Turbidite-reservoir architecture in complex foredeep-margin and wedge-top depocenters,Tertiary Molasse foreland basin system,Austria

We employ an integrated subsurface dataset, including >400 m of drill cores and three-dimensional (3D) seismic-reflection data from >530 km² of the Tertiary Molasse foreland basin system in Austria, to characterize turbidite-system architecture across structurally complex foredeep-margin and wedge-top depocenters and to interpret the influence of tectonic deformation and submarine topography on hydrocarbon-reservoir quality and distribution. Turbidite-system architecture and depositional processes were correlated with associated topographic features in order to identify zones of preferential sediment gravity-flow convergence or divergence. Zones of flow convergence facilitate flow acceleration and accumulative flow behavior, whereas zones of flow divergence facilitate deceleration and depletion. Zones of preferential flow convergence include narrow (<2 km) and steep (<20°) foredeep-margin slope channels along thrust front-segmenting tear faults, and steep, unchannelized piggyback-basin and foredeep margins (local gradients as great as 40° across piggyback-basin margins). The foredeep-margin gradient is exaggerated principally by tectonic deformation that post-dates turbidite-system development, based on a paucity of growth strata. Piggyback-basin-margin gradients are exaggerated as a result of deformation synchronous with and following turbidite-system development, judging from the presence of growth strata. Slope-channel topography facilitated the development of relatively coarse-grained, amalgamated turbidite reservoirs, whereas unchannelized basin-margin topography facilitated deposition of fine-grained, chaotic non-reservoirs. Zones of preferential flow divergence are flat (<1°), unconfined (i.e., large in comparison to sediment gravity flows) piggyback-basin floors, which facilitated the development of relatively coarse-grained, non-amalgamated, upward fining turbidite reservoirs, stratigraphically partitioned by fine-grained mass transport-complex deposits. The results of this study elucidate the influence of foredeep-margin and wedge-top tectonic deformation and topography on turbidite-system and associated reservoir character and distribution across the Molasse foreland basin system in Austria, and can be applied to oil and gas exploration in analogous, structurally complex settings.     

Gas accumulations in Oligocene–Miocene reservoirs in the Alpine Foreland Basin (Austria): evidence for gas mixing and gas degradation

International Journal of Earth Sciences - Two petroleum systems are present in the eastern (Austrian) sector of the Alpine Foreland Basin. Whereas oil and thermogenic gas in Mesozoic and Eocene...     

A Relative Moment Tensor Inversion Technique Applied to Seismicity Induced by Mining

Summary. Studies of source mechanisms of mining-induced seismic events play an important role in understanding the various modes of failure observed around underground excavations and enable the geometry of likely planes of failure to be determined. These planes can be mapped using conventional techniques, for example, geological fracture mapping. However, such an approach is often problematical due to limited access to the site and/or poor exposures (if any) of the failure plane. An added difficulty is that planes of failure often do not follow faults of geological origin, but are related to the geometry of the advancing stope face. For example, the development of face-parallel shear zones ahead of deep-level stope faces. In such cases, the stresses induced by mining dominate over the geological structure in the critical region close to the stope face. Seismic methods therefore have the potential of being a practical method of studying the development of seismic shear zones underground.Slip on such a failure plane generates a three dimensional elastic wave that propagates through the rockmass, carrying a wealth of information regarding the source rupture process. The ground motions caused by the passage of the wave can be recorded by arrays of sensitive instruments called seismometers. These sets of recordings (seismograms) provide the basic data that seismologists use to study these elastic waves as they propagate through the Earth. Conventional seismic analyses provide scalar measurements of the rupture size and intensity. However, through a process known as moment tensor inversion (MTI), the seismograms recorded from a seismic event can be used to calculate a moment tensor that describes the three dimensional nature of the source mechanism. Interpretation of the moment tensor gives insight into whether the rockmass failed in tension, compression or shear and indicates the direction of movement and the failure plane.Moment tensor solutions computed using conventional MTI methods are sensitive to noise and may be biased due to systematic errors in the measurements. The primary objective of this study was to develop a robust MTI method to estimate the moment tensors of clusters of seismic events recorded in the underground environment. To achieve this, three hybrid MTI methods were developed by the author. These methods involve different iterative weighting schemes designed to enhance the accuracy of the computed moment tensors by decreasing the effect of outliers (data points whose residuals lie far from the mean or median error). The additional information required for hybrid methods is obtained by considering a spatial cluster of seismic events and assuming that the waves generated by each event in the cluster follow a similar path through the rockmass and allowing a common ray-path to be assumed. Hence the unknown effect of the heterogeneous rockmass on the waveform is similar for all the events in the cluster.The final objective was to determine whether the techniques developed could be successfully applied to real data. The hybrid MTI methods using the median and the weighted mean correction were applied to a cluster of 10 events, having remarkably similar waveforms, recorded at Oryx Gold Mine. For comparative purposes, the more conventional absolute method was also applied. The solutions computed using the hybrid MTI with a median correction displayed a distinct improvement after the iterative residual correction procedure was applied, in contrast to the solutions obtained from the absolute method. The radiation patterns and fault-plane solutions from the hybrid method showed a high degree of similarity, and were probably more accurate reflections of reality. These observations are very encouraging and point towards the potential for using the hybrid MTI method with a median correction as a standard processing tool for mine seismicity.The implications of this work are that a robust method for calculating the focal mechanisms of clusters of seismic events induced by mining activities has been developed. Regular application will lead to a better understanding of rock fracture processes and to improved safety underground.     

Balancing lateral orogenic float of the Eastern Alps

Oligocene to Miocene post-collisional shortening between the Adriatic and European plates was compensated by frontal thrusting onto the Molasse foreland basin and by contemporaneous lateral wedging of the Austroalpine upper plate. Balancing of the upper plate shortening by horizontal retrodeformation of lateral escaping and extruding wedges of the Austroalpine lid enables an evaluation of the total post-collisional deformation of the hangingwall plate. Quantification of the north–south shortening and east–west extension of the upper plate is derived from displacement data of major faults that dissect the Austroalpine wedges. Indentation of the South Alpine unit corresponds to 64 km north–south shortening and a minimum of 120 km of east–west extension. Lateral wedging affected the Eastern Alps east of the Giudicarie fault. West of the Giudicarie fault, north–south shortening was compensated by 50 to 80 km of backthrusting in the Lombardian thrust system of the Southern Alps. The main structures that bound the escaping wedges to the north are the Inntal fault system (ca. 50 km sinistral offset), the Königsee–Lammertal–Traunsee (KLT) fault (10 km) and the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system (60 km). These faults, as well as a number of minor faults with displacements less than 10 km, root in the basal detachment of the Alps. The thin-skinned nature of lateral escape-related structures north of the SEMP line is documented by industry reflection seismic lines crossing the Northern Calcareous Alps (NCA) and the frontal thrust of the Eastern Alps. Complex triangle zones with passive roof backthrusts of Middle Miocene Molasse sediments formed in front of the laterally escaping wedges of the northern Eastern Alps. The aim of this paper is a semiquantitative reconstruction of the upper plate of the Eastern Alps. Most of the data is published elsewhere.