on recovery.

Early identification of important heterogeneities will allow for more efficient field exploitation. The strongest impact of this approach may be to focus data collection early in the life of a field on the most important types of heterogeneities to be expected for a given depositional and stratigraphic setting.
Stress effects on reservoir flow: — Numerical modelling used to reproduce field data
Authors: Heffer K. J. , Koutsabeloulis N. C.
Published: April 2008

Empirical field data show a strong correlation between the preferred directionality in many normal IOR floods and the local modern-day state of earth stress. The conjecture that this correlation is due to the activation of natural fractures by changes in the stress state induced by the flood has been tested with coupled modelling of fluid flow and geomechanical deformaton. The results of initial generic modelling incoroporating thermal stresses from cold water injection showed good agreement with the details of the field data, lending strong support to the conjecture. The modelling indicated that directionality trends increasingly towards the major horizontal stress axis as a flood progresses. The new coupled modelling capability provides an improved tool for planning well patterns, infill drilling and flood management.
Prudhoe Bay: infill drilling in gravity dominated WAG floods
Authors: Wilcox T. C. , Polzin M. W. , Kuo S. S. , Humphrey K. J.
Published: April 2008

Many horizontal miscible gas floods experience relatively low vertical sweep efficiency due to gravity segregation of the injected solvent and reservoir liquids. This is true for the Prudhoe Bay Miscible Gas Project (PBMGP) in which the enriched-hydrocarbon miscible injectant (MI) is expected to sweep only approximately 300f the reservoir volume in the project areas. The Prudhoe Bay reservoir has a relatively thick oil column, high inter-sand vertical permeability, high net-gross, and is generally developed on 80 acre (324 = 103 m2) well spacing.

An infill drilling project was implemented in the Northwest Fault Block (NWFB) area of Prudhoe Bay (Fig. 1) to improve solvent sweep efficiency and increase EOR reserves. This project differs from many EOR infill drilling projects in that most of the additional recovery is attributable to EOR, with minimal improvement in waterflood recovery.

The infill project is feasible because the reservoir is relatively thick, and sufficient EOR reserves are obtained from miscible displacement around new injectors. A significant increase in flood rate can also occur, resulting in production acceleration. Results of the project will guide analysis of infill drilling in other EOR areas at Prudhoe. The Prudhoe Bay Miscible Gas Project is the world’s largest enriched-hydrocarbon miscible gas flood, and is located in northern Alaska, USA.
Laboratory evaluation and field tests of a silicate gel system intended for use under North Sea conditions
Authors: Kristensen R. , Lund T. , Titov V. I. , Akimov N. I.
Published: April 2008

This paper describes results from laboratory experiments and field tests, using basic Na-silicate. The work described has been performed as a part of the programme to qualify chemical systems for large-scale in-depth permeability modification processes in the North Sea.

Gelling properties of basic silicate solutions have been measured as a function of pH, silicate concentration and temperature. The gelation times decrease with decreasing pH and increasing concentration. Increasing temperature will also give shorter gelation times. Adsorption of silicate is found to be linearly increasing with increasing silicate concentration.

The rock buffer capacity is found to be strongly dependent on mineralogy. As an example, kaolinite has a buffer capacity more than ten times higher than other minerals. Both adsorption and buffer capacity will strongly influence the composition and gelation properties of the silicate solution in an in-depth treatment.

In total, three field tests have been performed. The test area was chosen on the basis of Gullfaks reservoir data. From a one well formation test, field information was obtained on silicate adsorption and buffer capacity. The field results confirmed the laboratory data. The other two field pilots were run to obtain information on an optimum injection scheme for injection of silicate solutions into reservoirs with brines with high salinity and hardness. A preflush of fresh water was found to condition the critical reservoir parameters and allow for good injectivity of the basic silicate solution. The potential of water shut-off treatments of high watercut producers was examined in two well tests. The wells produced before treatment at a watercut of 96%. In one of the wells the watercut was reduced to less than 10% at the same fluid production rate and the watercut has been lower than 20% for more than eight months after treatment.
An investigation of the vertical sweep efficiency of steam drive in a layered reservoir
Authors: Godderij R. R. G. G. , Gümrah F. , Palmgren C. T. S. , Bruining J.
Published: April 2008

This paper presents results obtained in a theoretical and experimental study on the efficient use of steam injection for the recovery of oil from a layered, dipping reservoir.

For the experimental part, we use a scaled transparent vacuum model. An infrared camera follows the evolution of the steam condensation front. For the theoretical part, we use an interface model to describe the behaviour of the steam condensation front. Two approximations for the flow field are examined, leading to a Dietz-Dupuit model and a cross-flow model respectively.

The theoretical results are compared to the experimental values to investigate the validity of the models. We conclude that vertical sweep efficiency improves with dip. Furthermore the Dietz-Dupuit model leads to a reasonable agreement in the case with a high permeable layer at the top. With the high permeable layer at the bottom, however, discrepancies occur. The simple cross-flow model shows agreement with all of the gravity stable experiments, but lacks a firm physical basis.
Morgan pressure cycling in-situ combustion project: performance and modelling
Authors: Marjerrison D. M. , Fassihi M. R.
Published: April 2008

Amoco Canada has been operating a pressure cycling fireflood at Morgan, in the Lloydminster area of Alberta, Canada, since the mid-1980s. A pressure cycle consists of air injection into a central injector accompanied by production from its surrounding producers. The reservoir contains a 12° API heavy crude and the wells are laid out in 30 acre (12 ha), seven-spot patterns. The rapid drop in crude oil prices in 1986 led to the successful development and implementation of an innovative pressure cycling-combustion process. This paper looks at the performance of the project and the numerical modelling done to history-match the results. The numerical modelling effort was undertaken to explain the high oil rates (individual wells at 120 STB/D = 19 m
3
/D) and the extraordinarily low injected air to produced oil ratio (AOR) of less than 2 MSCF/STB (355 m
3
/m
3
). Mechanisms incorporated in the numerous model include: (1) a high-speed channel from the injector to the producer, (2) combustion gas entrainment by the oil phase and (3) oil upgrading. The physics involved with these mechanisms are explained and the model results are compared with the actual field data. Also, model sensitivity to variations in the assumptions regarding the mechanisms are discussed.
Cyclostratigraphy and correlation of the Cenomanian Stage in Western Europe
Authors: Gale A. S.
Published: April 2008

Basinal successions of Cenomanian age in Western Europe are dominated by pelagic and hemipelagic sediments, and display conspicuous primary bedding cyclicity (couplets) which is attributed to climatically-controlled variations in carbonate productivity. Within single basins individual rhythmic couplets are widely identifiable and enable basin-wide decimetre-scale correlation. A high resolution ammonite biostratigraphy (aided by inoceramid bivalves) is used to correlate successions in separate basins, and allows a comparison of couplet numbers to be made. Couplet numbers are similar across Western Europe, in spite of an order of magnitude thickness variation, and the carbonate: clay ratios of beds and groups of beds are persistent and diagnostic of particular levels. Additionally, beds with distinctive ichnofabrics (e.g. abundant dark
Chondrites
) are regionally extensive in the basins of northern Europe. It has proved possible to construct a composite cyclostratigraphy for the Cenomanian, graduated by 212 precession units (mode at 21 ka), which indicate a duration for the stage of 4.45 Ma. Corroboration of this cyclochronology comes from radiometric dating; the Middle Cenomanian basal
A. rhotomagense
Zone to the Upper Cenomanian basal
N. juddii
Zone contain 107 precession couplets in Western Europe, giving a duration 2.24 Ma. The equivalent biostratigraphic interval in the Western Interior Basin of the USA recently yielded Ar-Ar dates of 2.2 Ma from sanidines in bentonites The dominance of the precession cycle in the mid-Cretaceous is in keeping with results from climate sensitivity modelling.
Geochemical constraints from formation water analyses from the North Sea and the Gulf Coast Basins on quartz, feldspar and illite precipitation in reservoir rocks
Authors: Bjørlykke Knut , Aagaard Per , Egeberg Per K. , Simmons Scott P.
Published: April 2008

Formation water analyses from North Sea and Gulf Coast Basin reservoirs provide important constraints on burial diagenetic processes. Thermodynamic calculations show that aluminium solubilities are low (< 1 ppm at 130°C) even in the presence of organic acids. Illite therefore forms mainly at the expense of dissolving aluminous minerals such as smectite and kaolinite. The potassium values of the formation waters are always lower than K-feldspar saturation during burial diagenesis (> 1.5 km burial depth), suggesting that K-feldspar is continually being dissolved. The potassium concentrations indicate, in most cases, super-saturation with respect to illite. This is evidence that the failure of illite to form is probably because aluminium is not available. In the North Sea Basin, potassium concentrations approach equilibrium with kaolinite and illite at higher temperatures (140°C), indicating that illitization at that point keeps up with K-feldspar dissolution. In the subsiding parts of the Gulf Coast Basin, where kaolinite and smectite are being heated, the potassium values are clearly lower than in the uplifted (cooling) parts of the basin where illite is not forming. This suggests that the potassium concentrations are to a large extent controlled by the rate of illite precipitation, which again depends on the availability of aluminium from dissolving Al-minerals. There is considerable evidence that both illitization and quartz cementation may continue in the water-saturated parts of the pore after oil emplacement. This is also consistent with an almost closed-system burial diagenesis. The primary mineralogical composition of the sediments and early diagenetic processes strongly influence deeper burial diagenesis and in particular the precipitation of illite.
Compositional heterogeneities in oilfield formation waters: identifying them, using them
Authors: Smalley P. C. , Dodd T. A. , Stockden I. L. , Råheim A. , Mearns E. W.
Published: April 2008

Two new techniques are now available for gaining information on formation water composition from core samples: core centrifugation, in which preserved core samples, complete with their preserved fluid content, are ultracentrifuged to extract the oil and water; and residual salt analysis (RSA), where salts are redissolved from unpreserved dry core and parameters such as
87
Sr/
86
Sr are measured in the leachate. These methods are described and details given on the quality control measures that are crucial in order to avoid artefacts of core contamination by drilling fluids. Such methods have allowed formation-water compositional data to be obtained at the sub-metre scale, in some cases revealing significant small-scale variations (gradients and steps) in formation-water chemistry.

Resistivity (related to salinity) variations are important for interpretation of water saturation from electrical resistivity logs, which in turn affects the estimate of the total amount of oil contained in the field. In the Machar chalk-reservoired oilfield, formation-water data derived by RSA and core centrifugation both indicate variations in salinity (TDS 140 000–220 000 ppm) that significantly affect resistivity. In some oilfields, assumption of a single resistivity value for the whole field could thus lead to errors in petroleum reserves estimation.

Step changes in formation-water composition can be used as indicators of reservoir fluid compartmentalization, important for field development strategy. Examples are given of cases where RSA variations have been used to distinguish between laterally restricted and laterally extensive shales, both in the oil and water legs. Analysis of water compositional variation forms a valuable addition to the range of tools available for assessing reservoir compartmentalization during reservoir appraisal. This is because natural variations in fluid compositions provide quasi-dynamic data (i.e. how fluids have moved slowly during geological time), from which inferences can be made about more rapid fluid-flow patterns under dynamic (oil production) conditions.
Characterization of high molecular weight hydrocarbons (>C
40
) in oils and reservoir rocks
Authors: Philp R. P. , Bishop A. N. , Del Rio J.-C. , Allen J.
Published: April 2008

The presence of high molecular weight hydrocarbons in crude oils has been known for some time, particularly in view of their ability to precipitate and form barriers and blockages in pipelines. Recent advances in analytical technology have led to the development of capillary columns for gas chromatography capable of withstanding high temperatures (350 to 450°C), which allow the characterization of hydrocarbons with carbon numbers as great as C100 and above. Consequently, the individual high molecular weight hydrocarbon compounds in oils and waxes can now be identified. In the present paper, results are presented from high temperature gas chromatographic analyses of naturally occurring waxes. These clearly show the presence of significant concentrations of hydrocarbons with carbon numbers in excess of C
40
in many of these samples. In order to assist in the quantification of these high molecular weight hydrocarbons, we have also assessed the potential of thermal microdistillation coupled with a field ionization detector, to obtain more accurate molecular weight distributions in wax samples. This is based on the concept that high temperature gas chromatography has an upper carbon number limit in the range C
100
to C
120
. Thermal distillation may provide the ability to exceed that limit and determine the presence of compounds above C
120
. More detailed characterization of the wax fractions on a molecular level has the potential to provide clues as to the composition and origin of the residual components left in a petroleum reservoir at the end of its production history. A better understanding of the nature of this material will lead to improved secondary and tertiary recovery mechanisms, and other enhanced oil recovery procedures.
Overview of the geochemistry of some tar mats from the North Sea and USA: implications for tar-mat origin
Authors: Wilhelms A. , Larter S. R.
Published: April 2008

Tar mats and oil legs from several marine source-rock derived oil fields (Ula and Oseberg, North Sea and an unidentified North American example) have been geochemically characterized. Tar mats were derived from the same petroleum system as the oil-leg oils and are found in the most porous and permeable portions of the reservoir. It is possible to eliminate some potential tar-mat formation mechanisms using the data from case histories and subsurface asphaltene solubility characteristics. Adsorption of asphaltenes on reservoir clays and light to moderate levels of biodegradation can probably be eliminated as feasible tar-mat formation mechanisms. Thermal degradation of oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to formation of tar mats in the Ula Field, whereas increased gas solution in the oil leg may have contributed to deasphaltation in the Oseberg Field. Based on tentative mass-balance calculations, the extreme pressure dependence of asphaltene solubility in subsurface crude oils and the occurrence of tar mats in zones of elevated permeability, it is suggested that tar-mat formation is initiated during petroleum migration. While tentative, this model may provide a more generic initiation mechanism than others in the literature.