Magnus Sandstone Research Articles

The Penguins Cluster, Blocks 211/13a and 211/14, UK North Sea

Abstract The Penguins Cluster of fields are owned jointly (50:50) by Shell UK Ltd (Shell) and Esso Exploration and Production UK Ltd (Esso), with Shell as the operator. The cluster was discovered in 1974 and is composed of a combination of oil and gas condensate accumulations located 50–65 km north of the Brent Field, at the northern end of the North Viking Graben. Two main producing reservoirs are present: the Penguins West Field (Penguin A) consists of an Upper Jurassic Magnus Sandstone Member reservoir, while the Penguins East Field (Penguin C, D and E) consists of a Middle Jurassic Brent Group reservoir, underlain by currently undeveloped Statfjord and Triassic (Cormorant) reservoirs. The Magnus reservoir is composed of turbidite sands with an average porosity of 15% and permeabilities of 0.10–300 mD. The Brent reservoirs are composed of deltaic shoreface deposits with an average porosity of 14% and permeabilities of 0.01–1000 mD. The fields were brought on stream in 2003 as a subsea development via what at the time was the world's longest comingled tieback to the Brent Charlie facility. A total of nine producing wells have been drilled from four subsea manifolds, producing c. 78 MMboe to date through depletion drive.

Geochemical modelling of diagenetic reactions in a sub-arkosic sandstone

AbstractA reaction path model was constructed in a bid to simulate diagenesis in the Magnus Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea, UKCS. The model, involving a flux of source rock-derived CO2 into an arkosic sandstone, successfully reproduced simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite. Generation of CO2 occurred before and during the main phase of oil generation linking source rock maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to the earlier stages of oil charging. Independent corroborative evidence for the model is provided by formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO2 and the presence of petroleum inclusions within the mineral cements. The model involves a closed system with respect to relatively insoluble species such as SiO2 and Al2O3 but is an open system with respect to CO2. There are up to seven possible rate-controlling steps including: influx of CO2, dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO2 and Al2O3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg2+ and Ca2+. Given the large number of possible controls, and contrary to modern popular belief, the rate of quartz precipitation is thus not always rate limiting.

Abstract: New Insights into the controls on sediment dispersal and reservoir distribution of the Magnus Sandstone Member, UK, Northern North Sea

Abstract: New Insights into the controls on sediment dispersal and reservoir distribution of the Magnus Sandstone Member, UK, Northern North Sea

Micropalaeontological biostratigraphy of the Magnus Sandstone Member (Kimmeridgian-Early Volgian), Magnus Field, UK North Sea

Abstract The micropalaeontological biostratigraphy of the Magnus Sandstone Member of Kimmeridgian-Early Volgian age is described for the first time using data from 27 Magnus Field wells (blocks 211/12a and 211/7a). Despite extensive re-sedimentation within the Magnus Sandstone Member turbidite reservoir, a consistent sequence of nine bioevents is recognized field-wide. These include both regionally extensive bioevents and localized extinction and acme bioevents. The microfaunas are documented and key taxa, which include numerous radiolarian species, are described and figured. The bioevent scheme provides the basis for the chronostratigraphic and genetic sequence stratigraphic subdivision ( sensu Galloway) of the Magnus Sandstone Member. It allows the recognition and biostratigraphic calibration of three maximum flooding surfaces (MFS) which in ascending stratigraphic order comprise: the pre-Magnus Sandstone Member top J62-basal J63 ‘ Eudoxus ’ of Partington et al. (upper limit of the ‘Lower Kimmeridge Clay Formation’); the intra-reservoir top J63-basal J64 ‘ Autissiodorensis ’ (‘B Shale’); and the supra-reservoir top J64-basal J66 ‘ Hudlestoni ’ (upper limit of the ‘H Unit’). The erection of a field-specific biostratigraphic framework that characterizes and discriminates between the individual shales associated with the Magnus Sandstone Member has impacted on reservoir modelling, primarily in the area of fluid pressure distribution, where shales such as the ‘B Shale’ and those of the ‘F Unit’ form significant intra-reservoir pressure barriers, placing sands in pressure isolation. Using a bioevent-constrained well correlation in the central field area this pressure compartmentalization is illustrated with reference to repeat formation tester tool (RFT) pressure depletion data. At a broader scale, the biostratigraphically-defined ‘time slices’ are used in conjunction with three-dimensional (3D) seismic data to model Magnus Sandstone Member submarine fan evolution, which is interpreted in terms of three main depositional phases: (1) a latest Kimmeridgian (J63) single submarine fan lobe with a depocentre in the mid-field, sourced from the northwest and displaying laterally confined, fault-controlled canyon-fill; (2) Early Volgian (J64 lower) bilobate accretion with two depocentres forming either side of the N-S-trending Brent High; and (3) late Early Volgian (J64 upper) as before but with the southward migration of the northern lobe depocentre, with fault confinement. Micropalaeontology has also been used at the well site in the Magnus Field where it has influenced drilling operations in ‘real time’. The primary role to date has been the precise picking of casing points above high-pressured or low-pressured sands (e.g. ‘A Unit’) and calibration of supra-reservoir angle-build in deviated wells. It is concluded that the application of biostratigraphy to integrated reservoir description has considerably enhanced our knowledge of submarine fan heterogeneity in Magnus, which has in turn led to more effective reservoir management and an extended production life for the field.

A Magnus opus: Helium, neon, and argon isotopes in a North Sea oilfield

This study of the Magnus oilfield, located in the East Shetland Basin, northern North Sea, represents the most detailed investigation of noble gas isotope systematics in a liquid hydrocarbon reservoir yet undertaken. Samples from nine producing wells across this Middle Jurassic field were taken and the helium, neon, and argon isotopic ratios and abundances in the oil were determined. Both the helium and the neon isotope systematics require a contribution from a mantle source. If the mantle endmember is modeled using mid-ocean ridge (MOR) values, 2.3–4.5% of the 4He and 4.3–6.2% of the 21Ne is mantle-derived. The remainder of the 4He and 9.0–12.0% of the 21Ne is crustal-radiogenic and the remaining 21Ne is atmosphere-derived. The resolved mantle-derived He/Ne ratio is quite distinct from upper-mantle values estimated from MOR samples, but indistinguishable from values resolved in other regions of continental extension. This result provides strong evidence that the subcontinental lithospheric mantle not only has a different noble gas inventory to the convecting mantle under MORs, but also has a near uniform composition. The quantity of radiogenic noble gas associated with the Magnus oil/groundwater system can only be accounted for by production predominantly from outside the volume of the Magnus Sandstone aquifer/reservoir drainage area and the associated Kimmeridge Clay source rock formation and together with the mantle-derived noble gases, provides strong evidence for cross-formational communication with deeper regions of the crust. The 20Ne and 36Ar must have been input into the oil phase by interaction with an air-equilibrated groundwater. Noble gas partitioning between a seawater-derived groundwater and the oil phase at the average Magnus Sandstone aquifer temperature requires a subsurface seawater/oil volume ratio of 110 (±40) to account for both the 20Ne and 36Ar concentrations in the central and southern Magnus samples. The volume of groundwater which has equilibrated with the Magnus oil is indistinguishable from the static volume of water estimated to be in the down-dip Magnus aquifer/reservoir drainage volume. This suggests that the Magnus oil has obtained complete equilibrium with the groundwater in the reservoir drainage volume, probably during secondary migration, and further suggests that concurrent cementation of the Magnus sandstone aquifer has occurred with little or no large-scale movement of air-equilibrated groundwater through the aquifer system. Higher concentrations of 20Ne and 36Ar in the N. Magnus oil cannot be accounted for by equilibration with a seawater-derived groundwater. This is qualitatively consistent with the earlier and more mature oil in this section equilibrating with freshwater, which is known to have been trapped in the crest of the reservoir structure.

Thermogravimetry/evolved water analysis (TG/EWA) combined with XRD for improved quantitative whole-rock analysis of clay minerals in sandstones

AbstractCurrent methods of quantitative whole-rock clay mineral analysis of sandstones often provide little more than an estimate of clay mineral abundances, especially where the total clay mineral content is <10 wt% of the sandstone. More accurate determinations of clay mineral abundance in the whole rock can be made by combining thermogravimetry/evolved water analysis (TG/EWA) and X-ray diffraction (XRD) data. The TGA/EWA system incorporates a purpose built thermobalance linked to a water specific infrared detector which is used to measure quantitatively the clay mineral dehydroxylation water evolved from the whole rock when heated from 250°C to 900°C. This gives a measure of the total hydroxyl content of the clay minerals in the whole rock which, when combined with XRD analysis of a separated clay size-fraction, enables individual clay mineral abundances in the whole-rock sample to be determined. Results on artificial sand/clay mineral mixtures prepared with known amounts of different clay minerals (chlorite, illite and kaolinite) show that the accuracy of the combined method is most influenced by the accuracy of the XRD data. Errors associated with TG/EWA were found to be negligible by comparison. A case study is included in which the technique has been used to determine accurately the illite abundance in the Magnus Sandstone Reservoir, Northern North Sea.

Textural and Isotopic Variations in Diagenetic Kaolinite from the Magnus Oilfield Sandstones

AbstractIn Upper Jurassic sandstones of the Magnus Sandstone Member from the Magnus Oilfield in the North Sea, diagenetic kaolinite morphology and isotopic composition vary away from the Late Cimmerian unconformity. Near the unconformity, coarse-grained (15-30 μm) vermiform kaolinite morphology is most common, whereas downdip, 3 km away, fine-grained (∼10 μm) blocky morphology is prevalent. Kaolinite abundances increase downdip, away from the unconformity, and kaolinite near the unconformity has low δ18O (12·5-14·9%o smow) compared to that downdip (15·9−17·5%o). This reflects replacement of marine depositional pore-waters by meteoric water near the erosion surface. However, isotopic temperature estimates suggest that the majority of kaolinite formed at elevated burial temperatures of ~80°C, and not during subaerial exposure of the sandstone. The δD of kaolinite close to the unconformity is also lower than that downdip. Kaolinite morphology and isotopic composition record meteoric water ingress during Late Cimmerian subaerial erosion and retention of meteoric-derived water in the crest of the Magnus structure during burial diagenesis. Kaolinite formation during subaerial exposure is not of significance to the sandstone reservoir quality.

Synchronous oil migration and cementation in sandstone reservoirs demonstrated by quantitative description of diagenesis

We present a model that explains the patterns of sandstone burial diagenesis in certain oil reservoirs, in which petroleum migration and burial cementation were synchronous. The coincidence of these two processes controls the chemistry and distribution of major burial cement phases across the field, which in turn controls the distribution of reservoir quality, causing a rapid decline of porosity and permeability with depth. Such a rapid poroperm deterioration is observed in many North Sea sandstone oilfields; we highlight the Magnus Sandstone Member of the Magnus Oilfield, northern North Sea as a type example of such a reservoir. The two most significant elements of the synchronous cementation and migration model are that burial cementation in the reservoir occurs over a restricted time interval, probably less than 10 Ma and that rapid and widespread fluid circulation is not invoked to explain the concentrations of cements observed. We speculate that cementation takes place at, and in a series of zones below, the oil-water contact which descends as oil fills the reservoir, with little change to the bulk chemistry of the reservoir formation waters through time.

Implications of Linearly Correlated Oxygen and Hydrogen Isotopic Compositions for Kaolinite and Illite in the Magnus Sandstone, North Sea

AbstractAuthigenic kaolinite and illite are important diagenetic minerals in the Magnus Sandstone, a giant oil reservoir in the northern North Sea. These clay minerals, separated from three wells, show considerable ranges in their oxygen isotopic composition (δ8OSMOW = +9 to + 16%) and hydrogen isotopic composition (δDSMOW = - 55 to - 105%). The variations in δ18O and δD are positively linearly correlated with a high degree of statistical significance for both kaolinite and illite:Kaolinite:n=12;δD=6.1δ18O−169;r=0.66(>95%)Illite:n=11;δD=5.9δ18O−159;r=0.78(>99%).Formation of the clays in a pore fluid of uniform isotopic composition over a range of temperatures appears unlikely. It is suggested that the observed relationships between clay mineral δ18O and δD are perhaps best explained by a model of precipitation at more or less constant temperature from pore fluids which varied isotopically across the oilfield. The isotopic composition of the formation waters would then lie along the line: δDw = 6.2 δl8Ow - 50. This is most plausibly interpreted as a mixing line with suggested minimal endmembers at (δ18O, δD) values of (+4, -24) and (-4, -76). The first of these represents reasonable isotopic values for Magnus Sandstone formation waters. Although δ18O of the second is compatible with an evolved Cretaceous meteoric water, its δD value is difficult to understand in the context of the model.

Distribution, Chemistry, Isotopic Composition and Origin of Diagenetic Carbonates: Magnus Sandstone, North Sea

ABSTRACT Diagenetic ferroan carbonates grew in the Upper Jurassic reservoir sandstones of the Magnus oilfield in porewaters which differed in composition across the field. These porewaters remained compositionally different and stratified for at least 35 M.y. Variations in carbonate chemistry across the field are attributable to these porewater variations, which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late Cimmerian subaerial exposure. Original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution. Rare diagenetic calcite occurs as discrete rhombic crystals. Diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains. Poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones. Three compositional growth zones in siderite crystals are observed across the field from crest to flank. In all three wells studied, a similar trend of compositional evolution through time is observed in both two and three zoned rhombs. First-formed siderite is relatively magnesian, intermediate zones are more ferroan, and outer zones are at least as magnesian as the first stage. These individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples (up to 87 mol % Fe + Mn) and more magnesian downdip (up to 58 mol % Mg). This reflects the greater influence of relatively Fe-rich meteoric-derived water in the crest and the greater influence of marine-derived Mg-rich porewater downdip. Ankerite shows a similar variation in Fe and Mg abunda ces across the field (crest and flank maximum 25 and 17 mol % Fe + Mn respectively, 27 and 43 tool % Mg) and developed due to release of Mg, Fe, Ca and HCO3- ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions. Both siderite and ankerite have lower 18O at the crest of the oilfield than downdip (respectively siderite 16.0 and 17.6 SMOW; ankerite 17.7 and 21.4). These differences in 18O reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity, whereas downdip porefluid contained a larger marine-derived component. Strong organic influence on 13C (-8.0 to -14.6 PDB for magnesian siderite; -7.7 to -13.6 for ankerite), closed system 34S values (up to 15.7 for late cubic pyrite), and stratified 18O from crest to flank of the field argue against large scale porewater movements. Diagenetic porewater stratification is strongly supported by the parallel, but distinct, geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field.

Diagenetic Pore Waters Stratified for at Least 35 Million Years: Magnus Oil Field, North Sea: Geologic Note (1)

Isotopically distinct pore waters remained stratified in the Upper Jurassic Magnus Sandstone, reservoir for the Magnus oil field in the northern North Sea, throughout diagenesis. A petrographic and isotopic study of the reservoir sandstones in three wells indicates that a similar suite of diagenetic quartz overgrowths, kaolinite, magnesian siderite, ankerite, and illite developed across the field. However, the oxygen isotope ratios (delta{18}O) recorded in these diagenetic minerals from the crest of the field are consistently more negative (by about 3 o/oo) than those from the downdip flank. We propose that this consistent difference in delta{18}O values records the persistence of isotopically different and stratified pore waters in the Magnus sandstones. The sandstones w re deposited as marine turbidites, but were subaerially exposed during periods of structural uplift and tilting during the Early and middle Cretaceous. Meteoric water entered the structural crest, resulting in pore-water stratification. Deposition of thick Upper Cretaceous mudstones and marls sealed the Magnus structure during subsequent rapid burial. Diagenetic minerals in the crest of the oil field grew in a pore water with significantly more meteorically derived water than those downdip, assuming that each diagenetic phase developed at a similar temperature in each well. The pore waters remained stratified for at least 35 m.y., from middle Cretaceous (approximately 90 Ma) subaerial erosion of the sandstone until reservoir charging at 55 Ma. The sandstones and mudstones of the Kimmeridge Clay Formation behaved essentially as a closed system during diagenesis, in contrast to the open-system concept envisaged in most other diagenetic reconstructions.

Ancient subaerial exposure and freshwater leaching in sandstones

Research Article| December 01, 1990 Ancient subaerial exposure and freshwater leaching in sandstones D. Emery; D. Emery 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar K. J. Myers; K. J. Myers 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar R. Young R. Young 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar Author and Article Information D. Emery 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England K. J. Myers 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England R. Young 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (12): 1178–1181. https://doi.org/10.1130/0091-7613(1990)018<1178:ASEAFL>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. Emery, K. J. Myers, R. Young; Ancient subaerial exposure and freshwater leaching in sandstones. Geology 1990;; 18 (12): 1178–1181. doi: https://doi.org/10.1130/0091-7613(1990)018<1178:ASEAFL>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present an example of leaching by meteoric water in the Jurassic Magnus Sandstone Member of the northern North Sea. The interpretation is based on information from several independent techniques, including downhole natural gamma-ray spectrometry, detailed petrographic analysis, and cross-sectional time-slice reconstructions from seismic data. The initial lines of evidence that prompted further investigation into freshwater involvement in the Magnus Sandstone Member were brackish formation-water compositions and a potassium depletion shown by the gamma-ray spectrometry log, in sandstone beneath the unconformity at the base of the Cretaceous section. Subsequent detailed petrographic study confirmed that the potassium depletion had been brought about by potassium feldspar dissolution, resulting in precipitation of kaolinite and creation of secondary porosity, the latter also revealed by conventional core-analysis porosity and permeability measurements. Finally, cross-sectional time-slice reconstructions from a regional seismic line across the Magnus oilfield strongly suggested subaerial exposure of the Magnus Sandstone Member in Early Cretaceous time and consequent freshwater leaching of the Magnus Sandstone Member and generation of the unconformity at the base of the Cretaceous. The combination of several lines of evidence from the Magnus field supports the process of subaerial exposure and freshwater leaching to create secondary porosity in sandstones. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Relationship Between Permeability and the Morphology of Diagenetic Illite in Reservoir Rocks

The Relationship Between Permeability and the Morphology of Permeability and the Morphology of Diagenetic Illite in Reservoir Rocks Summary The permeability characteristics of two reservoir sandstones from the Magnus and West Sole fields in the U. K. North Sea are essentially similar and are strongly influenced by the occurrence of filamentous illitic clay. However, the amount of illite in the West Sole sandstone is 20 times that in the Magnus sandstone, suggesting that the distribution and form of the clay is more important than its actual amount. Scanning electron microscope (SEM) observations show that the clay mineral occurs as an open, tangled web of long, thin ribbons virtually filling the pore spaces, a morphology that is properly revealed only after critical point drying. Introduction Early work at the BP Research Center on the permeability characteristics of reservoir sandstones from the Magnus permeability characteristics of reservoir sandstones from the Magnus field in the U.K. North Sea revealed puzzling differences between results from well testing and those measured by routine core analysis. Core data always indicated a higher permeability, which, in the case of the water zone, permeability, which, in the case of the water zone, amounted to 20 to 30 times that found by well testing. Similarly, there was a very wide discrepancy between gas and brine permeabilities of sandstone plugs measured in the oven-dried state and in the wet, preserved state. A possible explanation for this anomaly was that there could possible explanation for this anomaly was that there could be dehydration and shrinkage of clay minerals within the sandstone pores. Indeed, conventional SEM observations did reveal the presence of diagenetic filamentous illite coating the pore walls, although at that stage it was thought to be unlikely that this mineral could be responsible for the large differences in permeability. Work at the Macaulay Inst., however, showed that the morphology of the illite in the Magnus sandstones, as inferred through SEM, depended crucially on the manner of drying the specimen. The in-situ morphology of the clay mineral was revealed only after critical-point drying, when it was observed as an open, tangled web of very thin, long ribbons virtually filling the pore spaces. In contrast, after air- or freeze-drying, the illitic mineral occurred in dense mats packed against the pore walls. Following these observations, it was demonstrated conclusively 1 that the "interface-sensitive" nature of the filamentous illite provided a complete explanation for the discrepancies between the well-test data and routine core permeabilities. it was further suggested that the permeabilities. it was further suggested that the indiscriminate adoption of routine core analysis procedures could lead to serious errors in the assessment of reservoir quality. Our investigation compares the permeability and clay mineralogy of Magnus sandstones with those of West Sole sandstones, also in the U.K. North Sea, to show that fluid flow characteristics cannot be predicted from the quantitative clay content of reservoir sandstones. The West Sole sandstones contain abundant filamentous illite, which amounts to about 10% of the rock, compared with about 0.5% in the Magnus sandstones. The SEM and X-ray diffraction characteristics of the separated illite from both sandstones are dissimilar, but despite these differences the fluid flow properties of the two sandstones are comparable. Permeability Measurements Permeability Measurements Core plugs were cut from preserved Magnus and West Sole cores using degassed, simulated formation water as the cutting fluid. The core plugs, 1 in. [2.54 cm] diameter and 1 in. [2.54 cm] long, were cut from the center of the drilled pieces to reduce possible mud contamination. They were stored under formation brine. Complete brine saturation was ensured before permeability measurements were made. The cores were evacuated under brine to remove any gas and a pressure of 1,000 psi [6895 kPa] was then applied for &amp;gt;72 hours to dissolve any remaining gas. After saturation, the initial brine permeability was measured, during which time the sample was confined in a Hassler core holder with a sealing pressure of 400 psi [2758 kPa] on the rubber sleeve. Then the samples were dried in a humidity oven. After drying, the gas permeability was measured. All the samples were then resaturated permeability was measured. All the samples were then resaturated with brine and the brine permeability redetermined. Table 1 shows that the permeability characteristics of the two sandstones are similar and that, in particular, the gas permeability measured after oven drying was 20 times higher than the initial brine permeability measured on preserved samples. The resaturated brine permeabilities are higher than the initial brine values, permeabilities are higher than the initial brine values, indicating a permanent effect caused by drying. JPT P. 2225