Kimmeridge Clay Research Articles

Detailed stratigraphy of the uppermost Kimmeridge Clay Formation (Upper Jurassic) from the Swanworth Boreholes, Dorset, UK

The higher parts of the cores of the Swanworth Boreholes, Dorset, allow detailed investigation of parts of the Upper Jurassic Kimmeridge Clay Formation that are difficult to sample on the coastal type-section. The member-level divisions set up by Buckman and Arkell are confirmed as readily recognisable. Body-fossils have been found throughout the beds examined and are recorded and confirm the ammonite zonal boundaries as proposed by Cope (1978). The extensive bioturbation of the sediments in the highest beds was primarily caused by decapod crustaceans whose remains are preserved in the borehole cores. Detailed examination of the cores has failed to confirm the existence of the so-called Chapman's Pool Pebble Bed, nor can its existence at nearby Gad Cliff be confirmed.

New cirripedes (Crustacea, Thoracica) from the Jurassic and Cretaceous of the United Kingdom

Extensive new material of scalpelliform cirripedes, including articulated individuals, from the Late Jurassic (Tithonian) Kimmeridge Clay of Dorset, southern England, is described and used as a basis for taxonomic revision of the family Zeugmatolepadidae. Two subfamilies, the Zeugmatolepadinae and Martillepadinae nov., are established and two new genera (Martillepas, Concinnalepas) in the latter subfamily are described from the Upper Jurassic and one (Icenilepas) from the Upper Cretaceous Chalk. Material from the Kimmeridge Clay of Dorset also includes the oldest fossil representative of the family Pollicipedidae for which a new genus and species, Etcheslepas durotrigensis, are erected, and the oldest calanticid, Cretiscalpellum sp. nov. Jurassic taxa established by T.H. Withers in 1928, on the basis of sparse material, are redescribed, and referred to the new genera as Martillepas ovalis, M. costata, M. hollisi, Concinnalepas concinna and Etcheslepas fragilis.

Anatomy of Colymbosaurus megadeirus (Reptilia, Plesiosauria) from the Kimmeridge Clay Formation of the U.K., and high diversity among Late Jurassic plesiosauroids

ABSTRACTWe describe the syntype postcranial skeletons of the cryptoclidid plesiosauroid Colymbosaurus megadeirus. C. megadeirus is the type species of Colymbosaurus, and one other valid species, C. svalbardensis, is known. Although it is in widespread usage, C. trochanterius is a not valid species, although the presence of an anteroposteriorly oriented ridge bisecting the distal surfaces of the holotype humerus allows its referral to Colymbosaurus sp. Some specimens previously referred to C. trochanterius, including the holotype of ‘Plesiosaurus’ manselii, lack this feature, or show other differences from our hypodigm, suggesting that they are taxonomically distinct from Colymbosaurus, and countering previous suggestions that the Kimmeridge Clay Formation represents a depauperate plesiosauroid fauna. Substantial differences between the atlas-axis complexes and cervical vertebrae of Kimmerosaurus langhami, ‘Plesiosaurus’ manselii, C. megadeirus, and specimens referred here to cf. Spitrasaurus indicate that at least four plesiosauroid taxa were present in the Kimmeridge Clay Formation. This proposition is also supported by coracoid morphotypes, and demonstrates that Late Jurassic plesiosauroids were taxonomically diverse. Most Late Jurassic–Early Cretaceous cryptoclidids belong to a monophyletic Colymbosaurinae, but Kimmerosaurus and its sister taxon Tatenectes originated independently from Middle Jurassic cryptoclidids.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Largest known specimen of the genus Dakosaurus (Metriorhynchidae: Geosaurini) from the Kimmeridge Clay Formation (Late Jurassic) of England, and an overview of Dakosaurus specimens discovered from this formation (including reworked specimens from the Woburn Sands Formation)

We report the largest known British specimen of the metriorhynchid crocodylomorph genus Dakosaurus discovered offshore from Chesil Beach, Dorset, England (Kimmeridge Clay Formation). This specimen is large toothed, which has extreme enamel spalling on the labial surface, enlarged carinae (‘carinal flanges’), carinal wear, macroziphodont denticles, and the crown retains much of its labiolingual width along most of its apicobasal length. This suite of morphologies is unique to Dakosaurus. All known Kimmeridge Clay Formation Dakosaurus specimens are isolated tooth crowns. A skull previously referred to Dakosaurus lacks all the cranial apomorphies of D. maximus and D. andiniensis, and cannot be referred to this genus. Furthermore, the vast majority of putative Dakosaurus tooth crowns from the ‘Potton Sands’ ( = Woburn Sands Formation) do indeed represent Dakosaurus (as well as two Plesiosuchus specimens), and they most likely originate from the Kimmeridge Clay Formation.

Volumetrics of Petroleum Generation: Implications for Expulsion and Overpressure

In this study, the author explores the volumetrics of petroleum generation using a well constrained set of source rock and Kimmeridge Clay Formation petroleum samples at maturities covering the complete generation window from the North Sea. Densities of pure kerogen isolates were determined using density gradient centrifugation. Best estimates were derived for the composition of petroleum as a function of maturity. The PVTX properties of the petroleum were derived using PVTSim v.10. Combining the kerogen density data with the petroleum PVTX data, and placing these data into an algebraic mass balance scheme, the volume and pressure changes attendant upon petroleum generation were determined as a function of maturity. For source rock temperatures up to ∼130°C, the volume expansion is about 21% of the immature kerogen volume with an attendant pressure increase of about 10 MPa within the uncertainty of the methods in a closed system. Significant increases in volume, up to 71% with a concomitant pressure increase of about 33 MPa only occur at temperatures >130°C, when generated fluids have high gas-oil ratio. The results suggest that a completely closed source rock system can become overpressured during the main phase of oil generation.

Compositional controls on early diagenetic pathways in fine-grained sedimentary rocks: Implications for predicting unconventional reservoir attributes of mudstones

Diagenesis significantly impacts mudstone lithofacies. Processes operating to control diagenetic pathways in mudstones are poorly known compared to analogous processes occurring in other sedimentary rocks. Selected organic-carbon-rich mudstones, from the Kimmeridge Clay and Monterey Formations, have been investigated to determine how varying starting compositions influence diagenesis. The sampled Kimmeridge Clay Formation mudstones are organized into thin homogenous beds, composed mainly of siliciclastic detritus, with some constituents derived from water-column production (e.g., coccoliths, S-depleted type-II kerogen, as much as 52.6% total organic carbon [TOC]) and others from diagenesis (e.g., pyrite, carbonate, and kaolinite). The sampled Monterey Formation mudstones are organized into thin beds that exhibit pelleted wavy lamination, and are predominantly composed of production-derived components including diatoms, coccoliths, and foraminifera, in addition to type-IIS kerogen (as much as 16.5% TOC), and apatite and silica cements. During early burial of the studied Kimmeridge Clay Formation mudstones, the availability of detrital Fe(III) and reactive clay minerals caused carbonate- and silicate-buffering reactions to operate effectively and the pore waters to be Fe(II) rich. These conditions led to pyrite, iron-poor carbonates, and kaolinite cements precipitating, preserved organic carbon being S-depleted, and sweet hydrocarbons being generated. In contrast, during the diagenesis of the sampled Monterey Formation mudstones, sulfide oxidation, coupled with opal dissolution and the reduced availability of both Fe(III) and reactive siliciclastic detritus, meant that the pore waters were poorly buffered and locally acidic. These conditions resulted in local carbonate dissolution, apatite and silica cements precipitation, natural kerogen sulfurization, and sour hydrocarbons generation. Differences in mud composition at deposition significantly influence subsequent diagenesis. These differences impact their source rock attributes and mechanical properties.

Oyster patch reefs as indicators of fossil hydrocarbon seeps induced by synsedimentary faults

The Late Jurassic deposits of the Boulonnais area (N-France) represent the proximal lateral-equivalent of the Kimmeridge Clay Formation; they accumulated on a clastic-dominated ramp subject to synsedimentary faulting as a result of the Atlantic Ocean rifting. In the Gris-Nez Cape area, i.e., close to the northern border fault zone of the Jurassic basin, the Late Jurassic sequence contains small-dimensioned oyster patch reefs (<1 m) that are specifically observed at the base of an abrupt deepening trend in the depositional sequence induced by well-defined pulses of normal fault activity. Petrographic analysis of these patch reefs shows that they are exclusively composed of Nanogyra nana embedded in a microsparitic calcite matrix. ™13C measurements, carried out within both the matrix and the shells, display significantly lower values in the matrix compared to the oyster shells which suggests that the carbonate matrix precipitation was involving a carbon source different from marine dissolved inorganic carbon, most probably related to sulfate reduction, which is evidenced by light ™34S in pyrites. Similarities but also differences with lucinid-rich bioconstructions, namely, the Late Jurassic pseudo-bioherms of Beauvoisin (SE-France) suggest that the patch reefs developed at hydrocarbon seeps are related to synsedimentary faults. The extensional block-faulting segmentation of the northern margin of the Boulonnais Basin in Late Jurassic times is thus believed to have induced a sort of small-dimension hydrocarbon seepage field, recorded by the patch reef distribution.

A new metriorhynchid crocodylomorph from the Lower Kimmeridge Clay Formation (Late Jurassic) of England, with implications for the evolution of dermatocranium ornamentation in Geosaurini

A new metriorhynchid crocodylomorph from the Lower Kimmeridge Clay Formation (Kimmeridgian, Upper Jurassic) of England is described. This specimen, a three-dimensionally preserved skull and left mandibular ramus, is referred to a new species: Torvoneustes coryphaeus sp. nov. Within the genus Torvoneustes, T. coryphaeus sp. nov. is unique as it has a long anteromedial process of the frontal, ornamented dermatocranium, and the supraorbital notch forms a strongly acute angle. Our phylogenetic analysis confirms the placement of this specimen in Torvoneustes. The dentition of T. coryphaeus sp. nov., like that of the type species, has a blunt apex, crown basal–mid regions with numerous tightly packed apicobasally aligned ridges, and apical region with an anastomosed pattern of ridges that interact with the carinae. Within Thalattosuchia these dental characteristics are only found in Torvoneustes and the teleosaurid Machimosaurus. The heavily ornamented dermatocranium of T. coryphaeus sp. nov. is in contrast to the unornamented (nasals and frontal)–lightly ornamented (maxillae and premaxillae) pattern seen in Torvoneustes carpenteri. Curiously, this pattern of reduction and loss of dermatocranium ornamentation is also observed in Metriorhynchus, Dakosaurus, and the subclade Rhacheosaurini. We hypothesize that the ‘smooth’ dermatocranium of Late Jurassic metriorhynchids evolved independently in each subclade (parallel evolution), and would have reduced drag, thereby making locomotion through water more energy efficient. © 2013 The Linnean Society of London

A new metriorhynchid crocodylomorph from the Lower Kimmeridge Clay Formation (Late Jurassic) of England, with implications for the evolution of dermatocranium ornamentation in Geosaurini

A new metriorhynchid crocodylomorph from the Lower Kimmeridge Clay Formation (Late Jurassic) of England, with implications for the evolution of dermatocranium ornamentation in Geosaurini

Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district

Abstract An account is given of a Geologists’ Association meeting in the Isle of Purbeck held on 28th–30th September 2012 and the stratigraphy and structures of the rocks examined during the weekend are described. Uppermost Jurassic Stage nomenclature and recent changes to stratigraphical nomenclature in the uppermost part of the Kimmeridge Clay Formation are discussed and the conclusion reached that the long-established divisions (Members) of this Formation are both readily recognisable and have nomenclatorial priority. The recent change to the position of Pallasioides-Rotunda zonal boundary ignores the ammonite fauna and is inappropriate. For the Lulworth district the stratigraphy of the uppermost Jurassic (Portlandian) through Lower and Upper Cretaceous formations are described and their associated structures discussed. The coastal evolution of the Lulworth coast is briefly discussed.

Evidence of the metriorhynchid crocodylomorph genus Geosaurus in the Lower Kimmeridge Clay Formation (Late Jurassic) of England

We report the oldest known specimen of the metriorhynchid crocodylomorph genus Geosaurus, and the first specimen discovered from the Lower Kimmeridge Clay Formation of England (Kimmeridgian stage, Rasenia cymodoce Sub-Boreal ammonite Zone). This specimen, an isolated tooth, has the tri-facetted labial surface morphology that is unique to Late Jurassic species of Geosaurus. Its dimensions suggest that it is one of the posterior teeth. Macrowear on the labial surface (enamel spalling) shows evidence of tooth–food abrasion. The presence of Geosaurus near the base of the lower Kimmeridgian, along with the recently re-described middle Oxfordian specimen attributable to Torvoneustes, shows that the subclade Geosaurini evolved and diversified prior to the Kimmeridgian. Members Geosaurini evolved a diverse array of sophisticated feeding mechanisms, including durophagous forms (Torvoneustes), barracuda-like forms with ‘scissor-like’ occlusion (Geosaurus), species reminiscent of false killer whales/Type 1 North Atlantic killer whales (Dakosaurus) and a large genus similar to extant Type 2 North Atlantic killer whales (Plesiosuchus). With the description of the isolated Geosaurus tooth, all four Geosaurini genera are now known from the Lower Kimmeridgian Clay Formation. The craniodental plasticity of Geosaurini, and the astonishing range of feeding mechanisms they evolved, is unparalleled amongst thalattosuchians.http://www.zoobank.org/urn:lsid:zoobank.org:act:36B1FB4C-C20E-41C2-BC19-BAAB4E9D1612.

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).

Evidence for the teleosaurid crocodylomorph genus Machimosaurus in the Kimmeridge Clay Formation (Late Jurassic) of England

We report the only definite specimen of the teleosaurid crocodylomorph genus Machimosaurus from the Kimmeridge Clay Formation of England. This specimen (an isolated tooth) is now the only evidence of Machimosaurus in the Kimmeridge Clay Formation because a large skull and mandible, previously considered to be of Machimosaurus mosae, was recently shown to pertain to a metriorhynchid crocodylomorph. The tooth described herein was originally figured in 1884 as a tooth crown from a metriorhynchid crocodylomorph. However, its conical shape, blunt apex and distinctive enamel ornamentation are characteristic of the teleosaurid genus Machimosaurus. That Machimosaurus, and teleosaurids in general, were so rare in the Kimmeridge Clay Formation suggests that these marine crocodylomorphs did not commonly use this seaway. Their rarity is in contrast to contemporaneous deposits from continental Europe where teleosaurid remains, including Machimosaurus, are far more common. These continental deposits were deposited in shallow-marine/brackish ecosystems, suggesting that teleosaurids were largely restricted to coastal marine environments.

Filling the ‘Corallian Gap’: re-description of a metriorhynchid crocodylomorph from the Oxfordian (Late Jurassic) of Headington, England

Re-description of a long-known, but little studied, metriorhynchid crocodylomorph from the Oxfordian of England begins to help fill the ‘Corallian Gap’. This ‘Gap’ represents a paucity of vertebrate remains from the Oxfordian of England. Filling the ‘Corallian Gap’ has important implications for our understanding of metriorhynchids from England, especially as there was a shift in this ecosystem from being metriorhynchine dominated in the older Oxford Clay Formation, to a geosaurine-dominated fauna in the younger Kimmeridge Clay Formation. The re-description of the specimen reveals that it is a member of the derived subclade Geosaurini, which most likely pertains to the genus Torvoneustes.

A hydrous pyrolysis study to ascertain how gas yields and the extent of maturation for a partially matured source rock and bitumen in isolation compared to their whole source rock

In order to ascertain whether kerogen and the heavy oil or bitumen generated during the initial stages of source rock maturation then go on to produce similar gas yields compared to the natural situation when they are present in source rocks, a study has been conducted on an immature type II Kimmeridge Clay Formation (KCF) source rock (Dorset, United Kingdom), with an initial vitrinite reflectance (VR) of 0.31% Ro and total organic carbon (TOC) content of 14.4%. Water (hydrous) pyrolysis experiments were conducted on the whole immature source rock, the isolated partially matured bitumen-extracted source rock and the isolated bitumen (bitumen generated and extracted from the initial source rock) and mixtures of the isolated partially matured source rock and bitumen using a 22ml Hastalloy vessel at 320–420°C for 7–48h over a pressure range of 115–500bar. Hydrocarbon gas yields and the increase in VR were greater for the whole rock experiments compared to the experiments on the isolated partially matured source rock and isolated bitumen (no mineral matter present), as well as the experiments on mixtures of the isolated partially matured isolated source rock and bitumen combined. Hydrocarbon gas yields and VR values were found to decrease at 500bar compared to 180bar at 350°C where the effects of pressure retardation were found to be much more significant for the partially matured source rock maturation and bitumen in isolation than for the whole source rock. At 420°C, gas generation was not affected by pressure, but VR decreased going from 310bar to 450bar. The results obtained demonstrate that the interplay of inherent mineral matter, reactant phase, source rock/kerogen, bitumen and pressure are key factors in determining the extent of hydrocarbon generation and source rock maturation in geological basins.

Oil source rocks at the mesozoic and cenozoic continental margins: Communication 1. Oil source rocks at continental margins in the Triassic-Jurassic and Neocomian-Aptian

The presence of rocks capable of generating hydrocarbons (HC) in the section of sedimentaryrock basins is an essential criterion for their qualification as structures with oil and gas pools. Although organic matter (OM) is always present as dissemination in genetically different sediments, it is believed that rocks enriched with OM of the sapropel series (2 to 3% Corg) can generate a significant amount of liquid HC. However, rock sequences with the Corg ranging from 3–5 to 15–20% are considered oil source formations. The rock section of large petroliferous basins usually includes one or two source sequences, which generated liquid and gaseous HCs after submergence to high temperature and pressures zones. In the basin confined to the Arctic slope of Alaska, one of the main producers of liquid HC is represented by the Upper Triassic clays and limestones of the Shublik Formation. In the Barents Sea and North Sea basins, such rocks are represented by the Spekk Formation and the Kimmeridge Clay, respectively; in the West Siberian basin, by the Bazhenovo Formation; in the Persian Gulf, by the Fahlian, Sargelu, and Garau formations; in basins of the Caribbean region, by marls and clayey limestones of the La Luna Formation. In perioceanic basins of the South Atlantic, the major source sequences are represented by the Neocomian and Barremian clays and marls. The source rocks are identified as the Lagoa Feia Formation in the Campos and Santos basins. They are cognized as the Black Marlstone or Bukomazi Formation in the Lower Congo, Kwanzaa-Cameroon, and Angola basins.

The nature of porosity in organic-rich mudstones of the Upper Jurassic Kimmeridge Clay Formation, North Sea, offshore United Kingdom

Abstract Analyses of organic-rich mudstones from wells that penetrated the Upper Jurassic Kimmeridge Clay Formation, offshore United Kingdom, were performed to evaluate the nature of both organic and inorganic rock constituents and their relation to porosity in this world-class source rock. The formation is at varying levels of thermal maturity, ranging from immature in the shallowest core samples to mature in the deepest core samples. The intent of this study was to evaluate porosity as a function of both organic macerals and thermal maturity. At least four distinct types of organic macerals were observed in petrographic and SEM analyses and they all were present across the study area. The macerals include, in decreasing abundance: 1) bituminite admixed with clays; 2) elongate lamellar masses (alginite or bituminite) with small quartz, feldspar, and clay entrained within it; 3) terrestrial (vitrinite, fusinite, semifusinite) grains; and 4) Tasmanites microfossils. Although pores in all maceral types were observed on ion-milled surfaces of all samples, the pores (largely nanopores with some micropores) vary as a function of maceral type. Importantly, pores in the macerals do not vary systematically as a function of thermal maturity, insofar as organic pores are of similar size and shape in both the immature and mature Kimmeridge rocks. If any organic pores developed during the generation of hydrocarbons, they were apparently not preserved, possibly because of the highly ductile nature of much of the rock constituents of Kimmeridge mudstones (clays and organic material). Inorganic pores (largely micropores with some nanopores) have been observed in all Kimmeridge mudstones. These pores, particularly interparticle (i.e., between clay platelets), and intraparticle (i.e., in framboidal pyrite, in partially dissolved detrital K-feldspar, and in both detrital and authigenic dolomite) are noteworthy because they compose much of the observable porosity in the shales in both immature and mature samples. The absence of a systematic increase in organic porosity as a function of either maceral type or thermal maturity indicates that such porosity was probably unrelated to hydrocarbon generation. Instead, much of the porosity within mudstones of the Kimmeridge appears to be largely intraparticle and interparticle (adjacent to inorganic constituents), so the petroleum storage potential in these organic-rich mudstones largely resides in inorganic pores.

Cranial anatomy, taxonomic implications and palaeopathology of an Upper Jurassic Pliosaur (Reptilia: Sauropterygia) from Westbury, Wiltshire, UK

Abstract: Complete skulls of giant marine reptiles of the Late Jurassic are rare, and so the discovery of the 1.8‐m‐long skull of a pliosaur from the Kimmeridge Clay Formation (Kimmeridgian) of Westbury, Wiltshire, UK, is an important find. The specimen shows most of the cranial and mandibular anatomy, as well as a series of pathological conditions. It was previously referred to Pliosaurus brachyspondylus, but it can be referred reliably only to the genus Pliosaurus, because species within the genus are currently in need of review. The new specimen, together with another from the same locality, also referred to P. brachyspondylus, will be crucial in that systematic revision, and it is likely that the genus Pliosaurus will be found to include several genera. The two Westbury Pliosaurus specimens share many features, including the form of the teeth, but marked differences in the snout and parietal crest suggest sexual dimorphism; the present specimen is probably female. The large size of the animal, the extent of sutural fusion and the pathologies suggest this is an ageing individual. An erosive arthrotic condition of the articular glenoids led to prolonged jaw misalignment, generating a suite of associated bone and dental pathologies. Further damage to the jaw joint may have been the cause of death.

Macrosemimimus, gen. nov. (Actinopterygii, Semionotiformes), from the Late Jurassic of Germany, England, and France

ABSTRACT We describe a new semionotiform genus, which was recognized while studying Macrosemimimus fegerti, gen. et sp. nov., from the Late Jurassic Solnhofen Archipelago, Germany. Macrosemimimus (Lepidotes) lennieri from the early Kimmeridgian of Cap de la Hève, France, including Lepidotes toombsi from the Lower Kimmeridge Clay in Cambridgeshire, England, as a junior synonym, also belong to the new genus. Though different species, the presence of the same fish genus in different biodiversity zones suggests that semionotiforms might have been less sensible to the ecological and physical factors that caused the endemisms recognized in the invertebrate faunas. Macrosemimimus is mainly distinguished by the peculiar shape of the antorbital portion of the frontal bones, a single pair of extrascapular bones that do not reach the dorsal midline, anteriorly open circumborbital ring, only two suborbital bones, edentulous maxilla, a large quadratojugal involved in the jaw articulation, four postcleithra, and the pectoral fins placed very low and joining ventrally. The two included species differ in the ornamentation of the skull bones, the morphology of the scales, the relative size and position of the extrascapular bones, the dorsal extension of the preoperculum, and the kind of dentition. Although the phylogenetic relationships of Macrosemimimus are not yet established, the particular shape of the antorbital portion of the frontals and the patterns of the extrascapular and suborbital bones indicate possible systematic affinity with the macrosemiids, “Lepidotes” gloriae from the Oxfordian of Cuba, and “Lepidotes” microrhis and “Lepidotes” tanyrhis from the Barremian of Las Hoyas, Spain.

A new method for top seals predictions in high-pressure hydrocarbon plays

The current model describing the sealing mechanism of high-pressure hydrocarbon traps and the ensuing methodology for predicting top seal integrity and capacity in high-pressure plays assumes that the caprock, defined as the low matrix permeability formation immediately overlying the reservoir, is the seal. This study challenges this assumption and proposes the existence within the caprock of a fluid waste zone consisting of a system of fractures cutting from the reservoir upwards into the caprock and, therefore, charged with reservoir fluids. Because of the waste zone fractures the reservoir fluids are not sealed at the base of the caprock. Instead the seal coincides with the fracture waste zone tip point, which occurs at an important stress and stratigraphic boundary termed the ultimate seal. Six case studies demonstrate that in the UK Central Graben the top seal for Mesozoic high pressure hydrocarbon accumulations lies between the Base Cretaceous Unconformity and the base of the Chalk Group and that the Jurassic Kimmeridge Clay Formation is not necessarily the seal for these traps. The data used for the construction of structural and stress models for the case studies include pore pressure measurements, formation integrity measurements, well logs and reflection seismic profiles. An important conclusion of this study is that the shorter the waste zone the higher the chance of finding a hydrocarbon column in the reservoir.