John Shimeld

Permeability prediction with artificial neural network modeling in the Venture gas field, offshore eastern Canada

Estimating permeability from well log information in uncored borehole intervals is an important yet difficult task encountered in many earth science disciplines. Most commonly, permeability is estimated from various well log curves using either empirical relationships or some form of multiple linear regression (MLR). More sophisticated, multiple nonlinear regression (MNLR) techniques are not as common because of difficulties associated with choosing an appropriate mathematical model and with analyzing the sensitivity of the chosen model to the various input variables. However, the recent development of a class of nonlinear optimization techniques known as artificial neural networks (ANNs) does much to overcome these difficulties. We use a back-propagation ANN (BP-ANN) to model the interrelationships between spatial position, six different well logs, and permeability. Data from four wells in the Venture gas field (offshore eastern Canada) are organized into training and supervising data sets for BP-ANN modeling. Data from a fifth well in the same field are retained as an independent data set for testing. When applied to this test data, the trained BP-ANN produces permeability values that compare well with measured values in the cored intervals. Permeability profiles calculated with the trained BP-ANN exhibit numerous low permeability horizons that are correlatable between the wells at Venture. These horizons likely represent important, intra-reservoir barriers to fluid migration that are significant for future reservoir production plans at Venture. For discussion, we also derive predictive equations using conventional statistical methods (i.e., MLR, and MNLR) with the same data set used for BP-ANN modeling. These examples highlight the efficacy of BP-ANNs as a means of obtaining multivariate, nonlinear models for difficult problems such as permeability estimation.

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.

Acquiring Marine Data in the Canada Basin, Arctic Ocean

Despite the record minimum ice extent in the Arctic Ocean for the past 2 years, collecting geophysical data with towed sensors in ice-covered regions continues to pose enormous challenges. Significant parts of the Canada Basin in the western Arctic Ocean have remained largely unmapped because thick multiyear ice has limited access even by research vessels strengthened against ice [Jackson et al., 1990]. Because of the resulting paucity of data, the western Arctic Ocean is one of the few areas of ocean in the world where major controversies still exist with respect to its origin and tectonic evolution [Grantz et al., 1990; Lawver and Scotese, 1990; Lane, 1997; Miller et al., 2006].

Submarine Landslides in Arctic Sedimentation: Canada Basin

Canada Basin of the Arctic Ocean is the least studied ocean basin in the World. Marine seismic field programs were conducted over the past 6 years using Canadian and American icebreakers. These expeditions acquired more than 14,000 line-km of multibeam bathymetric and multi-channel seismic reflection data over abyssal plain, continental rise and slope regions of Canada Basin; areas where little or no seismic reflection data existed previously. Canada Basin is a turbidite-filled basin with flat-lying reflections correlateable over 100s of km. For the upper half of the sedimentary succession, evidence of sedimentary processes other than turbidity current deposition is rare. The Canadian Archipelago and Beaufort Sea margins host stacked mass transport deposits from which many of these turbidites appear to derive. The stratigraphic succession of the MacKenzie River fan is dominated by mass transport deposits; one such complex is in excess of 132,000 km2 in area and underlies much of the southern abyssal plain. The modern seafloor is also scarred with escarpments and mass failure deposits; evidence that submarine landsliding is an ongoing process. In its latest phase of development, Canada Basin is geomorphologically confined with stable oceanographic structure, resulting in restricted depositional/reworking processes. The sedimentary record, therefore, underscores the significance of mass-transport processes in providing sediments to oceanic abyssal plains as few other basins are able to do.

Diagenesis and Porosity Reduction in the Late Cretaceous Wyandot Formation, Offshore Nova Scotia: A Comparison with Norwegian North Sea Chalks

Abstract Porous chalk units have produced significant oil and gas discoveries around the world, including the world-class Tor Field of the Norwegian North Sea. On the Scotian Shelf of eastern Canada, the Upper Cretaceous Wyandot Formation is widespread, extending approximately 500 km along the length of the margin, and consists primarily of limestone with major chalk intervals. These chalks have porosities of up to 30% and are the reservoir for a gas and oil discovery at the Primrose N-50 well (1972) and a gas show at the Eagle D-21 well (1972). However, despite its potential as a hydrocarbon reservoir and/or seal, the Wyandot Formation is under-studied. In this paper, the Wyandot chalk is studied using conventional core samples, petrophysical logs, isotope geochemistry and SEM images to enhance the understanding of the depositional history, diagenesis and porosity-reducing mechanisms within the Wyandot Formation. Results indicate that the Wyandot chalks are in situ pelagic deposits, as opposed to the allochthonous North Sea chalks, and that mechanical compaction and dissolution/re-precipitation are the dominant mechanisms of porosity reduction. Given that significant volumes of Wyandot chalk have been eroded on parts of the Scotian Shelf, it is possible that North Sea type allochthonous reservoirs exist is distal locations on the Scotian Slope, and therefore an increased understanding of the sedimentology and porosity distribution of the in situ Wyandot Formation is important for further exploration in this frontier area.

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

Abstract 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.

The Pliocene Shelburne Mass-Movement and Consequent Tsunami, Western Scotian Slope

Submarine mass-movement is a significant process along continental margins, even along passive margin slopes. Interpretation of seismic reflection profiles along the Scotian margin, for example, indicates the Cenozoic section is dominated by mass transport deposits (MTD) at a spectrum of scales. Occasional exceptionally large MTDs are observed which seem particularly foreign in a passive continental margin setting. The Shelburne MTD was recognized from exploration industry seismic reflection data along the western Scotian Slope. It is a buried Plio/ Pleistocene feature that extends in excess of 100 km from the upper slope to the abyssal plain and maps to an area in excess of 5,990 km2 and a volume >862 km3. Its features demonstrate that it is a frontally-emergent MTD with a slump portion and a debris flow/run-out portion. Tsunami simulations were generated for this event, one assuming the slump portion generated the tsunami, the other, both the slump and debris flow contributed. For a mass movement comparable in scale to the Shelburne MTD, these simulations demonstrate that the city of Halifax, Nova Scotia, would be impacted within 70 to 80 minutes by a 13–25 m high wave, depending on the MTD source volume (slump or slump and debris field).

Significance of Northeast-Trending Features in Canada Basin, Arctic Ocean

Synthesis of seismic velocity, potential field, and geological data from Canada Basin and its surrounding continental margins suggest that a northeast-trending structural fabric has influenced the origin, evolution, and current tectonics of the basin. This structural fabric has a crustal origin, based on the persistence of these trends in upward continuation of total magnetic intensity data and vertical derivative analysis of free air gravity data. Three sub-parallel northeast trending features are described. Northwind Escarpment, bounding the east side of the Chukchi Borderland, extends ∼600 km and separates continental crust of Northwind Ridge from high-velocity transitional crust in Canada Basin. A second, shorter northeast-trending zone extends ∼300 km in northern Canada Basin and separates inferred continental crust of Sever Spur from magmatically intruded crust of the High Arctic Large Igneous Province. A third northeast-trending feature, here called the Alaska-Prince Patrick magnetic lineament (APPL) is inferred from magnetic data and its larger regional geologic setting. Analysis of these three features suggests strike-slip or transtensional deformation played a role in the opening of Canada Basin. These features can be explained by initial Jurassic-Early Cretaceous strike slip deformation (phase 1) followed in the Early Cretaceous (∼134 to ∼124 Ma) by rotation of Arctic Alaska with sea-floor spreading orthogonal to the fossil spreading axis preserved in the central Canada Basin (phase 2). In this model, the Chukchi Borderland is part of Arctic Alaska.