Late Cimmerian Unconformity Research Articles

Cross-border stratigraphy of the Northern, Central and Southern North Sea: a comparative tectono-stratigraphic megasequence synthesis

Abstract The Devonian–Recent tectono-stratigraphic history of the Northern, Central and Southern North Sea is here reviewed at a regional scale and four novel cross-border pseudo-Wheeler diagrams are presented to summarize the stratigraphic evolution of the cycles of basin fill and uplift/erosion. In this scheme, six first-order megasequence boundaries have been defined, characterized by extensive and long-lasting erosional hiatuses and major coastal regressions: (1) Caledonian (or Base Devonian) Unconformity; (2) Variscan–Saalian (or Base Permian) Unconformity; (3) Mid Cimmerian (or Intra-Aalenian) Unconformity; (4) Late Cimmerian (or Base Cretaceous) Unconformity; (5) Atlantean (or Near-Base Tertiary) Unconformity; and (6) Eridanos (or Mid-Miocene) Unconformity. These surfaces have been linked to regional causal factors ranging from orogenesis-related compressional uplifts, in either active plate margin settings (1) or foreland basin settings (2), to intra-plate dynamically supported uplifts associated with the development of mantle plumes (3, 5 and 6) and the end-of-rifting and associated widespread erosion of tilted fault block crests (4). The aforementioned megasequence boundaries punctuate the geodynamic evolution of the North Sea area and facilitate the subdivision of the entire the North Sea sedimentary basin fill into six megasequences, named here A–F. All of the lithostratigraphic units of the North Sea (formations and members) have been described within the context of this first-order tectono-stratigraphic framework. The correlation powers of certain stratigraphic markers are also compared and contrasted, together with the potential cross-border equivalence of sedimentary units on different sides of the political median lines.

Erosional valleys at a major Late Jurassic–Early Cretaceous unconformity offshore Germany and The Netherlands: potential reservoirs or deteriorated seals?

Abstract Joint results of two independent international studies on the erosional impact of the ‘Late Cimmerian Unconformity’ (LCU), a Late Jurassic–Early Cretaceous erosional event, are presented. The potential impact on the reservoir and sealing potential of adjacent strata are discussed. Results are based on a seismic mapping campaign in the NE part of the Dutch offshore region and the Central German North Sea sector. The geological evolution of potential traps at the level of the LCU was studied to obtain a better understanding of reservoir potential. Erosion or leaching of the evaporitic Triassic intervals at the LCU caused a locally strong topographical relief with prominent elongated depressions along Permian salt structures (interpreted as erosional valleys). The infill of the erosional valleys may consist of remnants of initial strata as breccia or material from nearby eroded strata, possibly Lower Triassic sandstone or clays. These valleys may be overlain conformably by Upper Jurassic–Early Cretaceous sediments or may be filled with a combination of both. The lithological composition of the infill determines the sealing or the reservoir potential of the valleys, which is essential for further evaluation of the resource exploration possibilities in this locality and across the basin.

The origin and timing of multiphase cementation in carbonates: Impact of regional scale geodynamic events on the Middle Jurassic Limestones diagenesis (Paris Basin, France)

The Middle Jurassic carbonates of the eastern part of the Paris Basin display surprisingly low values of porosity and permeability ( Φ < 15‰ and K < 0.5 mD). The main objective of this study is to determine the causes and timing of the cementation that altered the petrophysical properties of these carbonates thereby destroying their potential as oil reservoirs; a fate that did not befall their equivalents in deeper, central parts of the Paris Basin. Using petrographic and geochemical analyses (stable O and C isotopes, Sr isotopes, major elements), we identify six calcitic spar stages, two dolomite stages, and several episodes of fracturing and stylolitization ordered in paragenetic sequence. Cement quantification shows the predominance of two blocky calcite cement stages (75% of total cementation). O and Sr isotopes from these calcite cements suggest that the parent fluids resulted either from a mixing of trapped Jurassic seawater and meteoric water, or from buffered meteoric waters. In the geological history of the Paris Basin, major meteoric water inputs were possible during the Early Cretaceous, when the Middle Jurassic carbonates cropped out at its northern and eastern borders. Lateral meteoric recharge may have occurred as a result of two separate uplift events (Late Cimmerian Unconformity and Late Aptian Unconformity) and the related exposures of carbonates to the north of the study area. This palaeohydrological circulation brought about a significant reduction of porosity (from 40% to 10%) through calcite cement precipitation. The Early Cretaceous events are of great importance in the diagenetic evolution of the sedimentary basins bordering the London-Brabant Massif across all of northwestern Europe. A subsequent dolomite and calcite cementation stage accounts for about 5% of the total cement volumes. This late cementation may have been caused by hydrothermal fluids ascending along permeable fractures during the Late Oligocene extension episode.

Geodynamic control on carbonate diagenesis: Petrographic and isotopic investigation of the Upper Jurassic formations of the Paris Basin (France)

The Oxfordian and Kimmeridgian carbonates of the eastern edge of the Paris Basin display poor reservoir properties ( Φ < 15% and K < 0.1 md). The petrographic investigation and the resultant paragenetic sequence show that this is due to extensive precipitation of 2 types of almost synchronous blocky LMC (Low Magnesian Calcite) cements during mesogenesis (burial). Stable isotope investigation of these non-recrystallized cements, through both microdrilling sampling and in-situ SIMS (Secondary Ion Mass Spectrometry) analysis, shows that they originated from slightly buffered meteoric fluids (− 3‰ SMOW to − 8‰ SMOW) flowing through the Malm carbonate aquifers of the eastern edge of the Paris Basin. After a review of the post-Upper Jurassic history of the Paris Basin, it appears that meteoric water supplies leading to such cementations were associated with the evolution of the basin's geodynamic setting, and notably with the LCU (Late Cimmerian Unconformity) major event which corresponds to the uplift of the London–Brabant massif. Denudation of the Oxfordian and Kimmeridgian carbonates on the southern flank of the London–Brabant massif, to the north of the study area, then lead to lateral recharging of the associated aquifers with meteoric fluids. This scenario is similar to the one proposed to explain cementations of the Jurassic carbonates in the Wessex–Weald Basins which emphasizes the main role played by the Cretaceous unconformities over carbonate diagenesis at the scale of the northwestern Tethys margin.

The context of lowstand events in the Kimmeridgian (Late Jurassic) sequence stratigraphic evolution of the Wessex–Weald Basin, Southern England

Abstract A depositional sequence stratigraphic framework for the Kimmeridgian Stage of the Wessex–Weald Basin, southern England, is applied in the investigation of the sedimentary and erosional response to proposed lowstand events. This framework recognises 13 sequence boundaries (Km1-Km13) and is based on the integrated assessment of geophysical, sedimentological, palaeontological and geochemical data. Changes in the nature and continuity of Kimmeridgian unconformity surfaces record the effect of the reorganisation of the onshore basins in response to eustasy and tectonic movements. During the Earliest Kimmeridgian, the basin geometry was such that falls in relative sea level generated widespread unconformities. Associated lowstand events are recorded in marginal areas by a number of sand units, derived from a variety of local sources. A tectonically enhanced deepening event during the late Early Kimmeridgian (Eudoxus Zone) led to unconformities being confined in their distribution, to the basin margins. As sea level fell during the Latest Kimmeridgian, a progressive reduction in the depositional area led to an amplification of the effects of lowstand processes at the basin margins where unconformity surfaces are amalgamated. On the most positive areas these surfaces are overprinted by a major interregional hiatus, the ‘Late Cimmerian Unconformity’. The timing and effects of Kimmeridgian lowstand events were intimately linked to the tectonic evolution of the Wessex–Weald Basin. Thus at times when rates and amounts of eustatic change are small (i.e. during ‘greenhouse times’) the tectonic signature of a basin will be the primary signal recorded in sedimentary successions.

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.

Diagenesis of Lower Jurassic sandstones, Block 211/13 (Penguin Area), UK northern North Sea

The Penguin area is a cluster of three hydrocarbon accumulations located in Block 211/13 in the UK sector of the North Sea. The reservoirs are principally Middle and Upper Jurassic sandstones, but include the Lower Jurassic Statfjord and Nansen formations. The Statfjord Formation was deposited in a fluvial environment and is overlain by sandstones deposited during a marine transgression (Nansen Formation). Fluvial channel sandstones of the Statfjord Formation are extensively cemented by carbonates that precipitated soon after deposition, whereas sandstones of the Nansen Formation were cemented by grain-rimming chlorite and dolomite. Subsequent diagenesis involved the extensive precipitation of kaolin and the formation of minor secondary porosity at the expense of detrital feldspars and carbonate cements. Petrographic, isotopic and structural considerations suggest that the latter diagenesis was caused by meteoric water ingress during the formation of the late Cimmerian unconformity. Meteoric water was not responsible for extensive porosity modification, and the reservoir quality of the Lower Jurassic sandstones is poor and has not been signficantly improved by the formation of secondary porosity.

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.

Early to mid-Cretaceous tectonics and unconformities of the Wessex Basin (southern England)

Sediment distribution patterns, time-subsidence plots, seismic data and outcrop analysis of the Lower Cretaceous (?Ryazanian-Albian) of the Wessex Basin, southern England, suggest that a number of unconformable horizons exist and that there is no single ‘late Cimmerian unconformity’ surface. A tectonic change, from areally restricted deposition in the Weald and Channel sub-basins to extensive sedimentation across the whole of southern England, began in the mid-Aptian. This change ended with transgression of basement ‘highs’ in the early Albian of the Dorset area, and later Albian of the Devon and London Platform basin margin areas: its onset can be linked to the Austrian tectonic phase of Europe. The onset of marine deposition in the Aptian is coincident with periodic transgression of the margins of the Wessex Basin throughout the Aptian-Albian. Mid-late Aptian and early/mid-AIbian transgressions were preceded by periods of erosion. The Aptian-Albian Lower Greensand is characterized by transgressive-regressive phases, suggesting a short-term controlling process (third-order eustatic changes or intra-plate mechanisms). Tectonic changes in the mid-Aptian include uplift followed by widespread subsidence. This change coincides with a time of plate reorganization in the European and North Atlantic areas.

The role of the late Cimmerian unconformity for the distribution of kaolinite in the Gullfaks Field, northern North Sea

ABSTRACTA study of the Middle Jurassic Rannoch Formation in the Gullfaks Field shows that there is no relationship between the content and distribution of kaolinite and location relative to the late Cimmerian unconformity. From petrographic data most of the kaolinite is interpreted to be detrital, and only traces of authigenic kaolinite are observed. Mass‐balance calculations are used to provide guidelines as to the likelihood of kaolinite being formed and preserved in sandstones under the unconformity. The result of the calculations shows that the propagation rate of the dissolution front in the sandstones was probably on average slower than the erosion rates during the formation of the late Cimmerian unconformity. Hence, transformation of significant amounts of feldspar and mica to kaolinite probably did not take place within the sandstones which at present underlie the unconformity in the Gullfaks Field. Periods of exposure during formation of unconformities may thus be less effective causes of kaolinitization in sandstones than has been assumed.

How Important is the Late Cimmerian Unconformity in Controlling the Distribution of Kaolinite in Sandstone Reservoirs of the North Sea?: ABSTRACT

A study of the kaolinite content in the Rannoch Formation in the Gullfaks Field and in the Statfjord and upper Lunde formations in the Snorre field, shows that there is no relationship between the content and distribution of kaolinite and the location relative to the late Cimmerian unconformity. These observations are supported by mass balance calculations, which include consideration of meteoric water flow rates, dissolved carbon dioxide, mineral dissolution rates and erosion rates. These calculations strongly support the idea that the propagation rate of the dissolution front in sandstones during meteoric flushing, on the average, was slower than the erosion rates during the late Cimmerian unconformity. Hence, the sandstones kaolinitized by transformation of feldspar and mica during meteoric flushing are not likely to be preserved in sandstones underlying the unconformity. This suggests that the kaolinite content and distribution in the sandstones underlying the late Cimmerian unconformity is not controlled by the unconformity. The calculations and observations from other areas suggest, however, that the amount of kaolinite may increase in the upper few meters below the unconformity.

The role of the Late Cimmerian unconformity for the distribution of kaolinite in the Gullfaks Field, northern North Sea: Bjørkum, P.A. et al., 1990. Sedimentology, 37(3):395–406. Rogaland Res. Inst., P.O. Box 2503, Ullandhaug, 4004 Stavanger, Norway

The role of the Late Cimmerian unconformity for the distribution of kaolinite in the Gullfaks Field, northern North Sea: Bjørkum, P.A. et al., 1990. Sedimentology, 37(3):395–406. Rogaland Res. Inst., P.O. Box 2503, Ullandhaug, 4004 Stavanger, Norway

The Fulmar Oil-field (Central North Sea): geological aspects of its discovery, appraisal and development

The Fulmar Oil-field (Central North Sea): geological aspects of its discovery, appraisal and development

Early Cretaceous

The Early Cretaceous sea primarily covered the same basinal regions as the Late Jurassic sea but, late in the Early Cretaceous the sea also covered Late Jurassic land masses. During Early Cretaceous time the topography of the North Sea region became gradually buried. The following major transgression comprises the transition Early/Late Cretaceous. At the Jurassic/ Cretaceous transition, the Late Cimmerian unconformity is a significant feature (fig. 24), known from large parts of the North Sea region. The subsequent transgression and sedimentation of marine clay (the Valhall Formation), and marine sand (the LC-1 Unit), started late in Late Jurassic. Therefore, the formations described in the present chapter also comprise sediments of Late Jurassic age. Thicknesses of the Lower Cretaceous sediments are given in fig. 15.

Latest Jurassic - Early Cretaceous Events and the Late Cimmerian Unconformity"in North Sea Area"

Each of several regional or local unconformities may have been identified locally as the Cimmerian unconformity, a supposed major break at the base of the Valhall Formation (or Roedby Formation). Over most of the North Sea the base of the Valhall Formation is isochronous, and conformable with underlying sediments. It is detected on seismic reflection profiles because it represents a widespread facies change marking the late Ryazanian transgression. Most of the unconformities are of eustatic origin and the effects of eustatic sea level changes were never completely masked by local tectonics. Thus, in the modeling of individual oil fields, the possibility of sedimentary breaks occurring can be predicted in part by reference to regional or eustatic events.--Modified journal abstract.