bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reservoir characteristics of a braid-plain depositional system: the Upper Carboniferous Pennant Sandstone of South Wales
Authors: Jones Julie A. , Hartley Adrian J. 
Published: April 2008

Abstract
Braid-plain depositional systems form important hydrocarbon reservoirs in Palaeozoic, Mesozoic and Cenozoic basins throughout the world. Production from these reservoirs is frequently complex due to the nature and scales of permeability barriers inherent to braid-plain deposits. An analysis of the nature and distribution of permeability barriers from the well exposed braid-plain deposits of the Upper Carboniferous Pennant Measures of South Wales has revealed a variety of scales of heterogeneity. Potential permeability barriers include: (1) conglomerate channel lag deposits, (2) thick accumulations of plant detritus, (3) distribution of fines on foreset, set and coset boundaries, (4) laterally discontinuous mudstone bodies parallel or transverse to channel azimuth and (5) periodic basin-wide coal and seatearth development. Comparison of the Pennant outcrop with producing braid-plain reservoirs indicates that these permeability barriers are commonly encountered during production and are likely to be present (to some extent) in all potential braid-plain reservoirs. Account of heterogeneity distribution during production/development of braid-plain reservoirs will aid cost-effective production strategy.
Cyclical discharge variation recorded in alluvial sediments: an example from the Devonian of southwest Ireland
Authors: Kelly S. B. 
Published: April 2008

Abstract
Cross-stratification is a common sedimentary structure in alluvial deposits and can often be recognized in core. The quantitative study of cross-strata can potentially yield information concerning the behaviour of alluvial systems through time. Such information is valuable to a reservoir geologist when assessing the architecture of an alluvial reservoir.
Several alluvial systems in the Munster Basin (Devonian), southwest Ireland, display the characteristics of terminal fans generated in a ‘hydrographically-closed’ basin. Analysis of three formations reveals cyclical variations (‘megacycles’) in the dimensions of cosets and sets within channel sandbodies. The ‘megacycles’ are 100–200 m thick and record increases and subsequent decreases in the dimensions of sedimentary structures. The megacycles are also reflected by the density of channel deposits such that high densities are associated with relatively large channels and low densities are associated with relatively small channels. Cyclicity is also locally developed in the flood-plain deposits as alternations of red beds with calcretes (oxidizing conditions) and green/grey beds with pyrite nodules (reducing conditions).
Established relationships between dune size and flow depth suggest that cyclical variations in the size of sedimentary structures reflect discharge fluctuations. This is supported by the close correlation between set thickness and coset thickness, the latter being closely related to sandbody thickness. The cyclical variations in channel sandbody density and flood-plain conditions are also thought to reflect discharge variation.
Available biostratigraphic data and radiometric dates suggest that the cycles represent periods of several hundred thousand years. The periodicity and regularity of the cycles together with their relationship to discharge variation suggests that they may be related to astronomical forcing of the Earth’s climate.
A review of aeolian bounding surfaces, with examples from the Permian Minnelusa Formation, USA
Authors: Fryberger Steven G. 
Published: April 2008

Abstract
The recognition and use of bounding surfaces continues to be a problem in the study of modern and ancient aeolian sediments. The economic problems involve the apparent partitioning of aeolian petroleum reservoirs by cementation along bounding surfaces and associated strata. Bounding surfaces have also become a significant part of outcrop description. Ideally, bounding surfaces in ancient rocks should be tied to modern analogs, of which many are now recognized.
The currently popular approach to bounding surface description involves breakdown of rock units into orders based on theoretical relationships to draa migration. As described in this report, however, there are numerous bounding surfaces within aeolian genetic units, and between aeolian and non-aeolian strata that are not the product of draa migration. Indeed, conditions in which modern bedforms climb, and are preserved in perfect sequence are relatively rare, occurring most commonly where sand seas pile up at mountain fronts. Further, aeolian deposition and preservation is more complex and episodic than can be accommodated by the restrictive requirements of the climbing bedform model.
The author’s preferred approach to the description of aeolian sequences is in terms of genetic units and subunits, with objective description of crossbed sets using either Campbell’s or Mckee & Wier’s methods. These methods tie in well to sequence stratigraphic concepts that are applied at higher heirarchial levels.
Some examples of the application of bounding surface geometries and terminologies can be found in the Permian Minnelusa Formation of Wyoming, USA. The Upper Minnelusa Formation comprises a parasequence set of at least four parasequences. Each parasequence boundary was formed by marine transgression over an aeolian dunefield. The entire sequence is bounded by unconformities at the top and based. Most oil exploration takes place within the individual parasequences. Development drilling, however, usually making use of core, focuses on bounding surfaces at a much smaller scale. Perhaps surprisingly, the most important form of bounding surface control on reservior quality that can confidently be identified is fabric-selective cementation along primary bounding surfaces and strata. Other forms of cementation and decementation, not directly tied to bounding surfaces are also important.
Mixed aeolian sandsheet and fluvial deposits in the Tumblagooda Sandstone, Western Australia
Authors: Trewin Nigel H. 
Published: April 2008

Abstract

Field investigation of the Late Silurian Tumblagooda Sandstone of Kalbarri, Western Australia demonstrates that the lower part of the formation comprises a unit of fluvial trough crossbedded sandstone (450 m) overlain by previously unrecognized aeolian sandsheet deposits (350 m) which include small (to 2 m) aeolian dunes. Interbedding of aeolian and fluvial facies is present and fluvial facies decrease in importance down the fluvial transport path which was to the west and northwest. Aeolian transport was to the southeast and resulted in formation of medium- to coarse-grained low-angle sandsheets and small parallel-crested dunes. Fluvial transport in aeolian-dominated areas was deflected to the southwest along interdune corridors. Interdune deflation areas were frequently flooded, probably by rising ground water. Traces fossils are abundant, including beds bioturbated by
Heimdallia
and numerous arthropod trackways which cross aeolian strata. The dominance of sandsheet relative to dune morphologies is ascribed to the following factors: (1) mediumcoarse sand grain size, (2) high water table, and (3) short aeolian transport distance between fluvial channels not permitting extensive dune formation. A lack of vegetation is essential for development of fluvial and aeolian sandsheet facies. Fluvial sandstones were cemented by quartz overgrowths with only minor compaction. Aeolian dune sandstones have only minor quartz cement but show greater chemical compaction effects. Porosity retention is better in the aeolian sandstones.

The Taylor Group of Antarctica which is thought to be of early Devonian age shows strong similarities to the Tumblagooda Sandstone in terms of trace fossil assemblages, and sedimentology.
Structural and climatic controls on facies distribution in a mixed fluvial and aeolian reservoir: the Triassic Sherwood Sandstone in the Irish Sea
Authors: Meadows Neil S. , Beach Alastair 
Published: April 2008

Abstract
Interpretation of nearly 5000 km of seismic data, and facies analysis of approximately 900 m of core from 11 wells, has produced structural and sedimentological models of the Triassic Sherwood Sandstone in the East Irish Sea Basin. Sedimentary facies recorded include moderate to low-sinuosity braided fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and non-evaporitic playa deposits. The establishment of facies associations and correlation of these with wireline logs has enabled extrapolation of sedimentary models into uncored intervals and between wells. The identification of differing seismic character associated with the various facies has made possible the mapping of these diagnostic seismic packages using a seismo-stratigraphic approach. The resulting distribution of seismically derived facies accords with proposed structural and sedimentological models, predicts the position and course of fluvial channels operating in the basin during late Sherwood Sandstone time, and shows that these are largely controlled by fault activity. Detailed facies analysis of sandflat sequences reveals a systematic variation in the proportion of sheetflood and playa to aeolian deposits. It is suggested that this corresponds to a climatic control with sheetflood- and playa-dominated sequences representing relatively wet episodes with a high water table, and aeolian-dominated sequences representing relatively dry episodes with a lowered water table. Correlation of these climatic events may introduce a chronostratigraphic element to the analysis of these continental ‘red-bed’ sequences.
The Rough Gas Field: distribution of Permian aeolian and non-aeolian reservoir facies and their impact on field development
Authors: Ellis D. 
Published: April 2008

Abstract
The Rough Gas Field lies in United Kingdom Continental Shelf Blocks 47/3d and 48/8b in the Southern North Sea and is operated in a gas storage mode by BGE&P involving the annual injection and production of large volumes of gas. To achieve full efficiency in this designated role it was necessary to carry out a detailed evaluation of the main producing reservoir, the Permian Rotliegendes Sandstone, after early development drilling demonstrated a need to re-evaluate the initial geological model. The present geological model is based on the detailed analysis of nine fully cored new development wells and well productivity surveys following an original six well conventional depletion. This resulted in the successful targeting of later development wells into specific geographic areas of the field for optimal field development.
Reservoir facies development can be described in terms of complex fluvial processes associated with the basin margin and aeolian influences associated with the basin as a whole. Within the Rough Field the Rotliegendes can be subdivided vertically into three reservoir units or layers (Units A, B, and C) and laterally into regions characterized by differing degrees of aeolian or non-aeolian influence. It is observed that the highest reservoir permeabilities are associated with aeolian or reworked aeolian sands while flow profile studies indicate close correspondence between injectivity/productivity and sedimentary facies. Aeolian facies or reworked aeolian sands of the lowermost ‘A’ unit facilitate the bulk of gas injection and production while fluviatile sands in all three units show reduced or even zero flow.
Diagenesis of Triassic Sherwood Sandstone Group rocks, Preston, Lancashire, UK: a possible evaporitic cement precursor to secondary porosity?
Authors: Strong G. E. 
Published: April 2008

Abstract
Boreholes at Kirkham [SD 4324 3247] and Weeton Camp [SD 3888 3603] in Lancashire penetrate Triassic sandstones of the Sherwood Sandstone Group that occur near the eastern margins of the Manx-Furness (East Irish Sea) Basin. Petrological analysis of sandstone samples from cores was undertaken to determine diagenetic characteristics.
Some very porous sandstones with porosities of the order of 25–30% have cement dissolution textures indicating that earlier extensive cements have largely been removed by aggressive pore fluids, probably during the Triassic. These early cements were calcite and non-ferroan dolomite, and possibly later evaporitic cements (anhydrite, gypsum, halite) introduced during the period of deposition of the Mercia Mudstones. Non-ferroan dolomite post-dates and locally replaces early calcite. There is minor ferroan dolomite, late calcite and pyrite.
Extensive early quartz overgrowths and cements have developed in some quartz arenites. Porosity values are about 10 to 13%, but permeabilities are likely to be low. These quartzcemented quartz arenite units may correlate between the boreholes.
The clay mineral assemblages of the sandstones of each borehole are significantly different. The Kirkham sequence is characterized by illite with Fe-rich chlorite and minor smectite, whereas the Weeton Camp sequence is dominated by kaolinite with minor illite and smectite. These differences are attributed to different pore fluid histories, possibly structurally controlled.
A new lithostratigraphy and depositional model for the Upper Rotliegend of the UK Sector of the Southern North Sea
Authors: George Gareth T. , Berry Jeremy K. 
Published: April 2008

Abstract
The lack of a pragmatic basin-wide lithostratigraphic subdivision for the Upper Rotliegend of the Southern North Sea Basin has presented numerous problems when attempting to correlate wells or implement reservoir zonation schemes within this important gas province. This deficiency has also made it extremely difficult to predict facies distribution, and thus reservoir quality, when generating or assessing new prospects. With this in mind, a detailed study of the cores and logs from 126 released wells from the UK sector of the gas province was undertaken. The results of this study suggest that it is possible to subdivide the Upper Rotliegend into five lithostratigraphic units (Units 1–5 from oldest to youngest) based on the recognition of drying-upward cycles. These drying-upward cycles are developed throughout the basin in both the Leman Sandstone and Silverpit Claystone Formations. They are considered to reflect long term fluctuations in the Rotliegend climate and thus they have important chronostratigraphic implications. In any given well the numbers of drying-upward cycles (or alternatively lithostratigraphic units) actually developed, the nature of their bounding surfaces and their component lithofacies are controlled by the position of that well within the depositional basin. Utilizing the new subdivision as a stratigraphic framework it has been possible to construct detailed lithofacies maps for the five units. These maps have revealed the complex tectonic and sedimentary evolution of the Rotliegend basin and they have also suggested a new depositional model for the fluvial facies association. The tectonic evolution of the basin has been traced by analysing the spatial and temporal distribution of the various units and their component facies. In areas such as the Sole Pit, where the onset of sedimentation was contemporary with early basin rifting, all five units (Units 1–5) can be recognized. However, in those areas where sedimentation was delayed due to progressive tectonic evolution (e.g. back-stepping of the Dowsing Fault Zone), one or more of the earlier units may be absent. The later post-rift sequences (Units 4 & 5) display the greatest areal distribution within the basin.
Ground penetrating radar: application to sandbody geometry and heterogeneity studies
Authors: Gawthorpe R. L. , Collier R. E. Li , Alexander J. , Bridge J. S. , Leeder M. R. 
Published: April 2008

Abstract
Ground penetrating radar (GPR) offers a high-resolution, shallow subsurface profiling technique for use in sedimentological and reservoir analogue studies. GPR is similar to seismic reflection profiling but uses electromagnetic radiation in the 50 to 500 MHz frequency range (in geological applications). By using these relatively high frequencies, high resolution data can be obtained. Short duration pulses of electromagnetic energy are transmitted into the ground, reflected from interfaces across which there are abrupt changes in dielectric properties, and are detected by a receiver. These received signals are displayed in nanoseconds two-way time and may be recorded digitally allowing subsequent processing.
Some 1000 m of 2D GPR profiles were collected from a modern point bar on the Madison River, Montana USA and have been interpreted using an approach similar to seismic stratigraphic analysis. This has allowed identification of a number of radar sequences and radar facies. Radar sequence boundaries are identified by reflector terminations (onlap, downlap, toplap and erosional truncation) and represent episodes of erosion during the development of the point bar. In contrast, radar sequences and their component radar facies record phases of accretion of the point bar. Each radar sequence is linked to a discrete accretionary unit that can be mapped on the surface of the point bar. Mapping of the radar sequences and radar facies has allowed quantification of their 3D geometry.
Quantification of variability in laminated sediments: a role for the probe permeameter in improved reservoir characterization
Authors: Corbett P. W. M. , Jensen J. L. 
Published: April 2008

Abstract
Fluvial and aeolian reservoirs are commonly laminated. Cross-bedding, for example, in these environments comprises foresets of contrasting grain size and sorting resulting from periodic fluctuations in depositional current strengths. These contrasts are commonly associated with variations in permeability. The recent development of a small-scale permeability measurement device (probe permeameter) allows quantification of these contrasts. Laminated sediments can now be described numerically in terms of permeability variability. Such measurements provide an appropriate quantity for comparison of lamination styles and are useful in reservoir characterisation.
We illustrate the measurement of permeability variability with probe permeameter data from the Etive and Rannoch Formations (Middle Jurassic, lower Brent Group, North Sea). These intervals, albeit from a shallow marine setting, reveal a relationship between permeability and depositional facies that was not clear from traditional permeability measurements (core plugs). The permeability variability is related to grain size variations (Etive) and mica content (Rannoch) which are controlled by depositional processes. The coefficient of variation (a statistical measure) has been used to quantify the variability. Using this measure, we compare permeability variation in the laminated shoreface sandstones with recently published examples from aeolian and fluvial sandstones.
Jurassic and Cretaceous orogenic events in the North Karakoram: age constraints from sedimentary rocks
Authors: Gaetani Maurizio , Jadoul Flavio , Erba Elisabetta , Garzanti Eduardo 
Published: April 2008

Abstract
At least two major episodes of tectonic deformation are recorded in the Mesozoic sedimentary succession of the North Karakoram. The older event took place between the Liassic and the early part of the Mid-Jurassic, as documented by tens to hundreds of metres thickness of dark grey litharenites (Ashtigar Fm.). The latter are sharply overlain by 35 m to 150 m thick red sandstones, interbedded with shales and siltstones (Yashkuk Fm.). Terrigenous bodies increase in thickness from west to east, and from north to south. Petrographic composition indicates the orogenic character of these quartzo-lithic sandstones, characterized by mafic volcanic and serpentinite detritus, being replaced upwards by sedimentary and subordinate metasedimentary clasts. The Ashtigar Fm. may have been deposited in a collisional basin close to a newly-formed suture belt. Contribution from allochthonous oceanic rocks was significant in the first stages of collision, whereas detritus was subsequently derived mainly from uplifted sedimentary successions. Terrigenous units were conformably transgressed by shallow-water limestones in the Aalenian, and later on marine conditions persisted until the earliest Cretaceous.
The second event of tectonic deformation occurred between the earliest and the Late Cretaceous, when the fan delta Tupop Conglomerate, mostly derived from uplifted carbonate and clastic rocks, sealed the strongly folded and faulted underlying succession. Mid-Cretaceous granitoid plutons are also widespread in the North Karakoram. This orogenic event is interpreted as related to the final welding of the Kohistan arc and Karakoram microplate to Asia.
The highly irregular topography of the Tupop Conglomerate is onlapped unconformably by reddish to grey nodular limestones, some hundred metres thick and containing phosphatized inoceramids and nannofossils of Campanian age (Darband Fm.). These are the youngest marine sediments detected in the Karakoram to date.
Collision-related granitoid magmatism and crustal structure of the Hunza Karakoram, North Pakistan
Authors: Crawford M. B. , Searle M. P. 
Published: April 2008

Abstract

The Cretaceous to Miocene Karakoram Axial Batholith runs E-W across the middle Hunza Valley in north Pakistan and separates a dominantly sedimentary terrain to the north from a regional metamorphic terrain to the south. A strong phase of deformation and metamorphism associated with the
c.
65–50 Ma collision of India with Asia affected the area. The southern part of the batholith and the metamorphic rocks to the south are affected by strong south-vergent thrusting which has brought deeper level, hotter rocks over higher level, cooler rocks so that metamorphic grade increases with structural height. In contrast, the northern part of the batholith is relatively undeformed and shows an intrusive contact with an andalusite-bearing thermal aureole.

Two groups of post-collisional magmatic rocks can be recognised in the area. The first group, the Hunza dykes, occurs as a massive dyke complex intruding the early phase of the batholith as well as the sillimanite-bearing gneisses to the south. These dykes consist of a co-genetic suite of granodiorites, monzogranites and leucogranites which appear to be related by feldspar-biotite fractionation. They are volatile-depleted, non-minimum melts with unevolved crustal isotopic ratios and enriched Sr and Ba signatures similar to the Miocene Baltoro pluton further east. They are considered to represent lower-crustal magmas derived by dehydration melting of metasediment, probably promoted by intrusion of mantle-derived magmas.
The second group is represented by the Sumayar pluton which intrudes the staurolite-grade metamorphic rocks and is a homogeneous, water-saturated minimum-melt leucogranite, with a major element composition similar to that of the Manaslu leucogranite of central Nepal. Abundant tourmaline and evolved Sr isotope ratios indicate a metapelitic source. Analogy with Manaslu suggests that this source was either fluxed by fluids to promote melting or produced melt under vapour-absent conditions via muscovite breakdown. In the case of the latter, low melt fractions require that an extensive area was tapped to generate a pluton the size of Sumayar. Consideration of the petrogeneses of the two groups of magmas allow constraints to be placed on the deeper crustal structure of the Hunza Valley.