Uncertainty analysis of subsalt overpressure development in offshore Louisiana, Gulf of Mexico

The confining pressure exerted by the prograding Tertiary clastic wedge has caused extensive salt deformation in the Gulf of Mexico. The high rates of lateral salt motion from the Louisiana shelf break to the Sigsbee Escarpment are expected to have significant impact on the dynamic evolution of the surrounding sedimentary formations. The evolution of fluid pressure in supra- and subsalt sediments has been modeled in a series of pseudo-wells along a profile intersecting numerous salt sheets and structures offshore Louisiana. Based upon the estimated rates of salt movement, the timing can be determined for insertion/depletion of salt in the sediments as the salt has moved through, and by, sediments on its way basinward. An execution of a series of I-D fluid/flow compaction models has enabled estimation of the spatial variation in overpressure build-up with time; indicating that overpressure uncertainty is about 10–20% about a mean of about 40 atmospheres, with uncertainties on salt thickness and salt speed roughly comparable in importance in contributing to the uncertainty. The quantitative behaviors suggest that the speed of lateral salt insertion and the thickness of the salt are the main factors causing anomalous overpressure build-up. A quantification of the dynamic behaviors is significant when modeling the timing of potential trapping of hydrocarbons beneath salt sheets and of the subsalt overpressure to be expected in subsalt drilling.     

Thermal impact of salt: Simulation of thermal anomalies in the gulf of Mexico

Salt and related structures have played important roles in controlling hydrocarbon accumulations in the Gulf of Mexico. Using a two-dimensional fluid flow/compaction model, which allows for both conduction and convection of heat, an examination is given of the effects on thermal patterns of the combined influence of multiple salt features, including diapirs, pillows, sheets and wedges. The focusing and defocusing of heat due to the higher thermal conductivity of salt are accounted for in the modeling. The results show that there could be as much as a 30°C anomaly above multi-salt bodies due to the focusing of heat by salt, and as much as 50°C temperature contrast between internal salt positions and sediments external to the salt in the deep part of a section. The magnitude of the thermal anomaly depends on the size (or width) of the salt and on the depth of the rooted salt. The modeled results provide estimates of the influence of salt in expanding the oil generation window by approximately half of the salt thickness.     

Magnetic susceptibility anisotropy as a strain indicator in the sudbury basin, Ontario

The magnetic susceptibility anisotropy of 181 samples from the Chelmsford Formation, the Onaping Formation and the norite of the Sudbury Basin has been measured. Magnetic foliation planes are almost exactly parallel to the planar structural elements (slaty cleavage and foliation) that are axial planar to the Basin. Magnetic lineations, where present, are parallel to linear structural elements (mineral elongation lineations). The shapes of the susceptibility ellipsoids of individual samples and the grouping of their principal axes are used to classify each site according to one of nine possible anisotropy types. In the Chelmsford Formation there is dominantly one anisotropy type, but several are present in the norite. At each site there is a variation in shape of the anisotropy ellipsoids, which can be interpreted using magnetic susceptibility axial ratio plots analogous to the deformation plots used by structural geologists. Using this technique the measured anisotropy patterns are interpreted in terms of progressive modification of original predeformation magnetic fabrics by various types of strain. In the norite the shape of the susceptibility ellipsoid and the anisotropy type vary systematically with distance from the Grenville Front.     

Geomagnetic reversal frequency since the Late Cretaceous

Models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the Heirtzler et al. [1] magnetic polarity time scale which has subsequently been superseded. Examination of newer time scales shows that the mean reversal frequency is dominated in the Cenozoic and Late Cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed. Subdivision into two periods of stationary behavior is no longer warranted. The distribution of polarity intervals is visibly not Poissonian but lacks short intervals. The LaBrecque et al. [2] polarity time scale shows the positions of 57 small-wavelength marine magnetic anomalies which may represent short polarity chrons. After adding these short events the distribution of all polarity intervals in the age range 0–40 Myr is stationary and does not differ significantly from a Poisson distribution. A strong asymmetry develops in which normal polarity chrons are Poisson distributed but reversed polarity chrons are gamma distributed with indexk = 2. This asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the Late Cenozoic. If short-wavelength anomalies arise from polarity chrons, the geomagnetic field may be more stable in one polarity than the other. Alternative explanations of the origin of short-wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons, however. The observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long-term memory.     

On the magnetic susceptibility anisotropy of deep-sea sediment

Susceptibility anisotropies in the form of vertically prolate ellipsoids have been reported in many deep-sea sediment cores. The results of the present investigation suggest that these anisotropies may not describe the original magnetic fabric of deep-sea sediment, but are more likely due to either a measurement effect or to deformation of the sediment during coring. Anisotropy measurements made on a spinner magnetometer sometimes were found to be greatly affected by the shape of the sample. This apparent “sample-shape effect” was not observed on a low-field torque meter. The anisotropy of samples taken near the base or the top of some piston cores often reflects sediment disturbance during the coring operation. Most samples of deep-sea sediment examined had weak anisotropies that could be interpreted as due to normal depositional processes, including bioturbation. The best-fitting susceptibility ellipsoids were usually oblate with near vertical minimum susceptibility axes.     

Lower Cretaceous magnetic stratigraphy in Umbrian pelagic limestone sections

The magnetic stratigraphy of the Lower Cretaceous, pelagic Maiolica limestone has been investigated in three partially correlative sections at Gorgo a Cerbara, Presale and Frontale in the northern Umbrian Apennines of central Italy. The white, well-bedded limestone has a magnetic mineralogy dominated by magnetite. Stable magnetic directions isolated by thermal demagnetization define alternating polarity zones in each section. The magnetozone patterns are distinctive and can be correlated with the geomagnetic reversal history derived from the M-sequence marine magnetic anomalies. The three new sections confirm the polarity sequence for anomalies M0 to M10N. Although the Maiolica is inadequately dated, the correlated anomalies, together with the results of other investigations, allow tentative associations of anomalies M0–M19 with individual stages in the Lower Cretaceous and Upper Tithonian.The investigations also demonstrate the usefulness of magnetic stratigraphy in basin analysis. They yield mean sedimentation rates, confirm that there is a hiatus between the base of the Presale section and the underlying Jurassic formations, and show that a large part of the Frontale section has been cut out by faulting.     

Large-scale slumps off southern India and Sri Lanka

Two suites of slumps from opposite margins of the Gulf of Mannar, between Sri Lanka and southern India, have met and coalesced. The “Eastern Comorin” Slump is the more coherent of the two with a length of 70 to 100 km. The “Colombo” side slump consists of two to four blocks 15 to 35 km in length. Both slump-suites decrease to the south. A paleoslump underlies the western toe of the East Comorin Slump at a depth of some 800 meters. To the south, an enlarging and deepening submarine canyon marks the area of slump coalescence.     

A review of: “The earth's magnetic field”

Abstract

By Ronald T. Merrill and Michael W. McElhinny. Academic Press, 420 pp. Paperback: $30.00/£17.50 (UK only) ISBN: 0 12 491242 7 Hardback $67.50/$39.50 (UK only) ISBN: 0 12 491240 0 1983.     

The origin of the White Beds below the Cretaceous-Tertiary boundary in the Gubbio section, Italy

Immediately below the Cretaceous-Tertiary (K-T) boundary clay layer in Umbrian pelagic limestone sections is a 0.5 m thick interval of white limestone beds. Geochemical and rock magnetic studies have been carried out to investigate the origin of these beds. Fe/Si ratios suggest that the iron abundance is a constant proportion of the detrital component in the red marlstone above the boundary and the pink limestone below the white beds. The stratigraphic variations of initial susceptibility and the IRM intensities acquired in different fields behave similarly to the Fe/Si ratio, and reflect primarily the changing concentrations of the ferromagnetic minerals caused by sedimentary fluctuations. The Fe/Si ratio, overall Fe abundance and the intensities of the magnetic parameters are all anomalously low in the white beds. The differing magnetic properties of the samples are due mainly to differing proportions and grain-size distributions of pigmentary hematite and detrital magnetite. The coercivity spectra of samples from the white beds are very similar to those of the pink beds, but are clearly distinct from those of the red beds. The white beds were probably deposited under the same conditions as the underlying pink beds. The anomalously low intensities in the white beds result from reduction of hematite in originally pink beds and subsequent removal of the Fe²⁺ ions. The reduction may be a consequence of downwards infiltration of reducing waters resulting from the large quantity of organic matter produced by the extinctions at the K-T boundary.     

Lower Cretaceous magnetic stratigraphy in Umbrian pelagic carbonate rocks

Summary. A record of geomagnetic field polarity for the Barremian, Aptian and Albian stages of the Early Cretaceous has been derived in three over-lapping sections of pelagic carbonate rocks in the Umbrian Apennines of northern Italy. The remanence carrier in the greyish-white Majolica limestone and Fucoid Marls is magnetite, with haematite also an important constituent in a zone of 'couches rouges' within the Fucoid Marls. The weak remanent magnetizations were measured with a cryogenic magnetometer. Alternating field or thermal demagnetization was used to isolate the characteristic remanent magnetization (ChRM) in 655 specimens from 248 stratigraphic levels. The samples respond positively to a tectonic fold test, indicating that the ChRM predates the Late Tertiary folding of the Umbrian sequence. The magnetic stratigraphy derived from variations of virtual geomagnetic pole latitude clearly defines the recognizable reversal pattern associated with Mesozoic marine magnetic anomalies M0 to M4. The sections have been zones palaeontologically on the basis of planktonic foraminifera and calcareous nannofossil assemblages. The ages of magnetic anomalies M0 to M4 determined in this way are somewhat older than those in the reversal time scale of Larson & Hilde (1975). Anomaly M0 is located in the Early Aptian, close to the Aptian/Barremian boundary. A long period of normal polarity in the Aptian and Albian corresponds to the early part of the Cretaceous magnetic quiet zone. It is interrupted in the Late Aptian by a reversal which we find in only one of the Fucoid Marl sections, and which has not been reported in oceanic magnetic anomaly investigations.