Etive Formation Research Articles

The value of core in mature field development – examples from the UK North Sea

Abstract Core-based studies have had material impacts on the understanding of a number of late-life, mature North Sea Brent Group hydrocarbon reservoirs. These studies have included sedimentological, diagenetic and reservoir quality focused evaluations of core. The primary objective of the studies has been to improve conceptual and qualitative models that can be utilized in reservoir modelling and also for infill drilling and well workover evaluations. Most of these studies have been undertaken on old core samples collected in the 1980s and 1990s. Two case studies are described here that provide examples of the utility of core in mature fields. (1) Heather Field calcite: to quantitatively assess the distribution of calcite cements and their impact on hydrocarbon volumes and reservoir quality distribution in Brent reservoirs. (2) Thistle Field Etive Formation barriers and baffles: to characterize and describe the origin and distribution of low-permeability intervals within the Etive Formation reservoir. These two studies used a wide variety of core-based techniques including core logging and description, optical microscopy and petrographical studies, isotope analyses, X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) (FEI Company analysis tool and software, QEMSCAN)-based mineralogy, portable-X-ray fluorescence (XRF), NDTr and Thermo Scientific Inc. NITON TM operational software (NDT) geochemical analysis, as well as image analysis of grain size and texture. These data were then integrated with other subsurface datasets, such as well log, seismic data and well performance data, in order to address the specific reservoir challenge. These new and focused reappraisals of core demonstrate the dual value of core-based studies, which can: improve the understanding of producing hydrocarbon reservoirs, leading to improved productivity and recovery. Core is a full asset life-cycle resource and provides critical insight at all stages of field maturity as production behaviour changes and alternative development strategies are considered; further our general knowledge and understanding of clastic sedimentology and diagenesis using rich and diverse core-based datasets backed up by substantial well log and seismic datasets.

E&P Notes (February 2022)

E&P Notes (February 2022)

3D seismic imaging of lower delta-plain beach ridges: lower Brent Group, northern North Sea

Abstract: Three-dimensional seismic reflection data are used to image littoral deposits within the lower Brent Group, northern North Sea. Seismic attribute maps within the unit indicate the development of parallel, high-amplitude stripes up to 15 km in length, 50–100 m wide and spaced 150–200 m apart. In map view these features trend NE–SW to ENE–WSW and are arranged into ‘sets' that display subtly different orientations. Well data in regions where these anomalies are well developed indicate pronounced anomaly-perpendicular thickness variations in sand-rich beach-ridge facies within the Etive Formation and coals and mudstones within the overlying Ness Formation. Based on these observations, the high-amplitude anomalies are interpreted as the seismic expression of coal-filled swales, whereas the adjacent zones of low amplitude are interpreted to represent the cores of sand-rich beach ridges. The geometry of beach ridges identified in the Etive Formation compares favourably with sedimentological and geometric data from modern beach ridges. The results of this study have implications for (1) the stratigraphic context and preservation of beach ridges, (2) datum selection when attempting stratigraphic correlations within the Brent Group, and (3) the exploration and production of hydrocarbons from beach ridge-type reservoirs.

Changes in physical properties of a reservoir sandstone as a function of burial depth – The Etive Formation, northern North Sea

Rock physical properties, like velocity and bulk density, change as a response to compaction processes in sedimentary basins. In this study it is shown that the velocity and density in a well defined lithology, the shallow marine Etive Formation from the northern North Sea increase with depth as a function of mechanical compaction and quartz cementation. Physical properties from well logs combined with experimental compaction and petrographic analysis of core samples shows that mechanical compaction is the dominant process at shallow depth while quartz cementation dominates as temperatures are increased during burial. At shallow depths (<2000–2500 m, 70–80 °C) the log derived velocities and densities show good agreement with results from experimental compaction of loose Etive sand indicating that effective stress control compaction at these depths/temperatures. This indicates that results from experimental compaction can be used to predict reservoir properties at burial depths corresponding to mechanical compaction. A break in the velocity/depth gradient from about 2000 m correlates with the onset of incipient quartz cementation observed from petrographic data. The gradient change is caused by a rapid grain framework stiffening due to only small amounts of quartz cement at grain contacts. At temperatures higher than 70–80 °C (2000–2500 m) the velocities show a strong correlation with quartz cement amounts. Porosity reduction continues after the onset of quartz cementation showing that sandstone diagenesis is insensitive to effective stress at temperatures higher than 70–80 °C. The quartz cement is mainly sourced from dissolution at stylolites reflected by the fact that no general decrease in intergranular volume (IGV) is observed with increasing burial depth. The IGV at the end of mechanical compaction will be important for the subsequent diagenetic development. This study demonstrates that mechanical compaction and quartz cementation is fundamentally different and this needs to be taken into consideration when analyzing a potential reservoir sandstone such as the Etive Formation.

Genesis of an over‐thickened shoreface sandstone tongue: The Rannoch and Etive formations of the Middle Jurassic Brent delta, North Sea

ABSTRACTProgradational, wave‐dominated shoreface sandstones are typically reported to have thicknesses of less than 30 m. The shoreface sandstones of the Rannoch and Etive formations in the North Sea Brent Group, however, comprise far greater thicknesses (in places exceeding 100 m). The genesis of these successions have not been well understood, and the purpose of this study is to investigate within the framework of facies geometries in both modern and ancient depositional systems how the Rannoch‐Etive sandstones could have been formed, on a local‐regional scale.The facies analysis concludes that the Brent delta advance represents a single progradational event, not punctuated by transgressions. In such a scenario, the vertical sandstone thickness would be determined by: 1) shoreline trajectory angle, 2) the horizontal (dip‐directed) length of shoreface sand (typically 1–2 km in modern systems), and 3) shoreface sand pinch‐out depth (typically 5–12 m in modern systems). Within a framework of modern facies geometries, a 100 m thick vertical shoreface sandstone succession could result from a regression characterised by a shoreline trajectory of 2.6–5.4° (implying 80–95 m rise in relative sea level per 1–2 km of progradation), a 5–12 m deep shoreface sand pinch‐out depth, and a 1–2 km shoreface sand length.If, however, one applies such a model for the entire Brent delta progradation in the study area (200 km) this implies a rise in relative sea level of c. 5000 m, which clearly would be associated with large amounts of deposits accumulating behind and in front of the advancing shoreline. Such deposits are not present, which implies that modern facies geometries do not apply and that rising sea level was not of major importance in defining stratigraphic architecture. It is more likely that the regional overthickening of the Rannoch‐Etive sandstones resulted from progradation with a deep shoreface sand pinch‐out depth.

Structural evolution of the Penguins Cluster, UK northern North Sea

Abstract The Penguins Cluster is a group of four oil and gas fields in the northern end of the East Shetland Basin. Its structural complexity is caused by the interaction between two or more fault trend populations, fault reactivation and the impact of faulting on the Brent reservoir architecture. This structural picture is further complicated by a NW–SE trending basement lineament interpreted as a Caledonian shear zone. The present day structural configuration is the result of two Mesozoic rifting episodes and their associated thermal subsidence phases. The Permo-Triassic rifting created a number of north–south-trending tilted fault blocks, and was followed by a period of tectonic quiescence until the Middle Jurassic, when a faulting episode coeval with the Brent Group deposition caused footwall rotation, uplift and erosion of the upper Rannoch Formation prior to the deposition of the Etive Formation across the area. The rifting climaxed in the late Jurassic, when the reactivation of pre-existing faults under oblique-slip conditions in the Penguin C Field created small-scale lozenge-shaped transpressional and transtensional fault blocks. The presence of reverse faults in the area is explained with a continuous kinematic model of structural evolution and oblique-slip fault reactivation rather than positive basin inversion.

Controls on facies architecture in the Brent Group, Strathspey Field, UK North Sea: implications for reservoir characterization

The Brent Group reservoir in the Strathspey Field, UK North Sea comprises shallow marine, marginal marine and non-marine strata of variable reservoir character. Controls on the distribution, geometry, connectivity and orientation of sandbodies within the reservoir have been assessed via the application of high-resolution sequence stratigraphic methods to an integrated dataset comprising core, wireline log, 3D seismic and reservoir production data. The base of the Brent Group is a major sequence boundary overlain by weakly wave-influenced, shallow marine sandstones of the lowstand-to-transgressive Broom Formation. The overlying Rannoch Formation represents highstand progradation of a wave-dominated shoreface, which is truncated across a sequence boundary by a near-field-wide fluvio-estuarine complex (Etive Formation). Marginal-marine strata of the overlying Ness Formation are divided into field-wide units bounded by mudstones that form vertical pressure barriers. Major, oil-bearing fluvial sandbodies overlie sequence boundaries and the widths and orientations of two such sandbodiesare controlled by syn-depositional, intra-reservoir faults. The distribution of these sandbodies is therefore predictable. The Ness Formation is truncated by a major sequence boundary overlain by stacked, tidally influenced, shallow marine sandstones of the Tarbert Formation. The resulting facies architecture has been successfully used to refine reservoir models, identify bypassed oil and target infill wells.

The significance of the Etive Formation in the development of the Brent system: distinction of normal and forced regressions

Abstract Recent sequence stratigraphic debate on the Brent system have focused on the interpreted nature of the progradational trajectory (horizontal, slightly upwards or downwards) of the shoreline (Rannoch/Etive Formations) through time, as this gives a direct measure of how late Aalenian-Bajocian relative sea level changed during regression. Early interpretations emphasized the unified shallowing-upward nature of the Rannoch-Etive-Ness depositional system, and implicitly accepted a uniform shoreline progradation, i.e. a shoreline trajectory that was horizontal or slightly rising, implying a stable or slightly rising relative sea level. No irregularities of the trajectory were noted, and unusual shifts in facies, grain size etc. were normally related to autocyclic processes. More recent work has suggested that in some instances there is evidence for more irregular shoreline progradation at certain times, and for fall(s) in relative sea level and forced regression. This evidence comes from incised valleys and deep erosion/subaerial exposure surfaces from the landward (Etive-Ness boundary) and basinward (Rannoch-Etive) reaches of the Brent system respectively. However, it is currently unclear if any of these downshift surfaces recognized in the strandplain/coastal plain and shoreface environments are in time-equivalent strata. Current debate is mostly handicapped by a lack of agreement on the origin and depositional facies of the Etive Formation. There is significant debate about the relative amounts of fluvial, tidal and wave influence detected in the strata of this formation, with some authors arguing for a dominance of fluvial distributaries and mouth-bar deposits, whereas others propose either tidal-channel and inlet deposits or wave-dominated shoreface and strandplain settings. The stratigraphy is impacted by this disagreement. The character and sharp base of the Etive Formation can be argued to be consistent with normal shoreline processes, where wave or tidal conditions can produce significant erosion in the shoreface, without the necessity of any forced regression. Other interpretations, particularly where the Etive Formation is seen in terms of fluvial facies and processes, require a significant basinward shift of the shoreline to explain the Rannoch-Etive superposition, and a fall of sea level to cause the erosive boundary between the two formations. However, there is now ample evidence, including new evidence presented here, that both of the end-member scenarios for the progradation of the Brent system are incorrect. The notion that the overall progradation was entirely a product of normal regression, during stable and/or slightly rising relative sea level, is negated by local evidence of incised valleys, of subaerial exposure and plant growth in lower shoreface strata in the Rannoch Formation, and of repeated erosion surfaces with coarse-grained lags at the base of the Etive Formation. On the other hand, the idea of continuous sea level fall or of a single, late-stage fall, such that there was regional valley incision of the Etive into the Rannoch Formation and that the former is entirely younger than the latter, is negated by local evidence of gradual upward facies change between the formations, of stratigraphic interfingering between the formations, and of time lines passing through the Etive into the Rannoch Formation. It is perhaps not surprising that the system’s overall regressive trajectory varied in time from being forced to being normally regressive, and that further detailed local studies are required before regional generalisations can be made.

Abstract: Bedsets Define Reservoir Compartments In The Shoreface, Coastal Barrier And Strandplain Sandstones Of The Rannoch And Etive Formations, Brent Field, Uk North Sea&amp;nbsp;

Abstract: Bedsets Define Reservoir Compartments In The Shoreface, Coastal Barrier And Strandplain Sandstones Of The Rannoch And Etive Formations, Brent Field, Uk North Sea&amp;nbsp;

Abstract: Bedset-Scale Facies Architecture and Reservoir Compartmentalisation in the Rannoch and Etive Formations, Lower Brent Group, UK North Sea

Abstract: Bedset-Scale Facies Architecture and Reservoir Compartmentalisation in the Rannoch and Etive Formations, Lower Brent Group, UK North Sea

Application of High-Resolution Sequence Stratigraphy to Northwest Hutton Field, Northern North Sea: Implications for Management of a Mature Brent Group Field

The Middle Jurassic Brent Group reservoirs in Northwest Hutton comprise shallow-marine, marginal-marine, and coastal-plain reservoir sandstones of both channelized and sheet geometries. The connectivity and production characteristics of these sandstones can be predicted through the application of high-resolution sequence stratigraphic correlation techniques. The Broom Formation overlies a basal Brent sequence boundary and is a lowstand to transgressive systems tract tidal-estuarine complex. The overlying Rannoch Formation comprises shoreface sands that represent a highstand systems tract. The overlying Etive Formation is a multilayer fluvial to estuarine channelized system of near fieldwide development interpreted as incised valley fills. The superjacent Ness Formation is genetically related to the Etive and is the main oil-bearing interval over much of the field. The Lower Ness comprises mouth-bar parasequences and is interpreted to be a transgressive systems tract to the Etive lowstand. The Upper Ness is a nonmarine delta plain succession, interpreted as a highstand systems tract. Upper Ness channel sands typically correlate between several wells and are oriented west-east. An interval of widespread, stacked channels indicates a change in drainage orientation and is related to a high-frequency relative sea level fall. The uppermost reservoir unit is the Tarbert Formation, related to a further major sequence boundary. Original repeat formation tester data support a stratigraphically well- connected reservoir model. Compartmentalization exists and is due to stratigraphic layering (laterally persistent mudstones creating low vertical permeability) and variable dimensions of channels and valley fills.

Sequence Stratigraphy and Stratigraphic Simulation of the Brent Group, Norwegian North Sea: ABSTRACT

Sequence stratigraphic analysis of a well log data base from the Norwegian North Sea has established that the Brent Group is a deltaic complex consisting of four third-order depositional sequences. Sequence 1 was deposited first during northwestern progradation of the delta from the Horda Platform, while sequences 2 and 3 were primarily aggradational. Sequence 4 is characterized by backslapping geometries and a basin wide drowning event which resulted from variably increasing subsidence rates across the basin. The stratigraphic events interpreted in this study are based on the identification of unconformities (sequence boundaries), transgressive surfaces, and maximum flooding surfaces. Dates for these events were established using biostratigraphy from 12 wells. Unlike earlier interpretations, the Oseberg Formation is considered to be part of Sequence 1 and is an incised-valley fill which is genetically related to the distal Rannoch and Etive formations. Also in this study, a new sea level curve is proposed for the Aalenian - Bathonian which has similar frequencies to the Exxon sea level chart, but much smaller amplitudes. This interpretation was simulated in order to estimate the sea level, subsidence, and sediment supply parameters involved in deposition of these sequences. Simulation results indicate that the main deposition of themore » Brent Group occurred during uniform subsidence, while drowning of the depositional system resulted from a sharp, but variable increase in subsidence rates across the basin. Quantitative comparisons between the lithologic percentages in each sequence at well locations and at representative points in the simulation confirm that the simulation presents a unique and reasonable answer.« less

Influence of Relative Sea Level on Facies, Stacking Patterns and Reservoir Heterogeneity: Middle Jurassic Brent Group, UK North Sea: ABSTRACT

The lower Brent Group, above the genetically distinct lowstand deposits of the Broom Formation, is a progradational sequence set made up of the lower- shoreface Rannoch, the upper-shoreface and beach-plain Etive, and the back-barrier Lower Ness Formations. Much of the reservoir heterogeneity in this sandstone-rich succession can be attributed to facies and stacking-pattern variations within higher frequency systems tracts. Highstand systems tracts have strongly offlapping progradational geometries. Sweep efficiency is influenced by: (1) seaward-dipping, mica-rich bed-sets; (2) high-quality ebb tidal-delta sandstone bodies; and (3) the sharp increase in reservoir quality at the sub-horizontal base of the upper shoreface. The increased permeability leads water in the Etive to overrun oil in the Rannoch. Horizontal wells are targeting unswept reserves in the lower-permeability facies. Relative falls in sea level punctuated highstand progradation and led to the subaerial erosion of the Etive and Rannoch formations. Lowstand deposits are contained within incised valleys and consist of fluvial sandstones, coastal-plain mudstones and coal. Well-performance anomalies and abrupt variations in sandstone thickness are associated with the heterogeneous valley-fills. Shoreface sedimentation resumed during transgressive systems tracts. High sediment flux enabled the nearshore system to stack aggradationally. Diachronous lithostratigraphy, aggradation of the heterolithic back-barrier Ness Formation, interbedding ofmore » Rannoch facies in the lower Etive and increased carbonate cement in the lower shoreface Rannoch mark this systems tract.« less

Influence of relative sea-level on facies and reservoir geometry of the Middle Jurassic lower Brent Group, UK North Viking Graben

Abstract Detailed sequence stratigraphy of several neighbouring fields in the East Shetland basin indicates that a higher order of cyclicity is superimposed onto the lower Brent Group. Both relative falls and rises in sea-level affected the strongly progradational nearshore/barrier-island system and induced important variations in the geometry, distribution and stacking patterns of the reservoir facies. The majority of the shallow marine strata belong to the transgressive and highstand systems tract and consist of the lower-shoreface Rannoch, the upper shoreface and strand-plain Etive and the back-barrier and lagoonal Lower Ness. High frequency increments of shoreline progradation are preserved in the Rannoch as gently seaward-dipping bedsets. The small-scale units mark asymmetric cycles in grain size and mica content and together, with stratiform cement, impart a strong permeability anisotropy on the Rannoch. The nearshore Etive Formation represents a sediment-laden, barred nearshore complex and is subdivided into (1) a uniformly thick upper shoreface and (2) an overlying emergent strand plain. The two subfacies are commonly separated by a thin interval of foreshore which is locally enriched in heavy minerals. The shoreface deposits show systematic variations in stacking geometries ranging from strongly progradational to aggradational. Two relative falls in sea-level led to the subaerial erosion of nearshore Etive deposits and the juxtaposition of coastal-plain mudstones, coals and fluvial deposits with shale-clast lags on lower shoreface Rannoch strata. Although lithostratigraphically part of the Etive, these deposits bear no resemblance to the well ordered nearshore-Etive deposits. The channel complexes are vertically and laterally heterogeneous and are confined to incised valleys separated by non-depositional interfluves. In Cormorant and Tern fields, well-performance anomalies such as early water breakthrough and unpredictable water injection and oil production rates are associated with these lowstand incised valley deposits. An enigmatic coarse-grained unit occurs in the lower Rannoch in Eider field and is provisionally interpreted as the distal expression of the lowstand shoreface deposits associated with the older incised valley. Following the lowstand deposition nearshore-marine sedimentation reestablished during the transgressive/early highstand systems tract. Under the influence of rising relative sea-level the barrier complex aggraded in place with minimal landward translation of the shoreline position. The volume of sediment supplied to the Etive shoreface was sufficiently high to keep pace with phases of increased accommodation. The shoreline system subsequently stacked with a strongly progradational geometry as the high sediment supply filled the available accommodation space during the latter part of the highstand systems tract.

Impact of sequence-based correlation style on reservoir model behaviour, lower Brent Group, North Cormorant Field, UK North Sea

Abstract An increasing percentage of remaining reserves in mature Brent Group producing fields is found in the Rannoch Formation, a lower shoreface succession characterized by low permeabilities, abundant mica and stratabound carbonate cements. The high contrast in permeability between the Rannoch and overlying Etive formations has resulted in severe water overrunning of oil-bearing Rannoch sandstones in a number of fields. Remaining Rannoch reserves will be difficult to recover and will require sophisticated reservoir management techniques. Differences in log correlation style which in the past may have been dismissed as inconsequential may now be important to consider in field depletion planning. The objective of this study was to assess whether, in a high net-to-gross shoreface section such as the lower Brent Group at Cormorant Field, a sequence-based correlation would result in a significant difference in reservoir model behaviour, particularly in oil displacement patterns in lower shoreface sandstones of the Rannoch Formation. The modelled interval includes the prograding shoreline succession of the Rannoch (lower shoreface), Etive (upper shoreface-strandplain) and Ness (coastal plain) formations. There is a significant amount of facies interfingering between the Etive and Rannoch within the modelled area. Permeability models were constructed from both sequence-based and lithostratigraphic well-log correlations and used as input for 2-D reservoir simulations. The results show a divergence in oil displacement behaviour between the two models, depending upon the degree of permeability anisotropy assumed. The principal reason for the difference lies in how the Rannoch layers are correlated. In the lithostratigraphic model, the Rannoch is treated as a tabular unit with layer boundaries parallel to the formation boundaries. Water injection into the upper Rannoch sweeps the upper Rannoch across the length of the model. In contrast, the sequence-stratigraphic approach treats the Rannoch-Etive as a series of clinoforms with layers offlapping toward the north in the direction of progradation. In this scenario, water injected into the upper Rannoch in the downdip area migrates parallel to layering into the lower Etive in the updip direction. The oil-bearing upper Rannoch in the updip areas correlates downdip to low-permeability facies which do not take water on injection; thus, the upper Rannoch remains unswept. Predicted recovery efficiencies from the Rannoch lower shoreface sandstones are consistently lower in the sequence stratigraphic model, and more closely match observed reservoir behaviour at Cormorant Field than predictions based on lithostratigraphic correlation. This study suggests that, even in sand-rich systems such as the Rannoch-Etive, a sequence-based approach to reservoir zonation can improve predictions of reservoir performance.

On the interaction of geology, measurement, and statistical analysis of small‐scale permeability measurements

ABSTRACTSedimentary rocks exhibit small‐scale (mm) permeability variations which can be detected with a probe permeameter. The structure of the laminations may provide evidence about the characteristics of the measurement device. The statistical interpretation of these measurements using averages and variograms is enhanced when the geological character of the rock is taken into consideration. The three aspects of rock characterization, namely geology, measurement, and statistical analysis, are best assessed in concert. This theme is illustrated using probe data taken from sections of the Rannoch and Etive Formations, Northern North Sea.

SCALES OF PERMEABILITY HETEROGENEITY WITHIN THE BRENT GROUP

A selection of Brent Group reservoir sequences from the Murchison field were chosen to study the nature of permeability variation within the Brent Group, with particular emphasis on the Etive Formation, which forms one of the main reservoirs in the field.Preliminary core examination of potential permeability barriers, as identified using wireline log response, has indicated that permeability heterogeneities can be placed into a five‐fold hierarchy. Permeability heterogeneities resulting from depositional or syndepositional processes comprise the first four orders of heterogeneity and include formations, facies transitions, bedding surfaces and laminations. Permeability heterogeneities which are the product of diagenetic processes are classified as fifth‐order heteorgeneities.Two major types of permeability barrier have been identified in core and on wireline logs within the Etive Formation. They include permeability heterogeneities associated with a transition between the two major facies associations (tidal channel and beach/foreshore sandstones); and heterogeneities associated with the development of inactive channel‐fill sandstones and siltstones within the tidal‐channel facies association.A detailed sedimentological model has been developed which demonstrates the complexity of reservoir zonation in the Etive Formation within the Murchison field.

SCALES OF PERMEABILITY HETEROGENEITY WITHIN THE BRENT GROUP

A selection of Brent Group reservoir sequences from the Murchison field were chosen to study the nature of permeability variation within the Brent Group, with particular emphasis on the Etive Formation, which forms one of the main reservoirs in the field.Preliminary core examination of potential permeability barriers, as identified using wireline log response, has indicated that permeability heterogeneities can be placed into a five‐fold hierarchy. Permeability heterogeneities resulting from depositional or syndepositional processes comprise the first four orders of heterogeneity and include formations, facies transitions, bedding surfaces and laminations. Permeability heterogeneities which are the product of diagenetic processes are classified as fifth‐order heteorgeneities.Two major types of permeability barrier have been identified in core and on wireline logs within the Etive Formation. They include permeability heterogeneities associated with a transition between the two major facies associations (tidal channel and beach/foreshore sandstones); and heterogeneities associated with the development of inactive channel‐fill sandstones and siltstones within the tidal‐channel facies association.A detailed sedimentological model has been developed which demonstrates the complexity of reservoir zonation in the Etive Formation within the Murchison field.

K-Ar dating of illites in Brent Group reservoirs: a regional perspective

Abstract This paper reviews existing K-Ar dates from diagenetic illites from the Brent Group and presents new age data from 11 wells from across the East Shetland Basin. K-Ar ages from diagenetic illites record illite growth in various parts of the Brent Group of the East Shetland Basin from about 60 Ma until 17 Ma. Diagenetic illites from the Cormorant field yield ages, which when combined with burial history simulation, suggest substantial illite growth from 75°C. This temperature is believed to be close to the temperature for the start of illite growth in the Brent Group. Other illite dates, when combined with burial history simulations, demonstrate continued illite growth to temperatures in excess of 110°C. Etive Formation samples of wells in which the overlying Kimmeridge Clay Formation source rocks are mature for oil generation, show a strong correlation between the K-Ar age of the finest size fraction of diagenetic illite and the calculated time at which the overlying source rocks reached a vitrinite reflectance of 0.62%. Data from other formations do not follow this trend and frequently give ages which are considerably older than those from the Etive Formation. It is considered likely that many of these older dates may arise as a result of contamination by detrital illites which are likely to be less common in the highly reworked sands of the Etive Formation barrier complex. No recent (&lt; 10 Ma) K-Ar dates have been obtained from diagenetic illites of the water zone of any wells so far studied in the Brent province. It is also apparent from many wells where diagenetic illites from both the hydrocarbon and water zones have been dated, that illite growth apparently stops in the water zone relatively soon after hydrocarbon emplacement. Furthermore, the time span for illitization, suggested by dating different illite size fractions of the same sample, is much shorter than that suggested by comparison of the range of temperatures illite apparently grew at and burial histories of Brent Group wells. Consideration of these points has led the authors to conclude that illite begins to form at a temperature of about 75°C probably at quite a low reaction rate. The process of oil migration at somewhat higher temperature perturbs the static aqueous pore fluid medium and increases effective water-to-rock ratio by fluid transport. This results in increased rates of illite growth. Subsequent to the peak of oil generation, fluid flow rates decrease, with a concomitant decrease in illite-forming reaction rates. Any attempt to isolate illite formed at lower or higher temperatures will inevitably also sample illite from the most rapid period of growth and so yield a false and reduced time span of illite formation and apparently old ages for most recently formed illite in the water zone.

The palaeoenvironments and dynamics of the Rannoch—Etive nearshore and coastal succession, Brent Group, northern North Sea

Abstract This paper investigates the palaeoenvironments and dynamics of the Rannoch Formation in the East Shetland Basin and the sedimentology of the Etive Formation in one particular part of the basin, the southern Cormorant area. The study utilizes core and well data from the North Sea as well as comparative outcrop data from Utah, western USA. Rannoch Formation facies, coupled with a slight coarsening and cleaning upward of sand-dominated profiles above offshore shales and below channelized delta top sediments, are indicative of a prograding shoreface. The middle shoreface is dominated by hummocky cross-stratified fine sand characterised by horizontal to very low angle, planar to undulatory laminations, often cut by steeper scour features. The Etive Formation is characterized in the southern Cormorant area by development of a microtidal, wave-dominated barrier cut by associated inlet channels. These associations are consistent with an interpretation of the Rannoch Formation as a high energy, storm-dominated environment, with strong wave oscillatory motion accompanied by weak translatory currents as the most likely depositional setting. In the overlying Etive Formation, shoreline characteristics are those of a dissipative coastal system with a complex three-dimensional inshore topography and at least one nearshore bar system developed during fairweather periods.