Late Jurassic Kimmeridge Clay Research Articles

Macroevolutionary trends in the genus Torvoneustes (Crocodylomorpha: Metriorhynchidae) and discovery of a giant specimen from the Late Jurassic of Kimmeridge, UK

AbstractThe metriorhynchid crocodylomorph fauna of the Late Jurassic Kimmeridge Clay Formation (KCF) of the UK was highly diverse. One genus from this Formation, Torvoneustes, was unique in evolving dentition similar to known chelonivorous crocodylomorphs and enlarged attachment sites for the pterygoideus adductor musculature. Here we report the largest known Torvoneustes specimen, the occipital region of a large cranium that was discovered on the shore of Brandy Bay, Dorset, UK (KCF). We also report three tooth crowns discovered during the 19th century from Oxfordshire that can also be referred to Torvoneustes. The partial braincase is unique in having: verticalized basioccipital tuberosities that have thickened ventral margins, the notch between the basioccipital tuberosities is a narrow inverted ‘U’-shape and a subrectangular-shaped carotid canal foramina. The presence of ‘occipital fossae’ (deep concavities with the hypoglossal foramina in their dorsomedial corners) and carotid foramina with raised rims relative to the basioccipital posterior surface, allow us to refer it to Torvoneustes. Although incomplete, the exceptional size of the specimen demonstrates that Torvoneustes attained larger body lengths than previously supposed (3.7–4.7 m). Comparing the dimensions of this specimen to other metriorhynchids suggests that at least some Torvoneustes specimens rivalled Plesiosuchus manselii in body length.

A giant pliosaurid skull from the late Jurassic of England.

Pliosaurids were a long-lived and cosmopolitan group of marine predators that spanned 110 million years and occupied the upper tiers of marine ecosystems from the Middle Jurassic until the early Late Cretaceous. A well-preserved giant pliosaurid skull from the Late Jurassic Kimmeridge Clay Formation of Dorset, United Kingdom, represents a new species, Pliosaurus kevani. This specimen is described in detail, and the taxonomy and systematics of Late Jurassic pliosaurids is revised. We name two additional new species, Pliosaurus carpenteri and Pliosaurus westburyensis, based on previously described relatively complete, well-preserved remains. Most or all Late Jurassic pliosaurids represent a globally distributed monophyletic group (the genus Pliosaurus, excluding ‘Pliosaurus’ andrewsi). Despite its high species diversity, and geographically widespread, temporally extensive occurrence, Pliosaurus shows relatively less morphological and ecological variation than is seen in earlier, multi-genus pliosaurid assemblages such as that of the Middle Jurassic Oxford Clay Formation. It also shows less ecological variation than the pliosaurid-like Cretaceous clade Polycotylidae. Species of Pliosaurus had robust skulls, large body sizes (with skull lengths of 1.7–2.1 metres), and trihedral or subtrihedral teeth suggesting macropredaceous habits. Our data support a trend of decreasing length of the mandibular symphysis through Late Jurassic time, as previously suggested. This may be correlated with increasing adaptation to feeding on large prey. Maximum body size of pliosaurids increased from their first appearance in the Early Jurassic until the Early Cretaceous (skull lengths up to 2360 mm). However, some reduction occurred before their final extinction in the early Late Cretaceous (skull lengths up to 1750 mm).

Astrochronology of the late Jurassic Kimmeridge Clay (Dorset, England) and implications for Earth system processes

The Late Jurassic Kimmeridge Clay Formation (KCF) is an economically important, organic-rich source rock of Kimmeridgian–Early Tithonian age. The main rock types of the KCF in Dorset, UK, include grey to black laminated shale, marl, coccolithic limestone, and dolostone, which occur with an obvious cyclicity at astronomical timescales. In this study, we examine two high-resolution borehole records (Swanworth Quarry 1 and Metherhills 1) obtained as part of a Rapid Global Geological Events (RGGE) sediment drilling project. Datasets examined were total organic carbon (TOC), and borehole wall microconductivity by Formation Microscanner (FMS). Our intent is to assess the rhythmicity of the KCF with respect to the astronomical timescale, and to discuss the results with respect to other key Late Jurassic geological processes. Power spectra of the untuned data reveal a hierarchy of cycles throughout the KCF with ∼ 167 m, ∼ 40 m, 9.1 m, 3.8 m and 1.6 m wavelengths. Tuning the ∼ 40 m cycles to the 405-kyr eccentricity cycle shows the presence of all the astronomical parameters: eccentricity, obliquity, and precession index. In particular, ∼ 100-kyr and 405-kyr eccentricity cycles are strongly expressed in both records. The 405-kyr eccentricity cycle corresponds to relative sea-level changes inferred from sequence stratigraphy. Intervals with elevated TOC are associated with strong obliquity forcing. The 405-kyr-tuned duration of the lower KCF (Kimmeridgian Stage) is 3.47 Myr, and the upper KCF (early part of the Tithonian Stage, elegans to fittoni ammonite zones) is 3.32 Myr. Two other chronologies test the consistency of this age model by tuning ∼ 8–10 m cycles to 100-kyr (short eccentricity), and ∼ 3–5 m cycles to 36-kyr (Jurassic obliquity). The ‘obliquity-tuned’ chronology resolves an accumulation history for the KCF with a variation that strongly resembles that of Earth's orbital eccentricity predicted for 147.2 Ma to 153.8 Ma. There is evidence for significant non-deposition (up to 1 million years) in the lowermost KCF ( baylei– mutabilis zones), which would indicate a Kimmeridgian/Oxfordian boundary age of 154.8 Ma. This absolute calibration allows assignment of precise numerical ages to zonal boundaries, sequence surfaces, and polarity chrons of the lower M-sequence.

Are the Kimmeridge Clay deposits affected by “burn-down” events? Palynological and geochemical studies on a 1 metre long section from the Upper Kimmeridge Clay Formation (Dorset, UK)

Two independent analytical approaches, palynology and inorganic geochemistry, were applied to identify potential oxygen “burn-down” events in the Late Jurassic Kimmeridge Clay Formation (KCF). The KCF interval of the rotunda ammonite zone, spanning 121.82–122.72 m depth was sampled from the Swanworth Quarry 1 borehole (Dorset, UK) at 2.5–5.0 cm resolution. Samples were analysed for total organic carbon (TOC), concentrations of elements that are known to be productivity- and/or nutrient-related (e.g. Cu, P), detrital (e.g. Al, Ti, Zr) and redox-sensitive/sulphide-forming (e.g. V, Mo, Fe, Mn, S), and palynofacies components including analysis of organic-walled dinoflagellate cysts (dinocysts) on a species level. The TOC contents generally exceed 2 wt.%, with a maximum of 8.8 wt.% at 122.37 cm depth and elevated values in the central part of the investigated interval. This interval of relatively higher TOC values correlates well with the maximum recovery of marine palynomorphs and low Al values, suggesting that the TOC is primarily of marine organic matter (OM). Changes in V/Al, Mo/Al, Fe/Al, Mn/Al and S patterns at 122.37 m depth mark a shift from anoxic conditions in the lower part of the studied interval to more oxic conditions in its upper part. Such a shift could explain the relatively high TOC and marine palynomorph concentrations in the lower part of the studied interval as a result of better preservation, and the subsequent decrease as an effect of a post-depositional “burn-down”, i.e. OM oxidation. As the amount of marine palynomorphs and TOC content diminishes from the middle part of the section upwards, species-specific changes in dinocyst assemblages can be observed. In particular, concentrations of Circulodinium spp., Cyclonephelium spp., Sirmiodinium grossi, Senoniasphaera jurassica and Systematophora spp. decrease rapidly in comparison to other species, such as Glossodinium dimorphum and Cribroperidinium sp. 1, which may suggest selective degradation of dinocysts due to oxidation. We suggest that post-depositional oxygenation of bottom and pore waters within the sediment was most probably the cause for decreasing TOC values and reduced recovery of marine palynomorphs towards the top of the studied interval in comparison to high TOC and marine palynomorph values in the central part of the studied interval due to anoxic conditions.

Designer wells to maximize recovery for Otter Field development, Northern North Sea

The 2002 to 2003 Otter Field development drilling campaign utilized a combination of detailed trajectory planning and integrated geosteering techniques. The objective of this work was to maximize oil recovery, with a minimal number of wells, from the complexly faulted Otter structure. To achieve this, subhorizontal production wells were planned to track near top reservoir, through the structural culminations, to connect adjacent fault blocks. Otter is the most northwesterly of the Brent Province fields of the Northern North Sea, located in UK blocks 210/15a and 210/20d, 530 km north of Aberdeen, operated by TOTAL with partners Shell U.K. Exploration and Production, ExxonMobil and Dana. The field was discovered by the Phillips 210/15-2 well in 1977 (then called Wendy) and appraised by Fina well 210/15a-5 in 1997, following 3D seismic acquisition in 1994. The decision to proceed with development was confirmed after the success of appraisal well 210/15a-6 drilled by TotalFina in 2000. The Otter structure is an easterly dipping tilted panel that is divided into four major blocks and several minor blocks by a network of subsidiary faults. The reservoir is the Middle Jurassic Brent Group, with the uppermost Tarbert Formation shallow marine sandstones comprising the main producing target. The oil source rock is the Late Jurassic Kimmeridge Clay, present in the off-structure areas, though locally absent over the Otter Field area. Top seal is provided by the Mid- to Late Jurassic Heather Shales. The Otter oil is a medium gravity crude (36.5° API) with a GOR of 79m 3 /m 3 (443 scf/bbl), in a normally pressured reservoir at a crestal depth of 1970m subsea. Otter well planning was conducted using a 3D geocellular model based on interpretation of both conventional and acoustic impedance inversion seismic datasets. Apilot study, prior to development drilling, included geochemical and petrophysical reservoir unit definition and the forward modelling of LWD log response in sub-horizontal wells. The results of these studies were used to aid geosteering, incorporating real time chemostratigraphy and LWD data at the wellsite. In addition, borehole resistivity images while drilling were used to assist in structural interpretation in real time and thus to guide the well trajectory to maximize the pay section. A key component in using these new technologies was the office-based integration of all the data via web-based monitoring of the operations. Three production wells target the culminations at the extremities of the Otter Field, supported by a downdip water injector, all drilled from a centrally located subsea template. Following the successful drilling of the first production well, 210/215a-T1, production start-up was in October 2002, via subsea tieback to the Eider facility.

Central North Sea hydrocarbon systems: Generation, migration, entrapment, and thermal degradation of oil and gas

The high-pressure and high-temperature (HPHT) areas of the central North Sea constitute an important hydrocarbon province. This includes the deep, Mesozoic reservoirs in United Kingdom quadrants 22, 23, 29, and 30. This study was undertaken to better understand oil and gas compositional histories in HPHT hydrocarbon systems and to help identify new exploration opportunities. The Late Jurassic Kimmeridge Clay Formation has been the source for both oil and gas over the entire area, with additional gas charge from the humic coals of the Middle Jurassic Pentland Formation in the western graben areas. The southern Forties Montrose high, with its southward-plunging Mesozoic terraces, is host to numerous oil and gas fields with temperatures ranging from 90 to 180°C and formation pressures whose gradient to the surface exceeds 0.8 psi/ft (0.192 MPa/m). Several of these oil accumulations have undergone in-reservoir thermal cracking, resulting in a lighter, single-phase fluid, together with a pyrobitumen residue in the pore volumes. With several traps at or near their leak-off pressure, the likelihood of top seal failure and gas leakage is prevalent. Such top seal failure is intermittent and, in some instances, is associated with gas chimneys. The main causes of pressure increase in Mesozoic sediments are thought to be volume increases associated with gas generation from source rocks, clay dehydration, and thermal cracking of oil. Top seal failure because of pressure buildup by salt diapirism and the buoyancy of large hydrocarbon columns has resulted in a series of compositionally fractionated oils and gases. A new technique is presented, whereby the geochemical character of a shallow (Tertiary) oil reservoir that has undergone fractionation can help lower the risk of detecting the presence of hydrocarbon at depth in potential, deep (Mesozoic) reservoirs. Gary Isaksen coordinates geoscientists and technical programs in ExxonMobil's Upstream Geoscience companies. Isaksen holds B.Sc. and M.Sc. degrees and a Ph.D. from the University of Bergen, Norway. Since he joined Exxon in 1985, he has worked on hydrocarbon system analyses in numerous sedimentary basins worlwide as well as the application of petroleum geochemistry to development and production.

Alwyn North Triassic Main gas-condensate: drilling deeper promotes production

Production of oil and gas in the Alwyn North Field, UKCS, commenced in 1987 from Middle Jurassic Brent and Lower Jurassic Statfjord reservoirs. In 1988, a deepened Statfjord producer well (3/9a-N21) detected gas in the underlying Triassic, but testing was not possible at that time. The 3/10b-2 exploration well, drilled in 1992 to the east of the field, tested gas at low rates from a poor quality Triassic reservoir section. In 1995 well 3/9a-N33, a deviated well, was drilled from the Alwyn platform to evaluate the productivity of the deep Triassic. This well penetrated 770 m of Upper Lunde section, which was tested at 1.3×10 6 m 3 per day of gas plus 4400 STB per day of condensate. The Alwyn trap comprises a westerly dipping fault panel structure. Erosion over the crest of the tilted panel and the eastern-facing scarp slope by the Base Cretaceous Unconformity (BCU) truncates the Jurassic section down to the Upper Triassic. Sealing of the Triassic Main accumulation is by uppermost Triassic shales of the overlying Statfjord Formation and by Cretaceous mudrocks onlapping onto the BCU. Hydrocarbon charging of the Triassic Upper Lunde sandstones is from mature Late Jurassic Kimmeridge Clay source rocks that are juxtaposed against the reservoir along parts of the eastern frontal fault of the Alwyn panel. The Upper Lunde (of Carnian to Rhaetian age) is a low net:gross reservoir section up to 800 m thick. The sediments are of semi-arid fluvial, floodplain and lacustrine origins, with the best reservoirs occurring in fluvial channel sandstones. The Late Triassic comprises four informal zones from A (top seal), through B and C to D reservoirs, above the Middle Lunde shale. The Triassic reservoir fluids are gas-condensate in the B and C units with an intraformational seal at the C–D boundary, beneath which lies volatile oil in the D unit. A total of eight further Triassic production wells were added to the AlwynNorth Field by the end of 2001, as the development plan for Triassic Main progressed. These wells contribute in the order of 60% to the Alwyn North daily production.

Annual layering in the Upper Jurassic Kimmeridge clay formation, UK, quantified using an ultra-high resolution SEM-EDX investigation

Abstract This paper reports the results of an ultra-high resolution investigation of the geochemistry of the Late Jurassic Kimmeridge Clay Formation (KCF) using scanning electron microscope/energy dispersive X-ray analysis (SEM-EDX). Sub-millimetre banding is revealed in which each band (approximately 112 μm thick) comprises a couplet of coccolith-rich and coccolith-poor sediment. Major element analysis reveals that regularly-spaced peaks in the relative amount of calcium are associated with coccolith-rich bands. It is argued that each couplet represents an annual layer, the calcium peaks corresponding to an annual bloom of coccolithophorids. There is also a broad correlation between the calcium/aluminium ratio and the ratios of sodium/aluminium, phosphorus/aluminium and potassium/aluminium to calcium/aluminium ratios. We tentatively suggest that there may be a link here, sodium, phosphorous, and potassium today being key nutrients for coccolith productivity. Aluminium is taken as representing the background terrigenous (clay) fraction. If each banded couplet is indeed annual and if the thickness of such bands were to be representative of the KCF as a whole, then the post-compactional thickness is broadly in agreement with sedimentation rates of 100 μm per year calculated by Oschmann (1990) . This study confirms the view that the KCF in southern England accumulated in a predominantly anoxic environment that favoured the preservation of organic material within the bottom sediment. Bundles of bands, clustering in groups of about 8–10 are tentatively related to short-term changes in insolation (possibly sun-spot cycles).

A comparison of pyrolysis products with models for natural gas generation

Hydrous pyrolysis, using progressive heating from 150 to 365°C, is used to study the generation and isotopic composition of hydrocarbon gases and carbon dioxide. Different source rocks from the Draupne, Heather, Dunlin and Brent Formations in the North Sea, a Miocene brown coal from Germany, a coal from the Middle Jurassic Ness Formation in the northern North Sea and a rich marine type source from the Late Jurassic Kimmeridge Clay Formation onshore UK are studied. The experimental results are related to vitrinite reflectance as a maturity reference parameter and to existing models for natural gas generation. Hydrous pyrolysis simulates the observed isotopic composition of natural systems fairly closely. The largest deviations are found for methane, which is most sensitive to fractionation effects. The obtained results mainly reflect variations in source rock type and initial isotopic composition. Different maturity-dependent trends are found between coal-derived and marine type II-derived gases. Trends in the experimental data fit well with basic assumptions of existing models for natural gas generation.

Thermal generation of carbon dioxide and organic acids from different source rocks

Carbon dioxide and organic acids are claimed to play an important role in sandstone diagenesis. In the present work hydrous pyrolysis was used for a qualitative comparison of the relative potential of different source rocks to generate organic acids and carbon dioxide. In addition, the carbon isotopic composition of the generated carbon dioxide was determined. Experiments with three immature source rocks are described; a Miocene brown coal from Germany, a coal from the Middle Jurassic Ness Formation in the northern North Sea and a rich marine type II source from the Late Jurassic Kimmeridge Clay Formation onshore U.K. Both coal samples are typically humic. A stepwise heating program from 150 to 365°C was used. Closed Pyrex tubes containing immature coal (% R mm = 0.37) were heated in order to measure vitrinite reflectance evolution. The experimental results were related to vitrinite reflectance, as a maturity reference parameter, and to a kinetic model using kinetic parameters for carbon dioxide generation (Burnham A. K. and Sweeney J. J. (1989) Geochim. Cosmochim. Acta 53, 2649–2657). Differences in the yiels and carbon isotopic composition were obtained between the marine type II source rock and the coals. Organic acid generation converted between 1 and 3% of the organic carbon. Carbon dioxide was the major oxygen containing component, with molar yields 5–10 times that of organic acids. The experimental yields of organic acids and carbon dioxide were compared to a first order kinetic model for carbon dioxide generation, and a reasonable correspondence in the transformation ratio curves was observed.

Quantification of Tertiary erosion in the Inner Moray Firth using sonic velocity data from the Chalk and the Kimmeridge Clay

Sonic velocities for the Chalk and Kimmeridge Clay were analysed to determine the importance and magnitude of Tertiary erosion for 26 wells in the Inner Moray Firth. Apparent erosion (height above maximum burial depth) was derived by the displacement, along the depth axis, of a given velocity-depth trend from the normal (undisturbed) trend. The similarity in apparent erosion results derived from the Cretaceous Chalk and Late Jurassic Kimmeridge Clay successions of the Inner Moray Firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and/or diagenetic process. Resultant apparent erosion estimates are variable across the Inner Moray Firth. Such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern Inner Moray Firth. However, the actual magnitude of erosion (equal to the sum of apparent erosion and post-erosional Tertiary burial) was approximately 1 km throughout the whole Inner Moray Firth area, indicating that Early Tertiary erosion was regional and not simply restricted to the inverted basin margins. However, a well defined mechanism sufficient to have generated the regional event is still sought.

The Clyde Field, Block 30/17b, UK North Sea

Abstract The Clyde Field, which was discovered in 1978, is located on the SW edge of the North Sea Central Graben. The reservoir is developed with Late Jurassic shallow marine sands of the Fulmar Sand Formation. An estimated 408 MMBBL of oil is present (Annex B), of which 154 MMBBL is considered recoverable. The structure of the Clyde Field takes the form of a rotated Jurassic fault block, truncated at its crest by a major unconformity. Oil is retained within a combination trap, sourced from Late Jurassic Kimmeridge Clay thermally matured in the highly productive basinal lows, adjacent to the field. Reservoir sand quality is highly variable, ranging from excellent with permeabilities in excess of Id, to poor with permeabilities of less than 1 md. The principal control on reservoir quality appears to be original depositional texture, although strong diagenetic effects are also present. Production is from a single, centrally located, platform provided with thirty slots. Aquifer support is insufficient to maintain reservoir pressure at the current plateau production rate of 50 000 BOPD and so a programme of water injection has been implemented.