We demonstrate that increasing erosion during the kinematic evolution of a thrust wedge will lead to out-of-sequence thrusting as a result of backwards critical taper movement. In-sequence Accepted ArticleThis article is protected by copyright. All rights reserved. thrusting in the Subalpine German Molasse Basin built a critical-tapered foreland Coulomb thrust wedge. Later out-of-sequence thrusts dissected all but the frontal duplex stacks. The footwall/hangingwall relation visible on seismic data proves the out-of-sequence nature of the latest thrusting stage. Establishing a stable drainage system leads to increased erosion in elevated areas of the thrust wedge, resulting in flattening of the critical wedge. In order to keep its predefined angle, the critical wedge repositions, and the tip of the taper moves towards the hinterland. Thus, thrusting will also reposition and move towards the hinterland.

Like the Alps and Western Carpathians, the Apuseni Mountains represent a fragment of the Variscan orogen involved in the Alpine crustal shortenings. Thus the more extensive Alpine tectonic unit in the Apuseni Mountains, the Bihor Autochthonous Unit is overlain by several nappe systems. During the Variscan orogeny, the Bihor Unit was a part of the Some terrane involved as the upper plate in subduction, continental collision and finally in the orogen collapse and exhumation. The Variscan thermotectonic events were marked in the future Bihor Unit by the large Muntele Mare granitoid intrusion, an S-type anatectic body. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating yielded a weighted mean age of 290.9 ± 3.0 Ma and a concordia age of 291.1 ± 1.1 Ma. U-Pb isotope dilution zircon analyses yielded a lower intercept crystallization age of 296.6 + 5.7/-6.2 Ma. These two ages coincide in the error limits. Thus, the Muntele Mare granitoid pluton is a sign of the last stage in the Variscan history of the Apuseni Mountains. Many zircon grains show inheritance and/or Pb loss, typical for anatectic granitoid, overprinted by later thermotectonic events.

OMV’s exploration efforts in Austria include the prospecting for new fields in units of the Alpine thrust belt below the Neogene Vienna Basin. Current exploration efforts are targeting the deep and underexplored parts of the Paleogene thrust belt with potentially large structural closures within the so-called Rhenodanubian Flysch units. Interpretation of fold-thrust structures is primarily based on new 3D seismic reflection data, which images the Paleogene nappes buried below the Neogene Vienna Basin fill and is supplemented by well data. In order to improve the understanding of the structural architecture, the results are compared to the regional structural framework of the Eastern Alps and West Carpathians to the W and NE of the Vienna Basin, respectively. Spatial seismic interpretation depicts the Rhenodanubian Flysch units being subject to three major phases of deformation during the Paleogene and Neogene: D1, Paleogene extension prior to thrusting; D2, Paleogene ~N-directed foreland propagating thrusting and subsequent Early Miocene ~N- and ~NW-directed out-of-sequence thrusting; D3, Middle and Late Miocene extension related to the formation of the Vienna pull-apart basin post-dating the thrusting episodes. Our interpretation depicts several large ~NE-trending structural closures within the deeper parts of the N-vergent Paleogene nappe stack with structural closures of up to 1000 m and areas of up to 5km². They include Paleogene turbidites, which are known dual porosity fractured reservoirs in producing fields within overlying nappes. The NE-trending structural closures result from both Paleogene N-directed thrusting and subsequent refolding of the N-vergent flysch nappes by Early Miocene NW-directed out-of-sequence thrusts. Comparison of data with the regional tectonic framework suggests that the NW-directed out-of sequence thrusts result from a local reorientation of thrusting and from basement buttressing during the Early Miocene, both being triggered by the shape and geometry of the underlying basement units. Our results highlight the exploration potential in the deeper parts of the Alpine thrust belt with target depths exceeding 3 km. Due to the complex deformation, challenging reservoir types, high formation pressures and limited amount of data, the exploration of these deep targets translates to higher geological/technical risks and uncertainties, compared to shallower, more traditional plays. However, though very challenging, the deep opportunities have the potential for finding significant resources in an already hypermature hydrocarbon basin.

Seal quality assessment is not only essential in petroleum systems studies but also in the context of other geo energy applications such as underground hydrogen storage. Capillary breakthrough pressure controls top seal capacity in the absence of faults or other discontinuities. In basins that lack measured capillary pressure data (e.g., from drill cores), regional compaction-porosity trends can be used as a first prediction tool to estimate the capillary properties of mudstones. Mathematical compaction models exist but need to be calibrated for each basin. This study aims to establish a compaction trend based on theoretical models, then compare it with theoretical maximum hydrocarbon column heights inferred from true measured capillary pressure curves. Middle to upper Miocene mudstone core samples from the Vienna Basin, covering a broad depth interval from 700 to 3400 m, were investigated by X-ray diffractometry, with an Eltra C/S analyzer, and by Rock–Eval pyrolysis for bulk mineralogy, total organic carbon, and free hydrocarbon contents. Broad ion beam—scanning electron microscopy, mercury intrusion capillary porosimetry, and helium pycnometry were applied to obtain pore structural properties to compare the mathematical compaction models with actual porosity data from the Vienna Basin. Clear decreasing porosity depth trends imply that mechanical compaction was rather uniform in the central Vienna Basin. Comparing the Vienna Basin trend to global mudstone compaction trends, regional uplift causing erosion of up to ~ 500 m upper Miocene strata is inferred. A trend of increasing Rock–Eval parameters S1 and production index [PI = S1/(S1 + S2)] with decreasing capillary sealing capacity of the investigated mudstones possibly indicates vertical hydrocarbon migration through the low-permeable mudstone horizons. This observation must be considered in future top-seal studies for secondary storage applications in the Vienna Basin.

No abstract