bluuebunny/crossref_metadata_2025 huggingface dataset:
A review of the petroleum geochemistry of the Precaspian Basin
Authors: Pairazian V. V.
Published: November 2010

On the territory of the former Soviet Union more than 30- oil and gas-bearing basins have been defined. Among them the Precaspian basin is one of the most prolific in terms of oil and gas exploration and production. The basin is situated in the southeastern part of the East European Platform and covers an area of about 500 000 km
2
. There are a number of source rocks of different ages, among which the principal ones are Lower Permian, Middle Carboniferous and Middle Devonian marine shales and carbonates. The post-salt Mesozoic sources are of less significance. Geological and geochemical data interpretation allows the main oil/gas accumulation zones and new exploration prospects to be defined. Two main petroleum systems are recognized in the Palaeozoic sequence of the basin, containing 90% of the recoverable hydrocarbon resources: Lower Permian-Middle Carboniferous and Lower Carboniferous-Upper Devonian. The new exploration targets are deep Middle Devonian clastic reservoirs on the passive continental margins and Middle Carboniferous (Bashkirian) carbonates in the Caspian offshore part of the basin.
An analysis of dynamic pseudo-relative permeability methods for oil-water flows
Authors: Barker John W. , Dupouy Philippe
Published: November 2010

The properties and limitations of six widely used dynamic pseudo-relative permeability methods are analysed for the case of incompressible, immiscible, two-phase flow. The example proposed by Stone is used to illustrate our findings. The analytical results are confirmed by numerical simulation.
Laboratory simulation of rock massif deformation over a decompression zone
Authors: Turuntaev S. B. , Kondratjev V. N.
Published: November 2010

An experimental, laboratory sand model study of the origin of a decompression zone due to fluid extraction in a rock mass was carried out. It was found that an arch structure of sand density changes was formed above the decompression zone. The arch structure, which bears a portion of the load of the upper layer of the sand, can be destroyed by the action of seismic waves. The destruction of the arch causes an increase in fluid pressure in the decompressed zone. The value of the pressure increase is directly proportional to the depth of the decompression zone and inversely proportional to the dimension of the zone.
Synthetic seismic models of stacked Upper Carboniferous carbonate platforms in North Greenland; comparison to Barents Sea seismic data
Authors: Stemmerik Lars , Willersrud Knut , Elvebakk Geir
Published: November 2010

Four synthetic seismic models have been generated for an Upper Carboniferous platform carbonate-siliciclastic-evaporite succession in North Greenland. They are based on a lithological model that allows recognition of units corresponding to third-order depositional sequences. The best seismic representation of the depositional geometry is obtained in synthetic models using velocity data from the Barents Sea. They clearly show the morphology of the carbonate platforms and the onlapping siliciclastic wedges. Furthermore the distribution of anhydrite and the absorption of energy below these layers is well documented. The models also show that velocity pull-ups should be expected below dolomitized platforms whereas velocity pull-downs may record siliciclastic lenses within the platforms. The seismic models help to a better understanding of the carbonate to evaporite transitions in the Upper Palaeozoic of the Barents Sea.
Litho-flow facies prediction in an alluvial fan/fluvial system, central North Sea
Authors: Vaughan A. , Hansen T. , Cardon H. , Radcliffe N.
Published: November 2010

An existing generic facies scheme consisting of a channel, sheetflood and floodplain facies was available from the cored wells. Bivariate and multivariate analysis of various wireline and core data indicated that each generic facies exhibits wide variations and significant petrophysical overlap with other facies. This scheme cannot therefore reliably be applied using wireline data, negating its application in uncored intervals through discriminant analysis. Due to the large variation in porosity and permeability of each facies, the existing scheme does not permit adequate determination of the probable reservoir performance of the sediments. In order to address these problems a new "litho-flow" facies scheme has been constructed which can clearly be identified from wireline data thereby enabling it to be reliably extended beyond cored intervals. Each facies in the scheme exhibits a discrete range in porosity and permeability by which the probable flow characteristics of the sediments can be assessed and has successfully been used by reservoir engineers as input to a probabilistic reservoir modelling package. Finally, the predicted facies are valid in terms of the environment of deposition, assisting geologists in the correlation of units between wells for the establishment of a depositional model. The new facies scheme was formulated from a careful selection and processing of data to achieve the above. This included core descriptions, core porosity and permeability measurements, mineral volume analysis and porosity estimation from wireline data, permeability predictions and electrofacies from cluster analysis.
Downflank hydrocarbon potential identified using seismic inversion and geostatistics: Upper Maastrichtian reservoir unit, Dan Field, Danish Central Graben
Authors: Vejbæk Ole Valdemar , Kristensen Lars
Published: November 2010

Porosities considerably higher than anticipated from porosity/depth trend models are encountered in a Maastrichtian reservoir unit on the western flank of the Dan Field. Because there is a good correlation between seismic impedance and well log porosity, inverted seismic data are used to infer that highly porous zones are widely distributed. The distribution of these high porosity zones is predicted using geostatistical methods based on the inverted seismic data. These predictions contradict the general assumption that porosity deteriorates with depth in the study area. Annealing cosimulation is applied using inversion-derived seismic impedances as soft data and well log porosities as hard data. The sensitivity of the porosity characterization to hydrocarbon in-place estimates is investigated through the calculation of water saturations using height above free water level and simulated porosity as input parameters. Multiple realizations show that calculated hydrocarbon in-place estimates are more sensitive to the location of the free water level than to uncertainties related to the geostatistical reservoir characterization.
Estimation of length distributions from outcrop datasets – application to the Upper Permian Cutler Formation, Utah
Authors: Visser Clemens A. , Chessa Antonio G.
Published: November 2010

Many outcrops have partially exposed sedimentary bodies that are cut by one or both edges of the outcrop. These so-called ‘partials’ contain important information on the size distribution of the bodies under study, and should be accounted for in a proper statistical sense. A new method was proposed elsewhere, based on the theory of conditional probability. The method estimates the expected complete length along outcrop for each ‘partial’, making use of the length distribution of completely exposed bodies in the same outcrop. The estimated lengths are then added to the set of observed complete lengths to yield a new, refined length distribution. This paper illustrates the practical application of the method on an outcrop dataset from the Permian Cutler Formation in southeastern Utah that may serve as an analogue for mixed fluvial–aeolian reservoirs. Solutions are proposed for some practical problems that can be encountered when dealing with outcrop data. The estimated length distributions show that ‘partials’ add considerable information about the larger length classes.
Tengiz oilfield: geological model based on hydrodynamic data
Authors: Anissimov Leonid , Postnova Elena , Merkulov Oleg
Published: November 2010

The Tengiz oilfield, in the Precaspian Basin, western Kazakhstan, was discovered in 1981, but the size of the field and its large oil reserves were not fully estimated until many years later. Presently, it is known that the oilfield stretches over an area of almost 400 km
2
but the thickness of the productive zone was not recognized untill recently. Formation pressure distribution in the Tengiz carbonate reservoir allows definition of the position of the oil–water contact. Reservoir characterization and hydrodynamic data are the basis for a dynamic model of the Tengiz oil accumulation which reflects the reservoir architecture, filling history of the reservoir and distribution of productive zones.
Reservoir modelling sand-rich deep-water clastics: the necessity of down-scaling
Authors: Hurst Andrew , Cronin Bryan , Hartley Adrian
Published: November 2010

Channels, lobes and levees are inadequate architectural elements to describe sand body geometry in sand-rich deep-water systems. Sand sheets are the most common architectural element. Elutriation modifies primary sedimentary structures by mobilizing and reorganizing grains. This changes pore size distribution and permeability characteristics. Bed amalgamation and offset stacking of sand sheets makes problematic the use of bed thickness in correlation. A down-scaling approach for reservoir modelling is recommended, as a hierarchy of architectural elements that define up-scaling relationships cannot be established.
The relationship between the composition and rank of humic coals and their activation energy distributions for the generation of bulk petroleum
Authors: Petersen H. I. , Rosenberg P.
Published: November 2010

Activation energy (
E
a
) distributions and pre-exponential factors (
A
) of 27 humic coals of varying composition, rank, age and origin are discussed. The petrography and rank of the coals have been correlated to the distribution of
E
a
and the Hydrogen Index (HI). The HI is positively correlated to vitrinitic and liptinitic constituents; in particular, the vitrinite maceral collotelinite has a strong influence on the HI. Inertinitic constituents have a negative effect on the HI. A higher value of the peak position of the
E
a
distributions is mainly controlled by the thermal maturity and to a lesser extent by the proportion of collotelinite. A number of other vitrinitic components result in a lower
E
a
peak position. The peak width of the
E
a
distributions is broadened by high vitrinite reflectance values and higher proportions of collotelinite and the microlithotypes vitrite and vitrinertite. Liptinitic and inertinitic constituents, in particular the microlithotype clarodurite, narrow the peak width.
Petrography and clay mineralogy of volcanoclastic sandstones in the Triassic Rewan Group, Bowen Basin, Australia
Authors: Bashari Alireza
Published: November 2010

The sandstones of the Early Triassic non-marine Rewan Group in the southern Bowen Basin represent a potential reservoir for hydrocarbons. They are mainly lithic in nature, with a large proportion of volcanic rock fragments and contain quartz grains of variable size and an authigenic clay matrix. The results of petrographic analysis show a dual provenance basin-fill pattern (cratonic–volcanic). Reservoir qualities vary considerably, but are best developed in the Quartzose and Conglomerate Intervals where partial quartz cement retarded compaction and secondary porosity developed by dissolution of labile material and flushing of clay minerals. The presence of different clay minerals in pore networks may have a significant impact on petrophysical properties of the reservoir and hence affect reservoir productivity.
The Miocene petroleum system of the Sava Depression, Croatia
Authors: Baric G. , Ivkovic Z. , Perica R.
Published: November 2010

The Sava Depression, located on the southern margin of the Pannonian Basin, contains a number of small oil and gas fields. The depression is filled principally with Neogene sediments resting on Palaeozoic crystalline basement. Rifting occurred in Early to Middle Miocene times. Geochemical data imply that Middle to Upper Miocene pelites are good oil-prone source rocks. Most of the oil and oil–gas fields are situated at depths of 370 to 2300 m. Weathered basement rocks and Miocene conglomerates and sandstones are reservoir rocks and the fields are mostly structural and/or structural–stratigraphic traps. Oils from the fields correlate well with the identified source rock facies and maturity level and we propose short migration distances. Sava oil properties show variations in bulk and specific parameters that can be attributed to post-generative processes.
An assessment of steady-state scale-up for small-scale geological models
Authors: Pickup Gillian E. , Stephen Karl D.
Published: November 2010

The calculation of pseudo-relative permeabilities can be speeded up considerably by using steady-state methods. The capillary equilibrium limit may be assumed at small scales (30 cm or less), when the flood rate is low. At high flow rates and larger distance scales, we may use a viscous-dominated steady-state method which assumes constant fractional flow. Steady-state pseudos may also be calculated at intermediate flow rates using fine-scale simulations, and allowing the flood to come into equilibrium at different fractional flow levels. The aim of this paper is to assess the accuracy of steady-state scale-up for small-scale sedimentary structures.

We have tested steady-state scale-up methods using a variety of small-scale geological models. The success of steady-state scale-up depends not only on the flow rate, but also on the nature of the heterogeneity. If high permeability zones are surrounded by low permeability ones (e.g. low permeability laminae or bed boundaries), oil trapping may occur in a water-wet system. In this case pseudo-oil-relative permeabilities are very sensitive to flow rate, and care must be taken to upscale using the correct viscous/capillary ratio. However, in permeability models, where phase trapping may not occur (unconnected low permeability regions), the pseudos are similar, whatever the viscous/capillary ratio.

The disadvantage of steady-state scale-up is that it cannot take account of numerical dispersion, in the manner in which dynamic methods can. However, we show examples of coarse-scale simulations with viscous-dominated steady-state pseudos which agree favourably with fine-scale simulations. Provided there are sufficient grid blocks in the coarse-scale model, the smearing of the flood front due to numerical effects is not serious.
Lower Cretaceous deep-water sandstone reservoirs of the UK Central North Sea
Authors: Garrett S. W. , Atherton T. , Hurst A.
Published: November 2010

The Lower Cretaceous deep-water depositional system of the Central North Sea is emerging as a significant economic target. It contains a broad range of sedimentary facies and architecture. Thick sands were deposited by high-density sediment gravity flows. Unusual banded and mixed slurried facies represent the products of processes transitional between turbidity currents and debris flows. Shale-prone units show evidence of debris flows and post-depositional down-slope movement. Geometrical architectural elements include narrow linear incised channels, broad linear sand-rich fairways, prograding sand lobes and laterally extensive sheets. Models for exploration and production are refined by core magnetic measurements, automated quantitative petrography, detailed structural analyses and biostratigraphical zonations. Key remaining challenges are refining depositional models to aid prediction of lateral facies variations, understanding trap mechanisms and geometry and improving images of sandstone units on seismic data.
Deformation processes in the Britannia Field, UKCS
Authors: Porter J. R. , Knipe R. J. , Fisher Q. J. , Farmer A. B. , Allin N. S. , Jones L. S. , Palfrey A. J. , Garrett S. W. , Lewis G.
Published: November 2010

The intensity and distribution of deformation within cores recovered from 24 Britannia Field wells has been quantified and used to assess the sub-seismic deformation present. Structural logging has been amalgamated with microstructural and petrophysical analysis of fault rocks and the results used to construct models of the fluid flow behaviour of fault zones and to evaluate the correlations between core-scale structures and the larger-scale seismically mapped fault arrays. Deformation within Britannia is controlled primarily by (1) early sediment disruption, dewatering, detachment and folding in semi-lithified sediments and (2) more long-lived clustered arrays of small faults developed within fault damage zones. Fault population analysis illustrates that seismic-based mapping alone does not provide a good platform for accurately predicting the number of sub-seismic faults present. Analysis of the effect of deformation features on fluid communication highlights the importance of small-scale faults in enhancing communication in complex and initially separate sand bodies.
The Kopervik fairway, Moray Firth, UK
Authors: Law Adam , Raymond Anne , White Gillian , Atkinson Alan , Clifton Madelaine , Atherton Tony , Dawes I.a.n. , Robertson Ernest , Melvin Angela , Brayley Sharon
Published: November 2010

The Albian–Aptian sand fairway in the Outer Moray Firth of the United Kingdom Continental Shelf, informally referred to as the Kopervik fairway, poses considerable challenges to the explorationist, but has the potential to generate significant commercial reward. After an initial spate of activity in the mid 1970s, the Kopervik Sandstone remained largely ignored as a primary exploration target until the late 1990s, despite its excellent reservoir quality and proven charge. This renewed exploration effort led to a number of successful discoveries. As a result of this activity – and a willingness among operators along the fairway to share information – a large regional database is developing. Drawing on this, we summarize our current and historical understanding of the fairway, using existing discoveries as examples.
Organic geochemistry of petroleum seepages within the Jurassic Bencliff Grit, Osmington Mills, Dorset, UK
Authors: Watson D. F. , Hindle A. D. , Farrimond P.
Published: November 2010

Occurrences of oil within the Bencliff Grit at Osmington Mills were studied through an integration of organic geochemistry and a consideration of the geological setting. Oil-stained sandstones dominate the cliff outcrop with localized regions of particularly concentrated oil impregnation. A second ‘live’ seep of oil occurs where the Bencliff Grit beds pass below high tide level at Bran Point. Organic geochemical analyses showed both oils to be at least moderately biodegraded, with the oils in the cliff outcrop showing enrichment in polar constituents compared with the active seep. Multivariate statistical analysis of the molecular composition identified an enrichment in diasterane biomarkers in the oils of the live seep; this difference is ascribed to source and/or maturity differences. The oil within the outcrop is considered to represent the residual staining of an unroofed oil field, whilst the live seepage at Bran Point represents a migration pathway towards the eroded anticline.
Thermal buffering of sedimentary basins by basement rocks: implications arising from numerical simulations
Authors: Lin Guichang , Nunn Jeffrey A. , Deming David
Published: November 2010

The Earth’s crust releases or absorbs heat energy in response to changes in the thermal regime. Numerical simulations of basin-scale heat transport which use the sediment–basement interface as a thermal boundary assume that near-surface temperature changes have no effect on basement rocks and vice versa (unbuffered model). We test this assumption by comparing numerical models of transient fluid flow and heat transport in the Arkoma foreland basin with and without thermal buffering by basement rocks. Thermal buffering by basement rocks reduces cooling (by fresh water recharge) of deep basin sediments near the fold–thrust belt and reduces warming (by upward fluid discharge) of basin margin sediments. The unbuffered model predicts a temporary warming of basin margin sediments which is largely an artefact of the model thermal boundary conditions. In all cases, the buffered model produces no significant thermal transient. Exaggerated temperature predictions also can occur in numerical simulations of uplift and erosion or heat refraction.
Britannia Field, UK North Sea: petrographic constraints on Lower Cretaceous provenance, facies and the origin of slurry-flow deposits
Authors: Blackbourn G. A. , Thomson M. E.
Published: November 2010

Deep-water sandstones of the Early Cretaceous Britannia reservoir are rich in ‘muddy’ material, with the development of unusual ‘slurry-flow’ deposits (
sensu
Lowe & Guy 2000
), including banded facies. The banding comprises couplets of pale sandstone containing microporous detrital chlorite and other clays, retaining substantial porosity, and dark sandstone in which biotite (now altered to chlorite) has promoted quartz pressure solution that has largely destroyed porosity. The main source of the abundant chlorite and biotite is the Jurassic Forties Igneous Province, underlying and surrounding the Britannia Field. Altered alkali basalts and other lithologies here are known to be rich in both minerals.

Grain-size distributions have been examined using image analysis. The banded facies are generally finer grained than the high-density turbidite and unbanded slurry-flow sandstones, and may be a distal equivalent. Dark bands in some cases contain no more fine clays than associated light bands, indicating that Lowe & Guy’s model for dark-band formation, invoking a cohesive sublayer, is incomplete. Gelation of chlorite-rich clays within each banded couplet is proposed as an alternative mechanism. These microporous pore-filling chlorites have restricted the precipitation of quartz overgrowths and other non-porous cements, although their presence inhibits permeability.
The ‘tight gas’ challenge: appraisal results from the Devonian of Algeria
Authors: Hirst J. P. P. , Davis N. , Palmer A. F. , Achache D. , Riddiford F. A.
Published: November 2010

Projected future increases in Algerian gas production will, in part, come from more complex reservoirs within the Ahnet-Timimoun Basin; here, conventional quality reservoirs (>1 mD) are interbedded with volumetrically significant, low permeability sandstones (<1 mD) – the ‘tight gas’ sandstones. The challenge has been to develop a programme of work which will establish reserves in this sub-millidarcy resource; on balance, the results of this evaluation programme are positive.

The main tight gas sections are of Lower Devonian age. Grain rimming chlorite cement within thin intervals locally inhibited later pervasive quartz cementation; the quartz is the cause of reservoir degradation in the bulk of the sandstone resulting in permeabilities of <1 mD, and often in the microdarcy range. Generally, the Lower Devonian has a low density of faulting and fractures; open and closed fractures are observed in core, whilst mud losses suggest some fractures are conductive in the subsurface.

One of the main structures, the Teguentour Field, has been evaluated through the drilling of two new wells. The first of these wells (Teg-14) evaluated interbedded tight and conventional sandstones, whilst the second (Teg-15) was a dedicated tight sandstone completion involving an induced fracture programme.

The permeability range in the tight gas sandstones extends below the resolution of conventional porosity/permeability measurement; determination has been improved through mercury injection derived permeabilities. Water saturations determined from core and log suggest gas may be present in the low permeability rock. The presence of gas could not be confirmed by formation tester samples and thus dynamic data were required from the Teg-15 well which was completed within the tight sandstone interval; only the top 5 m was perforated to limit the possibility of a communication pathway down to conventional quality sandstones. After nitrogen lift, the well flowed gas to surface at
c.
5 ×10
4
SCFD for several hours. The low leak-off coeffcient determined suggests there was no connection to a pervasive conductive fracture network, although fractures intersected by the wellbore may have contributed to flow. Also, large poro-elastic back stresses indicate that a connection to high matrix or fracture permeability was unlikely. On balance, the test is a positive indicator of gas presence in very low permeability/porosity rocks which may recharge associated conventional layers following suffcient drawdown.
Exploration for and development of hydrocarbons in the Chalk of the North Sea: a low permeability system
Authors: Megson Joan , Hardman Richard
Published: November 2010

It is evident that all or most North Sea Chalk fields have dipping oil–water contacts and that some also have dipping free water levels. Furthermore, there are Chalk fields, such as South Arne (Denmark) and Joanne (UK) with steeply dipping oil–water contacts and a large element of non-structural trapping, while other discoveries, such as the recent Halfdan discovery northwest of the Danish Dan Field, have no element of structural trapping and appear to lie on a Tertiary migration pathway. It is also evident that the presence of North Sea Chalk oil, both as fields and shows, is localized and, generally, confined to areas of early source maturation – although there are exceptions, such as the UK Fife and Flora Chalk oil accumulations, which are distant from a source basin.

We believe that these attributes reflect the same cause, that is the very low lateral and vertical permeability (typically 0.1 to 1 mD air permeability) of the Chalk system. Our hypothesis is that oil enters the Chalk from the Jurassic at a limited number of entry points within the early mature basin and, only if the oil reaches Chalk of relatively high permeabilities, normally the Tor Formation, does the oil migrate significant distances laterally. This oil migration occurs at geological timescales, i.e. kilometres over millions of years and we, therefore, have to consider the configuration of the Chalk – with respect to migration routes and traps – at the time the oil was injected into the Chalk and began to migrate through it, which was normally prior to the North Sea Miocene inversion and later tilting. There is considerable evidence for trapping in Chalk palaeostructural traps; oil migrated into a palaeohigh, segregated vertically and is trapped by the low oil relative permeabilities within the transition zone when the trap later tilts. Another trapping mechanism is the migration trap, as described for the Halfdan discovery in Denmark, where, in an area mature for Chalk exploration, oil has been discovered in continuity with the Dan Field but with a free water level 700 ft lower. This type of trap may contain significant reserves of oil in chalks of 1 mD permeability and, in recent years, it has become possible to exploit chalks of this nature commercially. It is also possible to develop these chalks when no effective natural fracture system exists.
The seismic evaluation of a naturally fractured tight gas sand reservoir in the Wind River Basin, Wyoming
Authors: Bates C. R. , Phillips D. R. , Grimm R. , Lynn H.
Published: November 2010

Sub-vertical natural fracturing is critical for ensuring economic gas production from tight sand reservoirs in many provinces. Over the last seven years, the US Department of Energy (DoE) has sponsored a series of programmes in the continental United States to develop cost-effective technologies for investigating naturally fractured reservoirs. The results from a study of a Laramide-age faulted anticline reservoir in the Lower Fort Union Formation of the Wind River Basin, Wyoming are presented. At the field site a multi-azimuth, multi-offset 3D compressional wave seismic survey was acquired and processed using azimuth-dependent processing followed by multi-azimuth attribute analysis. Geophysical attributes such as velocity, frequency, reflectivity and amplitude variation with offset, together with geological attributes such as depth below closure and distance from faults, were correlated to estimated ultimate recovery of gas for established wells within the field. The correlations were made using linear statistical methods, non-linear rank methods and neural networks to combine attribute results. The geophysical attributes and other geological information were combined into prospectivity maps for areas of highest fracture density. Based on the geophysical and geological results an 80% success rate was achieved at prospecting well locations.

A major aspect of the DoE programmes has been the investigation of costeffective protocols for fracture detection. From this study the following procedure is recommended for cost-effective mapping of fractures and associated gas: (1) perform field and remote-sensing reconnaissance of structural trends; (2) acquire 3D P-wave surveys with offsets equal to or greater than target depth in all azimuths; (3) process in at least two dominant structural azimuths; (4) combine best correlated geophysical and geological attributes to fractures for future fracture zone mapping. The implications of the results from this survey have important consequences for the design and implementation of multi-azimuth, multi-offset data acquisition on land and for marine surveys where multi-component ocean bottom cable surveying is used.
Effect of Positive Rate Sensitivity and Inertia on Gas Condensate Permeability at High Velocity
Authors: Henderson G. D. , Danesh A. , Tehrani D. H.
Published: November 2010

The authors have previously reported that gas condensate relative permeability will increase with increasing velocity when conducting steady-state measurements. The increase in relative permeability was referred to as ‘positive rate sensitivity’ or the ‘positive coupling effect’. A systematic series of core tests has since been conducted, generating data at IFT values ranging from 0.015 to 0.78 mN m
−1
, using core types ranging in permeability from 11 to 350 mD. The results confirmed that the positive coupling effect existed in low permeability cores and different lithologies at low and high IFT. The maximum tested velocity was in the region of 75 m per day, which was estimated to be at the boundary above which inertia would be significant.

To investigate the competition between inertia and the positive coupling effect, subsequent core tests have been conducted at velocities ranging from 7 to 700 m per day, with the higher velocity being in the region of fractions of a metre from the wellbore. The maximum test velocity was one order of magnitude above the velocity boundary with significant inertia. The test programme has generated previously unpublished data, showing that the positive coupling effect initially reduces the effect of inertia before becoming dominant in the core tests conducted using gas condensate fluids. Such unique data provide valuable information on the forces governing production at the wellbore to reservoir engineers.
Physical Reservoir Models: from Pictures to Properties
Authors: Peeters M.
Published: November 2010

Petrophysics and geophysics have not always worked together as closely as one might expect. This paper discusses current advances and sketches future developments. The major objective of geophysical research is to obtain sharper pictures. With the advent of 3D seismic, there is a strong trend to make displays both in time and depth and to convert seismic attributes into rock properties. Conversions based on seismic data alone give non-unique results due to limited resolution. Further progress hinges on calibration of acoustic attributes with parameters measured on core. For time-lapse seismic, the situation is even more complex, due to changing effects of temperature, pressure and compaction. All these effects have to be properly quantified if 4D seismic interpretation is to be integrated with dynamic reservoir models to realize the situation where all geoscientists work on one ‘unified’ 3D earth model. Biot–Gassman fluid substitution algorithms were successfully applied for decades, but suffer, like all mixing laws, from non-uniqueness. Recent investigations demonstrate that dispersion in the low frequency band could have strong implications for seismic attribute analysis. It is important to find out which relations measured on core at high frequencies can be used at seismic frequencies and which require additional laboratory work. The shock-tube is well suited to this task, because it handles large rock samples, and obtains responses below 1 kHz.
Quantitative modelling of basin subsidence caused by temperature-driven silica dissolution and reprecipitation
Authors: Walderhaug Olav , Bjørkum Per Arne , Nadeau Paul H. , Langnes Olaf
Published: November 2010

Equations describing the rate of vertical thinning of sandstones due to precipitation of quartz cement produced at stylolites are derived for constant temperature and for a linear temperature change. All temperature histories can be approximated by a series of linear segments, and the equations therefore enable the diagenetic thinning of sandstones undergoing quartz cementation to be calculated as a function of temperature history. Application of the equations to interbedded sandstone–shale sequences indicate that for heating rates and geothermal gradients commonly encountered in sedimentary basins, thermally driven diagenesis leads to cumulative rates of sandstone thinning in the range of 1–10 m Ma
−1
. This implies that when effects of thermally driven shale diagenesis, mechanical compaction and isostatic adjustment are taken into account, thermochemical diagenetic thinning may explain a large proportion of the total generation of accommodation space in many basins. The equations describing the rate of sandstone volume reduction and, therefore, rate of fluid expulsion, could also be utilized for calculating overpressure development.
Fluvial reservoir architecture in the Statfjord Formation (northern North Sea) augmented by outcrop analogue statistics
Authors: Dalrymple Mark
Published: November 2010

Outcrop data from the fluvial/incised valley systems of the Cretaceous Straight Cliffs Formation of southern Utah, USA, are applied to the subsurface fluvial reservoirs of the Statfjord Formation. Aspect ratios (width:thickness) for fluvial sandbodies have been measured from large-scale cliff exposures. This database provides the lateral extent, or width, of any fluvial sandbody, given the thickness for that sandbody and the net-to-gross of the interval in which it is preserved. This allows prediction of a probability distribution for fluvial sandbody width from one-dimensional vertical well data.

The outcrop data are applied to one well in each of the Brent, Statfjord and Snorre fields, through similar stratigraphic intervals, to allow comparison of the various sandbody architectures which result from the same outcrop dataset. Various fluvial architectures are identified: from high net-to-gross and high connectivity, to low net-to-gross with low sandbody connectivity.
High resolution geochemical and petrographical correlation of deep marine Palaeocene sandstones from the Foinaven Field, West of Shetland
Authors: Hutchison Alasdair , Hartley Adrian , Hole Malcolm , Whear Emma , Preston Jeremy
Published: November 2010

Cores from three wells in the Upper Palaeocene deep-marine reservoir interval of the Foinaven Field, West of Shetland have been analysed using trace element geochemistry and petrography. The 60 m thick turbidite-dominated succession is restricted to the T34 biozone and comprises a series of seismically defined channel and overbank deposits. Petrographic and trace element geochemical data indicate that wells 204/24a-3 and 204/25b-5 have strong similarities and that strata in the remaining well (204/24a-7) have distinctly different signatures. Of the two related wells, 204/25b-5 is dominated by a thick amalgamated sandstone succession, 204/24a-3 by more thinly bedded and finer grained sandstones considered to represent channel and associated overbank deposits respectively. Similarities in geochemical and petrographic data allow division of the biozone in the two related wells into four principal correlation zones with two additional sub-zones. The study has established a high resolution correlation scheme and constrained correlation lengths for reservoir and non-reservoir facies in a deep marine depositional system.
Isotope geochemistry of oils from fields and mud volcanoes in the South Caspian Basin, Azerbaijan
Authors: Guliyev Ibragim S. , Feizulayev Akper A. , Huseynov Dadash A.
Published: November 2010

This study is based on the isotope-geochemical characteristics of oils from 53 fields in the South Caspian Basin – from Upper Cretaceous, Eocene, Oligocene–Lower Miocene (Maykop), Middle and the Upper Miocene (Diatom) and Pliocene. The isotopic composition of carbon (δ
13
C) allows two main groups of oils to be identified: (1) isotopically heavy and (2) isotopically light oils. The first group includes oils assumed to be generated by Upper Cretaceous, Eocene and Oligocene–Lower Miocene sources. The second group consists of oils assumed to be generated by Diatom (Middle and Upper Miocene) sources. Isotope-geochemical and biomarker parameters demonstrated that oils in the Pliocene reservoirs are not syngenetic to their enclosing deposits. The isotopic composition of carbon in oils and hydrocarbon gases in mud volcanoes and fields and the biomarker parameters of oils allow several isolated stratigraphic oil- and gas-producing complexes (source rocks) in the Mesozoic, Palaeogene–Lower Miocene and Middle–Upper Miocene to be postulated. Oils in the Pliocene reservoirs and in mud volcanoes consist of mixed oils generated by the Pre-Diatom and Diatom deposits and of oils from only one of the above-mentioned complexes.
Investigation of the petrophysical properties of a porous sandstone sample using confocal scanning laser microscopy
Authors: Petford N. , Davidson G. , Miller J. A.
Published: November 2010

Confocal scanning laser microscopy (CSLM) is used to produce images of the two- and three-dimensional distribution and geometry of pore space in a reservoir sandstone and measure the 2D distribution of pore throat radii. Nondestructive serial sectioning of the rock using laser light at 100% illumination, combined with image thresholding and histogram equalization techniques allow the pore volume structure of the uppermost 100 μm of the sample to be reconstructed. Negative imaging of the pore volume gave superior depth and feature resolution compared to positive (reflection) imaging. Artefacts encountered in applying classical Medial Axial Transforms to CSLM images include branch networks dominated by coordination numbers of 3. Skeletonization using Euclidean distance maps gives increased accuracy in the description of the pore network. Measured pore throat size distribution in the rock is strongly exponential and described by the expression
y
=219
e
−0.25

x

where
y
is the number of pore throats.
A review of palaeomagnetic data on the timing and origin of multiple fluid-flow events in the Arbuckle Mountains, southern Oklahoma
Authors: Elmore R. Douglas
Published: November 2010

A synthesis of palaeomagnetic/geochemical studies on Palaeozoic rocks in the Arbuckle Mountains, Oklahoma, indicates that basinal and meteoric fluids, as well as burial diagenetic processes, caused multiple remagnetization events. Basinal fluids migrated laterally through palaeoaquifers and vertically through faults/fractures, causing localized remagnetization and alteration in, and around, the fluid conduits. These fluids migrated during multiple flow events that spanned an approximately 60 Ma time interval in the late Palaeozoic. Based on the timing of remagnetization, most of the basinal fluids can be considered orogenic fluids. The palaeomagnetic results from the Arbuckle Mountains are consistent with migration of orogenic-type fluids during and after orogeny, although there were multiple flow events rather than one event as has been hypothesized for the central Appalachians. Meteoric fluids also caused localized alteration and remagnetization whereas a burial diagenetic mechanism (e.g. maturation of hydrocarbons) is a likely explanation for pervasive chemical remagnetization in an impermeable unit.
The effect of allochthonous salt evolution and overpressure development on source rock thermal maturation: a two-dimensional transient study in the northern Gulf of Mexico Basin
Authors: Stover S. Chereé , Weimer Paul , Ge Shemin
Published: November 2010

Sequential two-dimensional forward modelling of source rock maturation along a north–south 600 km megaregional profile across the northern Gulf of Mexico Basin illustrates the coupled influence of allochthonous salt evolution and overpressure development on source rock maturation. Model results indicate that allochthonous salt evolution strongly affected local and regional source rock maturation, while overpressure development had a minor impact. Within the lower slope portion of the profile (south of the Sigsbee escarpment to the Tabular Salt/Minibasin province), localized thermal lows associated with salt stock development locally retarded the maturation of source rocks, while the slow development of overpressures effected little influence on the maturation. Within the centre of the profile (offshore Louisiana shelf), however, the evolution of allochthonous salt sheets and shallow salt structures locally and regionally lowered the oil generation window. The timing of overpressures in this region, related to the progradation of the Cenozoic marine depocentre, may have accelerated the maturation of middle Eocene source rocks, although the deeper Turonian source rocks had reached maturation at the time of greatest overpressure development. In the northern portion of the profile (southern Louisiana), the Terrebonne salt sheet lowered the oil generation window and retarded the maturation of the middle Eocene source rocks.
Overpressure associated with a convergent plate margin: East Coast Basin, New Zealand
Authors: Darby David , Funnell Robert H.
Published: November 2010

The East Coast Basin of New Zealand lies in the frontal arc of the Hikurangi subduction zone. Overpressure forms a major hazard for exploration in the basin. Fluid pressures of 12.8 MPa at 600 m depth have been encountered, equivalent to 90% of lithostatic pressure, and mud weights of up to 19.5 ppg are required to control formation fluids at subsurface depths of only 1400 m. The distribution and magnitude of overpressure shows no relation to present-day depth. Lithostratigraphy controls the distribution of overpressure and widespread low-permeability bathyal mudstones form seals to overpressure across the basin. The transition from normal pressure to overpressure is not associated with any single stratigraphic unit. The Neogene evolution of the plate boundary has given rise to rapid Miocene sedimentation (400 m Ma
−1
) in areas of the basin and has caused disequilibrium compaction. Late Neogene compression has subsequently uplifted the overpressured, undercompacted sediments by up to 3000 m at rates of 1000 m Ma
−1
. Lateral tectonic compression associated with the plate boundary has also caused undrained shear of thick mudstones, leading to extremely high overpressures in deformed sediments independent of depth.
Potential for subtle traps in the Permian Rotliegend of the UK Southern North Sea
Authors: Maynard James R. , Gibson John P.
Published: November 2010

The Permian, Rotliegend Group of the UK Southern North Sea is shown to consist of periodic, cyclic stratal packages which can be used to aid regional correlation in the zone of playa–erg interaction colloquially known as the Rotliegend Feather Edge. In this area, the depositional topography controls the sedimentary facies, with a concentration of aeolian sandstone deposition in the topographic depressions, especially in the lee of hills. These units onlap the contemporaneous intra-basinal and marginal highs. The palaeotopographic lows rapidly became infilled, with later lacustrine deposition occurring over a smoother surface. This allowed the deposition of finer-grained lake deposits over the sands. Prediction of the palaeotopography is key to understanding potential reservoir distribution and seal juxtaposition within the Rotliegend. This paper demonstrates how pre-Permian subcrop maps, Bouguer anomaly maps and the Rotliegend isopach can be used to reconstruct this palaeotopography. Two areas are studied which demonstrate some of the subtleties of the model. The southern Quadrant 42–43 area had a palaeotopography with gentle gradients and a number of subtle intrabasinal highs that allowed for isolated reservoirs overlain by shales with high sealing potential. In contrast, the northern Quadrant 49 area, contained only intra-basinal relief, with high palaeotopographic gradients. Aeolian sediments banked up against this barrier and no intra-Rotliegend seal was deposited over these sands.
Enhanced lithological description of a Jurassic mudrock sequence using geophysical wireline logs
Authors: Brereton N. R. , Gallois R. W. , Whittaker A.
Published: November 2010

Methods to combine individual wireline logs such as resistivity, sonic, bulk density, neutron porosity and natural gamma have been developed to enhance lithological interpretation, particularly of mudrock sequences. Multiple log data presentation to generate a synthetic, pseudo-lithology log is achieved by combining downhole geophysical log records using a colour cube. Well-documented lithological descriptions of borehole cores from the Kimmeridge Clay Formation (Jurassic) of Britain are compared with the newly processed combination geophysical log data. The colour cube logs confirm existing lithological descriptions and, importantly, enable considerable refinement of general lithological descriptions of ‘undifferentiated’ mudstone.
Characterization of heterolithic deposits using electrofacies analysis in the tide-dominated Lower Jurassic Cook Formation (Gullfaks Field, offshore Norway)
Authors: Gupta Renu , Johnson Howard D.
Published: November 2010

A range of heterolithic facies, comprising thinly interbedded (millimetre–decimetre scale) sandstones and mudstones, characterizes the heterogeneous Lower Jurassic Cook Formation, including the productive Cook-3 reservoir in the Gullfaks Field. These heterolithic facies were deposited in a tide-dominated estuarine to deltaic setting and show up as massive ‘shaly-sands’ on conventional wireline logs. This makes the recognition and discrimination of different heterolithic facies types virtually impossible, which severely limits detailed reservoir geological and petrophysical predictions. This problem has been addressed by undertaking a high resolution electrofacies analysis using core facies as a ‘training set’ and applying this, through multivariate statistical techniques, to the interpretation of the conventional logs.

An electrofacies database was created comprising five genetically linked rock types (ranging from lenticular-wavy bedding, through flaser bedding and into clean/massive sandstones). This electrofacies scheme was validated with reference to
c
. 125 m of cored section from five wells using gamma-ray, density and neutron logs. Multivariate statistical techniques have enabled probabilistic discrimination of the different types of heterolithic facies down to intervals of only 0.25 to 0.5 m thick, which is considerably greater than could be achieved using conventional well-log evaluation techniques alone.