Offset Models Research Articles

4-D evolution of rift systems: Insights from scaled physical models

The four dimensional (4-D) evolution of brittle fault systems in orthogonal, oblique, and offset rift systems has been simulated by scaled sandbox models using dry, cohesionless, fine-grained quartz sand. Extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model. The results of these analog model studies are compared with natural examples of rift fault systems. Rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction. Segmentation of the rift margin increases with increase in obliquity of the rift axis, resulting in a consequent increase in displacement on intrarift fault systems. Offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone. Along-strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones. High-relief, extension-parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems. Changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults. The second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low-relief oblique-slip zone or grabens. These typically are found in highly oblique rift systems (<45 degrees) and above right steps in offset rift models.

Extracting MOS parameter variations using dual-drain MOSFET offset

We have developed two offset models for dual-drain MOSFETs-one for surface-channel and one for buried-channel MOSFETs. By fitting the models to offsets measured at selected biases, estimates of various MOS parameter variations can be extracted. The models are originally intended to demonstrate that, contrary to current belief, channel implant variations can contribute as much offset to magnetic sensor MOSFETs as mobility variations. The utility of the method, however, extends beyond magnetic sensors. The same approach can be used to extract MOS parameter variations important in circuits such as MOS differential pairs.

Discovery of two cool magnetic white dwarfs

We report the discovery of magnetism in two cool white dwarfs, LHS 1044 and LHS 1734. Optical and infrared photometry, as well as high signal-to-noise spectroscopy, are presented for both objects. The observed energy distributions along with the measured trigonometric parallaxes are used to derive the effective temperatures and surface gravities. LHS 1734, with T(eff) = 5230 K, becomes the coolest observed magnetic white dwarf. The strength of each magnetic field is estimated from a detailed line profile analysis. Offset dipole models with dipole strengths of 16.7 and 7.3 MG, for LHS 1044 and LHS 1734, respectively, are shown to reproduce successfully the observed line profiles. In both objects, the dipole needs to be offset away from the observer by an amount equal to nearly one-fifth of the stellar radius.

A Dodsland - Hoosier Viking Waterflood Prediction

Abstract A waterflood prediction study has been developed for the Dodsland Viking field in Saskatchewan. This study demonstrates the importance Of the geological description used to account for fieldwide variations in PVT properties. The methodology identifies the significance of local wellbore effects (hydraulic fracture stimulations). The use of a modem simulator, which is fully implicit and uses improved Orthomin-type matrix solvers enables modelling of this problem where older technology fails. Finally, the results are compared against actual offset production performance and models which do not account for local wellbore effects. Introduction The Dodsland-Hoosier Viking field was discovered in 1953. Its location in southwestern Saskatchewan is shown in Figure 1. The field consists of a complex series of oil and gas pools, as depicted in Figure 2. Development occurred rapidly during the late 1950s followed by unitization and subsequent waterfloods. A resurgence of development occured during the early 1980s in response to government incentives. The Kiyiu Lake Voluntary Unit No. I was developed in 1983 and 1984. A waterflood was planned and government approval obtained during 1985. However, the dramatic dip in oil prices in early 1986 delayed implementation of the project. A detailed re--evaluation was commissioned by J.C. International Petroleum Ltd. to more closely evaluate waterflood economics under the more severe economic conditions of 1988. Geology The geology of the field was comprehensively documented in a paper by W.E. Evans(1). Two main factors control the occurrence of oil and gas:the separate linear sandstone bodies, which overlap; and,the structure, which is controlled by underlying solution collapse and post depositional compaction. Figure 3 shows the axes of the overlapping members and Figure 4 displays the stratigraphic relationship of the sandstone bodies. Cross sections through the centre of the members (Fig. 5) demonstrate how both of the above factors resulted in a system of oil and gas accumulations shown earlier in Figure 2. The lithology of the field has been studied in detail by Tooth et al(2). The vast majority of the reservoir rock consists of finely interlaminated sands, siltstone, and shales. These laminations are typically 13 mm (0.5 in.) thick. Core analysis, from wells near the study area, had porosities ranging from 16% to 24% and permeabilities ranging from 0.1 mD to 40.0 mD. The average porosity and permeability was 20.7% and 6.3 mD, respectively. A basal chert conglomerate is also found in some members, but is not thought to be present in significant amounts within the study area. Overall, the Viking formation in this field is of poor quality. Production Characteristics Production from such a low permeability reservoir required hydraulic fracture stimulation. These treatments, coupled with the low permeability of the reservoir, results in a characteristic production profile. Economic evaluations have been conducted for a number of companies with substantial production from this field. Reserves are determined using type curves, an example of which is shown in Figure 6. A production forecast for an individual well is determined by multiplying the initial production rate by the normalized rates. After two years an exponential decline is used.

Geomagnetic field intensity and reversal asymmetry in late Precambrian Keweenawan rocks

Summary. Fifty-four palaeointensity values, measured by the Thellier technique, are used to study the cause of reversal asymmetry in late Precambrian Keweenawan rocks. These data, obtained mainly from diabase dykes carrying primary TRM and their baked host rocks, show that ‘reversed’ palaeointensities are on average 40 per cent higher than their ‘normal’ counterparts. This result cannot be explained by any overall difference in remanence characteristics, grain size or cooling rate (dyke width) between the rocks comprising the two polarity groups. A significant difference in the local intensity of the geomagnetic field between times of normal and reversed polarity is thus indicated. This conclusion is reinforced by positive baked contact tests for both normal and reversed Thellier palaeointensities. Secondary component and single dipole offset models as explanations for Keweenawan reversal asymmetry are in conflict with the palaeointensity results. The difference in palaeointensity between normal and reversed rocks disappears when the data are reduced to the palaeoequator in accordance with a geocentric axial dipole field. The palaeointensity results are therefore consistent with the idea that reversal asymmetry is caused by the North American plate occupying a lower palaeolatitude when the field had normal polarity. Acceptance of a geocentric axial dipole would imply that the Keweenawan geomagnetic field intensity was about 45 per cent higher than at present. However, evidence at one locality of possibly three asymmetric reversals poses problems for a plate movement interpretation, and forces consideration of a model whereby the asymmetry is caused by persistent non-dipole field components. Rock magnetic studies show that reliable palaeointensity estimates can be obtained for rocks which have stable NRM with single domain or pseudo-single domain grain sizes and with a high Koenigsberger ratio. Baked contact rocks are the most reliable whereas hydrothermally altered lavas are unsuitable for Thellier intensity determinations. It is suggested that af demagnetization studies on companion specimens should be performed before Thellier experiments in order to select specimens which have high values of median destructive field and which lack viscous or other overprints.

Late Precambrian Keweenawan Asymmetric Polarities as analyzed by axial offset dipole geomagnetic models

The Great Logan Paleomagnetic Loop has been interpreted to be a paleomagnetic signature of the North American plate motion during the Keweenawan igneous activity (1200–1000 Ma). However, the presence of three successive asymmetric polarities, along with new geochemical data from the Mamainse Point area, makes this interpretation untenable. A new interpretation is presented according to which the asymmetric polarities represent long‐term zonal nondipole geomagnetic field components. Wilson's single‐dipole offset model can in general explain the inclinational asymmetries, but it yields intensity ratios (reversed/normal) significantly lower than those observed. A coaxial two‐dipole model is proposed that better takes into account the observed intensity data. This model consists of a geocentric dipole and an offset dipole located at the core‐mantle boundary. The offset dipole simulates the long‐term zonal average of the spherical harmonic nondipole field. If the geocentric dipole reverses while the offset dipole retains its constant polarity, nonantiparallel reversal results. A method is presented by which the depth of the offset dipole and the ratio of the dipole strengths can be determined using only the observed inclinations. The two‐dipole model suggests that inclinational asymmetries are due to a different nondipole/dipole ratio during the normal and the reversed polarity. The most probable variant of the two‐dipole models predicts that this ratio is 35% and 49% for the normal and the reversed polarity, respectively. A new interpretation for the evolution of the Logan Loop is described according to which at least 50% of the hairpin‐shaped loop is produced by oscillation of the dipole strength ratio. Apparent polar wander (APW) is still evident, but the shape of the loop and the APW speed along the loop is reduced. The two‐dipole model is tested by worldwide paleomagnetic data of Keweenawan age.