Fulmar Formation Research Articles

Quartz Veins Record Vertical Flow at a Graben Edge: Fulmar Oil Field, Central North Sea

The Fulmar oil field lies at the faulted western margin of the Central Graben in the central North Sea. Chalcedony and quartz, and later dolomite, occur as diagenetic cements in veins within the Upper Jurassic Fulmar Formation. Quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees C, with a mean of 117 degrees C. The measured d18O of the authigenic quartz ranges from +25.1 to +27.1o/oo SMOW. The d18O of the water from which most of the quartz precipitated is calculated to have been around +8o/oo, which is significantly more positive than the measured present-day formation water d18O of +4o/oo. Quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18O of +23.5o/oo SMOW. High salinities are recorded in the fluid inclusions in the quartz, the majority ranging from 15 to 19 wt. % NaCl equivalent. Also, the quartz contains inclusions of anhydrite. High-salinity aqueous inclusions, combined with anhydrite inclusions in the quartz and the 18O-enriched calculated water composition, suggest that the quartz precipitated from an evaporite-influenced fluid. The same fluid contributed Mg for dolomite precipitation. More deeply buried sections, including the Permian Zechstein evaporites underlying the Fulmar field, are the most obvious source of an 18O-enriched saline fluid. Mineralization of the vein resulted from cross-formational flow of warm fluids from depth through the Zechstein and into the Jurassic succession over vertical distances of 600-2000 m. Fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release.

The Fife Field, UK central North Sea

The Fife Field is located in the far southeastern part of the Central North Sea Basin close to the UK, Norwegian, Danish median line. The field is a shallow relief four-way dip closure formed by inversion during Late Cretaceous/ early Tertiary times. The reservoir consists of thick Upper Jurassic, heavily bioturbated sandstones which are considered to have been deposited in a similar setting to the Fulmar Formation. The depth to the top of the Upper Jurassic at the crest of the field is 8250 ft sub-sea with the oil-water contact at 8512 ft sub-sea. The seal to reservoir is provided by Volgian-Ryazanian shales of the Kimmeridge Clay Formation and Upper Cretaceous Chalk. Although Jurassic sandstones form the primary reservoir, additional hydrocarbons have been encountered in the Tor Formation of the Chalk Group which is fractured over the crest of the field. The Fife Field was discovered in 1991 and is currently under development. Production started in August 1995 via the 'Uisge Gorm' Floating Production Storage and Offloading facility (FPSO). STOIIP is estimated at 132 x 106BBL and ultimate recovery is predicted to be 34 x 106 BBL oil. The low mobility of the oil and the low vertical permeability of the reservoir contribute to the predicted low (26%) recovery efficiencies.

Facies characteristics and depositional models of highly bioturbated shallow marine siliciclastic strata: an example from the Fulmar Formation (Late Jurassic), UK Central Graben

Abstract An extensive core database from the Late Jurassic Fulmar Formation illustrates the practicalities of facies analysis and depositional modelling in highly bioturbated, heterolithic, shallow marine siliciclastic strata. Twelve major sedimentary facies (1–12) are defined on the basis of lithology, grain size, visually estimated sand-silt content and the presence or absence of primary sedimentary structures. Ichnofabric characteristics are also defined and have proved to be not always facies specific. For purposes of depositional modelling the 12 facies types are resolved into six facies associations (A-F), each representative of distinct shoreline/shallow marine environments. From these associations three broad depositional models have been constructed: storm-influenced shoreface (model 1), bioturbated shoreface (model 2) and a speculative bioturbated shelf model (model 3). Stratigraphical and palaeontological criteria suggest that models 1 and 2 are shoreline-attached; sedimentological data indicate that they occur as end-members to a spectrum of shoreline settings constrained by incident wave energy ‘bands’. For shorefaces subject to significant storm influence, well-preserved event beds occur seaward of the upper shoreface. In the bioturbated shoreface model, however, the lower shoreface and adjacent shelf deposits are characterized by intense infaunal reworking. The bioturbated shelf model displays no clear evidence of shoreline connection, with the resultant facies often exhibiting highly abundant siliceous sponge spicules, together with other open marine indicators such as belemnites and rare ammonites. Unlike the shoreface sand bodies, which received sediment from basin-margin fluvial systems, shelf sand bodies in the Fulmar Formation are more likely to have been intrabasinally sourced along relay ramps or other forms of transfer zone. This sediment may have undergone a long transit/residence period on the shelf prior to eventual burial in hanging-wall depocentres strongly influenced by Zechstein salt withdrawal. This study demonstrates that an ichnofabric approach (as opposed to one of simple ichnodiversity) is highly significant with respect to the description and modelling of highly bioturbated shallow marine siliciclastic strata. In the Fulmar formation, ichnofabric analysis is capable of providing sensitive information on fluctuations in depositional energy levels, water depth, sedimentation rates, erosion rates, substrate consistency and dissolved oxygen levels. Evidence of these fluctuations is often apparent at intra-facies scale, but would largely go unnoticed using a conventional sedimentological approach to data gathering, based on ichnodiversity at best. Depositionally significant changes in ichnofabric may occur without any significant change in ichnodiversity. Ichnofabric analysis also plays an important role in the hierarchical assessment of key stratal surfaces for purposes of sequence stratigraphic modelling.

The role of trace fossil (ichnofabric) analysis in the development of depositional models for the Upper Jurassic Fulmar Formation of the Kittiwake Field (Quadrant 21 UKCS)

Abstract The sedimentology and sequence stratigraphy of the Upper Jurassic (Upper Oxfordian — Middle Kimmeridgian) Fulmar Formation of the Kittiwake Field, Western Platform of the north Central Graben are investigated through an integrated study of core material and wireline logs. The Fulmar Formation, in this area, comprises largely finegrained sandstones which are intensely bioturbated such that the use of primary sedimentary structures for the identification of depositional environments is impractical. By using the approach of ichnofabric analysis presented here, the information provided by trace fossils can be fully utilized in the formulation of a depositional model for the Fulmar Formation. A depth and substrate-related succession of ichnofabrics was determined for the Fulmar Formation from more complete and progradational successions. This attached shoreface succession extends from a Chondrites ichnofabric (offshore), through Anconichnus (upper offshore), Anconichnus and spreiten burrows and ‘ Teichichnus zigzag ’ (upper offshore-offshore transition zone), bivalve tube ichnofabric (offshore transition zone to lower shoreface), Ophiomorpha irregulaire (lower shoreface), Ophiomorpha nodosa (middle shoreface) to a burrow mottling ichnofabric and associated high-energy laminated sandstones (upper shoreface). Anomalies in this succession form the basis for the identification of bounding surfaces, particularly omission surfaces ( Thalassinoides and Diplocraterion habichi ichnofabrics) and sequence boundaries. The distribution and evolution of the essentially retrogradational succession of the Fulmar sandstones is illustrated by analysis of a detailed core log of well 21/18-3, a general cross section through the Kittiwake Field and an Early-Mid-Kimmeridgian time slice facies distribution map.

The Puffin Field: the appraisal of a complex HP-HT gas-condensate accumulation

Abstract The Puffin Field was discovered by well 29/5a-1, drilled in 1981 to test a Base Cretaceous closure. The well encountered over-pressured gas-condensate-bearing Upper Jurassic shallow marine sandstones below 14,000 ft. The well flowed over 20 × 10 6 SCFGD with 2650 barrels per day of condensate. Appraisal of the discovery, which fell into five phases, involved the acquisition of two 3D seismic surveys and drilling of eight wells. This paper describes the appraisal of Puffin from an historical perspective. The initial geological model involved an intra-graben horst draped by a relatively uniform blanket of Fulmar Formation sandstones. The first two apprisal wells were disappointing and hinted at previously unsuspected complexities. The second pair of appraisal wells, although proving up further hydrocarbons, penetrated thick Triassic sequences, with the Pentland and Fulmar Formations being absent. These results were incompatible with the existing geological model and a new model was developed. This envisaged the presence of several large Triassic-cored pods, formed in response to salt withdrawal and local tectonism, separated by narrow elongate zones (inter-pods) in which Middle and Upper Jurassic sections were present. Four further appraisal wells were drilled and proved the model to be robust with respect to stratigraphy and structure, although it did not always accurately predict hydrocarbon distribution. The reservoir geology of the shallow marine Fulmar Formation and fluvial Skagerrak Formation reservoirs will be outlined. The wells indicated considerable differences in hydrocarbon contacts between wells and differences in fluid gradients and pressures between the pods. The geological evolution of the field will be described.

Sequence Stratigraphic Division and Associated Cement Distribution: Upper Jurassic Fulmar Formation, Central North Sea, UKCS: ABSTRACT

The Fulmar Formation is an important reservoir interval in the Central North Sea, UCKS. Because of the highly bioturbated nature of the sediments, primary sedimentary structures are rare. This has limited previous sequence stratigraphic interpretations. Detailed and integrated ichnofabric analysis allows the development of a superior sequence stratigraphic interpretation in which a relatively predictable distribution of early diagenetic cements is observed. Within the cored intervals examined, five recurring ichnofabric associations are recognized, and these reflect a range of environments, from offshore transition to lower shoreface. In the two fields studied, a series of sharp-based sandstones are recognized. Analysis of the ichnofabrics associated with these sharp-based sandstones indicates an abrupt shallowing, typically with a change from offshore transition to a proximal lower shoreface environment. The base of each sandstone is marked by an erosional surface commonly with a shell/pebble lag. A common ichnofabric expression of such an abrupt shoaling is the superimposition of clean sandstones with Ophiomorpha on silty sandstones with Cylindrichnus. Internally, the sandstones display a progradational stacking pattern. Ichnofabric analysis therefore suggests that these abrupt basinward shifts in facies represent deposition in response to punctuated falling sea-level i.e. forced regression. Within this sequence stratigraphic framework, abundant carbonate and pyritemore » cements are observed. Although the sedimentological relationship of these cements is unclear, their distribution is relatively systematic, being best developed at the tops of parasequences. This evidence suggests that they are related to minor flooding surfaces, and has important implications for predicting reservoir heterogeneity.« less

Late Secondary Porosity Associated with Overpressure Leak-Off: Evidence from the Fulmar Formation, Central North Sea: ABSTRACT

The Upper Jurassic Fulmar sandstone of the Central North Sea is characterized by deep burial (greater than 4000m burial depth) and extremely high levels of overpressure (up to 40MPa overpressure at 4500m depth). High overpressure may support high porosity at great depths, and so the relationship between overpressure and porosity is of considerable commercial importance in deep prospects. Simulating the porosity evolution and overpressure of the sandstones via a two-dimensional basin model reveals that for the majority of Fulmar Fm. sandstones, the porosity evolution is a simple pattern of reduction during burial to present-day values of 15-20%. However, in wells sited close to structurally-elevated overpressure leak-off points, the porosity is anomalously high (up to 35% at 4500m depth). We suggest that pressure-induced vertical fluid flow at over-pressure leak points allows removal of the products of feldspar dissolution. This process allows porosity to increase from an end-Miocene low of 15-20% to the present-day values of 30-35%. Developing voids require support by overpressure, and so this increase in porosity must have occurred as the sandstones became highly over-pressured during rapid Plio-Pleistocene subsidence. We infer that overpressured sandstones may behave as open geochemical systems, and that 10-15% porosity has formed in regions ofmore » the Central Graben during deep burial in the last 5 Ma.« less

Facies controls on reservoir quality in the Late Jurassic Fulmar Formation, Quadrant 21, UKCS

Abstract Fulmar Formation sandstones have been assessed for reservoir quality in terms of their primary textural and sedimentological characteristics with respect to conventional core analysis data. Some 6000 ft of core were characterized in total using a comprehensive lithofacies scheme and the results integrated with porosity and permeability data to create a statistically valid data set. From this dataset the primary influences affecting reservoir quality have been evaluated for a discrete area of the Central North Sea. Simple cross-plotting techniques indicate general trends in reservoir quality related to lithofacies type and hence depositional energy levels in shoreline/shallow marine shelf environments. Dominant grain size trends are identified that also influence reservoir quality. Broad prograding, aggrading and retrograding sequences are recognized which can be used to sub-divide the reservoir and locally predict reservoir quality.

A Kimmeridgian time-slice through the Humber Group of the central North Sea: a test of sequence stratigraphic methods

Abstract Reservoir quality of the Fulmar Formation in the UK Central Graben depends on the original distribution of facies as well as subsequent modification by partly facies-dependant diagenesis. Numerous sequence stratigraphical analyses of the Humber Group have been published recently, each of which has elements which are convincing geologically. Nevertheless, it is our experience that facies and reservoir prediction within the Humber Group remain difficult. Sequence stratigraphical interpretations in the Group are clearly non-unique due to major uncertainties in the data. (1) Biostratigraphical uncertainties include species concepts, recognition of agediagnostic taxa in thermally altered floras, validity of repeated micropaleontological events and equivocal paleoenvironmental information. (2) Sedimentological uncertainties are due to the gross facies similarity of the Fulmar Formation over a large geographic area and throughout the Upper Jurassic, which make the detection of potential maximum flooding surfaces and especially sequence boundaries difficult even in many cored wells. Furthermore strong syndepositional subsidence variations due to both salt movement and tectonics, lead to aliased sampling of the subsurface in wells, which are mostly situated on highs. (3) Seismic correlation difficulties are due to persistent seismic imaging problems below the Late Cimmerian (‘X’) Unconformity. (4) The depositional and sequence stratigraphical models for Humber Group deposition need to be set in the context of a transgressively skewed, back-stepping system with strong local subsidence control. The overall transgression is thought to be due to the combination of sea-level rise, widening and deepening of the basin due to tectonics, and reduced sediment supply through time. In this situation existing models have to be applied with even more care than normal if they are not to be misleading (e.g. concepts of forced regressions). These difficulties are clarified in the context of a Kimmeridgian time slice through the Humber Group. Multiple options exist in well log correlations. As a result palaeogeographic reconstructions of the time-slice have an error margin of ±15 km, which has serious implications on the validity of reservoir quality predictions.

Applications of High Resolution Sequence Stratigraphy in North Sea Syn-Rift Reservoir Correlation and Development: ABSTRACT

Tectonically active basins may host a spectrum of sequence stratigraphic expressions previously considered to be spatially mutually exclusive. In low accommodation areas with high sediment supply, fourth order eustatic cyclicity results in high frequency sequence sets while within rapidly subsiding areas, time-equivalent Type-2 sequences are expressed by highly asymmetrical coarsening upward successions, resembling large parasequences. In the shallow marine Fulmar Formation, of the U.K. North Sea Central Graben a sequence boundary and overlying lowstand deposits, which illustrate the effects of laterally variable subsidence rate and intrabasinal topography on the expression of a eustatic sea-level fall, lie between the Glosense and Serratum (J54a and J54b) maximum flooding surfaces. The syn-rift physiography comprises major tilted fault blocks, with the Central Graben dipping parallel to the major faults, simulating a ramp setting. Where the throw of the faults were greatest (SE), the structure acted as a local shelf-slope break. Adjacent to the basin margin, incised valley were cut at fluvial input points (structural transfer zones) and laterally, interfluvial sequence boundaries developed. During early lowstand, sand bypassed the footwall shelf and was deposited as lowstand fan sediments within the deepest part of the hangingwall, with the fault zone acting as a local shelf slopemore » break. Within the shallower water areas of the hangingwall a localised ramp geometry existed parallel to the fault zone. Forced regression deposits developed here were coeval but not physically related to the deep water lowstand turbidite fan deposits.« less

Stable isotopes in illite: the case for meteoric water flushing within the Upper Jurassic Fulmar Formation sandstones, UK North Sea

AbstractThe extent to which the diagenesis of sedimentary sequences within the UK North Sea is influenced by the passage of meteoric water is subject of current debate. Petrographic (SEM, TEM, thinsection) observation and stable isotope ratios from authigenic illite separates are used to deduce the pore-fluid evolution of the Fulmar Formation within well 29/10-2 from the Central Graben, with emphasis on the past importance of meteoric water flushing.Petrographic observation shows the illite to have two modes of occurrence (pore-filling and as pseudomorphs after feldspar). These can be equated with two authigenic crystal morphologies (plates and laths) observed by TEM within illite separates prepared for isotopic analysis. The laths are equated with late stage ‘hairy’ pore-filling illite, the plates with illite pseudomorphs after feldspar which are difficult to date petrographically.The δD (−55 to −87‰ SMOW) and δ18O (12.4–14.6‰ SMOW) analyses can be interpreted in terms of a conventional hydrogen-oxygen cross-plot to indicate illite precipitation from pore-waters of meteoric origin at 50–60°C. However, these predicted precipitation temperatures are irreconcilable with the observed petrographic sequence. The hydrogen isotope compositions may have undergone resetting during burial. Consequently, only the δ18O data may be confidently interpreted in terms of the conditions of illite precipitation. This implies illite growth at temperatures above 80°C from waters with δ18O between −2.5 and +6‰ SMOW, leaving the case for meteoric water flushing of the sandstones unproven.

Effective stratigraphical subdivision of the Humber Group and the Late Jurassic evolution of the UK Central Graben

New biostratigraphical analysis of the Humber Group sediments from the UK North Sea Central Graben, using integrated palynological and micropalaeontological zonation schemes, allows substage resolution and basin-wide discrimination of subunits of the Heather Formation/Kimmeridge Clay Formation, and associated sandstones. The integrated use of both zonation schemes is particularly important because thermally mature Humber Group sediments buried below 12 000 ft in the Central Graben cannot be dated consistently to substage level using palynology alone. Poor data and the undiagnostic log character of the mudstones and shallow marine sandstones have resulted in a simple lithological scheme being widely applied, with all sandstones assigned to the Fulmar Formation and mudstones to the Heather Formation and the Kimmeridge Clay Formation. Previous generalized usage of the Fulmar Formation for all Upper Jurassic shallow marine sandstones is inappropriate. The Upper Oxfordian Puffin Formation is present in the northern and western Central Graben. The largely Kimmeridgian Fulmar Formation is restricted to the south and east, and the coeval Ula Formation flanks the Jæren High. These three units are all interpreted as syn-rift sandstones, locally sourced from uplifted and eroded fault block crests within and marginal to the Central Graben. Latest Kimmeridgian or younger shallow marine sandstones are largely confined to platform areas outside the graben, and can be related to regressive phases.

Sequence stratigraphic control on Middle and Upper Jurassic reservoir distribution within the UK Central North Sea

An integrated sequence stratigraphic analysis, utilizing seismic data, well-log cross-sections, biostratigraphic information, and core descriptions, provides a chronostratigraphic framework to analyse and predict the distribution of reservoir sandstones within the Middle to Upper Jurassic section of the UK Central North Sea. The Callovian (Middle Jurassic) through Ryazanian (lowermost Cretaceous) section in the UK Central North Sea forms a depositional wedge that thins towards the western graben margin. This wedge is bounded by surfaces traditionally interpreted as the Middle Cimmerian and Late Cimmerian unconformities. Marginal marine (Fulmar) sandstones are present within this wedge, and can be subdivided into informal lower and upper Fulmar members. Each member has a unique spatial and temporal distribution within the basin that can be related to primary depositional palaeogeography, post-depositional erosion, and basin tectonics. The lower member of the Fulmar Formation is Callovian to Early Oxfordian in age, and is restricted to the Central Graben. This member overlies Middle Jurassic strata of the Pentland Formation, and is conformably overlain by Oxfordian-age strata of the Kimmeridge Clay. The upper member of the Fulmar Formation is Early Kimmeridgian in age, and is distributed along the western graben margin. The upper member is conformably overlain by Kimmeridgian-age strata of Kimmeridge Clay, and typically overlies Triassic deposits of the Skagerrak Formation. Post-depositional erosion, association with relative falls in sea-level and accentuated by basement and salt tectonism, is the major factor that modifies primary depositional patterns within the Fulmar Formation. Truncation associated with unconformities interpreted as the 142 Ma, 138 Ma, and 136 Ma sequence boundaries affect the distribution, fades patterns, and thickness of the upper member of the Fulmar. These unconformities, as well as the 155.5 Ma, 150.5 Ma and 144 Ma sequence boundaries, may also affect the lower member of the Fulmar.

Stratigraphic, structural and depositional history of the Jurassic in the Fisher Bank Basin, UK North Sea

The Fisher Bank Basin forms the northernmost segment of the eastern Central Graben. It is filled with Jurassic sediments and contains proven hydrocarbon reservoirs of Late Jurassic age. These sediments were deposited in response to rifting, subsidence and fault block tilting, superimposed on a background of eustatic sea-level rise. Sedimentation commenced in the Middle Jurassic (?Bathonian) with the establishment of coastal swamps, depositing shales, coals and sandstones which comprise the Pentland Formation. This was followed by the development of a coastal barrier/lagoon complex in the Callovian depositing sandstones and shales which are laterally equivalent to the Hugin Formation. Both the Pentland and Hugin formations are relatively uniform in thickness throughout the study area and, although the formations are separated by an unconformity, there is no evidence of contemporaneous fault movement. A phase of intense rifting commenced in the Early Oxfordian and continued until the latest Oxfordian. This resulted on a broad scale in the development of the Fisher Bank Basin as a half-graben, with a wedge of sediment deposited which thickens eastward towards the Jæren High. On a more local scale, the distribution of the Middle to Upper Jurassic appears to have been strongly influenced by salt withdrawal forming local depocentres. In the Early and Middle Oxfordian, a shoreface environment prevailed on the western side of the basin, with shallow-marine, more offshore conditions existing to the east. This resulted in the deposition of cross-bedded and bioturbated sandstones in the west, with a lateral transition into bioturbated shales in the east. In the Late Oxfordian, the sandstones were progressively onlapped by the shales, with aerobic, slightly deeper water conditions becoming established throughout the basin. Previous workers have attributed these sediments to the Fulmar and Heather formations, respectively. The sandstones are, however, demonstrated to be both older and geologically separate from the Fulmar Formation, and the shales younger and lithologically different to the Heather Formation. To emphasize these differences, new names are proposed for these units: the sandstones are referred to the informally termed ‘Frigate Formation’ and the shales the ‘Lower Kimmeridge Clay Formation’. A regional maximum flooding event of latest Oxfordian/earliest Kimmeridgian age marks the transition from the ‘Lower Kimmeridge Clay’ to the ‘Upper Kimmeridge Clay’. The latter ranges in age from Kimmeridgian to Volgian (and locally Ryazanian) and consists of olive-black to greyish-black, laminated shales that are envisaged to have been deposited in a largely anaerobic deeper-marine environment. Rifting appears to have become much less prominent in the latest Oxfordian/Early Kimmeridgian, with the ‘Upper Kimmeridge Clay Formation’ forming a fairly uniform blanket of sediment across the basin and adjacent highs. Erosion of Upper Oxfordian and Lower Kimmeridgian shales took place in the later Kimmeridgian, particularly over the Forties-Montrose High and Jæren High. This was accompanied by localized deposition of sands in the Kimmeridgian and Volgian, with the Kimmeridgian sandstones being of possible shallow-marine origin and those in the Volgian forming in deeper water as turbidites.

Late Jurassic plays along the western margin of the Central Graben

The distribution of the Lower Kimmeridgian Fulmar sands along the edge of the Western Platform of the Central Graben is intimately related to Triassic halokinetic activity. Withdrawal of Zechstein evaporites resulted in the development of thick packages of Triassic Smith Bank claystones, the so-called ‘Smith Bank Pods’. The crests of the Zechstein evaporite highs were later subjected to groundwater dissolution, resulting in topographic lows, which controlled the pattern of drainage during the deposition of the Triassic Skagerrak sands. The Triassic salt movements associated with this phenomenon also exercised the underlying control on later drainage patterns in the Jurassic. The relative sea-level rise in the Early Kimmeridgian flooded the margin of the Western Platform which reworked the original valley fill and, together with erosion products from the hinterland, resulted in the deposition of the shallow marine sands of the Fulmar Formation. 3D seismic can reveal the extent of these palaeovalley systems draining into the Central Graben. The Fulmar sand packages are effectively encased in shales; a lateral seal is formed by the Smith Bank Formation and the top seal consists of Kimmeridgian shales. They represent an attractive possibility for stratigraphically trapped hydrocarbons.

Sequence Stratigraphic Signatures in a Complex Shelf Sand System: Implications For Reservoir Modeling

The Upper Jurassic of the United Kingdom South Central Graben, is a poorly understood shallow marine succession. Problems associated with working in these sequences arise on a number of scales. Regionally, the Fulmar Formation comprises at least six individual sandbodies of varying ages; their distribution is controlled by a combination of late Jurassic rifting and halokenetic structuration. Biostratigraphic dating of marine flooding surfaces has revealed a southward onlap through time related to graben expansion. On a subregional and field scale, problems in predicting reservoir anatomy arise due to the predominance of thick, apparently aggradational successions, deposited within a poorly defined paleogeographic setting with complex localized salt structuration. These factors, combined with the high degree of bioturbation, the scarcity of siltstone/claystone horizons, the uniformity of the grain size, and rare evidence for subaerial exposure have led to the general belief that conventional facies based on sequence stratigraphic modeling may not be applicable to this succession. Detailed relogging of 1500 m of core combined with ichnofabric analysis have revealed the presence of parasequences and parasequence sets subtly defined by changes in sand/silt ratio, and the presence of shell hashes and early diagenetic fabrics. Stacking patterns within these reservoir elements have led tomore » the recognition of both type 1 and, most significantly, type 2 sequence boundaries related to localized salt movement. Field scale, three-dimensional computer modeling of these newly defined correlative surfaces has resulted in a greatly enhanced understanding of reservoir anatomy with direct application for drilling strategy in a developing field.« less

The Effects of Oil Emplacement on Diagenetic Processes: Examples from the Fulmar Reservoir Sandstones, Central North Sea: Geologic Note (1)

Sedimentary petrography and petroleum geochemistry studies have been done on Fulmar Formation (Upper Jurassic) cores from Fulmar field, central North Sea. The extent of quartz overgrowths and albitization of K-feldspars is lower in oil-saturated zones compared with water-saturated zones. However, microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil- and water-saturated zones (85-125 degrees C), with maximum recorded temperatures close to the current reservoir temperature (about 130 degrees C). This suggests that quartz overgrowth development in the oil-saturated zones continued after oil emplacement, but was retarded compared with development in the water-saturated zones. The quartz cement, therefore, mus have been essentially locally derived by pressure solution. Albitization of K-feldspars is significantly less (about 50%) in the oil-saturated zones compared with albitization in the water-saturated zones. Because albitization of K-feldspars depends on a supply of sodium and removal of potassium, this process is more likely to be terminated by oil emplacement. The observed difference in the albitization has been used to suggest that the Fulmar reservoir probably was filled during the Neogene. This age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins. The Fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates. This is probably because the Fulmar sandstones, being partly turbiditic, offshore sand deposits, were not exposed to meteoric water. Their close proximity to the marginally mature to mature Kimmeridge Clay Formation seems to have little effect in terms of developing secondary porosity, suggesting that the role of organic acids or CO[2] generated in these source rocks is of minor importance.

The Fulmar Field, Blocks 30/16 & 30/llb, UK North Sea

Abstract The Fulmar Field is located on the southwestern margin of the Central Graben in Blocks 30/16 and 30/11b of the UK sector of the North Sea. The field is a partially eroded anticline with steeply dipping flanks formed by the withdrawal of deeper Zechstein salt. The reservoir consists of thick Upper Jurassic, shallow marine, very bioturbated sandstones of the Fulmar Formation and deep marine turbidites interbedded within the Kimmeridge Clay Formation. Seal to the reservoir is provided by the Kimmeridgian shales in the west and Upper Cretaceous chalks which unconformably overlie the Fulmar Formation in the east. The reservoir section has been subdivided into seven members and 14 reservoir units. Reservoir quality is generally excellent, although there are lower-energy sandstone facies found in the eastern part of the field. The Fulmar oil is highly undersaturated and a secondary gas cap has been created by gas injection. Two exploration wells were drilled before the field was declared commercial. Development is from a 36 slot steel platform and a six slot template. Oil evacuation is by a floating storage unit and gas evacuation is via the Fulmar gas pipeline. Total STOIIP is 815 MMBBL and ultimate recovery is 462 MMBBL oil and 264 BCF gas. Production started in 1982 and 319 MMBBL oil and 121 BCF gas have been produced by year-end 1988. A total of 80 BCF gas has been re-injected for conservation purposes.

The Kittiwake Field, Block 21/18, UK North Sea

Abstract Kittiwake was discovered by well 21/18-2 within a 7th Round block, part of Production Licence P351. Highly undersaturated oil is present in the Fulmar Formation and Skagerrak Formation reservoir sequences; 70 MMBBL of reserves is in Fulmar sandstones whereas oil in the Skagerrak is mostly immovable. The field will be developed from a single 16-slot platform with initially 5 producing and 5 water-injection wells. Solution gas is removed via the Fulmar Field pipeline to St Fergus and, as from September 1990, the oil is loaded onto tankers from a single-buoy mooring.

Diagenesis of the shallow marine Fulmar Formation in the Central North Sea

AbstractThe diagenetic history of the Upper Jurassic Fulmar Formation of the Central North Sea is described with emphasis on the Fulmar Field. The Fulmar Formation was deposited on a variably subsiding shallow-marine shelf under the influence of halokinetic and fault movements. The sediments are extensively bio-destratified although large-scale cross-bedding is locally preserved. The dominant mechanism of deposition is thought to have been storm-generated currents. Soft-sediment deformation structures are common and are attributed to syn- and post-depositional dewatering of the sandstones. The dewatering was associated with fractures and shear zones which reflect tectonic instability resulting from periodic salt withdrawal and/or graben fault movements. The dewatering may have been initiated by repacking of the sediments during earth movements or by the gradual build-up and sudden release of overpressures due to compaction and/or clay mineral dehydration during rapid burial at the end of the Cretaceous. The formation is composed of arkosic sandstone of similar composition to Triassic sandstones from which it was probably derived. The sandstones also contain limited amounts of marine biogenic debris including sponge solenasters, bivalve shells, rare ammonites and belemnites. Initial diagenesis began with an environment-related phase during which quartz and feldspar overgrowths and chalcedony and calcite cements were precipitated. These cements appear to form concretions adjacent to local concentrations of sponge debris and shell debris, respectively, and were disturbed after their formation by fracturing and dewatering. This was followed by an early burial stage of diagenesis which resulted in extensive dolomite cementation and minor clay mineral authigenesis (illite and chlorite). The last phase of mineral growth was probably pyrite. During early burial diagenesis, secondary porosity after feldspar and/or carbonate was produced, although the exact timing is not clear. The lack of both stylolitic developments and extensive illitization indicates that the late burial diagenesis stage was never reached, although sufficient clay diagenesis occurred to destroy all traces of mixed-layer illite-smectite (present in some shallower wells). The main control on reservoir behaviour is primary depositional fabric. Diagenesis only overprints these controls. Locally-cemented fracture sets act as baffles to fluid flow, but they are not extensive and the reservoir acts as one unit.