Offshore Eastern Canada Research Articles

Petrology and tectonic significance of seamounts within transitional crust east of Orphan Knoll, offshore eastern Canada

Petrology and tectonic significance of seamounts within transitional crust east of Orphan Knoll, offshore eastern Canada

Detrital quartz sources in the Scotian Basin, eastern Canada, using hot-cathode cathodoluminescence: Availability of coarse-grained sand for reservoirs

Quartz is the principal framework mineral in clastic sediment reservoirs. In a frontier basin with sparse wells, the source of quartz in sandstones may be a predictor of the availability of medium- to coarse-grained quartz sand from plutonic sources, likely to provide good reservoirs. The Scotian Basin, offshore eastern Canada, was used to test this hypothesis because of its well-understood provenance history and geographic variability in known medium- to coarse-grained reservoir sandstones. The sources of detrital quartz in fine-grained sandstones were determined using hot-cathode cathodoluminescence (CL), supplemented by other petrographic techniques. The CL color shift for different quartz types was calibrated against the CL properties of representative source rocks in the hinterland, because generalizations in the literature do not precisely match our basin-specific observations. Grain size of sandstone exerts a strong control over quartz type, with plutonic-hypabyssal quartz and high-grade metamorphic quartz more abundant in coarse-grained sandstones and low-grade metamorphic quartz more abundant in fine-grained sandstones. Nevertheless, the analysis of fine-grained sandstones shows that plutonic-hypabyssal quartz is more abundant in fine-grained sandstones of the Sable subbasin than in those of the Abenaki subbasin. The abundance of plutonic-hypabyssal quartz correlates with the abundance of medium- to coarse-grained sandstone reservoirs in the Sable subbasin. This study suggests that, in frontier basins, the abundance of plutonic-hypabyssal quartz in fine-grained sandstones can be used as an indicator of available medium- to coarse-grained sandstone reservoir.

A revised biostratigraphic and well-log sequence-stratigraphic framework for the Scotian Margin, offshore eastern Canada 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin. 2Earth Sciences Sector Contribution 20120137.

In this study, we build on a foundation of previous biostratigraphic studies for the Scotian Basin, offshore eastern Canada, by conducting new quantitative multidisciplinary biostratigraphic studies of the Mesozoic sections in eight wells: Bonnet P-23, Chebucto K-90, Cohasset L-97, Glenelg J-48, Glooscap C-63, Mohican I-100, South Desbarres O-76, and South Griffin J-13. These wells were chosen to provide good spatial coverage, stratigraphic penetration, and correlation with the seismic grid. We have also evaluated pre-existing biostratigraphic data and undertaken the well-log sequence-stratigraphic interpretation of Upper Triassic to Cenozoic sediments in 16 additional wells using a consistent multidisciplinary event scheme derived from the new wells. Key to the dating of some horizons has been integration of the palynology and micropaleontology (disciplines mostly used previously on the Scotian Margin) with new nannofossil data. Simultaneously we have closely integrated the biostratigraphic results with lithofacies, well-log, and seismic interpretations. Using this integrated approach, we have identified nine regionally mappable sequence-stratigraphic events (unconformities and maximum flooding surfaces, or MFSs): the Intra-Oligocene Unconformity; the Ypresian Unconformity; the Turonian/Cenomanian Unconformity; the Late Albian Unconformity; the Aptian/Barremian Unconformity; the Intra-Hauterivian MFS; the Near-Base Cretaceous Unconformity; the Tithonian MFS; and the Top-Callovian MFS. Additional events of local extent were also recognized in some wells. Our study has led to significant revision of some previous lithostratigraphic picks. The Early Jurassic on the Scotian Margin remains poorly resolved and may be represented by an unconformity on much of the basin periphery.

Paleohydrogeological and thermal events recorded by fluid inclusions and stable isotopes of diagenetic minerals in Lower Cretaceous sandstones, offshore Nova Scotia, Canada

Upper Jurassic–Lower Cretaceous sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. Previous studies of fluid inclusions in Jurassic limestones and apatite thermochronology of Cretaceous sandstones have suggested a late Mesozoic thermal event. Fluid inclusions in different authigenic cements were analyzed to determine the temperature and composition of basinal fluid at the time of precipitation and the timing of hydrocarbon migration and entrapment. Fifty-one sandstone samples were analyzed for stable isotope composition (18O and 13C) of carbonate cements. Trapping temperatures for primary aqueous inclusions hosted in quartz overgrowths (89–175C) and in late carbonate cements (138–173C) are higher than predicted by two-dimensional modeling from burial alone based on postrift geothermal gradients. These inclusions contain high-salinity fluids (mostly 19–22 wt. % NaCl equivalent). Second, predominantly aqueous inclusions have much lower salinities (5.2–6.7 wt. % NaCl equivalent), and some contain liquid hydrocarbons. Late Fe-calcite cement in Hauterivian sandstone shows negative values of (13.17 to 9.2), whereas cements in deeper and shallower sandstones have higher 13C values. These data indicate that high-temperature, high-salinity fluids preceded hydrocarbon migration. The oldest and youngest rocks studied show less effect of high temperatures than do rocks that were buried to depths of at least 2 km (1.2 mi) after 135 Ma and before 100 Ma. Both fluid inclusions and 13C of carbonate cement suggest that highest temperatures were achieved during burial at 115 to 105 Ma. This Early Cretaceous thermal event had a geothermal gradient of at least 55C/km. It is earlier than the previously reported event based on apatite fission tracks but is consistent with the fission track data. The event is synchronous with regional evidence of volcanism, and its peak coincides with rapid salt-tectonic deformation in the deep basin. Hydrocarbon charge to the outer-shelf wells occurred after this thermal event and thus not before the Late Cretaceous.

New Approach to Deepwater-Drilling-Data Analysis Enhances Real-Time Capabilities

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 146624, ’A New Approach to Deepwater Drilling Data Analysis Offers Enhanced Real Time Capabilities in a Post Macondo World,’ by John F. Greve, SPE, Chevron Deepwater Gulf of Mexico Business Unit, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The paper has not been peer reviewed. Deepwater drilling activity in the Gulf of Mexico is no longer a given in the aftermath of the BP Macondo blowout. The new and ever-evolving process to apply for drilling permits and operate now greatly depends on thorough real-time monitoring of pertinent surface and downhole data. A new method for loading and analyzing live time and depth data showed merit during 2010 trials on two deepwater wells, one offshore eastern Canada and the other offshore west of the Shetland Islands. Rapid data access and standardized visualization of high-resolution time, depth, and survey data are vital to ensure that the occasional well-control event does not become today’s worldwide news headline. Background—Real-Time Data Collection The Wellbore Information Transfer Standard Markup Language (WITSML) well-data exchange protocol introduced in the mid-1990s continues to gain popularity as a standardized means to deliver drilling and logging data to the client operator. An increasing number of software programs are offering WITSML links to load depth-based data into programs for petrophysical analysis, pore-pressure estimation, and 3D visualization of well logs vs. seismic data. For roughly a decade, large service companies have offered Web-based viewing of drilling-gauge displays and real-time depth-based data. Even though Internet access to WITSML data has been available to client operators, little benefit has been derived from the time-based data because of inherent limitations of depth-focused Web-based viewers. A new vendor-neutral WITSML-based data viewer capable of easily connecting to multiple WITSML vendors combines real-time synchronous depth and time viewing of drilling, logging-while-drilling (LWD), and mud-logging data in a variety of graphical displays. Historical Perspective on Collection of Time-Based Drilling Data For many decades, time-based drilling data have been recorded with mechanical devices. Special 24-hour multiscale paper forms were loaded on a cylindrical drum every day. Because the chart was under a protective lid and sometimes covered by dirt and mud, it was considered more of a midnight chart-replacement chore for a roughneck than a useful tool. The charts rarely made it to the operator’s office and were often lost or thrown out.

Turbidite deposition and the development of canyons through time on an intermittently glaciated continental margin: The Bonanza Canyon system, offshore eastern Canada

Bonanza Canyon is a complex canyon system on the slope from the intermittently glaciated Grand Bank on the south side of Orphan Basin. A 3D seismic reflection volume, 2D high-resolution seismic reflection profiles and ten piston cores were acquired to study the evolution of this canyon system in relation to glacial processes on the continental shelf and the effects of different types of turbidity currents on the development of deep water channels. Mapped reflector surfaces from the 3D seismic volume show that the Bonanza Canyons developed in a depression created by a large submarine slide of middle Pleistocene age, coincident with the onset of glacigenic debris flows entering western Orphan Basin. Two 3–5 km wide, flat-floored channels were cut into the resulting mass-transport deposit and resemble catastrophic glacial meltwater channels elsewhere on the margin. Both channels subsequently aggraded. The eastern channel A became narrower but maintained a sandy channel floor. The western channel, B, heads at a spur on the continental slope and appears to have been rather passively draped by muds and minor sands that have built 1500-m wave length sediment waves. Muddy turbidites recorded by piston cores in the channel and on the inter-channel ridges are restricted to marine isotope stage (MIS) 2 and were deposited from thick, sheet-like, and sluggish turbidity current derived from western Orphan Basin that resulted in aggradation of the channels and inter-channel ridges. Sandy turbidites in channels and on inner levees were deposited throughout MIS 2–3 and were restricted to the channels, locally causing erosion. Some coincide with Heinrich events. Channels with well-developed distributaries on the upper slope more readily trap the sediments on Grand Bank to form sandy turbidity currents. Channel B dominated by muddy turbidity currents has wide and relatively smooth floor whereas channel A dominated by sandy turbidity currents has a sharp geometry.

Varietal heavy mineral analysis of sediment provenance, Lower Cretaceous Scotian Basin, eastern Canada

The origin of a major increase in sand supply in the Early Cretaceous of the Scotian Basin, offshore eastern Canada, has been investigated by analysis of heavy mineral varieties. The chemistry of detrital minerals from the Scotian Basin was compared with that of minerals from potential source rocks, sampled from older sedimentary successions, river sands and glacial tills on land. Heavy mineral varieties were established based on > 1000 high quality electron microprobe (EMP) analyses and used to classify heavy mineral types by Mineral Liberation Analysis (MLA), which uses a rapid scanning electron microscope particle analyser. Heavy mineral varieties were compared between the Cretaceous quartz arenites and subarkoses and samples from potential source areas. Different chemical varieties of the same mineral group have different grain size distributions, so that their relative modal abundances may be influenced by hydraulic sorting, but modal abundance of different varieties of tourmaline, garnet and spinel provides useful information. Widely used provenance-sensitive mineral ratios are difficult to interpret because of the abundance of polycyclic sources. Comparisons of the shape parameters of the individual mineral types are also useful for provenance identification. Aspect ratio in elongate grains, such as tourmaline and zircon, is a measure of polycyclic reworking, but is influenced by the initial shapes of minerals from different source rocks. Uniformity of varietal heavy minerals over a large area and stratigraphic range, and the covariance of spinel and zircon with quite different crystalline sources, indicate predominance of polycyclic sources to the Central Scotian Basin, probably principally from Carboniferous and Lower Paleozoic sandstones, in part sourced from western Newfoundland ophiolites. Heavy mineral varieties in the West Scotian Basin can be closely matched to sources in the adjacent Meguma Terrane of the Appalachians. All the data show that the sands of the Scotian Basin were derived from reworking of crystalline basement of the Appalachian orogen and its Paleozoic sandstone cover, uplifted during rifting of Iberia from the Grand Banks. There is no evidence of a significant supply of sand to the Scotian Basin from a proto-St Lawrence River.

Controls on diagenesis of Lower Cretaceous reservoir sandstones in the western Sable Subbasin, offshore Nova Scotia

Lower Cretaceous deltaic sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. The influence of several factors on diagenesis has been investigated: depositional lithofacies, sea level changes, chemistry of basinal sediments and basinal fluid flux during burial. The distribution and chemistry of diagenetic minerals was determined from nine wells located along a dip section of the Sable Subbasin. Mineral type and paragenesis were characterized using a combination of optical petrography, back-scattered electron images, and electron microprobe analyses. Siderite is unusually abundant in marine sediments of the Scotian Basin and has thus been studied in detail. Siderite occurs in several generations. Early and late siderites are similar in chemical composition, suggesting buffering by pre-existing siderite, but rare low-Mg siderite is related to a greater contribution of meteoric water. Siderite has locally dissolved to create microporosity and has suppressed quartz overgrowths. Siderite is most common in those muddy prodeltaic lithofacies where there is the highest availability of detrital ilmenite. Reactive Fe released by breakdown of this ilmenite is responsible for the unusual presence of early siderite in marine sediments, with the Ca and Mg content of the siderite indicating fully marine waters. Lithofacies have a strong influence on early diagenetic mineral assemblages. Lithofacies deposited in the transgressive system tract have abundant early Fe-calcite and siderite. Early kaolinite occurs principally in proximal (fluvial and river mouth) lithofacies, where meteoric water was most likely available during the deposition. Contrary to other studies, we find little impact of sequence stratigraphy on diagenetic minerals except in the transgressive system tract. Mesogenetic minerals are related to flux of formation water and maturing hydrocarbon products, resulting first in pyrite and siderite and later in ankerite and ferroan calcite. The principal controls are interpreted to be high Fe 2+ and low Ca 2+ of formation waters and vertical movement of the waters along faults. Overall, the bulk chemical composition of terrigenous sediments and the depositional lithofacies are the most important factors controlling diagenetic minerals in the Lower Cretaceous of the Scotian Basin.

Late Cretaceous and Cenozoic fossil dinoflagellates and other palynomorphs from the scotian margin, offshore eastern Canada

Synopsis Palynomorphs, especially dinoflagellate cysts (dinocysts), have been at the forefront of research carried out on Mesozoic‐Cenozoic sediments on the Scotian Margin over the last 30–40 years: this research has been driven by the need to develop a stratigraphical framework to better understand the region's petroleum systems. To support the compilation of a detailed event strati‐graphical scheme for the Late Cretaceous to Cenozoic of the margin, emphasising dinocysts but with information from other fossil groups and non‐biostratigraphical data, there is a need to formalise the dinocyst taxonomy. In this paper, we fulfill this need by illustratingand, where appropriate, describing and discussing taxa used in our event biostratigraphical scheme. The following taxa (dinocysts except where indicated) are new: Areoligeracircumsenonensis,Axiodinium,Axiodiniumprearticulatum, Cordosphaeridium delimurum, Glaphyrocysta extensa, Hafniasphaera delicata, Impletosphaeridium capitatum, Mendicodinium robustum (validation of previously proposed name), Minisphaeridium,Oli‐gokolpoma, Oligokolpoma tubulus, Palaeocystodinium obesum, Palaeocystodinium teespinosum, Palaeohystrichophora palaeoinfusa, Pentadinium sabulum, Pervosphaeridium granaciculare, Tall‐adinium, Wetzeliella caviarticulata and Cingutriletes tyriskos (a trilete spore). The following are newly proposed combinations (with a former name in parentheses): Cerebrocysta waipawaensis (Pyxidinopsis waipawaensis), Cerodinium glabrum (Cerodinium speciosum subsp. glabrum), Din‐opterygium alatum (Xiphophoridium alatum), Kleithriasphaeridium cooksoniae (Florentinia cook‐soniae), Kleithriasphaeridium perforatum (Florentinia perforata), Minisphaeridium latirictum (Hys‐trichosphaeridium latirictum), Nyktericysta tripenta (Balmula tripenta), Pentadinium granulatum (Pentadinium laticinctum subsp. granulatum) Talladinium? clathratum (Charlesdowniea clathrata) and Talladinium wulagenense (Charlesdowniea wulagenensis). Emendations of the following taxa are proposed: Apectodinium, Dinopterygium, Distatodinium, Glaphyrocysta, Hafniasphaera, Isa‐belidinium, Kleithriasphaeridium, Manumiella, Nyktericysta, Palaeocystodinium, Rhombodinium, Wetzeliella and Wetzeliella articulata. Material from the Kiowa Formation of Kansas supports our concept of Dinopterygium. We review several morphological terms already in the literature and introduce the following new ones: mesotabular, obtabular, contabular, penicontabular, epeliform, equi‐epeliform and lati‐epeliform.

The last 100 million years on the Scotian Margin, offshore eastern Canada: an event-stratigraphic scheme emphasizing biostratigraphic data

In order to provide a detailed stratigraphic framework for the Late Cretaceous and Cenozoic basin fill of the Scotian Margin (the continental shelf and continental slope off Nova Scotia, eastern Canada), we have developed an event-biostratigraphic scheme based mainly on new analyses of several exploration wells. These include the following shelf wells — Demascota G-32, Hesper I-52, Onondaga E-84 and Wenonah J-75 — as well as Shelburne G-29 and Shubenacadie H-100 on the slope. Several fossil groups are involved in this study, most notably dinoflagellate cysts (dinocysts) and nannofossils, with the former generally providing more diverse assemblages and the latter more precise calibration with global correlations. Because most of the material studied is from cuttings samples, we have relied largely on Last Appearance datums (LADs). The scheme incorporates information from about 250 microfossil species (or groups of species), delimiting about 180 events. Not all events occur in all wells and the scheme needs to be tested against future observations: nevertheless, it represents a considerable advance on previous schemes, which were based mostly on broad zones rather than detailed events. Moreover, it allows for a more refined assessment of ages of geological events, such as the early Eocene gamma spike and the incision of the Wenonah Canyon. And it provides an accurate age context for ongoing stratigraphic, sedimentological and paleoenvironmental studies, and ultimately for a fuller understanding of petroleum systems on the Scotian Margin.
 
 RÉSUMÉ
 
 Pour fournir un cadre stratigraphique détaillé du remplissage du Crétacé tardif et du Cénozoïque du bassin de la marge Néo-Écossaise (plateau continental et pente continentale au large de la Nouvelle-Écosse, est du Canada), nous avons mis au point une formule événementielle-biostratigraphique principalement basée sur de nouvelles analyses de plusieurs puits d’exploration. Ces derniers comprennent les puits Demascota G-32, Hesper I-52, Onondaga E-84 et Wenonah J-75 sur le plateau continental ainsi que Shelburne G-29 et Shubenacadie H-100 sur la pente continentale. L’étude touche plusieurs groupes de microfossiles, plus particulièrement les kystes de dinoflagellés (dinokystes) et les nannofossiles, les premiers procurant généralement des assemblages plus diversifiés et les derniers, un étalonnage plus précis avec des corrélations mondiales. Comme la majeure partie du matériel étudié provient d’échantillons de déblais, nous nous sommes largement appuyés sur les plans de référence des dernières manifestations. La formule incorpore des données provenant d’environ 250 espèces (ou groupes d’espèces) de microfossiles délimitant quelque 180 phénomènes, dont environ 50 % sont basés sur des nanofossiles, quelque 46 % sur des palynomorphes et 4 % sur des foraminifères. Les phénomènes en question ne se manifestent pas tous dans tous les puits et il faut encore soumettre la formule à des essais en vertu des observations futures: elle représente néanmoins un progrès considérable par rapport aux formules antérieures, qui étaient surtout basées sur des zones étendues plutôt que sur des phénomènes détaillés. Elle permet de plus une évaluation plus raffinée des âges des phénomènes géologiques, comme les pics gamma du Paléocène tardif et l’incision du canyon Wenonah. Elle procure par ailleurs un contexte de datation exact pour les études stratigraphiques, sédimentologiques et paléoenvironnementales courantes, et elle permettra en fin de compte une compréhension plus complète des systèmes pétroliers le long de la marge Néo-Écossaise. [Traduit par la redaction]

Glaciogenic Debris-Flow Deposits of Orphan Basin, Offshore Eastern Canada: Sedimentological and Rheological Properties, Origin, and Relationship to Meltwater Discharge

Abstract Glaciogenic debris-flow deposits (GDFs) have been recognized in the last decade seaward of many shelf-crossing ice streams. The rheology of GDFs remains poorly understood. Ultra-high-resolution sparker seismic profiles and 25 long piston cores were used to define the architecture, age, and sediment properties of the GDF deposits in Trinity trough-mouth fan (TMF), offshore northeast Newfoundland, and hence understand their origin and emplacement. The GDF deposits comprise poorly sorted gravelly mud. Individual GDF lenses are 5–30 m thick, 2–10 km wide, and up to 250 km long. Shear strength measurements and grain-size analysis indicate that GDFs have a different, more fluid rheology at their margins and tops, exhibiting a surging flow behavior. On the upper slope, the transition from hard over-consolidated till to thin proximal GDF deposits is exposed in a mid Holocene landslide scar. The transition between the two lithotypes appears gradual and no pre-Holocene failure scarps were detected. A process involving the continuous release of subglacial flowing material with high pore pressure and low shear strength is invoked for the production of GDFs. Five stacked GDF units (A–E) were deposited during the last glacial maximum (20.5–28 cal. ka), and can be correlated into a regional lithostratigraphy based on the presence of Heinrich beds and 8 local meltwater events (R1–8) represented by red plumite deposits south of the Trinity TMF. This correlation indicates that the timing of major GDF pulses corresponds to the early part of five meltwater discharge events (20.5–21, 23–23.5, 23.8–24.5, 25–27, and 27.5–28.5 cal. ka), so that GDF deposits represent only a small period of time during a major glacial advance. Three meltwater events (19.2–20, 23–23.5, and 25–27 cal ka) produced hyperpycnal flows that resulted in the formation of channel systems and distal sand turbidites. The presence of such erosional features in TMFs on the continental margins of the Norwegian–Greenland Sea suggests that this novel relationship between GDFs and hyperpycnal turbidites may be widespread and thus important for understanding the glaciological processes involved.

Submarine mass‐transport facies: new perspectives on flow processes from cores on the eastern North American margin

AbstractNo comprehensive scheme yet exists to describe the depositional products of submarine sediment failures at the scale of piston cores, resulting in misinterpretation of failure deposits and overuse of the genetic term ‘debris flow’. Ninety‐nine sediment cores (0·5 to 20 m in length), from offshore eastern Canada and the Gulf of Mexico, are used to propose a descriptive sedimentary facies scheme with genetic implications for mass‐transport deposits. Seven facies are distinguished: (i) allochthonous stratified sediment; (ii) distorted stratified sediment; (iii) clast‐supported hard‐mud‐clast conglomerate; (iv) matrix‐supported mud‐clast conglomerate; (v) thin mud‐clast conglomerate (<0·8 m thick); (vi) diamicton; and (vii) sorted sand‐gravel deposits (≥0·05 m thick).Seven genetic types of deposits are recognized. (i) Slumping of coherent sediment blocks (facies I). (ii) Slump and slide deposits (facies I and II). (iii) Debris‐avalanche deposits (hard sediment of facies I and II overlain by facies III). (iv) Low‐viscosity or large‐scale, high‐viscosity, cohesive debris flow deposits (facies IV, may have I, II, and III). (v) Very low‐viscosity debris flow deposits (facies V). (vi) Cohesionless debris flow deposits (facies VI). (vii) High‐density turbidity currents (facies VII). Vertical transitions between the genetic types were analysed by Markov chain analysis.Although sedimentological transitions are inferred between deposits of slides and cohesive debris flows, their spatial distribution indicates that a cohesive debris flow forms principally in the initial stages of a sediment failure, suggesting that transformation depends mostly on the strength of the sediments. A genetic link is suggested for cohesionless debris flow deposits, which originate from the disintegration of sandy sediment on the upper continental slope, and the closely related turbidity current deposits. Debris avalanches are common in sedimentary marine environments with steep slopes (>10°). In many cases, geometrical and seismic characteristics of debris avalanche, slide and debris flow are similar, requiring core data to verify transport process.

New insights on the Paleogene dinoflagellate cyst genera Enneadocysta and Licracysta gen. nov. based on material from offshore eastern Canada and southern Argentina

Enneadocysta is a biostratigraphically important Paleogene dinoflagellate cyst genus. Its original interpretation as a partiform gonyaulacoid (thus belonging to the gonyaulacalean suborder Cladopyxiineae) was based on the presence of two antapical processes. Another Paleogene genus, Areosphaeridium , is similar, at least superficially, to Enneadocysta , but has a single antapical process, placing it in the sexiform gonyaulacalean suborder Gonyaulacineae. The morphology of a new species of Enneadocysta from offshore eastern Canada, Enneadocysta magna , shows that the two antapical processes of the genus (unlike processes reflecting other plate series) are penitabular, not mesotabular, and indicates that the genus is gonyaulacinean, not cladopyxiinean. This new interpretation, as well as new material from southern Argentina, confirms the generic assignment of the Southern Hemisphere species Enneadocysta dictyostila (emended) and a new species, Enneadocysta brevistila . A new genus and species, Licracysta corymbus , from offshore eastern Canada is intermediate in morphology between Enneadocysta and Cleistosphaeridium and strongly suggests an assignment for all three genera to the gonyaulacinean family Areoligeraceae. The new combination Licracysta ? semicirculata is questionably proposed, and the descriptive terms licrate, dolabrate, entire clypeate, ragged clypeate, intratabular, mesotabular, obtabular, contabular and nontabular are either proposed as new or reviewed.

A new conceptual model for the structural evolution of a regional salt detachment on the northeast Scotian margin, offshore eastern Canada

In this study, we examine, using seismic data in conjunction with numerical modeling, a regional-scale salt detachment and associated synkinematic sediments from the Scotian margin, offshore eastern Canada. This part of the Scotian margin is characterized by an up to 4.5-km (2.8-mi)-thick and approximately 175-km (108-mi)-long synkinematic wedge of Jurassic sediments with internal sigmoidal, landward-dipping reflectors. The synkinematic wedge is laterally extensive, encompassing an area of approximately 30,000 km2 (11,600 mi2), and soles into an interpreted salt detachment. The Jurassic synkinematic wedge, which is interpreted to have formed as an open-ended allochthonous salt nappe, was loaded by prograding sediments during the Jurassic. This loading squeezed the salt seaward and caused the overlying sediments to undergo extension and gravity spreading and gliding, detaching on the salt sheet. The open-ended nappe model provides a mechanism for producing a large amount of extension with very little compensating contraction. Numerical model results indicate that high rates of extension, detaching on even a thin salt layer, can result in similar sigmoidal, landward-dipping strata. Based on the numerical modeling and seismic interpretation results, we propose a new conceptual model for the Jurassic–Paleogene structural evolution of the study area; this model may also have implications for other passive-margin salt basins with regional salt detachments.

Deepwater, Subsalt Drilling From Nova Scotia, Canada - Case Study

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98279, "Pore Pressure Prediction and Drilling Challenges: A Case Study of Deepwater, Subsalt Drilling From Nova Scotia, Canada," by C. Marland, SPE, and S. Nicholas, SPE, Halliburton; W. Cox, EnCana Corp.; C. Flannery, SPE, Murphy Oil Sdn. Bhd.; and B. Thistle, Nexen Inc., prepared for the 2006 IADC/SPE Drilling Conference, Miami, Florida, 21-23 February. Offshore eastern Canada is an environment where deep water and the need to penetrate thick salt sheets increase the difficulties faced by drillers. The full-length paper details a deepwater, subsalt-drilling case history and examines the challenges of dealing with high pore pressures and variable fracture gradients. During drilling, conditions were encountered that required use of unconventional borehole sizes and on-the-fly drilling-design modification to deploy unplanned equipment rapidly. Introduction The Weymouth A-45 well is approximately 160 miles south by southeast from Halifax, Nova Scotia, Canada, in the deepwater area of the Scotian shelf. Seismic analysis identified the presence of a subsalt anomaly, the 150-m-thick Argo salt sequence. This sequence presented a potential drilling challenge and also a problem for prewell pore-pressure and fracture-gradient planning. Because of the challenges posed by the deepwater environment and the thick salt deposit, the well included a complex well design with concentric hole openers and an unconventional casing design. Despite potential hazards and complex well design, the drilling program allowed flexibility in decision processes and in well design to deal with problems encountered. Flexibility was particularly important when drilling through the salt body with a point-the-bit rotary-steerable system (RSS). Although the RSS was not planned for use below the salt, the success of the system in the shallower parts of the well led to its use below the salt and highlighted the flexibility of rotary-steerable technology. Planning and Design In August 2002, three seismic lines and six offset wells were used for initial analysis of the Weymouth Prospect. These data were used to generate overburden, pore-pressure, and fracture-pressure predictions. The six offset wells, including two deepwater wells, showed two different pore-pressure regimes unrelated to water depth. The first pore-pressure regime was characterized by normal pore-pressure gradients to approximately 2900 m followed by a slow increase in pressure, averaging 22 kg/m3 equivalent circulating density (ECD) per 100 m through to total depth (TD). The second pore-pressure profile had a steeper pressure "ramp" from 3600 m with a rapid pore-pressure increase to 1800 kg/m3 and greater by well TD.

Late Cenozoic evolution of Laurentian Fan: Development of a glacially-fed submarine fan

The Late Cenozoic evolution of the upper Laurentian Fan, offshore eastern Canada, was studied by integrating single and multi-channel airgun seismic surveys collected over the past three decades. The upper Laurentian Fan has a complex history, strongly influenced by the growth and decay of continental ice sheets since at least the Middle Pleistocene. Four regional stratigraphic horizons (O, Q, B, and A, in order of increasing sub-bottom depth) were correlated throughout the single channel airgun data set and have been identified in low-resolution, multi-channel data. An additional deeper horizon, L, only identified in the multi-channel data, marks the Pliocene onset of Laurentian Fan development. Correlation of the seismic markers, in conjunction with seismic facies analysis, provided the basis for dividing the evolution of the fan into five major phases: i) initial development of a single, progradational leveed channel; ii) erosional over-deepening of this channel associated with the onset of glaciation; iii) major re-organization of the fan system coinciding with the glacial excavation of Laurentian Channel on the continental shelf; iv) further re-organization of the fan associated with ice streams flowing through Halibut Channel; and v) predominantly aggradational growth of the fan and a progressive westward shift of the upper fan depocentre. A chronology for the fan was determined indirectly using sparse biostratigraphic data from seafloor outcrops, extrapolation of sediment accumulation rates, and correlation to the global oxygen isotope curve. It shows that the regional reflectors O–A correspond to marine isotope stages 4, 6, 12 and (?) 22. Average sediment accumulation rates show a gradual increase through time consistent with the generally progradational character of the fan. Comparison of Laurentian Fan with the Northwestern Atlantic Mid-Ocean Channel in the Labrador Sea, both glacially-fed submarine channel–levee systems, suggests that Eastern Valley of Laurentian Fan is analogous to the submarine braidplain in the Labrador Sea. Ice-margin flow separation, similar to that inferred at the head of NAMOC, resulted in coarse-grained bedload flowing down Eastern Valley as hyperpycnal inflows, whereas Western Valley and the canyons of the East Scotian Slope were fed principally by finer-grained sediment from meltwater plumes. The turbidity currents associated with these plume deposits were initiated either by remobilization of this material due to upper slope failures or through hyperpycnal flows created by double diffusive sedimentation. The stratigraphic evolution of Laurentian Fan records a wealth of glaciogenic and paleoclimatic information that would be further revealed by the collection of long cores (e.g., Integrated Ocean Drilling Program boreholes).

Egret-Hibernia(!), a significant petroleum system, northern Grand Banks area, offshore eastern Canada

Egret-Hibernia(!) is a well-explored petroleum system (3.25 billion barrels oil equivalent [BOE]) located in the Jeanne d'Arc Basin on the Labrador–Newfoundland shelf. Rifting and sediment fill began in the Late Triassic. Egret source rock was deposited in the Late Jurassic at about 153 Ma. After this time, alternating reservoir rock and seal rock were deposited with some syndepositional faulting. By the end of the Early Cretaceous, faults and folds had formed numerous structural traps. For the next 100 m.y., overburden rock thermally matured the source rock when it reached almost 4 km (2.5 mi) burial depth. For 2 km (1.25 mi) below this depth, oil and gas were expelled, until the source was depleted. The expelled petroleum migrated updip to nearby faulted, anticlinal traps, where much of it migrated across faults and upsection to the Hibernia Formation (44% recoverable oil) and Avalon Formation (28%). Accumulation size decreased, and gas content increased from west to east, independent of trap size. These changes correspond to a decrease in source rock richness and quality from west to east.Almost all (96%) of the discovered petroleum resides in the Lower Cretaceous or older reservoir rock units. All accumulations found to date are normally pressured in structural traps. Fifty-two exploration wells found eighteen discoveries. Their size ranges from 1.2 to 0.01 billion BOE. Most discoveries were made between 1979 and 1991. The discovery cycle began with larger accumulations and progressed to smaller accumulations. The estimated sizes of the larger accumulations have grown since 1990. Estimated mean value for undiscovered hydrocarbons is 3.8 billion BOE, thereby raising the ultimate size of Egret-Hibernia(!) to 6.19 billion BOE.

Geochemistry and sequence stratigraphy of regional Upper Cretaceous limestone units, offshore eastern Canada

Hydrocarbons occur in two regional, Upper Cretaceous limestone units—the Turonian-Coniacian Petrel Member, and the Santonian-Maastrichtian Wyandot Formation. The units form important seismic markers beneath the Scotian Shelf and the Grand Banks of Eastern Canada. They mainly consist of bioturbated chalk and minor amounts of calcareous mudstone. A search for source rock using the Δ log R technique showed intervals with source potential, but testing of core and cuttings by Rock-Eval analysis showed no source potential. Three issues are the main cause for the inconsistency: (1) unconsolidated shales that likely included organic material were lost during sample washing; (2) severe contamination by mud additives; and (3) presence of gas. The organic matter found on the shelf has been strongly oxidised, but the distal facies of these limestone units and condensed shale units above and below may yet have potential to form source rock, beyond the studied areas.

ABSTRACT: Proto-Oceanic Crust: How Does it Influence Deep and Ultra-Deep Water Exploration Offshore Eastern Canada, Greenland, Iberia and Northwest Africa?

ABSTRACT: Proto-Oceanic Crust: How Does it Influence Deep and Ultra-Deep Water Exploration Offshore Eastern Canada, Greenland, Iberia and Northwest Africa?

Porosity and permeability prediction from wireline logs using artificial neural networks: a North Sea case study

Estimations of porosity and permeability from well logs are important yet difficult tasks encountered in geophysical formation evaluation and reservoir engineering. Motivated by recent results of artificial neural network (ANN) modelling offshore eastern Canada, we have developed neural nets for converting well logs in the North Sea to porosity and permeability. We use two separate back‐propagation ANNs (BP‐ANNs) to model porosity and permeability. The porosity ANN is a simple three‐layer network using sonic, density and resistivity logs for input. The permeability ANN is slightly more complex with four inputs (density, gamma ray, neutron porosity and sonic) and more neurons in the hidden layer to account for the increased complexity in the relationships. The networks, initially developed for basin‐scale problems, perform sufficiently accurately to meet normal requirements in reservoir engineering when applied to Jurassic reservoirs in the Viking Graben area. The mean difference between the predicted porosity and helium porosity from core plugs is less than 0.01 fractional units. For the permeability network a mean difference of approximately 400 mD is mainly due to minor core‐log depth mismatch in the heterogeneous parts of the reservoir and lack of adequate overburden corrections to the core permeability. A major advantage is that no a priori knowledge of the rock material and pore fluids is required. Real‐time conversion based on measurements while drilling (MWD) is thus an obvious application.