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Hidden Markov Models (HMMs) have been applied to predict reservoir lithologies using seismic inversion results as inputs. This approach takes into account the conditional probabilities between different lithologies, i.e. the vertical transitions in sedimentary sequences. These properties are used as prior geological information. In order to relate the seismic inversion results to the true well-log data, HMMs need to be trained based on the Expectation-Maximization theory. Application of the resulting model on a synthetic example from the Book Cliffs (Utah, USA) showed that most lithologies are classified correctly, even for some thin layers. A comparison with point-wise methods in which data samples are treated independently from each other, such as k -means and fuzzy logic classifiers, leads to the conclusion that the spatial correlation in HMMs allows better lithological predictions because the prior information accounts for the geological depositional processes. A real case study with data from the Vienna Basin (Austria) is performed, in which lithologies in a 3D cube are obtained based on properties from seismic inversions, via trained HMMs. While the vertical sequences are shown to be reasonably well predicted, the horizontal continuities are not. This indicates that the future research should focus on the lateral geological relationships. [ABSTRACT FROM AUTHOR]

Abstract: A high-resolution chronostratigraphic record established by [W.E. Paulissen, S.M. Luthi, P. Grunert, S. Ćorić, M. Harzhauser, Geol. Carp. (2011)] that takes into account variations in sedimentation rates and temporal gaps caused by unconformities and faulting in a research borehole in the Vienna Basin was used to investigate the possible presence of orbital, millennial and centennial periodicities. The sedimentary sequence covers Middle to Late Miocene shallow marine, fluvio-deltaic and lacustrine shales, silt- and sandstones deposited during the transition from a pull-apart basin to the final infill in a compressional regime. Spectral analysis was performed on gamma ray logs and high-resolution electrical borehole images using three different analytical methods over six suitable intervals where continuous and constant sedimentation was identified. The significant periods were found to closely match the orbital cycles of precession, obliquity and short eccentricity, providing a solid basis for the analysis of potential sub-Milankovitch cycles. The significant frequencies encountered at the millennial- to centennial-scale fell for 75% within a relatively narrow time period of 0.25 to 5kyr, with a concentration of peaks between 1 and 2kyr (29%) and 500 to 800years (17%). These periodicities relate closely to the millennial- (Dansgaard–Oeschger and Bond cycles) and centennial-scale climate cycles documented from the Quaternary. It is suggested that the high-frequency cycles observed in the Miocene of the Vienna Basin represent differences in grain size related to cyclic variations in regional precipitation. [Copyright &y& Elsevier]

Abstract: A regional multi-scale fracture network is prominently exposed in the Cambro-Ordovician sedimentary succession in south-western Jordan. This succession consists of marine deposits, overlain by heterogeneous deltaic sediments, culminating in 500 m of massive to cross-bedded fluvial sandstones. A data set acquired from high-resolution satellite imagery and field measurements enables the quantification of the geometry and spatial characteristics of the fracture network in three dimensions. Five directional fracture sets are identified from the satellite data with mean strikes of 000°, 040°, 065°, 120°, and 145°. Large fracture zones (longer than 600 m), composed of tightly spaced joints confined in fracture swarms, exhibit a log-normal trace-length distribution and the fracture swarms are regularly spaced. The trace-length distribution of intermediate fractures (40–600 m long) exhibits power-law characteristics and the fractures are clustered, reflecting different fracture system styles and stratigraphic controls as a function of scale. The small-scale fracture network is strongly dependent on the distribution of bedding plane discontinuities throughout different parts of the succession. The multi-scale fracture network is categorized in a three-tier hierarchical fracture model that reflects underlying stratigraphic controls in the study area. This type of model can be used to improve the representation of fracture networks in fluid flow simulations of fractured reservoirs with hybrid models that combine elements of discrete fracture networks and continuum models. [Copyright &y& Elsevier]

Stress distributions and deformation patterns in a medium with a pre-existing fracture set are analyzed as a function of the remote compressive stress orientation (σH) using finite element models with increasingly complex fracture configurations. Slip along the fractures causes deformation localization at the tips as wing cracks or shear zones. The deformation intensity is proportional to the amount of slip, attaining a peak value for α=45° (α: angle between the fracture strike and σH) and slip is linearly proportional with fracture length. Wing cracks develop for high deformation intensities for 30°<α<60°, whereas primary plastic shear zones develop for low deformation intensities. Additionally, two types of secondary shear zones develop for α<30° and α>60°, with constant angles of 135° and −60° with σH, respectively. Mechanical interaction between fractures in a fracture zone, quantified as change in slip compared to an isolated fracture, decreases with increasing fracture separation. Fracture underlap elongates the fracture length and therefore increases the amount of slip, while fracture overlap exhibits the opposite effect. Fracture slip decreases with an increasing amount of directly adjacent fractures. Mechanical interaction becomes negligible for fracture configurations with spacing-to-length and spacing-to-overlap ratios exceeding 0.5 and that in this case fractures are decoupled. Independent of the pre-existing fracture configuration, the development of a secondary systematic fracture set driven by a remote stress rotation is dominated by σH; development of wing cracks or shear zones is restricted to the fracture tips. Blocks with tapered geometries are present in models with a variable fracture strike, where the maximum principal stress (σ1, applying the geological convention that compressive stresses are positive) trajectories consistently deviate from σH; the presence of two systematic σ1 trajectory orientations suggests that two types of secondary features could develop in one re-activation phase. [ABSTRACT FROM AUTHOR]